J. Phys. Chem. 1992, 96, 3736-3741
3736
transfer. Our simpler model neglects any possible coherent effects. Summary
We have measured the electron-transfer rate of betaine-30 and tert-butylbetaine in various solvents, both polar and nonpolar. We find the electron-transfer rate depends on solvation dynamics for fast solvents, but does not scale with solvation dynamics. Rather, the rate remains fast even in very slowly relaxing solvents. Also, the temperature dependence of the electron-transfer rate in these very slowly relaxing solvents is small. Because the treatments of Sumi and MarcusI3 and of Jortner and Bixon16 are unable to reproduce all of the observed trends, we have introduced a hybrid model. The model includes two classical degrees of freedom: (i) an intramolecular vibrational degree of freedom of the solute with an arbitrarily fast correlation time and (ii) a solvent degree of freedom characterized by a single
relaxation time. The model also includes a single quantal degree of freedom corresponding to a high-frequency intramolecular vibrational mode of the solute. This vibrational mode serves to reduce the effective activation energy of the reaction, compared to the prediction of a purely classical model. Partitioning the classical energy into a low-frequency intramolecular degree of freedom and the solvent provides a mechanism for thermally activated electron transfer in rigid media. Acknowledgment. Support of this research by the National Science Foundation and the Office of Naval Research is gratefully acknowledged. N.E.L. was supported by ,a National Science Foundation Postdoctoral Fellowship and E.A. was supported by a Swedish Natural Science Research Council Postdoctoral Fellowship. Registry No. Betaine-30, 10081-39-7.
Electron Transfer from Ground-State Triethylamine to the Second and Lowest Excited Triplet States of Haloanthraquinones (1-Chloro, 2-Chloro, 1,5-DIchloro, 1,8-Dlchloro, and 1,8-DIbromo Compounds) in Acetonitrile at Room Temperature Studied by Picosecond and Nanosecond Laser Spectroscopy Kumao Hamanoue,* Toshihiro Nakayama, Satoshi Asada, and Kazuyasu Ibuki Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku. Kyoto 606, Japan (Received: October 9, 1991; In Final Form: January 2, 1992)
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Picosecond laser photolysis of haloanthraquinones (XAQ) at room temperature in acetonitrile-triethylamine (TEA) reveals the existence of two new absorptions (bands X and Y) which are different from the triplet-triplet (T” T2 and T’ TI) absorptions due to the second [3XAQ*(T2)]and lowest [3XAQ*(Tl)]excited triplet states of XAQ. The rise of bands X is faster than that of bands Y, which increase with accompanying decrease of T’ T I absorptions. Moreover, the intensity ratios of bands X to bands Y at 2-11sdelay increase with increasing T2 TI internal conversion times. Since nanosecond laser photolysis reveals that bands Y shift to bands X in the submicrosecond time regime with a rate matching increase of transient photocurrents and that bands X decay in the millisecond time regime following second-order reaction kinetics which yields haloanthrasemiquinoneradicals and triethylamineradical, it is concluded that bands X are the ahrptions of the free-radical anions of XAQ produced via the direct electron transfer from ground-state TEA to 3XAQ*(T2)and that bands Y are the absorptions of the exciplexes between 3XAQ*(Tl)and ground-state TEA or the ion pairs (or the contact ion pairs) between the free-radical anions of XAQ and the free-radical cation of TEA.
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Introduction In previous papers, we have reported that picosecond laser photolysis of 1,8-dichloroanthraquinone1 and 1,8-dibromoanthraquin~ne~*~ in solutions at room temperature give rise to the appearance of not only the triplet-triplet (T’ T I ) absorption bands of the lowest excited triplet states [3XAQ*(Tl)] but also the T” T2absorption bands of the second excited triplet states [3XAQ*(T,)] located below the lowest excited singlet (SI)states and that the T2 T1 internal conversion times are -700-750 ps for 1,8-dichloroanthraquinone and -70-1 10 ps for 1,8-dibromoanthraquinone. Although no clear T” T2 absorption bands have been observed for anthraquinone, l-chloroanthra-
quinone, 2-chloroanthraquinone, and 1,5-dichloroanthraquinone, analysis of the change of transient absorptions with time has led us to the conclusion that the T2states also exist below the SIstates and the T2 T1 internal conversion times are less than 70 P S . ~ In spite of these circumstances, an electron transfer from ground-state triethylamine to triplet anthraquinone and haloanthraquinones (XAQ) forming the exciplexes has been found to occur via 3XAQ*(TI)in both toluene and ethanol, and these exciplexes change to the contact ion pairs between the radical anions of XAQ and the radical cation of triethylamine, followed by the first-order proton transfer generating the anthrasemiquinone radicals and the triethylamine r a d i ~ a l ; ~neither - ~ * the free-radical
(1) Hamanoue, K.; Nakajima, K.; Kajiwara, Y.; Nakayama, T.; Teranishi, H. Chem. Phys. Lett. 1984,110, 178. Hamanoue, K.; Nakayama, T.; Shiczaki, M.; Funasaki, Y.; Nakajima, K.; Teranishi, H. J. Chem. Phys. 1986, 85, 5698. Hamanoue, K.; Yamamoto, Y.; Nakayama, T.; Teranishi, H. Physical Organic Chemistry 1986 Kobayashi, M.,Eds.; Studies in Organic Chemistry; Elsevier: Amsterdam, 1987; Vol. 31, p 235. (2) Nakayama, T.; Ito, M.; Yuhara, Y.; Ushida, K.; Hamanoue, K. UItrafast Phenomena VI, Springer Ser. Chem. Phys. 1988, 48, 489. (3) Hamanoue, K.; Nakayama, T.; Ito, M. J. Chem. Soc., Faraday Trans. 1991, 87, 3487.
(4) Nakajima, K. Master Thesis, Faculty of Engineering and Design, Kyoto Institute of Technology, 1983. ( 5 ) Hamanoue, K.; Yokoyama, K.; Kajiwara, Y.; Kimoto, M.; Nakayama, T.; Teranishi, H. Chem. Phys. Lett. 1985, 113, 207. (6) Ha,ma,noue, K.;Nakayama, T.; Yamamoto, Y.; Sawada, K.; Yuhara, Y . ;Teranishi, H. Bull. Chem. SOC.Jpn. 1988, 61, 1121. (7) Hamanoue, K.; Kimoto, M.; Kajiwara, Y.; Nakayama, T.; Teranishi, H. J. Photochem. 1985, 31, 143. (8) Hamanoue, K.; Nakayama, T.; Ibuki, K.; Otani, A. J. Chem. SOC., Faraday Trans. 1991, 87, 3731.
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Picosecond Laser Spectroscopy of Haloanthraquinones
The Journal of Physical Chemistry, Vol. 96,No. 9, 1992 3737
anions nor the exciplexes are produced via 3XAQ*(T,). For anthraquinone in acetonitrile, however, we have proposed that the free-radical anion of anthraquinone and the exciplex of triplet anthraquinone with ground-state triethylamine are produced via the T2and TI states of anthraquinone, respecti~ely.~If this proposal is correct, the yields of the free-radical anion and the exciplex should be affected by the T2 T1 internal conversion time. Thus, the present paper deals with the details of the photoinduced electron transfer from triethylamine to the 1-chloro, 2-chloro, 1,S-dichloro, 1,8-dichloro, and 1,8-dibromo compounds in acetonitrile at room temperature studied by picosecond laser photolysis, as well as the decomposition of the exciplexes into the free-radical anions of haloanthraquinones and the free-radical cation of triethylamine studied by nanosecond laser photolysis.
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Experimental Section
EP-grade 1-chloroanthraquinone (1-CAQ), 2-chloroanthraquinone (2-CAQ), and 1,s-dichloroanthraquinone(1S-DCAQ) were purchased from Wako Pure Chemical Industries, and 1,8dibromoanthraquinone (1,8-DBAQ) was synthesized from EPgrade 1,8-dichloroanthraquinone(1,8-DCAQ from Tokyo Kasei). The details of the methods of preparation of these compounds have been given in previous paper~.~JOGR-grade triethylamine (TEA, Wako) was refluxed over calcium hydride and distilled under a nitrogen atmosphere. The solvent used was spectral-grade acetonitrile (Dojin), which was dried using molecular sieves 3A (Wako) preheated in a crucible over a Bunsen burner and cooled in vacuo. All experiments were performed at room temperature; picosecond laser photolysis was carried out using the second harmonic (347.2 nm) from a mode-locked ruby laser with a mean pulse width of 30 ps and the sample solution in a cell of 2-mm path length was not degassed. The details of this laser photolysis system have been given elsewhere:" A doublebeam optical arrangement was adopted and the time-resolved absorption spectrum over a wavelength range of 200 nm was recorded using two multichannel photodiode systems (Unisoku USPS00) controlled by a personal computer (NEC 9801RA). For nanosecond laser photolysis, the sample solution in a cell of 10-mm path length was degassed by several freeze-pumpthaw cycles and the sample excitation was performed by the second harmonic (347.2 nm) from a Q-switched ruby laser with a pulse duration of 20 ns at half-maximum intensity:12 The time-resolved absorption spectrum over a wavelength range of 200 nm was recorded using a multichannel analyzer system composed of a polychromator (Unisoku M200), an image intensifier (Hamamatsu V3347U), and a linear positionsensitive detector (Unisoku USP501) controlled by the personal computer; with the image intensifier operated in gated or continuous mode, the time-resolved absorption spectrum in the nanosecond-millisecond time regime could be recorded very easily from one excitation laser shot. The decay of transient absorptions was analyzed by means of a combination of a photomultiplier (Hamamatsu R666 or RCA 8575) with a storage oscilloscope (Iwatsu TS-8123) controlled by the personal computer. For the measurements of transient photocurrents, a sample cell with two plane-parallel platinum electrodes (8 mm X 8 mm) separated by 10 mm and a fast preamplifier was constructed, and an electrode bias of 450 V was supplied by dry batteries. The voltage signal across the 50-52 load resistor was fed into the first preamplifier and displayed on the storage oscilloscope. The rise time of this preamplifier-oscillope system was ca. 10 ns. In order to find out the enhancement of photoreduction of haloanthraquinones by TEA, steady-state photolysis of the de(9) Hamanoue, K.; Nakayama, T.; Sugiura, K.; Teranishi, H.; Washio, M.; Tagawa, S.;Tabata, Y. Chem. Phys. Lett. 1985, 118, 503. (10) Hamanoue, K.;Kajiwara, Y.; Miyake, T.; Nakayama, T.; Hirase, S.; Teranishi, H. Chem. Phys. Lett. 1983, 94, 276. (1 1) Nakayama, T.; Tai, S.;Hamanoue, K.; Teranishi, H. Mem. Fuc. I d . Arts, Kyoto Tech. Univ.,Sci. Technol. 1980, 29,46. Hamanoue, K.; Hidaka, T.; Nakayama, T.; Teranishi, H. Chem. Phys. Lett. 1981,82, 5 5 . (12) Ushida, K.; Nakayama, T.; Nakazawa, T.; Hamanoue, K.; Nagamura, T.; Mugishima, A.; Sakimukai, S.Rev.Sci. Instrum. 1989, 60, 617.
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Figure 1. Time-resolved transient absorption spectra obtained by picosecond laser photolysis of I-CAQ, 2-CAQ, and 1,5-DCAQ in CH,CNTEA (1 M).
gassed sample solution in a cell of IO-" path length was performed using 313-nm monochromatic light, which was selected from an USH-SOOD super-high-pressure mercury lamp by a combination of two Toshiba color glass filters (UV-29 and UVD33S) and a filter solution (Ni$04.6H20, 50 g dm-3, path length 2 cm). The change of absorption spectra upon photolysis was recorded using a Hitachi 200-20 spectrophotometer, and the quantum yield of photoreduction was determined by measuring the decrease of reactant absorptions following the HatchardParker tris(oxalato)ferrate(III) actinometry method.I3
Results Figure 1 shows the time-resolved transient absorption spectra obtained by picosecond laser photolysis of 1-CAQ, 2-CAQ, and 1,S-DCAQ in acetonitrile containing 1 M triethylamine (TEA). Clearly, new absorptions (bands X) build up at first and then other absorptions (bands Y) increase gradually up to 2-ns delay; the overall spectral profile at this delay time looks like a superposition of bands X and Y. Although these results suggest that the precursors for the intermediates with bands X are different from those for the intermediates with bands Y, Le., the intermediates with bands X and those with bands Y are produced independently, this circumstance can be seen much clearly for 1,8-DCAQ as shown in Figure 2. At a delay time of 40 p, only the T" T, absorption (band C with ,A, = 539 nm) and the T' T, absorption (band D with ,A, = 508 nm) of 1,8-DCAQ are observed;' at 100-ps delay, however, no band C (the T" T2 absorption) can be seen that a new absorption (band X) is observed. At delay times longer than 100 ps, band D (the T' T1 absorption) decreases and another new absorption (band Y) increases accompanied by a clear isosbestic point at -539 nm. For 1,8-DBAQ, however, the spectral change is somewhat complicated? Although band E (& = 508 nm) and bands FI,2.(A,x = 485 and 536 nm) are the T" T2 and T' T1 absorptions of 1,8-DBAQ, respectively, bands X and Y appear even at a delay time of 100 ps owing to the shorter T2 T1 internal conversion time (-70-100 ps),s3 compared with that for 1,8-DCAQ (-700-750 ps).' The rise of band X at delay
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(13) Hatchard, C. G.; Parker, C. A. Proc. R. SOC.London, Ser. A 1956,
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