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We synthesised selenium-linked liquid crystal dimers and examined their phase behaviour to determine the effects of chalcogen linkage and spacer length on the formation of a twist-bend nematic (NTB) phase. Two new homologous series of nonsymmetric selenoether- and ether- or thioether-linked cyanobiphenyl-based dimers (CBSenOCB and CBSenSCB, respectively) comprising an alkyl spacer chain with a carbon number (n) of 3, 5, 6, 7, 9, and 11 were developed. We also synthesised n = 6 and 11 of previously reported symmetrically selenoether-linked CBSenSeCB homologues. Conventional nematic (N) and NTB phases were observed for nearly all odd-n CBSenSeCB, CBSenOCB, and CBSenSCB dimers. The phase transition behaviours of the three homologous series were compared with those of the known symmetric ether-linked CBOnOCB, thioether-linked CBSnSCB, and asymmetric thioether- and ether-linked CBSnOCB dimers. Replacing ether with thioether or selenoether resulted in lower LC phase-transition temperatures, particularly for selenoether. Single-crystal structural analyses revealed that CBS3SCB and CBSe3SeCB exhibited folded U-shaped molecular shapes that were bent more than the V-shapes of CBOnOCB (n = 3 and 5) and CBS5SCB. In addition, thioether and selenoether linkages rendered the molecular forms of odd-spacer dimers in the crystals more bent. [ABSTRACT FROM AUTHOR]

The high interest of isotopically labelled compounds is induced by two important factors: (i) improvement of highly sensitive and precise analytical methods for isotope identification and (ii) development of new synthetic approaches for isotopically labelled compounds. However, there is still a lack of efficient and cheap methods for the design of deuterium labelled liquid crystalline materials. Herein, the continuous flow system was adapted for the synthesis of deuterated Liquid Crystals using the H-Cube Pro reactor, where deuterium gas was generated in situ from heavy water. We designed and developed the synthesis of homologous series of 4,4ʺ-dialkyl-2′,3′-difluoro-[1,1′:4′,1ʺ]terphenyls where deuterium atoms are placed at carbon α and carbon β positions or only at carbon α positions of alkyl terminal chains. The synthetic strategy involves mainly selective deuteration reactions of C≡C bonds as well as reduction of carbonyl groups C=O using batch or continuous flow conditions. Theoretical calculation and experimental study show that deuterium labelled Liquid Crystals exhibit increased photochemical stability compared to protonated ones. Moreover, the comparison of physicochemical properties between deuterium labelled and non-labelled 4,4ʺ-dialkyl-2′,3′-difluoro-[1,1′:4′,1ʺ]terphenyls is presented. This work provides efficient methods to obtain deuterated liquid crystalline materials with much better photochemical stability compared to their fully protonated isotopologues. [ABSTRACT FROM AUTHOR]

Two homologous series of sulfur-containing thioether-linked 4-cyanoazobenzene-based liquid crystal (LC) dimers were developed: a symmetric series based on 4-cyanoazobenzene arms [(CN)AzoSnSAzo(CN)] and a non-symmetric series based on 4-cyanoazobenzene and 4-cyanobiphenyl arms [(CN)AzoSnSCB]. They contained different flexible oligomethylene spacers (CnH2n) consisting of odd numbers of carbon atoms (n = 3, 5, 7, 9, and 11). The symmetric (CN)AzoSnSAzo(CN) dimers with n = 5, 7, 9, and 11 showed an elusive monotropic twist–bend nematic (NTB) phase, while all the non-symmetric (CN)AzoSnSCB dimers exhibited the NTB phase. Notably, the NTB phase observed for non-symmetric (CN)AzoS7SCB could be supercooled to room temperature, leading to a glassy NTB phase. (CN)AzoS9SCB showed a similar tendency accompanied by partial crystallisation. These are the first reported examples of azobenzene-based LC dimers exhibiting a broad temperature range of NTB phases supercooled to room temperature. Such prominent stability of the NTB phase is due to the supercooling effect assisted by the synergy of molecular bend arising from thioether linkages and non-symmetric mesogenic arms. The phase-transition temperatures were overall higher for symmetric (CN)AzoSnSAzo(CN) than for non-symmetric (CN)AzoSnSCB. The difference in the mesogenic arm structures hardly influenced their molecular geometry or anisotropy in terms of entropy changes. [ABSTRACT FROM AUTHOR]

We synthesised a novel family of liquid crystalline 1,4-phenylene bis(4-alkylthio benzoates), of which homologues with different alkyl chain lengths of 1-8 were designed, to investigate the structure-property relationship. The mesogenic incidence and tendency were found to be strongly dependent on the carbon numbers in the terminal alkyl chains. Even members formed widely monotropic or enantiotropic nematic (N) phases compared to odd members that showed either none or monotropic-narrower ones. The temperature dependence of birefringence (Δn) for the hexylthio homologue was evaluated and compared to that for a hexyloxy counterpart. It was found that the hexylthio homologue exhibited higher Δn values than the hexyloxy counterpart over the entire range of near TIN-T. In addition, wide-angle X-ray diffraction measurements, using magnetically aligned specimens, found enhanced cybotactic nematic tendency with smectic (Sm) A-like temperature dependence for the hexylthio analogue, due to the intermolecular attractive S···S interaction, in comparison with the hexyloxy analogue exhibiting N phase with clear Sm C-type clusters. We revealed the actual benefits of alkylthio groups in the fluid N phase. [ABSTRACT FROM AUTHOR]

In this study, we offer a new family of alkylthio-containing diphenylacetylene-based liquid crystalline molecules (the so-called tolanes) showing nematic phases stable down to room temperature and high birefringence, with insights into the role for terminal alkyl chains in mesogenic incidence and tendency. A number of asymmetric tolane homologues having various alkyl chain lengths in the alkylthio and alkyl groups at each p-, p’-position were synthesised, and their phase transition behaviour was investigated by polarising optical microscope observation, differential scanning calorimetry and wide-angle X-ray diffraction measurement. Consequently, several homologues exhibited monotropic nematic or highly ordered smectic (soft crystal) phases stable down to room temperature. It is found that a long alkyl group on the side opposite to an alkylthio group is prerequisite for mesogenic incidence. In addition, a nematogenic homologue exhibited a higher birefringence value of 0.20 compared to that of 0.19 for an alkoxy counterpart. [ABSTRACT FROM AUTHOR]