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

Abstract

ABSTRACT 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.