Two-dimensional Reversible Supramolecular Poly-morphism Governed by Weak Solvent-Solute Interactions

Molecular self-assembly can be a powerful strategy for fabricating dynamic two-dimensional (2D) nanomaterials. However, framing a rational design principle that can satisfy the strict thermodynamic and kinetic criteria enabling a uniform in-plane growth, as well as offer a provision for dynamic structural rearrangement, presents a considerable challenge. We report a hydrophobic core-substituted naphthalenediimide that self-assembles in water-dioxane mixtures to form a pair of 2D poly-morphs, with distinct morphological, optical and surface characteristics. Changing the solvent composition or temperature prompts an in-situ transformation between the two polymorphs in solution. A comprehensive structural investigation using solid-state NMR spectroscopy, x-ray and electron diffraction analyses reveals not only the nature of molecular packing and noncovalent (n-pi* and pi-stacking) interactions stabilizing each 2D polymorph, but also the role of weak solvent-solute interactions in driving polymorphic interconversion. Different arrangements of perfluoroalkyl sidechains at the exposed 2D surface of the two polymorphs result in difference in solvent-solute interactions at the solid-liquid interface, which affects the surface free-energy and relative stability in different solvent compositions. Further, entropy change associated with surface ordering of perfluoroalkyl sidechains and desolvation dictates thermal reorganization between the polymorphs. How changes in dioxane-water microheterogeneity with solution composition and temperature may affect these weak solvent-solute interactions and the phase equilibrium is also discussed.