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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