Near-infrared absorption carboxylated chlorophyll-a derivatives for biocompatible dye-sensitized hydrogen evolution

Chlorophylls (Chls) are naturally occurring photosensitizers having an excellent visible and near-infrared light absorption property. Herein, we employ three Chl a derivatives as sensitizers in a TiO2-based photocatalytic system for H2 evolution with ascorbic acid as the sacrificial reagent under visible light (λ > 400 nm). The H2-evolution activity of these Chl a derivatives depends on a carboxy group in the C3- and/or C17-substituent(s). The concentration of 1 mg/ml dye/Pt/TiO2 powder suspended in an aqueous solution with ascorbic acid as the sacrificial reagent gives the highest H2-evolution rate. The fact that Chl-2 with a carboxy group on the C3 position of the chlorin macrocycle gives the highest H2-evolution rate of 0.79 μmol h−1 is ascribed to the lowest charge recombination rate between Chl-2 and TiO2 among all Chls investigated. This work provides us with important information in synthesizing more favorable molecular structure of Chl derivatives for the highly efficient photocatalytic H2 evolution from water splitting.