Excited-State Dynamics of [Ru(^S-Sbpy)(bpy)₂]²⁺ to Form Long-Lived Localized Triplet States

The novel photosensitizer [Ru(^S-Sbpy)(bpy)₂]²⁺ harbors two distinct sets of excited states in the UV/Vis region of the absorption spectrum located on either bpy or ^S-Sbpy ligands. Here, we address the question of whether following excitation into these two types of states could lead to the formation of different long-lived excited states from where energy transfer to a reactive species could occur. Femtosecond transient absorption spectroscopy identifies the formation of the final state within 80 fs for both excitation wavelengths. The recorded spectra hint at very similar dynamics following excitation toward either the parent or sulfur-decorated bpy ligands, indicating ultrafast interconversion into a unique excited-state species regardless of the initial state. Non-adiabatic surface hopping dynamics simulations show that ultrafast spin-orbit-mediated mixing of the states within less than 50 fs strongly increases the localization of the excited electron at the ^S-Sbpy ligand. Extensive structural relaxation within this sulfurated ligand is possible, via S-S bond cleavage that results in triplet state energies that are lower than those in the analogue [Ru(bpy)₃]²⁺. This structural relaxation upon localization of the charge on ^S-Sbpy is found to be the reason for the formation of a single long-lived species independent of the excitation wavelength.