Electronic Inefficiencies in Inter- and Intra-Molecular Energy and Electron Transfer

Abstract

The efficiency of exciton or charge migration from the center of photon absorption to a neighboring reaction center depends on the product of nuclear and electronic retardation factors Knu and Kel, respectively. Some relatively recent energy (ENT) and electron (ELT) transfer studies have delineated important features of the often elusive electronic factors. It has been shown that ENT and ELT processes can be electronically inefficient in bimolecular reactions of simple transition metal complexes. In such situations, superexchange couplings of the reaction partners with CT-donor or -acceptor species in the condensed phase matrix can lead to appreciable increases in the reaction efficiencies. In a simple treatment applicable in the surface crossing limit, K el 1/2 ≅ (JRP/ERP) exp (−αrDA) and JRP ≅ H RP ° + (HXCT)2 ∑(ECT)−1, values of the inferred transition state superexchange coupling parameters span a range 0.2 ≤ HXCT/eV ≤ 2.5, comparable to those found for donor-acceptor couplings in charge transfer complexes. Near resonance transfer may be important when reactants and products have the same nuclear coordinates. Some preliminary observations are presented regarding photoinduced ENT and ELT in covalently linked (bpy)2RuII[(CN)MIII (NH3)5] 2 6+ complexes.