Fundamental expressions for analyzing potential modulated fluorescence (PMF) responses were derived within the framework of a phenomenological model for adsorption and transfer of ionic species across polarized liquid|liquid interfaces. For small periodic perturbations of the Galvani potential difference, PMF signals can be linearized and the contribution of each process can be uncoupled in the frequency domain. The PMF response for kinetically controlled adsorption is expressed as a semicircle in the complex plane in which the characteristic frequency of maximum imaginary component is proportional to the adsorption and desorption rate constants. Considering that the potential dependence of adsorption exhibits opposite sign whether the process take place from the aqueous or organic phase, the corresponding PMF responses appear in different quadrants of the complex plane. The present model delivers useful diagnostic criteria for analyzing the nature of the various processes contributing to the periodic fluorescence response. The adsorption dynamics of mesotetrakis(N-methyl-4-pyridyl)porphyrinato zinc(II) at the water|1,2-dichloroethane interface were evaluated from the frequency dependent PMF responses. Studies performed at various Galvani potential differences clearly confirm that the adsorption can take place at two distinctive planes located at the aqueous and organic sides of the interface. Basic aspects in connection to the nature of the adsorption planes are briefly discussed.

The adsorption properties of ionic fluorescent dyes at the polarized water/1,2-dichloroethane interface were studied by potential modulated fluorescence (PMF) spectroscopy under total internal reflection. Analysis of the frequency-dependent fluorescence associated with modulation of the interfacial concentration of the ionic dyes proved to be a rather sensitive approach for separating interfacial phenomena from bulk responses. The combination of PMF and electrochemical techniques allows to uncover differences in the specific interfacial interactions of tris(2,2‘-bipyridyl)ruthenium(II) (Ru(bpy)3 2+), meso-tetrakis(N-methylpyridyl)porphyrinato zinc(II) (ZnTMPyP4+), and meso-tetrakis(p-sulfonatophenyl)porphyrinato zinc(II) (ZnTPPS4-). While Ru(bpy)3 2+ shows quasi-reversible ion transfer features, the charged zinc porphyrins exhibit adsorption properties at potential close to the transfer range. The anionic ZnTPPS4- appears to be adsorbed at the interface at potentials more positive than the formal transfer potential. On the other hand, the spectroelectrochemical data show that ZnTMPyP4+ is adsorbed at the interface at potentials either side of the formal transfer potential. Due to the difference in the potential dependence of the adsorption processes, PMF responses associated with interfacial accumulation from the aqueous side exhibit a different phase shift with respect to adsorption steps from the organic side. The experimental results clearly demonstrate that adsorption planes at the organic and aqueous side of the interface are physically distinguishable. Furthermore, PMF dependence on the polarization of the excitation beam allows to estimate average molecular orientation of the adsorbed species.

ABSTRACT:The self-assembly of the oppositely charged watersoluble porphyrins, cobalt tetramethylpyridinium porphyrin (CoTMPyP 4+ ) and cobalt tetrasulphonatophenyl porphyrin (CoTPPS 4− ), at the interface with an organic solvent to form molecular "rafts", provides an excellent catalyst to perform the interfacial four-electron reduction of oxygen by lipophilic electron donors such as tetrathiafulvalene (TTF). The catalytic activity and selectivity of the self-assembled catalyst toward the four-electron pathway was found to be as good as that of the Pacman type cofacial cobalt porphyrins. The assembly has been characterized by UV−visible spectroscopy, Surface Second Harmonic Generation, and Scanning Electron Microscopy. Density functional theory calculations confirm the possibility of formation of the catalytic CoTMPyP 4+ / CoTPPS 4− complex and its capability to bind oxygen.

Interfacial properties of zinc tetra(4-carboxyphenyl) porphyrin (ZnTPPC) specifically adsorbed at the water|1,2-dichloroethane (DCE) junction were elucidated from the photoelectrochemical behavior under linearly polarized light. The photocurrent responses originating from heterogeneous quenching by ferrocene did not show a significant dependence on the penetration depth of light into the aqueous phase. However, the photoresponses exhibited a remarkable dependence on the light polarization for illumination in total internal reflection. These studies provide a rather unique insight into the correlation between interfacial reactivity and molecular orientation. The average tilting angle of the transition dipole moment appears to be directly related to the surface coverage. Finally, photocurrent responses are strongly decreased with increasing pH of the water phase. This result suggests that the self-assembly properties of ZnTPPC at the water|DCE interface are linked to cooperative intermolecular hydrogen bonding involving partially protonated carboxyphenyl groups. The implication of these findings concerning solar-energy conversion as well as the molecular structure of liquid|liquid junction are briefly discussed.