Kinetics of return intersystem electron transfer in triplet radical ion pairs in solution and on silica. Surface effect on bell-shaped energy-gap dependence

Abstract

The decay kinetics of triplet radical pairs (RIPs) formed by electron transfer (ET) from aromatic amines to quinones in the triplet excited state in eight different solvents and absorbed onto porous surface of silica has been studied, using the laser flash technique with transmittance and diffuse reflectance set-up. The dependence of rates (1 μs–20 ns) of intersystem return ET within RIPs on the RIP free energy is bell-shaped on the surface as well as in homogeneous solutions, even in non-polar solvents. It is well described in terms of the simplest single-quantum-mode approximation within the non-adiabatic ET theory. The reorganization energy of the environment increases from 0.1 to 0.7 eV on going from hexane to octanol and up to 0.9 eV on the surface. The shift of the maximum of the bell-shaped dependence to higher energy is accompanied by the narrowing of its width. The role of the RIP conformation and the interactions between radical ions within the RIP as well as with the surroundings, and those of the diffusion and of other internal and external factors, are discussed.