Construction of chiral propeller architectures from achiral molecules

Of particular interest to the next generation of biological and electro-optical advances in materials science and technology is the recognition and translation of chirality to different length scales in self-assembled systems.([1-7]) Understanding and controlling this is essential for mimicking the development of biological structures and for achieving tailored macroscopic properties.([8-17]) Classically, chirality is generated by the incorporation of asymmetric tetrahedral carbons, but self-assembled liquid crystal (LC) materials formed through non-covalent interactions, such as H-bonding, have attracted much attention, as these materials may also possess chirality independent of molecular chirality. Amplification processes can enable chirality transfer from the nanometer scale to the macroscopic scale, as is often observed in Nature. Here, we show for the first time that chiral propellers can be constructed from achiral molecules (BPCA-Cn-PmOH, i.e., 4-biphenylcarboxylic, acid molecules connected to alkoxyl chains with varying number of carbon atoms (n = 6-10) and terminated with phenyl groups with hydroxyl groups at the meta-position, as shown in Fig. 1a) via H-bond-driven self-assembly. We have previously shown that this series of achiral BPCA-Cn-PmOH molecules, having neither molecular chirality([8-17]) nor a significantly bent core (like banana-shaped molecules),([18-23]) can form 3D helical supramolecular structures in a smectic C (SmC) phase.([24,25]) These helical structures can be caused by the molecular tilting of twisted head-to-head H-bonded dimers (Fig. 1a) during the SmA to SmC phase transition.([25]) The question becomes: can an achiral molecule that is not in a smectic phase construct phases with intrinsic cholesteric, chiral characteristics? The answer to this question would provide significant insight into the most basic requirements needed to form chiral phases, which are critical in biology and optics.