Potential-Induced Aggregation of Anionic Porphyrins at Liquid|Liquid Interfaces

The adsorption and self-aggregation of anionic porphyrins were studied at the polarized water|1,2-dichloroethane (DCE) interface by polarization-modulation total internal reflection fluorescence (PM-TIRF) spectroscopy. 5,10,15,20-Tetrakis­(4-sulfonatophenyl)­porphyrin diacid (H4TPPS2–) and protoporphyrin IX (H2PP2–) exhibited high surface activities at the interface. The selective excitation of interfacial species in PM-TIRF measurements elucidated the potential-induced aggregation mechanism of the porphyrins. The J-aggregates of H4TPPS2– were reversibly formed only at the water|DCE interface by applying appropriate potentials even when the porphyrins exist as monomers in the aqueous and organic solutions. In the H2PP2– system, the slow aggregation process was found in the negative potential region. The spectral characteristics and the signal phase of PM-TIRF indicated that the H2PP2– monomers were adsorbed with relatively standing orientation and that the long axis of the J-aggregates was nearly in plane of the interface. H2PP2– was also investigated at the biomimetic phospholipid-adsorbed water|DCE interface. The competitive adsorption of neutral glycerophospholipids effectively inhibited the potential-dependent adsorption and interfacial aggregation processes of H2PP2–. The results demonstrated that the aggregation state of the charged species can reversibly be controlled at liquid|liquid interfaces as a function of externally applied potential.