Photocatalytic Hydrogen Evolution with a Self-Assembling Reductant-Sensitizer-Catalyst System

Abstract: A noble-metal-free system for photochemical hydrogen produc- tion is described, based on ascorbic acid as sacrificial donor, aluminium pyridyl porphyrin as photosensitizer, and cobaloxime as catalyst. Although the aluminium porphyrin platform has docking sites for both the sacrificial donor and the catalyst, the resulting as- sociated species are essentially inactive because of fast unimolecular reversible electron-transfer quenching. Rather, the photochemically active species is the fraction of sensitizer present, in the aqueous/organic solvent used for hy- drogen evolution, as free species. As shown by nanosecond laser flash pho- tolysis experiments, its long-lived trip- let state reacts bimolecularly with the ascorbate donor, and the reduced sen- sitizer thus formed, subsequently reacts with the cobaloxime catalyst, thereby triggering the hydrogen evolution proc- ess. The performance is good, particu- larly in terms of turnover frequencies (TOF=10.8 or 3.6 min 1 , relative to the sensitizer or the catalyst, respec- tively) and the quantum yield (F= 4.6%, that is, 9.2% of maximum possi- ble value). At high sacrificial donor concentration, the maximum turnover number (TON=352 or 117, relative to the sensitizer or the catalyst, respec- tively) is eventually limited by hydro- genation of both sensitizer (chlorin for- mation) and catalyst.