One- and Two-Dimensional Polymorphism in a Hydrogen-Bonding Bis-Ureido-Squaramide

We report on a newly designed bis-ureido-squaramide (USq) (1) monomeric unit, capable to tune its conformation and regulate its self-assembly behavior in solution and in solid state, forming one- (1D) and two-dimensional (2D) assemblies. In chloroform, the bis-USq 1 exists as a folded monomeric form (Mon_F), stabilized by six intramolecular hydrogen bonds, and polymerizes by increasing concentration and decreasing the temperature. Microscopic observations, supported by theoretical calculations, revealed that 1 does not assemble into the expected main-chain polymers, and instead, a supramolecular 2D polymer (AggA) is formed by the interplay of USq-USq quadruple hydrogen bonds and additional π-π and van der Waals interactions. Remarkably, this 2D assembly was preserved in solid-state as a lamellar phase (1-Lam) after solvent evaporation. However, heating the sample to isotropic and cooling it down revealed a second and more stable polymorph based on a 1D columnar structure (1-Col). Interestingly, both solid-state polymorphs are stabilized by analogous hydrogen bonding patterns but different molecular packing. This work underpins the USq unit as an intriguing hydrogen-bonding moiety suitable for the development of new supramolecular materials but also as a platform to study pathway complexity and polymorphism in solid state and solution.