Role of Geometrical Frustration in Self-limiting Enantioselective Synthesis of Chiroptical Helices

Tunable rotation of light polarization across a wide wavelength spectrum is a desired property for optoelectronic materials. Nanoscale helices and their segments (bow-ties) are expected to exhibit some of the strongest chiroptical activity among nanoscale structures with mirror asymmetry. Successful efforts for fabrication of helices with desired geometry (pitch, length, width, twist angle and thickness) have been focused on top-down approaches such as ion-beam lithography. However, lithographic methods are limited by the fabrication efficiency of the instrumentation involved, thereby hindering bulk production. Herein, we present a colloidal synthesis route for enantiopure bow-ties by guiding the electrostatic and coordination interactions between cadmium ions and chiral amino acid cysteine (Cys). Cd-Cys bow-ties are self-assembled from sheet-like monomeric units, with individually controllable length, width and pitch, leading to tunable chiroptical activity across 500 nm to 3 μm wavelength range. This self-limiting structure of bow-ties can be described within the theoretical framework of geometrical frustration and provide chemical design rules for controlling the interaction and bending energy of monomeric units.