[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.
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