Photoinduced Hydrogen Evolution with Viologen-Linked Zinc Porphyrins

Viologen-linked water-soluble zinc porphyrins with different methylene chain lengths between porphyrin and viologen, Zn-PC3(CnV) (n=2–6), were both synthesized and characterized. The lifetimes of the photoexcited triplet states of these compounds were almost the same as those of zinc porphyrins without linked viologen. The fluorescence decay profiles, however, comprised two components with first-order decay (shorter and longer lifetimes) indicating the existence of two conformations: complexed and extended conformers. In the complexed conformer, photoexcited singlet state is quenched by linked viologen. These compounds were applied to photoinduced hydrogen evolution in a system containing NADPH, Zn-PC3(CnV), and hydrogenase under steady state irradiation. Upon irradiation of the sample solution hydrogen was generated, indicating that all of the compounds, Zn-PC3(CnV) (n=2–6), can be substrates of the enzyme hydrogenase, and that every compound participates as both a photosensitizer and an electron carrier in the same molecule.