Fluorescence monitoring of pH dependent complexation of chlorin p6 with surfactants

We have observed the pH-dependent formation of electrostatic complexes of chlorin p6 with surfactants in submicellar concentrations. Successive protonation of the ionizable groups cause a progressively increasing positive charge on the fluorophore as the pH decreases. Complexes are formed with the cationic CTAB at pH 7 and 5 and with the anionic SDS at pH 3, as manifested in a decrease in fluorescence quantum yield and radiative lifetime at low surfactant concentrations. The fluorescence quantum yield and lifetime are observed to increase beyond certain concentrations of the surfactants and saturate before the critical micellar concentrations are reached. The variation in the fluorescence parameters is rationalized by a competitive model involving pH-induced and surfactant-induced aggregates.