4CzIPN Photosensitizes the Cobaloxime-Catalyzed Light-Driven Hydrogen Evolution Reaction

Thermally activated delayed fluorescence (TADF) compounds have found great application as photocatalysts in recent years. Their use as photosensitizers (PS), however, is still limited although they have huge potential as abundant alternatives to traditional noble metal-based PSs, such as [Ru(bpy)₃]²⁺. We herein investigate the TADF compound 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) as a PS together with the catalyst cobaloxime (Co(dmgH)₂(py)Cl) in the photochemical hydrogen evolution reaction (HER). Systematic optimization of the solvent composition, 4CzIPN and catalyst concentration, and sacrificial electron donor (SED) identified ammonium ascorbate as the most effective SED under the conditions tested. Under optimized conditions, the system reached a TON of 6451 ± 82 after 20 h irradiation. Stern-Volmer quenching experiments and scalar-relativistic quantum-chemical calculations provided rate constants for the different processes in the photosensitization mechanism, demonstrating that the quenching of the excited singlet state (S₁) of 4CzIPN by ammonium ascorbate is by far more dominant than the quenching of its excited triplet state (T₁). This work demonstrates that 4CzIPN can sensitize the cobaloxime-catalyzed light-driven HER with ammonium ascorbate as the SED and provides mechanistic insight into the role of singlet- and triplet-state quenching in TADF photosensitization.