Adsorption and aggregation of meso-tetrakis(4-carboxyphenyl)porphyrinato zinc(II) at the polarized water vertical bar 1,2-dichloroethane interface

The specific adsorption of the water-soluble meso-tetrakis(4-carboxyphenyl)porphyrinato zinc(II) (ZnTppC(4-)) was studied at the polarizable water\1,2-dichloroethane (DCE) interface by surface second harmonic (SH) generation and quasi-elastic laser scattering (QELS). The intense SH responses were analyzed as a function of the ZnTppC(4-) concentration and of the Galvani potential difference. It was confirmed that the coverage of ZnTPPC increases as the Galvani potential difference approaches the formal ion transfer potential. In this potential range, the high interfacial concentration induces the formation of J-aggregates at the interface. The aggregation phenomena manifest itself by a red-shift of the SH response resonant to the Soret band and an overall increase in the SH intensity. At potentials more positive than the formal transfer potential, the SH spectrum exhibits a maximum at the same wavelength of absorption of ZnTPPC in solution, suggesting a low extent of aggregation in the porphyrin assembly. The ZnTPPC adsorption also manifests itself by changes in the frequency of the microscopic thermal fluctuation (capillary waves) at the liquid\liquid boundary. The potential dependence of the capillary waves frequency was estimated from QELS, allowing the construction of electrocapillary curves at various ZnTPPC concentrations.