Axially Chiral 4,4'-Bithiopyran Atropisomer Dimerized by Thiopyran-Fused [4]Helicene: Synthesis, Chiral Properties, and Supramolecular Behaviors.

Despite the progress in atropisomers and helicenes with axial and helical chirality, the construction of axially chiral compounds by directly dimerizing two helicenes remains challenging. Herein, a thiopyran-fused [4]helicene NCT is dimerized to 4,4'-bithiopyran atropisomer BNCT with axial and helical chirality. The variable-temperature 1H NMR and UV-vis spectroscopies indicate that atropisomer BNCT rotates around the axis upon temperature changes. The enantiomeric interconversion pathway of atropisomer BNCT is systematically investigated by density functional theory calculation. Although the dihedral angle between the two thiopyran rings is 63.0° revealed by single-crystal analysis, the dichloromethane solution of BNCT displays a significant bathochromic shift of 56 nm for the absorption maximum in comparison to that for NCT, suggesting the effective conjugation extension along the atropisomer axis. The R- and S-enantiomers of BNCT have been successfully separated, and their electronic circular dichroism spectra exhibit a mirror-image relationship. Most interestingly, octafluoronaphthalene (OFN) can be inserted between the BNCT molecules to form supramolecular interactions with a host-guest ratio of 1:1. The single cocrystal of BNCT and OFN suggested that the supramolecular behaviors originate from the donor-acceptor interactions between BNCT and OFN molecules, although BNCT presents a twisted molecular skeleton.