Long-range photoinduced electron transfer dynamics in rigid media

Abstract

In semi-rigid PEG-DMA550 films with added reductive quenchers, electron transfer quenching of the metal-to-ligand charge transfer excited state(s) of [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) occurs by both rapid, fixed-site, and slow, diffusional, quenching processes. Stern–Volmer analysis of diffusional quenching reveals diffusion-controlled quenching both in the fluid and film with the latter greatly inhibited by the high viscosity of the medium. The data for fixed-site quenching are consistent with electron tunneling with the expected exponential distance dependence. Based on this analysis long-range electron transfer occurs with a distance attenuation factor β of ∼0.47 Å⁻¹ with a notable decrease, β = 0.16 Å⁻¹, when the quencher is incorporated into the PEG backbone. Fixed-site electron transfer quenching varies with driving force. Back electron transfer is complex, as expected for a distribution of fixed sites, and varies with power law kinetics.