J. Phys. Chem. 1984, 88, 1865-1871
1865
Temperature Effects in Positive-Charge Transfer between Solute Molecules in Poly(methy1 methacrylate) Matrices Akira Kira* and Masashi Imamura The Institute of Physical and Chemical Research, Wako-Shi, Saitama 351, Japan (Received: August IO, 1983)
Pulse radiolysis measurements were done for positive-charge (PC) transfer from the biphenyl cation to neutral acceptors in poly(methy1 methacrylate) (PMMA) matrices at 77, 194, and 288-295 K. The compounds employed as the acceptor were pyrene, acenaphthene, phenanthrene, durene, naphthalene, triethylamine (TEA), N,N-dimethylaniline (DMA), and N,N-diethylaniline (DEA). Common kinetic features were observed in both PMMA and sec-butyl chloride matrices at 77 K with the difference that the reaction was slower in PMMA. The rate was independent of temperature when pyrene was used as an acceptor, but it increased with temperature for other hydrocarbon acceptors. The temperature dependence is stronger for an acceptor with a higher ionization potential if comparison is made among the hydrocarbon acceptors or among the amine acceptors. At room temperature, the decay is accelerated after a certain time, and the time and the extent of acceleration depend on the acceptor. These results are discussed qualitatively in terms of current electron-transfer theories with consideration of the relaxation of a PMMA matrix at a transition temperature.
Introduction
In experiments with plant tissues, temperature is practically the only factor that experimenters can vary, since other factors such as free energy and geometry are specific to a tissue. Thus, the studies on biologicai systems concentrate on temperature effects.'s2 In contrast, the free energy dependence4%* has mainly been studied in matrices of low-temperature glasses containing suitable solutes where the electron transfer occurs on irradiation with ionizing radiation. In such glassy solutions, the free energy can be controlled by the choice of both solute and matrix, but the temperature range is limited below the melting point of a matrix compound such as 2-methyltetrahydrofuran and sec-butyl chloride. The idea that the use of a polymer as a matrix would expand the temperature range has led t o the present pulse-radiolytic study using poly(methy1 methacrylate) (PMMA) matrices. PMMA was chosen because of its comparatively high glass transition temperature (120 "C), its good optical transparency, and the efficient formation of solute (radical) ions on exposure to ionizing radiation; cation formation is common to most solute compound~~ and ~ -anion ~ ~ formation occurs for solutes with high electron a f f i n i t i e ~ . ~Polyethylene ~.~~ and polystyrene are not suitable because the solute ion yields are very low in their maThe simplest charge transfer in matrix systems involves a trapped electron or a matrix hole; however, neither of these entities has been identified experimentally for PMMA, although the hole must exist as a precursor of the observed solute cation. Hence, focus was set on phenomenological PC transfers from the biphenyl cation to various organic acceptors added as the second solute. Biphenyl cations were preferentially produced in PMMA samples by matrix hole scavenging by excess biphenyl. The kinetics of the PC transfer was studied for various acceptors at 77 and 194 K and room temperature (288-295 K) by using the pulse radiolysis method. The results furnish a survey of the temperature
Intermolecular electron transfer in a broad sense including phenomenological positive-charge (PC) transfer has been observed in amorphous solids such as plant tissues',2 and low-temperature g l a ~ s e s ~where - ' ~ the diffusion of solute molecules is negligible on the time scale of the experiment. The long-range nature of this electron transfer has attracted attention since the first proposal of the naive electron-tunneling model by DeVault and Chance.' Electron- or hole-tunneling models have been applied extensively to the calculation of electron- or hole-transfer rates in glassy solutions at 77 K.3-'5 Theoretically, the electron-transfer rate is governed by both electronic interaction and vibrational overlap between the initial and final states. Current theories'6-2' mostly deal with the vibrational overlap on the basis of general electron-transfer theories, not referring to the long-range nature of the electron transfer in amorphous solids. The theories regarding the vibrational overlap predict the dependence of the rate on t e m p e r a t ~ r e , ' ~free - ~ ~en erg^,'^,'^^^' and isotope substitution.21 Not many theoretical studies have been made concerning the electronic interaction except for those based on the electron- or hole-tunneling models .22-24
DeVault, D.; Chance, B. Eiophys. J . 1966, 6, 825. Hales, B. J. Eiophys. J . 1976, 16, 471. Miller, J. R. J . Phys. Chem. 1975, 79, 1070. Miller, J. R. J . Phys. Chem. 1978, 82, 767. Beitz, J. V.; Miller, J. R. J . Chem. Phys. 1979, 71, 4579. Nosaka, Y.; Kira, A.; Imamura, M. J . Phys. Chem. 1979,83, 2273. Kira, A.; Nosaka, Y.; Imamura, M. J . Phys. Chem. 1980, 84, 1882. Kira, A. J . Phys. Chem. 1981, 85, 3047. Miller, J. R.; Beitz, J. V. J . Chem. Phys. 1981, 74, 6746. (IO) Kira, A,; Nosaka, Y.; Imamura, M.; Ichikawa, T. J . Phys. Chem. 1982, 86, 1866. (11) Teather, G. G.; Klassen, N. V. J . Phys. Chem. 1981, 85, 3044. (12) Cygler, J.; Teather, G . G.; Klassen, N. V. J . Phys. Chem. 1983,87, 455. (13) Namiki, A.; Nakashima, N.; Yoshihara, K.; Ito, Y.; Higashimura, T. J . Phys. Chem. 1978, 82, 1901. (14) Namiki, A.; Warashina, T. Chem. Phys. Lett. 1982, 85, 136. (15) Namiki, A.; Ito, Y.; Higashimura, T. Chem. Phys. Lett. 1982,88,492. (16) Hopfield, J. J. Proc. Natl. Acad. Sci. U.S.A. 1974, 71, 3640. (17) Jortner, J. J . Chem. Phys. 1976, 64, 4860. (18) Webman, I.; Kestner, N. R. J . Phys. Chem. 1979, 83, 451. (19) Sarai, A. Eiochim. Eiophys. Acta 1980, 589, 71. (20) Buhks, E.; Jortner, J. FEBS Lett. 1980, 109, 117. (21) Ulstrup, J.; Jortner, J. J . Chem. Phys. 1975, 63, 4358. (1) (2) (3) (4) (5) (6) (7) (8) (9)
0022-3654/84/2088-1865$01.50/0 , , I
(22) Brocklehurst, B. Chem. Phys. 1973, 2, 6. (23) Redi, M.; Hopfield, J. J. J . Chem. Phys. 1980, 72, 6651. (24) Doktorov, A. B.; Khairutdinov, R. F.; Zamaraev, K. I. Chem. Phys. 1981, 61, 351. (25) Borovkova, V. A.; Bagdasaryan, K. C.; Chepl, D. B.; Shemarov, F. V. Khim. Vys. Energ. 1971, 5 , 337. (26) David, C.; Janssen, P.; Geuskens, G. In?. J . Radiat. Phys. Chem. 1972, 4, 51. (27) Torikai, A.; Asai, T.; Suzuki, T.; Kuri, Z . J. Polym. Sci., Polym. Chem. Ed. 1975, 1 3 , 1 9 7 . (28) Torikai, A.; Kato, H.; Kuri, Z. J . Polym. Sci., Polym. Chem. Ed. 1976, 14, 1065. (29) Torikai, A,; Mishina, H. J . Polym. Sci., Polym. Chem. Ed. 1981, 19, 2291. (30) Ho, S. K.; Siegel, J . Chem. Phys. 1969, 50, 1142. (31) Partridge, R. H. J . Chem. Phys. 1970, 51, 2491. (32) Siegel, S.; Stewart, T.J . Chem. Phys. 1971, 55, 1775.
0 1984 American Chemical Societv -
Kira and Imamura
1866 The Journal of Physical Chemistry, Vol. 88, No. 9, 1984
5
[pyr].
0 0'
-6
I
I
I
I
-2
-4 log
I
-2
-4
-6
log
I
0
t
Figure 1. Absorbances of biphenyl cations at 680 nm in a PMMA matrix containing 0.3 M biphenyl at 77, 194, and 295 K plotted as a function of the logarithm of time after the pulse (time is measured in seconds).
--
30 rnM
0
t
Figure 2. Survival curves of biphenyl cations in PMMA matrices containing both 0.3 M biphenyl and 30 or 15 mM pyrene at 77 (0),194 ,).(
and 295 (0)K.
dependence of the PC transfer in systems with different free energies.
Experimental Section Our pulse radiolysis apparatus with the logarithmic time function has been described el~ewhere.~,'Solid samples with an optical path length of 1 cm (1 X 1 X 4 cm3) were placed in a Styrofoam cryostat6 and cooled directly by liquid nitrogen to 77 K and by dry ice to 194 K. The cryostat was also used for measurement at room temperature in order to keep the same geometry. Room temperatures were 288-290 K in early experiments done in winter and 295 K in later ones in spring. PC transfers from the biphenyl cation to several acceptors were measured at both temperatures, and no marked differences were observed within the experimental accuracy. A sample was irradiated with a 1-ps electron pulse of a 2.7 MeV-150 mA beam, which yielded an absorbance of 0.28-0.30 at 475 nm for the KCNS dosimeter (Gt = 22040)33in a 1-cm square Suprasil cell set in the cryostat. The PMMA samples were prepared by the method of thermal bulk polymerization. A methyl methacrylate monomer solution containing solute compounds and an initiator, a,a'-azobis(isobutyronitri1e) ( 3 mM) was degassed with a freeze-thaw method and then sealed off in a rectangular Pyrex cell, The solute concentration in the monomer solution was set, considering shrinkage on polymerization, 0.82 times as high as it should be in the polymer sample. The sealed solution was heated at 60 O C for 15 h, then at 70 'C for 24 h, and at 90 O C for 100 h; other conditions were also examined with respect to both the initiator concentration and the heating procedure in early experiments. The cell was broken in order to remove the sample a few hours before measurement. Aging of bare samples in the air for several days did not seem to affect markedly the phenomena of current interest. Several samples were prepared simultaneously in a set, which included a t least one reference containing only biphenyl. The biphenyl cation yields of the references of different sets did not always agree with one another even under seemingly identical preparation conditions; therefore, unknown subtle differences in preparation seem to influence the behavior of transient species. However, the yield and kinetics of the ions were reproducible between two samples of the same components within the same set. Methyl methacrylate monomer (Tokyo Kasei) was treated with sodium sulfite in the presence of sulfuric acid and then washed successively with 5% sodium hydroxide and 20% sodium chloride solutions. After this treatment, the monomer was stored on anhydrous sodium sulfate until vacuum distillation which preceded the sample preparation. Biphenyl and pyrene were twice recrystallized. Other chemicals of the best available grades including zone-refined ones (Tokyo Kasei) were used as received. (33) Jha, K. N.; Bolton, G. L.; Freeman, G . R. J. Phys. Chem. 1972, 76, 3876.
0
-6
I
1
I
-4
I
log t
-2
I
0
Figure 3. Survival curves in a PMMA matrix containing both 0.3 M biphenyl and 30 mM acenaphtheneat 77 (O),194 ( O ) , and 295 ( 0 ) K.
Results PC-transfer reactions from biphenyl (radical) cations, bip+, to various acceptor solutes S as in eq 1 were observed. Biphenyl bip+
+S
-
bip
+ S+
(1)
cations were produced by electron-pulse irradiation of a PMMA sample containing excess biphenyl and a small amount of a PC acceptor: PMMA'
+ bip
-
bip'
(2)
where PMMA+ denotes a positive hole in a PMMA matrix whose nature is unknown. The absorption spectra of both the cation34 and triplet state35of biphenyl were confirmed in the pulse radiolysis at room temperature in the absence of the acceptor; no absorption bands of the biphenyl anion were detected. These findings are consistent with the previous r e s ~ l t s . * ~ - ~ ~ The time change in the biphenyl cation absorption was measured at one of the peak wavelengths, 680 nm, where the biphenyl triplet state does not absorb. The absorbances at 680 nm of a sample containing only biphenyl are shown in Figure 1 as a function of the logarithm of time. Time t is measured from the end of the pulse in seconds: log t log(t/s). The absorbance slightly increases until t -30 ps and then decreases. The initial increase is probably due to reaction 2 involving residual holes. The reaction scheme for the subsequent decay is not elucidated yet, but charge r e c o m b i n a t i ~ n and ~ ~ - PC ~ ~ transfer to neutral radicals of PMMA fragments can be principal processes. The Gc value for the biphenyl cation at 680 nm measured at 30 ws in a PMMA glass sealed in a Suprasil cell (for the same irradiation conditions with the fluid dosimeter) was (3.3 0.3) X lo4 M-' cm-I at 288 K, and Gc at 77 K was estimated at (4.8 0.6) X lo4 M-' cm-I from the above value and a ratio of the absorbance at 77 K to the one at 288-295 K, 1.45 f 0.05. In the presence of an acceptor, the
*
(34) Shida, T.; Iwata, S . J . Am. Chem. SOC.1973, 95, 3473. (35) Porter, G.; Windsor, M. W. Proc. R . SOC.London, Ser. A 1958, 245, 238.
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Temperature Effects in Positive-Charge Transfer
The Journal of Physical Chemistry, Vol. 88, No. 9, 1984 1867 O.l
