The photophysical behavior of d 6 complexes having nearly isoenergetic MLCT and ligand localized excited states

Abstract

A large number of second- and third-row d 6 transition metal complexes with N-heterocyclic ligands have been prepared in which the lowest energy excited state is a metal-to-ligand charge transfer (MLCT) state. In some of these complexes ligand localized (IL) excited states exist which are of the same spin multiplicity as the MLCT state and are energetically accessible from the 1MLCT state. Several complexes have been examined which exhibit luminescence from two states in frozen matrices. In fluid solutions, however, only a few examples exist of complexes having two non-equilibrated triplet excited states. This article provides a brief overview of the experimental methods commonly used in the analysis of triplet excited state formation and relaxation in transition metal complexes having coexisting triplet excited states. In addition, descriptive examples are given of complexes for which both 3IL and 3MLCT states can be populated following excitation into the 1MLCT state.