X-ray transient absorption structural characterization of the 3MLCT triplet excited state of cis-[Ru(bpy)2(py)2]2+

The excited state dynamics and structure of the photochemically active complex cis-[Ru(bpy)2(py)2]2+ have been investigated using optical transient absorption (OTA) and X-ray transient absorption (XTA) spectroscopy, and density functional theory (DFT). Upon light-excitation in aqueous solution cis-[Ru(bpy)2(py)2]2+ undergoes ultrafast dissociation of one pyridine ligand to form cis-[Ru(bpy)2(py)(H2O)]2+. OTA measurements highlighted the presence of two major time components of 1700 ps and 130 ps through which the system decays to the ground-state and evolves towards the photoproduct. XTA data were acquired after 150 ps, 500 ps, and 3000 ps from laser excitation (λexc = 351 nm) and provided the transient structure of the 3MLCT state corresponding to the longer time component in the OTA experi ment. In excellent agreement with DFT, XTA shows that the 3MLCT geometry is characterized by an elongation of the dissociating Ru–N(py) bond and a shortening of the trans Ru–N(bpy) bond with respect to the ground state. Conversely, calculations show that the 3MC state has a highly distorted structure with Ru–N(py) bonds between 2.77–3.05 Å.