Viologen-Radical-Driven Hydrogen Evolution from Water Catalyzed by Co-NHC Catalysts: Radical Scavenging by Nitrate and Volmer-Heyrovsky-like CPET Pathway.

The factors controlling the catalytic activity in photochemical hydrogen evolution reaction (HER) are studied in detail for two macrocyclic cobalt compounds bearing two N-heterocyclic carbenes and two pyridyl donors ( and , where has a methoxy substituent on each pyridyl ligand). The present study adopts an aqueous photosystem consisting of EDTA, [Ru(bpy)] (bpy = 2,2'-bipyridine), and MV (MV = methylviologen) at pH = 5. Both catalysts are shown to promote HER in a similar efficiency (TON = 12-13 in 6 h), revealing a minor contribution of the electron-donating methoxy substituents.

Redox tuning in Pt(bpy)-viologen catalyst-acceptor dyads enabling photocatalytic hydrogen evolution from water.

A Pt(ii)-based photo-hydrogen-evolving molecular device tethered to dimethyl-substituted viologens (Pt(bpy)(dmMV)), providing higher driving force for hydrogen evolution reaction (HER) than the non-methylated analogue (Pt(bpy)(MV)), is found to exhibit improved photocatalytic performance. The observed behaviors are explained by the multiple HER pathways taken by evolving H by the doubly and triply reduced species generated via consecutive photo-driven steps. Although the activity is still killed by the Dexter-type energy transfer quenching, our results provide new design strategies towards the development of more finely tuned molecular devices controlling the photocatalytic HER.