Modulating self-assembly and polymorph transitions in bisdendronized squaramides

Supramolecular self-assembly is an advanced approach for constructing ordered nanoscale architectures with broad applications. While the principles of supramolecular polymerization have been thoroughly explored in artificial small molecules, polymer transformations remain barely explored, likely due to the lack of suitable reference models presenting well-defined and reversible transitions between aggregates. In this study, we introduce a series of bisdendronized squaramides (SQs) 1-3, showcasing complex self-assembly behaviours involving four distinct aggregates, three different interaction patterns, and various thermodynamically controlled polymorph transformations. Notably, SQ 3, with ethyl spacers between the SQ cores and the dendrons, exhibits a concentration and temperature-dependent equilibrium among three polymorphs: the particle-like Agg-A and fibrillar Agg-C, formed by slipped hydrogen bonds, and the fibrillar Agg-B, formed by head-to-tail hydrogen bonds. Additional solid-state experiments revealed that these SQs also form columnar liquid crystals, assembled by π-π interactions in SQ 1 and hydrogen bonding in SQ 2 and SQ 3. This work positions SQ units as valuable models for understanding polymorph equilibrium in solution and solid-state, which is crucial for developing stimuli-responsive supramolecular polymers.