Effect of delocalization and rigidity in the acceptor ligand on MLCT excited-state decay

In its most simple form, the energy gap law for excited-state nonradiative decay predicts a linear dependence of In k(nr) on the ground- to excited-state energy gap, where k(nr) is the rate constant for nonradiative decay. At this level of approximation, the energy gap law has been successfully applied to nonradiative decay in a wide array of MLCT excited states of polypyridyl complexes of Re-I, Ru-II, and Os-II. This relationship also predicts a dependence of k(nr) on the structural characteristics of the acceptor ligand. We report here a brief survey of the literature which suggests that such effects exist and have their origin in the extent of delocalization of the excited electron in the ligand pi* framework and on acceptor ligand rigidity.