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 <b>NCT</b> is dimerized to 4,4'-bithiopyran atropisomer <b>BNCT</b> with axial and helical chirality. The variable-temperature ¹H NMR and UV-vis spectroscopies indicate that atropisomer <b>BNCT</b> rotates around the axis upon temperature changes. The enantiomeric interconversion pathway of atropisomer <b>BNCT</b> 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 <b>BNCT</b> displays a significant bathochromic shift of 56 nm for the absorption maximum in comparison to that for <b>NCT</b>, suggesting the effective conjugation extension along the atropisomer axis. The <i>R</i>- and <i>S</i>-enantiomers of <b>BNCT</b> have been successfully separated, and their electronic circular dichroism spectra exhibit a mirror-image relationship. Most interestingly, octafluoronaphthalene (<b>OFN</b>) can be inserted between the <b>BNCT</b> molecules to form supramolecular interactions with a host-guest ratio of 1:1. The single cocrystal of <b>BNCT</b> and <b>OFN</b> suggested that the supramolecular behaviors originate from the donor-acceptor interactions between <b>BNCT</b> and <b>OFN</b> molecules, although <b>BNCT</b> presents a twisted molecular skeleton.