C13 NMR study of the director distribution adopted by the modulated nematic phases formed by liquid-crystal dimers with odd numbers of atoms in their spacers

The orientational order of the molecules in the bent mesogen CB6OCB has been studied throughout the range of temperature stability of both the $N_{\mathrm{U}}$ and $N_{\mathrm{TB}}$ liquid-crystal phases by ${}^{13}\mathrm{C}$ NMR spectroscopy. These spectra provide local order parameters for the para axes of both of the nonequivalent cyanobiphenyl groups and show how they change on entering the twist-bend nematic phase. A key feature of the order parameters is a weak, but clear maximum in the temperature variation of the order parameter prior to the $N_{\mathrm{TB}}$ phase. This suggests that the directors in both the $N_{\mathrm{U}}$ and $N_{\mathrm{TB}}$ phases are tilted with respect to the magnetic field of the spectrometer. Significantly the conformational states of the spacer are comparable in both phases, although the low temperature nematic is chiral but not that at high temperature. It is proposed that the higher temperature, tilted phase could be the splay-bend nematic phase.