Wan Ahmad Kamil Mahmood

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.

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.

Synthesis, thermal and optical behaviour of non-symmetric liquid crystal dimers α-(4-benzylidene-substituted-aniline-4′-oxy)-ω-[pentyl-4-(4′-phenyl)benzoateoxy]hexane

A series of six non-symmetric liquid crystal dimers α-(4-benzylidene-substituted-aniline-4′-oxy)-ω-[pentyl-4-(4′-phenyl)benzoateoxy]hexane have been synthesized and characterized. These dimers vary from one another in terms of the terminal substituent (H, CH3, C2H5, F, Cl and Br) with different polarizabilities and molecular sizes. Investigation on the thermal behaviour shows that the dimer with a more polarizable terminal substituent of Br atom exhibits a smectic A phase as well as an enantiotropic nematic phase. Influence of the molecular size of the terminal substituent in stabilizing the liquid crystalline properties is also studied.

Synthesis and mesomorphic properties of 7‐acyloxy‐3‐(4‐acyloxyphenyl)‐4H‐1‐benzopyran‐4‐one

A new homologous series of 7‐acyloxy‐3‐(4‐acyloxyphenyl)‐4H‐1‐benzopyran‐4‐ones was synthesized and characterized by elemental analysis and spectroscopic techniques along with thermal behaviour study using differential scanning calorimetry. Texture observation was performed under the polarizing optical microscope from which the derivatives were found to exhibit different molecular organizations as exemplified by various mesophases. Whilst the compounds containing butanoyloxy and hexanoyloxy groups behave as nematogens, the other compounds with heptanoyloxy and longer side chains show a SmC phase. The octadecanoyloxy derivative was the only member showing a monotropic SmC phase. The difference in phase behaviour is discussed based on the molecular structure and the presence of a lateral dipole which can be associated with the intermolecular interaction within the mesophase.

Non-Symmetric Liquid Crystal Dimers: High Thermal Stability in Nematic Phase Enhanced by Thiophene-2-Carboxylate Moiety

Series of non-symmetric liquid crystal dimers, α-(4-benzylidenechloroaniline-4′-oxy)-ω-[4-(thiophene-2-carboxyl)benzylideneaniline-4′-oxy]alkanes incorporating a thiophene-based moiety in one of the two mesogenic units have been synthesized and characterized. The nematogenic properties of the dimers were studied wherein the flexible spacers made up by n methylene units (–CH2-) ranging from n=5 to n=12. These dimers exhibit enantiotropic N phase with high thermal stability (>187.0°C). The N phase temperature range for the present homologues is found to be strongly dependent on the length and parity of the methylene spacer which connects the 4-benzylidenechloroaniline and 4-(thiophene-2-carboxyl)- benzylideneaniline mesogenic cores.

Novel nanohybrids of cobalt(III) Schiff base complexes and clay: synthesis and structural determinations.

The [Co(Me(2)Salen)(PBu(3))(OH(2))]BF4 and [Co(Me(2)Salen)(PPh(3))(Solv)]BF(4), complexes were synthesized and characterized by FT-IR, UV-Vis, (1)H NMR spectroscopy and elemental analysis techniques. The coordination geometry of [Co(Me(2)Salen)(PPh(3))(H(2)O)]BF(4) was determined by X-ray crystallography. It has been found that the complex is containing [Co(Me(2)Salen)(PPh(3))(H(2)O)]BF(4) and [Co(Me(2)Salen)(PPh(3))(EtOH)]BF(4) hexacoordinate species in the solid state. Cobalt atom exhibits a distorted octahedral geometry and the Me(2)Salen ligand has the N2O2 coordinated environment in the equatorial plane. The [Co(Me(2)Salen)(PPh(3))(H(2)O)]BF(4) complex shows a dimeric structure via hydrogen bonding between the phenolate oxygen and hydrogens of coordinated H2O molecule. These complexes were incorporated into Montmorillonite-K10 nanoclay. The modified clays were identified by FT-IR, XRD, EDX, TGA/DTA, SEM and TEM techniques. According to the XRD results of the new nanohybrid materials, the Schiff base complexes are intercalated in the interlayer spaces of the clay. SEM and TEM micrographs show that the resulting hybrid nanomaterials have layer structures. Also, TGA/DTG results show that the intercalation reaction was taken place successfully.

Synthesis and Phase Transition in New Chalcone Derivatives: Crystal Structure of 1-Phenyl-3-(4′-undecylcarbonyloxyphenyl)-2-propen-1-one

ABSTRACT A series of new chalcone derivatives with a general formula of CH3CnH2nCOOC6H4CH:CHCOC6H4 where n = 10, 12, 14, and 16 were well synthesized and crystallized from organic solution. The physical properties as well as the chemical formulations of these compounds were determined by the microanalysis and spectroscopic methods (FTIR, and 1H and 13C NMR). Crystal structure of CH3C10H20COOC6H4CH:CHCOC6H4 was determined by single-crystal X-ray diffraction analysis. The space group of this crystal is triclinic with a = 5.8570(3) Å, b = 8.5640(5) Å, c = 24.1592(12) Å, V = 1175.82(11) Å3, and Z = 2, and its structure was refined to R(F) = 0.055 and ωR(F 2 ) = 0.161. The elongated terminal alkyl chain is fully stretched in solid phase. Phase-transition temperatures and the thermal parameters were obtained from differential scanning calorimetry (DSC). The texture observation was carried out with a polarizing optical microscope (POM) over heating and cooling cycles. All of the title compounds except undecylcarbonyloxy analogue exhibit Cr1-Cr2 transition with smectic-like texture within the Cr2 phase.