Optimization of Photocatalyst Excited- and Ground-State Reduction Potentials for Dye-Sensitized HBr Splitting

Dye-sensitized bromide oxidation was investigated using a series of four ruthenium polypyridyl photocatalysts anchored to SnO2/TiO2 core/shell mesoporous thin films through 2,2′-bipyridine-4,4′-diphosphonic acid anchoring groups. The ground- and excited-state reduction potentials were tuned over 500 mV by the introduction of electron withdrawing groups in the 4 and 4′ positions of the ancillary bipyridine ligands. Upon light excitation of the surface-bound photocatalysts, excited-state electron injection yielded an oxidized photocatalyst that was regenerated through bromide oxidation. High injection quantum yields (Φinj) and regeneration quantum yields (Φreg) were essential to obtain efficient bromide oxidation yet required a photocatalyst that is both a potent photoreductant and a strong oxidant after excited-state injection. The four photocatalysts utilized in this manuscript ranged from unity Φinj (1.0) and minimal Φreg (0.037) to minimal Φinj (0.09) and unity Φreg (1.0). The photocatalyst that displayed the highest overall dye-sensitized photoelectrosynthesis cell performances exhibited near unity Φreg (0.99), while a significant Φinj was still preserved (0.59). Thus, these results highlighted the delicate interplay between the ground- and excited-state reduction potentials of photocatalysts for dye-sensitized hydrobromic acid splitting.