摘要:

We explore the valence charge distribution, equilibrium geometry and harmonic force fields of the 4-pentyl-4-cyanobiphenyl (5CB) molecule and the benzene (C6H6) molecule, which provides an important mesogenic fragment, using first principles techniques adapted from large scale electronic structure calculations of periodic solids. We present for the first time accurate structural data for the isolated 5CB molecule and observe subtle broken symmetries relative to the constituent mesogenic fragments. The dynamic properties of these molecules are determined by diagonalization of dynamical matrices, the elements of which are obtained directly from quantum mechanical Hellmann-Feynman forces. Results for both molecules are in excellent agreement with available spectroscopic data, and for benzene are comparable to the most accurate quantum chemistry calculations to date. For 5CB we also present values for the molecular dipole and quadrupole moments.

ISSN:0267-8292    

DOI:10.1080/026782997209199

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The conformational energy landscape of the prototypical nematic liquid crystal 4-pentyl-4cyanobiphenyl (5CB) is studied using first principles computer modelling. It is found that the most favourable conformation occurs when the two constituent phenyl rings are inclined at an angle of 31 with respect to each other. Also, the orientation of the alkyl chain is found to have an important influence on the ring-ring torsional potential. We fit the energy surface of these coupled torsions to yield an accurate intramolecular potential for use in empirical modelling. To test the strength of the coupling between the alkyl tail and the phenyl rings and the cyano group, we also calculate potentials for the relative orientation of the phenyl rings in biphenyl and cyanobiphenyl (0CB). Our calculations are performed using density functional theory using pseudo-potentials and the generalized gradient approximation to exchange and correlation. The molecular electronic wavefunction is expanded in terms of a plane wave basis set. We compare our results with recent NMR and Gaussian-based quantum chemistry calculations where available.

ISSN:0267-8292    

DOI:10.1080/026782997209207

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A series of hydroxy-functionalized semi-flexible polyazomethines has been synthesized and characterized to investigate the effect of the structure of the diamine monomer on the thermal and mesogenic properties. The diamine monomer introduces structural modifications such as flexibility, lateral substitution, kinks or alteration of chain coaxiality. The mesomorphic phase of the mesogenic polymers was characterized as nematic in nature, but an evolution of the nematic into a smectic mesophase was observed for the polyazomethine derived from 3,3dimethoxybenzidine by annealing in the mesophase temperature range. Polyazomethines derived from aliphatic diamines were complexed with copper(II) to study the dependence of type of complexation on length of the aliphatic diamine.

ISSN:0267-8292    

DOI:10.1080/026782997209216

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Recently we have been concerned with a systematic and fundamental investigation into the effects of the linking group on the lyotropic and thermotropic properties of aliphatic substituted acyclic carbohydrate systems. For this purpose, we chose to study the selfassembling behaviour of alkyl substituted xylitols where the aliphatic chain was attached to the xylitol moiety via ether, ester and thioether linkages. We have extended this work and, in this current study, we report the effects on the self-assembling properties of sequentially moving the position of a dodecyl chain in acyclic x-O-dodecyl-D-xylitols. This work was used for a direct comparison with results reported on cyclic systems such as the dodecyl x-O-beta-Dglucopyranosides.

ISSN:0267-8292    

DOI:10.1080/026782997209225

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The mesomorphic properties of conventional rod-like liquid crystals (diphenyl1,3,4-thiadiazoles, diphenylpyrimidines, diphenyltriazines, diphenyltetrazines and p-terphenyl derivatives), of macrocyclic liquid crystals and of dimesogens can be influenced by addition of the electron acceptor 2,4,7-trinitrofluorenone (TNF). Thereby nematic and smectic C phases are suppressed and smectic A phases can be stabilized or induced. Long and branched terminal chains result in a strong stabilization of the SA phase, whereas no smectic phase is induced to accompany the nematic phases of mesogens with short terminal chains.

ISSN:0267-8292    

DOI:10.1080/026782997209234

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

High pretilt angles of nematic liquid crystals (NLC) have been successfully generated on rubbed organic solvent soluble polyimide (PI) surfaces with a helical backbone structure and trifluoromethyl moieties. We have found that the pretilt angle of in 4-n-penty-4-cyanobiphenyl (5CB) on rubbed PI surfaces with trifluoromethyl moieties attached to the lateral benzene rings is larger compared to that on rubbed PI surfaces with trifluoromethyl moieties attached to the polymer backbone. The observed pretilt angle of 5CB on unidirectionally rubbed PI surfaces with trifluoromethyl moieties attached to the lateral benzene rings is about 15 degrees in the wide rubbing region. From these results, it is expected that the microscopic surface structure of the polymer strongly contributes to the pretilt angle generation of the LCs at the surface.

ISSN:0267-8292    

DOI:10.1080/026782997209243

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

It is possible to express the transition temperature nematic-isotropic (TNI) for nematogens with just two aromatic ring systems as the sum of incremental values of substructural elements, denoted as substituents and bridging moieties. Of the 1426 mesogens that can be constructed by the elements for which the increments have been given, the TNI values of most can be predicted to within 10 C, the only notable exception being the 1,2-diphenylethenes.

ISSN:0267-8292    

DOI:10.1080/026782997209252

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor