Advanced interfacial functionalisation strategies: supramolecular self-assembly of porphyrins at electrified aqueous|organic interfaces for solar energy conversion

Nanostructures that are inaccessible through spontaneous thermo-dynamic processes may be formed by supramolecular self-assembly under kinetic control. Immiscible liquid|liquid interfaces are an attractive platform to develop well-ordered self-assembled nanostrutures, unattainable in bulk solution, due to the templating interaction of the interface with adsorbed molecules. Here, the time-resolved in situ UV/vis spectroscopic observations of the self-assembly of zinc(II) meso-tetrakis(4-carboxyphenyl)porphyrin (ZnTPPc) and zinc(II) meso-tetrakis(4-aminophenyl)porphyrin (ZnTAPP) are reported at an immiscible aqueous|organic interface. In case of ZnTPPc the kinetically favoured metastable J-type nanostructures form quickly, but then transform into stable thermodynamically favoured H-type nanostructure, whereas ZnTAPP only forms a J-type. Subsequently, with ZnTPPC, the interfacial electrostatic environment was tuned using a kosmotropic anion (citrate) in order to control the influence of the pathway selection. At high concentrations, interfacial nanostructure formation was forced completely down the kinetically favoured pathway and only J-type nanostructures were obtained. These interfacial films exhibited photocurrent under light illumination and the presence of an organic electron donor. The profile of these photocurrents were explained by a mathematical model based on partial differential equations. Through this model it was found that the contribution of diffusion was negligible, with the photocurrent decay being primarily dictated by the rates of product separation and recombination.