Spontaneous chiral domains in the nematic phase of achiral trans-4-alkylcyclohexanecarboxylic acids

Abstract

We have discovered a spontaneous chiral ordering in thermotropic nematic liquid crystals forming hydrogen bonds, namely in trans-4-alkylcyclohexan ecarboxylic acids, which do not contain any chiral group in the molecular structure, and have suggested a possible explanation of this phenomenon. The investigation was performed by polarizing light microscopy, using cells with glass substrates treated to give a uniform planar alignment in the nematic phase. On cooling from the isotropic phase, two types of nematic domains with diVerent alignment were found to grow: some possessing a unidirectional planar orientation as expected, and others with a spontaneous twisted molecular arrangement. The appearance of the twisted domains and the sense of their twist were random, i.e. they always emerged in various regions of the cell with diVerent twist sense: thus, they cannot be regarded as induced by stable defects in the surface aligning ® lm. With the coalescence of the nematic droplets, the number of the domains with the spontaneous twist very quickly decreased, since two chiral domains characterized by diVerent handedness transformed into a racemic domain with uniform planar alignment. Here the explanation of the macroscopic texture is given in terms of molecular structure, based upon the ability of the carboxylic acids to organize through hydrogen bonds into not only dimers, but also trimers and oligomers. The microscopic structure in¯uences the macroscopic twisted organization of the director inside the domains, and the surface lies normally to the helix axis. A brief review is given of work done in the ® elds of hydrogen bonded liquid crystals and of order transitions, together with a Landau± de Gennes model describing the mechanism of the spontaneous chiral ordering.

Texture transitions in the liquid crystalline alkyloxybenzoic acid 6OBAC

“…1), with concentrations depending on the temperature. Near the nematic-isotropic phase transition, as reported in [4], a process of oligomerization, with the formation of trimers, quadromers and so on, is possible: these oligomers produce a chiral nematic domain in a material with achiral molecules. In the 4,n-alkyloxybenzoic acids (nOBAC), the monomers are composed of two sterically distinct molecular parts, the oxybenzoic acid residue and the aliphatic chain.…”

“…Peculiar behaviours, like textures transitions in the nematic phase and the appearance of domains of spontaneous twist and dendrite structures [1][2][3][4][5], makes liquid crystals with hydrogen bonded dimers, such as 4,nalkyloxybenzoic acids, very attracting materials for the research of chiral behaviour in nonchiral compounds. In the nematic phase, the material is a mixture of monomers and dimers (closed and open, Fig.…”

Übertragung und Amplifikation von Informationen und Eigenschaften in nanostrukturierten Flüssigkristallen

“…[104] Obwohl es also offensichtlich möglich ist, eine enantioselektive Phasentrennung in der nematischen Phase achiraler gebogener Moleküle zu beobachten, ist es interessant, dass Strigazzi et al ein vergleichbares Modell erstellt haben, um ihre Beobachtung chiraler Domänen in der nematischen Phase stabförmiger achiraler 4-(Alkyloxy)benzoesäuren zu erklären. [105] Sie vermuten durch Wasserstoffbrücken zusammengehaltene verdrillte offene Dimere als Ursache für die Bildung helicaler Strukturen. [106] Damit in Einklang ist die Beobachtung von Jeong et al, dass die Verbindung 52 eine Form von Chiralität zeigt.…”

“…[105] Sie vermuten durch Wasserstoffbrücken zusammengehaltene verdrillte offene Dimere als Ursache für die Bildung helicaler Strukturen. [106] Damit in Einklang ist die Beobachtung von Jeong et al, dass die Verbindung 52 eine Form von Chiralität zeigt. [107] Sie zeigten, dass Dimere von 52 chirale propellerartige Konformationen einnehmen können, die dann zu helicalen nematischen Strukturen führen.…”

Hydrogen Bond Influence on Physical Properties of p-n-Propyloxybenzoic Acid – p-n-Propyloxy-p'-Cyanobiphenyl System

“…Подобные взаимодействия реализуются в жидко-кристаллических карбоновых кислотах и системах на их основе [2,3]. По данным ИК-спектроскопии, межмолекулярная водородная связь в производных карбоновых кислот достаточно сильна и составляет 6-8 ккал/моль.…”