Redox-active ligands in artificial photosynthesis: a review

Abstract Given the rising socioeconomic issues of fossil fuels, efficient artificial photosynthesis would be an important milestone toward a sustainable world. A key step of photosynthesis is the catalytic photooxidation of water by photosystem II, which has a mean lifetime of 30 min under full sunlight. Since the efficiency of photosystem II is controlled by redox-active tyrosine–histidine pairs that regulate the light-induced flow of charges, research has recently focused on the utilization of redox-active ligands in artificial systems. Here we review the molecular catalysis of water oxidation with emphasis on redox cooperation modes between ligands and metal centers. Molecular systems involving redox-active ligands could achieve up to 100% efficiency with respect to oxygen production, overpotential of 200–300 mV and turnover frequency above 100 s −1 , which is comparable to the natural process. Nonetheless, molecular catalysts are often prone to degradation of the organic ligand. The oxidative activation of ligands can contribute to the water oxidation reactivity of a metal–ligand complex, or lead to controlled catalyst film formation. We discuss the design of functional analogs to the tyrosine–histidine pair that for the most part rely on abundant elements and exploit redox-active molecular moieties to assist the catalytic centers. We highlight analogies with the cooperation between the natural oxygen-evolving complex and the redox-active tyrosine–histidine pairs found in photosystem II.