Infrared vibrational spectroscopy of [Ru(bpy)₂(bpm)] ²⁺ and [Ru(bpy)₃]²⁺ in the excited triplet state

This work involved a detailed investigation into the infrared vibrational spectra of ruthenium polypyridyl complexes, specifically heteroleptic [Ru(bpy)₂(bpm)]²⁺ (bpy = 2,2′-bipyridine and bpm = 2,2′-bipyrimidine) and homoleptic [Ru(bpy)₃]²⁺, in the excited triplet state. Transient spectra were acquired 500 ps after photoexcitation, corresponding to the vibrational ground state of the excited triplet state, using time-resolved infrared spectroscopy. We assigned the observed bands to specific ligands in [Ru(bpy)₂(bpm)]²⁺ based on the results of deuterium substitution and identified the corresponding normal vibrational modes using quantum-chemical calculations. Through this process, the more complex vibrational bands of [Ru(bpy)₃] ²⁺ were assigned to normal vibrational modes. The results are in good agreement with the model in which excited electrons are localized on a single ligand. We also found that the vibrational bands of both complexes associated with the ligands on which electrons are little localized appear at approximately 1317 and 1608 cm^-1. These assignments should allow the study of the reaction dynamics of various photofunctional systems including ruthenium polypyridyl complexes.