The Chemistry of Excited Complexes: a Survey of Reactions

The Chemistry of Excited Complexes: a Survey of Reactions R. S . DAVIDSON Department of Chemistry, The City University, London, U.K. 1 Definitions 2 2 Introduction 2 3 Formation of excited complexes 2 Direct excitation 2 Electron-transfer reactions 5 Energetics of electron transfer 8 4 Role of excited complexes in producing species capable of giving rise to chemical reactions 13 Equilibrium and non-equilibrium exciplexes and excimers 13

Triple complexes (triplexes) and substitution reactions of exciplexes (SEXreactions) 43 Role in energy transfer 48 Formation of triplet states 50 Energy wastage 55 Exciplex-induced valence-bond tautomerism 55 5 Role of radical ions generated from excited singlet states 57 Ionic reactions of photogenerated radical ions 57 Redox reactions of photogenerated radical ions 74 Redox reactions of radical ions in oxidation reactions 76 Role of radical ions in chemiluminescent reactions 81 S,,1 Reactions . 83 6 Excited complex formation and electron-transfer reactions of triplet states 84 Electron-transfer reactions of carbonyl compounds 84 7 Excited complex formation and photo-induced electron-transfer reactions in organised systems 94 Micellar systems 94 Monolayers 98 Other ordered systems 98 8 Chemical reactions postulated as occurring via excited complex formation or an electron-transfer reaction 100 Intermolecular cycloaddition 100 Intramolecular cycloaddition 107 Other addition reactions 109 Reduction 111 Fragmentation 112 References 113 1

R. S. DAVIDSON

2

1

Definitions

The term EXCIPLEX (excited complex) is used to describe an electronically excited molecular complex of definite stoichiometry. Complexes which fall into this broad classification include : EXCIMERS (excited dimers) - electronically excited complexes formed between identical atoms or molecules. HETEROEXCIMERS - electronically excited complexes formed between two nonidentical atoms or molecules. EXCITED CHARGE-TRANSFER COMPLEXES - complexes produced by excitation of ground state molecular complexes for which there is conclusive evidence for association in the ground state.

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Introduction

Since the earlier reviews on the chemistry of excited complexes appeared (Lablache-Combier, 1972; Davidson, 1975) many further examples of reactions occurring via these species have been described. Scheme 1 outlines some of the many processes that excited complexes may undergo. Excited complexes formed between two chromophores can be described in the general way shown in (1) (Mataga and Ottolenghi, 1979). If ground state interactions are

+

M

C, 404TM2) + c,$(M,M;)

+ c,d(MYM:) + Ca4(M:MJ + c54WiM2)

(1) important, as they are in excited charge-transfer complexes, then the term C, becomes significant. The weighting of C1 and C2 relative to C, and C, determines the extent to which exciton resonance contributes to the stabilisation relative to charge transfer. There is now a whole spectrum of excited complexes at the extremities of which are complexes having little chargetransfer stabilisation and those in which it is of paramount importance. 3

Formation of excited complexes

DIRECT EXCITATION

The formation of intermolecular complexes in fluid solution requires that after excitation of one of the partners, collision with the other partner must occur within the lifetime of the excited state. Under such circumstances the maximum rate constant for the process is determined by the rate constant for diffusion control in the solvent employed. Furthermore, as the free energy for complex formation decreases, it becomes more and more necessary for multiple collisions to take place if excited complex formation is to occur. T o offset unfavourable energetics to some degree, one may resort to creating

Ds, + Aso

Dso+As,

Boxes indicate species which may lead to chemical reaction. In this scheme it is assumed that DT, or AT, are of lower energy than D+ A-. Dso. Aso represent the donor and acceptor molecules in their singlet ground state

+

W

4

R. S. DAVIDSON

local high concentrations of the reaction partners. This can be accomplished by using organised systems such as micelles or monolayers (Turro et al., 1980) or by linking the two groups together by means of a flexible chain (De Schryver et al., 1977a; Bouas-Laurent et al., 1980). However, when one utilises this latter strategy one has to ensure that thechain has sufficient flexibility and is of the appropriate length and that it does not perturb the electronic properties of the chromophores. Another way of creating a high local concentration in bimolecular reactions is to utilise a quenching molecule in which there is more than one unit of the quenching chromophore. Thus the quenching of naphthalene fluorescence by a,w-diaminoalkanes is far more efficient than when a monoamine is used (Beecroft et al., 1978). Furthermore, when more than four methylene groups separate the amino groups, the measured quantum yield of exciplex formation caused by the diamine relative to the monoamine (on a mole to mole basis) is considerably higher. Local high concentrations may also be created by utilising, where appropriate, the ability of a compound to form hydrogen bonds. Thus, N-heterocycles form complexes with phenols very readily when alkanes are used as solvents (Yamamoto et al., 1976). One might anticipate that the formation of excited complexes between partners, in which there is little or no ground state stabilisation, should not occur in rigid media unless, of course, extremely high concentrations are employed. However, in many crystalline matrices, e.g. cyclohexane, excimer formation (McDonald and Selinger, 1970; Mataga et al., 1967; Davidson and Lewis, 1981), processes occurring via an exciplex (Davidson et al., 1980a) and exciplexes (Mataga et al., 1966b) have been observed. It appears that the use of crystalline matrices can lead to the formation of aggregates and microcrystals of the solute molecules. Inclusion complexes have also been shown to aid excimer formation provided that the cavity dimensions and polarity are appropriate. Thus Ueno et af.(1980) have shown that the lipophilic cavity of y-cyclodextrin can accommodate two molecules of sodium (I-naphthy1)methyl acetate and that this leads to enhanced excimer emission. Excimer formation by diarylmethylammonium salts is promoted by y-cyclodextrin but not by a- and pcyclodextrins (Emert et al., 1981a). The best results were obtained with compounds having fairly large aryl groups e.g. 4-biphenylyl and I-naphthyl. The hydrophobic character of these groups helped to solubilise the compounds in the interior of the cyclodextrin. Another factor which influences the energetics of excited complex formation in solution is solvent polarity. Where charge transfer is important, eqn (2), due to Rehm and Weller (1970), applies. E,, and Eredare the oxidation

THE CHEMISTRY OF EXCITED COMPLEXES

5

potential of the donor and the reduction potential of the acceptor respectively, e2/E‘ais the Coulombic interaction energy between the two radical ions at the encounter distance a in the solvent of dielectric constant E’ and AE,, is the electronic excitation energy of the fluorescer. From this equation it can be seen that the bulk solvent dielectric constant can affect the energetics. Thus for compound [ 11 intramolecular exciplex formation could not be observed in methylcyclohexane, but when benzene CH,CH,N’

n

‘

W0

was used as solvent it was clearly visible (Beecroft and Davidson, 1981). Another interesting feature of eqn(2) is the parameter a, the distance separating the two molecules or chromophores in the solvent-relaxed excited complex. Irie et al. (1978) showed, in a beautifully designed experiment, that as the dielectric constant of the solvent is increased, so the value of a increases. Thus the quenching of the fluorescence of an enantiomer of 1,l’binaphthyl by an optically active amine in nonpolar solvents is efficient only when the correct enantiomer of the amine is used. As the solvent polarity is increased, this dependence becomes of decreasing importance. O’Connor and Ware (1979) have made a detailed study of fluorescent exciplex formation between 1-cyanonaphthalene and I ,2-dimethylcy.clopentene. By examination of the kinetics of exciplex formation and decay as a function of solvent polarity and temperature they concluded that as the solvent polarity is increased A S becomes less negative whilst the binding energy for the exciplex remains constant, i.e. the a term in the equation plays an important part in determining the probability of excited complex formation. ELECTRON-TRANSFER REACTIONS

In Scheme I , the solvent-separated radical ions ( D i and A;) are represented as having a combined energy which is lower than either the non-relaxed or equilibrium exciplexes. It is also implicit that the combined energy is lower than that of the species initially created in the excitation process. However, if one considers the genesis of the radical ions, one realises that two oppositely charged species are geminately produced and that, to obtain the solventseparated species, they have to diffuse away from each other, thereby overcoming the Coulombic attractive force. Such a process is likely to be less favourable than the exothermic back electron-transfer reaction to give neutral A and D. This energy may be released as heat, thus affording an

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R. S. DAVIDSON

efficient way of transforming light into low grade energy. A recently described example is the quenching of the fluorescence of rubrene by N,N,N',N'tetramethyl-p-phenylenediamine(Yee et al., 1979). Another possibility is that the back electron transfer will generate an excited state such as an exciplex or possibly an excited singlet state of one of the partners. Many radical-ion partners (i.e. a radical cation and a radical anion) have been generated electrochemically (Bard and Park, 1974) and also by chemical means (Zachariasse, 1974; Weller and Zachariasse, 1971). Formation of exciplexes by the reaction between radical ions can occur by the two routes (3) and (4). Examples are known where both mechanisms ZA; + 2Dt

-4

solvent separated ion pair

(ZA;---zDt)

-+

solvent shared ion pair

'(A'D?)

or 2A; + 2Dt

+

(2,4--2Di)

3A*

A

3A*

(3)

exciplex

+D

+ 3D*

+ 3D*+'(A;D?)

(4)

(a mixed triplet annihilation reaction)

operate, e.g. in the reaction between bitolyl radical anions with the radical cation of N,N,N',N',-tetramethyl-p-phenylenediaminein dimethoxyethane. Exciplex emission is observed on mixing the two radical ions. When a triplet quencher was added to the system, the yield of exciplex fluorescence was reduced but not totally suppressed, indicating that there is a route which does not involve triplets. It is also possible to choose systems where either both or one triplet state cannot be populated on energetic grounds. Such systems cannot therefore follow the triplet-triplet annihilation route. Examples of such systems include the reaction of the radical cation of tri-p-tolylamine with the radical anions of benzophenone and 1,4-dicyanobenzene. A particularly clever way of determining whether triplet states are involved in the production of exciplex fluorescence is to examine the effect of a varying magnetic field upon the yield of the fluorescence. One of the effects of an external magnetic field upon triplet states is upon the hyperfine interactions between the unpaired electron and the nuclear spins; consequently the lifetime of the triplet state is affected which in turn affects the yield of exciplex (Werner et al., 1978; Bube et al., 1978). Another interesting example of charge annihilation producing excited states is that of the solvated electron with a tris(2,2'-bipyridyl)ruthenium(II) complex (Jonah et al., 1978). The formation of excimers and exciplexes by reaction of radical ions generated electrochemically has been the subject of much research (Bard and

THE CHEMISTRY OF EXCITED COMPLEXES

7

Park, 1974). As in the reaction of chemically generated radical ions, the role of triplets in excited complex formation is determined by whether the energy of the triplet states be above or below that of the excited complex. Some l-amino-3-(9-anthryl)propanes which exhibit intramolecular exciplex emission on direct excitation also exhibit exciplex fluorescence when subjected to cyclical oxidation and reduction at a rotating platinum disk electrode in tetrahydrofuran (Ziebig et al., 1980). A study of the quantum yield as a function of applied magnetic field showed that triplet states are intermediates and it was suggested that a triplet exciplex is involved. The evidence for the occurrence of such species is somewhat tenuous. Excimer formation with anthracenic compounds appears to have been observed (Chandross et al., 1965; Parker and Short, 1967). The formation of the pyrene excimer by the triplet-triplet annihilation route appears to be well established (Tachikawa and Bard, 1974). Once again the influence of an external magnetic field upon the excimer fluorescence yield proved to be very useful in determining the role of triplets in these reactions. Exciplex formation has been observed in the reaction between the electrogenerated stilbene radical anion and the tri-p-tolylamine radical cation. On energetic grounds exciplex formation may occur by the triplet-triplet annihilation route or direct reaction of the radical ions. Since the triplet states of stilbene and tri-p-tolylamine have short lifetimes, it was concluded that exciplex formation must take place by the latter route. For the reaction of the radical anion of 1,2-benzanthracene with the radical cation of tri-p-tolylamine some of the observed exciplex emission is believed to occur by the triplet-triplet annihilation route. Electrochemical oxidation of the tris(2,2-bipyridyl)ruthenium(II) complex to the corresponding ruthenium(II1) species in the presence of reducing agents such as oxalate anions gives rise to emission from the excited ruthenium(I1) complex, (Rubinstein and Bard, 198 I). In principle, the thermolysis of high energy compounds can give rise to excited states which may in fact be excited complexes. Thus thermolysis of [2] (Nakamura and Goto, I979a,b) generates chemiluminescence, the emission being composed of fluorescence from the indole nucleus and fluorescence from an intramolecular exciplex formed between the indole and ester groups.

n

n

@JL&L&I

I

Me

PI

Me

a

R. S. DAVIDSON

Exciplexes have also been detected in the thermolysis of some 1 ,Zdioxetanes (Zaklika ef al.. 1978). ENERGETICS OF ELECTRON TRANSFER

The process of electron transfer from an excited state to a ground state molecule and the subsequent reactions are described by Scheme 2. Using this

A*

\../

kl2

+D +

(A*---D)

k23

k34

+

(2A;

encounter

+ 2Df) +

'i'Oe,"%yd

k,,

2A;

+ 2Df

solvent separated ion pair

A +B Scheme 2

scheme Rehm and Weller (1970) derived (5) which relates the observed quenching rate constant with the rate constants for the reactions contributing to the quenching process. If (6) holds, where Zijis the frequency factor, then, k,

=

kl2

+ (k21/k23) + (k21k32/k30k23) kij = Zijexp

[

-%I

(5)

assuming that Z3, can be taken as a common frequency factor for all the reactions, (5) can be re-written as (7). Provided that one can obtain a relationk12

ka = 1

+ (k,,/Z) [exp (AGS23/RT) + exp (AG,,/RT)]

(7)

ship between A c t z 3 and AGZ3one can calculate k,. AC,, is given by (2). Rehm and Weller, on the basis of many experimental results, derived an empirical

relationship (8). AG-(0) is the activation free energy for a reaction in which there is no free energy change and corresponds to the intrinsic barrier to electron transfer. According to Scandola and Balzani (1979) this equation predicts that, for a value of AGt(0) = 2 kcal mol-', A c t should vary as a function of AG as shown by the Rehm-Weller (RW) line in Fig. 1.

THE CHEMISTRY OF EXCITED COMPLEXES

30

9

t

I

i

A G kcol mot-’

FIG. 1 Graphical representation of the free energy relationships for electrontransfer processes (calculation performed with AG*(O) = 2 kcal mol-’). (From Scandola and Balzani, 1979)

An alternative relationship (9), based on absolute reaction-rate theory, has been derived by Marcus (1956, 1960, 1964) and Hush (1961, 1967, 1968, 1975). Scandola and Balzani (1979) found that the use of this equation gave AG:

=

AG; (0)

[

1

+ 4AGi (0)

(9)

the Marcus line shown in Fig. 1. It is obviously desirable to have a theoretically derived relationship which accounts for all the experimental observations and in this respect the Marcus-Hush theory proves inadequate and particularly so for exoergonic reactions. Scandola and Balzani (1 979) therefore developed a further relationship based on a “thermodynamic-like treatment of concerted reaction kinetics” due to Agmon and Levine (1977) and Levine (1979). Agmon and Levine (1977) assumed a standard free energy profile (10) for concerted reactions. The change from reactants to products is represented by a progress variable n which ranges from 0 to I . For an electron-transfer G(n) = nAC

+ [Act (O)/ln 2]M(n)

(10)

reaction, n is related to the fraction of charge along the reaction co-ordinate. The term M(n) is assumed to be of the form M(n) = -(I - n) In (1 -n) - n In n Scandola and Balzani (1979) utilising these equations obtained (11). This

}

(11)

R. S. DAVIDSON

10

equation predicts A c t to vary with AG,, (as shown in Fig. 1) in a similar way to the Rehm-Weller empirical relationship. Utilising (1 l), Balzani et al. (1980) computed plots of log k, versus redox potentials (Fig. 2) and showed how the AGt(0) term affects the shape. The term AGt(0) is affected by two main reorganisational processes as shown in (12): (i) changes in internal

EOO ( * D D ) - E O O ( * A A

IM

FIG.2 Influence of the intrinsic reorganisational energy on the shape of the plot of log k , vs Eoo(D*D)from eqn ( 5 ) using k , = 1Olo M-ls-l k-d = 1.2 x 101os-l, k”,, = 1 x 10l1s-l, T = 293 K ; AG*(O) = 250 ( a ) ; 1000 (b); 1250 cm-’ (c). (Adapted from Balzani et al., 1980)

nuclear co-ordinates of the molecule (“inner sphere” reorganhational energy AG;) and (ii) from changes in the solvent arrangement around the molecule (“outer sphere” reorganisational energy AGE). It was pointed out that AGE

A 0 (0) = AG!

+ AGi

(12)

may well be negligible for electron-transfer reactions from a delocalised (e.g. n) orbital but significant when a localised (e.g. o * ) orbital is involved. Thus plots of log k, versus redox energies may well have different shapes when aromatic and aliphatic amines are used as quenchers. As far as practising chemists are concerned, the difficulty of utilising eqn (1 I ) is that one has to employ curve-fitting to obtain the relevant parameters. Thus the simple relationships (13) and (8) empirically derived by Rehm and Weller still hold wide appeal. Bock et al. (1979b) have shown in a most convincing way that the k,/lmol-* s-1 =

2.0 x 1010 I + 0.25 exp (AGtlRT) + exp(AG/RT)

(13)

degree of reorganisation (“inner” and “outer sphere”) plays an important part in determining the energetics of electron-transfer reaction, and hence the

THE CHEMISTRY OF EXCITED COMPLEXES

11

profiles of the plots of log k, against the energy of the appropriate redox couple. Figures 3, 4 and 5 show the plots for quenching the emission of tris(2,2'-bipyridyl)ruthenium(l I ) species by amines, bipyridinium ions and aromatic nitro-compounds. In Fig. 3 the slope of the line is approximately 4 whereas the line in Fig. 4 has a slope of approximately I . Bock ef al. pointed out that for the quenching by aromatic nitro compounds (14) the ieactions having rate constants k,, and k,,, do not involve similar electron-transfer processes and will therefore have different inner sphere reorganisational energetics. For the back electron-transfer reaction having (RuBpt+)* + ArNO, (Bp

=

Bipyridyl)

k,,

k,,, RuBp:'

+ ArNO,

+ (RuBp;? . . . . . ArNO;)+

(14)

k32

rate constant k,, the transferred electron is entering the ligand whereas for the process with rate constant k,,, it is transferred to the metal. For all the quenching reactions k,,, will be virtually independent of the system since the process is so exoergonic. On the other hand, the rate constant k:32will be very sensitive to the energetics of the light-induced electron-transfer process. It has been shown (Ware et al., 1974) from a study of the quenching of fluorescence of 9,lO-disubstituted anthracenes by substituted I ,I-diphenylethylenes that, as the light-induced electron transfer becomes less.and less exoergonic, the process having rate constant k,, becomes more and more important. Bock et al. (1979b) suggest that the slopes of the lines in Figs 3, 4 and 5 are

--> -*"-

I

06-o

0 5-

c

'

.I

04-

0 3-

02

-06

-04

-02

AG,, /

in CH,CN , ~ (I FIG.3 Plot of RTln k , V S ~ G

00

02

2

0.4

( V )

0.1 M) at 22k 2°C. The theoretical line is that obtained by using the Marcus and Hush equation, =

RTlnk,= RTlnk,(O)-&G,,(I

where h

=

+&)

2 2h 11 kcal mol-' and k,(O) = 8.8 x 10-M-ls-'. (From Bock et al., 1979b)

12

06

R. S. DAVIDSON

c

AGZ3 / V

FIG.4 Plot of RT In k, vs AG23 in CH,CN (p = 0.1 M) at 22 & 2°C for the quenching of [Ru (bipy)i+]* by viologens. The theoretical line is that obtained by using the Marcus-Hush equation having k,(o) = 8.4 x lo6 M-ls-l and A = 17 kcal mo1-I. (From Bock et al., 1979b)

0.58

c

0

"t

ArNO,

'

0.42

0.341

I

t

-0.6

I

-0.4

I

-0.2

I

0.0

I

0.2

o\ I

0.4

AG,,/V

FIG.5 Plot of RT In k , vs AC23 for quenching of [Ru (bipy):']* CH,CN ( I = 0.1 M) at 22 f 2°C. (From Bock et al., 1979b)

by ArNO, in

determined by the importance of k,, relative to k,,. The difference between k,, and k,, reflects not only the thermodynamics of the actual electron-

transfer process, but also the reorganisational process. The treatment by Balzani et al. in ( I 1) places a different emphasis upon the reorganisational processes compared with that by Marcus and Hush. Bock et al. on the other hand utilised the Marcus-Hush approach and point out that any attempt to predict k , must take account of the relative importance

THE CHEMISTRY OF EXCITED COMPLEXES

13

of k,, and k30.It is unfortunate that steady state kinetics do not directly give values of k,, and k,, and one is forced to resort to time-resolved experiments which have the problem that multi-exponential (fortunately usually only biexponential) decays have to be analysed. A most useful generalisation, due to Rehm and Weller, is that quenching rate constants approaching the diffusion-controlled limit indicate that AG is strongly negative, rate constants a factor of ten below the diffusion-controlled limit that AG 0 and values less than a tenth of the diffusion-controlled limit indicate that AG is positive. This generalisation has been applied to show that electron transfer from excited singlet and triplet aromatic to methyl viologen is exothermic excepting when the aromatic hydrocarbons contain strongly electron withdrawing substituents such as cyano and nitro (Davidson et d.,1981a).

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4 Role of excited complexes in producing species capable of giving rise t o chemical reactions

EQUILIBRIUM A N D N O N - E Q U I L I B R I U M EXCIPLEXES A N D EXCIMERS

Non-relaxed complexes As can be seen from Scheme I , excited complex formation can lead to a number of species which are capable of undergoing chemical reactions. For systems involving donor-acceptor interactions, the initially created complex is termed either a non-relaxed exciplex, an encounter complex, or a FranckCondon excited complex. The essential feature of this complex is that it has been created by the collision of two molecules and that solvent molecules have not had time to reorganise so as to afford it maximum stabilisation. The extent of charge transfer in such complexes is open to question. Halide ions quench the fluorescence of polycyclic aromatic hydrocarbons (Watkins, 1973; Beer et a / . , 1970). Although the quenching efficiency increases with ease of oxidation of the anion, no spectroscopic evidence could be obtained for the formation of a fluorescent complex or radical ions. Formation of triplet aroniatic hydrocarbon was observed. I t seems probable that in these systems the halide ion forms an encounter complex with the excited singlet state of the aromatic hydrocarbon having some charge-transfer character. However, its binding energy is extremely low and non-radiative decay routes such as formation of the triplet aromatic hydrocarbon (which is no doubt aided by the heavy atom effect) compete with formation of a stabilised or equilibrium exciplex. A similar situation arises with [3] (Davidson et al., 1980a). The halogeno group intramolecularly quenches the fluorescence of the naphthalene nucleus without the formation of a fluorescent complex. Once again triplet formation is enhanced as a result of this interaction.

14

R. S. DAVIDSON

In the ensuing sections many more examples will be cited, such as the cycloaddition reactions of alkenes to benzene and related compounds (Cantrell, 1977; Gilbert and Heath, 1979; Bryce-Smith et al., 1980b) where the quenching of fluorescence can be related to the redox properties of the quencher and yet it has proved impossible to detect the formation of fluorescent complexes or radical ions. On the basis of this negative evidence it has to be assumed that reaction proceeds via a non-relaxed exciplex. If the non-relaxed exciplex has a reasonable degree of binding energy and therefore has a lifetime longer than that required for solvent reorganisation, it should in theory be possible to detect such a species. This is the case for fluorescent exciplex formation in the pyrene-tri-n-butylamine and pyreneN,N-diethylaniline systems (Nakashima et al., 1972). These systems were examined by the technique of time-resolved fluorescence spectroscopy. It was shown that when solvent reorganisational processes are slowed down by increasing the viscosity of the solvent by lowering its temperature, the fluorescence spectrum of the exciplex showed a time dependence. This dependence demonstrated that solvent reorganisation stabilised the exciplex. The point has already been made that non-relaxed exciplexes may well be responsible for chemical reactions. In some cases the lack of fluorescence for an equilibrium exciplex may be due to the chemical reaction of the nonrelaxed exciplex opening up a highly efficient non-radiative decay route for such a species. This appears to be the case for reaction of excited singlet aromatic hydrocarbons with primary and secondary aliphatic amines. N-Methylindole and related compounds form fluorescent exciplexes with aromatic hydrocarbons such as naphthalene (Davidson and Whelan, 1977) but undergo no chemical reaction. Pyrrole [4] on the other hand undergoes an addition reaction with naphthalene (Scheme 3) but does not form a fluorescent exciplex (McCullough et al., 1970, 1972). In this system the acidic hydrogen of the N-H bond is efficiently transferred to the basic radical anion of naphthalene. Many aromatic amino acids undergo decarboxylation reactions with the excited singlet states of aromatic hydrocarbons and such systems exhibit little or no exciplex fluorescence (Brimage and Davidson, 1973). Once again, the presence of an appropriately situated acidic hydrogen aids the chemical reaction to such an extent that fluorescent complex formation is virtually suppressed. Primary and secondary aromatic amines quench the fluorescence of the excited singlet states of polycyclic aromatic hydrocarbons but this is not accompanied by the formation of fluorescent exciplex forma-

THE CHEMISTRY OF EXCITED C O M P L E X E S

15

m+n

H

H

N

H

[41

Scheme 3

tion (Okada et al., 1976a). Flash photolytic studies showed that the reactions lead to production of neutral radicals (presumably via radical ions). The products of the reaction (1 5 ) between secondary aromatic amines, e.g. N-methylaniline, and anthracene (Yang and Libman, 1973) can be readily rationalised as being formed by initial electron transfer between the excited aromatic hydrocarbon and the amine followed by rapid proton transfer to give neutral radicals; these then undergo radical-radical combination and disproportionation reactions.

H

I

N-Me

H

H

Q Transformation of non-relaxed into relaxed exciplexes

So far, two reasons have been advanced for the failure of non-relaxed

exciplexes to give equilibrium exciplexes, namely, lack of binding energy (stability) and chemical reactivity (product formation).

R. S. DAVIDSON

16

TABLE 1 Bimolecular constant for self-quenching of the fluorescence of substituted anthracenes

Solvent An thracene 9-Methylanthracene 9-Propylanthracene 9,lO-Dimethylanthracene

Benzene Benzene Toluene Benzene

kDIM/l OBM-' s- '

2.3 2.8

0.6 0.04

ksQ/lOBM-'s6.6 9.2 8.4

1.8

For the equilibrium complex to be formed the two groups or molecules have to reorientate with respect to each other. Such a process may be hindered by (a)unfavourable steric interactions due to substituent groups present within the participating molecules, (6) in the case of bichromophoric compounds the lack of conformational mobility in the linking chain, and (c) if a group has to undergo rehybridisation in the electron-transfer reaction, e.g. pyramidal tertiary amines becoming planar, this reorganisation may not be able to occur within the lifetime of the excited state. Steric factors. An interesting example of steric effects upon excimer formation is demonstrated by the anthracenes. Many anthracenes exhibit selfquenching of fluorescence and these results are shown in Table 1 (from Bouas-Laurent, et al., 1980). As one moves from anthracene to 9-methylanthracene and then to 9,10-dimethylanthracene one finds that the rate constant for self-quenching of fluorescence (ksQ)decreases and so does the rate constant for photodimerisation (kDIM).For anthracene, the latter reaction is very efficient and excimer formation in fluid solution cannot be observed. The much lower rate of dimerisation for 9,lO-dimethylanthracene allows excimer formation to be observed. This excimer is of lower stability than 9-methylanthracene and this is attributable to the greater steric effects with the former compound (Barnes and Birks, 1966). In the case of 9,10-diphenylanthracene, in which the phenyl groups lie out of the plane of the anthracene ring, both excimer formation and photodimerisation are suppressed. This photochemical stability makes 9,IO-diphenylanthracene an excellent standard for fluorescence quantum yields. The importance of steric effects shows up in the entropy of formation for excimers. Zachariasse et al. (1978) have studied the temperature dependence of intramolecular excimer fluorescence of 1,3-di(4-biphenyl)propane, and obtained a value of -64.8 J K-'mol-' for -AS. The intramolecular excimer of 1,3-di( I-naphthy1)propane has a -AS value of 4 1 . 8 J K-lm-'. The much higher A S value for the biphenyl system can be attributed to its non-planar aromatic framework which has to become planar for excimer formation.

THE CHEMISTRY OF EXCITED COMPLEXES

17

Steric effects have also been shown to be important in intramolecular exciplex formation (Pragst et al., 1978). It was found that the intramolecular exciplex formed by 3-(N-methyl-N-p-tolyl) amino-] -(9-anthraceno)propane had a dipole moment of 12.2 D whereas 3-(N-p-tolyl)amino-I-(9-anthraceno)propane gave an exciplex having a dipole moment of 15 D. It was suggested that in the former compound the exciplex deviates from the sandwich conformation and consequently has less charge-transfer character. Conformational mobility. The effect of conformational mobility upon the ability of bichromophoric compounds to exhibit intramolecular excimer and exciplex formation has been the subject of much debate (Bouas-Laurent et al., 1980; De Schryver et al., 1977a). It is very apparent that the relative orientation of the two groups is of far greater importance for excimer formation than it is for exciplex formation. In the latter process much of the stabilisation comes from the Coulombic interaction of two oppositely charged groups and the efficiency of interaction is governed more by the distance between the groups than whether the two groups can form a sandwich complex. Hirayama (1965) was the first to demonstrate that excimer formation by a,w-diphenylalkanes was most efficient when there were three methylene groups interposed between the two phenyl groups. The linking propyl chain allows the molecule to adopt a strain-free conformation (in which there are no eclipsing interactions in the side chain) in which the two phenyl groups overlie each other, adopting a sandwich configuration. Chandross and Dempster (1 970a,b) demonstrated .that a,w-di(1-naphthyl)alkanes and a,w-di(2-naphthyl)alkanes exhibit similar behaviour. In these systems the binding energy of the excimer is insufficient to overcome the conformational barriers and gauche interactions which occur for shorter chains (one and two methylene groups) and for longer chains (four or more methylene groups). The binding energy of the pyrene excimer is greater than that of the phenyl and naphthyl systems and Zachariasse and Kiihnle (1976) have been able to demonstrate that u,wdipyrenylalkanes exhibit excimer formation for a variety of chain lengths (Fig. 6). A similar study has been made with pyrenyl groups linked by an a,wdicarboxylic acid system (Yamamoto et al., 1978). a,w-Bisdimethylaminoalkanes show fluorescent intramolecular excimer formation when one, two, three and four methylene groups are interposed between the amino groups, but a further increase in chain length inhibits fluorescent excimer formation (Halpern et al., 1979). However for these compounds with five and more methylene groups interposed, the fluorescence decay of the excited amino group is non-exponential which suggests that the amino groups do interact but cannot attain the equilibrium conformation necessary for fluorescent excimer formation. One of the problems associated with using a polymethylene chain is that the motion and the final folded conformation required

R. S. DAVIDSON

18

n--

x

g

5

0 2 4 6 8101214161820222426: B

FIG.6 Ratio of intramolecular excimer (1’) and monomer (I)emission intensities (a) and energy of excimer emission (b), as function of n in a,o-di(1-pyrenyl)alkanes, Pyr-(CH,),-Pyr. (Reproduced with permission from Zacchariasseand Kuhnle, 1976)

may involve unfavourable eclipsing interactions between protons. This point has been made in an elegant study by Ito et al. (1981b). They found that there is a ten-fold difference in the rate of excimer formation between the racemic and meso forms of 2,4-di( 1-naphthy1)butane. To overcome eclipsing interactions an oxygen atom can be interposed between two methylene groups. The naphthyl ethers [5] and [6] exhibit excimer formation even though in [6] there are four atoms in the chain linking the naphthyl groups (Davidson and Whelan, 1977). Interestingly [5] photodimerises to give two products [7] and [8] (Todesco et al., 1978). If the structures of [7] and [8] reflect the structure of an excimer intermediate, it appears that [5] can form two excimers. one having the naphthalene rings overlying each other and the other having the two naphthalene groups parallel to each other but with overlap only between one ring of each group. Polyethylenoxy groups can be used to link aryl groups (Davidson and Whelan, 1977). The dynamics of the flexibility of polyethylenoxy chains has been studied by examining the kinetics of fluorescence quenching and excimer formation of compounds in which two anthryl groups are linked at the 9-position by such a chain (Desvergne et al., 1980). The compound [9] photodimerises quite readily in contrast to its close relative [lo] in which the anthracene groups are linked by a polymethylene chain; the oxygen atoms are thus enabling the molecule to

THE CHEMISTRY OF EXCITED COMPLEXES

19

fold so as to allow the anthracene rings to approach close to each other (Desvergne and Bouas-Laurent, 1978, 1979). Another significant finding is that the fluorescence quantum yield of [9] is 0.14, far lower than the value of 0.75 for [lo]. This again reflects the greater flexibility of the polyethylenoxy chain compared with the polymethylene chain. The intramolecular photodimer of [9] is thermally unstable but it can be stabilised if it is prepared in the presence of lithium ions which leads to the production of the crown ether complex [l 11. [9] X = O [lo] X

=

CH,

R. S. DAVIDSON

20

Polyethylene glycols having pyrenyl groups attached have been used to study the dynamics of the polyethylene oxide chain (Cuniberti and Perico, 1977). The long lifetime of the excited singlet state of pyrene and the stability of the pyrene excimer make the pyrenyl group an extremely useful probe, As might be expected, the quantum yield for intramolecular excimer fluorescence increased as the molecular weight of the polyethylene oxide was lowered from 10000 to 1000. A study has been made (Davidson and Whelan, 1977) of the fluorescence quantum yield of a number of a,w-di(1-naphthy1)alkanes and related compounds. It was found that the formation of a fluorescent excimer was accompanied by a decrease in monomer fluorescence. However in many cases (Table 2) fluorescence quenching was observed which was not accompanied by any fluorescent excimer formation. It was suggested that in these compounds, the two aryl groups, one being in an excited singlet state, can interact but the linking chain precludes them adopting a sandwich configuration and hence stable excimer formation. Since this decay route is blocked the non-relaxed complexes decay non-radiatively. Some further results are given in Table 3 (Davidson and Lewis, 1981a,b) which show how important nonradiative decay can be. With these examples it is also assumed that radiative decay is suppressed because of the inability of the appropriate molecules to adopt the sandwich configuration. The results of Halpern et al. (1979) from their study on diaminoalkanes also demonstrate that fluorescence quenching TABLE 2 ) excimer ( Quantum yields" for fluorescence from the naphthalene unit ( 4 ~and for a,w-dinaphthylalkanesin degassed cyclohexane solution at 20°C

Compoundb

1-Substituted naphthalenes 4M

0.22 NPCH2NP 0.19 Np(CHz)aNp NP(CH~)J'JP 0.19 NP(CHz)aNP NP(CH~),NP 0.05 NpCH(OH)CH,CH,Np NpCH ,OCH,Np 0.015 0.09 NPCH~OCH~CH~NP NPCHZOCHZCH~OCH~NP 0.10 N~CH~(OCH~CH~)~OCH~NP -

2-Substituted naphthalenes

4E

4M

4E

-

0.33 0.23 0.018 0.21 0.21

-

0.033

0.0085 0.098 0.14 0.13

0.04 -

-

4~)

-

0.10

-

0.0375 0.07

-

Determined using 1- and 2-methylnaphthalenes as standards (4 0.21 and 0.30 respectively).The ethers 2-NpCH,OMe and 1-NpCH,OMe have quantum yields of 0.11 and 0.10 respectively * Np = naphthyl

21

THE CHEMISTRY OF EXCITED COMPLEXES

TABLE 3

Fluorescence quantum yields of some naphthyl esters (1 x 10-4M) relative to methyl 1naphthylacetate in ethanol at 20°C" Estef 1-NpCHzC02CH2Np-1 1-NpCH2COZNp-l 2-NpCHzCOzNp-2

4F

0.1 1 0.18 0.34

" Lewis, 1974

* 1-Np = 1-Naphthyl;2-Np = 2-naphthyl occurs even when excimer formation is difficult or impossible to detect. Thus the formation of fluorescent excimers and fluorescence quenching appear to be inextricably bound up with each other. Indeed the intramolecular quenching of fluorescence by the diaminoalkanes is probably just a rather specific example of concentration quenching, such as that found for triethylamine (Halpern et al., 1977). Thus a disorganised array of molecules which can interact, but without adopting the conformation required for excimer formation, can dissipate the energy given to the system by exciting an appropriate molecule. In most of the cases presented so far (but with exception of [5]) it has been suggested that there is only one particular conformation that two molecules or groups may adopt if fluorescent excimer formation is to occur. However, examples are being uncovered in which it is very clear that fluorescent excimer formation may arise from several conformations. The pyrene ester [ 121 has

been incorporated into three polymer systems and the wavelength of maximal emission from the excimer determined. Table 4 shows that [I21 does form a fluorescent excimer in various polymers and that the emission is blue-shifted 40 f 5 nm compared with that observed in fluid solution. This blue shift is attributed to the polymer restraining the pyrene molecules from adopting the sandwich conformation which gives rise to the characteristic emission at

R. S. DAVIDSON

22

TABLE 4

Wavelengths of maximum excimer emission (nm) exhibited by [12] in polymers and fluid medium“ Fluid medium Toluene Methyl isobutyrate Methyl benzoate a

hmax

480 480 485

Polymeric medium Polystyrene Polymethyl methacrylate Polyvinyl benzoate

&nax

445 440 440

Qmx

35

40 45

Martie et al., 1979

480 nm. Although pairs of molecules cannot adopt this conformation, they can adopt less stable conformations which exhibit fluorescence. Polyvinylcarbazole exhibits excimer emission from the carbazole unit. By use of picosecond pulse radiolysis, Tagawa et al. (1979) showed that excitation produces two excimers, the normal “sandwich” excimer which fluoresces at 420 nm and another which fluoresces at 375 nm. The latter high energy species decays to the lower energy species very quickly and therefore good time resolution is necessary in order to see both. Another way to examine excimers having unusual conformations is to generate molecular pairs in matrices of varying rigidity and then study the fluorescence from such species as a function of temperature. In the earliest work (Chandross et al., 1966; Chandross and Ferguson, 1966) the anthracene photodimer contained in a rigid matrix was photolytically cleaved to give two anthracene molecules. Because of the rigidity of the medium, the two molecules so produced can undergo little or no movement; a pair of molecules is thus situated close enough to each other to exhibit excimer fluorescence on excitation. In this way fluorescence from the excimer of anthracene could be observed since the matrix not only kept the pair of molecules together but also slowed down the photodimerisation reaction. The photolytic dissociation of dianthracene has been re-examined (Ferguson and %a, 1978) and the course of events shown to depend upon temperature and solvent viscosity. The study revealed that cleavage leading to excimer fluorescence has a thermal activation barrier and the intermediate [13] resembles a benzyl radical. Evidence was presented that the excimer produced in this way had a

-

* + A

A

~

(a an anthracene excirner

(

THE CHEMISTRY OF EXCITED COMPLEXES

23

conformation different from that which is responsible for photodimerisation (Ferguson and Miller, 1975). Compounds [I41 and [I51 undergo photolytic cleavage on irradiation in ethanol glass to give compounds in which the two

anthracene groups are close enough to exhibit excimer emission when excited (De Schryver e l al., 1977b). Because of the disposition of the two groups relative to one another, only excimer emission could be observed. The cycloadduct [161 undergoes cleavage (1 7) to give a heteroexcimer and once again its conformation is determined by the rigidity of the glass (Ferguson et al., 1979).

1161

Hetero excimer

Solution phase studies have also shown that anthracene can.form more than one fluorescent excimer (Hayashi et al., 1977a). The I ,Zdianthrylethane [17] and 1,2-di(9,9'-dianthryl)ethane exhibit excimeric emission, the amount increasing relative to monomer emission as the solvent polarity is increased. Compound [17] undergoes an intramolecular cycloaddition reaction giving a product which, when cleaved in a matrix by photolysis with low wavelength light affords [I71 in a conformation in which the anth,racene rings overlie each other (the type I1 conformation). This conformation [17b] exhibits excimer emission which is to the red of that shown by [17] in fluid solution. It is suggested that the excimer of [I71 in fluid solution has the type I conformation [17a] and that charge transfer makes some contribution to its stability. The lack of observation of emission due to [17b] in fluid solution

R. S. DAVIDSON

24

.

A

[17a] Type I conformation

may be because this conformation lies on the reaction pathway for the cycloaddition reaction. Another approach to studying conformational effects upon excimer formation has been to synthesise cyclophanes. The rigidity of the structure of these compounds enables one to probe very precisely the effect of the relative orientation of the two groups upon excimer emission. Thus the [3,3]-cyclophanes [18]and [I91 (in which, for clarity, the unsaturation of the naphthalene rings is not shown) interconvert, and each has its own photo-chemistry (Kawabata

et al., 1979); for example, they undergo intramolecular cycloaddition reactions to give different products. Similarly the two stereoisomeric naphtheno[2,3]phanes give different cycloaddition products upon irradiation (Blank and Haenel, 1981). In the [3,3]-systems another problem arises; because of the constraints imposed upon the systems by the trimethylene bridges, the two aromatic rings are forced so close together that there is electronic interaction in the ground state. When even shorter bridges are used (e.g. dimethylene) it appears that the aromatic system becomes distorted. A major synthetic triumph has been the synthesis of [20] (Sekine et a/., 1979), and calculations have been carried out on the degree of distortion in this compound (Iwamura et af., 1980). Benzenoid [2,2]-cyclophanes (e.g. [2 I]) exhibit excimer fluorescence and cleave on photolysis as shown in (18) to give p-quinodimethanes (Kaupp and Zimmermann, 1976).Irradiation of [21] in a rigid matrix produces a benzyl radical pair by homolytic cleavage of one of the bridging ethane groups (Ishikawa et al., 1980). This intramolecular radical pair (i.e. a

25

THE CHEMISTRY OF EXCITED COMPLEXES

Q biradical) exhibits fluorescence the wavelength of which is strongly dependent upon the solvent viscosity. It was suggested that the fluorescence was due to intramolecular excimer formation by the radical pair. The photolytic cleavage of cyclophanes has proved to be of synthetic value e.g. in the generation of [22] (Kaupp, 1976). Kaupp and Zimmermann (1976) have also examined the photochemistry of the [2,2]-napthenophane [23]. This compound also exhibits

h0,

I

~ 3 1

290 nm 20°C)

nm)and ; >

Scheme 4

excimeric emission (cf. the reactions of [5]) and undergoes wavelength- and temperature-dependent reactions (Scheme 4). Anthracenophanes have been synthesised and also cyclophanes containing one anthracene unit and one unit of another hydrocarbon e.g. [24] and [25] (Shinmyozu et al., 1978). These compounds undergo photocycloaddition reactions and in the case of [25] this results in the loss of resonance in the benzene ring (19). The pyrenophanes [26] and [27] have been synthesised and their photophysics examined (Hayashi et

R. S. DAVIDSON

26

[24; R

R

=

CO,Et]

[25; R = CO,Et]

al., 1977b). The absorption spectrum of [26] shows that there is little transannular interaction in this compound, whereas in [27] there is a new absorption band attributable to such an interaction. Not surprisingly the fluorescence spectra of the two compounds are markedly different. The transannular interaction in [27] gives rise to a structureless band at 560 nm shifted to the

red compared with methylpyrene. In nonpolar solvents [26] exhibits fluorescence typical of the pyrene chromophore. In solvents of higher polarity, e.g. methyl isobutyl ketone, a structureless red-shifted fluorescence band appears at 475 nm which was assigned to an intramolecular excimer. To attain the sandwich configuration, [26] has to undergo anti-syn isomerisation and it is suggested that this occurs via a charge-transfer state, such a state being favoured by the use of polar solvents. The excimer of [26] appears to have a looser structure than [27] and not surprisingly has a shorter fluorescence lifetime. This work clearly shows that a particular compound may not necessarily give rise to a unique excimer but even so excimers are highly organised structures. The foregoing discussion shows that when a molecule in its excited singlet state starts to interact with a ground state molecule, provided that the energetics are favourable the two molecules will try and orientate themselves to

THE CHEMISTRY OF EXCITED COMPLEXES

27

form an excimer which may lead to product formation. The conformation of the excimer (which is linked to its stability) determines the relative efficiencies of radiative and non-radiative decay pathways. It has been proposed by Beddard et al. (1976) that the concentration quenching of chlorophyll is due to singlet energy migration between chlorophyll molecules which is arrested by interaction with a pair of chlorophyll molecules which produce a nonfluorescent excimer (Beddard and Porter, 1976). The reaction centres in the photosynthetic apparatus appear to be chlorophyll molecular pairs having a specific orientation. To try and mimic these reaction centres, a compound containing two chlorophyll molecules linked by an ethylene glycol unit has been synthesised (Pellin et al., 1979, 1980). The conformations of such a compound in fluid solution may not of course reflect the conformation adopted by the molecular pair in the photosynthetic system. However, in this and in a related system in which three pheophorbides (two adjacent ring systems containing a central metal atom) are linked together by an ethylene glycol unit, it appears that the polychromophoric systems act as good models for the reactive centres (Boxer and Bucks, 1979). Proposals have been made concerning the mechanism ot' energy wastage in the photocyclomerisation of compounds containing two anthracene nucleii connected by a chain. Bergmark et al. (1978) propose that the photocycloaddition reactions occur via biradicals (Fig. 7) and that these can give rise to either product formation or revert to starting materials. That biradicals are involved has been challenged by Ferguson (1980) and defended by Jones et al. (1980a). The lack of direct experimental evidence for the involvement of such species makes it difficult to assess the validity of the generality of biradical intermediates. Before leaving bichromophoric systems which exhibit excimer formation, it is worthwhile considering the significance of the rate constants determined from fluorescence-decay measurements. In nearly all cases the interaction in a bichromophoric system is assumed to be a unimolecular process, i.e. the interaction of the two terminal groups is assumed to be a pseudounimolecular process. Using this assumption one can use the classical equations developed by Birks (1970) for intermolecular excimer formation. This treatment does not lead to true rate constants for intramolecular excimer formation. Firstly, there is the problem that there will be an ensemble of conformations each of which may be able to give the excimer but by routes having different activation energies. A second point is that the system is not truly unimolecular. Thus a conformation in which the two chromophores are aligned in the ground state in a similar fashion to that found in the excimer will approximate to the unimolecular model. On the other hand if a fairly lengthy chain links the two units, the extended conformation of the molecule will not behave in a unimolecular fashion. The best approximation one can make is the active sphere

R. S. DAVIDSON

A=

A* = A,,

B= B'

C=

=

B in an excited state

c* = c,,

so

FIG.7 Energy profile &gram for the photodimerisation of 9,9'-dianthrylmethane. (Adapted from Bergmark et al., 1978)

THE CHEMISTRY OF EXCITED COMPLEXES

29

model, adapted by Halpern er al. (1979) from a procedure developed by Shimade and Szwarc (1975). Thus the rate constants for excimer formation, evaluated in this way, may well reflect the conformational mobility of the linking chain, but it is impossible to compare the conformational mobility of one type of folding chain with another, e.g. polymethylene with polyoxyethylene. Rehybridisation. Because many aliphatic tertiary amines have an sp3hybridised nitrogen atom, the quenching of excited singlet states by such compounds will have a different AG:(O) than quenching by planar amines such as aromatic amines in which nitrogen participates in p-x conjugation. Beecroft et al. (1978) found that the quenching of naphthalene fluorescence in a nonpolar solvent by triethylamine and N-methylpiperidine occurs at well below the diffusion-controlled limit. Meeus et al. (1979, 1980b) made a similar study using 2-methylnaphthalene and obtained a similar result. They also studied the effect of temperature variation upon the quantum yield of exciplex fluorescence and efficiency of quenching the 2-methylnaphthalene fluorescence. In this manner they were able to determine AH" and AS" values for exciplex formation (Table 5). Complex formation is less efficient for 5 TABLE Thermodynamicparameters for the quenching of 2-methylnaphthalenefluorescence by tertiary amines"

Amine

Solvent

Triethylamine Isoctane N-Methylpiperidine Isoctane N-Methylpiperidine n-Butylether

AHo

kcal mol-l -5.0 - 3.3

- 10.0

AGO

ASo

kcal mol-l

cal mol-' k-l

-0.5

- 14.8

-0.9 -0.7

-7.7 -31.4

Meeus et al., 1979, 1980b N-methylpiperidine than for triethylamine. This difference was attributed to the difference in the geometric restrictions in the approach of the amine. These will of course be the most severe in nonpolar solvents where the hydrocarbon and amine have to be situated very close to each other. The greater difficulty experienced by N-methyl-piperidine in forming a tight complex results in an overall lower rate constant for fluorescence quenching and quantum yield for fluorescent exciplex formation. The more planar compound N-methylpyrrolidine is a better quencher than either triethylamine or Nmethylpiperidine. The quantum yield of exciplex formation is higher. For a pyramidal amine to form a fluorescent exciplex it has to become planar and therefore energy has to be expended, i.e. rehybridisation costs energy. Intramolecular complex formation between tertiary amines and aromatic hydrocarbons is strongly affected by the hybridisation of the nitrogen. Van

R. S. DAVIDSON

30

der Auweraer et al. (1980a,b) have studied the fluorescence of w-phenyla-N,N-dimethylaminoalkanes. Exciplex emission was observed for n = 2, 3, and 4. Of particular interest was the finding that the decay of the fluorescence from the benzenoid nucleus had a longer lifetime than that of the exciplex. Beddard et al. (1972) found the same was true for 2-(N,N-diethylamino)1-(1-naphthy1)ethane in acetonitrile solution. Meeus et a/. (1980a) have obtained a similar result for 2-(N-piperidino)- 1-(2-naphthyI)ethane. If one adopts the usual kinetic scheme for exciplex formation [due to Rehm and Weller (1970)], the exciplex fluorescence should always have a longer lifetime than the emission from the aromatic nucleus. Van der Auweraer et a/. (1980a) pointed out that in these compounds only certain conformations can readily form the exciplex; in the other conformations exciplex formation can only occur if rotation about the C,-N bond or inversion at nitrogen occurs (Fig. 8). Both these processes have activation energies of 5-10 kcal mol-I which

\:

o,= M e

Rotation about CC ,p

bond gives exciplex

@ r t M e

Rotation about C,-Cp bond does not give exciplex provided that inversion at nitrogen does not occur nor rotation about the CB-N bond FIG.8 Effect of inversion at nitrogen upon exciplex formation. (From Van der Auweraer et a]., 1980)

means that they are slow on the time scale of the initially created excited state. Thus, such conformations do not participate in exciplex formation and some of the emission from the aryl group remains totally unquenched. Normally w-amino-a-arylalkanes can adopt a conformation which produces exciplex fluorescence by a process involving C-C-bond rotation which is very fast. Both 3-(4-dimethylaminophenyl)- 1 -(9-anthracenyl)propane and 3-(4-dimethylaminophenyl)-1-(1-pyrenyl)propane form fluorescent exciplexes, which by means of picosecond time-resolved fluorescence spectroscopy have been shown to take a few nanoseconds to be formed (Migita et al., 1980, 1981). The rate of intramolecular fluorescent exciplex formation has also been shown to be dependent upon the length of the linking chain, the polarity of the solvent (the build up time decreases as solvent polarity is increased)

THE CHEMISTRY OF EXCITED COMPLEXES

31

and upon solvent viscosity (Okada et al., 1981). These results emphasise the point made earlier, that the kinetics of bichromophoric systems cannot be analysed on the basis of a “unimolecular” model since time is required to reach the conformation which gives rise to excimer or exciplex fluorescence.

Eflect of solvent upon the formation of relaxed exciplexes and excimers The early work by Weller and co-workers (Knibbe et al., 1967) on intermolecular exciplex formation between aromatic hydrocarbons and tertiary amines established that as the solvent polarity was increased one observed (a) a red shift in the exciplex fluorescence, ( 6 ) a decrease in quantum yield for exciplex fluorescence (becoming zero in highly polar solvents), and (c) a decrease in the lifetime of the exciplex. All these observations were accommodated by the suggestion that, as the solvent polarity increased, there was an increasing tendency for the exciplex to dissociate into radical ions. This effect works in opposition to the increase in solvating power of the solvent which stabilises the exciplex due to favourable dipole-dipole interactions. For the most part this rationalisation holds. However, intramolecular amine-aromatic hydrocarbon exciplexes show some anomalies. Unlike most of their intermolecular counterparts they show exciplex fluorescence in highly polar solvents (Brimage and Davidson, 1971; Beddard et al., 1972). The quantum yield for exciplex fluorescence is lowered as the solvent polarity is increased. Surprisingly, examples were found in which the exciplex lifetime increased as the solvent polarity was increased. Thus in these systems the dissociation of the relaxed exciplex into radical ions may not be an important process. It is more likely that the electron-transfer process occurs from the non-relaxed exciplex. The fact that intramolecular exciplexes give rise to fluorescence in solvents of widely differing polarity and that the wavelength of the emission depends upon the solvent polarity makes them ideal candidates for studying solvation phenomena. It has been found that the fluorescenke spectra of some naphthylalkylamines in nonpolar solvents are drastically affected by the addition of small quantities of polar solvents (Chandross, 1974; Beddard et al., 1975). The time-resolved experiments showed that the initially formed exciplex has a nonpolar environment and that during its lifetime small polar molecules diffuse to it and solvate it so causing a shift in the maximum of fluorescence. The steady state measurements indicate that the addition of small amounts of polar solvents causes a red shift in the fluorescence and in the shape of the fluorescence band. These results show how dangerous it is to use mixed solvent systems (and particularly the bulk solvent properties of these systems) to examine exciplex phenomena or as a diagnostic mechanistic tool. A straightforward Stern-Volmer plot of quenching of fluorescence will be perturbed in a way that is difficult to describe mathematically.

32

R. S. DAVIDSON

2,6-Di(dimethylaminomethyl)naphthalene has been used to study the solvating properties of mixed solvent systems. It was found (Ibemesi and El-Bayoumi, 1980) that this compound exhibits exciplex fluorescence when nonpolar solvents contain > 25 % of a polar solvent, e.g. water or ethanol. The fluorescence wavelength indicates that the exciplex is generated in a polar environment, i.e. much of the amine is solvated by the polar molecules in the ground state. When lower concentrations of polar solvent are present there is a contribution from the dynamic interaction of the initially created excited state with polar molecules that diffuse to it. Ibemesi and El-Bayoumi (1979) also suggest that in mixed solvents there is a specific interaction between small polar molecules and the excited state as well as the overall influence of the dielectric constant of the solvent mixture. One of the most bizarre small molecule quenching effects came to light during an investigation of exciplex formation between benzene and triethylamine. Leismann and Mattay (1978) observed exciplex formation in a nonpolar solvent but utilised a high concentration of benzene. Beecroft and Davidson (1981) found that, in nonpolar solvents, triethylamine quenched the fluorescence of benzene (present at low concentration) and that this was accompanied by sensitised amine fluorescence. Under these conditions, very little exciplex fluorescence could be observed. Increasing the benzene concentration led to the result obtained by Leismann and Mattay. It was concluded that the use of a high concentration of benzene led to exciplex formation, the initial encounter complex between the benzene and the amine being solvated by the polarisable benzene molecules thereby lowering the energy of the exciplex below that of the singlet state of the amine. Consequently, exciplex and not amine emission was observed. A most interesting example of intramolecular exciplex formation is afforded by 1-naphthyl-pentamethydisilanes(Shizuka et al., 1981). Exciplex fluorescence is observable in solvents of widely differing polarity. Presumably the disilane cannot adopt a sandwich configuration and therefore the apparently high quantum yields for exciplex formation are somewhat surprising. The finding that the wavelength of exciplex emission is solvent dependent has been used to calculate the dipole moments of a variety of complexes (Beens and Weller, 1968b). Most exciplexes have dipole moments which correspond to nearly complete electron transfer. Very few systems correspond to the intermediate region. One such system is N-methylbenzimidazole plus 1-cyanonaphthalene (Davidson et al., 1977). In this intermolecular system, exciplex emission can be observed in highly polar solvents. It appears that for most systems electron transfer is so facile that use of high polarity solvents leads to complete electron transfer giving radical ions; as a result, the formation of fluorescent complexes is not observed.

THE CHEMISTRY OF EXCITED COMPLEXES

33

Recently there have been several observations of exciplex formation in the gas phase. Exciplexes between 9,lO-dicyanoanthracene and 1,5-dimethylnaphthalene (Itoh et al., 1981) and 1,Zdimethoxybenzene, hexamethylbenzene and 2,5-dimethylhexa-2,4-diene(Hirayama and Phillips, 1981) have been studied. The work of Hirayama and Phillips showed that the wavelength of exciplex emission in the gas phase is very similar to that obtained in cyclohexane solution. Measurement of wavelengths of exciplex emission in the gas phase enable one to probe solvent polarity properties using eqn (20) more accurately since v,, is now known. Here vmax is the frequency of emission (20)

in solvent, p the dipole moment of the complex, b Planck’s constant, c the

. speed of light, a the radius of the complex, E the dielectric constant, and n the

refractive index. It was pointed out that the emission spectra of exciplexes are often perturbed by the use of high concentrations of donor molecules since these can act as “solvent molecules”. 1,4-DicyanonaphthaIenealso forms an exciplex with 2,4-dimethylhexa-2,4-dienein the gas phase and the enthalpy of formation was determined as 8 kJ mol-’ (Abbott et al., 1981). Fluorescence emission from the majority of excimers is virtually independent of solvent polarity, indicating that little of their stability is due to chargetransfer interactions. Exceptions to this rule have been found, e.g. [17] and [26]. The norm is that excimer stability is little effected by change in solvent polarity. Compound [28] is interesting in that it can form an intramolecular

excimer and also an intramolecular exciplex (Beddard et al., 1977b). By means of time-resolved fluorescence spectroscopy it was found that, in cyclohexane, emission from the excimer and exciplex can be observed and that the latter is to the blue of the former. On changing the solvent to benzene the exciplex fluorescence is red-shifted and occurs at similar wavelengths to the excimer. In solvents of higher polarity, only exciplex emission can be observed. It appears that in cyclohexane and benzene the excimer and exciplex have similar energies, whereas in more polar solvents the solvent polarity lowers the exciplex energy below that of the excimer. When the nitrogen atom is protonated, [28] shows intramolecular excimer emission irrespective of solvent polarity. Similar observations have been made with benzylamines

R. S. DAVIDSON

34

(Goldenberg e f al., 1978; Liao et al., 1979) and the efficiency of excimer formation used to probe the structure of molecules. Change in solvent polarity has been shown to affect the relative contribution of exciton resonance and charge transfer to the stabilisation of excited complexes (Eunice et al., 1979). It was found, for example, that the quenching of the fluorescence of anthracene by amines and phosphines in nonpolar solvents showed a better correlation between log kguench and the singlet energy of the quencher than with the oxidation potential of the quencher. The reverse is true when polar solvents are used, showing, as had been postulated in earlier work (Davidson and Lambeth, 1969). that charge transfer is important in such solvents. Twisted excited charge-transfer states 9,9’-Bianthryl is interesting because the two ring systems are not coplanar due to bad steric interactions between the protons at the 1- and 8-positions of one ring with those on the other ring. The absorption spectrum resembles that of anthracene indicating the lack of x-overlap between the two aromatic systems. In nonpolar solvents the fluorescence of 9,9’-bianthryl resembles anthracenes but on increasing the solvent polarity a red-shifted structureless emission appears (Schneider and Lippert, 1968). The structure of this emitting species was proved by laser flash photolysis (Nakashima et al., 1976). The absorption spectrum corresponded to a mixture of the anthracene radical cation and anion, showing that increasing the solvent polarity had enabled the bianthryl molecule to be stabilised by charge transfer. By means of picosecond laser flash photolysis it was shown that the locally excited anthracene fluorescence decays (in propan-2-01) in 80 psec and this is matched by the growth in the fluorescence due to the complex (Migita et al., 1981). The question arises as to the reason for the rise time of the emission. The time scale precludes it being due to reorientational relaxation of the propan-2-01. However, it could be due to the reorganisation necessary to solvate the highly ionic species being produced. Another possibility is that the charge-transfer process is associated with rotation about the 9,9’-bond of bianthryl so as to bring the two ring systems closer to coplanarity. As the polarity of the solvent is further increased, e.g. when acetonitrile is used, the rise time of the fluorescence decreases to about 20 psec, indicating that charge transfer is more favoured. It was proposed that the charge-transfer state resembles the FranckCondon state, both having the two anthracene rings perpendicular to each other. In such a conformation, stabilisation by charge transfer will be favoured over electronic delocalisation. As one might expect, when a donor and an acceptor group are linked by a 0 bond, then charge-transfer stabilisation becomes very important. One of the earliest systems to be investigated was 4-dimethylaminobenzonitrilein which the amino group is tilted slightly out of the plane of the benzene ring.

-

THE CHEMISTRY OF EXCITED COMPLEXES

35

This compound fluoresces and as the solvent polarity is increased a new redshifted structureless fluorescence band appears. The quantum yield of fluorescence and the fluorescence lifetime decrease as the solvent polarity is increased. It appears that as the solvent polarity is increased several processes take place. Charge transfer will be favoured and this may give rise to an internal charge-transfer (ICT) complex and/or a twisted intramolecular chargetransfer state (TICT). Calculations of the energetics associated with going from a coplanar to a twisted state have been carried out (Cowley et al., 1978) and the results are shown in Fig. 9. Similar calculations have been carried out for the more general case of molecule D-A by Grabowski el al. (1979a) and they also showed that the twisted internal charge-transfer state is at a lower energy than the Franck-Condon state. Furthermore, the triplet twisted internal charge-transfer state lies close to the singlet state and it is anticipated that intersystem crossing will be important in these compounds. Several other predictions were made. Two of these are (a) conversion of ICT --f TICT is not expected to occur with extremely strong donor and acceptor groups and ( b ) the radiative transition 'TICT + So should be strongly forbidden due to

FIG.9 Effect of rotation about the Me,N-C bond upon the electronic energy levels of 4-N,N-dimethylaminobnzonitrile. (From Cowley et al., 1978)

36

R. S. DAVIDSON

minimal overlap between D and A in the transition. Although many of the predictions are based on calculations for 4-dimethylaminobenzonitrile,they should obviously hold for many closely related compounds, such as 4dimethylaminobenzoic acid. Such a system has been investigated (Grabowski et al., 1979b). Care has been taken to select compounds in which either the amino group is forced to lie in the same plane as the benzene ring and without any freedom to rotate, e.g. [29], or as in the case of [30] the nitrogen lone pair is forced out of the plane of the benzene ring. For [31], the two

o-methyl groups create such a large steric interaction with the dimethylamino group that coplanarity between it and the benzene ring is impossible. Compounds [30] and [31] show, in solvents of medium polarity, a red shifted, structureless band in their fluorescence spectra (Grabowski et al., 1979a,b). By way of contrast, the planar compound [29] shows no such fluorescence. These results clearly show that for the creation of an excited charge-transfer state that coplanarity between the amino group and the benzene ring is not a prerequisite. Other processes can also come into play when donor acceptor compounds are photolysed in polar solvents. 1-Anilinonaphthalene8-sulphonic acid shows a very low quantum yield of fluorescence in water (4F= 0.003) and a relatively high one in ethanol (q5F = 0.41). Laser flash photolysis studies revealed that on increasing the polarity of the solvent by changing from ethanol to water, the rate of photo-ionisation to give a solvated electron increased rapidly (Fleming et al., 1977). Another class of compounds which appear to exhibit twisted charge-transfer character is the aroylanthracenes (Tamaki, 1978b). The fluorescence emission of such compounds moves to the red and loses its structure as the solvent polarity is increased. Compound [32] is of interest because it has been shown by X-ray crystallography to be non-planar ; the anilino and phthalimido groups being very nearly orthogonal to each other (J. H. Barlow er al., 1979). Both solid [32] and its solution exhibit a charge-transfer absorption band although fluorescence from [32] could not be detected. Thus coplanarity is not a prerequisite for the occurrence of charge-transfer absorption bands. Many compounds having potential donor and acceptor groups linked to each other by a o-bond do not constitute a donor-acceptor molecule in the

THE CHEMISTRY OF EXCITED COMPLEXES

37

ground state. However, on excitation, the excited group becomes an acceptor or donor group. An example of such a compound is [33]. The absorption spectrum of this compound shows little evidence of any charge-transfer NMe,

NMe2

I

interaction in the ground state. However, the fluorescence spectrum of [33] shows a broad structureless band to the red of the normal anthracene fluorescence and this band moves to the red as the solvent polarity is increased (Okada el al., 1976b). The quantum yield of fluorescence is high in all solvents (varying from 0.3 to 0.8) and the lifetime of fluorescence is longer in highly polar solvents compared with nonpolar solvents. The strange feature is that the lifetime suddenly increases on change of solvent from diethyl ether to isobutyl acetate. This change in lifetime indicates that there is a considerable change in electronic structure on changing the solvent from one to the other. The nature of this change is not known, but it is clear that the broad structureless emission is due to an excited charge-transfer state. The wavelength dependence of the fluorescence of [33] is similar to that exhibited by [34; n = 1, 2, 3 and 41 which is further confirmation of the charge-transfer nature of the emitting state. More recent picosecond flash photolysis studies have shown that in propan-2-01 the initially created anthracene excited singlet state decays to give the charge-transfer state. The decay time is 70 psec and the rise time for the charge-transfer fluorescence is 110 psec. The absorption spectrum of the excited state responsible for the long wavelength emission appears to be made up from the spectrum of the anthracene radical anion and the dimethylaniline radical cation. Thus charge transfer can occur in the excited state of [33] and co-planarity of the two systems is not a prerequisite. It was suggested that emission may occur from more than one twisted charge-transfer state and that the conformational population of these states is affected by change in solvent polarity. A comparison is made in Table 6 of the fluorescence properties of [35] and [36; n = 1 and 21 (Beddard et al., 1982). Like [33], [35] shows little interaction between the pyrrole and the naphthalene ring. The fluorescence of [35] is broad, structureless and moves to the red on increasing the solvent polarity.

-

-

R. S. DAVIDSON

38

TABLE 6

Fluorescence quantum yields and lifetimes for [35] and [36] Solvent Cyclohexane Benzene Acetonitrile

1351

4~

0.03 5.4 0.02 4.6 < 0.01 6.2

rp/nsec 4F

rp/nsec

4~

TF/nSeC

[36; n

=

0.03 6.4 0.02 6.8 < 0.01 6.3

11

[36; n = 21 0.37 15.3 0.21 14.3

-

0.04 11.3

The quantum yield for fluorescence is low and decreases on increasing the solvent polarity. This may be due to photoionisation playing a more important part in solvents having a high polarity. The fluorescence lifetime remains short in all solvents. The fluorescence characteristics of [36; n = I], low quantum yield of fluorescence, short fluorescence lifetimes and the wavelength

MeO

M

e

Me

I

of emission, are very similar to those of [35]. Compound [36; n = 21 shows higher quantum yields of fluorescence, the fluorescence lifetimes are longer (in cyclohexane and benzene) but the wavelength of emission in a particular solvent is similar to those for [35] and 136; n = I]. All three compounds obviously form excited charge-transfer complexes but there is no unique conformation. Compound [36; n = 21 can adopt a sandwich conformation, but for the compound having n = 1 this is impossible. Similarly the possible conformations that [36; n = 11 can adopt are very different from those available to [35]. Despite these differences the wavelength of fluorescence from the complexes is similar. The small differences in the emission spectra of [36; n = 1,2] and [35] are probably due to the operation of the inductive effect in [35]. It has been previously shown that the wavelength of fluorescence of an excited complex is dependent upon the redox properties of the system, the Coulombic interaction between the charged groups and the solvation of the species (Mataga et af., 1966a; Beens et al., 1967). Since [35] and [36; n = 1 and 21 contain the same donor and acceptor groups it perhaps is not surprising that the wavelength of fluorescence is similar for all three com-

THE CHEMISTRY OF EXCITED COMPLEXES

39

pounds. Thus the wavelength of emission does not provide information about the conformation of the excited state responsible for emission. However, the quantum yields and lifetimes of excited complex formation do reflect the conformational requirements for excited complex formation. For [36; n = 21 which can adopt the preferred sandwich arrangement the quantum yield and lifetime are quite high. In [36; n = I] and [35] where such a conformation cannot be attained the quantum yield and lifetime are much lower. It therefore seems that although there may be many conformations which give rise to fluorescence from excited complexes, the relative efficiencies of the radiative and non-radiative pathways are highly dependent on conformation. Porphyrins carrying aryl groups in the meso position are non-planar. The aryl group has to twist out of the plane of the ring to overcome the steric repulsion between its orfho-protons and the spatially close protons on the pyrrole rings. Dalton and Milgrom (1979) have synthesised porphyrins carrying quinones at the meso positions. The quinone groups cause very efficient intramolecular fluorescence quenching i.e. the fluorescent quenching is efficient even though the donor and acceptor groups are not in the same plane and cannot, of course, adopt a sandwich configuration. Similarly, porphyrins carrying aryl groups (substituted with electron withdrawing groups) at the meso position exhibit highly efficient intramolecular fluorescence quenching (Harriman and Hosie, 1981). It has been pointed out that the diarylindenes can be considered as bichromophoric systems; irradiation leads to sigmatropic shift (21) and migratory aptitude studies have been carried out (Manning, e f al., 1981). When one

";li \ /

R

hv

4

Q R

of the aryl groups carries a strongly electron donating or accepting group, then charge transfer between the styryl system and the aryl group is favoured. This stabilisation favours the migration of the unsusbstituted aryl group. Conformational requirements f o r fluorescent exciplex formation It was stated earlier that the fluorescent exciplex of [36; n = 21 has a sandwich conformation. This statement was based on earlier studies of exciplex formation of naphthylalkylamines (Chandross and Thomas, 1971 ; Brimage and Davidson, 1971). It was found that when a dimethylene and trimethylene chain was interposed between the amino and hydrocarbon groups that

R. S. DAVIDSON

40

exciplex formation was very efficient. For longer chains, the quantum yield of exciplex formation dropped as the chain was lengthened. This presumably is a reflection of the fact that the binding energy of the exciplex is not sufficiently great to overcome the energy barriers associated with orientating the hydrocarbon chain. Not surprisingly, when either the electron availability of the donor is increased or a more powerful acceptor group is used, exciplex emission can be observed with relatively long chains connecting the groups. Thus Borkent et al. (1978) have shown that [37; n = 71 exhibits chargetransfer fluorescence. Compounds [38; n = 1-5 and 101exhibit excited chargetransfer formation (Hatano ef d.,1978). When n = 1 and 2, the complex is

formed via ground state conformations in which there is a direct interaction between the carbazole and ester groups, i.e. static quenching of fluorescence occurs. For n = 3, 4 and 5 the quenching occurs by both dynamic and static processes, whereas with n = 10 it is totally dynamic. It was estimated that the radius of the complex is -9 A" and for n = 3 there is an energy of activation for formation of I kcal mol-'. Chains longer than trimethylene can be used to link aryl and amino groups provided that they contain atoms or groups which relieve the eclipsing interactions which occur on folding the chains. Thus an ester group has been successfully used to make up a chain composed of six linking atoms (Beddard et al., 1972). The results already described for [36] are similar to those obtained for N-methyl-"-( 1-naphthylmethy1)aniline in that exciplex can be observed in relatively nonpolar solvents. This is not the case for [34; n = I] and for [39; n = 11. In these systems the amino group cannot directly interact with the x system of the anthracenyl and pyrenyl ring systems, but some interaction

-

THE CHEMISTRY OF EXCITED COMPLEXES

41

might have been expected to occur between the x orbitals of the aromatic amine (in which there is px conjugation with the amino group) and the polycyclic aromatic. In these systems the energy gained by solvation of the chargetransfer state is insufficient. When more polar solvents are used, these compounds and [34; n = 2 and 31 and [39; n = 2 and 31 also exhibit excited complex formation, the absorption spectra of which have been obtained by nanosecond laser flash photolysis and shown to be the summation of contributions from the hydrocarbon radical anion and the amine radical cation (Hinatu et al., 1978). From a study of fluorescence lifetimes and quantum yields of fluorescence it was concluded that, in polar solvents the exciplexes of [34; n = 2 and 31 and [39; n = 2 and 31 do not necessarily have to adopt the sandwich conformation (Okada et al., 1977). Studies of fluorescent complex formation by picosecond time-resolved experiments showed that the complex formed initially had a loose structure and that this underwent reorientation to give the sandwich structure (Migita et al., 1978). From an examination of the rise time of fluorescence it was shown that for the pyrenyl compound [39; n = 31 exciplex formation can occur from compounds having an extended conformation in nonpolar solvents. A similar result, contrary to the findings of Gnadig and Eisenthal (1977), was obtained for [34; n = 31 (Migita et al., 1980). The naphthoate ester [40] exhibits exciplex formation the extent of which is dependent on whether the amine or the naphthoate residue is excited (Costa

et al., 1980a). Time-resolved fluorescence spectroscopy showed that on excitation of the amine both fluorescence from the amine and the exciplex were visible during the first few nanoseconds. After ten nanoseconds, naphthalene fluorescence appeared. It was suggested (Costa et al., 1980b) that the exciplex seen at the early gate time is produced by static quenching and that the exciplex can act as an intermediate in transferring energy from the amine to the naphthoate. This explains why excitation of the amine portion of the compound leads to exciplex formation whereas excitation into the naphthoate residue leads to little exciplex formation. Another feature of [40] is that exciplex formation can be observed in glasses at 77 K indicating that there is a static interaction between the ester and amino group. Quantum yield measurements of naphthalene and exciplex fluorescence for a series of w-(N,N-diethy1)aminoalkylnaphthalenes have been measured

R.

42

S. DAVIDSON

(Davidson and Trethewey, 1976b). It was found that the quantum yield of fluorescent exciplex formation was highly dependent on the length of the linking chain whereas the efficiency of quenching of the naphthalene fluores; cence remained high for all the systems studied. This led to the suggestion that the conformational requirements for fluorescent exciplex formation and for quenching are different. It appears that the quenching can occur over greater distances than the distance which separates the amine and aromatic hydrocarbon in the exciplex. Whether or not this quenching involves a nonrelaxed exciplex or direct electron transfer is not known. The distance over which an electron can be transferred must be related to solvent polarity. It has been suggested that in MeCN solution I-(4-dimethylaminophenyl)-3(9-anthry1)propane undergoes direct electron transfer from the extended

=P

NMez

NC

[411

CN

NC 142~

K CN

conformation (Crawford et al., 1981). Compound [41] was studied in order to discover whether the quenching process could be stopped by separating the amine and aromatic hydrocarbon with a spacer (Davidson et al., 1979). When nonpolar solvents were used it was found that energy transfer from the excited phenyl to the amino group occurred. Since the molecule is rigid, this cannot have occurred by movement of the amino group close to the aryl group, and since a nonpolar solvent is used it is hardly likely that electron transfer would take place to give a zwitterionic intermediate. There is insufficient solvation energy available to enable such a species to be formed. In polar solvents, such as acetonitrile, electron transfer does occur and in methanol electron ejection into the solvent occurs. One of the problems of using the bicyclo[2.2.2]octane framework as a separator is that substituents at the bridgehead positions may be able to interact by a through bond process. That charge transfer can occur by through bond processes has been amply demonstrated (Pasman et al., 1976, 1978), e.g.

THE CHEMISTRY OF EXCITED COMPLEXES

43

with [42a,b]. Both absorption and emission spectra show that intramolecular charge transfer occurs in such compounds. Measurements of the quantum yields of fluorescence and phosphorescence of [3] (Davidson et af., 1980a) showed that for many of the compounds fluorescence quenching in rigid matrices is efficient. However, in all cases & + & # 1 and therefore on cooling many of the molecules must adopt conformations in which fluorescence quenching can take place but the interaction does not lead to triplet production. Thus there is apparently a conformational requirement for triplet production via the external heavy atom effect. When primary and secondary w-arylalkylamines are excited, exciplex fluorescence is not observed, but fluorescence quenching does take place (Shizuka et al., 1979). Presumably chemical reaction competes effectively with exciplex formation. However, by means of picosecond laser flash photolysis, the time-resolved absorption spectra of the species produced on reaction of pyrene with diphenylamine have been obtained (Okada et af., 1980b). It was shown that the reaction leads to neutral radicals via an exciplex. Although the formation of neutral radicals in such systems had been previously identified (Okada et af.,1976a) the role of exciplexes had been purely speculative. TRIPLE COMPLEXES (TRIPLEXES) AND SUBSTITUTION REACTIONS OF EXCIPLEXES (SEX REACTIONS)

In the early work on the quenching of the fluorescence of aromatic hydrocarbons by tertiary amines it was found that there was an optimal concentration of amine for fluorescent exciplex formation (Beens and Weller, 1975). Use of high concentrations of the amine led to diminution in the intensity of the exciplex and a further emission band appearing to the red. It was suggested that this new band was associated with a termolecular complex formed between two molecules of the amine and one of the hydrocarbon. A similar observation was made for the quenching of naphthalene fluorescence by 1,Cdicyanobenzene in toluene (Beens and Weller, 1968a,b). The fluorescence of the 1 : 1 complex formed between the two compounds is quenched by naphthalene to give a new emitting species which was thought to be a triple complex, formed as shown in (20). There are two possible sandwich structures D+A- + D + (DD+A-) (20) for such a complex, DDA and DAD. The symmetrical arrangement of the donor groups in the latter complex should lead to it having no dipole moment whereas if the complex has the former structure it should have a high dipole moment. Examination of the effect of solvent upon the emission wavelength of the triple complex showed that it has a very high dipole moment indicating that it has the DDA structure. In a slight variation of this system, the quenching of the excimer emission from 1,3-di(I -naphthyl)propane by 1,6dicyano-

R. S. DAVIDSON

44

benzene has been examined (Mimura et al., 1977). In this system triple complex formation is more favoured due to the saving in reorganisational energy by linking the two naphthalene units. It was found that the triple complex was -0.4 eV more stable than the naphthalene excimer and -0.2 eV more stable than the naphthalene- 1,4-dicyanobenzene exciplex. A particularly effective way of comparing bichromophoric versus trichromophoric interactions has utilised the cyclophanes [43], [44], and [45] (Masuhara, et al., 1977b). As can be seen, [43] corresponds to a DA complex, [44] to a DAD CN

/ \

[441

[451

complex and [45] to a DDA complex. All three show structureless fluorescence bands but in the case of the symmetrical compound [44] the wavelength for maximal emission is virtually solvent independent. This confirms the earlier surmise that a symmetrical triple complex will have a zero dipole moment. The emission exhibited by [45] is to the red of that shown by [43] as is expected from the earlier work of Beens and Weller (1968a,b). Interestingly the fluorescence lifetime of [43] and [45] increase on increasing the solvent polarity whereas [44] shows a very small decrease. Other cyclophanes, e.g. [46], have

THE CHEMISTRY OF EXCITED COMPLEXES

45

been synthesised which show charge-transfer absorption bands and these move to the blue on increasing the solvent polarity due to the greatw stabilisation of the ground state compared with the excited state (Horita et al., 1977). The early work on triple complexes related to the observation that tertiary amines can quench the fluorescence of 1 : 1 aromatic hydrocarbon-amine exciplexes. This quenching is usually attended by a red shift in the exciplex fluorescence. It can be argued that an amine behaves as a small polar molecule and therefore the quenching and red shift are associated with solvation of the exciplex by amine molecules rather than formation of a complex having a definite stoichiometry. In order to examine this question, the quenching of the fluorescence of naphthalene by a,o-diaminoalkanes has been examined (Beecroft et al., 1978). It is known that a,w-diaminoalkanes containing a chain of two and three methylene groups can fold, and therefore these compounds should be capable of forming a (DDA)* complex with naphthalene. Diamines containing diethylamino, pyrrolidino and piperidino groups were examined and when two, three and four methylene groups were interposed between the amino groups fluorescence quenching was very efficient. The quenching was attended by the appearance of a weak structureless naphthalene-amine exciplex emission. This red-shifted emission was attributed to fluorescence formed by the interaction of two amino groups with the naphthalene. The low quantum yield of exciplex fluorescence was attributed to the increased likelihood of radiationless decay when one tried to bring three groups together in an ordered array compared with trying to order two groups relative to each other. Further evidence in support of these suppositions came from a study of [47]. In nonpolar solvents the intramolecular quenching of the

a’ CH,CH,NMe,

naphthalene fluorescence is highly efficient and a weak red-shifted fluorescence band is observed. Once again, the interaction of three groups leads to extensive radiationless decay. With polar solvents exciplex emission is unobservable, and this contrasts with the finding that related naphthylalkylamines form fluorescent exciplexes in such solvents. a,w-Diaminoalkanes containing two, three and four methylene groups are highly efficient quenchers of the fluorescence from Rose Bengal (Davidson et al., 1978) and it was suggested that this involved interaction of two amino groups with the excited dye. Excited complex formation between two carbazole units and benzanthrone

R. S. DAVIDSON

46

has been observed in a study of the quenching of polyvinylcarbazole fluorescence by benzanthrone (Siegoizynski et a/., 1978). The exciplex fluorescence emission of many aromatic hydrocarbons with aliphatic and aromatic tertiary amines undergoes a red shift on change of solvent from mesitylene to p-xylene to benzene (Basu, 1978; Purkayastha and Basu, 1979). If these hydrocarbon solvents had been acting as electron donors one would have expected the opposite order, the greatest shift being found with mesitylene. It was therefore suggested that the aromatic hydrocarbons are behaving as poiar solvent molecules and may give rise to some specific exciplex-solvent interaction. Similar effects have been observed for the interaction of methylpyridines with heteroexcimers (Itoh and Takita, 1979; ltoh ef al., 1979). It was found that the fluorescence of the heteroexcimer of [48] in tetrahydrofuran was quenched by pyridines with the following order of efficiency : pyridine > 4-methylpyridine > 3,5-dimethylpyridine > 2-methylpyridine > 2,6-dimethylpyridine. Once again there is no correlation CN

between the efficiency of quenching by these added compounds with their ionisation potentials. Nevertheless the authors, on the basis of kinetic measurements, attribute the quenching to triple complex formation. The inefficiency of 2,6-dimethylpyridine was attributed to the methyl groups hindering the participation of the pyridine in the complex. In all cases, the quenching caused by the pyridines was not accompanied by the appearance of any new fluorescence bands. Thus whether or not the quenching is really due to triplecomplex formation or to the pyridine acting as a small polar molecule is open to question. However, the experimental observations have been put to good use. Thus 1 -cyanonaphthalene and 2,5-dimethylfuran form an exciplex and cycloaddition (22) also takes place to give [49] (Sugioka et al., 1972). The

WI question arises as to whether the exciplex lies on the reaction path to the product. A very good indication that this is the case comes from the finding I

THE CHEMISTRY OF EXCITED COMPLEXES

47

that product formation shows a negative temperature dependence and this is matched by the effect of temperature upon the fluorescence quenching efficiency and quantum yield of exciplex formation (Sugioka et al., 1972). Further proof for the exciplex came from the observation that the addition of pyridines quenches the fluorescence of the exciplex and in so doing quenches the product formation (Majima et al., 1978a). The intermediacy of an exciplex in the cycloaddition of 2,5-dimethylhexa-2,4-dieneto 9-cyanophenanthrene was also shown by utilising the quenching effect of added pyridines and imidazoles. Anthracene is photoreduced by N,N-dimethyi-aniline (Davidson, 1969) and it has been shown that a high concentration of the amine facilitates this reaction (Yang et al., 1976). It was suggested that the proton-transfer step between the hydrocarbon radical anion and the amine radical cation is aided by a second molecule of the amine which acts as a base catalyst. Perhaps a more surprising finding is that the efficiency of N,N-dimethylaniline to quench anthracene photodimerisation is dependent upon the anthracene concentration (Saltiel et al., 1977). It was suggested that anthracene was able to interact with the anthracene-dimethylaniline exciplex to give a triple complex which collapsed giving the anthracene photodimer. Phenanthrene undergoes a cycloaddition reaction (23) with dimethyl fumarate to give a cyclobutane [50] and an oxetane (Creed and Caldwell, CO,Me Me0.C’

,

8

C0,Me

C0,Me

\

1974). In nonpolar solvents exciplex emission can be seen. Electron-rich alkanes such as 2,3-dihydropyran quench the exciplex fluorescence and also the formation of the cycloadduct. Triethylamine behaves in a similar manner. 9-Cyanophenanthrene forms cycloadducts with styrenes (Caldwell and Smith, 1974), and this reaction can be quenched by the addition of electron acceptors such as dimethyl acetylenedicarboxylate. Similar reactions have been shown to be quenched by electron acceptors, e.g. cyanoethylenes (Caldwell et al., 1975). The effect of the added electron acceptors and donors is interpreted in terms of triple complex formation via their interaction with the exciplexes formed by the phenanthrenes. In the case of the reaction of 3,lO-dicyanophenanthrene and trans-anethole, it was shown that these two compounds form an exciplex which is quenched by N,N-diethylaniline forming an

R. S. DAVIDSON

48

exciplex between the amine and 3,lO-dicyanophenanthrene(Ohta et al., 1976). It was suggested that the aniline displaces the anethole from the anethole-phenanthrene exciplex. Such a substitution has been denoted as a SEXreaction. The formation of triple complexes and the role of SEXreactions has been treated by Frontier Molecular Orbital Theory (Creed et al., 1977). trans-Stilbene has been found to undergo cycloaddition with dimethyl fumarate to give cyclobutanes and oxetanes (Lewis and Johnson, 1978; Green, et al., 1979). Alkenes bearing electron accepting groups quench the fluorescence of trans-stilbene and weak exciplex fluorescence can be observed. It was proposed that the exciplex lies on the reaction pathway to the cyclobutanes. The formation of the oxetane is favoured by a high concentration of trans-stilbene and therefore it was suggested that oxetane formation involves reaction of the ester with the trans-stilbene excimer to give a triple complex. The reactions of 1,4-diphenylbuta-l,3-dienewith octafluoronaphthalene (Libman et al., 1978) and of 9,10-dichloroanthracene with 2,5dimethylhexa-2,4-diene (Smothers et al., 1979) have been proposed as involving triple complexes. ROLE IN ENERGY TRANSFER

In principle exciplexes can play a part in energy transfer (24). This appears to

+ B-+(At---B;)*-+A

+ B* exciplex take place with [40] when the amine is primarily excited since fluorescence from the naphthoate residue can be observed. Whether or not energy transfer takes place will be determined by the relative energies of A*, B* and the exciplex. Since each of these may respond to a change in solvent to a different extent it may be possible to go from a situation of energy transfer in one solvent to exciplex formation in another. A particularly interesting situation arises with compounds such as [5 11 and the intermolecular equivalent (e.g. for [51; n = 41 the equivalent being benzene and N-methylpyrrolidine). For A*

n

Ph(CHz)zN (CH,). W

[51; n

= 4,

5, 6 and 7)

[51; n = 41 exciplex formation is observed in solvents of varying polarity. By way of contrast N-methylpyrrolidine quenches the fluorescence of benzene

M) in cyclohexane solution. The quenching is accompanied by the appearance of fluorescence from the amine. Thus for [51] the intramolecular exciplex is of lower energy than the excited singlet state of the amine whereas for the intermolecular counterpart the singlet state of the amine is of lower i

THE CHEMISTRY OF EXCITED COMPLEXES

49

energy than the exciplex. Intramolecular exciplex formation should be less energy demanding than intermolecular exciplex formation as shown by (25) since the entropy requirements are less (Beecroft and Davidson, 1981). In ACC

=

-AEW

+ [E(D/D+)- E(A-IA)] - TAS- + C

(25)

(25), AGc and AS, are the free energy and entropy change for exciplex forma-

tion, AE, = energy of the zero-zero transition, &,ID+) and E(A-IA)are the appropriate redox potentials and C = free energy gained when the radical ions are brought to the encounter distance. Intermolecular exciplex formation between benzene and N-methylpyrrolidine can be observed when a high concentration of benzene is used and this has been attributed to benzene not only participating in exciplex formation but also acting as a polarisable solvent. The effect of lowering the temperature upon the intermolecular system was examined to see if fluorescence from the exciplex would grow at the expense of the amine fluorescence. This was found not to be the case and therefore the intermediacy of a fluorescent exciplex in the energy-transfer process was ruled out. One strange feature of the energy-transfer process is that it is exothermic if one considers the energy transfer taking place to the planar amine but is endothermic for energy transfer to the Franck-Condon pyramidal state of the amine. Presumably contact charge-transfer complex formation between the benzene and the amine would allow the amine to become planar. However, such a mechanism cannot explain the energy transfer which was observed with [41] since with thiscompound it is impossible for thearyl and amino group to come into contact. In another study, Van der Auweraer et al. (1980b) have shown that [52; n = 111 in tetrahydrofuran solution exhibits energy transfer from the Ph(CH,).NMe,

[=I phenyl to the amino group at room temperature. However, as the temperature is lowered a new fluorescence band attributable to exciplex formation appears. It seems that for [52; n = 111 in tetrahydrofuran the energy of the excited planar amine and exciplex are very similar and on the basis of the observations described it was proposed that the energy transfer occurs via the exciplex. It should be noted that the fluorescence emission of amines undergoes a large bathochromic shift on changing the solvent from a nonpolar one (e.g. cyclohexane) to a more polar one (e.g. tetrahydrofuran) and the amines are non-emitting in highly polar solvents (e.g. acetonitrile) (Muto et al., 1971). Since the emission from the exciplex also undergoes a red shift on changing from cyclohexane to tetrahydrofuran it is often difficult to preditt the relative energies of the two species. From the work on [41] it appears that in nonpolar

R. S. DAVIDSON

50

solvents the exciplex has the higher energy of the two. The difficulty of interpreting emission spectra in this series is illustrated by [52; n = 51 where for the tetrahydrofuran solution one cannot tell if the emission is due to the amine or the exciplex. The intermediacy of exciplexes has been used to explain the occurrence of non-vertical energy transfer from methyl benzoate to cyclo-octa-l,5-diene in reaction (26) (Goto et al., 1980). When (-)-menthy1

benzoate is used instead of methyl benzoate the intermediate cis,trans-diene is found to be optically active, i.e. asymmetric induction has been achieved. Chemically induced dynamic nuclear polarisation (CIDNP) is becoming a popular and useful technique for examining electron-transfer reactions. A particularly useful feature is that it can often differentiate between whether products (or regenerated starting materials) have been formed via an excited singlet or triplet route. Although the electron-transfer reactions of triplet ketones will be dealt with in a later section (Section 6) the investigation of the reaction between 4-methylacetophenone and 4-methyl-N,N-dimethylaniline (in acetonitrile) is particularly illustrative here (Hendriks et al., 1979). It was shown (see Fig. 10) that the excited singlet states of both the amine and the ketone and their triplet states were responsible for generating a radical-ion pair. Furthermore energy transfer from the excited singlet amine to the ketone occurs and this does not result in formation of the radical-ion pair. FORMATION OF TRIPLET STATES

The formation of electron donor-acceptor complexes from excited singlet states can lead to triplet formation. In highly polar solvents where radical-ion formation readily occurs, triplets may be produced by recombination (25) of solvent-separated radical ions and of geminate radical ions (Schulten et al., 1976): A; + D t - t 3 A + D. Such an electron-transfer reaction occurs in many of the electrochemiluminescent reactions discussed earlier. There is also evidence that, in some solvents of medium polarity, triplet production occurs via an exciplex (Orbach and Ottolenghi, 1975). The extent to which each of these processes contributes is obviously highly dependent upon the solvent. The formation of triplet aromatic hydrocarbons by quenching of aromatic hydrocarbon fluorescence by amines is known from the pioneering work of Leonhardt and Weller (1963; Ottolenghi, 1973). The mechanism of triplet formation in nonpolar solvents has been shown, e.g. by picosecond flash

,

THE CHEMISTRY OF EXCITED COMPLEXES

51

3.a

3 .€

34

2 3 .i

3.c

2.8

+

Ground state

FIG.10 Rate constants k1) for energy and electron transfer for the reaction of 4-methylacetophenone with N,N-dimethyl-p-toluidine in acetonitrile. (From Hendriks et al., 1979)

~

photolysis studies (Nishimura et af., 1977), to occur via relaxed exciplexes. It is also possible for triplets to be generated via the non-relaxed complexes. This probably occurs when the fluorescence of aromatic hydrocarbons is quenched by primary and secondary amines (Okada et af.,,1976a). Studies of the quenching of 9,lO-dicyanoanthracene by aliphatic and aromatic amines showed that there is no relationship between the triplet yield and the ionisation potential of the amine (Soboleva et af., 1978). However, there is a difference in effectiveness in triplet production between aromatic and aliphatic amines. There have been claims for the intervention of triplet exciplexes in the formation of triplets in the reaction of tetracyanobenzene with mesitylene (Craig et al., 1977) and in the reaction of pyrene with N,N-diethylaniline (Bell and Rodgers, 1976). In the former system the triplet exciplex yield was measured by energy transfer to anthracene. As yet there is little evidence for triplet exciplexes, whereas there is evidence, e.g. from phosphorescence, for the formation of triplet excited charge-transfer complexes (Iwata et al.,

I?. S. DAVIDSON

52

1967, Grellmann et al., 1972). In many systems, e.g. [53], the energy of the excited charge-transfer complex is below that of the lowest triplet state (in the case of [53] the triplet state of the phthalimido-group) and consequently this prevents triplet production (J. H. Barlow et af., 1979). 0

Tetracyanoethylene forms a charge-transfer complex with I , 1’-binaphthyl which exhibits two charge-transfer absorption bands (Yorozu et al., 1978). Excitation via the band of higher energy leads to the formation of the triplet state of binaphthyl which, unlike the ground state, is planar. When optically active 1,l’-binaphthyl is used, this change in geometry can be measured by the fact that the production of the triplet state is attended by racemisation. Mention has already been made of triplet formation via the external heavy atom effect. Studies on o-haloalkylnaphthalenes showed that the influence of the heavy atom is determined by the length and flexibility of the linking chain (Davidson et af.,1980a). It was shown by phosphorescence and triplettriplet absorption.spectroscopy that triplets were produced by this interaction although in the case of iodo compounds homolysis of the carbon-iodine bond is an effective non-radiative decay pathway. The recent finding that intramolecular fluorescence quenching by the bromine atom in w-phenoxyalkyl bromides is dependent on chain length shows the’similarity between these and the more classical aromatic hydrocarbon/amine systems (Davidson et af., 1980b). It is anticipated that the presence of halogeno groups in quenchers such as alkenes, dienes, amines, etc. should favour intersystem crossing in the exciplexes. The quenching of the fluorescence of a number of naphthalenes by chloroacetonitrile in acetonitrile has been shown to give the radical cation of the aromatic hydrocarbon and also the triplet hydrocarbon (Quina et af., 1977). Hydrogen chloride is evolved in these systems. The fluorescence of acridizinium ions is quenched by bromoalkanes, the effect being more marked for 1,Zdibromoethane than bromoethane (Bendig et af., 1977). It was found that the quenching efficiency is related to redox potentials. Triplet production in the quenching of aromatic hydrocarbon fluorescence by amines has usually been monitored by flash photolysis. A particularly good example is the study of the quenching of the excited singlet state of pyrene by 1,4-diazabicyclo[2.2.2]octane in cyclohexane solutions (Delouis et af., 1979). Fluorescent excipkx formation is not observed with this system but triplet formation does occur. By triplet-triplet absorption spectroscopy

THE CHEMISTRY OF EXCITED COMPLEXES

53

it was shown that increasing the amount of amine increased the triplet yield until it approached unity. The triplet yield for a variety of polycyclic aromatic hydrocarbon/aromatic amine systems have been determined by Watkins (1979b). When the amine contains a halogen substituent the triplet yield is increased. In an investigation of the photoreactions of amines with acenaphthylene, it was found that in nonpolar solvent the addition of amines favoured the formation of the trans-photodimer of acenaphthylene (Davidson, 1969). It is known that the trans-photodimer comes predominantly from the excited singlet hydrocarbon. It was proposed that the amine promoted intersystem crossing in the acenaphthylene via exciplex formation. In the case of styrenes and stilbenes triplet formation can also be monitored by the cis-trans isomerisation of the arylalkene (cis-trans isomerisation is a well known reaction of triplet arylalkenes). Thus, the addition of N,N-diethylaniline leads to an increase in triplet yield of styrylnaphthalene (Aloisi et al., 1977). This trans -+ cis isomerisation can also be induced by irradiation of styrylnaphthalene in the presence of various electron acceptors (Gennari et al., 1980). The fluorescence of trans-stilbene is quenched by dimethyl fumarate and exciplex fluorescence can be observed. The quenching leads to the formation of a (2 2)-cycloaddition product and also to cis-stilbene. In all probability the cis-stilbene has been produced via triplet trans-stilbene (Green et al., 1979). Lewis and Simpson (1979) have shown that trans-stilbene forms exciplexes with a variety of electron acceptors, e.g. fumaronitrile, diethyl fumarate, maleic anhydride, and they found that the exciplexes lead to triplet production, quantum yields for which were determined. The technique of chemically induced dynamic nuclear polarisation (CIDNP) has been used to show that the excited singlet state of trans-stilbene reacts with electron acceptors such as 1,2-dicyanotthylene and 9,lO-dicyanoanthracene to give radical ion pairs which react to give triplet stilbene (Arnold and Wong, 1979). In a related study, Caldwell and Creed (1979) examined the effect of temperature upon the interaction of phenanthrene with dimethyl fumarate. It was found that the cycloaddition reaction and internal conversion were temperature independent, but this was not the case for intersystem crossing. Triplet formation via exciplex formation was again invoked. Benz(a)anthracene reacts with cisand trans-penta-l,3-diene to give [54] and [55]. From quantum-yield measure-

+

R. S. DAVIDSON

54

ments it was shown (Saltiel et af.,1978) that [54] and [55] cannot be produced directly from the singlet exciplex and therefore it was proposed that they arise by reaction of the triplet arene with the 1,3-diene (possibly via a triplet exciplex). A thorough study has been made of the cycloaddition reactions of triplets generated via excited charge-transfer complexes (Wong and Arnold, 1979). Irradiation of 1,2,3-triphenyIcyclopropenein the presence of dimethyl fumarate, utilising light which only enters the charge-transfer band produces [56], [57], and [58]. It was shown that population of the excited chargetransfer complex produced the triplet cyclopropene which led to [56]-[%]. A

A

Ph

+

Ph

> 350INnm

Ph t561

2 0 z M e

r Me0,C

px

Phwph+phJ$

Ph

C0,Me

P71

Ph

Ph

~581

similar reaction takes place when dimethyl maleate is used instead of the fumarate (Arnold and Morchat, 1977). The excited singlet state of the cyclopropene [59] reacts with a number of electron acceptors, e.g. 9,lO-dicyanoanthracene, and tetracyanobenzene in polar solvents to generate the triplet cyclopropene via radical-ion pairs. In nonpolar solvents exciplexes can be Me0,C

CN

1611

'

observed (Brown-Wensley et al., 1978). With 9,lO-dicyanoanthracene the exciplex leads to cycloaddition and the formation of [60]. When a polar solvent is used, the intervention of the triplet, state leads to the formation of [61] (Farid and Brown, 1976). If dimethyl fumarate is added to the reaction

THE CHEMISTRY OF EXCITED COMPLEXES

55

mixture the cyclopropene radical cation can be intercepted. Cis- and trans1,2-Diphenylcyclopropanes undergo interconversion on irradiation in the presence of electron acceptors such as tetrachlorobenzo-1 ,Cquinone: CIDNP studies show that this isomerisation does not occur directly but rather via the triplet cyclopropane (Roth and Schilling, 1980). The formation of the triplet cyclopropane can occur via an initially formed excited singlet exciplex between the cyclopropane and the electron acceptor. CIDNP studies have also shown that the isomerisation of stilbene sensitised by aromatic hydrocarbons occurs via a singlet exciplex which ultimately leads to triplet stilbene which then undergoes isomerisation (Leshina et al., 1980a,b). The role of exciplexes in assisting intersystem crossing for systems in rigid glasses has been discussed (Hatano et al., 1981). As has been mentioned earlier, it is often very difficult to distinguish between and identify the roles of exciplexes (and excimers) and biradicals in cycloaddition reactions. Caldwell and Creed (1978b) have studied the cycloaddition of dimethyl fumarate to phenanthrene and found that the quantum yield of the cyclobutane photoaddition product is increased in the presence of oxygen. It was suggested that oxygen enhances intersystem crossing in the triplet biradical formed between the two reactants. Nitroxide radicals have also been found to increase intersystem crossing (S, + TI) in carbocyanines when nonpolar solvents are used (Kuzmin et al., 1978). When polar solvents are employed full electron transfer takes place. The formation of triplets via excimers is still an area which is little explored. Medinger and Wilkinson (1966) have shown that pyrene excimer formation leads to a decrease in the quantum yield of triplet formation. A similar result has been obtained for benz(a)anthracene (Heinzelmann and Labhart, 1969). On the other hand, benzene excimer formation leads to an increased triplet yield (Cundall et af.,1972; Hentz and Thibault, 1973). There has been a recent theoretical study of intersystem crossing in molecular pairs (Bowman and Norris, 1978). ENERGY WASTAGE

Excimer and exciplex formation often leads to energy wastage. Usually there is a partitioning between chemical reaction and quenching. However, in some systems, excited complex formation has been shown to lead totally to energy wastage. Such chemical reaction as is observed, occurs via routes not involving complexes. The di-x-methane rearrangement of [62] is just such a case. It was found that when X was an electron withdrawing group (e.g. CN) the reaction was far more efficient than when it was electron donating (e.g. methoxy) (Ferreira and Salisbury, 1978). Another example is the cycloaddition of 9-cyanophenanthrene to some substituted styrenes (Caldwell and

R. S. DAVIDSON

56

x Creed, 1978a). It was found that as the charge-transfer character in the transition state increased the rate constant for cycloaddition decreased. The oxidation of crystal violet to its cation radical can be initiated by reaction of the dye with the excited singlet states of many polycyclic aromatic hydrocarbons. This reaction was found to be far less efficient for polymer-bound pyrene than for free pyrene and this was attributed to excimer formation occurring in the polymer system which ultimately led to energy wastage (Tazuke et al., 1979). EXCIPLEX-INDUCED VALENCE-BOND TAUTOMERISM

Murov and Hammond (1968) observed that the strained hydrocarbon quadricyclene quenched the fluorescence of many polycyclic aromatic hydrocarbons and in doing so underwent valence-bond tautomerism to give norbornadiene. It was postulated that the excited singlet states of the aromatic hydrocarbon formed an exciplex with the quadricyclene and that the isomerisation was an effective radiationless decay pathway for the complex. Subsequently Taylor (1976) showed that hexamethyl-Dewar benzene quenched the fluorescence of 1-cyanonaphthalene and this was attended by the appearance of two new emission bands. One of these was identified as the exciplex formed between hexamethylbenzene and cyanonaphthalene. The valence-bond tautomerism to give hexamethylbenzene takes place along the hypersurface associated with the initially created excited state, i.e. this is another example of an adiabatic excited state reaction. Jones and Chiang (1979) have shown that many electron acceptors promote such reactions and confirm that the exciplex is generated in the reaction between the Dewar benzene and the excited singlet state of the polycyclic aromatic hydrocarbon. Jones and Becker (1981) have shown that electron acceptors sensitise the isomerisation of hexamethyl-Dewar benzene to hexamethylbenzene. The quantum yield for the process is greater than unity indicating that a chain HMDB + A --f HMDB+ + A' HMDBf --f HMBi (27) HMBt + HMDB-+ HMDBt + HMB [HMDB = hexamethyl-Dewar benzene; HMB = hexamethylbenzene]

THE CHEMISTRY OF EXCITED COMPLEXES

57

process occurs (27). Various quadricyclenes undergo valence bond tautomerism on irradiation in the presence of electron acceptors such as fumaronitrile (Jones et al., 1980b). Geometrical isomerisation of the acceptors does not occur. Interestingly, norbornadiene and quadricyclene react with electron acceptors to give distinct radical cations (Roth et al., 1981). Recent work has shown that a whole variety of strained hydrocarbons undergo valence-bond tautomerism on irradiation in the presence of naphthalene (Gassman et al., 1981). The strained hydrocarbons quench the fluorescence of naphthalene and the efficiency of this process correlates with the oxidation potentials of the strained aromatic hydrocarbons. This is a strong indication that the isomerisation reactions occur via an exciplex. 5

Role of radical ions generated from excited singlet states

IONIC REACTIONS OF PHOTOGENERATED RADICAL IONS

The earlier sections have described the evidence for relaxed and non-relaxed exciplexes giving rise to full electron transfer with the formation of solvated radical ions. With many systems, the possibility exists for electron transfer to occur without prior formation of a contact charge-transfer complex if a solvent of sufficiently high polarity is used; such is the case for [41]. The intramolecular radical ion of [41] in acetonitrile has a relative short lifetime (-30 nsec) and this illustrates the point that to obtain stable reaction products via electron-transfer reactions (28) one has to overcome the important back electron-transfer reaction (28b). A*

+ B + At. + B; + B;+A + B

(4 (b) (28) + B; -+Products (4 Some of the earliest reactions to be studied include the photoreduction of aromatic hydrocarbons by amines and amino acids and these have been reviewed (Davidson, 1975; Lablache-Combier, 1972). One of the ways for checking for the intermediacy of radical ions is to carry out the reaction in a solvent containing deuterium oxide. If radical anions are involved they should be deuteriated to give neutral radicals which ultimately will produce products containing deuterium. A quantitative examination has been made ofdeuterium and tritium exchange in the reactions of the excited singlet state of anthracene with N-deuterio- and N-tritio-diethylamine and with triethylamine in deuterium and tritium oxide (Gebicki er al., 1978). As one might expect, labelling occurs in the 9- and 10-positions but also isotope incorporation into positions 1 and 2 was observed when halogenated polycyclic aromatic hydrocarbons were irradiated in polar solvents. Dehalogenation is also observed At At.

I?. S. DAVIDSON

58

and this reaction is accelerated by the addition of tertiary amines (Bunce et al., 1980). To account for this observation it was suggested that the amine alters the course of the reaction by reducing the haloaromatic to a radical anion which should rapidly fragment to give halide ion and an aryl radical

+

+

ArHal,, R3N --f (ArHal)' (R3N)t (ArHal); --f Ar' + HalAr' + R,N -+ ArH + Amine radicals

(29)

(29). If the reaction is carried out in the presence of deuterium oxide, deuterium should be incorporated into the product (Ar,H) as indicated in (30),and (ArHal)' + D,O -+ (ArHalD)' + DO(ArHalD)' DO- % HOD (Ar,Hal)' (Ar,Hal)' + Arb + HalArb + R3N -+ Ar,H + Amine-derived radicals

+

+

(30)

this was found to be the case (Davidson and Goodin, 1981). In many instances more than one deuterium atom was incorporated into the product. It proved necessary to check that the deuterium incorporation was not due to reaction of the product with amine leading to isotope incorporation. In the case of [63], the presence of the amino group has a retarding effect on the dechlorination reaction (Bunce and Ravanal, 1977). It is possible that

excitation of 1631 generates an intramolecular radical anion in which the back electron transfer is so efficient as to compete successfully with the dechlorination reaction. It was also found that 3-(4-chlorophenyl)-I-phenylpropane dechlorinates less efficiently than chlorobenzene, but in this case heteroexcimer formation is probably responsible for energy wastage. Primary, secondary and tertiary amines have been found to photocatalyse the transformation of I-phenyl-1-(I-naphthyl) ethene into phenylacenaphthene (Lapouyade et al., 1977). It was found that the catalytic efficiency of the amine was not related to its ionisation potential but rather to its basicity. That an ionic process was involved was demonstrated by deuterium labelling studies. Deuterium incorporation in the aromatic hydrocarbonlamine systems is usually considered as involving the radical anion of the aromatic hydrocarbon. CIDNP studies (Gardini and Bargon, 1980) have drawn attention to the fact that trialkylaminium ions undergo fast proton exchange and hence

THE CHEMISTRY OF EXCITED COMPLEXES

59

lead to deuterium incorporation into the amine (3 I). Conjugated dienes are

+.

(CH,),NCH,

+.

-H+

% (CH,),N +H+

ROD

- kH, --+

+. (CH,),N CH,D

(31)

also known to accelerate the dechlorination of chloroaromatic hydrocarbons. Smothers et al. (1979) demonstrated in a very convincing way that these reactions involve radical ions. In particular they were also able to show that [64]can be formed via deuteriation of the radical anion followed by loss of a CI

CI

CI

CI'

+

D

/

/

chlorine atom (32); thus loss of chlorine does not exclusively occur from a radical anion. 9-Hydroxyfluorene and its acetate on irradiation in the presence of amines give fluorene (Ohashi et al., 1980). Tertiary amines are more effective in facilitating the reaction than secondary, which in turn are more effective than primary amines. This order of reactivity suggests that the reaction is occurring via an electron-transfer process (Scheme 5 ) to give the9-hydroxyfluorene radical anion which undergoes elimination to give a fluorenyl radical.

0

/

i)R [R

=

H, Ac]

5

m \

+

R

t)R

a

0 l

N

R. S. DAVIDSON

60

Addition of (2H,)-methanol to the reaction mixture leads to deuterium incorporation into the product. Deuterium labelling studies have also been used to investigate the reaction of stilbenes and related compounds with amines (Lewis, 1979). It is known that tertiary amines form fluorescent exciplexes with stilbenes in nonpolar solvents and that polar solvents are necessary for chemical reaction to occur (Lewis and Ho, 1977). This suggests that radical ions are involved in product formation. When secondary amines are used, reaction occurs in solvents of widely differing polarity and this is presumably due to the acidity of the secondary N-H bond. N-deuteriated diethylamine reacts with 1,2-diphenylcyclobutene in benzene to give products [65], [66] and [67] incorporating deuterium (Scheme 6). For the reaction with unsymmetrically substituted

Scheme 6

tertiary amines it was shown that there was little selectivity in the protontransfer reaction since, after appropriate statistical corrections, the reactivity of tertiary, secondary and primary C-H bonds were found to be similar. However there was a marked kinetic isotope effect in the proton-transfer step (Lewis and Ho, 1980). A particularly illuminating study involving isotopic substitution concerns the reaction of N-tritiated diethylamine with methyl benzoate (Kitamura et af., 1977). It was found that tritium was preferentially substituted into the p-position. There was also substantial substitution in the o-position but more surprising, the methyl group of the ester was labelled. It was proposed that reaction occurs as shown in Scheme 7 via radical ions [68] and [69]. 4-Cyanobenzyl-acetate, on irradiation in the presence of triethylamine is cleaved to give 4-cyanotoluene and 4,4'-dicyanobibenzyl (Ohashi et af., 1977d). When O-deuteriated methanol was used as solvent, deuterium was incorporated

THE CHEMISTRY OF EXCITED COMPLEXES

PhC0,Me

6'

+ Et,NT

61

708%

k

Et&T

+

COZCH,

0

+

COzCH,

Et,NH

c

T

Q

CO,CH,T

Scheme 7

into the methyl group of the 4-cyanotoluene. The products appear to arise by cleavage of radical anion [70] (see Scheme 8). One of the most surprising mechanisms to be discovered using labelling studies concerns the reaction of 9,lO-dicyanoanthracene with amines in acetonitrile solution which gives 9-amino-10-cyanoanthracene[71]. Use of

NCOCH,OCOCH,

k

+ Et,N

CN

Et,N

,/

+ CH,CO,-

+

6

[70]

CH, I

*

62

R. S. DAVIDSON

l5N acetonitrile showed that the amino group was derived from the acetonitrile (Ohashi et af.,1979a), a possible route being outlined in Scheme 9. The presence of one or more cyano groups in an aromatic or heterocyclic ring

CN

CN

Me

+CN-

. . ;@.&+-

CN

\

- @jy* Me

CN

\

/

H CN

H CN

FN CN

NHz

Scheme 9

6 - 6- 0 CH,tHNEt,

CH,CHNEt,

+ Et,N

ions

CN

H CN

1

k P~NH,

Radical ions

s"' 6 II

CN

1"

CHzCH,

I

CH, CH,

CH, CH,

\

[731

CN

CN

Scheme 10

EtaN

CN

+HCN

THE CHEMISTRY OF EXCITED COMPLEXES

63

appears to lower the reduction potential to such an extent that these compounds undergo electron-transfer reactions with primary and secondary aliphatic amines. In many of the reactions the cyano group is replaced by a reaction involving ips0 attack. 1,4-Dicyanobenzenereacts with tertiary amines (Tsujimoto ef ul., 1976) and with primary and secondary amines (Ohashi and Miyake, 1977). In these reactions the I-alkyl-4-cyanobenzenes [72] and [73] are produced (Scheme 10). Reaction of dicyanobenzenes with dimethylamides also leads to displacement of a cyano group to give [74] as shown in Scheme 1 1

0- 0

CH\J N

Rco/ HCN

+ 1741

CH,

‘CH,

CN

NC CH,-N

H

I

;OR

CN

Scheme 11

(Tsujimoto el al., 1977). Pyridines and 4-cyanopyridine react with triethylamine, but in these cases secondary photolysis reactions do not play a dominant role. and the amines [75a,b] (Bryce-Smith ef al., 1977) and [76] (Caronna et al., 1978)are produced (Scheme 12). The cyano group is obviously effective in making the reaction regio-selective. 2,3-Dicyanopyridines suffer decyanation upon irradiation in the presence of tertiary amines in acetonitrile solution (Tada et al., 1980). 2-Cyanopyridine and 4-cyanopyridine also undergo photoreactions with alkenes (e.g. cyclopentene) to give adducts in which the cyano group is replaced by the alkene (Bernardi et al., 1981). In the reaction to produce [74] it will be seen that an amide was used. Reactions of carbamates (Tsujimoto et al., 1979b) and ureas (Miyamoto et al., 1978) with l,l-di(4-cyanophenyl)ethene have been postulated as occurring via radical ions, although as yet there is no spectroscopic evidence for such intermediates. Tetracyanobenzene (Ohashi et al., 1979b) and 1,2-dicyano,3,4,5,

R. S. DAVIDSON

64

MeCHNEt, I

Et,N(?HMe

NEt,

I

-+

H

H

0

Et,NCHMe HCN

+

[761

Scheme 12

6-tetrachlorobenzene (Al-Fakhri and Pratt, 1976) undergo photo-reactions with ethers in which a cyano group is replaced by an a-alkoxymethylenegroup. 1,2-Dicyan0-3,4,5,6-tetrachlorobenzene reacts with alkyl aryl ethers to give biaryls and 1,2-dicyano-3,4,6-trichlorobenzenes(A!-Fakhri et al., 1980). It was proposed that the dechlorination occurred by formation of the radical anion of the cyanoaromatic which fragmented giving a chloride ion and an aryl radical. Hydrogen abstraction by the latter leads to the observed product. A similar reaction to that observed by Al-Fakhri and Pratt (1976) takes place between tetracyanoethylene and ethers (Ohashi et a]., 1977b). Presumably these reactions involve radical ions. Tertiary amines react with the exicted singlet state of stilbenes to give addition products (Lewis et al., 1981). When highly unsymmetrical amines such as di-isopropylmethylamine are used, the predominant reaction is at the least substituted group. It is thought that this preferential attack is not due to the relative acidity of the protons within the various groups but rather that the bulky groups determine the orientation of the amine and stilbene groups in the exciplex which in turn determines which protons are correctly disposed for reaction with the stilbene radical anion. Benzene is photoreduced by primary, secondary and tertiary amines (Bellas, et al., 1977) and the various products have been identified. The reaction with tertiary amines is accelerated by the addition of small amounts of protic solvents and use of CH,OD leads to deuterium incorporation. These findings are strongly indicative of the participation of radical ions, Primary and secondary amines quench the fluorescence of benzene but whether or not this leads to radical ions is not known. Product studies have been made of the

65

THE CHEMISTRY OF EXCITED COMPLEXES

photoreactions of a wide variety of substituted benzenes with primary, secondary and tertiary amines (Gilbert et a/., 1981). Carbon tetrahalides are good electron acceptors and react with electrons to give trihalomethyl radicals and halide ions. Amines are known to react with carbon tetrahalides by an electron-transfer mechanism (Wyrzykowska et af., 1978; lwasaki et ul., 1978). Some careful product studies have been made of the reaction between carbazole and carbon tetrachloride (Zelent and Durocher, 1981). Poly [bis(p-tolylamino)phosphazene] forms charge-transfer complexes with halogenated solvents and upon irradiation of these, cleavage to give p-tolylamine occurs (Gleria er al., 1981). The dibutyl nitroxide radical forms a weak charge-transfer complex with carbon tetrachloride and on irradiation acts as an electron donor (Anderson er al., 1979). N,N-Dimethylaniline reacts with methylchloracetic acid to give methyl-2-dimethylaminophenylaceticacid and its 4-substituted isomer (Numao et al., 1978). Formation of charge-transfer complexes between aromatic amines and nitro-compounds is well known. Compound [77] undergoes an interesting photorearrangement to give [78] (Mutai and Kobayashi, 1981). The reactivity

gL

JCHZ). PhN O z N ~ O ( C H , ) . N H P h5 [771

" / No -0

Ph O Z N G N : (CHz)mOH [781

of [77] depends upon the value of n and surprisingly the order is 4 > 5 > 3 > 2. The efficiency of interaction of the anilino group with the aromatic nitro group is found to be n = 2 > 3 > 4. This order of efficiency was determined from fluorescence lifetime measurements. Thus the order of reactivity appears to be the opposite of that required for electron-transfer quenching. It could be, therefore, that the chemical reaction does not occur via an electrontransfir process but by ips0 nucleophilic attack. There are many reports of the photoreactions of aromatic compounds with alkenes and dienes and many of these have been previously reviewed (Davidson, 1975) and are also discussed later in this chapter (Section 8). Some interesting newer developments include the reaction of 1,6dicyanobenzene with tetramethylethylene to give [79] and [80] (Arnold et a/., 1980). The course of this reaction is somewhat similar to that of cyanopyridines with alkenes (Bernardi et af., 1981) referred to on p. 63. When 1,4-dicyanonaphthalene is used in place of 1 ,Cdicyanobenzene [81], [82] and [83] are formed and these can be readily accounted for in terms of intermediate radical ions. Clear

g 46 a+)(-@$ ++H R. S. DAVIDSON

66

+

+

CN

CN

\

CN

Me Me

CN

NCMe

WI

Me Me

\

NC Me

1821

H

NC Me Me Me ~ 3 1

spectroscopic evidence for exciplex formation in such systems comes from the finding that [84], [85] and [86] exhibit intramolecular exciplex fluorescence (McCullough et al., 1980). Irradiation of [84] leads to intramolecular cycloaddition products [87] and [88]. The radical cations generated in some electron-transfer reactions can often be trapped by nucleophiles. Many of these reactions are of synthetic value. It has been shown that many phenylalkenes react with electron acceptors under the influence of light to give styryl cations which can be trapped by nucleophilic alcohols or cyanide ions; for example, 1-phenylcyclopentene

THE CHEMISTRY OF EXCITED COMPLEXES

67

cation reacts with methanol (Scheme 13) to give [89] (Shigemitsu and Arnold, 1975) and the phenylnorbornene cation reacts with cyanide ions (Scheme 14) to give [90a] and [90b] (Maroulis et al., 1978). Anisole undergoes an electron-

Ph

Ph

+

aoMe I

1

AT

+A

I

A=

Ph

CN

4

4' -

Scheme 13

+A

MeCN, b' CF,CH.OH KCN

Ph

+

Ph

+

&+AT

Ph

4."+&iN &;+. Ph

H Pal

Ph

Ph

[Nbl

Scheme 14

transfer reaction with 1,Cdicyanobenzene in dichloromethane and when potassium cyanide is present in the reaction mixture benzonitrile is produced (Suzuki et al., 1980). The reaction is aided by the addition of phase transfer catalysts such as polyethylenglycol and 15-crown-ether. Rather surprisingly, the reaction also occurs in the absence of an electron acceptor, indicating that the benzonhrile can be formed by direct nucleophilic attack of cyanide ions upon anisole. The reaction of 2-cyanonaphthalene with alkenes in nonpolar solvents leads to a (2 + 2)-cycloaddition reaction and an exciplex appears to be an intermediate (McCullough ef al., 1977). When polar solvents are used the

68

R. S. DAVIDSON

reaction takes a different course and [91a] and [91b] are produced (Scheme 15). Deuterium labelling studies confirmed the intermediacy of anionic species. The conversion of [92] into [93] (Neidigk and Morrison, 1978) is sensitised by

t33cN+w&+[~cN]+[4 ?

H H WaI

-

OMe

1 9 ~

Scheme 15

electron acceptors and that of [94] into [95] occurs on direct irradiation (Nylund and Morrison, 1978). In this latter reaction it was postulated that intramolecular electron transfer occurs to produce the radical cation which is captured by methanol. This type of intramolecular electron transfer between alkenes and the benzenoid nucleus may explain why non-radiative decay is so efficient in such compounds as [94] (Scully et at., 1978).

THE CHEMISTRY OF EXCITED COMPLEXES

69

The reaction of phenanthrene and unsaturated compounds, such as furan, 1,l-diphenylethylene and indene, in the presence of electron acceptors and a nucleophile leads to products incorporating the nucleophile (Majima et al., 1981). Thus furan gives [96]. Excitation generates the radical cation of phenanthrene (Phent) and the radical anion of 1 ,Cdicyanobenzene. Phent oxidises furan to its radical cation which reacts in the manner as shown in Scheme 16. CN k

CN

NC t961

i v i a Phcnf

CN

Scheme 16

Indene gives [97] and [98]. The benzylic radical [99] is probably reduced by the dicyanobenzene radical anion to give'its anion which is protonated by the methanol to give [97]. In these reactions phenanthrene is acting purely as a sensitiser. Phenanthrene has been found (Yamada et al., 1977) to react with

tetracyanobenzene to give [IOO] which is indicative of the intermediacy of radical ions (Scheme 17). Phenanthrene also reacts with electron acceptors such as 1,4-dicyanobenzene in the presence of cyanide ions to give 9-cyanophenanthrene and 9-cyano-9,10-dihydrophenanthrene(Yasuda et al., 1981a). Under these conditions, the photogenerated phenanthrene reacts with the

R. S. DAVIDSON

70

R

O

M

@

e

-

-

-CN-

\

\

Me0

-

CN

CN

’

CN

\

\

Me0 NC

/

CN CN

“001

Scheme 17

cyanide ion in preference to the acceptor radical ion. 2-Methoxynaphthalene reacts in a similar way to phenanthrene and gives 1-cyano-2-methoxynaphthalene. Irradiation of naphthalene and biphenyl in aqueous acetonitrile solutions containing cyanide ions leads to cyanation of the aromatic hydrocarbons (Bunce er al., 1981). To account for the formation of these products it was proposed that the hydrocarbons form excimers which dissociate into radical ion pairs, the cation reacting with cyanide ions to give the products. However, as was pointed out earlier, there is little evidence for the excimers of hydrocarbons, such as naphthalene, having any substantial charge transfer character which therefore casts some doubt upon the proposed mechanism. Cyanation of the aromatic hydrocarbons also occurs when electron acceptors such as 1,bdicyanobenzene are incorporated into the reaction mixture. Electron-transfer reactions have been used to accomplish the cyanation of heterocyclic compounds e.g. to make [I011 (Yoshida, 1978) as shown in Scheme 18. Radical cations have also been trapped with the hydride ion. Irradiation of aromatic hydrocarbons such as phenanthrene, anthracene and naphthalene in aqueous acetonitrile containing sodium borohydride and an electron acceptor produces the dihydroarenes (Yasuda er al., 1981b). With monoalkyl naphthalenes reduction in both the unsubstituted and substituted rings occurs

71

THE CHEMISTRY OF EXCITED COMPLEXES k

I

I

Ph

I H Ph

Ph

+ A;

.1

A = N C +N

+

A

Scheme 18

to give I ,4 and 5,8-dihydronaphthalenes. I-Methoxynaphthalene behaves anomalously in that it gives 1,2 and I ,Cdihydro adducts. An interesting useful variant upon the above reactions uses an electron donor as the sensitiser and in this way the nucleophile adds to the alkene in the opposite sense to that which occurs with an electron acceptor as sensitiser (Arnold and Maroulis, 1977). Using this method, alcohols and the cyanide ion can be made to add to alkenes such as I,]-diphenylethylene (Scheme 19). OMe

DMN Ph,CCH,

I

CN

CN-

Ph,d-CH, Ph,&H,

J

+ tDMN1t + DMN

Ph,6-CH3

+;[DMN]i

Phk-CH,

1

+ DMN

/ M ~ H

Ph,CCH,

I

OMe

Scheme 19

In the absence of nucleophilic species, the radical cations generated in the photo-induced electron-transfer reactions may undergo other reactions. Thus, 1-phenoxypropene undergoes cis-trans isomerisation on irradiation in the presence of electron acceptors such as dicyanobenzene (Majima et al., 1979). Some alkenes undergo dimerisation giving cyclobutanes on irradiation in the

R. S. DAVIDSON

72

presence of electron acceptors such as metal complexes of iron(II1); indenes and styrenes have been dimerised in this way (Mizuno et al., 1979). Organic electron acceptors induce the dimerisation of 1,l -diphenylethylene (Neunteufel and Arnold, 1973). Cases are known of the radical cations reacting with added alkenes; thus the indene radical cation can be trapped with furan (Scheme 20) to give [lo21 and [lo31 (Mizuno et al., 1978). It can also be m

+

A

\

(A = 1,4-Dicyanonaphthalene)

+ A;

\

Scheme 20

trapped with enol ethers to give cyclobutanes (Mizuno et al., 1977a,b). The radical cation of 1,l-diphenylethylene reacts with alkenes to generate a sixmembered ring (Scheme 21) e.g. in the formation of [I041 (Maroulis and Arnold, 1979). Product studies suggest that dimethyl fumarate can be used to capture radical cations (Brown-Wensley et al., 1978). A most interesting

+ Ph,C=CH, A

(A =

MeCN

a

-

[Ph,C=CH,]: + A'

Mc,C=CH.

1.4-Dicyanobenzene)

Me Me

+ A'

bh

1

@ Ph Po41

Scheme 21

Me Me +A;

Ph

THE CHEMISTRY OF EXCITED COMPLEXES

73

example of capturing radical cations relates to the reactions of photogenerated phenylacetylene radical cations (Mattes and Farid, 1980a). 9,IO-Dicyanoanthracene is not a sufficiently powerful electron acceptor in its excited singlet state to oxidise the alkyne, but 2,6,9,10-tetracyanoanthracene (TCA) is. The radical cation reacts with the alkyne to give [I051 and if a foreign alkyne is added this can also participate, e.g. to give [I061(Scheme 22). PhCECH

+

k

RCN

r 1 n6i

PhCsCH

[PhCECH]?

+

L

f

CH,=NH

+

PhCOCH=CHCOPh

Scheme 22

The nitrile that is used as the solvent can also participate and .gives a pyridine [107].Perhaps the most surprising trapping reaction is that observed when nitromethane is added. The adduct formed in this reaction fragments to give 1,Zdibenzoylethene. Examples are also known of photogenerated radical cations undergoing fragmentation reactions. Arnold and Maroulis (1976) showed that the ether [I081cleaves on irradiation in the presence of electron acceptors (Scheme 23).

Scheme 23

Benzyl aryl ethers are also cleaved when irradiated in the presence of e.g. tetracyanoethylene (Timpe and Weschke, 1980).

74

R. S. DAVIDSON

REDOX REACTIONS OF PHOTOGENERATED RADICAL IONS

Perylene and tetracene both undergo photo-induced electron-transfer reactions with pyromellitic anhydride (Levin, 1976). If a mixture of perylene and tetracene is used, and the light absorbed by the perylene, the perylene radical cation will be formed which because of the relative oxidation potentials will react with tetracene to give the tetracene radical ion. Thus the photogenerated perylene radical cation has undergone a redox reaction with tetracene. In effect, the perylene has acted as a sensitiser for the production of the tetracene radical cation. This type of sensitisation has been used to effect a number of reactions. Phenanthrene and 1,4-dicyanobenzene (DCNB) undergo light-induced electron-transfer reactions in acetonitrile to generate the phenanthrene radical cation (Majima et al., 1978b). If the photodimer of indene (a substituted cyclobutane) is added, the phenanthrene radical cation reacts with it to generate the radical cation of the indene photodimer. The latter cleaves generating indene. The [2 + 21 cycloaddition products formed from 1,2dihydronaphthalenes and phenyl vinyl ethers are stereospecifically cleaved to give alkenes upon irradiation in the presence of electron acceptors (Majima et al., 1980). Pyrylium and trityl salts are good sensitisers for cleaving cyclobutanes (Okada et al., 1981). Phenanthrene and 1,3-dicyanobenzene undergo a photo-induced electron transfer reaction to give the phenanthrene radical cation and the cyanobenzene radical anion. The cation will oxidise 1,2-diphenylcycIobutane giving a radical cation which reacts with methanol to ultimately give 1methoxy-l,2-diphenylpropane(Gotoh et al., 1981). A reaction which accompanies the formation of ring opened products is the isomerisation of the 1,2-diphenylcycIobutane.Sensitising mixtures such as phenanthrene 1,3dicyanobenzene can also be used to synthesise 2 + 2 cycloaddition products (Asanuma et al., 1977). Irradiation of phenanthrene in the presence of 1,3dicyanobenzene (DCB) in acetonitrile produces the phenanthrene radical cation which reacts with styrene giving the styrene radical cation. This reacts with a-methyl-styrene to give [lo91 and [110] (Scheme 24). Thus the redox sensitising reaction has been used to photodimerise the styrene without it directly absorbing light. The dimerisation of 1,l-diphenylethylene sensitisedby electron acceptors (33) has been shown to involve a dihydro-species [ l l I] which can be trapped by electron acceptors in an ene reaction (Arnold et al., 1981). The reaction of furan with 1,Cdicyanobenzene to give [I121 can be sensitised by phenanthrene (Scheme 25). The photogenerated phenanthrene radical cation reacts with the furan to give the furan radical cation (Pac et al., 1977). Another example of a redox sensitised reaction is the dimerisation of N-vinylcarbazole in which the redox couple is perylene + 1 ,Cdicyanobenzene (Tazuke and Kitamura, 1977). The photogenerated perylene radical cation

+

75

THE CHEMISTRY OF EXCITED COMPLEXES

Ph Me

Scheme 24 CN ph\/c = c H , + Q(DcNB)

A

Ph

@-

Ph CHzCHzX

A x

CN

Ph

Ph

(33)

X=CN or COaMe

['*'I

Ph Ph

reacts with the carbazole to give the carbazole radical cation which leads to dimer formation. An ingenious method of preparing radical cations involves irradiating photodimers having a cyclobutane structure in the presence of semiconductors Phen

+ DCNB

Phenf +

MeOH

00

[8]: a.ooM,

Phenr

Phen +

+ DCNBY

CN

(1 121

Scheme 25

N $

Me0

R. S. DAVIDSON

76

such as zinc oxide and cadmium sulphide (Okada et al., 1980a). The photogenerated holes in the semiconductor oxidise the cyclobutane which cleaves to generate alkenes. REDOX REACTIONS OF R A D I C A L IONS I N OXIDATION REACTIONS

The two main and most explored mechanisms of sensitised photo-oxidation involve the light-driven formation of radicals between substrate and sensitiser with the subsequent reaction of the radicals with oxygen [Type I reaction (34)] and the sensitiser reacting with oxygen to give singlet oxygen which then reacts with the substrate [Type TI reaction (35)]. hV

+ Subs -+

Sens Subs.

0 2

-+

Senso1 Subs'

Products ISC 3

Sens %kens --+Sens 3Sens + O2 -+ Sens 'Ago, 'Ago2 + Subs -+ Products

+

I

Type 1

(34)

Type IT

(35)

[Sens r= Sensitiser, %ens = Excited singlet sensitiser, 3Sens = Triplet sensitiser, Subs = Substrate and 'Ago2 = Singlet oxygen] These various processes have been recently reviewed (Davidson, 1979 ; Wasserman and Murray, 1979). The discovery that the reactions of excited states can occur by an electron transfer raises the question as to whether these reactions participate in the Type I reaction by providing a source of radicals or whether the radical anion generated in the reaction can reduce oxygen giving the superoxide ion which reacts with the radical cation to give products as indicated in (36) and (37).

H-A-H 0 2

HA' -+Products HB'

+.

+ B ~ H - A - H + B - + H A * + HB-

3 Products

1 1

By + 0 2 - + B +02; (37) +. H-A-H + 0, + Products Whether or not the radical anion B' will reduce oxygen can be readily determined from standard tables of redox potentials. Another possible route to a radical cation-superoxide anion radical pair is by excitation of chargetransfer complexes and some of these processes were detailed in an earlier review (Davidson, 1975). The propensity for excited states to undergo redox

THE CHEMISTRY OF EXCITED COMPLEXES

77

reactions raises the question as to whether singlet oxygen may react in this way. Reactions via excitation of charge-transfer complexes.formed between a donor and oxygen The earlier review drew attention to photo-oxidation reactions which ensue on irradiation of complexes formed between hydrocarbons and oxygen, sulphides and oxygen, and amines and oxygen. Triarylmethanes are photooxidised (38) to give triarylmethyl cations (Kuder et al., 1979). The ease hv

Ar,CH - - - 0, -+ (Ar,CH)t + 0 ,' (Ar,CH)t + O,-tAr,C+ + HO,;

(38)

of photo-oxidation is directly related to the oxidation potential of the triarylmethane. Direct irradiation of sulphides in the presence of oxygen produces sulphoxides (Tezuka et al., 1978). Diary1 sulphides and alkyl aryl sulphides are oxidised with equal facility indicating that cc-C-H bonds are unnecessary for the reaction to occur. The reaction is also relatively insensitive to the nature of the solvent; there is little difference in rate between reactions carried out in methylene chloride and methanol. If singlet oxygen had been an important intermediate, the rate should have been higher in the chlorinated solvent due to the longer lifetime of singlet oxygen in this solvent. Irradiation of styrene in the presence of oxygen leads to polymerisation (Kodaira et al., 1978). Although radical cations may well be intermediates in such reactions it has proved impossible to detect them in the reactions of polycyclic aromatic hydrocarbons with oxygen in acetonitrile solution (Watkins, 1979a). Reduction of oxygen by radical ions Several years ago it was reported that many polycyclic aromatic hydrocarbons sensitised the photo-oxidation of amines when polar solvents are employed (Bartholomew, et al., 1971). Whilst it was shown that radical ions are generated in these reactions it proved impossible to tell whether oxidation occurred as a result of the radical ions reacting to give neutral radicals which ultimately reacted with oxygen or whether the radical anion reduced oxygen giving the superoxide anion which then reacted with the radical cation to give the oxidation products. More definitive reactions have been carried out on the sensitised oxidation of alkenes by aromatic hydrocarbons such as 9,lO-dicyanoanthracene. Cyclohexa-1,3-dienes are oxidised to endoperoxides (Eriksen et al., 1977). Since such products are also formed on reaction of singlet oxygen with dienes, it is clear that the type of product formed does not give any indication as to which mechanism is operating. However, 9,IO-

R. S. DAVIDSON

78

dicyanoanthracene also sensitises the photo-oxidation of many compounds, e.g. tetraphenylethylene and stilbenes, which are very unreactive towards singlet oxygen. The finding that cyclohexadienes form exciplexes with 9,lOdicyanoanthracene (Eriksen and Foote, 1978) and that stilbenes undergo electron-transfer reactions with the anthracene to give detectable concentrations of the stilbene radical cation and the anthracene radical anion (Spada and Foote, 1980) is indicative of the reactions occurring via the electrontransfer route. The radical anion has been detected optically and by esr (Spada and Foote, 1980; Schaap et al., 1980). Use of these techniques showed that the radical anion reacts very rapidly with oxygen, presumably to give the superoxide anion. The alkene [ I 131 is oxidised to a 1,Zdioxetane [l 141 on Ar . . ~

Ar

Ar

(DCA

w31 Dicyanoanthracene)

:

EtSCH=C(CH,),CH,

I

Et

9.10-DCA

0 [1141

dye2r:cA EtCO(CH,),Me + EtSCHO [I 161

[1171

irradiation in the presence of 9,IO-dicyanoanthracene and oxygen (Schaap et a/., 1980). Once again this is the product one would expect from reaction of [113] with singlet oxygen. However, under the reaction conditions employed, the anthracene radical anion is formed. It was demonstrated that the alkene radical cation reacts with the superoxide anion to give the 1,2-dioxetanes. Thus it is very clear that the 1,2-dioxetane can be formed by routes other than one involving singlet oxygen. 9,IO-Dicyanoanthracene has been found to sensitjse the photo-oxidation of diphenylacetylene to benzil and benzoic acid (Mattes and Farid, 1980b). There are several interesting mechanistic aspects of this reaction; for example, it was found that the reaction is catalysed by the addition of an acid such as trifluoracetic acid. It was proposed that the acid protonates the radical anion of 9,lO-dicyanoanthracene produced in the photo-induced electron transfer, thereby inhibiting the wasteful back electron transfer. The products were postulated as arising by reaction of the alkyne radical cation with oxygen. The reaction is also accelerated by the addition of bases such as pyridine. In this case it was proposed that the base reacts with the alkyne radical cation to give a radical which then reacts with oxygen. 9,lO-Dicyanoanthracene also sensitises the oxidation of alkylbenzenes to carbonyl compounds (Saito et al., 1979). These hydrocarbons

THE CHEMISTRY OF EXCITED COMPLEXES

79

do not react with the superoxide anion. Photo-oxidation of 4-isopropyltoluene gave products which indicated a ratio of reactivity of methyl to isopropyl as being 1 : 1.3. If the reaction occurred solely via an outer sphere electron-transfer pathway the reactivity should have been 19 : 1. It was proposed that the reaction occurred by light-induced charge transfer to give the anthracene radical anion and the hydrocarbon radical cation. Proton transfer follows to produce benzylic radicals which react with oxygen to give the products. Whether it is valid to compare the selectivity of attack which results from light-induced charge transfer and transition-metal ion oxidation reactions is an open question. The photo-oxidation of sulphides can also be sensitised by 9,lO-dicyanoanthracene and the same products are obtained on dye-sensitised oxidation. Thus [I 151 gives [ 1 161 and [117], while, as shown in Scheme 26, [118] gives [119] and [I201 (Ando et al., 1979). It is apparent

q-w qJ /

0

0

~

9

OH

1

/

Et'

S

'OH

OH

HO, SEt

1

Scheme 26

that both direct and dye-sensitised reactions involve a common intermediate and for [118] this is suggested to be [121]. The results of this work thus suggest that sulphides undergo electron-transfer reactions with singlet oxygen. Tris(2,2'-bipyridyl)ruthenium(II) chloride is known to sensitise the photooxidation of amines and this reaction was shown to occur via an excited state of the complex which reacts with the aniine (Davidson and Trethewey, 1976~). Spectroscopic evidence has been obtained to show that the complex photo-

80

R. S. DAVIDSON

sensitises the oxidation of aromatic amines to amine radical cations (Anderson et al., 1977). It has been proposed that product formation occurs via the ruthenium(1) complex produced in the electron-transfer reaction reducing oxygen to the superoxide ion which in turn reacts with the amine radical cation to give the products. Aromatic hydrocarbons sensitise the decarboxylation of amino acids in the presence of oxygen and once again it proved impossible to tell if the products are formed via the superoxide anion or whether they are derived from radicals reacting with oxygen (Brimage and Davidson, 1973). Tris(2,2'-bipyridyl)ruthenium(II) chloride sensitises the decarboxylation of ethylenediamine tetraacetic acid in the presence of oxygen (Kaneko et af., 1980). Whether or not the superoxide anion is involved is not known. Recently it has been shown that methylene blue sensitises the oxidation of stilbene (Manring et al., 1980). It is known that stilbene is unreactive towards singlet oxygen. For the reaction conditions employed it appears that the excited singlet state of the dye undergoes an electron-transfer reaction with stilbene to give stilbene radical cation and methylene blue radical anion. The latter can reduce oxygen to the superoxide anion which is believed to react with stilbene radical cation to give benzaldehyde. The radical anion of rose bengal has too high an oxidation potential to reduce oxygen and does not therefore participate in this way. The ability of dyes such as methylene blue to participate in electron-transfer reactions (Davidson and Trethewey, 1976a) means that one cannot automatically assume that a dye sensitised photooxygenation reaction occurs via singlet oxygen. Phenazines photosensitise the oxidation of amines and it was proposed that the superoxide anion is involved (Nishikimi et al., 1978). Pyrylium salts photosensitise the oxidation of leuco crystal violet (Saeva and O h , 1976). The salts should be good electron acceptors and oxidise the leuco-dye to its radical cation. It was proposed that this reduces oxygen in a dark reaction to superoxide anion. The dye-sensitised photo-oxidation of amines has been shown to occur by a dye-amine reaction and by reaction of singlet oxygen with the amine (Davidson and Trethewey, 1976~).It appears that the same products are formed in both reactions. This suggests that singlet oxygen can undergo electron-transfer reactions with electron donors. Such a reaction has been used to explain the ability of certain amines, e.g. 1,4-diazabicyclo[2.2.2]octane,to act as physical quenchers for singlet oxygen (Ogryzlo and Tang, 1970). It was found that if tertiary amino groups were attached to groups sensitive to photo-oxidation by an insulating chain then the amino group protected the compound towards photo-oxidation (Atkinson et al., 1973). However, the introduction of some diethylaminomethyl groups into some azo-dyes apparently increased the rate of photo-oxidative degradation (Griffiths and Hawkins, 1977). The oxidation of tertiary amines produces iminium ions and these can be trapped with good nucleophiles such as cyanide ions (Herlem and Khuong-Huu,

THE CHEMISTRY OF EXCITED COMPLEXES

81

1979). It appears that not all good electron donors react with singlet oxygen by an electron-transfer process. It has been shown that the rate constants for reaction of furans and indoles with singlet oxygen are temperature independent and that the rate constants are not related to the oxidation potentials of these compounds (Gorman et al., 1979). These compounds are, however, highly susceptible to photo-oxidation. N-Vinylcarbazole, on the other hand, is not very susceptible to photo-oxidation and this compound reacts with singlet oxygen to give a dimer which is apparently produced via the carbazole radical cation (Nishimoto and Kagiya, 1978). The iodide ion has also been shown to be oxidised by singlet oxygen (Gupta and Rohatgi-Mukherjee, 1978). The oxidative dealkylation of some alkyl dihydroflavins has been proposed to occur by a charge-transfer process (Blankenhorn and Wemmerich, 1979). Another way of carrying out electron-transfer mediated oxidation reactions is to use semiconductors as catalysts (Mozzanega er al., 1977). Titanium dioxide will. photocatalyse the oxidation of substituted toluenes to benzaldehydes by electron transfer from toluene into the photogenerated hole. The electron in the conduction band will reduce oxygen giving the superoxide anion. Reaction of the superoxide anion with the hydrocarbon radical cation produces the aldehyde. A similar mechanism has been used to explain the observation that dealkylation of Rhodamine B (which contains N-ethyl groups) occurs when the dye is irradiated in the presence of cadmium sulphide (Watanabe d al., 1977). ROLE OF RADICAL IONS IN CHEMILUMINESCENT REACTIONS

In an earlier section (p. 6 ) it was pointed out that electron transfer between a radical cation and a radical anion liberates a large amount of energy and this may be in the form of light. Generation, using electrochemical methods, of the trans-anethole radical cation and the 9-cyanophenanthrene radical anion in the vicinity of each other, results in exciplex emission (Park and Caldwell, 1977). Compounds such as [34; n = 0, 1 and 21 give rise to electrochemiluminescence and the emission is from an intramolecular exciplex (Itaya and Toshima, 1977). It was proposed that the electrochemical reduction and oxidation cycles gave rise to radical anions and cations respectively and when these interacted they generated an intramolecular radical ion pair which underwent electron transfer to generate a neutral molecule together with exciplex fluorescence. The study of the production of excited states by the thermolysis of 1,2dioxetanes is quite extensive (e.g. Nakamura and Goto, 1979a,b). It has been found that the addition of electron donors, e.g. amines and aromatic hydrocarbons (in the ground state), accelerate the decomposition. The process has

R. S. DAVIDSON

82

been termed chemically induced electron exchange chemiluminescence (CIEEL) and has been the subject of a review (G. B. Schuster, 1979; G. B. Schuster et ul., 1979; G . B. Schuster and Schmidt, 1982). Thus, aromatic hydrocarbons sensitise the decomposition of phthaloyl peroxide [ 1221 as shown in (39), and the efficiency of sensitisation increases as the ease of

(1221

oxidation of the hydrocarbon increases and as the solvent dielectric constant is increased (Koo and Schuster, 1978). These findings are consistent with the proposed electron-transfer mechanism. It has been found that the decomposition of 1,2-dioxetanes can be intramolecularly sensitised. Thus with compounds such as [ 1 141 in which the aryl group are 4-substituted phenyl groups, the yield of excited singlet states can be dramatically increased if powerful electron donating substituents are used (Zaklika et ul., 1979). Of particular interest is the finding that, when the aryl groups are deprotonated 4-phenoxy groups, the system shows a high quantum efficiency (Schaap, 1981). This suggests that the high light yield in many bioluminescent systems (e.g. firefly luciferin) may well be due to the phenoxyl group in the active compounds catalysing the decomposition of the 1,2-dioxetanones (White et al., 1980). It is known that the decomposition of 1,2-dioxetanones can be intramolecularly sensitised by aromatic hydrocarbons and again the electron-transfer mechanism appears to be important (Schmidt and Schuster, 1978; Adam and Cueto, 1979). Amines are particularly effective sensitisers for the decomposition of these compounds (Schmidt and Schuster, 1980). The flavincatalysed decomposition of N-3-aminophthalyl hydrazide has been proposed as occurring viathe CIEEL mechanism (McCapra and Leeson, 1979). In many of the chemiluminescent systems, amines are particularly effective catalysts. It is well known that amines aid the decomposition of peroxides and that these reactions can be catalysed by light. Dibenzoyl peroxide forms a complex with N-vinylcarbazole and on irradiation initiates polymerisation of the vinylic amine (Al-Abidin and Jones, 1979). Walling (1980) has drawn attention to the fact that in the amine-assisted decomposition of peroxides it is difficult to tell what part and at what stage nucleophilic substitution and electron transfer play a part. Many compounds do not have sufficiently low oxidation potentials to catalyse the thermal decomposition of peroxides but in their excited singlet

THE CHEMISTRY OF EXCITED COMPLEXES

83

state they have more than sufficient energy. The excited singlet state of pyrene sensitises the decomposition of [122] and phthaloyl peroxide (Horn and Schuster, 1979). By means of the technique of flash photolysis it was possible to show that these sensitisation reactions produced the pyrene radical cation and heme that the CIEEL mechanisms was operating.

sRN1 REACTIONS SR,l reactions (substitution-radical nucleophilic) are light-catalysed reactions of synthetic utility. Many of the reactions are detailed in a recent review (Bunnett, 1978). The extent to which these reactions are photo-initiated chain reactions is not known. In the reaction of diethyl phosphite anion the first (EtO),PO-

b

(EtO),PO

+e

Scheme 27

step appears to be electron ejection with the electron being captured by the haloaromatic. In theory this radical anion can participate in a chainpropagating reaction with phosphite ions. It is found that iodoaromatics are far more reactive than bromoaromatics (Bunnett and Traber, 1978). Reaction of phosphite anions with m-iodobromobenzene leads to preferential replacement of the iodo group (Bunnett and Shafer, 1978a). Even greater selectivity is obtained with m-chlorobromobenzene (Bunnett and Shafer, 1978b). A variety of phosphite anions and related groups have been introduced into aromatic rings by this reaction (Swartz and Bunnett, 1979; Bard et al., 1979). Selenide and telluride anions (Pierini and Rossi, 1979) as well as arsenide ions (Rossi et al., 1981) also participate in these reactions. Enolates of many carbonyl compounds react via the SR,l mechanism and this reaction has found use in the synthesis of cycloalkanones (Semmelhack and Bargar, 1980). A review of these reactions is available (Bunnett, 1978).

R. S. DAVIDSON

a4

6

Excited complex formation and electrowtransfer reactions of triplet states

ELECTRON-TRANSFER REACTIONS OF CARBONYL COMPOUNDS

Several years ago it was found that many aromatic carbonyl compounds are photoreduced by aliphatic and aromatic amines (Cohen et al., 1973; Davidson, 1975). It was suggested that these reactions proceed by electron transfer from the amine to the triplet carbonyl compound. Evidence in support of this mechanism was that the ability of amines to quench the phosphorescence of carbonyl compounds, and to react with them, correlated with the ionisation potential of the amines. On the basis of this evidence the reaction scheme in A~,COT, + (RCH,),N --f (Ar,CO); +. (Ar,CO); + (RCH,),N --f Ar,bOH

+ (RCH,),N.t + RdHN(CH,R)2

(40)

(40) was proposed. Conventional microsecond flash photolysis and use of esr spectroscopy confirmed the formation of neutral radicals but did not pick up the formation of radical ions. It was argued that if radical ions were to be detected, polar solvents such as acetonitrile would have to be used and also an amine that did not contain appropriately situated hydrogens which could undergo proton transfer to give neutral radicals. For these reasons, tri-ptolylamine was used and, by means of microsecond flash photolysis, was shown to react with benzophenone in acetonitrile to give the benzophenone radical anion and the amine radical cation (Bartholomew et al., 1972). The detection of radical-ion formation with reactive amines such as N,N-diethylaniline required greater time resolution. By use of nanosecond laser flash photolysis it was shown that this amine reacts with benzophenone in acetonitrile to give radical ions, whereas in nonpolar solvents only the ketyl radical (Ph,kOH) could be detected (Arimitsu and Masuhara, 1973). More recent studies (Masuhara et al., 1978) have shown that even in polar solvents such as acetonitrile both radical ion and ketyl radicals are present immediately after the laser flash. Thus with amines such as N,N-diethylaniline electron transfer and hydrogen-atom transfer are in direct competition. In nonpolar solvents ketyl-radical formation will be favoured. This has been found to be the case with [123]. Picosecond flash photolysis showed that in polar solvents both radical ions and neutral radicals were present at 133 psec after the

[123; n

=

1,2,3]

THE CHEMISTRY OF EXCITED COMPLEXES

85

flash. In benzene, neutral radicals were present (Masuhara et at., 1980). Such studies have also shown that 1,4-diazabicyclo[2.2.2]octaneand triethylamine undergo an electron-transfer reaction with triplet benzophenone (Peters et al., 1980; Schaeffer and Peters, 1980). The fact that solvent-separated radical ions have not been detected in the reaction of aromatic carbonyl compounds with amines in nonpolar solvents does not necessarily mean that the electrontransfer mechanism is inoperative. If the electron transfer followed by proton transfer occurred within the same solvent cage, the radical ions would be extremely difficult to detect by optical means. This situation is somewhat analogous to the reactions of excited singlet states which occur via nonrelaxed exciplexes. Searches have been made for triplet exciplexes. Arimitsu and Tsubomura (1972) examined the emission spectrum of benzophenone in a rigid glass (at 77 K) containing a high concentration of N,N-dimethylaniline. They observed a new structureless emission at 500 nm which was attributed to emission from a ground-state complex between the amine and carbonyl compound. Wolf et al. (1977) have made a time-resolved emission study of this system and agree that the emission originates from a ground-state complex rather than being due to an exciplex. Similar studies have been made with [109; n = 21 (Masuhara et al., 1978). Although this compound shows no exciplex emission at room temperature, in ethanol at 77 K a new broad structureless emission is present with a maximum around 470nm. This emission has a short lifetime ( 30 nsec) and it was therefore proposed that it came from a singlet exciplex. An emission having,,,A at 500 nm is also present and this was attributed to the triplet exciplex. Recent studies on the benzophenone N,N-dimethylaniline system have indicated the formation of a singlet exciplex but a search for a triplet exciplex was unfruitful (Masuhara et at., 1981). In the reaction of a benzoin acetate with triethanolamine, it was found that the aminoalkyl radicals were produced at a slower rate than the decay of the triplet ketone. It was suggested that a triplet exciplex acted as a precursor for the radicals (Salamassi et al., 1980). Optical detection of intermediates produced in the reactions of triplet carbonyl compounds with electron donors has some obvious limitations. However, the technique of CIDNP is proving particularly effective at elucidating the reaction pathways in these systems. The outstanding work of Hendriks et al. (1979) illustrates the power of the technique. Not only was the role of radical ions in the reactions of alkyl aryl ketones with aromatic amines defined but the rate constants for many of the processes determined. The technique has been used to show that trifluoracetyl benzene reacts with electron donors such as 1,4-diazabicyclo[2.2.2]octaneand 1,Cdimethoxybenzene by an electron-transfer process (Thomas et al., 1977; Schilling et al., 1977). Chemically induced dynamic electron polarisation (CIDEP) has been N

N

86

R. S. DAVIDSON

used to show that both the excited singlet and triplet states of biacetyl react with triethylamine and that the triplet state is the more efficient at undergoing chemical reaction (McLauchlan et ul., 1977). In many cases, the reactions of carbonyl compounds are interpreted in terms of the reactivity of the triplet carbonyl compound. However, the work on [123] in which a fluorescent excited charge-transfer complex was detected, and the finding that some amine radical cations react with the radical anions of carbonyl compounds to produce exciplex fluorescence (Zachariasse, 1974) shows that, although intersystem crossing in carbonyl compounds is usually highly efficient, they may participate in excited singlet-state reactions. A thorough study has been made of the reaction of N-methylindole with aromatic carbonyl compounds (Wilkinson and Garner, 1977, 1978). Of particular interest in this work is the assessment of the role of triplet-triplet energy transfer versus electron transfer. In many cases, the observed quenching of excited states was found to be due to electronic energy transfer. In some cases, e.g. N-methylindole plus xanthone in ethanol, the quenching was shown to be by electron transfer. It was shown that the relative importance of the two processes is dependent upon whether the energy of the chargetransfer complex lies below or above the lowest triplet state of the system. When it lies below, electron transfer occurs. Obviously the energy gap and ordering of the states will be highly solvent-dependent. When energy transfer takes place, the reaction which ensues is hydrogen-atom transfer. As mentioned earlier, one of the methods used to try and deduce whether or not a triplet state is being quenched by electron or electronic energy transfer is to see if the quenching efficiency is related to the appropriate redox properties. A particular problem with this approach is ascertaining what part chemical reaction plays in the quenching, i.e. the relative importance of k,, to k,, (see p. 8). The phosphorescence of triplet benzophenone is quenched by primary, secondary and tertiary amines (Abbott and Phillips, 1977) but it is hard to tell what role a triplet exciplex plays. The effect of solvent upon the efficiency of quenching triplet aromatic ketones by triethylamine has been thoroughly investigated (Gorman et ul., 1978). It was found that in nonpolar solvents log kquenchcorrelated with ,AE,,,, + E(A-/A) extremely well, but this was not the case for more polar solvents. The measurements made by earlier workers (Guttenplan and Cohen, 1972) were also subjected to similar treatment. It was concluded that the correlation observed for low polarity solvents was due to the lower reaction probability in these solvents. However, in view of the recent results on the reactions of benzophenone with aliphatic amines, in which it was found that radical yields are often extremely high, even in cases where quantum yield for product formation is low (Inbar, et a]., 1980), it would seem advisable to check the chemical reactivity in all systems before attempting to delineate a particular mechanism. Diphenylamine is

THE CHEMISTRY OF EXCITED COMPLEXES

a7

known to react with benzophenone extremely efficiently to give ketyl radicals and yet very little product formation can be observed (Davidson et al., 1972; Stone and Cohen, I98 I). Thus, until time-resolution or CIDNP experiments are carried out, one cannot be sure of the role of electron-transfer processes in this reaction. Particular caution should be exercised for reactions involving primary and secondary aliphatic amines which have high ionisation potentials. Charge-transfer stabilisation in reactions with these compounds may be minimal, and therefore the transition state may be one in which essentially hydrogen-atom transfer is taking place. The fluorescence and phosphorescence of aliphatic ketones are quenched by amines and, although electron transfer has been invoked as playing a part in the process, the extent to which it participates is not clear. Attempts have been made to construct a theoretical model for such a system (Maharaj et al., 1977). Thiols and sulphides quench triplet carbonyl compounds. Evidence (including that from CIDNP studies) indicates that these reactions occur by a radical rather than an electron-transfer pathway (Cohen et al., 1979; Vermeesch et al., 1978). It is interesting to note that sulphides will deoxygenate ketones producing sulphoxides, sulphones and presumably carbenes (Fox et al., 1979). Phosphines quench triplet carbonyl compounds (Davidson and Lambeth, 1969). They also deoxygenate carbonyl compounds to produce phosphine oxides and carbenes, and in this case, the reaction was proposed as occurring by an electron-transfer process (Fox, 1979). Recently there has been a revived interest in the reaction of phenols and phenolates with triplet carbonyl compounds. In the case of phenols, there is the possibility that they act as electron donors and reduce triplet benzophenone, but it is difficult to establish unequivocal evidence for this process (Manion and Marcia, 198 1). When deuteriated benzene is used as solvent for the reaction, the carbonyl compound and phenol appear to aggregate, which favours reaction from the excited singlet state of the carbonyl compound. Flash-photolysis studies have shown that phenolates donate electrons to excited carbonyl compounds to give ketyl radical anions and .phenoxy radicals (Das and Bhattacharyya, 1981). The rate constants for this process are close to diffusion-controlled and the quantum yield of ketyl radical anion formation close to unity. 4-Bromophenoxide and 44odophenoxide give lower radical yields, possibly because they enhance intersystem crossing of the triplet ketone although cleavage of the carbon-halogen bond may play a part. As detailed in the earlier review (Davidson, 1975), the reaction of alkenes and conjugated dienes with ketones has been extensively investigated. Both electron-rich and electron-poor olefins act as quenchers. The quenching of triplet acetone by a range of alkenes has been studied using flash photolysis (Loutfy et al., 1979). As one moves from alkenes bearing electron-donating groups to those with electron-accepting groups, the rate constant for quench-

88

R. S. DAVIDSON

ing drops and then ultimately rises again. For quenching rate constants lower than the diffusion-controlled limit it was proposed that back electron transfer (k3.J is important. From these measurements exciplex binding energies were estimated. The reaction between triplet cyclohexanone and p-xylene has been investigated using 9,lO-dibromoanthracene as a triplet monitor (Wilson and Halpern, 1981). Energy transfer to the T, state of the anthracene results in thermally activated delayed fluorescence. Thus, by examining the sensitised fluorescence of the anthracene, details concerning the triplet sensitiser can be obtained. In the case of the cyclohexanone-p-xylene reaction it was found that the grow-in of the delayed fluorescence from the anthracene was relatively slow and it was suggested therefore that the sensitising species was a triplet complex composed of the ketone and the aromatic hydrocarbon. Triplet thiones are also quenched by electron-deficient alkenes (Turro and Ramamurthy, 1976). Many triplet amines and carbonyl compounds have energies far higher than that of singlet oxygen and yet the quenching of these triplets by oxygen does not produce singlet oxygen with a quantum yield of unity (Garner and Wilkinson, 1977). The reason for the inefficiency appears to lie in the diversion of some of the energy via complexes of oxygen with the amine and with the triplet carbonyl compound. Thioketones undergo cycloaddition reactions with electron-deficient alkenes (Turro and Ramamurthy, 1976) and also undergo efficient self-quenching (Brucklmann and Huber, 1978). This phenomenon of self-quenching is very common ; both aliphatic (Schuster and Stoute, 1978/79) and aromatic ketones (Wolf et al., 1975, 1977) behave in this way. Wolf et al. (1975, 1977) made a very thorough study of substituted benzophenones and found that for electrondonating substituents there was a good correlation with the substituent constant oP+. It was suggested that the quenching occurs via an exciplex rather than by an excimer mechanism. Benzenes bearing electron-withdrawing and electron-donating substituents quench benzophenone phosphorescence (Schuster et al., 1972), the extent of the effect being determined by the ability of the substituent to facilitate electron transfer from or to the ketone. Triplet exciplexes have been invoked as intermediates in the benzophenone sensitised cycloaddition of phenanthrene to dimethyl fumarate (Creed et al., 1978). The phosphorescence of aromatic ketones such as benzophenone is quenched by Lewis acids such as boron trichloride and a new phosphorescence emission is produced (Snyder and Testa, 1979). This has been attributed to chargetransfer phosphorescence. Interest has been shown in the electron-transfer reaction of 1,2-dicarbonyl compounds. The fluorescence of methyl pyruvate is quenched by good electron donors such as tertiary aliphatic amines although no new emission

THE CHEMISTRY OF EXCITED COMPLEXES

89

attributable to a triplet complex could be observed (Encinas and Lissi, 1981). Irradiation of pyruvic acid can, when the appropriate solvent is used, lead to decarboxylation (Davidson et af., 1981b). Evidence was presented showing that the decarboxylation is not a unimolecular reaction and it was suggested that it was a bimolecular electron-transfer process, as indicated in (41).

2CH3CO.CO,H

--

(CH3C0.C02H)i+ (CH,COCO,H);

C H , ~ O+

(CH,CO.CO,H)~ (CH,COCO,H)' CH,60

+ H+

co, + H+

(41)

CH,dOHCO,H

+ CH,kOH C0,H

CH,CHO

+ CH,CO.CO,H

When methyl viologen (MV2+) is incorporated into the reaction mixture, the formation of reduced viologen can be observed. The formation of this species is slower than the decay of the singlet and triplet pyruvate, which can be accounted for if the reduction occurs as shown in (42). Another

(CH,CO.CO,H)T

+ MV2+-+

CH,C0.C02H + M V i

(42)

interesting facet of the reactions of pyruvic acid is that it quenches triplet naphthalene even though its triplet energy is higher than that of the hydrocarbon. Flash photolysis studies indicated that the quenching probably involves an electron-transfer process. Not surprisingly, quinones undergo a variety of electron-transfer reactions. Triplet duroquinone reacts with tertiary amines (triethylamine, N,N-diethylaniline) to give an exciplex which may give radical ions (detected by laser flash photolysis) in polar solvents or hydrogen-atom transfer in nonpolar solvents (Amouyal and Bensasson, 1977). Triphenylamine quenches the triplet by exciplex formation in nonpolar solvents and by electron transfer in polar solvents. 2,5-Diphenylbenzoquinone reacts with aromatic amines via its triplet state to give radical ions which can be detected by laser flash photolysis (Kuzmin et al., 1979). The reaction of the quinone with diphenylamine has been studied by CIDNP and the conclusion reached that in weakly polar solvents electron and hydrogen-atom transfer compete (Levin et al., 1980). A triplet exciplex was postulated as being an intermediate. Chemically induced dynamic electron polarisation has been used to show that naphtha1,Cquinone reacts with triethylamine in propan-2-01 by an electron-transfer process (Wong, 1978). In the reactions of carbonyl compounds, including quinones, with amines the question as to what part nucleophilic attack plays (Walling, 1980) has never been raised. Since quinones are particularly susceptible to nucleophilic attack, this process could participate in the early stages of the reaction, with electron transfer following.

90

R. S. DAVIDSON

Scheerer and Gratzel(l977) have shown that triplet duroquinone reacts with a variety of electron donors, e.g. amines, ferrocyanide ions, 1,3,5-trimethoxybenzene, to give radical ions. Charge separation can be facilitated by the use of a suitable micelle system. Triplet quinones are powerful oxidising agents and it has been shown that 9,lO-anthraquinone oxidises chloride ions to chlorine atoms (Scharf and Weitz, 1979). Since in aqueous aerated solution this ultimately produces chlorine, this system has been advocated for storage of light energy. Tetrachlorobenzo- 1,Cquinone photosensitises the isomerisation of substituted styrenes (Roth and Schilling, 1979). By use of CIDNP it was shown that the quinone and styrene react to give radical ions and that the radical cation undergoes bond rotation which when followed by reduction gives the isomerised styrene. The chlorinated quinone [124] reacts with furan to give [125] in 92% yield (Maruyama and Otsuki, 1977). Evidence that product formation occurs via a radical pair comes from CIDNP studies.

Wilkinson and Schroeder (1979) have shown that the triplet states of aromatic hydrocarbons are quenched by quinones, the efficiency of quenching being related to the electron affinity of the quinone and the ionisation potential of the triplet hydrocarbon (Schroeder and Wilkinson, 1979). It was concluded that the quenching did not involve full electron transfer in nonpolar solvents. Photolysis experiments have shown that in propionitrile tetrachlorobenzo- 1,Cquinone reacts with naphthalene to give radical ions (Gschwind and Haselbach, 1979). The naphthalene radical cation reacts with naphthalene to give a detectable intermediate. The triplet state of styrene is peculiar in that the initially created form relaxes to give a twisted triplet. The twisted state undergoes electron transfer with methyl viologen and this finding has been used to calculate the triplet lifetime (Caldwell and Pac, 1970). Many triplet aromatic hydrocarbons behave in a similar way (Davidson et d.,1981). The efficiency of the process is related to the sum of the triplet energy and the oxidation potential. With aromatic hydrocarbons for which there is little intersystem crossing, electron transfer occurs from the excited singlet state. With pyrene, laser flash photolysis experiments enabled the electron-transfer process from both the excited singlet and triplet states to be observed.

THE CHEMISTRY OF EXCITED COMPLEXES

91

Triplet excimers of aromatic hydrocarbons have proved very difficult to detect and hence their role in deactivation of excited states is largely speculative. However, on the basis of emission experiments (Subudhi and Lim, 1976; Okajima et a/., 1977; Chandra and Lim, 1977; Webster et al., 1981), it has been suggested that some di(1-naphthy1)alkanes form such species. It is suggested that the favoured conformation of the triplet excimer does not have the two naphthalene rings lying parallel to each other. The electron-transfer reactions of triplet dyes has been extensively investigated because these reactions often complicate the mechanism of photooxygenation and because of the possibility of using these reactions to harness solar energy. Kayser and Young (1976a,b) have determined the rate constants for reaction of triplet methylene blue with a variety of amines, including aniline and 1,4-diazabicycl0[2.2.2]octane. Some of these rate constants have been determined by a less direct method (Davidson and Trethewey, 1976~). Ethylenediaminetetracetic acid reacts with triplet dyes and in the case of oxinine the reactivity of its excited singlet and triplet states has been assessed (Bonneau, 1977). Although the rate constant for quenching the excited singlet state is greater than that for the triplet, very much more chemical reaction takes place from the triplet state; only 2.3% of the excited singlet states lead to product formation. The relative reactivities of excited singlet and triplet states in electron-transfer reactions has been investigated using flavins. The excited singlet and triplet states of these compounds are quenched by aromatic hydrocarbons (Traber et al., 1981). It was found that the difference in reactivity exceeded that predicted by the Rehm-Weller equation (2). The reason for this discrepancy is not known. Triplet methylene blue undergoes electron transfer with aromatic hydrocarbons as well as aromatic amines (Kikuchi et al., 1977). Triplet dyes also undergo electron-transfer reactions with metal ions; thus thionine is reduced by iron(I1) and manganese(II1) (Ferreira and Harriman 1977; Duncan et al., 1978; Wildes et al., 1977). Rose Bengal, the dye which is frequently used as a sensitiser for photo-oxygenation has a triplet state which forms an exciplex with duroquinone (Hermann et al., 1978). Addition of duroquinone to oxygenation reactions quenches the oxygen uptake via the exciplex mechanism. Flavins react with aminopurines to generate radical ions, their formation being detectable by CIDNP (Kaptein et al., 1979). The reduced form of many dyes react with platinum in the presence of protons with the liberation of hydrogen. In many cases the process is facilitated by the addition of methylviologen which acts as a relay in transferring the electron from the reduced dye to protons. Reduced flavins have been shown to react in this way (Krasna, 1980; Kalyanasundaram and Gratzel, 1979). The triplet states of porphyrins undergo oxidation and reduction reactions. Zinc and magnesium tetraphenylporphyrin in their triplet states are oxidised

92

R. S. DAVIDSON

by europium ions (Potter and Levin, 1979). Manganese(II1) porphyrins sensitise the reduction of quinones (Harriman and Porter, 1980). The manganese (IV) and reduced quinone produced by the reaction were identified spectroscopically. Not surprisingly, excited metal phthalocyanines are oxidised by methyl viologen and this reaction can be used for hydrogen production (Tanno et al., 1980). Intramolecular charge transfer has also been investigated (Dalton and Milgrom, 1979). Triplet porphyrins are reduced by hydrazines (Chernikov et al., 1977). Aliphatic amines produce dihydro- and tetrahydro-products and products containing the amine, e.g. [126; R = CH(Me)NEt,] (Hare1 and Manassen, 1978). A thorough study has been made of. the reduction of palladoporphyrins by aromatic amines which leads to the metallated equivalents of [126] having for example R = CH,N(Me)Ph (Mercer-Smith et al.,

1979). It was proposed that these reactions involve exciplexes in which there is little stabilisation by charge transfer. Phthalocyanines are reduced by amines such as ethylendiamine tetracetic acid : the reduced phthalocyanine can reduce oxygen with the ultimate formation of hydrogen peroxide (Harbour et al., 1980). For effective reaction it is necessary to control the surface charge of the phthalocyanine. The redox reactions of chlorophyll have been well studied because of their important role in photosynthesis; the subject has been reviewed (Seely, 1978). Chlorophyll is oxidised to its cation by ferric chloride and benzo-1,Cquinone (Tollin and Rizzuto, 1978). Chlorophyll has been shown to photoreduce carbon dioxide and this process is enhanced by p-carotene (Fruge et al., 1979). Beddard et al. (1977a) proposed that the excited singlet state of chlorophyll undergoes one-electron reduction by p-carotene and some recent evidence lends credence to this suggestion (Dirks et al., 1980). Chlorophyll fluorescence is also quenched by electron acceptors such as nitrobenzene (Beddard et al., 1978). The redox reactions of ruthenium(I1) complexes have been extensively investigated because of their potential use in systems for harnessing solar

THE CHEMISTRY OF EXCITED COMPLEXES

93

energy (Sutin, 1979). The tris(2,2’-bipyridyl)ruthenium(II) chloride has been subjected to the closest scrutiny. In its excited state (a charge transfer to ligand excited state) it can reduce electron acceptors giving a ruthenium(II1) speciesand is reduced to give a ruthenium(1) species (Abruna et al., 1979). The ruthenium(II1) species is capable of reducing protons to hydrogen (usually methylviologen is used as an electron relay) and the ruthenium(1) species will oxidise, somewhat inefficiently, hydroxide ions to give oxygen. Rate constants for photo-induced reactions with a variety of donors and acceptors have been compiled (Bock er al., 1979a) and the data analysed in terms of the MarcusHush theory (Bock et al., 1979b). The excited complex efficiently reduces methylviologen (a 4,4‘-bipyridinium compound) (DeLaive et al., 1978; Durham et al., 1979). The back electron transfer involving reduced methylviologen and ruthenium(II1) ions can be suppressed by adding electron donors such as triethanolamine and ethylenediaminetetracetic acid. If a precious metal such as platinum is present, the reduced methylviologen can reduce protons giving hydrogen. Such systems ‘have provided models for hydrogen production using solar energy (Moradpour er al., 1978, 1980; Kalyanasundaram 1978b; Keller et al., 1980). However, in all these systems the amine is used as the fuel and in effect the light is being used to dehydrogenate the amine. A wide variety of 4,4’-bipyridinium and 2,2’-bipyridiniumcompounds have been synthesised so that a series of bipyridinium compounds having ,??+-valuesvarying between -0.24 and -0.78 V is available (Takuma et al., 1978; Amouyal et al., 1980). With this range of compounds it is possible to make an extensive investigation of the relationship between the logarithm of quenching of excited states by bipyridinium compounds. The excited tris (2,2’-bipyridyl)ruthenium(II) complex is oxidised to the ruthenium(II1) complex by quinones (Darwent and Kalyanasundaram, 1981). The measured yield of ruthenium(II1) and semiquinone radical anions is small owing to the efficiency of the back electron-transferreaction. Furthermore, the quinone radical anions after protonation disproportionate to generate the quinol. The quinol acts as a quencher for the excited ruthenium(11) complex and also the photoproduced ruthenium(II1) ions. It has also been shown that the oxidised and reduced forms of the ruthenium(I1) complex react together to generate chemiluminescence, the emission coming from the charge-transfer state of the complex (Glass and Faulkner, 1981). A particularly ingenious system has been devised in which the ruthenium is co-ordinated to two 4,4‘-hipyridyls in which only one of the nitrogen atoms of each of the bipyridyls is linked to the ruthenium (Sullivan et al., 1978). The quenching of the emission from bis(4,4’-bipyridyl)dicyanoruthenium(II) salts by bipyridinium compounds has also been investigated, and as might be expected quenching via electron transfer is presumed to occur (Gaines, 1979).

R . S. DAVIDSON

94

7

Excited complex formation and photo-induced electron-transfer reactions in organised systems

M I C E L L A R SYSTEMS

Extensive studies have been made of photo-induced electron-transfer processes in amphiphilic micelles (Turro et a/., 1980). The micelle structure is considered to be spherical, having an inner hydrocarbon core and an outer, highly charged and densely packed Stern layer which is surrounded by a charged but less densely packed Gouy-Chapman layer. Excimer formation has been used to probe the fluidity of the hydrocarbon interiors of amphiphilic micelles. One of the problems associated with adding a probe material is that it may distort or even totally change the micellar structure. It is thought the aromatic hydrocarbons such as pyrene do not do this. Normally the chances of obtaining a double occupancy of the micelle interior by a probe molecule are very low and hence intermolecular excimer formation is not often observed (Kalyanasundaram, 1978a). However, excimer formation from pyrene has been observed in cationic micelles (Dorrance and Hunter, 1972; Miller et a/., 1977). The distribution of the pyrene was found to be affected by the length of the alkyl chain of the molecule which forms the micelle. When pyrene is modified so that it is attached to a long alkyl chain bearing an ionic group, such as a carboxylate ion, excimer formation readily occurs since the micelle interior is made up of alkylpyrenes (Atik and Singer, 1978; Atik et al., 1979). Kinetic analysis of the excimer systems studied by Dorrance and Hunter (1972) indicated that the micelle interior had a rather rigid structure. Recent work has shown that the use of intramolecular excimer formation for measuring microfluidity is more reliable; compounds which have been used include 1,3-diphenylpropane, 1,3-dipyrenyIpropane (Zachariasse, 1978) di( 1 -naphthylmethyl)-ammonium salts and di(arylmethy1)ethers (Goldenberg et a/., 1978; Emert et ul., 1979). In order to obtain a value of the microviscosity of the micelle interior, a comparison has to be made of the kinetics of intramolecular excimer formation within the micelle with the kinetics of the process in solvents of known viscosity. Turro et al. (1979) used this approach to demonstrate the greater reliability of intra- versus inter-molecular excimer formation. Similar methods have been employed to measure the microviscosity of micelles formed by non-ionic (amphiphatic) compounds (Watkins and Selinger, 1979). Diarylmethylammonium compounds exhibit intramolecular excimer formation in sodium dodecyl sulphate micelles (Emert et d., 1981b). The micelles enhance excimer formation by, presumably, favouring the conformation of the ammonium compounds in which the aryl groups lie close together. If the nitrogen atom is alkylated with long chain alkyl groups, long alkyl groups appear to be attached to the micelle interior and the arylmethyl groups

THE CHEMISTRY

OF EXCITED COMPLEXES

95

being at the surface. Under these circumstances it is difficult for the molecule to adopt a conformation which allows excimer formation. Consequently, the introduction of long chain N-alkylgroups negates the enhancing effect of the micelle upon excimer formation. When cationic micelles are employed, the counter ions are often halide ions. It is found that the fluorescence lifetime of micellised aromatic hydrocarbons is reduced by bromide ions (Hautala et al., 1973; Miller et al., 1977). Whether the quenching is due to the heavy atom effect, electron transfer, or nucleophilic attack has not been determined. When anionic micelles are employed the counter ions can often have a profound effect upon the photophysical processes of micellised compounds. Thus if the counter ion is a heavy atom, e.g. silver, intersystem crossing can become highly efficient and make it possible for phosphorescence to be readily detected at room temperature (Kalyanasundaram et al., 1977; HumphryBaker et al., 1978). If, on the other hand, the counter ion does not interfere with photophysical processes, high triplet yields can be observed, e.g. for N-methyl-phenothiazine (Moroi et al., I979a), zinc(I1) tetrasulphophthalocyanine (Darwent, 1980), because the high likelihood of single occupancy of micelles precludes triplet-triplet annihilation. Fluorescence quantum-yield and lifetime measurements have established that oxygen penetrates micelles (Geiger and Turro, 1975). Entrance- and exit-rate constants have been determined for a cationic (CTAB) and an anionic (SLS) micelle system using 1,5-dimethylnaphthalene as the fluorescing species (Hautala et al., 1973). Of general importance is that the solubility of oxygen is higher in the micelle than in the aqueous phase. Many of the systems leading to excited complex formation' and chargetransfer reactions exhibit fluorescence quenching of either the donor or the acceptor. To ascertain the bimolecular rate constant for such a process one employs the standard Stern-Volmer equation. The situation is very much more complex for micellar systems since the quenching may involve for instance the donor being totally or only partially micellised. When a donor and acceptor molecule are both present in the same micelle then static quenching occurs. There are now available several mathematical models which enable one to extract rate constants such as quenching rate constants, entrance- and exit-rate constants, etc. (Yekta et a]., 1979; Waka et al., 1979, 1980; Atik and Singer, 1978). The fluorescence of many micellised aromatic hydrocarbons is quenched by electron donors (amines) and acceptors (dicyanobenzenes) and in no case has exciplex emission been detected. Laser flash photolysis showed that in all cases the quenching led to electron transfer, presumably because the polar complex can migrate to the polar Stern layer in a very short space of time (Katusin-Razem et al., 1978). For systems which form intramolecular exciplexes, e.g. [39], charge separation across the space-charge layer cannot

96

R. S. DAVIDSON

occur and therefore usually neither radical ion formation nor exciplex fluorescence can be detected, although efficient fluorescence quenching occurs (Masuhara et al., 1977a). For [39; n = I ] there is some evidence for the quenching occurring via an exciplex when dodecyltrimethylammonium chloride micelles are employed. Many surfactants, e.g. benzenesulphonates, contain aryl groups and it is found that they will form charge-transfer complexes with 1,2,4,5-tetracyanobenzene which can be detected by ultraviolet absorption and fluorescence spectroscopy (Masuhara et al., 1979); a similar result was obtained with an amphiphatic system. Fluorescence quenching in such micelles has been studied, an example being the quenching of the fluorescence of benzyl anthroate by triethylamine in Triton X (Costa and Macanita, 1978). Perhaps one of the most thoroughly studied aspects of the photochemistry of ionic micelles is that of photo-induced charge separation. The use of laser flash photolysis has proved invaluable in probing this type of process (Gratzel, 1977). A particularly illuminating example is that of the reaction of triplet chlorophyll with duroquinone in an anionic micelle such as sodium lauryl sulphate (Wolff and Gratzel, 1977). Both the donor and acceptor are solubilised in the micelle but on flash illumination charge transfer occurs. The duroquinone radical anion is ejected from the micelle and the negative charge on the surface of the micelle prevents it re-entering. Consequently the duroquinone radical anion h a s a long lifetime. If pycocyanine is added, which is reduced by the duroquinone radical anion to a species which bears a positive charge, the chlorophyll radical cation is quickly reduced when the reduced pycocyanine entering the micelle. This type of experiment has been carried out with porphyrins as donors and quinones as acceptors (Kano et al., 1978) and pyridinium compounds (Ogata et al., 1979). Zinc tetrasulphophthalocyanine in a cationic micelle is efficiently reduced by cysteine (Darwent, 1980). Other systems which have been considered include aromatic hydrocarbons and aromatic amines (Waka et al., 1978; Thomas et al., 1978). N-Ethyl carbazole and duroquinone undergo a photoinduced electron-transfer reaction (Yamaguchi et al., 1981). When the reaction is carried out with sodium lauryl sulphate micelles, efficient charge separation occurs and high yields of radical ions can be measured. When cetyl trimethylammonium bromide micelles are used, radical ion formation could not be observed. However, high yields of radical ions are observed when cetyl trimethylammonium nitrate micelles are used. These results illustrate the importance of the nature of the counter-ion. It was suggested that the bromide ions quench the excited quinone which is presumably located at the surface of the micelle. Charge separation can be observed when microemulsions containing cetyl trimethylammonium bromide is used as surfactant. The lack of quenching by the bromide ions indicates that the quinone and carbazole are located in the

THE CHEMISTRY OF EXCITED COMPLEXES

97

hydrocarbon core of this system. Systems utilising aromatic hydrocarbons and amines as electron donors have also been studied using reversed micelles, i.e. hydrocarbons containing micelles which entrain pools of water. Incorporation of anthra-9,10-quinone-2-sulphonateinto cationic micelles enables its photoreduction by hydroxide ions to be studied (Inoue and Hida, 1979). Radical ion formation can also be observed when reversed micellar systems are employed. Use of anionic surfactants enables easily reducible metal ions to be incorporated at the micelle surface. Incorporation of a donor into the micelle allows light-induced electron transfer to take place across the space charge layer and this is followed by intramicellar and intermicellar back electron transfer (Moroi et al., 1979a,b; Moroi et al., 1979); the two processes are kinetically distinguishable. Electron transfer from N-methylphenothiazine to tris(2,2'bipyridyl)ruthenium(ll) chloride with the aromatic amine contained in a micelle has been examined (Maestri et al., 1978). Ruthenium complexes bearing long alkyl chains which can be incorporated into micelles, both anionic and cationic (Kalyanasundaram, 1978b; Schmehl and Whitten, 1980). Incorporation of these complexes into cationic micelles containing N,N-dimethylaniline leads to the production of a ruthenium(1) complex (Tsutsui et al., 1979). When methylviologen is present in aqueous solution, the ruthenium(1) species is oxidised to give the reduced methylviologen and in this way efficient charge separation is obtained. A particularly interesting viologen based on the 4,4'-bipyridyl system has one nitrogen carrying a CI4 chain and the other a methyl group. The viologen undergoes electron transfer with triplet porphyrins to give a reduced viologen which is hydrophobic. In the presence of a cationic micelle, the viologen is trapped in the hydrocarbon core of the micelle with the oxidised porphyrin remaining in the aqueous phase. In this way efficient charge separation is obtained (Brugger et al., 1980). Another way of utilising micelles is to use the charged surface to bind (electrostatically) both the electron donor and acceptor or to bind one of the redox partners to the surface and have the other contained within the micelle. (Martens and Verhoeven, 1981). When pyrene is dissolved in sodium dodecyl sulphate and methyl viologen is bound to the'micelle surface, the hydrocarbon and viologen are located sufficiently close to each other that ground state charge-transfer (identified by the appearance of new absorption bands) complex formation takes place. This complex formation affects the kinetics of quenching the excited pyrene. Electron ejection from a variety of aromatic amines, e.g. 3-aminopyrene and tetramethylbenzidine, occurs via a monophotonic process when the amines are incorporated into anionic micelles (Thomas and Piciulo, 1978, 1979). A rather interesting system, which has been recently studied employs caffeine to solubilise aromatic hydrocarbons in aqueous solution (Nosaka et al., 1981). Two caffeine molecules intercalate one molecule of the hydrocarbon. Quenching of the excited singlet state of

98

R. S. DAVIDSON

hydrocarbons solubilised in this way by various ions such as Cu2+, Eu3+, Fe3+was studied and the hydrocarbon radical cation yield found to be quite high (0.68, 0.39 and 0.2 respectively). With many ions e.g. Ag+ and TI+ the yield of radical cations was very low due to the ions acting as heavy atom quenchers and thereby inducing intersystem crossing rather than electron transfer. MONOLAYERS

The construction and properties of monolayers has been well documented by Kuhn (1979) and the photochemical reactions which occur in such systems reviewed (Whitten et al., 1977). Molecules in monolayers are usually ordered and in the case of trans-azastilbenes irradiation of the ordered array produces excimer emission and dimers (Whitten, 1979; Quina er ul., 1976; Quina and Whitten, 1977). This contrasts with what is found when the trans-isomers of such compounds are incorporated into micelles. In such systems the predominant reaction is cis-trans isomerisation; excimer emission is lacking. It is suggested that the lack of isomerisation in the fatty acid monolayers is due to the tight packing and consequent high viscosity of such systems. Styrene also dimerises in a fatty acid monolayer. Interestingly, the products formed on photo-oxidation of protoporphyrins are dependent upon whether the reaction is carried out in a monolayer or a micelle (Whitten et al., 1978). Zinc octaethylporphyrin exhibits excimer emission in monolayers (Zachariasse and Whitten, 1973). Porphyrins are photoreduced by amines in monolayers (Mercer-Smith and Whitten, 1979). Electron-transfer reactions have been carried out with monolayers of stearic acid containing chlorophyll and electron acceptors such as quinones (Janzen et al., 1979; Janzen and Bolton, 1979). OTHER ORDERED SYSTEMS

A dipalmitoylphosphatidylcholine labelled with pyrene has been synthesised. In all organic solvents this compound exhibits pyrene excimer emission but in aqueous solution there is total fluorescence quenching. It is suggested that aggregation leads to the fluorescence quenching (Sunamoto et al., 1980). Photodimerisation reactions in cholesteric phases have been examined (Nerbonne and Weiss, 1978, 1979). The high viscosity of such systems probably precludes the participation of excited singlet states in bimolecular reactions. The concentration quenching of the excited singlet state of chlorophyll in lipid liposomes and vesicles has been shown to occur (Beddard et ul., 1976). Electron-transfer reactions between diphenylamine and duroquinone and

THE CHEMISTRY OF EXCITED COMPLEXES

99

between N-methylphenothiazine and methylviologen in microemulsions have been studied (Kiwi and Gratzel, 1978). With the former system the donor and acceptor are both solubilised in the lipid interior whereas with the latter the phenothiazine is in the lipid but the viologen is in the aqueous phase and charge separation can be effected. Pyrene excimer formation has been used to probe the structures of microemulsions and the conclusion was reached that pyrene is located in a more hydrophobic environment in microemulsions than it is in micelles (Gregoritch and Thomas, 1980). The formation of radical ions in the reaction of the excited singlet state of pyrene with N,N-dialkylanilines has been used to compare the efficiency of charge in micelles with microemulsions and vesicles. (Atik and Thomas, 1981a). It was found that increasing the size and rigidity of the molecular assembly decreased the yield of radical ions. In the case of microemulsions and vesicles the reaction partners are located in the hydrocarbon core where the higher viscosity and low polarity of the environment mitigate against the electron-transfer process. When pyrene, substituted with an ionic group, e.g. SO, is used the chargetransfer process occurs with similar efficiency in all three systems. The polar group locates the pyrene in the polar interfacial region thereby facilitating electron transfer. Cationic oil in water and water in oil microemulsions have also been examined (Atik and Thomas, 1981b). The oil in water system consisted of a cetyl trimethylammonium bromide, hexanol and dodecane mixture. Efficiency of photo-induced charge separation was again assessed by using the N,N-dialkylaniline pyrene reaction. It was found that the yield of pyrene-radical anion decreased as the amount of hexanol was increased. This was attributed to the hexanol causing the pyrene to adopt an increased hydrophobic location. For the water in oil system i.e. a water pool system, the amount of water located in the pools was found critically to affect the efficiency of charge-transfer process. As the water pools are decreased in size, so their rigidity increases and the water loses its bulk solvent properties. Pyrene and dimethylaniline mixtures in such systems exhibit weak exciplex fluorescence indicating that they are mainly located in a hydrocarbon region. If, however, a charged pyrene is used such as pyrenesulphonic acid, electron transfer is efficient since this occurs at the polar water hydrocarbon interface. If the tris-(2,2‘bipyridyl)ruthenium(II) complex is dissolved in the water pools together with heptyl viologen photo-induced electron transfer occurs in the aqueous phase but the reduced viologen migrates to the hydrocarbon phase thereby achieving efficient charge separation. This type of reaction has been used to probe “inter-water pool quenching” (Atik and Thomas, 1981~). The distribution of additives to microemulsions and reversed micelles has been shown to obey Poisson statistics. Pyrene has been used to probe the structure of the bilayers formed by dihexadecyl phosphate (Escabi-Perez el al., 1979). Many of the classical electron-transfer reactions carried out in

R. S. DAVIDSON

100

micellar systems have been repeated using vesicles (Infelta et al., 1980). Charge separation in the reaction of methylene blue with iron(l1) compounds has been studied using liposomes to prevent the energy-wasting back electron transfer (Sudo and Toda, 1979). Charge transfer between amphipathic ruthenium(I1) complexes and Nbutylphenothiazine in micelles, synthetic bilayers and liposomes has been studied by flash photolysis (Takayanagi et al., 1980). It was shown that the energy wasting back electron-transfer reaction is less efficient in the vicinity of the charged surface and that it is disfavoured by an increase in temperature. Cationic vesicles have been used to accomplish charge separation (Monserrat and Gratzel, 1981). The photosensitiser was a water-soluble porphyrin and electron acceptor was a modified, water-soluble viologen. The porphyrin photo-reduced the viologen which in its reduced form is lipid soluble but water insoluble. Consequently, the reduced species enters the vesicle. So effective is the charge separation that multimer formation of the reduced species in the vesicle can be observed. Another method which has been employed is to immobilise donors and acceptors on the surface of latex particles (Frank et al., 1979). 8

Chemical reactions postulated as occurring via excited complex formation or an electron-transfer reaction

INTERMOLECULAR CYCLOADDITION

One of the most extensively investigated systems is that of the reaction of alkenes with benzene and related compounds. Some general rules have now been formulated (Bryce-Smith et al., 1980b; Gilbert, 1980). The formation of 1,2- and 1,4-cycloadditionproducts is found to occur when there is a substantial difference in ionisation potential between the alkene and benzene. When this condition prevails, the reaction is postulated as occurring via an exciplex derived from the excited singlet state of benzene. The formation of 1,3cycloaddition products is thought to involve prefulvene. The reaction of this species with alkenes is regioselective (Gilbert and Heath, 1979). The orientation of addition is highly dependent upon the nature of the alkene, although on the whole there is a tendency for the reaction to produce the product having the endo stereochemistry (Bryce-Smith et al,, 1978~).The addition of 2,3dihydropyran (Gilbert and Taylor, 1977) and other oxygen-substituted alkenes (Atkins et al., 1978) produces 1,Zcycloaddition products. Furan and benzene form a [4 + 41 photocycloaddition product as well as a [2 + 21 cycloaddition product (Berridge et al., 1980). The two products are thermally interconvertible. Many other electron-rich alkenes, e.g. 1,Zdialkoxyethenes, add to benzene giving 1,2-, 1,3- and 1,Ccycloaddition products

THE CHEMISTRY OF EXCITED COMPLEXES

101

(Mattay et al., 1979a,b). When relatively polar solvents are employed, exciplex fluorescence can be observed (Leismann et al., 1978). This fluorescence is quenched by triethylamine, and addition of triethylamine quenches product formation. Provided the amine does not affect the triplet yield and triplet lifetime, this could constitute evidence for the reaction occurring via an exciplex. Alkylbenzenes and alkoxybenzenes form cycloaddition products with alkenes (Srinivasan and Ors, 1976; Gilbert and Heath, 1979) and the role of exciplexes has been discussed. The exciplexes formed between benzene and the alkenes probably have little binding energy in nonpolar solvents where presumably exciton resonance is the stabilising factor. In more polar solvents, the solvent polarity will aid stabilisation via charge transfer. The reactions involving electron-deficient alkenes are typified by the reaction of benzene with methyl acrylate (Atkins et al., 1977) and of anisole with acrylonitrile (Ohashi et al., 1976, 1977~).The reaction with methyl acrylate produces two stereoisomeric 1,2-cycloaddition products. It was proposed that the stereochemistry of the product is determined by the orientation of the alkene relative to benzene in the ground state complexes. There is good evidence, e.g. from studies of deuterium incorporation, that the reaction with acrylonitrile to give [127] and [128], and probably [I291 involves electron transfer (Scheme 28). Anisole forms ground state complexes with maleimide

J

J

I

Scheme 28

which can be detected by absorption spectroscopy (Bryce-Smith et al., 1978b). Irradiation in this band produces three isomeric 2 : 1 (2 imide : 1 anisole) adducts which are derived from initial 1 : 1 adducts formed via 1,2-, 2,3- and 3,4-modes of addition. The relative yields of the adducts are dependent upon solvent composition.

R. S. DAVIDSON

102

Diphenylmethane does not undergo cycloaddition reactions on direct excitation in the presence of electron acceptors (Gilbert and Lane, 1981). Reaction only occurs when the radiation is absorbed into the intermolecular charge-transfer band. With maleic anhydride [2 + 21 cycloaddition to one of the phenyl rings of diphenylmethane occurs and then the cyclohexa-l,3-diene so created reacts thermally with a further mole of the anhydride. Benzonitrile forms a number of cycloaddition products (Cantrell, 1977), e.g. with 1,2dimethylcyclohexene [ 1301, [ 1311 and [ 1321 are formed (Scheme 29). Products Me

Me

Me [1311

HNQ Ph ~321

Scheme 29

are formed by reaction with the nitrile group rather than the benzene ring. Formation of [132] can be formulated as occurring through an ene reaction. In all cases the alkenes quenched the fluorescence of benzonitrile, suggesting that reaction probably occurs via an exciplex. In aqueous acetonitrile 2-cyanopyridine undergoes an interesting photoaddition reaction with alkenes as shown in Scheme 30 to give [I331 (Saito,

0

Scheme 30

THE CHEMISTRY OF EXCITED COMPLEXES

103

et al., 1980). 1 ,CDicyanoaphthalene does not photoreact with toluene in cyclohexane solution. However, in a polar solvent such as acetonitrile reaction takes place to give 1-benzyl-4-cyano-naphthalene plus 1,2-dihydronapthalenes in which the toluene has added to the 1- and 2-positions of the naphthalene ring (Albini er al., 1981). The powerful electron acceptor hexafluorobenzene reacts with cyclopentene to give a 1,2-cycloadduct (Sket and Zupan, 1977) and with cis- and trans-cyclo-octene to give I ,2- and 1,3-addition products (Bryce-Smith el al., 1978a). It was also found that hexafluorobenzene reacted with tetramethylethylene to give the insertion product [135] (Scheme 31).

C,F,

+ Me,C=CMe,

h

F

F

' F F

F

e

F

'6 F

F

+

7F '

HF

11341

Me

Scheme 31

This is quite an unusual reaction although 3-chlorotetrafluoropyridine (M. G. Barlow er al., 1979) has been found to react in a similar way. Both bromobenzene and iodobenzene do likewise (Bryce-Smith et al., 1980a). There is abundant evidence that the reaction does not involve free aryl radicals and all the evidence points to exciplexes as intermediates. The photo-cycloaddition of alkenes and dienes to styrenes and stilbenes has been recently reviewed by Lewis (1979). trans-Stilbene adds to many electron-rich alkenes to give [2 2]-cycloaddition products (Kaupp et al., 1978). Similar products are formed with vinylene carbonate (Lewis and Hoyle, 1977). Conjugated 1,3-dienes give [2 + 21-cycloaddition products (Lewis and Hoyle, 1977; Lewis and Johnson, 1977). When nonpolar solvents are used, the reaction is modestly regioselective, but as the polarity of the solvent is increased the selectivity increases. Since the dienes quench the fluorescence of the stilbenes, it appears that exciplexes are involved. [2 + 21-Cycloaddition takes place when indene (Sket and Zupan, 1976) and imidazoles (Ito and Matsura, 1979) are irradiated in the presence of hexafluorobenzene and

+

R. S. DAVIDSON

104

acrylonitrile respectively. When the imidazole contains an N-H bond, products such as [ 1361 are formed (43). This together with fluorescence-quenching studies indicates that exciplexes are intermediates in these reactions. Me

..

~361

Naphthalene reacts with acrylonitrile (44) to give [137] and [138] (LeNoble and Tamura, 1977). The relative yield of the two products is independent of pressure which suggests that the reaction has a late transition state. The trimethylsilyl ethers of 1 - and 2-naphthol undergo [2 + 21-cycloaddition

[I371

[138]

with cyanoethylene (Akhtar and McCullough, 1981). 2-Cyanonaphthalene gives [2 + 21-cycloaddition products with electron-rich alkenes (Mizuno et al., 1977b). It has also been shown to photodimerise (Teitei et al., 1976). 9-Cyanophenanthrene forms fluorescent exciplexes with styrenes (Caldwell et al., 1978; Caldwell et al., 1979). The complexes lead to [2 21-cycloaddition and formation of the triplets which also yield cycloaddition products. The exciplex fluorescence is quenched by dimethyl acetylenedicarboxylate which also quenches the cycloaddition reactions, indicating that some of the products are formed via the exciplex. 9-Cyanophenanthrene forms [2 21cycloaddition products with 9-alkoxyphenanthrenes and, since heteroexcimer fluorescence can be observed, they are presumably intermediates (BouasLaurent et al., 1976). Benzylanthracene-9-carboxylate forms a head to tail photodimer (Costa and Melo, 1980) presumably via a singlet excimer. The fluorescence of the ester is quenched by triethylamine and in polar solvents radical ion formation has been detected. The photodimer is also formed in acetonitrile containing triethylamine and it was suggested that this occurred via a triple complex composed of two molecules of the anthracene and one of the amine. Conjugated dienes, e.g. 2,5-dimethylhexa-2,4-diene,quench the fluorescence of anthracenes and form adducts, e.g. [139], as shown in (45) (Yang and Shold, 1978). It was found that the Stern-Volmer quenching constant increased as the temperature was lowered and this increase in quenching efficiency is matched by an increase in the quantum yield of the

+

+

105

THE CHEMISTRY OF EXCITED COMPLEXES

cycloaddition reaction. Such experimental evidence strongly supports the contention that exciplexes are intermediates in the reaction. Several examples of [4 + 41-cycloaddition reactions between anthracenes and conjugated 1,3-dienes are now known (Kaupp and Teufel, 1980). 9-Cyanoanthracene reacts in a similar way (Kaupp and Schmitt, 1981). Acenaphthylene forms [2 + 2]-cycloaddition products with electrondeficient alkenes such as acrylonitrile and methyl acrylate (Nakamura et a/., 1978). Cycloaddition is favoured by carrying out the reaction in a micellar solution owing to the inefficiency of the dimerisation in such an organised system. Phthalimides participate in a number of photo-induced electron-transfer processes. They have been found to react with alkenes with the formation in some cases of oxetanes, e.g. [140], and ring expansion products [I411 (Scheme 32) (Mazzocchi et al., 1978a, 1979; Maruyama and Kubo, 1978a). The reaction can also be carried out intramolecularly (Maruyama et al., 1978;

qMe OEt

1

[lm]

Scheme 32

Machida et al., 1980a). When 1,l-diphenylethylene is used as the alkene and methanol is used as solvent, adducts such as [142] are produced which indicate that the reaction occurs by an electron-transfer process (Scheme 33) (Maruyama and Kubo, 1978b). Phthalimides also react with conjugated dienes to give ring expansion products (Mazzocchi et al., 1978b, 1977); only one of the

106

R.

S. DAVIDSON

+ (Ph,C=CH,): 0

0

& Ph

Ph

& Ph

NMe ,OMe

MeOH

Ph

N-Me H,

\

\

0

0

~421

Scheme 33

double bonds is incorporated into the newly constructed ring. Phthalimides also react with electron-rich alkenes. When methanol is used as solvent it is incorporated into the product e.g. [143a,b]. This, together with the effect that the substituent X has upon the ratio of the isomers of [143] is a strong indication that radical ions participate in the reaction as shown in Scheme 34

r

X

0

0

m

e

0 o

,

M

[143a]

Scheme 34

e + x&oMe [143b]

THE CHEMISTRY

OF EXCITED COMPLEXES

107

(Mazzochi and Khachik, 1981). I-Aminoanthraquinone forms oxetanes with conjugated dienes presumably via an excited singlet exciplex (Inoue et al., 1979). Evidence has been presented for the formation of exciplexes between arylketones and norbornadiene (Barwise et af., 1980). INTRAMOLECULAR CYCLOADDITION

Studies have been made of the intramolecular cycloaddition (40)of alkenes to benzene, e.g. [I441 gives [145] and [I461 (Gilbert and Taylor, 1978). The related compound [ 1471 cyclises to give m-cycloaddition products, [ 1481 gives p-cycloaddition and [149] is very unreactive (Gilbert and Taylor, 1979). In

Ph(CHz) .O(CH,).CH=CH, [I471

Ph(CH,),OCH=CH, ~481

PhO(CH,),CH =CH, [I491

many cases the alkene quenches the fluorescence of the aryl group, although exciplex fluorescence has not been detected. Anthracenes linked together by insulating chains at the 9-position form intramolecular dimers (BouasLaurent et al., 1980). The retrocycloaddition reaction (47) in which [I501 gives [I511 and [152] is particularly interesting (Becker and Sandros, 1978).

[1521

Thermolysis of [ 1501 produces chemiluminescence due to emission from [151] and [152]. In the case of [151] the emission is due to an exciplex (Becker et al., 1981). Pyrimidines linked by a polymethylene chain and dihydropyridines similarly linked undergo intramolecular cycloaddition (Koroniak and Golankiewicz, 1978; Potts et af.,1977).

R. S. DAVIDSON

108

A variety of intramolecular hydrogen-abstraction reactions from amino groups by carbonyl groups have been studied. There is no direct evidence that these reactions involve an intramolecular electron-transfer reaction. However, on the basis of studies on intermolecular systems, this is likely to be the case when the carbonyl group is part of an electron-accepting system, e.g. a quinone or phthalimide group. Quinone [153] was found to give [154], [155] and [156] (Scheme 35) (Maruyama et al., 1977; Falci et al., 1977).

.-aN

O 1 3 [ 1531

OH

1

*

c3F

0

a

NH(CH,),CHO

[I541

O h

0

CQD \

P551 ~561

Scheme 35

Phthalimido compounds also photocyclise, e.g. [I571 gives [158] (Coyle et al., 1978; Coyle and Newport, 1980; Machida et al., 1977, 1980b). Azetidines can be prepared by intramolecular photocyclisation of a-amino methyl ketones (Hesabi et al., 1980). Some Mannich bases have been photocyclised to give a

cyclopropane (Abdul-Baki et al., 1978). Hydrogen abstraction from amides by triplet carbonyl groups has been postulated as occurring by a chargetransfer process (Hasegawa et d., 1979). The reactions of [I591 shown in (48) have also been interpreted as involving an intramolecular transfer process (Tada el al., 1978). Cyclisation of chloracetylamides, e.g. [160], has been studied and it has been found that the quantum yield for reaction (49) is highest when fluorescence quenching is maximal. This further substantiates

THE CHEMISTRY OF EXCITED COMPLEXES

109

the claim that such reactions occur by electron transfer (Hamada et al., 1977). The trans conformation of the amide retards cyclisation. Use of N-chloroacetyl-N-methyl amides favours cyclisation by making the amide group adopt a favourable conformation (Hamada et al., 1981).

Q

0 Ph

0

OTHER ADDITION REACTIONS

Irradiation of methanolic solutions of N,N-dimethylanilines containing tetracyanoethylene (TCNE) leads to the formation of biaryls (50), e.g. [I611 (Ohashi et al., 1977a; Ohashi et al., 1979~).Benzo(a)pyrene, the powerful

0 NMe,

+ Me,N + CN

McOH

Me

Me

-

Me@Me \

(so) NMe, [I611

carcinogen reacts with alkanoic acids upon irradiation to give 6-(acy1oxy)benzo(a)pyrenes (Logan, et al., 1981). Whether or not this reaction involves electron transfer has not been established. An extremely interesting reaction of potential synthetic value occurs when a quinone is irradiated in the presence of an alkenoic acid, e.g. [162] (Scheme 36) (Wilson and Musser, 1980). Of particular note is that the reaction with [I631 gives the cyclic product [164]

R. S. DAVIDSON

110

Scheme 36

in 96% yield. Irradiation of flavones, e.g. [165], in the presence of sulphite ions leads to [I661 via the radical anion of [I651 (Yokoe et al., 1979). Dihalogenomaleimides react with indoles to give 2-substituted indoles (Matsuo PhC~C.CH,CH,CO,H ~

3

1

0

I

OH

[I641

so; +

et al., 1976) and with furans and thiophens to give the appropriate 2-substituted product (Wamhoff and Hupe, 1978). In the case of the reaction with indoles the quantum yield of the reaction increases as the solvent polarity is increased, but in highly polar solvents reaction does not .occur. This suggests that chemical reaction occurs from an exciplex rather than radical ions. Irradiation of bromouracil in the presence of 1,4-dimethoxynaphthalene leads to a 2-uracyl-l,4-dimethoxynaphthalene (It0 et al., 1981a). Pyrrolinium

THE CHEMISTRY OF EXCITED COMPLEXES

111

salts, e.g. [l67], react with electron-rich alkenes to give adducts, e.g. [I681 and [169] (Scheme 37). There is good evidence that the reaction involves radical ions (Mariano et al., 1978; Stavinoha and Mariano, 1981). When electrondeficient alkenes are used, [2 + 21-cycloaddition products are formed (Marian0 and Leone-Bay, 1980). H

1

Scheme 37

Allyliminium compounds such as [ 1701 undergo intramolecular cycloaddition reactions (51) in protic solvents. (Stavinoha et al., 1981). The fact that when methanol is used as solvent the products contain a methoxyl group is good evidence for the reactions occurring via radical ions. Irradiation of

[ 1701

aromatic hydrocarbons in the presence of chloracetonitrile leads to cyanomethylation of the aromatic hydrocarbon (Lapin and Kurz, 1981). A study ArH* + C1CH2CN --f [ArH]' + [CICH,CN]' [C1CH2CN]' + C1- + kH2CN [ArH]t + CH,CN -+ ArCH,CN + H+

(52)

of substituent effects supported the view that the reaction occurred via an electron-transfer process as indicated in (52). REDUCTION

9-Cyanophenanthrene is reduced by methylureas to the 9,10-dihydro-9cyanophenanthrene (Tsujimoto et al., 1979a). Since the ureas do not quench

R. S. DAVIDSON

112

the fluorescence of the phenanthrene it was suggested that reaction occurs from the triplet state of the phenanthrene which reacts with the urea to give an exciplex. The ability of aromatic amines to reduce polycyclic aromatic hydrocarbons has been used to probe the mobility of polymers containing pendant anthracenyl groups (Kozel et al., 1978). Aromatic carbonyl compounds are photoreduced by hydrazines (Inbar and Cohen, 1978). Laser flash photolysis studies have shown that the low quantum yield for photoreduction of many carbonyl compounds by aliphatic amines is not due to physical quenching but rather that the photogenerated radicals react to regenerate the starting materials (Inbar et al., 1980, 1981). Laser flash photolysis studies of the intramolecular reactions of 2-dimethylaminoethyl phenyl ketone have shown that the electron-transfer process has an efficiency which is considerably less than unity (Encinas and Scaiano, 1979). In the reaction of trifluoromethyl phenyl ketone with p-cymene there appears to be only about 20 % electron transfer in the transition state (Wagner and Puchalski, 1978). The reduction of thymine and uracil by tryptophan to yield dihydro products appears to involve electron ejection from the tryptophan with the pyrimidines acting as electron scavengers (Reeve and Hopkins, 1979). Some phenazine N-oxides are reduced to phenazines by tertiary amines and it appears that this occurs by an electron-transfer process (Pietra et al., 1978). FRAGMENTATION

Di-( 1-naphthylmethyl)sulphone forms an excimer but does not react to give an intramolecular cycloaddition product like the corresponding ether but rather fragments to give sulphur dioxide and (1-naphthy1)methyl radicals (Amiri and Mellor, 1978). 1-Naphthylacetylchloride has a very low quantum yield of fluorescence and this is possibly due to exciplex formation between the acyl group and the naphthalene nucleus (Tamaki, 1979). Irradiation leads to decarbonylation. It is known that acyl chlorides quench the fluorescence of aromatic hydrocarbons and that this process leads to acylation of the aromatic hydrocarbon (Tamaki, 1978a). The decarboxylation of anhydrides of phenylacetic acids [171] has been interpreted as shown in (53), involving

ArCH,CO,CH,Ar

-

1

ArCH,CO;

+ A k H , + CO

THE CHEMISTRY OF EXCITED COMPLEXES

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electron transfer from the phenyl group to the anhydride (Roof et al., 1976). N,N-Dialkylaminoethyl benzoates undergo the Norrish Type I1 reaction from their triplet states and these reactions may involve electron transfer (Coyle and Kingston, 1976). Phenacyl sulphonium and phosphonium salts are reduced on irradiation in the presence of many electron donors, e.g. dyes and tris(2,2'-bipyridyl)ruthenium(Il) chloride (Hedstrand er al., 1978). The irradiation of many arylacetic acids in the presence of heterocyclic compounds leads to decarboxylation. Porphyrinylacetic acids undergo self-sensitised decarboxylation (Callot, 1979). 1,1,2,2-Tetraphenylcyclopropaneundergoes ring opening on irradiation in the presence of electron acceptors such as 1-cyanonaphthalene and tetracyanoethylene (Arnold and Humphreys, 1979). cis-Arylcyclopropylcarboxylates,e.g. [ 1721,also fragment on irradiation (54) (van Noort and Cerfontain, 1979); the trans-isomer is stable.

ArPco OMe Phenylcyclopropanes undergo ring open upon irradiation in methanol solution containing copper (1 1) ions to give 1,3-dimethoxy-l-phenyl propane. (Mizuno et al., 1981). Electron transfer from the excited cyclopropane to the copper ions was proposed as being the initial electron-transfer reaction. Triplet carbonyl compounds sensitise the fragmentation of hydroperoxides and this has been suggested to occur via a triplet exciplex (Ng and Guillet, 1978). The reaction could play an important part in the photoinduced degradation of polyalkenes. References

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