Conformational Chirality of Single-Crystal Covalent Organic Frameworks.

Abstract

The crystallization of organic polymers is often hindered by chiral units, hence resulting in chiral organic polymers typically existing as amorphous or partially crystalline phases such as natural rubber and cellulose. Similarly, as an emerging crystalline chiral polymer, chiral covalent organic frameworks (COFs) also inevitably face a delicate balance between chiral units and crystallization, limiting their production and applications in separation, catalysis, and optics. Here, we present a general strategy for producing a series of conformational chiral COFs with high crystallinity through breaking the meso conformation of achiral COFs. Conformational chirality of COF-300 was constructed by involving chiral amino-acid derivative templates during synthesis and was proven to have excellent thermodynamic (200 °C annealing in air) and dynamic stability (61% cell volume change). The stereochemistry of the conformational chiral crystals can be controllably tuned by chiral templates, resulting in wide-range circular dichroism signals from ultraviolet to infrared wavelengths and absorption dissymmetry factors (g_abs) varying by up to 300%, with a maximum of g_abs = 0.012. This strategy paves the way for stereochemistry modification, property enhancement, and exploration of new applications of crystalline chiral materials.