DEUTERIUM NMR INVESTIGATION OF A DISCOTIC MESOGEN BASED ON HEXASUBSTITUTED TRUXENE

We report a H-2 nuclear magnetic resonance (NMR) investigation of 2,3, 7,8,12,13-hexakis(octadecanoyloxy)truxene (TxHA17) specifically labele d in the ct-position of the alkyl chain. Results from these measuremen ts, together with polarizing microscopy, show that the substance is in a crystalline state up to 90 degrees C. At this temperature the compo und melts and a uniaxial discotic mesophase appears. The liquid crysta lline phase persists until the clearing point at 191 degrees C. The di fferential scanning calorimetry thermograms confirmed these transition s but exhibited additional peaks below 90 degrees C. The phase behavio r at low temperatures (<90 degrees C) differs significantly from previ ous reports by Warmerdam et al. (Liq. Cryst. 1988, 3, 149 and 1087). S olid-state deuterium Line shape simulations were used to examine the m otions of the labeled methylene site. A Gaussian libration model gave excellent agreement between experimental and calculated H-2 NMR spectr a. A pronounced spatial inhomogeneity of the motion was found in the h igh-temperature region of the solid phases. By combining Line shape si mulations with nuclear spin relaxation measurements, mean correlation times for the alpha-site could be estimated. It was shown that there e xist, even at -100 degrees C, fast motions of the alpha-deuterons with rates relatively close to the Larmor frequency. Deuterium NMR spectra of benzene-d(6) dissolved in protonated TxHA17 are also reported. The se spectra were interpreted using a model in which pre benzene molecul es occupy two solvation sites: (i) intercalated within the columns of TxHA17 and (ii) dissolved in the aliphatic chains. The exchange of C6D 6 between the two sites is slow on the NMR time scale in the crystalli ne phase whereas it is fast in the mesophase.