Rational design of bromine-modified Ir(III) photosensitizer for photocatalytic hydrogen generation

The facile design of highly efficient photosensitizers is fundamentally crucial for the construction of efficient photocatalytic H2-evolving systems. In this work, we employed substituted 2-(thiophen-2-yl) pyridine (C^N) and 2,2′-bipyridine (N^N) ligands to afford a series of iridium complexes. Interestingly, by introducing heavy halogen group -Br, the triplet state lifetime of affording photosensitizer was significantly enhanced to 4273 ns, which is 54 times to that of unmodified one (77 ns). While coupling with Ni-substituted polyoxometalate Ni4(SiW9)2 catalyst and triethanolamine electron donor, the bromine-modified photosensitizer can effectively drive hydrogen generation with a turnover number of ∼ 1400 under visible light irradiation. Spectroscopic studies reveal that the -Br modified iridium complex exhibits larger Stern − Volmer constants for both reductive and oxidative quenching processes. Mechanistic analyses demonstrate the advantages of long-lived excited states and good photostability of bromine modified photosensitizer, the efficient electron transfer process, and the well-matched energy levels between all three catalytic components for efficient hydrogen production.