Thermotropic Liquid Crystals from Planar Chiral Compounds: [2.2]Paracyclophane as a Mesogen Core

Abstract

New thermotropic mesomorphic compounds containing a [2.2]paracyclophane (PC) unit were synthesized and investigated (Scheme). Carboxylic acids were selected as the starting PC building blocks. The influence of structural features on the stability of the mesomorphic phases was studied (Figs. 1 and 2): for this purpose, the structures of the PC-carboxylate unit and the organic fragment of the aryl-ester moiety were varied systematically. Esters derived from PC-monocarboxylic acid did not exhibit liquid-crystalline (LC) properties, while diaryl PC-dicarboxylates favored mesomorphism. Dicarboxylate substituents arranged in the paraposition provided LC phases in a broad temperature range and considerably increased the mesomorphic interval in comparison with that of the structurally related pseudo-para PC derivatives.Introduction. ± Unique physical properties, providing wide application in electrooptic devices and particularly the recent discovery of new mesophases, such as antiferroelectric, ferrielectric, and twist-grain boundary phases, have put chiral liquidcrystalline (LC) materials in the centre of liquid-crystal research [1] [2]. Historically, up to now, most chiral LC compounds contain molecules incorporating a centre of chirality. More recently, compounds with planar and axial chirality are beginning to attract the growing interest of researchers dealing with LC materials. The unique geometry of this type of compounds provides a rigid interaction between the origin of the symmetry-breaking chirality and the mesogenic unit, thus resulting in a definite fixation of the source of chirality in a LC matrix.In principle, planar chirality could drastically change the physical properties of the resultant LC material: 1) In terms of molecular structure, a rigid steric coupling between the transverse dipole, chiral centre, and the mesogen rod should maximize the local ferroelectric polarization [3], allowing consideration of the above compounds as possible chiral dopants for ferroelectric compositions, which are able to confer a high value of spontaneous polarization (P s ) to the achiral host [4]. 2) Electro-optical effects exhibited by LC materials such as electroclinic effects or nonlinear optical (NLO) properties are expected to be larger for compounds with rigid interaction between the chiral and optical portions of the molecule [5].However, the bulkiness of the structural units necessary to realize an axis or a plane of chirality significantly disturbs the desired rigid rod-like shape of the molecule, thus creating a serious obstacle to achieving stable mesomorphic properties. So far, only a

Thermotropic Liquid Crystals from Planar Chiral Compounds: Optically Active Mesogenic [2.2]Paracyclophane Derivatives

“…Elemental analysis calcd for C 18 H 16 O 4 (296.322): C 72.96, H 5.44; found: C 72.82, H 5.41. (All spectral data are identical with those of racemic 1 10…”

“…In our first attempt to investigate the ability of the PC unit to promote mesomorphism we used several racemic derivatives 10. As shown by these exploratory investigations, the mesogenic potential of the PC core depends on the number and position of substituents in the PC moiety: of the structures investigated, para ‐substituted PC derivatives had superior mesogenic properties with a broad interval of either nematic or smectic A (SmA) phases.…”

“…As mentioned above, para ‐substitution of the PC moiety was shown to best induce mesomorphism,10 but known examples of specific para‐ PC derivatives are still scarce and a general synthetic approach to such structures is not yet available. We decided to use PC carboxylic acids, rather than the possible (and available) PC phenols or aldehydes, as the most promising starting materials to construct LC compounds.…”

“…Optical resolution of [2.2]paracyclophane‐4,7‐dicarboxylic acid ((±)‐ 1 )10 was performed with ( S )‐phenylethylamine (PEA) through formation of the diastereomeric salt (−)‐ 2 . The enantiomeric excess ( ee ) of (−)‐ 1 was determined by converting it into the corresponding amide (−)‐ 3 with ( S )‐PEA.…”

Synthesis and characterization of side-chain liquid crystalline polysiloxanes containing α-terpineol in mesogenic group

“…In some cases the 1,4-cyclohexyl ring or 2,5-disubstituted 1,3-dioxane is also used to replace one of the phenylenes [9]. Over the past few years several modifications have been done on mesogenic group, as well as new meogens have been synthesized, which includes phenolphthalein as a part of the mesogen [10], 1,3,4-thiadiazole ring as mesogen [11], banana-shaped mesogen [12], disubstituted ferrocene ester as mesogen [13], [2.2]parafluoroalkyl-substituted carbohydrate-based mesogen [14], new mesogens with fluoro-substituted bent core [15], new smectic C mesogens containing a benzyl, phenylethyl or phenylpropyl moiety [16], new sheet-shaped mesogen based on 1,3,5-triazines [17], mesogens with cyclopentanone moiety [18] and others: the list is only illustrative but never exhaustive.…”