Ultrafast excited state dynamics of [Cr(CO)₄(bpy)]: Revealing the relaxation between triplet charge-transfer states

Ultrafast excited state dynamics of [Cr(CO)₄(bpy)] upon metal-to-ligand charge-transfer (¹MLCT) transition have been studied by pump-probe absorption spectroscopy in CH₃CN, pyridine and CH₂Cl₂ solvents. Intersystem crossing (ISC) was found to be very fast (∼100 fs) and efficient, while the formation of the photoproduct with one axial CO dissociated is significantly less competitive, indicating a barrier along the dissociative coordinate. As a refinement of the previous dynamic model [I. R. Farrell, et al., J. Am. Chem. Soc., 1999, 121, 5296-5301], we show that a conventional downhill energy relaxation concept dominates the observed dynamics. Experimentally, we have identified the consecutive population of two triplet states as a result of triplet electronic relaxation convoluted with vibrational and solvent relaxation (the overall time is 2.7-6.9 ps depending on solvent), as well as the overall depopulation of the excited state through the lowest triplet state (57-84 ps). Adaptive excitation pulse shaping could not achieve optimization of the photoproduct quantum yield via re-distribution of only low-frequency vibrational modes during excitation, indicating that the two low-lying ¹MLCT states, Cr(3d) → π∗bpy and Cr(3d) → π∗CO, are not coupled.