Synthesis, aggregation and photoinduced electron transfer processes of cationic water-soluble 21-thia and 21-oxaporphyrins

Abstract

The meso-pyridyl 21-thia- and 21-oxaporphyrins containing three pyridyl groups at the meso-positions have been methylated to obtain water-soluble porphyrins. The porphyrins are highly soluble in water and the water-soluble 21-thiaporphyrins exhibit aggregation behaviour above 10−4 M whereas the water-soluble 21-oxaporphyrins strongly aggregate at very low concentrations (10−7 M). The interaction of water-soluble ionic porphyrins with ionic surfactant in aqueous solutions has been studied as a function of surfactant concentration by means of absorption, fluorescence and resonance light scattering (RLS) techniques. It has been observed that, at the initial premicellar surfactant concentration, the oppositely charged surfactant induces aggregation and above cmc these aggregates dissociates into monomeric form and gets micellised. The self- and SDS-induced aggregation of these cationic porphyrins is found to depend on the number and position of the positive charge with respect to the porphyrin core. The micellar encapsulation technique is used to promote the photoinduced electron transfer between the porphyrins and neutral aromatic amines and it has been observed that the electron transfer rate becomes slowing down at the micellar surface compared to the bulk acetonitrile solution.