RADIATIVE PROPERTIES AND CHARGE-TRANSFER CHARACTER OF THE PHOSPHORESCENT TRIPLET-STATE IN MIXED RHODIUM CHELATES

The kinetics of the lowest excited 3pi-pi* states of the complex cations [Rh(thpy)2(bpy)]+ and [Rh(thpy)-(phpy)(bpy)]+ (thpy = 2,2'-thienypyridine, phpy = 2-phenylpyridine, bpy = bipyridine), co-doped in a single crystal of [Rh(phpy)2(bpy)]PF6, has been studied by means of optically detected microwave recovery and adiabatic rapid passage experiments. The population decay of the ligand-localized triplet-state sublevels in the complex cations is found to be 3 orders of magnitude faster than in the free ligand molecules. The decay of the 3pi-pi* states of the complex cations is largely radiative in nature. The enhanced radiative decay is discussed to arise from selective spin-orbit coupling-induced mixing with 1d-pi* charge-transfer configurations. In these 1d-pi* configurations, metal ion d-orbitals forming a molecular orbital with ligand orbitals via a-bonding are of particular relevance for explaining the differences among the emissive properties of the triplet-state sublevels as determined experimentally.