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 ((MC)-M-3) state is important in this complex, since it dominates the phosphorescence lifetime and photoreaction processes. Despite this, the (MC)-M-3 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 (MC)-M-3 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 ((MLCT)-M-3) state. This unique behavior was assessed under various experimental conditions and it was concluded that the band arises from the short-term population (similar to 23 ps) of the (MC)-M-3 state during relaxation to the bottom of the (MLCT)-M-3 state. These results agree with [Fe(bpy)(3)](2+) spectra, which show that the (MC)-M-5 state is the most stable excited state.