Ligand photodissociation in Ru(ii)-1,4,7-triazacyclononane complexes enhances water oxidation and enables electrochemical generation of surface active species

Although some ligand transformations involved in metal complexes have been reported during water oxidation (WO) ((photo)chemical/electrochemical), such as ligand decomposition, partial oxidation, or complete dissociation, ligand photodissociation has not been reported yet. Here, we report the first example of ligand photodissociation in [Ru([9]-aneN3)(bpy)(DMSO)]²⁺,1(bpy is 2,2′-bipyridine), under the frequently used conditions for screening water oxidation catalysts, and its relation to the catalyst activity. We demonstrate that the observed photodissociation in complex1boosts its catalytic performance towards electrochemical WO and enables surface generation of electrochemically active species. Conversely, the absence of such a phenomenon in [Ru([9]-aneN3)(pic)(DMSO)]⁺,2(pic is 2-picolinate), resulted in an unreactive catalyst towards WO. DFT calculations could explain the observed difference between complexes1and2aqueous solutions towards ligand partial-dissociation when exposed to light based on the activation barrier between triplet metal-to-ligand charge transfer (³MLCT) and dissociative metal-centered (³MC) excited states as well as the stability of the³MC state. Whereas the³MLCT-³MC transition for complex2requires higher activation energy than that for complex1, the resulting³MC state is less stable than its³MLCT state. Calculations involving the ligand exchange process reveal that a water molecule can replace one of the pyridine rings with a considerably lower activation barrier in the triplet state as compared to the singlet state.