Ion-pairing effects by organic anions on the supramolecular assembling kinetics of sulfonated porphyrins

Camphorsulfonic (CSA) and p-toluenesulfonic (p-TsOH) acids promote the self-assembling process of the diacid meso-tetrakis(4-sulfonatophenyl)porphyrin (H2TPPS4) into J-aggregates. A general decrease of the rate constants on increasing the acid concentration has been observed for these organic acids. For camphorsulfonic acid the formation of chiral J-aggregates, whose handedness is related to that of the acid, has been revealed by circular dichroism spectra. Changes in the aggregation rates, kinetic profiles and g-factors but no chiral discrimination have been observed for the two different enantiomers, when using different mixing protocols. Moreover, at high acid concentration, aggregation is inhibited and the formation of a new porphyrin species has been detected for both acids. This species has been characterized by steady-state and time-resolved spectroscopic techniques and has been ascribed to an ion pair between the diacid porphyrin and the organic counter-anion. Its role in the aggregation of the initial diacid species has been discussed.