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Journal article Open Access

Pathway complexity in supramolecular porphyrin self-assembly at an immiscible liquid–liquid interface

Robayo-Molina, Iván; Molina-Osorio, Andrés F.; Guinane, Luke; Tofial, Syed A.M.; Scanlon, Micheál D.

Nanostructures that are inaccessible through spontaneous thermodynamic processes may be formed by supramolecular self-assembly under kinetic control. In the past decade, the dynamics of pathway complexity in self-assembly have been elucidated through kinetic models based on aggregate growth by sequential monomer association and dissociation. Immiscible liquid|liquid interfaces are an attractive platform to develop well-ordered self-assembled nanostructures, unattainable in bulk solution, due to the templating interaction of the interface with adsorbed molecules. Here, we report time-resolved in situ UV/vis spectroscopic observations of the self-assembly of zinc(II) meso-tetrakis(4-carboxyphenyl)porphyrin (ZnTPPc) at an immiscible aqueous|organic interface. We show that the kinetically favoured metastable J-type nanostructures form quickly, but then transform into stable thermodynamically favoured H-type nanostructures. Numerical modelling revealed two parallel and competing cooperative pathways leading to the different porphyrin nanostructures. These insights demonstrate that pathway complexity is not unique to self-assembly processes in bulk solution, and equally valid for interfacial self-assembly. Subsequently, the interfacial electrostatic environment was tuned using a kosmotropic anion (citrate) in order to influence the pathway selection. At high concentrations, interfacial nanostructure formation was forced completely down the kinetically favoured pathway and only J-type nanostructures were obtained. Furthermore, we found by atomic force microscopy (AFM) and scanning electron microscopy (SEM) that the J- and H-type nanostructures obtained at low and high citric acid concentrations, respectively, are morphologically distinct, which illustrates the pathway-dependent material properties.

L.G and S.A.M.T would like to acknowledge the funding contributed by the Irish Research Council's Enterprise Partnership Scheme under the project ID: EBPPG/2016/271. The NTEGRA-SPECTRA Hybrid Nanoscope was funded by Science Foundation Ireland (SFI).
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