Study of molecular order and dynamics in calamitic and discotic liquid crystals by 2H NMR
Deuterium nuclear magnetic resonance (NMR) spectroscopy was used to explore molecular motions in the mesophases of calamatic liquid crystals 4-' n'-pentyloxybenzylidene-4'-heptylaniline (50.7), ' '-(methoxybenzylidine)-'p'-n-butylaniline(MBBA or 10.4), 4-'n'-hexyloxy-4'-cyanobiphenyl (60CB), a mixture of 60CB and 4-'n'-octyloxy-4'-cyanobiphenyl (80CB), and discotic liquid crystals hexakis('n'-hexyloxy)triphenylene(HAT6). The Zeeman and quadrupolar spin--lattice relaxation times were measured as a function of temperature at 15.1 MHz and 46 MHz using a broadband multiple--pulse sequence. In addition, quadrupolar splittings were measured for 60CB, 60CB/80CB mixture and columnar phase of HAT6. The TZ model was used to interpret the deuteron relaxation of biaxial molecules in the uniaxial medium. For 50.7, the analysis of the relaxation data in the nematic and smectic A phases supports a model which includes director fluctuations and rotational diffusion of an asymmetric rigid rotor in a biaxial potential of mean torque. The molecular biaxiality of the molecule is found, based on the relaxation data, to give a small positive molecular biaxial order parameter 'S'' xx'-'Syy'. In addition, the activation energy for the tumbling motion of the molecule is found to be larger than that for the spinning motion. For MBBA, the zero-frequency spectral densities ' J'0(0) data is quantitatively interpreted using a model that includes director fluctuations and rotational diffusion of symmetric rotors in a nematic phase. The contribution to 'J'0(0) from director fluctuations has mainly a second-order component, whereas the first-order contribution to 'J'1([omega]) is suppressed in the megahertz region (Larmor frequencies are 15.1 and 46 MHz) due to the high-frequency cutoff, which is estimated to be around 3-10 MHz for MBBA. For 60CB and 60CB/80CB mixture, the data analyses were carried out for both samples in order to achieve a consistent physical picture. The additive potential method is employed to model the quadrupolar splittings of 60CB, from which the potential of mean torque is parametrized, and the order parameter tensor for an "average" conformer is determined. A decoupled model is used to describe correlated internal motions of the end chain, which are independent of the molecular reorientation. The latter motion is treated using the small-step rotational diffusion model of Tarroni and Zannoni, while the former motion is described using a master rate equation.