Higher Order Cooperativity Emerging from the Helicoidal Growth of Curved Supramolecular Polymers

Biological supramolecular assemblies harness the cooperative action of multiple noncovalent interactions to achieve high degrees of structural order and functional complexity. For instance, microtubules, biological supramolecular polymers, stabilize their tubular architecture through a synergetic interplay between longitudinal interactions along the tubular axis and lateral interactions between adjacent helicoidal loops. In contrast, synthetic supramolecular polymers have rarely demonstrated such cooperative behavior emerging from higher order structures of self-assembled architectures. Herein, we report helicoidal supramolecular polymers of aryl barbiturate derivatives that exhibit cooperative self-assembly driven by the concerted action of two distinct noncovalent interactions: π-π stacking between adjacent molecules and alkyl side-chain interaction between adjacent helicoidal loops (i.e., nonadjacent molecules). By intentionally minimizing the π-conjugated core in the monomer design, we attenuated the π-π stacking interactions, thereby promoting the emergence of alkyl-alkyl interactions between neighboring helicoidal loops. This strategy favored the formation of extended helicoidal structures, in contrast to the closed-end toroidal structures previously observed for analogs with a large π-conjugated core. Moreover, the temperature sensitivity of the interloop alkyl-alkyl interaction led to its cancellation at elevated temperatures, inducing a phase transition from helicoidal structures to linear fibers lacking alkyl-mediated secondary interactions. The absence of interloop interactions rendered the linear fibers dynamic and allowed occasional ring-closing events of short fibers in equilibrium. Continuous occurrence of such ring-closing events during thermal annealing led to the emergence of toroidal architectures. These findings highlight the potential of integrating orthogonal noncovalent interactions in molecular design to achieve dynamic and hierarchical organization in synthetic supramolecular systems.