Electronic coupling in inter- and intramolecular donor-acceptor systems as revealed by their solvent-dependent charge-transfer fluorescence

The solvent dependence of the radiative-rate constant of both inter- and intramolecular donor-acceptor systems reveals the occurrence of significant intensity borrowing from one or more local transitions. This phenomenon appears to be especially pronounced in systems of the type D-3σ-A, where donor and acceptor interact via through-bond interaction over a relay of three σ bonds. Within the context of a three-state model, involving the no-bond state (DA), the charge transfer state (D+A−) and a single locally excited state [(DA)*], a simplified method is given to analyse the electronic coupling elements between these states.