J- and H-aggregates of porphyrin-surfactant complexes: time-resolved fluorescence and other spectroscopic studies

Abstract

The interactions of several water-soluble ionic porphyrins with different ionic or neutral surfactants in aqueous solutions were studied as a function of surfactant concentration. The interaction leads to the formation of porphyrin aggregates and/or micelle-encapsulated monomers with the exception of those porphyrin-surfactant pairs for which the interaction is Coulombically repulsive. The premicellar surfactant-porphyrin aggregate is identified by absorption and fluorescence spectroscopy, fluorescence lifetime and anisotropy, and resonance light scattering. The spectroscopic results are used to characterize the premicellar aggregates as J-type, H-type, or nonspecific aggregates. All premicellar surfactant-porphyrin aggregates dissociate to form micelle-encapsulated monomers when the surfactant concentration approaches cmc (critical micellar concentration). The interaction of tetrakis-(4-sulfanatophenyl)porphine dianion (H₄TPPS^2-) at pH < 3.5 with cetyltrimethylammonium cation (CTAB) is described by the following sequential equlibria controlled by the surfactant concentration: M_⇋J_⇋H_⇋ M_m. The stoichiometric ratio of porphyrin/surfactant is 1:2 for the J-aggregate and ∼1:4 for the H-aggregate. Kinetic intermediates were also observed prior to the formation of the J-aggregate. The J-aggregate exhibits circular dichroism (spontaneous chirality, not seen in H-type or micellar aggregates), intense resonance light scattering, low fluorescence quantum yield and lifetime, and unusually high fluorescence anisotropy.