Investigation of Vibrational Cooling in a Photoexcited Dichloro-Ruthenium Charge Transfer Complex Using Transient Electronic Absorption Spectroscopy

Vibrational cooling of molecules in excited electronic states is ubiquitous in photochemical reactions in solution but challenging to infer in time-resolved electronic absorption experiments. We report the ultrafast photophysics of cis-dichlorobis(2,2′-bipyridine)ruthenium(II), Ru(bpy)₂Cl₂, a precursor molecule commonly utilized in synthetic modifications of a vast array of ruthenium complexes. Femtosecond time-resolved electronic absorption spectroscopy is used to track an ultrafast spectral narrowing of the excited-state absorptions at 475 nm (21,050 cm^–1) and 505 nm (19,800 cm^–1) due to the reduced ligand in the photoexcited molecular complex. These sharp features, which overlap with a broader ground-state bleach spanning 450 nm (22,220 cm^–1) to 600 nm (16,670 cm^–1), evolve rapidly with time constants of 16 ± 5, 15 ± 3, and 18 ± 2 ps, respectively, for ligand-centered (π → π*, 266 nm) and charge-transfer (t₂ → π*, 400 and 550 nm) excitations and constitute a direct signature of picosecond vibrational cooling.