Proton-Coupled Electron Transfer on Mixed-Valent Copper Ions Containing S- and N‑Rich Triazine-Dimercapto Thiadiazole Coordination Polymer Photocathodes

Visible light-active semiconductor photoelectrocatalytic interfaces that are stable in protic electrolytes are essential for photoassisted multiproton-coupled electron transfer in electro-synthetic processes. Herein, we report a photocathode material with nitrogen- and sulfur-rich functional ligands and a mixed-valent copper coordination polymeric structure. A simple strategy of nucleophilic substitution reaction between triazine trichloride and 2,5-dimercapto-1,3,5-thiadiazole followed by coordination with mixed-valent copper ions is used to prepare the metallopolymer. Spectral analysis evidences the systematic bathochromic shift toward the midvisible region with the increase in the extent of polymerization and increasing coordination of mixed-valent copper ions. This p-type photoactive metallopolymer exhibits an optical band gap of ∼2 eV with a repeatable photocurrent response of 80 to 100 μA cm^–2 in strong protic media. This is significantly high in correlation to metal-free polydimercapto thiadiazole, polycopper dimercapto thiadiazole, and Cu₂O. The incorporation of triazine units and Cu^II/I in the thiadiazole polymeric network enables enhanced photoelectrochemical activity and stability. In comparison to neutral electrolytes, the photocurrent values observed are nearly 3 to 4 times higher in protic electrolytes. The metallopolymer exhibits a nearly 30 to 40% increase in photoinduced oxygen reduction current and good stability in protic electrolytes.