Direct Observation of Temperature-Dependent Excited-State Equilibrium in Dinuclear Ruthenium Terpyridine Complexes Bearing Electron-Poor Bridging Ligands

Dinuclear ruthenium–terpyridine complexes, bearing highly conjugated bridging ligands, have been investigated by time- and temperature-dependent emission spectroscopy, femtosecond time-resolved transient-absorption spectroscopy, and time-dependent density functional theory calculations. These terpyridine compounds show room-temperature emission around 640 nm and lifetimes between 140 and 230 ns. Reduction of the temperature results in an overall increase of emission lifetime. However, the specific temperature dependence of the luminescence lifetime depends on the particular bridging ligand. This ligand-specific behavior is found to correlate with the electronic structure of the ligand, which indicates an excited-state equilibrium between a highly delocalized 3MLCT and a ligand-centered 3ππ* excited-state. Due to this equilibrium a prolonged room-temperature lifetime is observed.