Study of Liquid Crystals Showing Two Isotropic Phases by 1H NMR Diffusometry and 1H NMR Relaxometry.

In this work, we report a study of two thermotropic liquid crystalline samples showing a not common mesophase behavior. The samples, namely a di-benzyloxy biphenyl derivative labelled 9/2 RS/RS, and a bimesogenic liquid crystal labelled L1, show a direct transition between two isotropic phases followed, at lower temperatures, by the optically isotropic, 3D structured, cubic phase. These systems have been investigated by means of ¹H NMR diffusometry and ¹H NMR relaxometry in order to characterize their isotropic–isotropic'–cubic mesophase behavior, mainly on the dynamic point of view. In particular, the temperature trend of the self-diffusion coefficients measured for both samples allowed us to significantly distinguish between the two isotropic phases, while the temperature dependence of the ¹H spin-lattice relaxation time (T₁) did not show significant discontinuities at the isotropic–isotropic' phase transition. A preliminary analysis of the frequency-dependence of ¹H T₁ at different temperatures gives information about the main motional processes active in the isotropic mesophases. [ABSTRACT FROM AUTHOR]