2H NMR Spectroscopic Investigation of para-Nitroazobenzene Liquid Crystals

The orientational order of two liquid crystals , namely, 6-［4-(4-nitrophenylazo)phenyloxy］ hexyl diethanolamines(C6) and 10-1-bromo［4-(nitrophenylazo)phenyloxy］ alkane(B10) was studied by means of ²H NMR spectroscopy with hexamethylbenzene-d18 as the probe molecule. The results show that the directors in the smectic A phase of C6 and the nematic phase of B10 could be aligned, which was parallel to the magnetic field. The orientational order parameter of the solute molecules in C6 was about 0.2, while it is only 0.1 in B10, which is expected because the more ordered smectic phase tends to align solute molecules to a high level. Compared to the orientational order parameter of the solute in the SmC phase of 4［3,4,5-tris(4-dodecyloxybenzyloxy)-benzoyloxy］-4-(4′-dodecyloxybenzoyloxy)biphenyl (Ⅰ) (P₂=0.14 ), it is larger in SmA phase of C6. The relatively higher orientational order parameter of the solute in C6 is attributed to the formation of intermolecular H-bonds in the SmA phase of C6.