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 kinetically stable interfacial nanostructures formed only at pH values close to the pKa 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 XRD analysis and in situ UV/vis and steady state fluorescence indicates 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.This publication has emanated from research by M.D.S. and A.F.M.-O. supported by the European Research Council through a Starting Grant (agreement no. 716792) and in part by a research grant from Science Foundation Ireland (SFI) (grant number 13/SIRG/2137). M.D.S. and A.F.M.–O. acknowledge funding through Irish Research Council New Foundations Awards (2014 and 2015) to facilitate the research. A.M.O., M.D.S., G.H. and M.D. are grateful to the support of the Irish Research Council and Campus France for travel support between the French and Irish groups through their joint ULYSSES programme. G. H. is grateful to the French Programme Investissement d'Avenir (PIA) "Lorraine Université d'Excellence" (Reference No. ANR-15-IDEX-04-LUE) for the partial financial support of this work. C.O.D. acknowledges support from Science Foundation Ireland (SFI) under Grant Numbers 13/TIDA/E2761, 14/IA/2581 and 15/TIDA/2893. Computational facilities and support for the molecular dynamics simulations were provided by the Irish Centre for High-End Computing (ICHEC). Ivan Robayo-Molina (University of Limerick) is acknowledged for assistance in carrying out the potentiometric titrations