Rate expressions for excitation transfer. IV. Energy migration and superexchange phenomena

General microscopic mechanisms of electronic excitation (energy) transfer (EET) in multichromophoric assemblies are investigated. Aspects of superexchange-mediated EET and energy migration (EM) and their contribution to the efficiency of donor-to-trap energy transport processes in macromolecules are discussed from a quantum mechanical viewpoint. The possibility of superexchange pathways for EM via higher excited states of the intermediate chromophores is introduced. The role of quasicoherent EM pathways, and how they are manifested in the quantum mechanical rate expression, is investigated and the significance of contributions to the rate arising through quantum mechanical interference between pathways is elucidated. The theory indicates conditions under which coherent EM pathways may significantly increase the efficiency of energy transport and trapping and the applications to natural and synthetic light-harvesting systems are outlined.