We developed twist-bend nematogens based on cyanobiphenyl dimers with thioether or sulfur linkage, as opposed to the commonly used methylene and ether linkages. Based on the smaller bond angle of thioether C–S–C than methylene C–CH₂–C and ether C–O–C, we designed two dimer homologous series that are more bent, symmetric CBSnSCB with two thioether linkages and asymmetric CBSnOCB with a thioether and an ether linkage, containing odd carbon numbers n = 3, 5, 7, 9, and 11, and also synthesized even-n members n = 6 for each system. Both CBSnSCB with n = 3, 5 and 7 and CBSnOCB with n = 5 and 7 formed a broad range of twist-bend nematic (N_TB) phases that were stable down to room temperature and showed remarkably enhanced glass-forming ability. In addition, CBS9OCB also formed N_TB phase at a fast scan rate. DFT calculations revealed that twist-bend nematogens CBS7SCB and CBS7OCB have smaller bend angles made of the para axes of the two cyanobiphenyl moieties and larger dipole moments than those of the conventional nematogen ether-linked CBO7OCB.

We designed three new bistolanes with alkylsulfanyl groups. A symmetric derivative contains short terminal methylsulfanyl groups (–SCH₃) on both ends, while two asymmetric derivatives contain a methylsulfanyl group on one end, as well as a highly polarizable cyano (–CN) or isothiocyanate (–NCS) group on the other end. These materials exhibited a well-defined enantiotropic nematic phase, as well as an extremely high birefringence (Δn > 0.6). Most notably, the NCS derivative achieves an extremely high value of 0.77 at 550 nm. These results should be helpful for the design and synthesis of novel nematic materials with high birefringence.

We synthesized two series of diphenyl-diacetylene (DPDA)-based materials with alkoxy and alkyl tails of length m (DPDA–OCm and DPDA–Cm, respectively), and measured their nematic-phase birefringence (Δn) as a function of wavelength and temperature. We found that Δn decreases with an increase in m, possibly by a dilution effect of the low-Δn alkyl tail. Further, of the two series, Δn was found to be relatively higher in the DPDA–OCm materials, with the highest value of 0.4 obtained for DPDA–OC1 at 550 nm at 10 °C below the isotropic-to-nematic transition temperature. Further, we observed the temperature dependence for Δn, which is proportional to the order parameter (s). From extrapolation to s = 1 (the perfect orientation state), it is speculated that the DPDA–O moiety has the potential to afford a very large Δn of 0.9.

In order to generate high-birefringence liquid crystal (LC) materials, the introduction of highly polarisable groups into the terminal positions of the mesogen represents one of the most important design strategies. Even though the alkylsulfanyl group is a potentially interesting option for this purpose, it has so far been very difficult to obtain enantiotropic mesophases for LCs containing this group. Herein, we report novel high-birefringence LC molecules based on 1,4-bis(2-phenylethynyl)benzene (bistolane) with alkylsulfanyl groups in the terminal positions. The incorporation of a fluorine atom into the central benzene ring of an alkylsulfanyl-substituted bistolane led to the formation of a well-defined enantiotropic nematic phase. In contrast, the nonfluorinated analogue with alkylsulfanyl groups did not exhibit a mesophase. In comparison with an alkoxy-substituted derivative, the birefringence of the alkylsulfanyl-substituted analog was substantially higher (Δn = 0.42 at 550 nm). Furthermore, the birefringence properties of the alkylsulfanyl-substituted derivative were observed to be proportional to the order parameter and largely temperature-dependent over the entire temperature range. These results thus provide not only fundamental insights into structure–reactivity relations, but also furnish practical design principles for the synthesis of new sulfur-containing, rod-shaped LC materials with optical applications.