3460
J. Am. Chem. SOC. 1994,116, 3460-3464
Nonvertical Triplet Excitation Transfer to Arylalkene Acceptors: Further Evidence That Double Bond Torsion Is Unimportant C. M. Brennan,? R. A. Caldwell,’ J. E. Elbert,* and D. J. Unett Contribution from the Department of Chemistry, The University of Texas a t Dallas, Richardson, Texas 75083 Received November 26, 1993”
Abstract: Time-resolved kinetic absorption spectrophotometry and photoacoustic calorimetry have been employed in a study of the kinetics and energetics of triplet excitation transfer to a series of arylalkene acceptors. Both the cis and trans isomers of @-methylstyreneare able to achieve significant relaxation in the triplet state. Nonvertical behavior is observed only for the cis isomer, for which single-bond torsions are expected to contribute to the measured relaxation. A comparison of the behavior of tetraphenylallene and its model, 1,l-diphenylethylene, is also revealing. Relaxation through double bond torsion in the allene triplet state leads to an “allyl-vinyl” biradical of particularly low energy. Measured energy transfer rate constants for the two molecules display a close correspondence, These results provide further compelling evidence for a recently proposed model for the nonvertical excitation transfer process dominated by single bond torsional modes. Introduction In nonvertical triplet excitation transfer (NVT), spectroscopic energies of donor and acceptor triplets predict an endothermic process, but the rate constant for excitation transfer is enhanced relative to that to beexpected were thespectroscopic energy deficit simply to be made up by thermal activation. The initial proposals1-3 for the process were intended to explain the rate enhancements. They postulated that a geometrical change in the acceptor triplet toward some conformation at which its energy was relaxed, i.e. lower than the known spectroscopic energy, occurred during the excitation transfer. For cis-stilbene (c-S), the motion suggested was torsion of the double bond toward a perpendicular geometry, in line with the long-understood torsional relaxation of the ethylene triplet. Subsequent discussions of the origins of the NVT process, as it pertains to flexible ?r-systems, have focused extensively on the details of the mode of access to these perpendicular triplets.4-11 Recently Gorman et al.12-l4 have offered a different intermetation. Thev noted that all acceDtors for which NVT was ~
Present address: I.C.I. Specialties, Blackley, Manchester, U.K. t Present address: Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. Abstract published in Aduance ACS Absrracrs, March 15, 1994. (1) Hammond, G. S.;Saltiel, J. J . Am. Chem. Soc. 1963, 85, 2516. (2) Hammond, G. S.;Saltiel, J.; Lamola, A. A.; Turro, N. J.; Bradshaw, J. S.;Cowan, D. 0.;Counsell, R. C.; Vogt, V.; Dalton, J. C. J . Am. Chem. f
SOC.1964,86, 3197. (3) Herkstroeter, W. G.; Hammond, G. S.J . Am. Chem. SOC.1966,88, 4769. (4) Saltiel, J.; Charlton, J. L.; Mueller, W. B. J . Am. Chem. SOC.1979, 101, 1347. (5) Balzani, V.; Bolletta, F.; Scandola, F. J. Am. Chem. Soc. 1980, 102, 2152. (6) Orlandi, G.; Monti, S.; Barigelletti, F.; Balzani, V. Chem. Phys. 1980, 52, 313. (7) Saltiel, J.; Marchand, G. R.; Kirkor-Kaminska, E.; Smothers, W. K.; Mueller, W. B.; Charlton, J. L. J. Am. Chem. SOC.1984, 106, 3144. (8) Ramamurthy, V.; Liu, R. S.H. J . Am. Chem. Soc. 1976, 98, 2935. (9) Wagner, P. J.; Scheve, J. J. Am. Chem. SOC.1977,99, 2888. (10) Lamola, A. A. Energy Transfer and Organic Chemistry; Interscience: New York, 1969. (1 1) Saltiel, J.; D’Agostino, J. D.; Megarity, E. D.; Metts, L.; Neuberger, K.; Wrighton, M. S.;Zafiriou, 0. C. Org. Phorochem. 1973, 3, 1. (12) Gorman, A. A.; Beddoes, R.; Hamblett, I.; McNeeney, S.P.; Prescott, A. L.; Unett, D. J. J. Chem. Soc., Chem. Commun. 1991, 963-964. (13) Gorman,A.A.; Hamblett,I.;Rushton, F.A. P.;Unett,D. J. J . Chem. SOC.,Chem. Commun. 1993,983-984. (14) Forward, P. J.; Gorman, A. A.; Hamblett, I. J. Chem. SOC.,Chem. Commun. 1993, 250-251.
claimed possessed significantly nonplanar mystems in the ground state as a result of steric crowding.15-17 The exception, transstilbene,4-7is discussed below. The strain induced by this crowding is relieved primarily through single bond torsions (phenyl-vinyl torsion in cis-stilbene). Low-frequency oscillations about these single bonds a t ambient temperatures enable these systems to explore more nearly planar ground state conformations which are expected to possess reduced spectroscopic triplet energies as a consequence of excited state stabilization. This interpretation therefore outlines a simple mode by which these systems achieve access to a lower entry point on the triplet surface and the rate enhancements characteristic of NVT. Taking c-S as a case in point, Gorman et showed that its analog 2,3-diphenylnorbornene (DPN), differing from c-S in being incapable of significant double bond twisting but like c-S in being able to stabilize in the triplet state by phenyl-vinyl single bond torsion, afforded a near-exact correspondence with c-S in rate constants of excitation transfer from standard triplet donors. Subsequent workls showed that relaxed DPN triplet was some 10 kcal mol-’ lower than the spectroscopic DPN triplet state, the relaxation being assigned primarily to phenyl-vinyl torsion. This paper reports further studies of excitation transfer kinetics to arylalkene acceptors in which the fully relaxed triplet state of the acceptor differs substantially in geometry from the spectroscopic state. We have sought a more rigorous test of the lack of importance of double bond torsion in assisting NVT to alkenes. Of great interest as an acceptor in this connection is trans-@methylstyrene (t-MS), expected to be planar in the ground state yet known to be perpendicular a t the double bond in the triplet state.19.20 It possesses the capability to relax in the triplet from some 60.5 (spectroscopic) to 53.2 kcal mol-I by double bond torsion, but it has no single bond torsion in the ground state able to assist NVT. We find that t-MS behaves as a classical vertical acceptor whereas cis-b-methylstyrene (c-MS), a nonplanar (15) Gorman,A. A.; Hamblett, I.; Jensen, N.-H. Chem. Phys. Lerr. 1984, -/ -I _1 291 (16) Gorman, A. A.; Hamblett, I.; Irvine, M.; Raby, P.; Standen, M. C.; Yeates, S . J. Am. Chem. SOC.1985, 107, 4404. (17) Gorman, A. A.; Lambert, C.; Prescott, A. L. Photochem. Phorobiol.
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1990, 51, 29. (18) Caldwell, R. A.; Riley, S.J.; Gorman, A. A,; McNeeney, S.P.; Unett, D. J. J . Am. Chem. SOC.1992, 114,4424. (19) Caldwell, R. A.; Cao, C. V. J . Am. Chem. SOC.1982, 104. 6174. (20) Bonneau, R.; Herran, B. Loser Chem. 1983, 4, 151-70.
OOO2-7863/94/1516-3460$04.50/00 1994 American Chemical Society
Triplet Excitation Transfer to Arylalkene Acceptors
