FT-IR study of liquid crystal molecules on alignment layers

Alignment control at the molecular level is crucial for realizing high-performance LCDs. In particular, the structure of the interface between the liquid crystal and the alignment layer must be clarified. By utilizing RAS (reflection-absorption spectroscopy), a highly sensitive FT-IR spectroscopic technique, we have obtained the following information about the orientation of liquid crystal molecules; (1) the cyano group in a liquid crystal molecule behaves like an electron donor with respect to an SiO layer, but like an electron acceptor with respect to a polyimide layer; (2) with the use of polarized IR spectroscopy, it was discovered that the liquid crystal 8CB is aligned with a slant away from the aligning direction, regardless of the type of alignment layer. The angle between the molecular short axis within the plane of the core and the surface normal was found to be smaller on polyimide layers than on SiO layers.     

Langmuir-Blodgett films as aligning layers for homeotropic alignment of liquid crystal molecules

The first single crystal structure of a Group VA halide salt with three equivalent long n-alkyl chains, benzyltrioctadecylammonium bromide (BzN18Br), is reported. It consists of alternating interdigitated and non-interdigitated regions of alkyl chains separated by ionic planes. Two chains per molecule are paired and extend to one side in a non-interdigitated region. The third chain is on the opposite side of the ionic plane and pairs intermolecularly to form an adjacent, interdigitated region. The thickness of two nearly extended molecules defines the bilayer unit-two ionic planes flanked by a region with intramolecularly paired chains and separated by an interdigitated chain region. Powder X-ray diffraction and optical microscopy data of liquid crystalline BzN18Br are consistent with an enantiotropic smectic A₂ (SmA₂) phase: the three n-alkyl chains of each molecule are projected from one side of an ionic plane, and head groups of neighbouring molecules are oriented head-to-head, in a non-interdigitated bilayer assembly. The structure of BzN18Br fills an important gap in our knowledge about the crystal packing of ammonium and phosphonium salts with one-four equivalent long n-alkyl chains. A comparison of their packing arrangements is made and the transitional nature of the BzN18Br structures is demonstrated. Although salts with one, two, or three long n-alkyl chains form SmA₂ phases, each is distinctive in its molecular packing. A large molecular reorganization accompanies the crystal-to-liquid crystal transition of BzN18Br.     

Constrained force constants for octahedral molecules

A set of constrained force constants has been derived from experimental vibrational frequency data for eighty three octahedral molecules. Superimposing the condition that the larger value for f_d—f_dd′ be used when f_dα—f_dα″ is a maximum on the six equations relating vibrational frequencies to force constants generates a seventh. This provides a uniform set of results for all 83 molecules. The values of the force constants have a simple rationale in terms of chemical bonding theory. Some preliminary calculations for SF₆ show that these force constants are suitable for use in generating reliable molecular dynamical trajectory data.     

Time-dependent CO2 sorption hysteresis in a one-dimensional microporous octahedral molecular sieve

The development of sorbents for next-generation CO(2) mitigation technologies will require better understanding of CO(2)/sorbent interactions. Among the sorbents under consideration are shape-selective microporous molecular sieves with hierarchical pore morphologies of reduced dimensionality. We have characterized the non-equilibrium CO(2) sorption of OMS-2, a well-known one-dimensional microporous octahedral molecular sieve with manganese oxide framework. Remarkably, we find that the degree of CO(2) sorption hysteresis increases when the gas/sorbent system is allowed to equilibrate for longer times at each pressure step. Density functional theory calculations indicate a "gate-keeping" role of the cation in the tunnel, only allowing CO(2) molecules to enter fully into the tunnel via a highly unstable transient state when CO(2) loadings exceed 0.75 mmol/g. The energy barrier associated with the gate-keeping effect suggests an adsorption mechanism in which kinetic trapping of CO(2) is responsible for the observed hysteretic behavior.     

Hydrogen molecules trapped in irradiated solid ethanol

Raman spectroscopy of γ-irradiated solid ethanol at 77 K revealed that trapped H₂ is produced mainly by the H atom abstraction at the alkyl group by H radicals which are dissociated from the alkyl group of ethanol. Comparison of the peak wavenumbers and the band widths of H₂ between the glassy and crystalline phases of ethanol suggests that the trapping sites of H₂ in the glassy phase are smaller in the average size and less homogeneous in the size distribution. The molar ratio between ortho- and para-H₂ trapped in irradiated crystalline ethanol is in rough agreement with the thermodynamic equilibrium between 5–120 K, intimating that the ortho-para conversion is allowed under the existence of an electro-magnetic gradient due to paramagnetic radicals.     

Characterization of force fields for lipid molecules: Applications to crystal structures

The nonbonded portion of a force field for lecithins was characterized by application to the study of the crystal packing geometry and energetics of eight different molecules. The molecules were either lecithin fragments or chosen to isolate particular intermolecular features to test the accuracy of the force field specifically for those interactions. In particular, the hydrocarbon interactions, hydrogen bonding, electrostatics, and phosphate interactions were critiqued. The results support previous findings that indicated that this force field is reasonably accurate for lecithins. For all molecules, a minimum was found near the experimentally determined crystal structure. Using D-glucitol as an example, it is shown that the structural effect of hydrogen bonding is better represented by a nonelectrostatic force-field model than by a purely electrostatic model. Results obtained with glycerylphosphocholine and four smaller organic phosphate molecules suggest that further study of nonbonded interactions of phosphate groups is needed. © John Wiley & Sons, Inc.     

Vibration-rotation spectroscopy of molecules trapped inside C60

A simple model is developed to treat the energy levels and spectroscopy of diatomic molecules inside C 60. The C 60 cage is treated as spherically symmetric, and the coupling to the C 60 vibrations is ignored. The remaining six degrees of freedom correspond to the vibrations and rotations of the diatomic molecule and the rattling vibration of the molecule inside the cage. By using conservation of angular momentum, we can remove two of these motions and simplify the calculations. The resulting energy levels are simple and can be labeled by a set of quantum numbers. The IR and Raman spectra look like those of gas-phase diatomic molecules at low temperatures. At higher temperatures, hot bands due to the low-frequency rattling mode appear, and the spectrum becomes congested, looking like a solution spectrum.     

Time-dependent multiconfiguration theory for electronic dynamics of molecules in intense laser fields: a description in terms of numerical orbital functions

The equations of motion (EOMs) for spin orbitals in the coordinate representation are derived within the framework of the time-dependent multiconfiguration theory developed for electronic dynamics of molecules in intense laser fields. We then tailor the EOMs for diatomic (or linear) molecules to apply the theory to the electronic dynamics of a hydrogen molecule in an intense, near-infrared laser field. Numerical results are presented to demonstrate that the time-dependent numerical multiconfiguration wave function is able to describe the correlated electron motions as well as the ionization processes of a molecule in intense laser fields.     

Liquid crystal properties resulting from synergetic effects between non-mesogenic organic molecules and a one nanometre sized octahedral transition metal cluster

This work presents the synthesis and studies of a red-NIR luminescent liquid crystal compound based on an octahedral metallic cluster core orthogonally bounded to six non-mesogenic organic ligands. It evidences synergetic effects between the organic and inorganic parts of the hybrid, resulting in the generation of liquid crystal properties on cooling from the isotropic melt.     

Self-constructed stable liquid crystal alignment in a monomer-liquid crystal mixture system

Here, we demonstrate excellent liquid crystal (LC) vertical alignment without using an alignment layer printing process by introducing octadecyltrichlorosilane (OTS) into the LC mixture. Further, we investigated the alignment mechanism by analysing the surfaces of the substrates. The optimum concentration of OTS was found to be about 0.03 wt%, which is 1/100 of that in the previously reported polyhedral oligomeric silsesquioxane (POSS)–LC system. Moreover, the OTS–LC system exhibited a more stable LC alignment compared with the POSS–LC system. These differences may arise from the different strengths of surface–dopant interactions; that is, the covalent bond in the OTS–LC system and the van der Waals interactions in the POSS–LC system. We also demonstrated that the method can be used in a capillary tube, which may serve as a new method facilitating the application of LCs with curved surfaces.     

Super crystal structures of octahedral c-In2O3 nanocrystals

The three-dimensional self-assembly of a nanocrystal superlattice, i.e., a super crystal, has attracted increasing attention. The small building blocks for assemblies are usually spherical nanocrystals. Recent progress indicates that it is possible to achieve a super crystal using cubic nanocrystals. We further analyze and describe two-dimensional and some three-dimensional assemblies of uniform cubic-phase In2O3 nanocrystals with an octahedral shape. In this article, we demonstrate our amazing observations on these kinds of super crystals (or superlattices) as a model system, report their scale in at least tens of microns, and show other interesting features such as steps, terraces, kinks, and vacancies which are similar to those from a single crystal. Based on electron microscopy observations, three types of well-defined octahedral nanocrystal packed structures in such super crystal systems are also identified. The investigation of octahedral super crystal systems provides an alternate direction in research that may extend the interest of superlattice study to a broad spectrum by enriching and varying the shape of elemental building blocks. This may potentially result in new concepts and more challenging applications such as soft X-ray photonics.     

Photoaddressable polymers for liquid crystal alignment

We demonstrate reversible photoinduced in situ reorientation of low molecular mass liquid crystals (LCs) by means of photoaddressable polymers (PAPs). These polymers contain mesogenic azobenzene side chains optimized to reorient cooperatively and effectively upon illumination with polarized light. Various low molecular mass LCs were introduced between two PAP layers and these sandwich devices were tested with respect to stability and reversibility of photoinduced orientation. Dissolution of the PAP layer by the low molecular mass LC was observed for several material combinations and systematically investigated. Different anisotropic dyes were added as fluorescence markers in order to monitor the photoinduced LC orientation. With an optimized material combination, more than 10 reversible reorientation processes could be realized with polarized light of either 514 or 405 nm wavelength, without any reduction in alignment quality. Further, microscopic polarized fluorescence patterns could be produced and erased within short exposure times.     

Electric fields around molecules

The molecular cavity surface proposed by Hermann has been used to define electrical indices for each atom in a molecule. A method of calculating these is proposed which appeals to fractal ideas. To illustrate the utility of these indices an approximate calculation of their values integrated over the surface of the molecule has been made for a variety of molecules and the results correlated with experimental solubility data. The resulting calculated solubilities are in good agreement with the observations. The value of these indices in discussions of molecular recognition is argued.     

Photoaddressable polymers for liquid crystal alignment

We demonstrate reversible photoinduced in situ reorientation of low molecular mass liquid crystals (LCs) by means of photoaddressable polymers (PAPs). These polymers contain mesogenic azobenzene side chains optimized to reorient cooperatively and effectively upon illumination with polarized light. Various low molecular mass LCs were introduced between two PAP layers and these sandwich devices were tested with respect to stability and reversibility of photoinduced orientation. Dissolution of the PAP layer by the low molecular mass LC was observed for several material combinations and systematically investigated. Different anisotropic dyes were added as fluorescence markers in order to monitor the photoinduced LC orientation. With an optimized material combination, more than 10 reversible reorientation processes could be realized with polarized light of either 514 or 405 nm wavelength, without any reduction in alignment quality. Further, microscopic polarized fluorescence patterns could be produced and erased within short exposure times.     

Reactions of photogenerated fluorine atoms with molecules trapped in solid argon

Isolated radicals^.NH₂ and radical-molecule complexes^.NH₂−HF, which are products of the reactions of mobile fluorine atoms with NH₃ molecules in solid argon, were identified by EPR spectroscopy. The isotropic HFC constants of the complex (a _N=1.20,a _H=2.40, anda _F=0.70 mT) were determined experimentally. The constant of isotropic HFC with the nucleus of hydrogen atom of the HF molecule is less than 0.1 mT. This assignment was confirmed in the experiments on isotope substitution of atoms (H→D),¹⁴N→¹⁵N) in the NH₃ molecule. According to quantum-chemical calculations, the free complex^.NH₂−HF has a planar structure withC ₂, summetry and a binding energy of 12 kcal mol⁻¹. Optimization of the arrangement of the complex in the crystal showed that its structure is only slightly distorted in the Ar lattice so that the equilibrium configuration is close to that obtained from gas-phase calculations. Different ratios of relative intensities of the proton triplet lines in the EPR spectra of isolated^.NH₂ radicals and^.NH₂−HF complexes were qualitatively explained by different heights of the barriers to rotation of the NH₂ fragment in the Ar lattice.     

Hyperbranched polyimide application in liquid crystal alignment layers

Several hyperbranched polyimides (HBPIs) were applied in liquid crystal (LC) alignment layers and exhibited outstanding performance for LC alignment. The maximum pretilt angle was above 8°, and the minimum pretilt angle was 4.2°. The results of atomic force microscope measurement disclosed that a lot of grooves were aligned parallel to the rubbing direction and found that the grooves were not main factor for LC alignment. The LC alignment and pretilt angles are unambiguously associated with the intrinsic HBPI chemical structures. The results of thermal gravimetric analysis and ultraviolet–visible spectra showed that the HBPIs had good thermal stability and excellent transmittance. T₅ and T₁₀ were higher than 360°C and 400°C, respectively. Copyright © 2012 John Wiley & Sons, Ltd.