Self-association of free base porphyrins with aminoacid substituents in AOT reverse micelles

Abstract

The aggregation properties of aminosulfonyl porphyrins with aminoacid substituents: methionine with the acid function methylated, PMetCH3; phenylalanine, PPhe; attached to the para positions of the meso phenyls of the porphyrin, was followed in organic solvents, water-dioxane mixtures and in reverse micelles of aerosol OT (AOT RM), using absorption, steady-state and time-resolved fluorescence. In AOT RM in the absence of water, both porphyrins are mainly present as monomers with the Soret band around 420 nm and with emission maxima around 649 and 717 nm. In the presence of water accounted for by parameter [omega]0, [omega]0 = [H2O]/[AOT], the solubility of both porphyrins is enhanced and followed by important spectral changes similar to those observed in the solvent mixtures: new maxima in absorption around 405 nm and in emission around 670 nm. A concomitant increase of the aggregates' contribution is found for PMetCH3, whereas for PPhe the effect is much more pronounced until [omega]0 = 10, above which a de-aggregation process occurs, due to competition between intra- and inter-molecular interactions. The small resonant light scattering signals observed points to the existence of aggregates of small dimensions whose nature is discussed in terms of exciton theory.