Properties of CB7CB-D4 as derived from 129Xe and 2H NMR experiments and computations

Abstract

Twist-bend nematic liquid crystal phases are fundamentally interesting as chiral macroscopic structures arising from achiral molecules, as well as from the perspectives of photonics and switching applications. Conical angle and orientational order are the key properties of the twist-bend nematic phase. In the present case, we investigate the conical angle in 1″,7″-bis(4-cyanobiphenyl-4′-yl)heptane (CB7CB), which displays ordinary nematic (N) and twist-bend nematic (N_TB) phases, utilizing ¹²⁹Xe shielding and ²H quadrupole splitting in CB7CB-D₄. When applying the ¹²⁹Xe shielding, measured with respect to an external gaseous xenon sample, the bulk susceptibility effect must be considered. For this purpose, the diamagnetic susceptibility tensor was computed. The analysis of the ²H NMR data necessitates the knowledge of the ²H quadrupole coupling tensor element along the C–D bond. Computations and data analyses show that the commonly used value of 168 kHz is slightly underestimated. Another property investigated is the temperature dependence of the ¹²⁹Xe linewidth (ν_1/2). It reveals two things: (i) when approaching the phase transitions from the high-temperature side, ν_1/2 increases abruptly, but decays, after the transition, exponentially toward the values in the isotropic phase, and (ii) upon cooling under ca. 355 K in the N_TB phase, ν_1/2 increases exponentially. Subtraction of the linewidth distributions at the phase transitions reveals that ν_1/2 remains independent of temperature in the isotropic and nematic phases, and partially also in the twist-bend nematic phase. The assumption that the ¹²⁹Xe linewidth reveals the T₂ relaxation rate, 1/T₂, leads to the conclusion that the rate is independent of temperature in the isotropic and nematic phases and partially also in the twist-bend nematic phase.