Cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase: on the role played by the length and parity of the spacer

Six members of the 1,omega-bis(4-cyanobiphenyl-4 '-yl) alkanes are reported and referred to as CBnCB in which n = 1, 15, 16, 17, 19 and 20 and indicates the number of methylene units in the spacer separating the two cyanobiphenyl units. The behaviour of CB3CB is revisited. The temperature dependence of the refractive indices, optical birefringence and dielectric permittivities measured in the nematic, N, phase for selected homologues are reported. The dimers with n >= 15 showed an enantiotropic N phase, and for the odd members the twist-bend nematic, N<INF>TB</INF>, phase was observed. CB3CB shows a direct N<INF>TB</INF>-isotropic, I, transition, whereas for CB1CB a virtual N<INF>TB</INF>-I transition is found. The temperature dependence of the bend elastic constant, K<INF>33</INF>, measured in the oblique helicoidal cholesteric state formed by mixtures of CBnCB with a chiral additive S811, shows strong non-monotonous behaviour with a deep minimum near the transition point to the N<INF>TB</INF> phase. The minimum value of K33K33 decreases as n increases. The long even members of the CBnCB series show similar values of T<INF>NI</INF> to their odd-membered counterparts, but their estimated values of T<INF>N<INF>TB</INF></INF><INF>N</INF> are considerably lower. This is attributed to molecular shape and its effect on K<INF>33</INF>. {GRAPHIACAL ABSTRACT}