Bimolecular Electron Transfer in the Marcus Inverted Region

The rates for the photoinduced bimolecular reactions of a homologous series of Ru II diimines with cytochrome (cyt) c in its oxidized and reduced forms have been measured. The electronic coupling and reorganization energy of the system have been adjusted such that the inverted region may be accessed at reasonable driving forces. The electron transfer (ET) rate constants for *Ru II diimine/Fe II cyt c reaction increase monotonically and approach the diffusion limit of 8.8 × 10 8 M - 1 s - 1 at Δ G ° = −0.7 V. At a higher driving force, which may be accessed with the powerfully oxidizing *Ru(diCF 3 -bpy) 3 2+, the rate for ET is observed to drop off. Similarly, the high driving forces achieved with *Ru II diimine/Fe III cyt c (−Δ G ≥ 1.12 V) are manifested in a decrease of the ET rate constant with increasing exergonicity. The observed ET rates for both systems are well described by a bimolecular model for ET occurring over an equilibrium distribution of reactant separation distances, each having a different formation probability and weighted by the first-order ET rate constant. The unique observation of bimolecular ET in the inverted region is not due to a peculiar reaction pathway engendered by the Ru II diimines, which react as do other small-molecule cations at the solvent-exposed edge of the heme. The inherent ET properties of cyt c engender a Marcus curve that is displaced below the diffusion limit and shifted to smaller driving forces.