Fig. 6. Schematic showing the HOMO/LUMO levels and two different photocatalytic reactions driven by 1 under different pH conditions. Under acidic conditions (pH 4 ascorbic acid buffer) the hydroxyl group remains protonated and 1 can photosensitize a proton reduction catalyst ([Mo₃S₁₃]^2–). Under basic conditions (pH 9–10 ascorbate) 1′ can sensitize an iron-porphyrin catalyst which selectively reduces CO₂ to CO.