Hydrogen was successfully produced under visible light irradiation in a tin porphyrin (SnP)-sensitized TiO2 system in the wide pH range (pH 3–11) although SnP hardly adsorbs on TiO2. The number of H2 produced in the SnP/TiO2 system after 9 h irradiation corresponds to the turnover number of 410. The apparent photonic efficiency for H2 evolution was estimated to be 35% with the monochromatic radiation of 550 ± 10 nm. The photochemical production of hydrogen is mediated through the formation of the π-radical anion (SnP˙−) that subsequently transfers electron to TiO2. The photogenerated SnP˙− was monitored by transient absorption spectroscopy and its lifetime is long enough to survive the slow diffusion from the solution bulk to the TiO2 surface, which makes the adsorption of SnP on TiO2 not required for hydrogen production. This is clearly contrasted with the common ruthenium complex-sensitized TiO2 system where the adsorption of the sensitizer complex is essentially required and the hydrogen production is limited to the acidic condition where the adsorption of the sensitizers on TiO2 is allowed. The photocatalytic activity of SnP was mainly attributed to the Q-band (500–650 nm), not to the Soret band (420–430 nm) of which absorption intensity is much higher.
