Nuclear magnetic resonance studies of smectic liquid crystals

Abstract

Nuclear magnetic resonance measurements in the smectic liquid crystalline phases of seven thermotropic compounds are reported. The compounds investigated are: terephthalbis(butylaniline); ethyl(methoxybenzylidene)amino] cinnamate; diheptyloxyazoxybenzene; octyloxybenzoic acid; dipentylazoxycinnamate; diethylazoxycinnamate; diethylazoxybenzoate. Proton spectra at 100 MHz of the neat liquid crystal molecules, as well as of simple probe molecules dissolved in small amounts in the liquid crystal, were studied in their dependence on temperature and on sample orientation with respect to the magnetic field. The results show that good alignment of the smectic phases can be obtained by cooling from the isotropic or the nematic phase in a magnetic field; once aligned, the smectic planes conserve their original orientation on rotation of the magnetic field. The results are consistent with the following models of the smectic phases: Smectic‐A is uniaxial and there is fast rotational and translational molecular diffusion. Smectic‐C is similar to smectic‐A, except that the molecules are tilted with respect to the smectic layers. A model, in which the direction of tilt is free to reorient, is used to interpret the behavior of the NMR spectra of smectic‐C on rotation of the magnetic field. The magnitude of the tilt angle can be estimated from such an interpretation. Two of the compounds studied have a smectic‐B phase. In one it is uniaxial, while in the other it is biaxial. The NMR results are consistent with this classification. They also show that molecular diffusion is considerably slower in the smectic‐B than in the smectic‐A and ‐C phases: the correlation time is of the order of 10⁻⁶ sec. In an appendix the formal theory of molecular orientation in a biaxial liquid crystal is given.