Structural Water-Enabled Helicity Emergence From an Asymmetric Achiral Molecule.

Helical supramolecular architectures are ubiquitous in nature yet remain challenging to construct from asymmetric achiral molecules. Here, we demonstrate that structural water acts as a symmetry-breaking and frustration-generating element in artificial self-assembly. Using an asymmetric achiral naphthalene derivative (N1) as a model system, we show that trace water fundamentally redirects its assembly pathway. In the presence of structural water, N1 forms racemic P/M helical fibers, whereas only non-helical aggregates are obtained under anhydrous conditions or with control molecules lacking sufficient hydrogen-bonding capability. Single-crystal x-ray analysis reveals that each water molecule functions as a tetravalent hydrogen-bonding node, bridging four N1 molecules into a nonplanar C2-symmetric tetramer. This water-centered motif introduces geometric incompatibility with optimal π-π stacking, generating packing frustration that is relieved through hierarchical helical twisting. The hydrogen-bonding network can be reversibly modulated by acid-base stimuli, enabling interconversion between helical and non-helical morphologies. Moreover, the resulting helices can be biased into homochiral states by chiral aromatic amino acids, revealing a water-gated chirality transfer mechanism that is absent under anhydrous conditions. This work extends design principles for supramolecular helicity beyond conventional symmetric monomers and highlights the role of structural water in controlling complex self-assembly pathways.