Interaction of nonconjugated chromophores in rigid model compounds

JOURNAL OF LUMINESCENCE 1,2 (1970) 489-501 © North-Holland Publishing Co., Amsterdam

INTERACTION OF NONCONJUGATED CHROMOPHORES IN RIGID MODEL COMPOUNDS NICOLAE FILIPESCU Department of Chemistry, The George Washington University Washington, D.C. 20006, USA

Spectroscopic techniques and photochemistry were used to evaluate the interaction be. tween an electronically excited chromophore and a different one in its ground state in rigid model compounds. These inflexible compounds are better suited for transfer work than binary mixed solutions of donor and acceptor since both the separation distance and the mutual orientation of the two chromophores are known, and complicating factors such as intermolecular transfer and complex formation are excluded by work at high dilution. Phenanthrene (P) and p-dimethoxybenzene (D) were rigidly connected to an inert spirocyclopropane-norbornane a-frame. The S 1 —~ So and Ti —~ So emissions of the two chromophores are found at different wavelengths and therefore can be detected separately. The emission and excitation spectra of the model compound were compared to those of an equivalent one-to-one mixture of each chromophore rigidly attached to the same inert frame. The dilute mixture shows the fluorescence and phosphorescence of each of the two components. In the model compound, however, the D fluorescence is definitely absent. The mechanism is consistent with efficient singlet energy transfer from the selectively excited D to P combined with partial back transfer from the Tz state of P to the lowest triplet of D.

The interaction between an electronically excited molecule (or chromophore) and another molecule in its ground state is a basic phenomenon in photochemistry, radiochemistry, and molecular spectroscopy. Evaluation of this interaction is done primarily by observation of three kinds of consequences: (1) changes in the additivity of separate electronic absorptions. (2) photochemical reactions in which bonds between the two chromophores may be formed (e.g. cycloaddition) or broken (e.g. H-abstraction), and (3) energy transfer from the excited to the unexcited chromophore. All these effects are critically dependent on the separation distance between the two molecules and their mutual orientation. One can obtain only fragmentary information about the interaction mechanism from experiments with binary mixed solutions because, for these systems, one can use only an arbitrary “average” separation and a random orientation. The relative spatial disposition of two chemically different chromophores is better defined in model compounds in which the two groups are connected to the same molecular frame. Furthermore, experiments with such model 489

F.6

490

N (‘OLAF Fill PES(iJ

compounds can be carried Out in dilute solutions, avoiding intermolecular proximity complications SLich as complexation and coprecipitation. In many model compounds purposely synthesized for intramolecular energy transfer studies’’ 2) the donor (D) and acceptor (A) chrornophores were joined by a saturated hydrocarbon chain of one or more methylene Linits, D-(CH

2)~-A.The

main advantage of these compounds over binary

solutions is that one can specify a maximum D-A separation distance. On the other hand, the actual separation and the mutual orientation of D and A are essentially random because of free rotation about the a-bonds of the alkane chain. In all these compounds, the transfer of electronic excitation energy was virtually complete at either singlet or triplet level and no significant differences were found on increasing the number of methylene spacers from one to three’). The relative position of the two chromophores is better known in model compounds in which the interconnecting bridge isoccurring an inflexible polycyclic 3) and naturally fused-ring netstructure. Thus. steroidal frames works4) have proved valuable in restricting not only the maximum separation between the attached donor and acceptor, but their minimum approach as well. Although the frame itself was rigid in these compounds, the exact relative orientation of the chromophore pair was not defined since rotation aboLit the chromophore-frame a bond allowed a multitude of conformers and some variation in the D-A distance. For accurately known interchromophore separation and orientation, the model compounds must have the two chromophore rigid/i’ attached to the in//evihie molecular network.

Synthetic ability is not the only criterion for the preparation of such model compoLinds. One must also consider the compatibility of the energy

levels of the two chromophores (relative energy, overlap, possibilities of selective excitation) and their ernittive properties in the model compound. which may be qLiite different from those of the “disconnected” chromophores. It is always possible, after going throLigh a tedious synthesis seqLience. to discover that the model compound is non-emitting. This immediately preclLides the evaluation of energy by spectrofiLiorometric means. On the other hand, it may happen that the energy transfer from donor to acceptor is practically complete. In such cases, the only conclusion is that the relative disposition of the two chromophores in the model compound allows strong interaction. This may happen frequently at small separation distances. Probably the most interesting rigid model-compounds are those in which the two nonconjugated chromophores, though in close proximity. exhibit

only partial or no interaction. Spectroscopic and photochemical studies of such systems are expected to help in the understanding not only of the energy

F.6

491

iNTERACTION OF NONCONJUGATEI2 CI-IROMOPIIORES

transfer mechanism but also of the detailed properties of excited organic chromophores. The first energy transfer results on model compounds with distinct donoracceptor relative orientation were reported by Keller last year5). He found total transfer between the anthrone and naphthalene chromophores of spirocompounds i and ii. The S 1 and T1 states ofanthrone are situated between the same respective states of naphthalene; therefore, anthrone is a suitable singlet-

acceptor and triplet-donor in bothtand it. Since the two chromophores are very closely spaced, exchange interaction is probably responsible for the total transfer inland ii. Other conclusions about the separation-orientation dependence of transferare difficult to extract because ofsome flexibility in the frame, lack of planarity of the anthrone moiety, and identical results for both iand it. At this laboratory, we have been able to synthesize several model compounds which are totally inflexible. En compound itt, the planar p-dimethoxybenzene (D) and fluorene (F) it-systems are connected by a saturated 6). norbornane-spirocyclopropane network in a perpendicular orientation

OMe

OMe III

tv

V

The S 1 and T1 energy levels of D are above the lowest singlet and triplet of F respectively.

The original interpretation of the spectrophotofluorometric data was that itt in rigid matrix at

there is no apparent interaction between D and F in

492

NICOLAE FILIPESCLJ

77 K. This inference was based mainly on the detection of two emissions centered at 415 and 440 nm. Since these emissions had different excitation spectra and since their ~ coincided with the phosphorescences of isolated

iv and V respectively, the conclusion of non-interaction seemed appropriate. However. Lamola reexamined the emission from iii and pointed out that the emission centered at 4l5 nn’i is actually short-lived fluorescence7). He attributed it to another species, possibly a photoproduct, and correctly reinterpreted the spectrofluorometric results on iii as consistent with efficient

D to F at the singlet level. We have actually identified this other species to be dibenzofulvene vi and established that it is formed energy transfer from

photochernically in trace amounts on prolonged ultraviolet excitation of iii

in rigid matrix. The dibenzofulvene chromophore of the rearranged photoproduct v~exhibits intense fluorescence at 415 nm.

4~:b1I OMe Vt

It should be mentioned that the spirocyclopropane-fluorene group in iii and v is a rather inefficient emiltor despite the high luminescence (~ > 0.5) of the parent fluorene molecule7). Extensive quenching of F luminescence by the 9-spirocyclopropane frame and potential formation of a highly fluorescent photoproduct diminish the utility of the spiro-linked F as an emitting acceptor. On the other hand, the photorearrangement of the Spirocyclopropane-fluorene chromophore to a dibenzofulvene was successfully used for photochemical evaluation of the energy transfer process in fluid solutions (see below). We have also synthesized rigid niodel-compouncl vii and its separatedchromophore “fragments’’ viii and iv. Structurally. vii resembles iii. the only difference being

o Mc

vii

viii

F.6

493

INTERACTION OF NONCONJUGATED CHROMOPHORES

that the F chromophore has been replaced by 4, 5-linked phenanthrene (P).

Unlike F, P retains its efficient emission from both S1 and T1 levels after being attached to the frame in vii and viii. Another major advantage of the D-P pair is that the characteristically structured fluorescence and phosphorescence of P are found at different wavelengths than the corresponding broad-band emissions of D. This enables convenient spectroscopic detection. At the same time, the energy levels of D and P are appropriate for both S-S and T-T transfer (see figs. 1 and 2). 70 60

200

I

I

I

I

-

P.PH E NANTHREN E D.p-OIMETHOXYBENZENE -

//7//~JIA~ FL.-D

/

FL-P

300

PHOS.-D

400 WAVELENGTH,m~

500

600

Fig. 1. Fluorescence (FL), phosphorescence (PHOS), and excitation (EXCIT) spectra of P and D attached to the frame (compounds viii and Iv, respectively).

cm’~xio~ 35

c~D I

SP

>-

~

~rI2P

Fig. 2. Energy level diagram of phenanthrene (P) and p-dimethoxybenzene (D) chromophores in compound vii. Arrows show the paths of energy migration on excitation of the D chromophore. The same emission is obtained on selective excitation of Sop.

494

NiCOLAE FILiPESCU

Comparison of emission and excitation spectra of vii with those of an equimolar dilute mixtLire of vui and iv showed an interesting case of total intramolecular energy transfer at the singlet level, partial back transfer from the second excited triplet and lack of interaction between the lowest triplets of two perpendicular chromophores less than 7 A apart. Excitation of vii either at ~ of D absorption or at wavelengths absorbed exclusively by P resulted in emission from S~,T~,and T?. However, no D fluorescence could be detected from vii regardless of the excitation frequency. In contrast. the S~ —o 5~ and T 1 —o S~, emissions of both chromophores were easily recorded from the equivalent dilute mixture of viii and iv. The only interpretation consistent with these observations is that S~is strongly coupled to S~resulting in total D-to-P transfer followed by partial return at the triplet level from T~to T~in competition with T~—- - T~internal conversion. The conclusion is confirmed by the relative intensity of the three emissions detected from compound vii. Actually, this result constitutes the first spectroscopic evidence for substantial transfer of T, energy. In addition, it substantiates the concept~’10) that, whenever the second triplet is below the 5, level, as in P. intersysteni crossing from 5, to T, takes place via the T2 state. It is intriguing that we do not observe total back transfer of triplet energy from T~to the lower-lying T~’despite the small separation distance (< 7 A). The explanation is probably found in the perpendicularity of the two irsystems; if the lowest triplet of one chromophore is symmetric with respect to the molecular plane of symmetry and the other triplet is antisymmetric. the exchange integral will vanish. The absence of transfer from T? to T~ illustrates the critical dependence of close-range interactions on the relative orientation of the two chromophores. Triplet transfer from tetralin- 1 .4-dione to the .s’pirocyclopropane-lin ked 2) fluorene investigated in model was compound ix. both spectroscopicallyi i) and photochemicallyi

As for the previous rigid model compounds

iii

and vu, the tiv absorption

F.6

INTERACTION OF NONCONJUGATED CHROMOPHORES

495

spectrum of ix resembled that of an equivalent mixture of “separated” chromophores V and x. Since the lowest singlet and triplet states of the diketone are energetically located between those of F, selective n —o excitation of the diketone is possible. Despite substantial spectral overlap, the phosphorescence emission of the two chromophores can be differentiated because that of F is almost unstructured whereas that of the tetralindione

shows characteristic carbonyl vibrational progression. The phosphorescence spectrum of ix in rigid matrix was similar to that of the isolated acceptor v except for a minor contribution from the diketone ir’°—o n phosphorescence. This result indicates efficient (~ 80 %) but not total transfer of energy from the n, ir’°diketone triplet to the lower-lying it, it’0 triplet of F. It also shows that, for the orientation and separation imposed by the rigidity of model compound ix, the transfer process is competitive with the natural phosphorescence of aromatic carbonyl compounds. Although most work on energy transfer was carried out with luminescent acceptors, one can also use acceptor chromophores which undergo a definite photochemical reaction. Thus, the cis trans isomerization of an olefinic acceptor was accomplished by intramolecular sensitization in model compounds in which the olefinic moiety was separated from the absorbing aromatic donor by methylene-type spacers2). We have been able to use the photorearrangement of the spirocyclopropanefluorene to dibenzofulvene for quantitative detection of intramolecular energy transfer in rigid model compounds°2’13) Thus, selective n, ir’0 excitation of the dione chromophore of ix in fluid solution led to the formation of xi by rearrangement of the F chromophore°2).The photo~—*

3000

A

~

chemical quantum yield of this reaction was comparable to that obtained on direct it —o ir’~ excitation of F. The photorearrangement of the isolated acceptor, the v xii reaction, was studied separately under direct and intermolecularly sensitized excitation. Quantum yield values obtained for these reactions were used in the quantitative evaluation of the transfer efficiency in the ix —o xi transformation. The photochemical results confirm -+

496

F6

NiCOLAE FILiPESCU

s~z~

that efficient — but not total — triplet transfer from the ketone chromophore to F takes place and thLis agree with the spectrophotometric measurements

on ix. The v —o xii photoisomerization was also employed to demonstrate I 3) intramolecular energy transfer from the T 2 state of cyclopentenone to F in compound xiii. The forniation of dibenzofulvene xiv on selective n —o excitation of the ketone was followed spectroscopically. The energy levels

i.

3430 A

XIII

Xlv

of the two chromophores in xiii are such that the only acceptable interpretation for the occurrence of the F isomerization is based on energy transfer from the second triplet of cyclopentenone to the T, of F (see fig. 3). This result is particularly interesting in view of a recent report that at least kcai/moie 00 -

_____

I~

-

S~I~

90 S’~~’

~:: ~

~

F Fig. 3. Energy level diagram for the cyclopentenone (C) and spirocyclopropanefluorcne (F) chromophores in compound xiii. Arrows show path ofenergy deactivation on excitation of either chroniophore. R represents rearrangement of F to dibenzofulvene,

F.6

INTERACTION OF NONCONJUGATED CHROMOPHORES

497

one major photoreaction of cyclopentenone, cycloaddition, originates at the T2 level, whereas its lowest triplet, T1, is ineffectiv&4). Both the spectroscopic investigation of model compound vii and the photochemical results obtained with xiii strongly suggest substantial triplet sensitization originating at the T 2 state of the donor. It is conceivable that for compounds in which the T2 .-‘-oT~internal conversion is made improbable by structural (or symmetry) considerations, the lifetime of the T2 state may

be long enough to be responsible for some reported transfer 15 16)“non-vertical” A nonvertical intercases or departures from “ideal” curves pretation of triplet transfer in the Saltiel above-mentioned model compounds vii and xiii is ruled out by the rigidity of the molecules, by the rather large endothermicity of such a process, and by intermolecular sensitization experiments13). Another inflexible model compound which exhibited interesting spectroscopic and photochemical behavior was the Diels-Alder adduct of 1,4-

with cyclopentadiene, xv. As usual, the 5, and T, states of the diketone are energetically between those of the norbornylene double naphthoquinone

>3000

xv

A

Xvi

bond (N). Consequently, selective excitation of the dicarbonyl chromophore is easily accomplished and N is an appropriate triplet acceptor’7). For these reasons, it was surprising to find out that the phosphorescence quantum yield of xv was about the same as that of its saturated analog x. The fact that the olefinic N chromophore, less than 4 A away, does not quench the T 1 —+ S~emission of the dicarbonyl chromophore is quite unexpected and

at odds with the “sphere of action” concept put forward for the interpretation of triplet transfer 18). measurements in vii, frozen solutions ofa mixed Again, as for molecule xv has plane donor and acceptor molecules of symmetry. It seems that the exchange integral could vanish only if the wave functions in the product are symmetric-antisymmetric or if there is no overlap between the lowest-triplet of the donor and the ground state of the acceptor. In either case, it is obvious that the relative orientation of two closely-spaced chromophores is highly critical for their interaction.

Another important conclusion derived from experiments with molecule xv

498

F,6

NICOLAE FiLl PESCU

is that excited triplets of the same species can have quite different properties in fluid solutions than in rigid matrix. Thus, intra- or intermolecular triplet sensitization of xv in liquid solution leads to formation of cage-product xvi. Actually, the quantum yield for the reaction under direct excitation of the diketone group is unity. We have been able to show that this photorearrangement takes place via triplet transfer from the diketone to N followed by or concurrent with intramolecular cycloaddition. Finally, we have prepared rigid model compounds in which the two chromophores are forced by the interconnecting network in a very close proximity’9). Thus, in compounds xvii and xviii, the mutually perpendicular naphthalene and phenanthrene (fluorene) it-systems show extensive trans-

xvii

Xvut

annular overlap. This is illustrated in their uv absorption which is substantially different from that of an equimolar acenaphthene-4,5-methylene-

phenanthrene

(or acenaphthene-9-spirocyclopropane-fl uorene)

mixture.

Molecular orbital calculations are presently carried out in an attempt to correlate theoretical predictions with observed absorption and emission spectroscopic properties of these last two compounds. Most of the work reported in the present paper was sLipported by the Atomic Energy Commission on Contract AT(40-l )-3797 and by the National Aeronautics and Space Administration under Grant NGR-09-OlO-008. Their assistance is gratefully acknowledged.

References I) 0. Schnepp and M. Levy, i. Am. Chem. Soc. 84(1962) 172; A. A. Lamola, P. A. Leermakers, U. W. Byers and U. S. Hammond, ibid. 85(1963) 2670; D. E. Breen and R. A. Keller, ibid. 90(1968) 1940. 2) H. Morrison, ibid. 87(1965) 932; H. Morrison and R. Peiffer, ibid. 90 (1968) 3428; H. Krislinsson and U. S. Hammond, ibid. 89 (1967) 5968; P. A. Leermakers, J. Montiller and R. D. Rauh, Mol. Photochern. 1(1969) 57.

F.6

INTERACTION OF NONCONJUGATED CHROMOPHORES

499

3) 5. A. Latt, H. T. Cheung and E. R. Blout, J. Am. Chem. Soc. 87 (1965) 995: R. A. Keller and L. J. Dolby, ibid. 89 (1967) 2768; ibid. 91 (1969) 1293. 4) R. D. Rauh, T. R. Evans and P. A. Leermakers, ibid. 90 (1968) 6897. 5) R. A. Keller, ibid. 90 (1968) 1940. 6) J. R. DeMember and N. Filipescu, ibid. 90 (1968) 6425; N. Filipescu, Molecular Luminescence Ed. E. C. Lim (W. A. Benjamin, Inc., New

York, 1969) p. 697. 7) A. A. Lamola, J. Am. Chem. Soc. 91(1969) 4786. 8) N. Filipescu, J. R. DeMember and G. R. Howard, J. Chim. Phys. (in press), (presented at the symposium on Radiationless Transitions in Molecules, Paris, France, May, 1969). 9) W. Siebrand, The Triplet State, Ed. A. B. Zahlan (Cambridge University Press, Oxford, 1967) p. 31. 10) M. Bixon and J. Jortner, J. Chem. Phys. 48 (1968) 715. ii) N. Filipescu, J. R. DeMember and F. L. Minn, J. Am. Chem. Soc. 91 (1969) 4169. 12) J. M. Menter and N. Filipescu, J. Chem. Soc. (London) (1969) B 616. J. M. Menter, Ph.D. Thesis, George Washington University, 1969. 13) N. Filipescu and J. R. Bunting, submitted for publication; J. R. Bunting, Ph.D. Thesis, George Washington University, 1969. 14) P. de Mayo, J. P. Pete and M. Tchir, J. Am. Chem. Soc. 89 (1967) 5712. 15) See for example: N. J. Turro, Molecular Photochemistry (W. A. Benjamin, Inc., New York, 1967) pp. 181—1 83. 16) For intermolecular T, sensitization, see also: R. S. H. Liu and R. F. Keliog, J. Am. Chem. Soc. 91(1969) 250; R. S. H. Liu and J. R. Edman, ibid. 91 (1969) 1492. 17) N. Filipescu and J. M. Menter, J. Chem. Soc. (London), (1969) B 6t6. 18) A. N. Terenin and V. L. Erniolaev, Trans. Faraday Soc. 52 (1956) 1042; V. 1.. Ermolaev, Soviet Phys.-Usp. 80 (1963) 333. (English translation); S. Siegel and H. Judeikis, J. Chem. Phys. 41 (1964) 648. 19) N. Filipescu, F. L. Minn and A. Thomas, in preparation.

Discussion on paper F6 Question I: K. B. Eisenthal

You state that the transfer from phenanthrene to p-methoxy benzene arises from an exchange transfer from T2 of phenanthrene, which is at a slightly lower energy than S~of phenanthrene, to T1 of p-methoxy benzene. Since one would expect that the internal conversion rate T2 to T1 in phenanthrene for their given energy separation, would be of the order of 10h1_1012 sec 1 is it not possible that transfer occurs from S~of phenanthrene (T ~ 10_8 see) to T~of p-methoxy benzene by the admixture of triplet state into S~and exchange transfer to T1 of p-methoxy benzene? It seems that your data cannot distinguish between these two possibilities. Ansii’er: N. Filipescu

In principle, your comment is correct. Our results do not rule out thepossibility of singlet-to-triplet transfer from S~to T?. However, such an

500

F.6

NiCOLAE FILiPESCU

alternative would be highly unusual and, to my knowledge, has never been reported or postulated for either inter or intramolecular transfer experiments.

The figure you mention for the lifetime of the T2 state of phenanthrene is an order-of-magnitude estimate. For anthracene, Liu and Kellogg found the lifetime of the T2 date to be about 10b0 sec. It seems quite possible thal the lifetime ratio of the S~and T~states is only l0 L.102 In this case the T~— to —T~transfer should dominate that of S~’— to T? caused by the admixed triplet character of S~’.Further experiments may establish whether there is actual transfer from S~directly to T~. Question 2: R. A. Keller A. The possibility of SI (acceptor) to T0 (donor) transfer should not he ruled out as an alternative to the proposed T, (acceptors) to T0 (donor) transt’er. It is well known that S~ —o T11 energy transfer is a very efficient process in many molecules (intersystem crossing) and, after all, the compound studied is a single molecule. B. We have studied energy transfer in the molecule shown below.

Cl-I2

The energy levels for this molecule can be represented as shown. S,(A) ___________S(D) T0(D) T0( A) __________________________________SO

We observed that T0(A) did not quench the fluorescence of S1(D). This evidence is in support of Dr. Filipescu’s contention that Si to T,, transfer

is not important between two different chromophors. Ans,,er: N. Filipescu The first comment was answered in the reply to Dr. Eisenthal. The answer to the third comment is as follows: The reason for the lack of T? to T~ transfer may well be found in the symmetry properties of the four wave functions in the exchange integral. Since the molecule has a plane of symmetry which coincides with the

F.6

INTERACTION OF NONCONJUGATED CHROMOPHORES

501

phenanthrene plane, i~I1)and ~ are symmetric with respect to this plane. The lowest triplets of phenanthrene and benzene are anti-symmetric in their own plane and therefore anti-symmetric and symmetric respectively with 1A

respect to the molecular plane of symmetry of the model molecule. The exchange integral may simply vanish because of orientation. We found absence of triplet transfer between proper donor and acceptor chromophores at 3 A apart in another model compound with similar symmetry properties:

(see text). Question 3.’ A. Wachtel

Have you observed a similar interaction through absorption by an organic (fluorescent) molecule to which an inorganic ion such as a rare earth is

attached, either by covalent bond, or in the form of a salt? Answer.’ N. Filipescu

We have used extensively rare earth ions as fluorescent acceptors of triplet excitation energy for both intra and intermolecular energy transfer. Many chelates with /3-diketones, heterocyclic bidentate ligands (dipyridyl, I ,lO-

phenanthroline, etc.) or salicylaldehyde have been investigated in detail by several groups, and they fall in the category you describe. If your question refers specifically to model compounds in which the organic donor and the lanthamide-ion-acceptor are connected separately to the same rigid molecular frame, the answer is negative. We have attempted to synthesize such

compounds but have not been successful. Nonrigid (free-rotating) model compounds of this type are prepared relatively easy, but we are not very

interested in these.