J . Phys. Chem. 1991, 95, 955-960 value smaPalong with the rise function, SBPP(t), and both parts are affected by the approximations in the theory. Indeed, for the values of e and g listed en Table I, the values smaPPused in the low-field theories
are different from the correct values. The errors in the steady-state and timedependent values may cancel out in some cases, resulting in an unexpected behavior of x2. Anyhow, our overall conclusions is that the r(7) expressions of Benoit performs rather well up to moderate fields (s, = 0.3),and the equation of Matsumoto et al. produces a slight improvement in some cases. Simulation results are compared with those from the seriesexpansion equations of Nishinari et al. in Figure 3. The T~~~ values that characterize the deviation of the buildup curves are listed in Table 1. The detection of the 10% deviation cannot be made precisely due to the statistical uncertainty of the simulation results. Anyhow the results in Table I are of semiquantitative interest and indicate that the equations of Nishinari et al. are valid in many cases beyond 7 = 0.20, for orientational strengths as high as those corresponding to s, = 0.66. As the s ( ~ ~ ~ ) /values s , in Table I indicate, this may typically cover the rise up to half the saturation value. We finally discuss the performance of the high-field approximations (eqs 14 and 16). The comparison with simulation results
955
is displayed in Figure 4 for the case of an induced dipole. A similar diagram for the case of a permanent dipole has the same aspect. As for the low-field expression, the x2 deviations between the analytical approximation and the simulated results is listed in Table I. The deviation are rather acceptable for values of s, as low as 0.66 and probably even lower. Summarizing the findings in the two cases of field strength, we have shown that the low-field and high-field equations can be used safely in conditions such that the steady-state order parameter and s, > 2/3, respectively. It seems reasonable to is s, < propose the use of eq 9 or 12 for s, < and eq 14 or 16 for s,
> l/2.
Apart from these conclusions, we have illustrated the applicability of the Brownian dynamic simulation technique to characterize this type of dynamic electrooptic process. Thus, simulation results can be particularly helpful for the interpretation of the experimental buildup curves in the intermediate region of s,, where the validity of the available approximations is unclear. Brownian dynamics simulation can be applied with the same simplicity to cases with more complex, time-dependent fields, and it is therefore hoped that this technique will be also helpful in the analysis of other electrooptical phenomena.
Acknowledgment. This work has been supported by Gzant PB87-0694 from the Direccidn General de Investigacidn Cientifica y TEcnica to J.G.T.
Intramolecular Electron Transfer in Polymers with Aromatic Side Groups as a Stepping Stone Migaku Tanaka, Hiroshi Yoshida, and Masaaki Ogasawara* Faculty of Engineering, Hokkaido University, Kitaku. Sapporo, 060 Japan (Received: March 31, 1990; In Final Form: July 2, 1990)
Intramolecular electron-transfer reactions in polymers with aromatic side groups were confirmed by the pulse radiolysis of solutions containing poly(4-vinylbiphenyl-co-1-vinylpyrene). First, the optical absorption spectra and decay reactions of the anions produced by electron pulses in 2-methyltetrahydrofuran containing homopolymers poly(4-vinylbiphenyl) or poly( 1vinylpyrene) were studied. Steady-state optical absorption spectra of these polymer anions produced by y-radiolysis at 77 K were also studied for comparison. Second, solutions containing the copolymers in which a small number of biphenylyl side groups of poly(4-vinylbiphenyl) were substituted by pyrenyl groups were examined. The transient absorption spectra observed by pulse radiolysis of these solutions clearly showed the existence of the intramolecular electron-transfer process along the polymer chain with the biphenylyl side groups as a stepping stone. The results were analyzed on the basis of a simple one-dimensional random-walk mechanism.
Introduction Electron transfer (ET) from organic anion radicals to neutral molecules in solution has been studied for a long time. Weissman and co-workers determined the rates of electron-exchange reactions between aromatic anion radicals and their neutral forms in solutions by measurement of line broadening of ESR spectra.l Pulse radiolysis studies by Arai and Dorfman demonstrated the ET from aromatic anion radicals to various electron acceptors? Recently, Calcaterra, Closs, and Miller measured the intramolecular ET rates in systems with donor and acceptor groups linked by rigid saturated spacer^.^.^ The ET rates observed in these studies are very fast in general provided appropriate exothermicity of the These studies have prompted us to construct reaction is ~~
~
( I ) Ward, R. L.; Weissman, S. 1. J . Am. Chem. SOC.1957, 79, 2086. (2)Arai, S.;Grev, D. A.: Dorfman, L. M. J . Phys. Chem. 1967,46,2572. (3)Calcaterra, L.T.;Closs,G.L.; Miller, J. R. J . Am. Chem. Soc. 1983, 105, 670. (4)Miller, J. R.; Calcaterra, L. T.; Closs, G. L. J . Am. Chem. Soc. 1984, 106, 3047. ( 5 ) Huddleston, R. K.; Miller, J . R. J . Chem. Phys. 1983,79, 5337.
chemical systems for one-dimensional ET in which e x e s electrons migrate along polymer chains with aromatic side groups as a stepping stone. Previous studies confirmed that anion radicals of polymers were generated by the reaction of excess electrons when ethereal solutions of the polymers were irradiated with pulsed electron beams.”-” Although the anion radicals thus produced simply (6) Hush, N. S.;Paddon-Row, M. N.; Cotsaris, E.; Overing, H.; Verhoven. J. W.; Heppener, M. Chem. Phys. Leu. 1985,117,8. (7) Pasman, P.; Mes, G. F.; Koper, W.; Verhoven, J. W. J . Am. Chem. SOC. 1985,107,5839. ( 8 ) Domingue, R. P.; Fayer, M. D. J . Phys. Chem. 1986, 90,5141. (9)Warman, J. M.; de Haas, M. P.; Overing, H.; Verhoven, J. W.; Paddon-Row, M. N.; Oliver, A. M.; Hush, N. S.Chem. Phys. Lett. 1986,128, 95. (IO) Closs. G.L.;Calcaterra, L. T.; Green, N. J.; Penfield, K. W.; Miller, J. R. J . Phys. Chem. 1986,90, 3673. (1 1) Penfield, K. W.; Miller, J. R.; Paddon-Row, M. N.; Cotsaris, E.; Oliver, A. M.; Hush, N. S.J . Am. Chem. Soc. 1987,109,5061. (12)Overing, H.; Paddon-Row, M. N.; Heppener, M.; Oliver, A. M.; Cotsaris, E.; Verhoven, J. W.; Hush, N. S.J . Am. Chem. Soc. 1987,109, 3258.
0022-3654/91/2095-0955$02.50/00 1991 American Chemical Society
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The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
disappear by the recombination reaction with solvent cations, the excess electrons may hop to the neighboring side groups of the polymers before the disappearance of the polymer anions. As the ET in the polymers to identical chromophores was difficult to be confirmed, we synthesized poly(4-vinylbiphenyl-co-1 -vinylpyrene) in which a small number of biphenylyl side groups were substituted by pyrenyl groups. If the excess electron attached to one of the biphenylyl groups transfers to a pyrenyl group in the polymer, a drastic spectral change due to the loss of biphenylyl anion and the formation of pyrenyl anion should be observed. The pyrenyl group was chcsen as indicator because the spectral overlap between anion radicals of biphenyl and pyrene was the minimum among the possible combinations of aromatic compounds.2 In the present study, we examined first the optical absorption spectra and decay reactions of the anions produced by electron pulses in 2-methyltetrahydrofuran containing homopolymers poly(4vinylbiphenyl) or p l y ( 14nylpyrene). Steady-state optical absorption spectra of these polymer anions produced by y-radiolysis at 77 K were also studied for comparison. Then, solutions of poly(4-vinylbiphenyl-co- I-vinylpyrene) were studied by pulse radiolysis, and evidence for intramolecular ET in the polymers was obtained. So far ion radicals of polystyrene,18-22poly(2~ i n y l n a p h t h a l e n e ) ,poly(4-~inyIbiphenyl),~~-~’ ’~~~~ poly(9-vinylphenanthrene),% poly( 1-vinylpyrene),25poly(9-vinylcarbgzole),2637 poly~-nitrostyrene),2*,~ poly(alky1 m e t h a ~ r y l a t e ) , ~poly(or*~~ g a n ~ s i l a n e ) ,polya~etylenes,~~ ~~,~~ and viologen polymers36 have been investigated by pulse radiolysis, laser photolysis, and ESR methods. However, the direct measurement of intramolecular excess electron transfer in polymers containing no heteroatoms has not been reported yet.
Experimental Section 1 . Materials. 4-Vinylbiphenyl (from Aldrich Co. Ltd.) was purified by repeated vacuum sublimation. 1-Vinylpyrene was synthesized by the method of Tanikawa et ale3’ Obtained crude product was purified by adsorption chromatography on silica gel with benzene as solvent. The resulting product was dried under (13) Matsushima, M.; Kato, N.; Miyazaki, T.; Fueki, K. Radiar. Phys. Chem. 1987,29, 231. (14) Tanaka, M.; Yoshida, H.;Opsawara, M. Radiar. Phys. Chem. 1988, 32, 719. ( I 5 ) Tanaka, M.; Yoshida, H.; Opsawara, M. Radiar. Phys. Chem. 1989, 34, 591. (16) Tanaka, M.; Yoshida. H.;Ogasawara, M. Radiar. Phys. Chem., in press. (17) Kato, N.; Miyazaki, T.; Fueki, K.; Saito, A. Inr. J. Chem. Kincr. 1988, 20, 877. (18) Irie, S.; Horii, H.; Irie, M. Macromolecules 1980. 13, 1355. (19) Tagawa, S.;Schnabel, W.; Washio, M.; Tabata, Y. Radiat. Phys. Chem. 1981, 18, 1087. (20) Washio, M.; Tagawa, S.; Tabata, Y. Radiar. Phys. Chem. 1983,21, 239. (21) Tabata, Y.; Tagawa, S.;Washio, M.; Hayashi, N. Radiar. Phys. Chem. 1985, 25, 305. (22) Tagawa, S.Radiat. Phys. Chem. 1986, 27,455. (23) Irie, S.; Irie, M. Macromolecules 1986, 19, 2182. (24) Tamai, N.; Masuhara, H.; Mataga, N. J. Phys. Chem. 1983, 87, 446 I . (25) Tanaka, J. A.; Masuhara, H.;Mataga, N. Polym. J . 1986, 18, 181. (26) Washio, M.; Tagawa, S.; Tabata, Y. Polym. J. 1981, 13, 935. (27) Masuhara, H.;Yamamoto, K.; Tamai, N.; Inoue, K.;Mataga, N.J. Phys. Chem. 1984,88, 3971. (28) Veregin, R. P.; Harbour, J. R. Macromolecules 1988, 21, 1349. (29) Albery, W. J.; Compton, R.G.; Jones, C. C. J. Am. Chem. Soc. 1984, 106. 469. (30) Torikai, A.; Ami, T.; Suzuki, T.; Kuri, Z . J. Polym. Sci., Polym. Chem. Ed. 1916, 13, 797. (31) Torikai, A.; Kato, H.; Kuri, Z . J. Polym. Sci., Polym. Chem. Ed. 1916. 14. -. . -, . . 106s .- - - . (32) Ogasawara, M.; Tanaka, M.; Yoshida, H. J. Phys. Chem. 1987,91, 937. (33) Ban, H.;Sukegawa, K.; Tagawa, S.Macromolecules 1987,20, 1775. (34) Ban, H.; Sukegawa, K.; Tagawa, S.Macromolecules 1988, 21, 45. (35) Yamaoka, H.;Ogasawara, M.; Tanaka, M., to be published. (36) Sakamoto. T.; Ohsako, T.; Matsuo. T.; Mulac. W.A,: Meisel. D. Chem. Len. 1984. 1893. (37) Tanikawa, K.; Ishizuka, T.; Suzuki, K.; Kusabayashi, S.;Mikawa, H. Bull. Chem. SOC.Jpn. 1968, 41, 2719.
.
Tanaka et al. A
1.0
lh
L L L d
0
4.
IC.
I
Wavelength1 nm
Figure I . Optical absorption spectra observed on y-radiolysis of MTHF solutions containing P4VBp and its model compounds at 77 K. (A) P4VBp: 30 base mmol dm-’; irradiation dose 2.8 kGy. (B) Biphenyl: 10 mmol dm-’; irradiation dose 0.6 kGy. (C) 4-Ethylbiphenyl: 10 mmol dm-); irradiation dose 0.6 kGy.
IA 1.0
c
A
Wavelengthlnm
Figure 2. Optical absorption spectra observed on y-radiolysis of M T H F solutions containing PlVPy and pyrene at 77 K. (A) P1VPy: 30 base mmol dm-’; irradiation dose 2.8 kGy. (B) Pyrene: 10 mmol dm-’; irradiation dose 0.6 kGy.
vacuum for 24 h and characterized by measuring melting point (mp = 89 0C),38UV absorption spectra in THF,39 and NMR spectroscopy. Poly(4-vinylbiphenyl) (P4VBp), poly( 1-vinylpyrene) (PIVPy), and poly(4-vinylbiphenyl-co- 1-vinylpyrene) (P(4VBp-co- 1VPy)) were synthesized by the following method: Benzene solutions containing monomers and 2,2’-azobisisobutyronitrile were introduced into Pyrex tubes, degassed by repeated freezepumpthaw cycles, and sealed in a vacuum (ca. Torr). Polymerization was carried out at 70 O C (in an oven) for 24 h. Polymers were precipitated in methanol. Further purification was carried out by reprecipitation from T H F solution into methanol. This pro(38) Kamat, P. V.; Basheer. R. A.; Fox, M. A. Macromolecules 1985,18, 1366.
(39) Todesco, R. V.;Basheer, R. A.; Kamat, P. V. Macromolecules 1986,
19, 2390.
Intramolecular Electron Transfer in Polymers cedure was repeated at least six times to remove residual monomers. Purified polymers were dried under vacuum for 24 hours. The molecular weight distribution was measured by gel permeation chromatography. The contents of pyrenyl groups in copolymers were estimated by the measurement of UV absorption spectra in T H F. Biphenyl (Wako Co. Ltd.), 4-ethylbiphenyl (Aldrich Co. Ltd.), and pyrene (Wako Co. Ltd.) were purified by vacuum sublimation. 2-Methyltetrahydrofuran (MTHF, Wako CooLtd.) was refluxed over sodium metal, distilled, and dried rigorously over a sodiumpotassium alloy. Cyclohexane (Wako Co. Ltd.) was distilled and dried over molecular sieves. 2. Irradiation and Measurements. MTHF solutions containing solutes were prepared under vacuum (ca. Torr) and sealed into quarts cells with light paths of 10 mm for pulse radiolysis and 1.5 mm for y-radiolysis, respectively. For steady-state measurements, the samples were irradiated with y-rays from 6oco a t 77 K in the dark. Optical absorption spectra were measured at 77 K with a Shimadzu MPS-5000 spectrophotometer. The Hokkaido University 45-MeV electron linear accelerator was used as the source of the electron beam for pulse radiolysis experiments. The half-width of the electron pulse used in this experiment was 50 ns. The optical systems and the data processing system were as described elsewhere.40 Pulse radiolysis measurements were made at ambient temperature (19 f 1 "C).
Results I . Anion Radicals of Homopolymers. Figures 1 and 2 show steady-state optical absorption spectra observed at 77 K in yirradiated MTHF matrices containing P4VBp. 4-ethylbiphenyl, or PlVPy. Steady-state absorption spectra of the anion radicals of biphenyl and pyrene observed in MTHF matrices are also shown for comparison." It is obvious that the anion radicals of P4VBp, and PlVPy are formed by the reaction of excess electrons with the aromatic side groups of the polymers. The absorption maxima of the polymer anions are slightly shifted toward the longer wavelength side compared with those of the model compounds (biphenyl and pyrene). This shift is attributed to the effect of substitution of the aromatic ring carbons by alkyl groups, because the same tendency is observed in the absorption spectrum of the 4-ethylbiphenyl anion. There is no evidence for the existence of the interaction between the negatively charged aromatic rings and the neighboring neutral ones, which contrasts to the positively charged side groups of P4VBp or PlVPy.'s*16Js The spectra were not affected by the molecular weight of the polymers. It follows that the excess electrons are not spread over the main chains but distributed mainly on aromatic side groups of the polymers. Figures 3 and 4 show transient absorption spectra observed on pulse radiolysis of MTHF solutions containing homopolymers and their model compounds. In all cases, the absorption bands due to the anion radicals of solutes appeared at the pulse end, but that of the solvated electrons (A,, = 1200 nm in MTHF42)was not detected; the anion radical formation was completed within the time duration of the electron pulse. The absorption band in the visible due to the anion radicals disappeared within several microseconds, but that in the UV survived much longer. The UV absorption has been assigned to the TI T, transition of the aromatic groups on the basis of the position and lifetime of the absorpti0n.4~ Strong emissions, probably due to fluorescence from the excited states of the isolated and associated aromatic groups, were observed in the solutions of model compounds and polymers, respectively." We monitored the decay reactions of the biphenyl anion and the P4VBp anion at 650 and 660 nm, respectively. The absorption at around 400 nm was much stronger and easily monitored.
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 957
I*
ou C
0
n
"I 0.5
1
i
IC Waveleng t h I nm Figure 3. Transient absorption spectra observed at ambient temperature on pulse radiolysis of MTHF solutions containing P4VBp and its model compounds. (A) 5 base mmol dm-' P4VBp: (0)pulse end; ( 0 )0.8 p after a pulse; (---) 4.0 ps after a pulse. (B) 5 mmol dm" biphenyl: (0) pulse end; ( 0 )0.5 p after a pulse; (-- -) 2.0 ps after a pulse. (C) 5 mmol dm-3 4-ethylbiphenyl: (0)pulse end; ( 0 )0.5 ps after a pulse; (---) 2.0 ps after a pulse.
-
(40)Ogasawara, M.;Kajimoto. N.; Izumida, T.; Kotani, K.; Yoshida, H. J . Phys. Chem. 1985,89, 1403. (41) Shida, T. In Electronic Absorption Spectra of Radical Ions; Shida, T., Ed.; Elsevier Science Publishers: New York, 1988; pp 134, 85. (42) Hamill, W . H. In Radical Ions; Kaiser, E. T., Kevan, L., Ed.; Intencience Publishers: New York, 1968; pp 345. (43) Carmichael, J.; Hug,G. L. J . Phys. Chem. Ref. Data 1986, IS, 1.
Wavelength/ nm Figure 4. Transient absorption spectra observed at ambient temperature on pulse radiolysis of MTHF solutions containing PlVPy and pyrene. (A) 5 base mmol dm-] PlVPy: (0)pulse end; ( 0 )2 ps after a pulse; (---) 8 ps after a pulse. (B) 5 mmol dm-] pyrene: (0)pulse end; ( 0 )0.5 ps after a pulse; (- - -) 2.0 ps after a pulse.
-
However, we avoided this wavelength because it was partially T, transition. For pyrene anion and overlapped by the T, PlVPy anion, the measurements were carried out at 495 and 500 nm, respectively. The decay reactions of polymer anions followed second-order kinetics; this was confirmed by the apparent lifetime of the polymer anions, which varied in proportion to the reciprocal dose of the electron pulse. The recombination reaction with solvent cations is the most probable for the decay of the polymer anions in solutions. The second-order rate constants, k,, determined for
958 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
TABLE I: Second-Order Rate Constants of the Decay Reactions of Polymer Anions Produced by Electron-Pulse Irradiation in MTHF degree of solute Dolymerization k,/ lOlo dm3 mol-’ s-I biphenyl 16.4 11.0 4-ethylbiphenyl P4VBp 89 1 7.3 759 5.I 288 3.9 162 3.5 pyrene 20.1 PlVPy 138 7.3 91 4.0
Tanaka et al.
8
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