Adsorption behavior of charged zinc porphyrins at the water/1,2-dichloroethane interface studied by potential modulated fluorescence spectroscopy

The adsorption properties of ionic fluorescent dyes at the polarized water/ 1,2-dicfiloroethane interface were studied by potential modulated fluoresce nce (PMF) spectroscopy under total internal reflection. Analysis of the fre quency-dependent fluorescence associated with modulation of the interfacial concentration of the ionic dyes proved to be a rather sensitive approach f or separating interfacial phenomena from bulk responses. The combination of PMF and electrochemical techniques allows to uncover differences in the sp ecific interfacial interactions of tris(2,2'-bipyridyl)ruthenium(II) (Ru(bp y)(3)(2+)), meso-tetrakis(N-methylpyridyl)porphyrinato zinc-(II) (znTMPyP(4 +)), and meso-tetrakis(p-sulfonatophenyl)porphyri zinc(II) (ZnTPPS4-). Whil e Ru(bpy)(3)(2+) shows quasi-reversible ion transfer features, the charged zinc porphyrins exhibit adsorption properties at potential close to the tra nsfer range. The anionic ZnTPPS4- appears to be adsorbed at the interface a t potentials more positive than the formal transfer potential. On the other hand, the spectroelectrochemical data show that ZnTMPyP4+ is adsorbed at t he interface at potentials either side of the formal transfer potential. Du e to the difference in the potential dependence of the adsorption processes , PMF responses associated with interfacial accumulation from the aqueous s ide exhibit a different phase shift with respect to adsorption steps from t he organic side. The experimental results clearly demonstrate that adsorpti on planes at the organic and aqueous side of the interface are physically d istinguishable. Furthermore, PMF dependence on the polarization of the exci tation beam allows to estimate average molecular orientation of the adsorbe d species.