Conformational Chirality\nof Single-Crystal Covalent\nOrganic Frameworks

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