Phase transitions in a high magnetic field of an odd, symmetric liquid crystal dimer having two nematic phases, N U and NTB, studied by NMR spectroscopy

Both H1 and C13 NMR spectra have been obtained in a static magnetic field of 23.5 T on a bent-shaped dimer molecule, 1′′,7′′-bis(4-cyanobiphenyl-4′-yl) nonane (CB9CB), which shows the sequence of liquid crystal phases twist-bend nematic, NTB, and uniaxial nematic, NU, before entering the isotropic phase. The H1 spectra are used to locate the temperature at which the sample melts to form a twist-bend nematic, TCrNTB, and then TNUI when the isotropic phase is entered, both in a magnetic field of 23.5 T, and to compare these with those measured at the Earth's field. The differences between these transition temperatures are found to be zero within the error in their measurement, in stark contrast to previous measurements by Salili et al. [Phys. Rev. Lett. 116, 217801 (2016)10.1103/PhysRevLett.116.217801]. In the isotropic phase in the presence of the field the sample exists in a paranematic phase in which the molecules of CB9CB are partially ordered. The H1 and C13 NMR spectra in the paranematic phase are used to measure the critical temperature T∗ below which this phase is unstable. The spectra are also used to study the structure, molecular orientational order, and distribution of molecular conformations in the paranematic phase.