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

Abstract

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 (λ_exc = 351 nm) and provided the transient structure of the ³MLCT state corresponding to the longer time component in the OTA experiment. 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.