Hirokazu Toriumi

Time-resolved vibrational spectroscopy of an electric field-induced transition in a nematic liquid crystal by use of step-scan 2D FT-IR

Abstract A study of the kinetics of reorientation of a uniaxially aligned nematic liquid crystal (4-pentyl-4-′-cyanobiphenyl) under the influence of an external ac electric field is reported. Step-scan FT-IR was employed for the acquisition of the dynamic linear dichroism spectra. 2D FT-IR frequency correlation maps indicate that the rigid core of the nematic molecule reorients as a unit and suggest that the phenyl chain may reorient more rapidly than the core.

Molecular flexibility and orientational ordering of nematic liquid crystals

The ‘‘chord model’’ of flexible alkane solute ordering in the nematic phase [D. J. Photinos, E. T. Samulski, and H. Toriumi, J. Phys. Chem. 94, 4688 (1990); 94, 4694 (1990)] is extended to prolate mesogen molecules with a single alkyl tail, and the physical properties of two homologous series, 4,4’‐n‐alkylcyanobiphenyls and 4,4’‐n‐alkyloxycyanobiphenyls, in neat nematic phases are evaluated. The chord model of the mesogen’s anisotropic interactions yields a consistent description of the essential features of the chain order profile (derived from deuterium nuclear magnetic resonance) and the even–odd variation of thermodynamic quantities associated with the nematic–isotropic transition. Very accurate agreement with experimental observations is obtained using the standard rotational isomeric state description of the tail–chain’s flexibility along with a minimal form of the self‐consistent anisotropic potential (i.e., only two coupling constants determine the strength of orientation dependent interactions, o...

Alkyl chain flexibility in discotic columnar mesophases

A steric model with a single adjustable parameter is employed to compute configurationally averaged quadrupolar splittings of deuterium labeled alkyloxy chains in a homologous series of hexaalkyloxytriphenylenes (THEn). A comparison with new experimental data [Goldfarb et al., J. Chem. Phys. (in press)] permits inferences about the alkyl chain flexibility in discotic mesogens. The parameter, a control of the constraints on the chain mobility, is first optimized by fitting deuterium NMR data in a homolgous series of nematogens with labeled alkyloxy chains. The same optimum value of the parameter is subsequently used to model the quadrupolar splittings in the columnar phase of THEn discotigens and succeeds in qualitatively accounting for the experimental observations. This result indicates that alkyl chain mobility is quite comparable in these two classes of liquid crystals. The distinctly different features of the quadrupolar splitting patterns observed for nematogens and columnar discotigens are shown to ...

Molecular ordering in nematics : the modular formulation of the potential of mean torque

Abstract The general framework for the description of orientational ordering of (flexible) molecules in the nematic phase in terms of ordering properties of submolecular units in a modular fashion is presented. Various choices of the elementary submolecular units (segments) leading to different representations of the potential of mean torque are discussed. Transformations among such representations are studied in order to clarify the physical significance of intersegmental correlation terms. It is shown that for n-alkane solutes and chains pendant to nematogenic cores in neat materials, the uncorrelated bond representation (each C‒C bond segment individually interacts with the nematic mean field) used in most of the previous studies of this type gives a poor description of chain ordering. The introduction of orientational correlations among bond segments (particularly nearest neighbor correlations) improves the quality of fits of the order parameter profiles and explains the difficiency of previous models...

Observation of Nematic Liquid Crystal Director Reorientation at the Interface between Substrate and Liquid Crystal Layer by Total Reflection Ellipsometry.

The liquid crystal (LC) director reorientation near the interface between the substrate and the LC layer was studied using total reflection ellipsometry (TRE). The dynamic and static responses of the ellipsometric angles to the electric field were measured by TRE. It was experimentally confirmed that total reflection occurs in the LC cell. The reflected light is dominantly modified by the interfacial LC director reorientation. The polar anchoring strength of the cell was estimated by analyzing the electrical response of the phase difference response. We confirm that the TRE is useful for studying the LC director reorientation near the interface between the substrate and the LC layer.

Time-Resolved FT-IR Analysis of Electric Field-Induced Reorientation Dynamics in Nematic 4-Pentyl-(4-cyanophenyl)-Cyclohexane Liquid Crystals

Abstract The application of rapid-scan FT-IR time-resolved spectroscopy to the study of molecular reorientation dynamics in a nematic liquid crystal during the electric field-induced homogeneous-to-homeotropic transition is reported. The average orientation of bond segments in a homogeneously oriented 4-pentyl- (4-cyanophenyl)-cyclohexane liquid crystal is determined from static infrared dichroism measurements. The observed temperature dependence of the molecular orientational order parameter agrees with the prediction of the Maier-Saupe theory. Analysis of time-resolved dichroic spectra shows that the homogeneous-to-homeotropic transition requires an induction period of several milliseconds and is completed in 10-20 ms. The cyanophenyl group reorients as a unit in response to the applied electric field and the reorientation rate increases as temperature is increased.

Anisotropic dispersion interactions in liquid crystals

Abstract The polarizability and shape anisotropics of a new class of aliphatic liquid crystals, 4,4′-di-n-alkyl-bibicyclo[2.2.2]octanes (n, n-BBCO) have been calculated as a function of the alkyl chain length and its degree of flexibility to inquire about the relative importance of attractive and repulsive interactions in stabilizing the liquid crystalline phase. The calculated polarizability anisotropics are found to be much smaller than that of conventional mesogens. In the context of the Maier-Saupe theory, this indicates that anisotropic dispersion interactions make only a minor contribution to the stability of this class of liquid crystals. The cylindrical symmetry exhibited by the n, n-BBCO homologues and their rigid molecular structure suggest that they might be ideally suited for critical testing of existing theories of the nematic-isotropic phase transition.

Design and Implementation of Near-Field Scanning Optical Microscope for Observation of Interfacial Liquid Crystal Orientation

A near-field scanning optical microscope (NSOM), optimized for observation of liquid crystal (LC) orientation at the LC/alignment layer interface, has been developed. The NSOM operates in the illumination mode using a metal-coated optical fiber probe together with a probe-positioning system based on the optical feedback technique. This novel system enables the depth profiling and two-dimensional imaging of the LC orientation distribution under an applied local electric field with high spatial resolution. In this paper we describe the design and implementation of the developed NSOM system and report its first application to the microscopic analysis of LC orientation at the LC/alignment layer interface.

Analysis of Totally Reflected Light from Liquid Crystal Cell using Renormalized Ellipsometry

Standard ellipsometry is usually applied for the evaluation of an optical isotropic system, but it is insufficient to analyze light reflected from an anisotropic system such as a liquid crystal (LC) cell. In order to analyze the light reflected from the LC cell, the standard ellipsometry system combined with the photo elastic modulator was extended to renormalized ellipsometry by introducing the definition of the renormalized reflection coefficients without any modification of the conventional experimental procedure. The experimental results such as the ellipsometric angles versus the rotation angle of the LC cell and the dependence of the ellipsometric angles on the applied electric field were in good agreement with the theoretical results based on renormalized ellipsometry. From these results, it was confirmed that renormalized ellipsometry was quite a useful technique to analyze the totally reflected light from the LC cell.

Molecular flexibility in nematics: from alkanes to dimer mesogens

The ‘chord model’ of flexible alkane solute ordering in the nematic phase (D. J. Photinos, E. T. Samulski and H. Toriumi, J. Phys. Chem., 1990, 94, 4688; 4694) is extended to dimer molecules. Using the same parametrization of the modular potential of mean torque that successfully describes alkane solutes in the nematic field and monomer mesogens in neat phases, we are able to generate correctly the segmental orientational ordering profiles of a variety of mesogenic and non-mesogenic dimer solutes in a nematic solvent. The effects of the linkage between the mesogenic core and flexible spacer on the ordering properties of the dimers are explored by studying the homologous series of cyanobiphenyl (CB) ethers, CBO—(CH2)N—OCB, (N= 2–10) with different primary structural topologies, i.e. with para, meta and ortho linkages. The observed 2H-NMR spectra of para-linked dimers with N= 9, 10 and of ortho-linked dimer with N= 10 are readily reconstructed using the simplest parametrization of the chord model potential. This parametrization is then used to predict the 2H-NMR spectra of the respective meta-linked dimers and also to make inferences about how the propensities of the topologically different series of dimers for conforming to the nematic field depend on spacer length and spacer parity. Finally, the effect of the core size on the overall ordering of dimers is studied by considering para-cyanophenyl (CP) ether linked dimers, CPO—(CH2)N—OCP. It is found that, although the NMR spectra of the spacer chain in the CPO dimer series are not profoundly different from those of the respective CBO dimers, the average value of their orientational energy is indicative of substantially reduced (self) orientability of these dimers. Generally, these findings corroborate the predictive power of the chord model and its utility for describing orientational ordering of flexible solutes, even when molecular topology is complex.