Saltiel, J; Marchand, GR; Kirkor-Kaminska, E; Smothers, WK; Mueller, WB; Charlton, JL
Rate constants, kobsd, for triplet excitation transfer from anthracene (ET = 42.7 keal/mol) and from 9,10-dichloroanthracene (ET = 40.3 keal/mol) to the stilbenes have been measured as a function of temperature in toluene. These are converted to rate constants, ken, for the excitation transfer step within the donor/acceptor encounter cage by using empirical values for the rate constants of diffusion, kdif, and the rate constants of back energy transfer, k-en, from stilbene triplets to the donors. Activation parameters for ken are as follows: for trans-stilbene ÎSâ =-12.3 ± 0.1 and-20.5 ± 0.6 eu and ÎHâ = 5.3 ± 0.1 and 5.2 ± 0.5 keal/mol for anthracene and 9,10-dichloroanthracene, respectively; and for czs-stilbene ÎSâ =-16.3 ± 0.1 and-22.49 ± 0.53 eu and ÎHâ = 3.95 ± 0.08 and 4.30 ± 0.22 keal/mol, in the same order. The striking result is that the difference in triplet energy between the two donors is reflected almost entirely in the activation entropy and not in the activation enthalpy in sharp contrast to predictions from recent classical and quantum mechanical treatments. This result, together with the observation that all ÎSâ values are smaller than expected for vertical triplet excitation transfer, strongly suggests that endothermic triplet excitation transfer is nonvertical to both stilbene isomers. It follows that twisted stilbene triplet conformations, 3pâ, must be lower in potential energy than either cisoid, 3câ, or transoid, 3tâ, conformations. The same conclusions can be reached somewhat more crudely by a consideration of Arrhenius parameters obtained directly from the temperature dependence of kobsd. Modified potential energy curves for twisting about the central bond of stilbene in S0 and T1 are proposed. © 1984, American Chemical Society. All rights reserved.