Redox and Excited-State Modulation in Porphyrin Photosensitizers Controls H 2 Evolution at pH 7

Abstract While ligand design for photocatalytic hydrogen evolution has attracted much attention, replacing rare elements with more abundant ones in molecular photosensitizers without losing photocatalytic efficacy remains challenging. The key is to obtain appropriate ground-state redox potentials and excited-state properties. We report a series of four new tetraanionic tin(IV)-porphyrin complexes bearing electron-donating or electron-withdrawing substituents and, compared to their zinc(II) analogues, systematically probed the influence of electronic effects on photocatalytic properties. Tin(IV) complexes proved more electron-poor than their Zn(II) analogues, resulting in distinct photocatalytic behavior. Under green light irradiation in pH 7.0 water/acetonitrile mixtures with a cobaloxime catalyst and a triethanolamine donor, the electron-rich octamethyl-substituted photosensitizer [Sn-MeP]4– achieved the highest photocatalytic turnover number of 430 molH2/molPS after 20-h irradiation and a maximum photocatalytic turnover frequency of 35 h–1 in the Sn series. Conversely, under identical conditions, the electron-poorest zinc(II) complex [Zn–F16P]4– yielded the best performance (photocatalytic turnover number of 850 and photocatalytic turnover frequency of 59 h–1). Notably, both series in this study operated via a reductive quenching pathway. This work establishes critical structure–property relationships and provides new insights into tuning excited-state redox chemistry in molecular photocatalytic systems based on earth-abundant elements.