Multidimensional photochemistry in flexible dye systems

.I. Photochern.

Photobiol.

A:

Chem.,

Multidimensional

62 (1992)

415427

41s

photochemistry in flexible dye systems

Wolfgang Rettig and Wilfried Majenz Iwan-N. -Stranski Institute, Technische Unlversitiit Berlin, Strasse des 17. Juni 112, W-1000 Berlin I2

(FRG)

Rem5 Lapouyade Laboratoire Bordeaux

de Photophysique 4 351, Cows

et Photochimie

de la Lib&ration,

Mok!culaire,

F-33405

Talence

WA du CNRS

348, Universite’ de

(France)

Giin ter Haucke Fachbereich

Chemie,

Fn-edrich

SchiIler

Universitiil;

Am

Steiger 3, O-6900

Jena

(FRG)

Abstract of many flexible dye systems can be understood on the basis of adiabatic photoreactions to emissive or nonemissive product states. Nonemissive product states lead to fluorescence quenching and are often the reason for what is conventionally called “internal conversion”. For compounds with several flexible bonds, competing photoreaction pathways can exist. This is shown for the example of benzopyrylium dyes and for donor-

The photophysics

acceptor-stilbenes. Chemical bridging of bonds and/or substituent changes can control the competition between the various channels. This opens a new view on the multidimensional nature of the excited-state hypersurfaces and gives access to control dye properties by molecular engineering.

1. Introduction

and scope

Many well-known dyes and several phutobiological systems undergo photochemical transformations after photoexcitation which lead to an excited state with mutually perpendicular 7~ systems and complete charge separation. Examples for the importance of these “twisted intramolecular charge transfer” (TICIJ states [l-5] can be found in most classes of laser dyes, in triphenylmethane dyes, in the visual process and in photosynthesis. Rhodamine laser dyes, for instance, show a non-radiative loss process (usually termed “internal conversion”) which can be shown [6-101 to be linked (a) to twisting of the dialkylamino groups, (b) to the donor character (ionization potential) of the amino group, (c) to the acceptor character of the xanthene moiety (including the effect of the carboxyphenyl substituent), (d) to pretwisting effects of the dialkylamino groups, (e) to the medium polarity, (f) to specific salvation by prutic solvents and (g) to a possible pyramidalization component of the amino groups. Points (a)-(e) can be understood in terms of the TICT model (internal conversion caused by a photochemical transition towards a non-radiative TICT state); points (f) and (g) indicate that other factors are also important. “Internal conversion” is even faster in the series of triphenylmethane (TPM) dyes: these show, under low viscosity conditions, ultrafast and activationless decay to the

0 1992 - Elsevier Sequoia. AU rights reserved

416

ground state. However, similarly as for the rhodamines, the decay rate is governed by the donor-acceptor properties of the molecular subgroups and involves a twisting motion [ll, 121. The TICT model can be successfully used to predict new dyes with desired properties (e.g. faster-switching dyes [13]). TPM dyes are a special class of cyanine dyes. Therefore also the photophysics of cyanine dyes should be understandable within the TICT model. The classical model of Ruli&re [14] for cyanine isomerization, for example, involves twisted species strongly coupled to the ground state. Theoretical calculations [15] can even give a guideline as to which of several possible bonds is the most active one in photo-isomerization. Coumarine laser dyes also show non-radiative losses due to the formation possibility of non-radiative TICT states under certain conditions [16-181. In Section 2 another class of potential laser dyes, substituted benzopyrylium dyes, are presented which show similar losses due to TICT formation, in some cases even through several parallel channels. Not in all cases does TICT formation lead to fluorescence losses; there is even an important class of laser dyes, the donor-acceptor-stilbene derivatives such as the well-known laser dye DCM, where TICT formation is the source of good fluorescence properties [19]. This can occur because these dyes possess a non-radiative decay channel, that of double-bond twisting, the importance of which is diminished by the competing formation of a fluorescent TICT state. Such multidimensional photochemical processes will be treated in Section 3. Finally, in Section 4 the current theory of biradicaloid charge transfer (BCT) states being able to cope with these multidimensional processes will be briefly reviewed and exemplified.

2. Competing

non-radiative

photocbemical

channels

in benzopyrylium

dyes

Benzopyrylium dyes with several flexible substituents [20] are interesting as laser dyes but show vastly differing fluorescence quantum yields I$~ depending on both substituent pattern and solvent polarity. Their non-radiative decay processes are linked to a twisting motion of the substituent. groups and are describable within the TICT model [21]. Figure 1 shows as an example fluorescence and fluorescence excitation spectra for a pair of benzopyrylium dyes EM and (B-M),, with a flexible and with a rigidized anisole substitutent respectively_ The spectral properties hardly differ. Table 1, however, reveals that there exists an overwhelming non-radiative decay channel in polar solvents for the flexible dye which is absent for the rigidized compound. Figure 2 shows fluorescence decay traces for the flexible dye EM measured with BESSY synchrotron radiation for different temperatures. It is clearly seen that by decreasing temperature or increasing viscosity, this decay channel can be slowed down. Taking both experiments together establishes that the intramolecular fluorescencequenching channel is linked with a large-amplitude twisting motion of the substituent. A-B-M

Interestingly, addition of a second flexible substituent at the other end of the leads to reduced non-radiative losses. This is at first sight molecule (system A-B-M)

417

I

1 600

I

500

400

nQEz

Fig. 1. Fluorescence and fluorescence excitation spectra of B-M in dichloromethane (-) and acetonitrile (---) [ZO]. TABLE

and (B-M),,

(lower)

1

Fluorescence

quantum yieldsa of EM

B-M (B-W,, * ZJZ 20%

(upper)

absolute,

f 10%

and (B-M),

in solvents of different polarity [20]

Dichloromethane

Acetonitrile

0.63 1.03

0.004 1.04

relative.

astonishing if the conventional explanation for non-radiative decay by the loose bolt theory [22-241 is taken, because additional degrees of freedom should lead to increased non-radiative losses. This “anti-loose-bolt effect” is, however, readily explained within the TICT model. Table 2 collects the necessary data of k,, for a number of model compounds, derived from eqn. (1) using room (RT) and low temperature (77 K) fluorescence decay times [21]. Figure 3 subdivides the molecular structure into donor and acceptor parts which will be used within the TICT model. k,,, = rf-’ (RT) - 7r-‘(77

K)

(1)

The A-EM system possesses two different TICT channels: the M channel ieading to a twisted anisole group and the A channel involving twisting of the diethylamino group. For the M channel the donor is the same as for system B-M but the acceptor has changed from B to A-B, i.e. to a system with weakened acceptor character due to the presence of the in-plane amino group with donor properties_ This raises the energy of the corresponding TICI’ state, and the rate constant of TICT formation (intramolecular fluorescence quenching, k,,,) will consequently drop- The A channel,

418

13(snY------

y__-,

t/ns

,

--.;_.

_r

.;

.~.y&

,

,

1

,

I 5

la

.

10

Fig. 2. Fluorescence decay curves for EM in n-butyronitrile as a function of temperature. The lifetimes strongly shorten with increasing temperature, characteristic of a temperature- or viscositycontrolled fluorescence-quenching reaction. Iterative reconvolution fits with a mono- to biexponential model (-) yielded mean lifetimes of 3.07 ns (- 198 “C), 0.45 ns (-79 “C) and 0.14 ns (-32 “C). The instantaneous response function of BESSY synchrotron radiation (FWHM about 600 ps) is also shown.

TABLE

2

Fluorescence quantum yields, lifetimes and non-radiative decay at room temperature for various benzopyrylium dyes 1211 Compound

6

TF

B-M P--W,r A-B-M A-P-M)b,

0.007 1.00 0.03 0.07

O-035 4.1 0.52 1.19

ens)

rate constants

in butyronitrile

k,, (10’ s-l) 2800 0 lS7 78

on the other hand, involves an acceptor system B-M. By suitable comparison of the data in Table 2, the separate contributions of the M and A channels to the total non-radiative decay rate can be quantified. By taking the difference between systems

A-(B-M),,,

(A channel only, k,,,= 78X 10’ s-l) and A-EM (A and M channels, total the M channel alone can be determined to contribute 109X lo7 s’rl , ,i.e. about half. A-B-M with both decay channels acting in parallel can thus be viewed as a multidimensional photochemical system (Fig. 3). Of course, the A channel can also be modulated by varying the substituents on M (D& For example, the system lacking the methoxy substituent on M is predicted to possess a faster A channel, in accordance with experiment [ZO]. The TICT model can therefore serve as valuable guideline in planning the synthesis of laser and fluorescence dyes with improved properties_

k = 187 X 10’ s-l>,

419

A channel

M channel

/J

OCH]

Fig. 3. Schematic drawing of an acceptor system with two different donor substituents, each of which can be active photochemically to lead to TICT states. The specific example of A-B-M is also shown.

3. Multiple

fluorescing states in donor-acceptor

Another family of compounds related to donor-acceptor-substituted used laser dye [193.

and related dyes

with multidimensional photochemistry are dyes stilbenes, such as DCS or DCM, a commonly C%

NC

,CH3

/“3

‘CH]

DCS W

stilbenes

Q1: @c”I= xl H

N\ CN

CH3

DCM

,CH3

N

‘CH3

DNS DCS possesses a double and three flexible single bonds, and twisting about the double bond is commonly believed to constitute the main non-radiative decay channel (via the non-luminescent product state P*). Twisting about the different single bonds can lead to TICT states A*, B*, C*, . .. which are highly luminescent in some cases. If one or more of these TICT states are sufficiently low lying, they can act as photochemical traps reducing the intramolecular fluorescence quenching via P’ and leading to dyes with outstanding fluorescence properties.

420

Quantum chemical model calculations (Table 3) indeed show that two of these TICI’ states, the ones involving twisting of ‘the single bonds adjacent to the ethylene moiety (A* and C*), can be suficiently low lying, especially if solvation and the large dipole moments are taken into account. The large dipole moment change between ground and excited state of 23 Debye units determined in solution [25] indeed favours such an interpretation because it is larger than expected for the planar conformation alone. Note that the calculation shows a very small energy gap between S1 and SO for the twisted double-bond conformation P*. This is consistent with the experimentally observed fluorescence quenching through this channel (high probability for interna conversion)_ The calculation also indicates a very small dipole moment for P*. This can be understood within the model of biradicaloid states (see Section 4) and is experimentally supported by the observed solvent polarity dependence of intramolecular fluorescence quenching [ 19]. Table 4 shows calculated results for the related dimethylamino-nitro-stilbene (DNS). The results are similar. The TICT states reachable by twisting the phenyl groups are predicted to be most active in photochemistry; twisting of the dimethylamino group is less favourable. However, in contrast to DCS, DNS possesses a further TICT channel, namely twisting of the nitro group. Interestingly, this TICT state is predicted to be the lowest one. It should, however, be borne in mind that these calculations can only serve as a rough guideline to understanding the photophysics and photochemistry of these dyes, because they neglect the coupling of the TICT with the allowed states. As will be shown below, the emission from the TICT state in the case of DCS is highly allowed and its experimental dipole moment is similar to that of the delocalized excited state E* measured in a planar model compound 1261. This is in contrast to the results in Table 3 and points to the strong importance of vibronic coupling, which is mainly induced by the width of the intramolecular rotational distribution function around the perpendicular (forbidden} geometry [27]. Thus, for a proper assessment of the relative importance of the TICX channels in DCS and DNS, TICI’ energies and dipole moments relative to those of the E* state have to be combined with some knowledge about the rotational distribution function within the TICT state. An alternative approach is to use experiment and to investigate selectively bridged model compounds. The corresponding studies on DCS [26, 281 will be outlined below. Regarding DNS, the fluorescence quantum yield measurements reveal a behaviour

TABLE

3

Energy gaps (electronvolts) between TICF and ground state derived by the CNDO/S method (modified programme QCPE #333) for different twisted conformations of DCS compared to the energy of the allowed state E* (planar conformation). Dipole moments {Debye units) are also given. The calculation is for the gas phase. In polar solvents the energy lowering of the TIC’F and other states is proportional to the square of the dipole moment EX (planar)

A* (twisted anilino group)

AE p (excited state) p (ground state)

3.82 18.8 6.8

4.76 38.3

B* (twisted dimethylamino group) 5.30 37.3

C* (twisted benzoni Wile group) 4.66 29.9 -

P* (twisted double bond) 0.14 3.3 -

421 TABLE

4

Energy gaps (electronvolts) between TICT and ground state derived by the CNDO/S method for different twisted conformations of DNS compared to the energy of the allowed state E* (planar conformation). Dipole moments (Debye units) are also given. The calculation is for the gas phase. In polar solvents the energy lowering of the TICT and other states is proportional to the square of the dipole moment. Similar to DCS, the energy difference AE of the hvisted double-bond species P* is close to zero and the dipole moment is considerably smaller than that of the planar conformation E* (planar)

A* (twisted anilino group)

B* (hvisted dimethylamino group)

C* (twisted nitrobenzene

D* (twisted nitro group)

group) AE p (excited state) p (ground state)

3.5 37 10

4.4 47 -

5.2 45

3.9 48

3.3 45 -

similar to DCS for low polarity solvents, namely reduced non-radiative decay losses for increasing solvent polarity. However, at higher polarity the fluorescence quantum yields for DNS decrease again, unlike those of DCS [19, 29, 251. This could be the result of the nitro group TICT channel as previously suggested [29]. To simplify the discussion, we will for the moment not discriminate between the various TICT channels, but use for interpretation of the data the simplified three-

state kinetic scheme [19, 261 presented in Fig. 4. Using this scheme, the fluorescence properties of DCM, DCS and other stilbene dyes can be understood_ Excitation from the ground state leads to the planar state E* which can either fluoresce (in a sufficiently rigid environment [261) or decay to the photochemical product states A* (fluorescent) and P* (non-fluorescent). A* and E* are polar, while P* is nearly non-polar (Table 3). The branching ratio from E* to P* and A* will therefore be strongly solvent polarity dependent, disfavouring nonradiative decay via P* and leading to very good fluorescence properties in highly polar solvents. For stilbene dyes with weaker donor groups (e.g. methoxy) the A* state is raised in energy and therefore less important. These dyes consequently decay predominantly via the non-radiative channel P*.

Fig. 4. Three-state kinetic scheme for the discussion of singlet photochemistry in substituted stilbenes. The primarily excited state E* can relax either to A* (can be emissive or non-emissive) or to P* (non-emissive). The dipolar nature of the planar species (E*) and of the twisted doubIebond species (P*) depends strongly on the substituents. The TIC3 state (A*) is always very polar and lowered energetically by donor-acceptor substitution. This one-dimensional representation does not show, however, the possible direct transition between A* and P* (see also Fig. 5).

422 Consideration of the multidimensional nature of the potential surfaces in Fig. 4 leads to the possibility of molecular engineering of the dyes (and of the dimensionality of the excited state reactive hypersurface) as shown with the examples DCSBS and DCSBD in Fig. 5. The relevant fluorescence quantum yield and non-radiative decay rate data are collected in Table 5. Whereas k,, increases by about two orders of magnitude for DCSBS with respect to DCS, because the “metastable” state A* is taken away, it is nearly absent for DCSBD lacking the P* state and leading to outstanding fluorescence properties (fluorescence quantum yields near unity). DCS

DCSBS

b)

a)

E*-

0

double

bond

twist

P*

90 NC

DCSBD

Fig. 5. Schematic two-dimensional cross-section of the SI potential for DC.5 M corresponds to an energy maximum, SEA and S,, to saddle points. The numbers indicate (arbitrary) energy units. In the model compound DCSBS A* cannot be formed and the relaxation is forced into the single channel E* -+ P*. For DCSBD the quenching by P* is stopped but the channel towards A* remains open. TABLE

5

Florescence diethylether

4f

k,,

(10’ s-l)

quantum yields at room temperature and non-radiative for DCS and bridged model compounds [26]

decay rates at 200 K in

DCS

DCSBS

DCSBD

0.03 15

0.002 1320

0.74 0

423

1 .o

‘LU,

-

o.o-.~‘..-~‘-.~~‘-~~.‘.~.-‘.-.~‘.~~~’~~’ #O

Fig. 6. Fluorescence

IZO

lifetimes

‘.-.-I. i60

measured

temperatures

and/or

higher

at BESSY

viscosities

880

320

TtK1 for DCS

At about 190 K a lifetime maximum is observed TICT (A*) emission. For higher temperatures lower

..‘..-.‘..*.I...*‘.. aso

ZOO

in ethanol

as a function

of temperature.

which is interpreted as being due to predominant

A*

P*

formation

formation

quenches becomes

the slowed

fluorescence.

For

down because it

involves a twisting motion, and at 77 K only E* fluorescence remains [26].

The three-state kinetic scheme involves the possibility for simultaneous fluorescence from E* and A*. However, no dual fluorescence can be detected for DCS under !ow concentration-low photon density conditions (in contrast to high concentration-high photon density conditions where two bands with precursor-successor relationship are detected and are connected with dimer and bicimer formation but also with single-bond rotations 126, 30, 311). This is probably the consequence of the strong coupling of E” and A* states and of their similar fluorescence energies. Nevertheless, variations in the relative contributions of the simultaneous emission from E’ and A* should result in a variation of the effective fluorescence rate constant kf measurable by &/rr if the intrinsic kf values for E* and A* differ. This has in fact been found 132) and is most obvious in the complicated temperature-lifetime behaviour displayed in Fig. 6 1263. Related stilbazolium dyes exhibit similar lifetime maxima [33]. The maximum lifetime observed in Fig. 6 is 2.2 ns and is connected with a quantum yield close to unity. This points to a rather allowed nature of the A* state which is in contrast to the usual model of forbidden TICT states. However, as pointed out before, TICT emission involves in every case vibronic coupling with allowed states [27] and the next cornerstone in the understanding of TICI’ states will be the development of a theory able to quantify and to predict these effects.

4. The biradicaloid double bonds

charge transfer

(BCT)

model describing

twisted single and

The theory of TICX states, as exemplified above, is just a special case of a more general theory, that of the biradicaloid states as developed by Salem, BonaEie-Kouteclj and coworkers [2-5,34-37]. This theory describes both twisted single and double bonds

424

and also biradicaloid planar systems [38]. It can be visualized in a simplified way (Figs. 7 and 8). Four cases can be distinguished: that of the “classical” TICT excited state (dot-dot nature with charge separation) and three further ones classified in Table 6. In the following we will have a closer look at the examples for each of these cases. (a) Twisting of a double bond in a symmetric system such as ethylene leads to frontier orbitals HOMO and LUMO; therefore the dot-dot state a pair of degenerate with less electronic repulsion is lower in energy (Fig. 7) and constitutes the ground state of the twisted conformation. Its electronic structure is symmetric; hence a vanishing dipole moment results. The excited state is of hole-pair nature, with charge separation for slight perturbations, the so-called zwitterionic state [34, 351. The same reasoning holds for twisting the double bond in stilbene. (b) The isoelectronic system aminoborane (Fig. 7), on the other hand, possesses a sufficiently large energy gap 6 between HOMO and LUMO such that the hole-pair state becomes So in the twisted conformation. However, owing to the different core charges (even on each subsystem) with respect to ethylene (odd on each subsystem),

Ft-i-

fx25 i

. . 70

hole -paw H

. . 8-9 CA

dot -dot

/H

hole -par

H,

dot-dot H

H;;.C,gcA\R dot dot (TICTI

Pig. 7. Upper part. Construction of confiwrations by filling two electrons into the frontier orbit& HOMO (H) and LUMO (L). In the case of twisted ethylene H and L are degenerate: for twisted aminoborane they are spaced by an energy gap S. Lower part. Assessment of the relative energies of hole-pair and dot-dot configurations by taking into account the electron-electron repulsion energy .I. Because J is smaller for dot-dot than for hole-pair states (owing to the increased average distance of the electrons), the dot-dot state becomes S, in the symmetric ethylene case. The preferential stabilization of dot-dot is overcompensated by S in the donor-acceptor system aminoborane.

425

-I-----2e ro

averlap

\

/c

overlap

overlap

\

C<

s-s,

/

Fig. 8. As the systems deviate from 90” twisting (zero overlap of the r orbitals), the BCT hole-pair and dot-dot states start to interact with each other and with 7nr* states and therefore repel each other energetically, creating a minimum in S, and a maximum in S,, for the zerooverlap conformation. The minimum in S1 and maximum in S, occur regardless of the hole-pair or dot-dot nature of the states. If the 7rr* states are very low lying, further minima at other twist angles may occur. Fig. 9. As 6 is varied from zero (left-hand side) to larger values, the S1 and So states approach, touch and sepaz-ate again for 90” twist. To the left of the critical HOMO-LWMO separation S,, systems with a hole-pair excited state are found; to the right, systems with a dot-dot excited state exist.

TABLE

6

Classification Character

(a) (b) (c) (d)

of systems with BCT states

of S,

Hole-pair Dot-dot Dot-dot Hole-pair

Core charge of subsystems

Charge separation properties of S,

Examples

Odd Even Odd Even

Large Large Small Small

Ethylene, stilbene Aminoborane. DMABN DCS Merocyanines

this hole-pair state is only weakly polar whereas the corresponding excited state of dot-dot nature shows charge separation. This state corresponds to the TICT state in dimethykuninobenzonitrile. (c) Upon twisting the double bond in a donor-acceptor stilbene such as DCS, a situation similar to aminoborane arises (large HOMO-LUMO gap S, So of hole-pair nature), but the core charges (odd on each subsystem) are such that the hole-pair state is connected with charge separation whereas the dot-dot excited state is only weakly polar. This leads to the good fluorescence properties of DCS in strongly polar solvents as outlined above.

426 (d) This fourth case will be reached for a system possessing nearly degenerate HOMO and LIMO while possessing subsystems with an even core charge. This will be reached by twisting a formal single bond in appropriate asymmetric cyanine or strong merocyanine dyes.

Acknowledgment The

support

by BMFT

within

project

05414

FABl

is gratefully

acknowledged.

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