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 (Φ_inj) and regeneration quantum yields (Φ_reg) 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 Φ_inj (1.0) and minimal Φ_reg (0.037) to minimal Φ_inj (0.09) and unity Φ_reg (1.0). The photocatalyst that displayed the highest overall dye-sensitized photoelectrosynthesis cell performances exhibited near unity Φ_reg (0.99), while a significant Φ_inj 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.