Sulfur-linked cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

Abstract

The synthesis and characterisation of two series of cyanobiphenyl-based liquid crystal dimers containing sulfur links between the spacer and mesogenic units, the 4',4''-[1,ωalkanediylbis(thio)]bis-[1,1'-biphenyl]-4-carbonitriles (CBSnSCB), and 4'-({ω-[(4'cyano[1,1'-biphenyl]-4-yl)oxy]alkyl}thio)[1,1'-biphenyl]-4-carbonitriles (CBSnOCB), are described. The odd members of both series show twist-bend nematic and nematic phases whereas the even members exhibit only the nematic phase. An analogous cyanoterphenylbased dimer, 3 4-{6-[(4'-cyano[1,1'-biphenyl]-4-yl)thio]-hexyl}[1 1 ,2 1 :2 4 ,3 1-terphenyl]-1 4carbonitrile (CT6SCB), is also reported and shows enantiotropic NTB and N phases. The transitional properties of these dimers are discussed in terms of molecular curvature, flexibility and biaxiality. The same molecular factors influence also birefringence of nematic phases. Resonant X-ray scattering studies of twist-bend nematic phase at both the carbon and sulfur absorption edges were performed, which allowed for determination of critical behaviour of helical pitch at the transition to nematic phase, the behaviour was found independent on molecular structure. It was also observed that despite the different molecular bending angle and flexibility, in all compounds the helical pitch length far from the N-NTB transition corresponds to 4 longitudinal molecular distances.

Ether‐ and Thioether‐Linked Naphthalene‐Based Liquid‐Crystal Dimers: Influence of Chalcogen Linkage and Mesogenic‐Arm Symmetry on the Incidence and Stability of the Twist–Bend Nematic Phase

“…Recently, Cruickshank et al. likewise reported CBS n SCB and CBS n OCB . To the best of our knowledge, all solely chalcogen‐linked twist–bend nematogens developed so far are based on phenyl rings as the π‐conjugated moieties in the mesogenic arms; other π‐conjugated cores have never been addressed.…”

“…[58] Recently, Cruickshank et al likewise reported CBSnSCB and CBSnOCB. [59] To the best of our knowledge,a ll solely chalcogen-linked twist-bend nematogens developed so far are based on phenyl rings as the p-conjugatedm oieties in the mesogenic arms; other p-conjugatedc ores have never been addressed. In addition, molecular bendingr esulting from the different chalcogen linkagesa nd the symmetry of mesogenic arm structures have yet to be analyzed systematicallyt ou nderstand the necessary molecular design features to obtain twist-bend nematics.…”

Molecular Flexibility and Bend in Semi‐Rigid Liquid Crystals: Implications for the Heliconical Nematic Ground State

“…[38] In this work, the materials studied feature am ethylene spacer with varying linking group;i ti st herefore to be expected that as linkersa re used which introduce greater flexibility (increased FWHM) the average bend angle decreases further away from the tetrahedral angle imposed by the methylene spacer.A3D plot of scaled transition temperature versus FWHM versus average bend shows how these three properties are related ( Figure 3c). Clearly, both averageb enda nd flexibility play ap art in dictating the scaled transition temperature and this can explain the previously unexpectedly low values of dT of materials such as CBS3SCB [27,39] and CBO5OCB, [24,40] both of which satisfy the angular dependency of the N TB phase but are highlyf lexible. Furthers tudies into the relationship between molecular bend and the incidence of the N TB phase will be expected to be complicated by the difficulty in preparing spacers which give bend angles larger than those presented here without introducing flexibility (i.e.…”

“…independently reported the synthesis and transition temperatures of these two materials, [39] in this work we chose to use transition temperatures for compounds 6 and 7 reportedb yA rakawa et al [27] because the clearing points are marginally higher. independently reported the synthesis and transition temperatures of these two materials, [39] in this work we chose to use transition temperatures for compounds 6 and 7 reportedb yA rakawa et al [27] because the clearing points are marginally higher.…”

Twist-Bend Nematogenic Supramolecular Dimers and Trimers Formed by Hydrogen Bonding

“…From a chemist's perspective, understanding the molecular features influencing the formation and stabilisation of the N TB phase is of paramount importance, and allows for the design of new materials that have targeted properties. These studies have involved systematically varying the molecular structure of twist-bend nematogens and have included varying the spacer length [11,12], the nature of the link between the spacer and mesogenic groups [13][14][15][16][17], the structure of the mesogenic units [18,19], and of the terminal substituents [12,[20][21][22][23]. By far the greatest number of twist-bend nematogens reported so far may be described as odd-membered liquid crystal dimers having a similar molecular curvature to CB7CB.…”