Superexchange and Sequential Mechanisms in Charge Transfer with a Mediating State between the Donor and Acceptor

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Abstract
The rate of intramolecular charge transfer from biphenyl to naphthalene was determined for the radical anions and radical cations of molecules with the general structure: (2-naphthyl)-(steroid spacer)-(4-biphenylyl). Varied degrees of unsaturation (one double bond, NSenB; two double bonds, NSen2B; and the b-ring completely aromatized, NSarB) were incorporated into the steroid spacer to examine the effect it would have on the charge transfer rate. The charge transfer rate, as inferred from the decay of the biphenyl radical ion absorption, increased in all cases relative to the completely saturated 3-(2-naphthyl)-16-(4-biphenylyl)-5α-androstane (NSB) reference molecule. For the anion charge transfer, the decay rates increased by factors of 1.4, 4.2, and 5.1, respectively, and for the cation, the decay rates increased by factors of 5, 276, and 470. To explain the results, the charge-transfer process was viewed as a combination of two independent mechanisms: a single-step, superexchange mechanism, and a two-step, sequential charge transfer. Using a low level of theory, simple models of the superexchange and two-step mechanisms were developed to elucidate the nature and differences between the two mechanisms. The critical variable for this analysis is the free energy of formation (ΔGI°) of the intermediate state: (2-naphthyl)-[spacer]1±-(4-biphenylyl). The conclusion from this treatment is that superexchange is the dominant mechanism when ΔGI° is large, but at small ΔGI°, the sequential mechanism will dominate. This is because the superexchange rate is shown to have a weak dependence on ΔGI°, changing 10-fold for a change in ΔGI° of 2 eV, compared to the sequential mechanism in which the rate can change over 103 for 0.5 V.