The influence of structural changes of symmetrical dimers containing two phenyl groups on liquid crystalline behaviour
TL;DR: In this paper, the mesogenic properties of symmetrical dimeric compounds were investigated by optical microscopy, calorimetric and X-ray methods, and it was shown that the location and direction of the ester bonds has a crucial significance in mesophase formation.
Abstract: Some symmetrical dimeric compounds containing biphenyl, biphenylcarboxylic acid or benzoiloxyphenyl moieties and polymethylene spacers were synthesised. The mesogenic properties of the synthesised compounds were investigated by optical microscopy, calorimetric and X-ray methods. It was shown that the location and direction of the ester bonds has a crucial significance in mesophase formation.
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TL;DR: In this article, six new non-symmetric dimers belonging to the family of compounds the (R,S)-(benzylidene-4-substituted-aniline)-2ʺ-methylbutyl-4ʹʹ-phenyloxy) benzoate-oxy) hexanoates have been prepared and their thermal behaviour characterised.
Abstract: Six new non-symmetric dimers belonging to the family of compounds the (R,S)-(benzylidene-4ʹ-substituted-aniline)-2ʺ-methylbutyl-4ʹʹʹ-(4ʹʹʹʹ-phenyloxy) benzoate-oxy) hexanoates have been prepared and their thermal behaviour characterised. The terminal substituents on the 4-benzylideneaniline fragment are H, CH3, C2H5, Cl, Br and I. All six dimers exhibit an enantiotropic nematic phase, and the efficiency of the terminal substituent in enhancing the nematic–isotropic transition temperature is largely consistent with that established for conventional low molar mass liquid crystals. This trend is discussed in terms of the size of the terminal substituent and its effect on the shape of the molecule. The halogen-substituted and ethyl-substituted dimers also exhibit smectic behaviour. For the halogen-substituted dimers, the driving force for smectic phase formation is considered to be, at least in part, the specific anisotropic interaction between the unlike mesogenic units. In contrast, for the ethyl-substitute...
102 citations
TL;DR: In this article, the α,ω-bis(4-cyanobiphenyl-4′-yl) alkanedioates have been synthesized and their transitional properties characterised.
Abstract: Eleven members of the homologous series of liquid crystal dimers, the α,ω-bis(4-cyanobiphenyl-4′-yl) alkanedioates, have been synthesised and their transitional properties characterised. These dimers consist of two cyanobiphenyl units connected by an alkyl spacer attached via ester linkages. All eleven members exhibit exclusively nematic behaviour. The nematic–isotropic transition temperatures, TNI, and associated entropy changes, ∆SNI/R, exhibit pronounced alternations as the length and parity of the spacer is varied; this is characteristic behaviour of liquid crystal dimers. The transitional properties of the ester-linked dimers are compared with the corresponding materials having either ether, methylene or carbonate linkages between the spacer and mesogenic units. For short spacer lengths and both odd- and even-membered dimers, the ester-linked materials show the highest values of TNI and the methylene-linked the lowest. For longer spacer lengths, TNI of the carbonate-linked dimers fall between those o...
80 citations
TL;DR: A series of new asymmetrical liquid crystal dimers, RnO-S2O-ORn (n = 2−10), linked by an ethylene spacer having carbothiol and carboxyl linkages to the core centre of the mo...
Abstract: A series of new asymmetrical liquid crystal dimers, RnO–S2O–ORn (n = 2–10), linked by an ethylene spacer having carbothiol – COS – and carboxyl – COO – linkages to the core centre of the mo...
11 citations
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...arrangement of molecules in liquid crystal phases [44]....
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TL;DR: In this article, six members of homologues series of the liquid crystal dimers, 4n-alkoxybenzylidine-4′-bromoaniline disulphides, which contain a sulphur-sulphur (S-S) link in the flexible spacer have been synthesized and their liquid crystalline properties were characterised.
Abstract: Six members of homologues series of the liquid crystal dimers, 4-n-alkoxybenzylidine-4′-bromoaniline disulphides, which contain a sulphur–sulphur (S–S) link in the flexible spacer have been synthesised and their liquid crystalline properties were characterised. The dimers possess a general formulation in which a S–S group situated between two identical bromo-substituted aromatic imines have bonded, respectively, to S atom through the methylene unit of –(CH2)n– in which n denotes the number of carbon atoms. The number of carbon atoms in the spacers were varied following ascending order from 3 to 8. The dimers with a shorter spacer (n = 3 and 4) displayed nematic phase as well as smectic phase. The higher homologues in which n = 5–8 exhibited smectogens. The odd–even effect can be ascribed to the torsion angle of S–S linkage.
10 citations
TL;DR: In this paper, a series of methylene-linked symmetrical liquid crystal dimers were synthesized and their structures were confirmed by physicochemical techniques, including differential scanning calorimetry and polarizing optical microscopy.
Abstract: A series of methylene-linked symmetrical liquid crystal dimers were synthesized. Their structures were confirmed by physicochemical techniques. Differential scanning calorimetry and polarizing optical microscopy verified the liquid crystal behaviors and their transition temperatures. The thermodynamic data for phase transition, melting transition (Tm), and isotropic transition temperatures (Ti) of the dimers were decreased with increasing the length of spacer. Both Tm and Ti and enthalpy change for isotropic transition (δHi) for dimers with even numbers of methylene units were higher than those with odd numbers. Dimers AC-OnO-AC (n = 2–10) formed enantiotropic nematic phase only, while the unsymmetrical dimer formed a smectic phase.
8 citations
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TL;DR: The underlying theme of this Critical Review is the relationship between molecular structure and liquid crystalline behaviour in a class of materials referred to as liquid crystal oligomers, and how this molecular architecture has been exploited to address issues in a range of quite different areas and has given rise to potential applications for these materials.
Abstract: The underlying theme of this Critical Review is the relationship between molecular structure and liquid crystalline behaviour in a class of materials referred to as liquid crystal oligomers. For the purposes of this review, a liquid crystal oligomer will be defined as consisting of molecules composed of semi-rigid mesogenic units connected via flexible spacers. Much of the review will be devoted to structure–property relationships in the simplest oligomers, namely dimers, in which just two mesogenic units are connected by a single spacer. Along the way we will see how this molecular architecture has been exploited to address issues in a range of quite different areas and has given rise to potential applications for these materials. On the whole, only compounds in which the mesogenic units are linked essentially in a linear fashion will be considered while structures such as liquid crystal dendrimers and tetrapodes fall outside the scope of this review. The review will be of interest not only to scientists working directly in this area but in particular to those interested in understanding the relationships between structure and properties in polymers, and those designing materials for new applications (231 references).
465 citations
TL;DR: In this article, the α,ω-bis (4n-alkylanilinebenzylidine-4′-oxy)alkanes have been synthesized and their transitional properties characterized.
Abstract: Eleven series of dimeric liquid crystals, namely the α,ω-bis(4-n-alkylanilinebenzylidine-4′-oxy)alkanes, have been synthesized and their transitional properties characterized. The number of methylene groups in the flexible core was varied from one to twelve while the length of the terminal alkyl chains was increased from a methyl group to a decyl substituent. The rich smectic polymorphism of this family of compounds was studied using X-ray diffraction and polarizing microscopy; it includes several examples of rare phase transitions, for example, smectic F-smectic A and crystal G-isotropic, as well as novel modulated hexatic phases based on a two dimensional lattice. The transitional properties of a given series, where the length of the terminal chain is held constant while the length and parity of the flexible spacer is varied, exhibit a pronounced alternation. For example, the entropy change at the nematic-isotropic transition exhibits a strong alternation and this has been interpreted in terms ...
265 citations
TL;DR: In this article, the most recent developments in the understanding of structure-property relationships in liquid crystal dimers and higher oligomers are discussed, including trimers and tetramers.
Abstract: A liquid crystal dimer is composed of molecules containing two mesogenic groups linked via a flexible spacer. Initial interest in these materials stemmed from their ability to act as model compounds for semi-flexible main chain liquid crystal polymers but are now of fundamental interest in their own right because their behaviour is significantly different to that of conventional low molar mass liquid crystals. Recently research has begun to focus also on higher monodisperse oligomers such as trimers and tetramers consisting of molecules containing either three or four mesogenic units, respectively, linked via flexible spacers. In this review the most recent developments in our understanding of structure–property relationships in liquid crystal dimers and higher oligomers is discussed.
264 citations
TL;DR: This article focused on structure-property relationships in liquid crystal oligomers, which consist of molecules containing two or more mesogenic units linked via flexible spacers essentially in a linear fashion and so did not consider, for example, liquid crystal dendrimers and tetrapodes.
Abstract: This review focuses on structure-property relationships in liquid crystal oligomers, which consist of molecules containing two or more mesogenic units linked via flexible spacers essentially in a linear fashion and so does not consider, for example, liquid crystal dendrimers and tetrapodes. Previous reviews have tended to focus mainly on liquid crystal dimers in which just two mesogenic units are interconnected by a single spacer. By contrast, this review is largely devoted to higher oligomers such as liquid crystal trimers and tetramers containing three or four mesogenic units connected by two or three spacers, respectively.
263 citations
TL;DR: In this paper, the synthesis and characterisation of three sets of non-symmetric liquid crystal dimers consisting of a cholesteryl-based unit and either 4-methoxybiphenyl, 4-cyanobiphensyl or (S)-2-methylbutyl 4′oxybipphenyl-4-carboxylate are described.
Abstract: The synthesis and characterisation of three sets of non-symmetric liquid crystal dimers consisting of a cholesteryl-based unit and either 4-methoxybiphenyl, 4-cyanobiphenyl or (S)-2-methylbutyl 4′oxybiphenyl-4-carboxylate are described. The transitional properties of these non-symmetric dimers are compared to those of the corresponding parent symmetric dimers. The symmetric dimers exhibit only chiral nematic or nematic phase behaviour. By contrast, members of the non-symmetric dimer series containing either 4-cyanobiphenyl or (S)-2-methylbutyl 4′-oxybiphenyl-4-carboxylate groups exhibit smectic behaviour. The former series show an interdigitated smectic A phase while for the latter the structure of the smectic A phase depends on the length of the flexible spacer. Specifically, for short spacer lengths a monolayer arrangement is seen while for long spacers an intercalated smectic A phase is formed. For an intermediate spacer length, the small-angle X-ray scattering pattern for the intercalated smectic A ph...
151 citations