Photosensitization of thin SnO2 nanocrystalline semiconductor film electrodes with metallodiporphyrin

Sensitized photocurrent generation is observed with a porphyrin dyad (P-Zn-P) and its structural moieties: 5-(4-carboxyphenyl)-10,15,20-tris(4-methylphenyl) porphyrin (P) and Zn(II) 5-(4-carboxyphenyl)-10,15,20-tris(4-methylphenyl) porphyrin (P-Zn). The dyes were adsorbed to saturation on a nanocrystalline SnO2 thin film, employed as working electrode in a photoelectrochemical cell. The metallized and unmetallized moieties possess different singlet state energies and redox properties. In both, solution and adsorbed state, nearly complete singlet-singlet energy transfer from the P-Zn to P has been determined in the dyad. P-Zn is less efficient than P in the photocurrent generation, but is a suitable energy donor in the dyad molecule. The generation of photoelectrical effects by the dyad is less effective in comparison with P. Considering the oxidation potentials of the two moieties in P-Zn-P, a mechanism is proposed where the oxidized metallized porphyrin enhances the back electron-transfer process.