On the blue phase of cholesterogenic liquid crystals. A deuterium NMR study

Abstract

Deuterium NMR measurements of cholesterogenic systems exhibiting the blue phase are reported. The measurements were performed on specifically deuterated cholesteryl nonanoate (CN) and cholesteryl myristate (CM) as well as on deuterated probe molecules dissolved in normal CN and CM. The blue phase exhibits NMR spectra that are distinctly different from both those of the cholesteric and isotropic phases. The spectral line shape is consistent with the following model for the blue phase: It is a liquid crystal made up of cholestericlike chiral segments whose elementary units are arranged in a cubic symmetry. The dimension of the elementary units are of the order of the cholesteric‐phase pitch length, i.e., several thousand angstroms. Translational diffusion of molecules along or between the chiral segments results in reorientational fluctuations of the director, which in turn affects the NMR line shape. The correlation times for the director reorientation calculated from this model and estimated translational diffusion coefficient for the various probe molecules are in excellent agreement with those derived from the experimental NMR line shape.