Dimerization effects on spectroscopic properties of water-soluble porphyrins in aqueous and micellar media

Abstract

The effects of cation-crown ether induced dimerization on the optical and fluorescence spectra of free base 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (H₂tcpp⁴) and 5,10,15,20-tetrakis(4-sulphonatophenyl)porphyrin (H₂tpps⁴) and their metal derivatives (Cu²⁺, Ni²⁺ and Zn²⁺) have been studied in aqueous and micellar media. Dimerization induces a red shift of Q-bands in both optical and fluorescence spectra with quenching of fluorescence intensity. However, the Soret band experiences a blue shift in Cu²⁺ and Ni²⁺ derivatives, a red shift in Zn²⁺ derivatives and no shift in the free bases. The magnitude of these shifts depends on the structure of the metalloporphyrin as well as on the nature of the crown ether and cation present in solution. The formation constants for the dimer follow the order Cu²⁺≈ Ni²⁺ > H₂tcpp⁴ > Zn²⁺. In micellar media the dimerization effect is observed only in anionic sodium dodecyl sulphate. In hexadecyltrimethylammonium bromide and Triton X-100, the porphyrins remain monomeric. An analysis of the data suggests that four cation-crown ether complexes are involved per dimer unit. The exciton formalism coupled with π-π interaction accounts for the observed shifts in Cu²⁺, Ni²⁺ and free-base porphyrins while a satisfactory interpretation has to take charge-transfer contributions into account for Zn²⁺ derivatives.