Homochiral versus Racemic 2D Covalent Organic Frameworks

The synthesis of homochiral two-dimensional Covalent Organic Frameworks (2D COFs) from chiral pi-conjugated building blocks is challenging, as chiral units often lead to misaligned stacking interactions. In this work, we introduce helical chirality into 2D COFs using configurationally stable enantiopure and racemic [5]helicenes as linkers in the backbone of 2D [5]HeliCOFs. Through condensation with 1,3,5-triformylbenzene (TFB) or 1,3,5-triformylphloroglucinol (TFP), our approach enables the efficient formation of a set of homochiral and racemic 2D [5]HeliCOFs. The resulting carbon-based crystalline and porous frameworks exhibit distinct structural features and different properties between homochiral and their racemic counterparts. Propagation of helical chirality into the backbone of the crystalline frameworks leads to the observation of extended chiroptical properties in the far-red visible spectrum, along with a less compact structure compared to the racemic frameworks. The present study offers insight into general chiral framework formation and extends the Liebisch–Wallach rule to 2D COFs.