An Electron-Rich Helical Host for the Exclusive Removal of a Planar Electron-Deficient Organic Compound

In this study, a series of electron-rich helical hosts, viz. Pyr-HAC, Anth-HAC and Ben-HAC, containing pyrene, anthracene and benzene residues, respectively, at their periphery, were screened for their interaction with different planar electron-deficient organic guests (PEDOGs). A strong and highly selective charge-transfer interactions (CTI) was observed between the host Pyr-HAC and the guest 1,2,4,5-tetracyano-benzene (TCNB), leading to a yellow-to-bright-red color change in both the solubilized and the solid state. The interaction between Pyr-HAC and TCNB also induced profound structural and morphological changes. Pyr-HAC self-assembled into belt-like morphology created by homochiral stacking of the host molecules, but in the Pyr-HAC⊃TCNB complex, square bipyramids containing intertwined heterochiral C₂ -double helices of Pyr-HAC were observed. Other PEDOGs did not induce any of the above changes in Pyr-HAC. Detailed UV/Vis absorption and fluorescence spectroscopy, NMR, and X-ray diffraction studies confirmed this selectivity, which arises due to CTI assisted by complementary, directional intermolecular hydrogen bonding (DIHB) between Pyr-HAC and TCNB. This allowed for the exclusive extraction of TCNB from a solution enriched in other PEDOGs. Thus, this study provides a ground work for designing responsive helical hosts towards CTI-driven selective "catch-and-release" of guests.