Potential-Dependent Adsorption and Orientation of meso-Substituted Porphyrins at Liquid|Liquid Interfaces Studied by Polarization-Modulation Total Internal Reflection Fluorescence Spectroscopy

Potential-dependent adsorption behavior of meso-substituted water-soluble porphyrins at the polarized water|1,2-dichloroethane (DCE) interface was studied by polarization-modulation total internal reflection fluorescence (PM-TIRF) spectroscopy. In the PM-TIRF experiments, the fluorescence signal from the interfacial region was analyzed as a function of the periodic modulation of linear-polarizations (p and s) of the incident excitation beam. The potential-dependence of PM-TIRF responses for meso-substituted porphyrins, 5,10,15,20-tetrakis­(N-methylpyridyl)­porphyrin (H2TMPyP4+) and 5,10,15,20-tetrakis­(4-sulfonatophenyl)­porphyrin (H2TPPS4–), indicated that both free base porphyrins were adsorbed with relatively lying orientations at the polarized water|DCE interface. The average orientation angles (θ) were estimated as θ = 61 ± 1° for H2TMPyP4+ and θ = 65 ± 1° for H2TPPS4– with respect to the interface normal. The wavelength-dependence of polarization-modulated fluorescence signals (PM-TIRF spectrum), which corresponds to “pure” emission spectrum of interfacial species, clearly indicated that H2TMPyP4+ and H2TPPS4– are adsorbed with a modification of the solvation at the interface. These results demonstrated a high ability of the PM-TIRF spectroscopy for the direct characterization of fluorescent species adsorbed at polarized liquid|liquid interfaces.