Carbon-13 nuclear magnetic resonance of ferroelectric liquid crystals with off-magic-angle spinning

Abstract

The orientational ordering of three ferroelectric liquid crystals 4′-(3-methyl-2-chloropentanoyloxy)-4-alkyloxybiphenyls has been studied by 2D carbon-13 nuclear magnetic resonance (NMR) with variable angle sample spinning. The three homologs have hexyloxy (C6), heptyloxy (C7), and octyloxy (C8) chains, respectively, at the 4 position. They exhibit smectic A (SA) and chiral smectic C (S*C; ferroelectric) phases and have unusually large spontaneous polarization of the electric dipoles. In the SA phase, macroscopic alignment of the molecular director along the spinning axis can be achieved by cooling the sample rapidly from the isotropic liquid. For the S*C phase, the molecular alignment is different for the three compounds. At a magnetic field of 7.05 T, the helical structures of C6 and C8 unwind and the molecular directors align along the spinning axis at a spinning rate of 1 kHz. However, under the same conditions, the helical structure of C7 is retained and its carbon-13 NMR spectrum shows a partial powder pattern. On the other hand, the racemate of C7 has a normal smectic C (SC) phase without a helical structure, and the molecular director can be easily aligned for the NMR study. The racemate of C8 was also synthesized and studied for comparison. The 2D NMR technique used was separated local field (SLF) spectroscopy with an efficient proton–proton dipolar decoupling sequence BLEW-48 in the evolution period. The carbon–proton dipolar coupling constants were determined and the order parameters for different molecular segments of the liquid crystal were calculated. The carbon-13 chemical shifts were also measured as functions of temperature and the data were used to probe the temperature dependence of order parameters. The results show that the C–H bond at the first chiral center in these compounds has an unusually large negative order parameter. This could be due to restricted rotation of the molecular segment at the chiral center, which would be related to the high spontaneous polarization of these compounds.