ESR study of photoexcited triplets of some tetra(4-sulfonatophenyl)porphyrin dimers

Tetra (4-sulfonatophenyl)porphyrin (H<SUB>2</SUB>TPPS), and the metalloporphyrins ZnTPPS and PdTPPS dimerize in aqueous solution. An ESR study has been made of the photoexcited triplets of the [ZnTPPS]<SUB>2</SUB>. [H<SUB>2</SUB>TPPS/ZnTPPS] and [H<SUB>2</SUB>TPPS/PdTPPS] dimers. It is found that there is a strong dimerization effect on the values of the zero field splitting parameters of the triplets. This contrasts with what has been reported for photoexcited triplets of a series of covalently-linked tetrapyrroles. The experimental data indicate that the dimerization effect on the TPPS triplet parameters must be attributed to charge transfer contributions to the triplet electronic state