Guan-Yeow Yeap

Liquid crystal oligomers: going beyond dimers

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.

Why do non-symmetric dimers intercalate? The synthesis and characterisation of the α-(4-benzylidene-substituted-aniline-4′-oxy)-ω-(2-methylbutyl-4′-(4″-phenyl)benzoateoxy)alkanes

Ten new non-symmetric liquid crystal dimers belonging to the family of compounds α-(4-benzylidene-substituted-aniline-4′-oxy)-ω-(2-methylbutyl-4′-(4″-phenyl)benzoateoxy)-alkanes have been synthesised and their transitional properties characterised. The dimers contain either a hexamethylene or octamethylene spacer, while the terminal substituents on the 4-benzylideneaniline fragment are H, CH3, F, Cl and Br. The unsubstituted dimers are not liquid crystalline, while the remaining compounds exhibit enantiotropic nematic behaviour. The trends in the clearing temperatures, according to the chemical nature of the terminal substituent, are largely consistent with those established for conventional low molar mass liquid crystals. Three of the dimers also exhibit an intercalated smectic A phase, specifically the two bromo-substituted dimers and the chloro-substituted dimer containing a hexamethylene spacer. The driving force for the formation of this phase is considered to be, at least in part, the specific anisotropic interaction between the unlike mesogenic units. The absence of smectic behaviour for the isosteric methyl-substituted dimers reveals that steric factors alone cannot stabilise the intercalated smectic A phase.

Synthesis, crystal structure and spectroscopic study of Para substituted 2-hydroxy-3-methoxybenzalideneanilines

Abstract Eight Schiff bases derived from 2-hydroxy-3-methoxybenzaldehyde and different para substituted anilines have been synthesized and their structures were elucidated by physical measurements and FT-IR. NMR assignments were made using 1H, 13C NMR and aided by 2D COSY homonuclear, HMQC and HMBC heteronuclear correlation techniques. IR spectral analysis of the model compounds was found useful in understanding the degree of stabilization upon this series of enol-imino tautomers, which possess different substituents in the aniline fragment. In order to rationalize the stabilization of tautomer in solid state, the crystallography data of 1-{(4-methylphenylimino)methyl}- and 1-{(4-chlorophenylimino)methyl}-2-hydroxy-3-methoxyphenol were adopted wherein the 4-chloro (4-Cl) and 4-methyl (4-CH3) containing compounds crystallized into orthorhombic lattice with a non-centrosymmetric space group P212121. The relationship between the stabilization of bonding involved in heteronuclear six-membered ring of the tautomer and the conformation of the molecules in crystal phase was reported.

Cholesteryl-based liquid crystal dimers containing a sulfur–sulfur link in the flexible spacer

Seven members of the homologous series of the liquid crystal dimers, the bis(ω-(cholesteryloxycarbonyl)alkyl)disulfides, which contain a sulfur−sulfur link in the flexible spacer have been synthesised and their liquid crystal properties characterised. The dimers are referred to using the acronym Chol-n-SS-n-Chol in which n denotes the number of carbon atoms linking the cholesteryl-based groups and the sulfur atoms, and was varied between 3, 5, 6, 8, 10, 11 and 12. All seven homologues exhibit a chiral nematic phase and for the longest three members a smectic A phase was also observed. An odd−even effect is apparent in both the transition temperatures and the values of the entropy change associated with the chiral nematic−isotropic transition, ΔSN*I /R, in which dimers with even values of n show the higher values. This is interpreted in terms of the average molecular shapes in which the C−S−S−C dihedral angle is around 90°. The values of ΔSN*I/R shown by these dimers are very small for liquid crystal dimers and this is attributed to the increased molecular biaxiality arising from the C−S−S−C dihedral angle. The smectic A phase exhibited by the homologues with n = 10, 11 and 12 is proposed to consist of an intercalated arrangement of the dimers which is driven by the mismatch in cross-sectional areas of the cholesteryl-based groups and alkyl chains.

SYNTHESIS AND MESOMORPHIC PROPERTIES OF SCHIFF BASE ESTERS ORTHO-HYDROXY-PARA-ALKYLOXYBENZYLIDENE-PARA-SUBSTITUTED ANILINES

A series of new compounds o-n-hydroxy-p-n-hexadecanoyloxybenzylideneaniline and its substituted derivatives was synthesized from the esterification of palmitic acid with intermediary Schiff bases, which posssess various terminal substituents X (X = H, F, Cl, Br, OCH3, CH3, and C2H5). All compounds possess mesomorphic properties wherein compounds with X = H, F, Cl, Br, and OCH3 exhibit smectic A (SmA) phases and those with terminal substituents CH3 and C2H5 are monotropic liquid crystals showing smectic C phases (SmC). Correlation studies of the clearing temperature (Tc) of the title compounds and polarizability related to mesophase stability and molecular anisotropy are reported.

Inner coordination sphere tin(IV) complexes with some O,N,N-terdentate {N-(2-hydroxybenzaldehyde)-1-amino-2-phenyleneimine and N-(2-hydroxy-1-naphthaldehyde)-1-amino-2-phenyleneimine} and O,N,N,O-quadridentate {N,N′-bis(2-hydroxybenzaldehyde)-1,2-phenylenediimine and N,N′-bis(2-hydroxy-1-naphthaldehyde)-1,2-phenylenediimine} Schiff bases

Abstract From the reactions of SnCl4 with organotin(IV) chlorides (RSnCl3, R2SnCl2 and R3SnCl, where R = Bu, Me and Ph) with N-(2-hydroxybenzaldehyde)-1-amino-2-phenyleneimine, N-(2-hydroxyl-1-naphthaldehyde)-1-amino-2-phenyleneimine, N,N′-bis(2-hydroxybenzaldehyde)-1,2-phenylenediimine and N,N′-bis(2-hydroxy-1-naphthaldehyde)-1,2-phenylenediimine a series of complexes have been synthesized and characterized, respectively, by microanalytical, IR, and 1H NMR spectroscopic methods. The Ph2SnCl2 reacted with N-(2-hydroxy-1-naphthaldehyde)-1-amino-2-phenyleneimine giving Ph2Sn(NAPPDI) [where NAPPDI = deprotonated N,N′-bis(2-hydroxy-1-naphthaldehyde)-1,2-phenylenediimine], wherein the former Schiff base exhibit a facile intramolecular CN bond cleavage and intermolecular CN bond formation.

Synthesis, physical and mesomorphic properties of Schiff's base esters containing ortho‐, meta‐ and para‐substituents in benzylidene‐4′‐alkanoyloxyanilines

Three series of Schiffs base esters, 2‐hydroxy‐4‐methoxybenzylidene‐4′‐alkanoyloxyaniline, 2‐hydroxy‐3‐methoxybenzylidene‐4′‐alkanoyloxyaniline and 3‐methoxy‐4‐alkanoyloxybenzylidene‐4′‐alkanoyloxyaniline, which possess mono‐ and di‐substituted moieties at both ends of the molecules have been synthesized and their mesomorphic properties investigated along with physical characterization (elemental analysis, mass spectrometry, Fourier transform IR and high resolution NMR). The effect of lateral methoxy and polar hydroxy groups on the mesomorphism of the title compounds was studied, based on the thermal properties and textural analysis. The results indicate that compounds with the larger number of carbons in the alkyl chain prefer smectic phase formation and possess higher transition temperatures and molecular polarizability along the long axis. Further analysis shows that the greater shielding effect (through the formation of intramolecular interaction) leads to lower molecular broadening and results in higher clearing points. Increase in melting point due to the lengthening of the alkyl chain is associated with the increase in van der Waals attraction between the molecules. These compounds exhibit nematic, smectic (SmA and SmC) phases depending on the type and position of the substituents.

Synthesis and Characterization of Some New Mesogenic Schief Base Esters N-[4-(4-n-Hexadecanoyloxybenzoyloxy)-Benzylidene]-4-Substituted Anilines

Series of elongated Schiff base esters containing three aromatic rings with palmitoyl moiety as one of the terminal carbon chain and various substituents R(R=H, F, Cl, Br, OCH3, CH3, and C2H5) at the other end of molecule have been isolated, and their structures were proposed via physical measurement. The mesomorphic properties of these compounds were investigated via differential scanning calometry and polarizing optical microscopy. The thermal data indicate that all of these compounds exhibit mesomorphic properties. Although compounds with R=H, OCH3, CH3, and C2H5 show nematic phases, the remaining compounds containing halogen F, Cl, and Br are smectogenic in nature. The structural changes that result in the variation of transition temperature and the electronic polarizability of the respective molecules are discussed.

Non-symmetric dimers: effects of varying the mesogenic linking unit and terminal substituent

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-substituted dimers, the driving force is thought to be the enhanced molecular inhomogeneity arising from the longer terminal chains. The properties of these new dimers are broadly similar to those of the corresponding dimers in which the ester linkage between the spacer and mesogenic unit is an ether link. This strongly suggests that the ether- and ester-linked materials have similar average molecular shapes.

Nematic and Smectic a Phases in Ortho-Hydroxy-Para-Hexadecanoyloxbenzylidene-Para-Substituted Anilines

Four Schiff base esters containing the different substituents of R=CN, OH, SH, and NO2 were synthesized, and the physical characterization was carried out along with spectroscopic techniques (FT-IR and NMR). Thermal properties were analyzed by using differential scanning calometry (DSC), and their liquid crystal textures were observed under polarized optical microscope. All compounds thus obtained exhibit mesomorphic properties of which the alignment of molecules are inclined to either nematic, N (R=SH), or smectic, SmA (for R=CN, OH, and NO2). The unidentifiable subphases in crystal region were recorded in all compounds except that with R=OH. The presence of a lateral hydroxyl group has been claimed to contribute to the enhanced molecular ordering, leading to a higher clearing temperature.