Direct Observation of the Triplet Metal-Centered State in [Ru(bpy)₃]²⁺ Using Time-Resolved Infrared Spectroscopy

[Ru(bpy)₃]²⁺ is well-known as a prototype for the Ru(II) complexes used in a wide variety of photofunctional materials. The triplet metal-centered (³MC) state is important in this complex, since it dominates the phosphorescence lifetime and photoreaction processes. Despite this, the ³MC state has not yet been observed by spectroscopic methods. In the present study, we demonstrated that time-resolved infrared vibrational spectroscopy enables observations of the ³MC state. A vibrational band at 1599 cm⁻¹ was found to exhibit unique temporal behavior that differed from that of other bands assignable to the triplet metal-to-ligand charge-transfer (³MLCT) state. This unique behavior was assessed under various experimental conditions and it was concluded that the band arises from the short-term population (∼23 ps) of the ³MC state during relaxation to the bottom of the ³MLCT state. These results agree with [Fe(bpy)₃]²⁺ spectra, which show that the ⁵MC state is the most stable excited state.