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-methylp henyl) porphyrin (P) and Zn(II) 5-(4-carboxyphenyl)-10,15,20-tris(4-methylp henyl) porphyrin (P-Zn). The dyes were adsorbed to saturation on a nanocrys talline SnO2 thin film, employed as working electrode in a photoelectrochem ical cell. The metallized and unmetallized moieties possess different singl et state energies and redox properties. In both, solution and adsorbed stat e, nearly complete singlet-singlet energy transfer from the P-Zn to P has b een determined in the dyad. P-Zn is less efficient than P in the photocurre nt generation, but is a suitable energy donor in the dyad molecule. The gen eration of photoelectrical effects by the dyad is less effective in compari son with P. Considering the oxidation potentials of the two moieties in P-Z n-P, a mechanism is proposed where the oxidized metallized porphyrin enhanc es the back electron-transfer process.