2-Dimensional Porphyrin Self-Assemblies at Molecular Interfaces

The heterodimer formed by electrostatic association of zinc(II) meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP) and zinc(II) meso-tetrakis(p-sulfonatophenyl)porphyrin (ZnTPPS) exhibits a strong affinity for the interface between water and 1,2-dichloroethane (DCE). Surface tension measurements using the quasi-elastic light scattering (QELS) technique reveal that the heterodimer adsorption can be described in terms of a Langmuir isotherm with standard Gibbs energy of adsorption of -45.5 kJ mol-1. The orientation of the heterodimer transition dipole moment, as estimated from light polarization modulated reflectance (LPMR), shows a marked dependence on the bulk concentration of heterodimer. On the other hand, changes in the Galvani potential difference between the two phases have little effect on the heterodimer organization at the water |1,2-dichloroethane interface when the surface coverage is close to maximum. This behavior suggests that the ZnTPPS-ZnTMPyP heterodimer forms an adsorbed layer of aggregated molecules which affects the physical properties of the interface. Indeed, capacitance and surface tension measurements reveal that the dielectric properties of the water DCE interface are significantly altered in the presence of heterodimer species.