Proton NMR Relaxation Study on the Nematic–Nematic Phase Transition in A131 Liquid Crystal

NMR studies of molecular ordering and molecular dynamics in a chiral liquid crystal with the SmC_{α}^{*} phase

Molecular dynamics of the antiferroelectric liquid crystal 4'-(octyloxy)biphenyl-4-carboxylate2-fluoro-4-[(octyl-2-yloxy)carbonyl]phenyl (abbreviated as D16) was investigated using different nuclear magnetic resonance (NMR) techniques. D16 molecules form a smectic-C_{α}^{*} phase (SmC_{α}^{*}) in an extremely wide temperature range (∼10 °C). Due to a small tilt of the molecules, this phase is characterized by short switching times, important for new photonic applications. The proton spin-lattice relaxation times were measured in isotropic (Iso), smectic-A (SmA), and SmC_{α}^{*} phases over a wide frequency range of five decades, with conventional and fast field-cycling NMR techniques. This approach allowed a comparison of the essential processes of molecular dynamics taking place in these phases. On the basis of NMR relaxometry measurements, we present a description of the motional behavior of liquid crystal molecules forming SmC_{α}^{*}. Pretransitional effects were observed in wide temperature ranges in both the isotropic and SmA phases in D16. The ^{1}H fast field-cycling NMR measurements were supplemented with NMR diffusometry and ^{19}F NMR spectroscopy.

1H NMR study of molecular order and dynamics in the liquid crystal CB-C9-CB

Molecular order and dynamics of the CB-C9-CB liquid crystalline dimer exhibiting the nematic (N) and the twist bend nematic (Ntb) phases were investigated by proton NMR spectroscopy, using fields of 0.78 T and 7.04 T, and relaxometry. The first relaxometry experiments for a very wide Larmor frequency domain (8 kHz-300 MHz) on this system, using a combination of standard and fast field cycling NMR techniques, were performed. The spectroscopy results in the Ntb phase allowed us to probe the local molecular orientation relative to the Ntb helix axis. The relaxation data were analyzed considering order director fluctuations (ODF), molecular self-diffusion (SD) and local molecular rotations/reorientations (R) relaxation mechanisms. Global fits of theoretical relaxation models, as a function of temperature and Larmor frequency, for the phases under investigation, allowed for the determination of rotational correlation times, diffusion coefficients, viscoelastic parameters, correlation lengths and activation energies (in the case of thermally activated mechanisms). A clear difference between the structures of the N and Ntb phases was detected from the results of proton spin-lattice relaxation through distinct temperature and frequency dependencies' signatures of the collective modes. Significant pre-transitional effects were observed at the N-Ntb phase transition both from relaxometry and spectroscopy data. The experimental results correlate to data and models for comparable liquid crystalline systems.

Milestone in the NTB phase investigation and beyond: direct insight into molecular self-assembly

Although liquid-crystalline materials are most widely exploited for flat-panel displays, their ability to self-organize into periodically ordered nanostructures gives rise to a broad variety of additional applications. The recently discovered low-temperature nematic phase (N(TB)) with unusual characteristics generated considerable attention within the scientific community: despite the fact that the molecules from which the phase is composed are not chiral, the helicoidal structure of the phase is strongly implicated. Here we report on combined experimental, computational and spectroscopic studies of the structural aspects influencing formation of the N(TB) phase as well as on the molecular organization within the phase. In an extensive DFT study, the structure-property prerequisite was traced to a "bent-propeller" shape of the molecule. We also demonstrate the first utilization of liquid state NMR for direct analysis of intermolecular interactions within thermotropic liquid-crystalline phases, providing new insight into molecular packing that can lead towards design of novel chiral functional materials. The synergy of experimental, computational and NMR studies suggests a syn-parallel helical molecular organization within the N(TB) phase.

Proton and deuterium nuclear spin relaxation study of the SmA and SmC* phases of BP8Cl-d17 : a self-consistent analysis

A self-consistent analysis of proton and deuterium nuclear spin relaxation times in the smectic phases of a partially deuterated smectogen is presented here. Proton spin-lattice relaxation times T(1Z) were measured as a function of Larmor frequency over a range of 1 kHz to 300 MHz at selected temperatures. Deuterium spin relaxation times T(1Z) and T(1Q) were measured as a function of temperature at two different magnetic fields in the smectic A phase. The deuterium data provide dynamic parameters such as rotational diffusion constants and internal jump rates as well as the nematic order parameter S. The proton data are analyzed using a number of relaxation mechanisms, one of which is the molecular reorientation. It is found helpful in these latter analyses to use the nematic order parameter and to fix the contribution from molecular reorientations determined by the deuterium spin relaxation. The fits to the proton T(1) frequency and temperature dispersions by the remaining relaxation mechanisms such as layer undulations and translational self-diffusion will be discussed for the smectic A and chiral smectic C phases.

NMR of bent-core nematogens: a mini-review

This review focuses on two of our studied bent-core liquid crystals 2-methyl-3-[4-(4-octylbenzoyloxy)-benzylidene]-amino-benzoic acid 4-(4-dodecylphenylazo)-phenyl ester A131, and a shape-persistent fluorenone mesogen, F1a by means of temperature-dependent (13)C NMR spectra and deuterium NMR spectra, respectively. Orientational order parameters, molecular packing, and/or conformations in the nematic phases or nematic glass can be extracted from the observed NMR spectra. These will be discussed in terms of certain motional models. In particular, F1a has been found to form a biaxial nematic glass. The derived orientational order parameters would put the biaxial system of phase symmetry lower than orthorhombic.