Photoelectrochemical properties of a series of electrostatically self-assembled films based on sandwich-type polyoxometalates and a bichromophore hemicyanine dye.

A series of photoelectric conversion films consisting of sandwich-type polyoxometalates [P(2)W(18)M(4)(H(2)O)(2)O(68)](10-) (P(2)W(18)M(4), M=Ni, Cu, Zn) and a bichromophore hemicyanine of (E)-1,1'-(hexane-1,6-diyl)bis(4-(4-(dimethylamino)styryl)pyridinium) bromide (H(6)) were prepared on quartz and indium-tin-oxide (ITO)-coated glass substrates by electrostatic self-assembly. The UV-visible absorption spectroscopy showed that the films were uniformly deposited with the visible absorption bands for H(6) in the films exhibiting different extents of bathochromic shifts with respect to H(6) aqueous solution. The (P(2)W(18)M(4)/H(6))(n) films exhibited cathodic photocurrents as irradiated with white light. The photocurrent action spectra were found to match with the absorption spectra of the films, indicating that the cathodic photocurrents were generated based on intramolecular charge transfer excitation of the H(6) in the films. The photocurrents were found to strongly depend on the transition metal ions M in (P(2)W(18)M(4)/H(6))(n) films. The effects of applied bias voltages, electron acceptor added in the electrolyte, and layer numbers of the films on photocurrent generation of (P(2)W(18)M(4)/H(6))(n) films were also examined, and photoelectric mechanism discussed.