Self-Assembly of Porphyrin Nanostructures at the Interface between Two Immiscible Liquids

One of the many evolved functions of photosynthetic organisms is to synthesize light harvesting nanostructures from photoactive molecules such as porphyrins. Engineering synthetic analogues with optimized molecular order necessary for the efficient capture and harvest of light energy remains challenging. Here, we address this challenge by reporting the self-assembly of zinc(II) meso-tetrakis(4-carboxyphenyl)porphyrins into films of highly ordered nanostructures. The self-assembly process takes place selectively at the interface between two immiscible liquids (water|organic solvent) with the kinetically stable interfacial nanostructures formed only at pH values close to the pK_a of the carboxyphenyl groups. Molecular dynamics simulations suggest that the assembly process is driven by an interplay between the hydrophobicity gradient at the interface and hydrogen bonding in the formed nanostructure. Ex situ X-ray diffraction (XRD) analysis and in situ UV-vis and steady-state fluorescence indicate the formation of chlathrate type nanostructures that retain the emission properties of their monomeric constituents. The self-assembly method presented here avoids the use of acidic conditions, additives such as surfactants, and external stimuli, offering an alternative for the realization of light-harvesting antennas in artificial photosynthesis technologies.