Iridium(III) Polypyridine Artificial Metalloenzymes with Tunable Photophysical Properties: a New Platform for Visible Light Photocatalysis in Aqueous Solution

Visible light absorbing iridium(III) complexes have been widely employed as photocatalysts in organic synthesis. Their catalytic reactivity and selectivity are generally optimized by modifying the structures of coordinated ligands to obtain desired photophysical properties. Artificial metalloenzymes (ArMs) can combine the unique features of both metal complexes and enzymes by incorporating a cofactor within a protein scaffold, which offers another strategy to improve the performance of metal catalysts. Herein, we describe a panel of Ir(III)-ArMs constructed by covalently embedding iridium(III) polypyridyl complexes into a prolyl oligopeptidase scaffold. A series of spectroscopic methods were used to examine how properties of the resulting ArMs are influenced by structural variation of the cyclometalated ligands and the protein scaffold. Visible light photocatalysis by these hybrid catalysts was also examined, leading to the finding that they catalyze inter/intra-molecular [2+2] photocycloaddition in aqueous solution and indicating that they can serve as new bio-photocatalysts for further exploration.