The temperature dependence of electron transfer over a wide range of driving forces extending from far in the "normal" to deep into the "inverted" region is studied with the aid of a limited number of closely related and fully rigid bridged donor-acceptor systems. The interpretation of temperature-dependent electron-transfer kinetics is shown to be complicated by the relatively large influence of the temperature dependence of the solvent dielectric properties. This problem becomes especially evident if the barrier is small, when it may lead to an overall nullification or even inversion of the temperature effect on the experimental rate. Upon correction for the temperature dependence of the solvent properties, the rate of electron transfer is found to be independent of temperature in the inverted region, where nuclear tunneling becomes dominant, in contradiction to expectations based on the classical Marcus treatment which, however, is shown to be capable of giving a qualitative description of the temperature dependence in the normal region, even under close to "optimal" conditions.
The dipolar transients formed on photoexcitation of a series of molecular assemblies consisting of a dimethoxynaphthalene donor and a dicyanoethylene acceptor separated by rigid, nonconjugated hydrocarbon bridges have been investigated by using time-resolved microwave conductivity (TRMC). The edge-to-edge donor to acceptor separation, Rt, is varied from 4.6 to 13.5 A in steps of approximately 2.3 A. The dipole moments of the corresponding excited-state intermediates increase from 26 to 77 D. The lifetimes of the charge-separated states increase markedly with increasing separation, from 1.0 to 740 ns in benzene. The lifetimes in p-dioxane are in general approximately an order of magnitude shorter than in benzene. The lifetimes in cyclohexane are approximately an order of magnitude longer than in benzene for the two shortest compounds. The lifetime toward direct charge recombination to the ground state is quite well described by an exponential dependence on Rj, i.e. tcr = A exp(Rt/a) with a = 1.13 A for all solvents and A = 1.3 X 10-10, 1.3 X 10-11, and 1.1 X 10-12 s for cyclohexane, benzene, and dioxane, respectively. For R, = 9.4 A in cyclohexane and 13.5 A in benzene back electron transfer resulting in indirect charge recombination via the local excited donor state begins to dominate the overall decay kinetics.
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