Influence of Chromophore Organization on Triplet Energy Migration in

Chapter 19

Influence of Chromophore Organization on Triplet Energy Migration in Amorphous Polymer Solids Richard D. Burkhart and Norris J. Caldwell

Downloaded by CORNELL UNIV on October 19, 2016 | http://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch019

Department of Chemistry, University of Nevada, Reno, NV 89557

A study of the comparative rates of triplet exciton migration has been carried out on molecularly doped polymers and on vinyl aromatic polymers. In both cases specific rate constants for triplet exciton migration were estimated from rate constants for triplet-triplet annihilation. The rate data were obtained by using a laser pulse-optical probe method to determine triplet concentrations directly by triplet-triplet absorption. It is found that triplet exciton migration rates for polymers are ten-fold to one-hundred-fold larger than those for doped polymer matrices probably due to the more dense local chromophore concentrations in the former. Natural chemical and physical processes occurring i n l i v i n g organisms have t r a d i t i o n a l l y been a source of c u r i o s i t y to practicing s c i e n t i s t s and have often provided the jumping-off place for fundamental studies. Photochemists, in p a r t i c u l a r , have been led to f e r t i l e f i e l d s for investigation after contemplating the various light-induced biological processes such as photosynthesis and the visual process. One of the very clear lessons which these investigations have brought to l i g h t i s the importance of proper organization of molecules or chromophobe groups in order to accomplish the task a t hand. Somehow the organism manages to achieve the proper structural arrangement taking advantage of chemical and physical laws and i t s own evolutionary propensities. Chloroplasts in the leaves of green plants provide the c l a s s i c a l example. The challenge which these naturally occurring processes presents, of course, involves the laboratory construction of organizational units which also are able to perform some pre-determined, photo-initiated task. It seems to be widely accepted that a necessary f i r s t step to this end requires a clear understanding of the interaction between the mechanism of electronic energy migration and the role played by the positioning of 0097-6156/87/0358-0242506.00/0 © 1987 American Chemical Society

Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


19.

BURKHART AND CALDWELL

Triplet Energy Migration

243

chromophoric groups(l). Most of the examples to be discussed here and, i n f a c t , the majority of experimental studies have focussed upon organic chromophores which e x i s t in the ground state with zero electron spin angular momentum. Thus, spin allowed s i n g l e t - s i n g l e t and t r i p l e t - t r i p l e t transitions and spin forbidden s i n g l e t - t r i p l e t transitions are the primary ones to be considered. In the present work a major emphasis has been placed on those processes involving excited t r i p l e t states. A p a r t i a l j u s t i f i c a t i o n for this particular choice i s that many d i f f e r e n t experimental methods e x i s t for evaluating the rate of t r i p l e t exciton migration. These include the use of s p a t i a l l y intermittent excitation (also c a l l e d the transient grating method)(2), time-dependent optical a n i s o t r o p y O ) , luminescence quenching(4), time dependent spectral s h i f t s ( 5 ) , and rates of t r i p l e t - t r i p l e t a n n i h i l a t i o n ^ ) . In addition, the r e l a t i v e l y long l i f e t i m e of t r i p l e t states places a l i g h t e r burden of performance on detection equipment and makes the d i r e c t observation of these species a r e l a t i v e l y simple task. A severe and sometimes c r i t i c a l weakness associated with the study of t r i p l e t states i s that i t i s usually d i f f i c u l t to prepare them by d i r e c t photon absorption. Usually an intermediate excited s i n g l e t state must be formed followed by intersystem crossing or else a s e n s i t i z a t i o n technique i s used. In either case, one must be careful to distinguish between energy migration among t r i p l e t s on the one hand and among precursor states on the other. Doped Polymer Matrices The Secluded P a i r . An example of the effects of energy migration involving excited electronic states which precede t r i p l e t formation i s provided by experiments on time-dependent optical a n i s o t r o p y O ) . For example, when polystyrene matrices doped with 1,2-benzanthracene (BZN) are photoexcited using a v e r t i c a l l y polarized beam, the resulting phosphorescence polarization increases i n absolute magnitude with increasing time over a period of 50 msec as does the delayed fluorescence p o l a r i z a t i o n . The f a c t that these delayed luminescence polarizations are near zero when observed at f i f t y ysec after the excitation pulse i s understandable i f rapid scrambling of the transition dipole vector has occurred among the precursor excited s i n g l e t states. But why, then, does the polarization not maintain this zero value at longer times? A plausible answer to this question leads to a f i r s t glimpse of some rudimentary structural features appearing even i n these molecularly doped matrices. In any solute-solvent system, even an ideal one, there w i l l be some d i s t r i b u t i o n function describing average intermolecular separation distances. Thus, i f the d i s t r i b u t i o n i s frozen at some instant in time there w i l l e x i s t certain regions which are more densely populated than others. The model adopted here f o r the doped polymer matrix i s e s s e n t i a l l y that of a f l u i d solution, but not necessarily an ideal one, which has been instantaneously frozen so as to immobilize both solute and solvent molecules. In densely populated regions, t r i p l e t exciton migration w i l l give r i s e to r e l a t i v e l y rapid t r i p l e t - t r i p l e t annihilation and a faster than average rate of t r i p l e t migration and decay. I t i s these same regions in which there w i l l be the most complete scrambling of


PHOTOPHYSICS OF POLYMERS


244

the transition dipole both for precursor s i n g l e t states and t r i p l e t s . As the emission intensity decreases the t r i p l e t states which remain at longer times tend to be those in less populated regions with few neighbors properly situated to be an energy acceptor. Thus, at long times, t r i p l e t emission comes primarily from those molecules which are the same ones excited by the i n i t i a l excitation pulse. Upon closer examination i t i s found that the delayed fluorescence (DF) polarization from these BNZ/polystyrene matrices approaches a value near 0.25 which i s one-half the theoretical maximum of 0.5. This f a c t suggests the existence of another type of structural feature, a so-called secluded p a i r , in which two molecules are s i g n i f i c a n t l y closer to each other than to any t h i r d species(7). A t r i p l e t exciton l o c a l i z e d at such a pair would be expected to spend, on average, f i f t y percent of i t s time at the molecule which o r i g i n a l l y absorbed the exciting photon and f i f t y percent at the partner s i t e . Thus, t r i p l e t - t r i p l e t annihilation involving interaction of a second exciton with this pair-trapped species w i l l , with equal p r o b a b i l i t y , produce a DF photon nearly co-linear with the excitation polarization or else randomly oriented with respect to i t . Thus, BNZ-doped polystyrene appears to possess two structural features which influence the c h a r a c t e r i s t i c s of the delayed luminescence. One of these involves clusters of BNZ molecules consisting of r e l a t i v e l y densely populated regions along with others which are much less densely populated. The other consists of molecular pairs which are capable of trapping excitons by virtue of k i n e t i c barriers as opposed to energetic ones. Although i t might seem that features of this sort would be common to a l l molecularly doped polymers, a search for them in N-ethylcarbazole (NEC) doped polystyrene revealed no indication of t h e i r presence. In p a r t i c u l a r , there was found to be no time dependence of the delayed luminescence polarization on the millisecond time scale(8). With NEC, however, one does find a dependence of the delayed luminescence polarization on dopant concentration. That i s , the polarization decreases monotonically as the concentration increases. T r i p l e t Decay and Exciton Migration. The rate of t r i p l e t state decay following a photoexcitation pulse i s conveniently followed by monitoring the time dependence of the delayed fluorescence. A l i m i t a t i o n of this approach i s that absolute t r i p l e t concentrations cannot usually be evaluated and so rate constants for processes having a kinetic order greater than unity cannot be determined. More information about these important higher order decay processes i s available by d i r e c t measurements of absolute t r i p l e t concentrations. For example, in the case of BNZ-doped polystyrene i t was possible to obtain k in the rate equation 2

-dT/dt = k T + k T x

(1)

2

2

by measuring the time dependence of Τ using optical absorbance of this species(9). In this equation Τ i s the t r i p l e t concentration, k i s the rate constant for the f i r s t order decay of t r i p l e t s and k i s the s p e c i f i c rate for t r i p l e t - t r i p l e t a n n i h i l a t i o n . The cluster model which was proposed to explain the time 2


1

19.

B U R K H A R T AND CALDWELL

245


dependent increase of the absolute delayed luminescence polarization would also predict that apparent k values should decrease monotonically with time. T r i p l e t - t r i p l e t absorption studies on BNZ-doped polystyrene, i n f a c t , were carried out to test this very point. I t was indeed observed that in the f i r s t 25 msec following excitation there i s a steady decline i n k and that the f i n a l asymptotic value of k increased with increasing solute concentration^). Such experiments do not, of course, prove that the suggested model i s correct but are, at l e a s t , consistent with the model's predictions and provide evidence f o r the power of this type of technique. An additional benefit to be derived from d i r e c t measurements of k involves i t s r e l a t i o n to rates of exciton migration using the tneory of diffusion controlled processes. The rate constant for a purely diffusion controlled reaction may be written i n the form (10) 2

2

2


2

k

d f

= %rDN/1000

(2)

where r i s the encounter diameter, D i s the diffusion c o e f f i c i e n t and Ν i s Avogadro's number. If D i s expressed in cm /sec and r in cm, then k has the units of M sec . An estimate of 15 A was suggested by B i r k s ( l l ) for the encounter diameter for t r i p l e t - t r i p l e t a n n i h i l a t i o n . The rate constants k . and k are linked by an e f f i c i e n c y factor, that i s , k =qk. where q has upper and lower l i m i t s of 2 and 0 respectively(12?. Thus, measurements of k y i e l d experimental values of qD. Measurements of t h i s sort have been made for a number of d i f f e r e n t polymer/dopant systems. These have been collected over the l a s t several years in these laboratories and are summarized in Table I. f

2

2

f

2

Table I . Second Order Rate Constants for T r i p l e t Decay Found i n Various Polstyrene/Dopant Systems -

Dopant Molecule

Concentration jM)

N-carboethoxycarbozole 0.0944 1,2-benzanthracene 0.230 1,2-benzanthracene* 0.114 naphthalene 0.047 *Using poly( -methylstyrene) as the matrix •Long time asymptotic value of k

Τ (K)

—

-

k xl0 (M^sec" ! a

Ί

1

77 298 298 298

1.8 11.1 29.2

2

For the polystyrene matrices i t i s clear that k values on the order of 10 to 10 seem to be t y p i c a l . Values of qD were calculated from these rate constants using as encounter r a d i i either the 15 A suggested by Birks or the average intermolecular separation distance calculated from the equation derived by Chandrasekhar (13), whichever was smaller. The results are presented i n Table I I . Again very l i t t l e i n d i v i d u a l i t y among the dopant molecules i s noted. 2



246 Table I I .

Values of qD for Various Dopant/Polystyrene Systems

Dopant Molecule


N-carboethoxycarbazole 1,2-benzanthracene naphthalene

r* (A) 14.4 12.8 18.2

Concentration (M)

qDxlO (cm /sec)

0.0944 0.134 0.047

12

1.8 1.6 1.6

As a f i n a l step in this analysis i t may be assumed that these exciton migrations occur by a sequence of random f l i g h t s for which the mean square displacement in unit time i s equal to 6D. The mean square displacement i s the product of the frequency of migratory hops (f) and the square of the length of each hop U ). Thus, fil = 6D and, using the mean separation distance of solute molecules a j I and a b i t r a r i j y setting q = 1, one finds f values between 500 sec and 600 sec" for the three systems of Table I I . These results may well be typical of most polymer/dopant systems. Certainly the behavior of these systems with respect to t r i p l e t exciton migration seems quite similar in spite of their rather d i s t i n c t i v e molecular architecture. Let us now turn to an examination of systems i n which chromophore units are regularly arranged by virtue of their being bonded to the backbone of a polymer chain. In this way, i t may be possible to assess the effects of chromophore organization on the mechanism of t r i p l e t exciton migration and decay. Pure Polymer Systems General Observations. The t r i p l e t photophysical behavior of most polymeric systems i s quite dependent on the p a r t i c u l a r physical state. I t i s important to d i s t i n g u i s h , for example, whether one i s dealing with neat polymer films or with solutions. If the l a t t e r , then the f l u i d i t y of the solution w i l l also be important. One reason for this preoccupation with the physical state concerns the ease with which t r i p l e t excimer formation occurs. Phosphorescence from t r i p l e t excimers, f o r example, i s common in s o l i d polymeric films but much less common in r i g i d solutions. For this reason, the interpretation of rate processes involving t r i p l e t states tends to be simpler to handle and more susceptible to quantitative treatment for r i g i d polymer solutions. An interesting feature of delayed luminescence from polymers which sets them apart from t h e i r monomeric analogues i s the very prominent delayed fluorescence (DF) emission which i s usually observed. This, in f a c t , may be taken as a primary feature of photophysical behavior which may be d i r e c t l y linked to chromophore organization. For many d i f f e r e n t polymers i t has been shown that the process of t r i p l e t - t r i p l e t annihilation i s responsible for the prominent DF emission observed. In f a c t , for d i l u t e polymer


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247

solutions the a n n i h i l a t i v e process appears to be almost exclusively intramolecular in character(14,15). Since i t w i l l be necessary to distinguish between t r i p l e t excitons which are mobile from those which are trapped a t an excimer forming s i t e , the symbol Τ w i l l be used to represent molar concentrations of the former and T. for the l a t t e r . If more than one type of trapped t r i p l e t i s present, an additional numerical subscript w i l l also be used, i . e . T^ T or in general, Ty


9

t 2

9

t

Neat Polymer Films. I t i s interesting to compare the t r i p l e t photophysical properties of poly(N-vinylcarbazole) (PVCAM16) on the one hand and poly(l-vinylnaphthalene) (P1VN)(17) on the other when each i s examined as a pure polymer f i l m . Both polymers exhibit a prominent excimer phosphorescence band as well as a d i s t i n c t delayed fluorescence emission. In addition, the delayed fluorescence arises by a process of t r i p l e t - t r i p l e t annihilation for both polymers. Furthermore, the luminescence decay k i n e t i c s suggest that e q u i l i b r i a of the type Τ. ^ Τ + 4 ° t m

(3)

e x i s t where ^E i s an excimer forming s i t e . That i s , excimer formation i s a reversible process. One manifestation of this equilibrium i s the s h i f t of the center of gravity of the phosphorescence band to longer wavelengths as the temperature i s raised. Presumably t h i s occurs because of the selective loss of the highest energy excimer species by detrapping as the temperature r i s e s . The mobile excitons produced are then free to probe available trap sites and, of course, w i l l tend to populate the lower energy ones s e l e c t i v e l y . Thus, the average energy of populated excimer sites decreases with increasing temperature. An important difference i n photophysical behavior between the naphthalenic and carbazole polymers i s the lifetime of the mobile exciton compared with that of the t r i p l e t excimer. For PVCA the mobile exciton i s much shorter lived than the excimeric species but for P1VN just the reverse i s true. In both polymers the primary mode of delayed fluorescence production involves a hetero-annihilation of the type 0

Τ

m

+ T. —> E * + M° t X

1

(4)

Thus, the delayed fluorescence l i f e t i m e i s e s s e n t i a l l y equal to that of Τ for PVCA and to T. for P1VN. The l a t t e r i s easy to prove since excimer phosphorescence and delayed fluorescence lifetimes are the same for P1VN. For PVCA at 77 Κ τ « τ but no d i r e c t and independent measure of the lifetime of T T n PVCA has been accomplished at this time. Delayed fluorescence lifetimes for these two polymers indicate that there i s more than one type of excimeric species present. I t had been pointed out by Siebrand(18) that when this situation obtains and when the lifetime of mobile the exciton i s less than that of the p

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1155 16th St., N.W. Hoyle and Torkelson; Photophysics of Polymers Washington, D.C. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

248



excimeric species, then one should observe that the delayed fluorescence intensity goes through a maximum when plotted as a function of the absolute temperature. This predicted maximum i s , in f a c t , found for s o l i d films of PYCA and, as expected, i s not observed for s o l i d films of P1VN. Polymers in Rigid Solution. The emission spectrum of PCVA in 2-methyltetrahydrofuran (MTHF) at 77 Κ consists of prominent delayed fluorescence and phosphorescence bands(19). For this reason i t was decided to investigate the rate of t r i p l e t exciton decay in these r i g i d solutions and to treat the data in terms of concurrent f i r s t and second order processes. For systems in which an equilibrium d i s t r i b u t i o n of potential reactants may be assumed, eq 1 may be employed for data analysis. I t i s not c l e a r , however, that such a d i s t r i b u t i o n i s v a l i d for polymer solutions especially in l i g h t of evidence suggesting that T-T annihilations occur p r i n c i p a l l y by i n t r a - c o i l processes(14-15). A recent investigation was carried out to find a valid method for analyzing data on the rate of t r i p l e t state disappearance using models involving only intramolecular migration of t r i p l e t excitons (Burkhart, R. D. Chem. Phys. L e t t . , in press). A one-dimensional model allowing only nearest neighbor migrations was compared with a three-dimensional random f l i g h t model. Physically r e a l i s t i c results were obtained only with the three-dimensional model. Furthermore, the diffusion c o e f f i c i e n t s for t r i p l e t exciton migration extracted from this three-dimensional intramolecular model were nearly the same as those obtained using the conventional kinetic equation ( i . e . eq 1 ) . The hopping frequencies for t r i p l e t exciton migration in PYCA for these three models are summarized in Table 111(20). Neither the electron exchange mechanism(21) nor the Forster

Table I I I . Frequencies of T r i p l e t Exciton Migration for Rigid Solutions of PYCA in MTHF at 77 Κ Using Various Models for Data Analysis Assumed Model

Migration Frequency (cm" ) 1

Conventional I n t r a - c o i l , one dimensional I n t r a - c o i l , three dimensional

3xl0

5

4 2x10* 6 7

dipole-dipole mechanism(22) can account for the one-dimensional model. For the other two models, however, the frequencies are on the order of ten-fold larger than those found for doped polystyrene matrices. Thus, for the polymer systems investigated to this point, there are only modest enhancements of the exciton migration frequency


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compared with molecularly doped polystyrene. Furthermore, the enhancements which are found may be due simply to a higher local density of chromophores i n the case of the polymeric systems. Time-Dependent Phosphorescence Spectra of Polymers. Although a strong s i m i l a r i t y exists between phosphorescence spectra of vinyl aromatic polymers and the corresponding monomeric analogues, i t i s interesting to focus attention upon the differences which e x i s t between these spectra. A sample comparison i s provided in Figure 1 between NEC and PVCA both as d i l u t e solutions in MTHF at 77 Κ (23). Both spectra are recorded using comparable conditions and instrument parameters. In the spectrum of PYCA i t seems that the 0-0 emission band i s present only as an unresolved shoulder and that the remaining structural features of this spectrum are somewhat broadened compared with those of NEC. An attempt was made to achieve better resolution of the 0-0 band of PYCA using narrow monochromator s l i t s and multiple scans of the emission band. In addition, spectra were recorded using variable delay times following e x c i t a t i o n . These results are presented in Figure 2. Two observations about the 0-0 phosphorescence band of PYCA emerge from these experiments. In the f i r s t place a clear resolution of this band i s , i n f a c t , achieved. In addition, the intensity of this band decreases r e l a t i v e to longer wavelength components by using long delay times on the order of several hundred milliseconds. An obvious corollary to this e f f e c t i s that apparent phosphorescence decay times w i l l depend upon the wavelength chosen for the measurement. Such effects are not large but they are readily measurable. Evidently these interesting t r i p l e t state phenomena are recorded here f o r the f i r s t time f o r a polymer system. Similar observations have been made, however, by Bassler and coworkers on amorphous benzophenone(5). The interpretation put forward to account for the e f f e c t involves t r i p l e t exciton migration i n the inhomogeneously broadened p r o f i l e . As indicated above for s o l i d f i l m spectra, the tendency i s for the migrating excitons to seek out the lowest energy s i t e s . Therefore, excitons at r e l a t i v e l y high energy s i t e s may not only relax to the ground state but may also migrate to lower energy s i t e s releasing some f r a c t i o n of their energy to the l a t t i c e . The rate of disappearance of such excitons w i l l c l e a r l y be greater than that of lower energy ones which have fewer channels available f o r energy d i s s i p a t i o n . In order to demonstrate this e f f e c t to best advantage i t was necessary to choose a PYCA sample having a r e l a t i v e l y low molecular weight. In this way interference of the phosphorescence emission by delayed fluorescence i s minimized. These are provacative r e s u l t s because they indicate that there may be no well defined lowest t r i p l e t state i n vinyl aromatic polymers unless special s t e r i c or electronic effects are present which n u l l i f y inter-chromophore interactions. On the other hand, they may provide an additional tool with which to investigate rates of energy migration in polymers and in some polymer/dopant systems as w e l l .




250

Figure 1. Phosphorescence spectra of NEC (upper) and PYCA (lower) in solution at 77 K. Both spectra are recorded at 50 msec following the excitation pulse using a monochromator band pass of 6 nm.

Ζ

400

420

440

460

480

W A V E L E N G T H (nm)

Figure 2. Time resolved phosphorescence spectra of PYCA in solution at 77 Κ using a monochromator band pass of 2 nm. Delay times of 400 msec (upper) and 800 msec (lower) were used.


19. BURKHART AND CALDWELL Triplet Energy Migration

251

Acknowledgment This work was supported by the U.S. Department of Energy under Grant Number DE-FG08-84ER45107.

Literature Cited 1. 2.


3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

Guillet, J. Polymer Photophysics and Photochemistry; Cambridge University Press: Cambridge, 1985; pp 241-251. (a) Meyer, E. G.; Nickel, Β. Z. Naturforsch. 1980, 35A, 503. (B) Burkhart, R. D. J. Am. Chem. Soc. 1974, 96 6276. Burkhart, R. D.; Abia, A. A. J. Phys. Chem. 1982, 86, 468. David, C.; Demarteau, W.; Geuskens, G. Eur. Polym. J. 1970, 6, 537. Richert, R.; Ries, B.; Bässler, H. Phil. Magazine Β. 1984, 49, L25. Caldwell, N. J.; Burkhart, R.D. Macromolecules 1986, 19, 1653. These pair structures had earlier been postulated on the basis of luminescence decay kinetics. For details see Burkhart, R. D. Chem. Phys. 1980, 46, 11. Abia, A. A. Ph.D. Thesis, University of Nevada, Reno, 1984. Burkhart, R. D. J. Phys. Chem. 1983, 87, 1566. Noyes, R. M. Progr. Reaction Kinetics 1961, 1, 130. Birks, J. B. Photophysics of Aromatic Molecules; Wiley: New York, 1970; p 390. Values of this efficiency factor have been evaluated for only a few molecules. For a detailed discussion see, Saltiel, J.; Marchand, G. R.; Smothers, W. Κ.; Stout, S. Α.; Charlton, J. L. J. Am. Chem. Soc. 1981, 103, 7159. Chandrasekhar, S. Rev. Mod. Phys. 1943, 15, 1. Pasch, N. F; Webber, S. E. Chem. Phys. 1976, 16, 361. Klöpffer, W.; Fischer, D.; Naundorf, F. Macromolecules 1977, 10, 450. Burkhart, R. D.; Avileś, R. G. Macromolecules 1979, 12, 173. Burkhart, R. D.; Avileś, R. G.; Magrini, K. Macromolecules 1981, 14, 91. Siebrand, W.; J. Chem. Phys. 1965, 9, 234. Klöpffer, W.; Fischer, D. J. Polymer Sci., Part C 1973, 40, 43. The rate constants providing the basis for this analysis were taken from the data of reference 6. Inokuti, M.; Hirayama, F. J. Chem. Phys. 1965, 43 1978. Förster, T. Z. Naturforsch. 1946, 4a, 321. The data of Figures 1 and 2 were obtained in this laboratory by Gregory Haggquist using a locally constructed phosphorimeter described in reference 6.

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Influence of Chromophore Organization on Triplet Energy Migration in

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