Synthesis of porphyrin dyads with potential use in solar energy conversion

A convenient procedure for the synthesis of porphyrin derivative dyads is described. The dyads consist of a free base porphyrin covalently linked to a zinc porphyrin or ferrocene by an amide bond. 5-(4-Substituted phenyl)-10,15,20-tris(4-methylphenyl) porphyrins were synthesized from meso-(4-methylphenyl) dipyrromethanes 1, which was obtained with appreciable yield (83%). The reaction of dipyrromethane 1 with a mixture of two appropriate substituted benzaldehydes affords the desired meso-substituted porphyrins, which can be easily separated by flash chromatography. These porphyrins bearing either one 4-acetamidophenyl group 2 or 4-carboxymethylphenyl group 3, and three 4-methylphenyl peripheral functional groups, were prepared with notable yields (15-17%) in a two-step one-flask reaction. Basic hydrolysis of the porphyrins 2 and 3 yielded amino 4 and acid porphyrin 5, respectively. Treatment of 5 with zinc acetate afforded the corresponding metal complex Zn-acid porphyrin 6. The dyads 7 and 8 were obtained by the coupling reaction between the acid chloride derivatives of either Zn-acid porphyrin 6 or ferroceneacetic acid and amino porphyrins 4, respectively. The present strategy may be easily used for preparation of other similar dyad derivatives. These compounds could have interesting applications in electronic materials. Preliminary studies of light energy conversion by SnO2 electrodes coated with porphyrin dyads 7 and 8 were performed. The results show that dyads 7 and 8 may be suitable for solar energy conversion devices.