Energy transfer, charge transfer, and proton transfer in molecular composite systems.

Models for the fundamental mechanisms of excitation energy transfer, including cases involving singlet oxygen states, twisting-intramolecular-charge-transfer (TICT) states, and intramolecular proton transfer, are described in terms of elementary concepts and energy diagrams. Three limiting cases of energy transfer are distinguished, Davydov free excitons (Simpson and Peterson strong coupling) and localized excitons (weak coupling), and the Förster mechanism of vibrational-relaxation energy transfer. The prominent rôle of the singlet molecular oxygen states is described, together with the rôle of simultaneous transitions for molecular oxygen pairs. The origin of sudden polarization via the TICT-state potential is discussed and the generality of this phenomenon emphasized. The intramolecular proton-transfer phenomenon is outlined, and its role in molecular excitation transient phenomena is described. The complex interaction of all of these excitation mechanisms in determining photochemical and radiation chemical pathways is suggested.