Visible light-driven hydrogen evolution using dye–semiconductor hybrids: effects of peripheral substituents and central metals of chlorophyll-a derivatives as photosensitizers

Chlorophyll (Chl)-a derivatives possessing a carboxylated substituent at the peripheral 20-position were prepared as photosensitizers. The synthetic pigments were adsorbed onto the surface of titanium oxide particles loaded with platinum nanoparticles through the bonding of the carboxy group of the Chls with the oxygen functional groups of TiO2. In an aqueous ascorbic acid solution, the resulting Chl-attaching Pt-TiO2 was illuminated with orange and red light to generate hydrogen gas. The hydrogen evolution reaction was initiated by the electron injection of the Chl photosensitizers excited with the visible light to the TiO2 semiconductor and was completed by the reduction of aqueous protons into H2 on the Pt cocatalyst, while ascorbic acid functioned as an electron donor to the Chl species. The hydrogen evolution activity was dependent on the spacer between the core chlorin π-system and the terminal carboxy group in the Chls. An increase in the spacer length suppressed the undesired back electron transfer from the TiO2 surface to the Chl cation radical to enhance the amount of evolved H2. The facile insertion of a copper atom at the central position of a Chl sensitizer boosted the photocatalytic activity, but its zincation and nickelation diminished the photosensitizing abilities.