Photosensitizers for H2 Evolution Based on Charged or Neutral Zn and Sn Porphyrins.

We report a comparison between a series of zinc and tin porphyrins as photosensitizers for photochemical hydrogen evolution using cobaloxime complexes as molecular catalysts. Among all the chromophores tested, only the positively charged zinc porphyrin, [ZnTMePyP4+]Cl4, and the neutral tin porphyrin derivatives, Sn(OH)2TPyP, Sn(Cl2)TPP-[COOMe]4, and Sn(Cl2)TPP-[PO(OEt)2]4, were photocatalytically active. Hydrogen evolution was strongly affected by the pH value as well as the different concentrations of both the sensitizer and the catalyst. A comprehensive photophysical and electrochemical investigation was conducted in order to examine the mechanism of photocatalysis. The results derived from this study establish fundamental criteria with respect to the design and synthesis of porphyrin derivatives for their application as photosensitizers in photoinduced hydrogen evolution.