Adsorption Behavior of Charged Zinc Porphyrins at the Water/1,2-Dichloroethane Interface Studied by Potential Modulated Fluorescence Spectroscopy

• Published in: The journal of physical chemistry. B Vol. 104; no. 29; pp. 6869 - 6876
• Main Authors: Nagatani, Hirohisa, Iglesias, Rodrigo A, Fermín, David J, Brevet, Pierre-François, Girault, Hubert H
• Format: Journal Article
• Language: English
• Published: American Chemical Society 27.07.2000
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp001106j

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.