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

TADF compounds have found great application as photocatalyst in recent years. Their use as photosensitizers (PS), however, is still limited although they have a huge potential as abundant alternatives for traditional noble metal-based PSs, such as [Ru(bpy)3]2+. We herein investigate the TADF compound 4CzIPN as a PS together with the catalyst cobaloxime (Co(dmgH)2(py)Cl) in the photochemical hydrogen-evolution reaction (HER). After optimization using a multi-batch screening photoreactor, we achieved turnover numbers (TONs) of over 5,000 with ammonium ascorbate as an abundant sacrificial electron donor (SED). 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 is capable of sensitizing the cobaloxime-catalyzed light-driven HER with ammonium ascorbate as the SED and gives an excited insight into the quenching mechanism of TADF compounds as PSs.