7042
J . Phys. Chem. 1988, 92, 7042-7043
Cage Escape Yields and Direct Observation of Intermediates in Photoinduced Electron-Transfer Reactions of cis- and trans-Stilbene Frederick D. Lewis,* Ruth E. Dykstra, Department of Chemistry, Northwestern University, Evanston. Illinois 60201
Ian R, Gould,* and Samir Farid* Corporate Research Laboratories. Eastman Kodak Company, Rochester, New York 14680 (Received: August 19, 1988)
The cation radicals of cis- and trans-stilbene have been generated in solution under conditions where they do not interconvert by means of pulsed-laser-induced electron transfer from biphenyl to singlet 9,lO-dicyanoanthracene followed by secondary electron transfer from the stilbenes to the biphenyl cation radical. The cis-stilbenecation radical is found to be configurationally stable on the microsecond time scale. Cage escape quantum yields have been measured for the photogenerated ion pairs of 9,lO-dicyanoanthracene with cis-stilbene (0.134) and trans-stilbene (0.254), and these values have been used to analyze the quantum yields for electron-transfer-induced isomerization and oxygenation of the stilbenes.
The cation radicals of cis- and trans-stilbene have been proposed as intermediates in the electron-transfer-induced one-way photoisomerization of cis-stilbene'.2 and the photooxygenation of cisand tran~-stilbene.'*~The trans-stilbene cation radical has been characterized in nanosecond laser flash photolysis experiments by means of its visible absorption4 and resonance Raman spectras and in steady-state irradiation by its ESR spectrum.6 While the cis-stilbene cation radical has been observed in frozen matrices,' it has proven to be an elusive species in fluid solution or the gas phase. Attempts at its generation have yielded the trans-stilbene cation radical or mixtures of the cis- and trans-stilbene cation radicals."I0 The formation of trans-stilbene cation radical has been attributed to unimolecular one-way thermally9 or photochemical'~*isomerization or bimolecular reactions such as reaction with neutral cis-stilbene resulting in chain isomerization1 or secondary electron-transfer quenching by trans-stilbene" We report here the generation and configurational stability of the cis-stilbene cation radical formed via indirect photoinduced electron transfer. This method has also been used to determine the cage escape efficiencies of the cis- and trans-stilbene cation radicals generated by direct electron transfer to singlet 9,lO-dicyanoanthracene. These values permit analysis of the previously reported quantum yields for cation radical isomerization and oxygenation. We recently reported a time-resolved method for obtaining the efficiencies of free radical ion formation in electron-transfer reactions in which a low concentration of a substituted stilbene was used as a secondary electron donor.'* This method can readily (1) Lewis, F. D.; Petism, J. R.; Oxman, J. D.; Nepras, M. J. J. Am. Chem. SOC.1985, 107, 203. (2) Tanimoto, Y.; Takayama, M.; Shima, S.; Itoh, M. Bull. Chem. SOC. Jpn. 1985, 58, 3641. (3) (a) Eriksen, J.; Foote, C. S.; Parker, T. C. J. Am. Chem. SOC.1977, 99,6455. (b) Eriksen, J.; Foote, C. S. J. Am. Chem. SOC.1980,102,6083. (c) Manring, L. E.; Kramer, M. K.; Foote, C. S. Tetrahedron Lett. 1984,25, 2523.
(4) Spada, L. T.; Foote, C. S. J . Am. Chem. SOC.1980, 102, 391. (5) Hub, W.; Kliiter, U.: Schneider, S.; Dorr, F.; Oxman, J. D.; Lewis, F. D. J. Phys. Chem. 1984,88, 2308. (6) Courtneidge, J. L.; Davies, A. G. Ace. Chem. Res. 1987, 20, 90. (7) (a) Shida, T.; Hamill, W. H. J. Chem. Phys. 1966, 44, 2375. (b) Suzuki, H.; Koyano, K.; Shida, T.; Kira, A. Bull. Chem. Soc. Jpn. 1982, 55, 3690. (c) Suzuki, H.; Ogawa, K.; Shida, T.; Kira, A. Bull. Chem. SOC.Jpn. 1983, 56, 66. (8) Ebbesen, T. W. J. Phys. Chem. 1988, 92, 4581. (9) Kuriyama, Y.; Arai, T.; Sakuragi, H.; Tokumaru, K. Chem. Lett. 1988, 1193. (10) Gooden, R.;Brauman, J. I. J. Am. Chem. SOC.1982, 104, 1483. (1 1) Rapid electron exchange has recently been observed between the cation radicals of cis- and rrans-1,2-bis( 1-methyl-4-pyridin0)ethylene.Ebbesen, T. W.; Akaba, R.; Tokumaru, K.; Washio, M.; Tagawa, S.;Tabata, Y . J. Am. Chem. Soc. 1988, 110, 2147.
SCHEME I 'DCA*
II\
Biphenyl
DCA'-/Biphenyl'+
-
DCA'-
+
Biphenyl" Stilbene
DCA'. + Stilbene''
DCA
be applied to characterize the cation radicals of other secondary electron donors present in low concentrations, thereby avoiding potential bimolecular reactions of the cation radical. Pulsed laser excitation of an air-saturated acetonitrile solution of 9,lO-dicyanoanthracene (DCA) containing 0.1 M biphenyl as the primary electron donor and 5 X lo4 M cis- or trans-stilbene results in the transient absorption spectra shown in Figure 1." In the absence of stilbene, the biphenyl cation radical (Ama = 640 nm) is formed with a quantum yield of ca. 0.75. Exothermic secondary electron transfer from trans-stilbene (Eox = 1.95 V) or cis-stilbene (Eox = 2.06 V) to biphenyl (Eox= 2.40 V) cation radical is complete within 0.5 ps of the exciting pulse.I4 The absorption bands obtained for the trans-stilbene (Amax = 472 nm) and cisstilbene (A, = 508 nm) cation radicals are remarkably similar in band shape and maxima to the spectra obtained by Shida and co-workers in 77 K halocarbon matrices.' Extinction coefficients of 21 400 and 59600 for the cis- and trans-stilbene cation radicals, respectively, were determined with the cation radical of tri= 668 nm, = 2 6 2 0 0 9 , generated under tolylamine (A,, (12) (a) Gould, I. R.; Ege, D.; Mattes, S. L.; Farid, S. J. Am. Chem. Soc. 1987, 109, 3794. (b) Gould, I. R.; Moser, J. E.; Ege, D.; Farid, S . J . Am. Chem. SOC.1988, 110, 1991. (1 3) Pulsed laser experiments were performed using an excimer pumped (Questek 2000) Lumonics dye laser (EPD-330) (DFS dye, 410 nm, ca. 2 mJ, ca. 15 ns) as described previously (ref 12). cis-Stilbene was purified by vacuum distillation and chromatography and was found to contain less than 0.1% trans-stilbene. The time-resolved decay of biphenyl radical cation at 640 nm was accompanied by a time-resolved growth at shorter wavelength for each stilbene with a time constant (ca. 200 ns) consistent with a rate of quenching of biphenyl radical cation of ca. 1.5 X loio M-I s-' by both stilbenes, i s . , diffusion controlled. The dicyanoanthracene radical anions which are formed are quenched by the oxygen present in solution to form superoxide which is transparent in the visible. Thus, the absorptions observed in the visible are due to the cations only (ref 4). (14) Peak potentials for irreversible oxidation in acetonitrile solution were measured vs Ag/AgI with ferrocene ( E , = 0.875 V) as a standard reference. The reported values can be converted to SCE by subtraction of 0.39 V.
0022-3654/88/2092-7042$01.50/0 0 1988 American Chemical Society
J. Phys. Chem. 1988,92, 7043-7045
I
I \
w 20 10
0 400
450
500
550
600
Wavelength (nm) Figure 1. Transient absorption spectra of cis- and tram-stilbene radical cations obtained 0.5 ps after excitation. identical conditions of secondary electron transfer, as an actinometer. Under the secondary electron-transfer conditions used to generate the stilbene cation radicals (Scheme I), their decay is mainly second order. After 10 ps, when the absorption due to the cis cation has decreased by ca. 1 order of magnitude, the spectrum is essentially unchanged, with only a slight shoulder observable at ca. 470 nm, the absorption maximum of the trans cation. Thus, we conclude that thermally activated unimolecular isomerization of the cis-stilbene cation radical does not occur on the microsecond time scale in solution. At the higher concentrations of cis-stilbene (0.05-0.1 M) used in previous attempts to generate its cation radical by photoinduced electron transfer,2s6secondary electron transfer to trans-stilbene present as an impurity or generated photochemically is most likely responsible for the observation of the trans-stilbene cation radical." Reaction of cis-stilbene cation radical and neutral to form a dimer cation radical may also contribute to the disappearance of the monomer cation radical a t higher cis-stilbene concentrations. The efficiency of reactions of photogenerated radical ions is determined by the quantum yields for separation of the initially Determined by electrochemical generation of the stable cation radical. Lenhard, J. R.,private communication. (1 5 )
7043
formed radical ion pair to form free radical ions and of their subsequent reactions. Quantum yields for cage escape1*of the stilbenes determined with DCA as the acceptor, 0.05 M stilbene as the primary donor, and 5 X lo-" M 4,4'-dimethoxystilbene as the secondary donor are 0.134 and 0.254 for cis- and trans-stilbene, respectively. Quantum yields for DCA-sensitized isomerization under conditions of steady-state irradiation are 0.32 and 0.002 for 0.05 M cis- and trans-stilbene, respectively.' Thus, isomerization efficiencies of the free radical ions are 2.4 and 0.008 for cis- and trans-stilbene, respectively. The isomerization efficiency for the cis but not the trans isomer increases with stilbene concentration, in accord with a chain mechanism. Quantum yields for benzaldehyde formation from the DCA-sensitized oxygenation of cisand trans-stilbene are 0.12 and 0.18, respectively.'J6 The higher observed quantum yield for photooxygenation of trans- vs cisstilbene is consistent with its higher cage escape efficiency. The smaller cage escape efficiency for the cis- vs trans-stilbene cation/DCA anion pair might not have been expected since cisstilbene has a higher oxidation potential than that of trans-stilbene, and it has been shown that cage escape yields for ion pairs of similar structures are higher for donors of higher oxidation potential due to the energy gap effect.I2 However, we have also shown recently that cage escape is quite sensitive to small changes in molecular structure and that lower escape yields are characteristic of molecules in which electron delocalization is smaller.12b The present results suggest that the cis-stilbene radical cation might adopt a conformation in which the charge is less delocalized due to steric crowding and that this cation is thus more "benzene like" than the less crowded trans cation. In conclusion, we have generated the cis- and trans-stilbene cation radicals in solution under conditions where they do not interconvert and have measured their cage escape efficiencies. This has permitted for the first time the correlation of cation radical structure with both cage escape and product formation efficiency. Acknowledgment. We thank A. G. Davies and T. W. Ebbesen for informing us of their results prior to publication. Work performed at Northwestern University is supported by the National Science Foundation (Grant CHE-8618994). (16) Photooxygenation quantum yields are dependent upon oxygen concentration light intensity and conversion, and these values ( I atm of 0 2 , P = 7.4 x lo8 einstein s-I) are not optimized.
An Application of Optical Waveguldes to Electrochemistry: Construction of Optical Waveguide Electrodes Kiminori Itoh* and Akira Fujishima Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo- ku, Tokyo 113, Japan (Received: July 1, 1988; In Final Form: October 13, 1988)
The optical waveguide (OWG) method was applied to electrochemical measurements for the first time. OWG electrodes were fabricated by coating conductive Sn02 onto a glass OWG. Basic OWG characteristics and optical sensitivity of the OWG electrodes were examined. Optical changes associated with redox reactions of a dye (methylene blue) adsorbed onto the electrode surface were sensitively monitored on the OWG electrodes.
Optical waveguides (OWGs) are essential components of integrated optics. An interesting feature of the OWGs is that a light wave propagating through the OWG layer is very sensitive
to optical constants of the OWGs and of the circumstance surrounding them. For instance, temperature sensors can be made with the OWGs,' and molecules a t the surfaces and/or at the
0022-3654/88/2092-7043$01 .50/0 0 1988 American Chemical Society
