Recent progress in ligand photorelease reaction mechanisms: Theoretical insights focusing on Ru(II) ³ MC states

The elucidation of reaction mechanisms taking place in the excited state is a current challenge for experimental and theoretical chemists. Ru(II) complexes have a long history for photophysics, and there is currently an increasing interest in their photochemistry. Ru(II) complexes provide a vast field of exploration, whether for synthetic purposes, to trigger molecular motions or to release biologically active components. The excited states involved, especially those of MLCT and MC character, are key to the rationalization of their photophysical and photochemical properties. This review focuses on the recent progress in the latter field through several case studies: i) the archetypes [Ru(bpy)3]2+ and [Ru(tpy)2]2+ first serve for the validation of the theoretical methods we are using; ii) then the study of photorelease of a monodentate ligand provides us with novel mechanistic hypotheses; iii) one step further, studying the photorelease mechanism of a bidentate ligand provides us with novel 3MC states of peculiar flattened geometry; iv) finally, returning to [Ru(bpy)3]2+ itself, we will show that the existence of these states can be generalized and probably represent a major player in the description of photoreactivity mechanisms, for ruthenium and possibly several other transition metals.