Synthesis of a diporphyrin dyad bearing electron-donor and electron-withdrawing substituents with potential use in the spectral sensitization of semiconductor solar cells

Anew dyad 9 (ZnP-P) has been synthesized linking 5,15-bis(4-carboxyphenyl)-10,20bis(4-nitrophenyl) porphyrin 4 (P) and Zn(II) 5-(4-aminophenyl)-10,15,20-tris(4-methoxylphenyl) porphyrin 8 (ZnP) by an amide bond. The structural moieties of dyad 9 (ZnP-P) present both different singlet state energy and redox properties. Dyad 9 was designed to improve the intramolecular electron transfer capacity. The ZnP moiety bears electron-donating methoxy groups and a zinc ion, while the other porphyrin structure, P, is substituted by electron-withdrawing nitro groups. On the other hand, structure P bears a carboxylic acid group, which is able to benefit from the orientation of dyad 9 adsorbed on the SnO2 electrode. Absorption spectroscopic studies indicated only a very weak interaction between the chromophores in the ground state. The fluorescence analysis shows that both porphyrin moieties in dyad 9 are strongly quenched and that the quenching increases in a polar solvent. The ZnP moiety acts like an antenna for porphyrin P, but, singlet-singlet energy transfer is not complete. Thermodynamically, dyad 9 presents a high capacity to form the photoinduced charge-separated state, ZnP+-P-. Dyad 9 sensitizes the SnO2 electrode and the photocurrent action spectrum closely matches the absorption spectrum, which confirms that light absorption by dyad is the initial step in the charge transfer mechanism. The photocurrent efficiency of dyad 9 is considerably higher than those of porphyrin monomers used as models of ZnP and P structures. Two processes may be contributing to enhance the charge injection efficiency in dyad 9; one involves an antenna effect that produces energy transfer from ZnP to P and the other includes electron transfer from the ZnP moiety to the photooxidizable free-base P. This dyad design, with P in direct contact with the substrate through the free carboxylic acid group, is a promising architecture of organic material for spectral sensitization of semiconductor solar cells. Copyright (C) 2003 Society of Porphyrins & Phthalocyanines.