Surfactant-like Behaviour of Short-Chain Alcohols in Porphyrin Aggregation

UV/vis absorption and time-resolved fluorescence measurements on alcoholic solutions of meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4) in neutral and acid form have been performed as a function of the alcohol polarity. These solutions show a peculiar behavior that mimics porphyrin in confined water solutions. In alcohols, TPPS4 exhibits a metastable phase showing the formation of new species in analogy with the confined water environment of AOT microemulsions. Various species have been detected at different pH values and on aging the solutions. Under neutral pH conditions, the porphyrin is present as free base monomer (S415) with a small amount of H-dimeric species (S400). On aging, the S415 leads to the formation of a new species (S423), which has been assigned as a J-type dimer of the neutral porphyrin. The species S400 and S423 are not present in bulk water solution but are typical of TPPS4 in confined water. On decreasing pH, the S415 almost immediately converts into the diacid form (S438), which evolves toward red-shifted J-aggregates (S490) and blue-shifted H-aggregates (S420). On decreasing alcohol polarity, the kinetic evolution from fresh to aged solution and from the monomeric diacid species to H- and J-aggregates speeds up. Exploiting the amphiphilic character of short chain alcohols and widely varying their polarity, we were able to enhance in bulk conditions the peculiar behavior observed in close proximity to the microemulsion wall.