Chiral cyanobiphenyl dimers – significance of the linking group for mesomorphic properties and helical induction

Abstract

Cyanobiphenyl (CB) mesogenic molecules are a focal point in liquid crystal (LC) research and applications due to their chemical stability, ease of synthesis, broad mesophase temperature range, and other advantageous properties. The introduction of chirality into LC systems offers unique optical and mechanical properties with great potential, and exploring the CB building block in chiral LC dimers presents a promising avenue for further research. In this study, we present the synthesis of chiral and racemic dimers bearing a chiral center as part of their flexible spacer and two CB moieties at the ends, i.e. the first chiral CB dimers. The investigation includes ester and amide linking groups near the chiral center to evaluate the impact of molecular flexibility on mesomorphic behavior and helical twisting power (HTP). The synthesis of racemic and enantiomerically pure dimers enabled the comparison of their mesogenic properties. Our results revealed CB dimers bearing ester linking group exhibit rich nematic polymorphism, including the rarely obtained room-temperature twist-bend nematic (N_TB) phase. In contrast, the amide linking group suppresses LC properties but significantly enhances the HTP. These findings highlight the importance of intermolecular interactions and the balance of flexibility and rigidity near the chiral center in achieving desirable mesomorphic properties.