Hale Ocak

Effects of molecular chirality on superstructural chirality in liquid crystalline dark conglomerate phases

The first examples of polyphilic bent-core compounds consisting of a biphenyl central core, a chiral terminal chain and an oligo(dimethylsiloxane) end group at the other terminus have been synthesized and characterized. The mesomorphic behaviour of the compounds was investigated by differential scanning calorimetry, optical polarizing microscopy, X-ray diffraction and electro-optic methods. The olefinic precursors show monotropic or enantiotropic B1 type mesophases. The siloxane substituted analogues containing a racemic chain exhibit dark conglomerate phases (DC[*] phases) which are composed of chiral domains with opposite chirality, whereas the siloxane derivatives with a homogeneously chiral moiety show “dark enantiomer” phases (DE* phases) with uniform chirality. Under electric fields, different types of ferroelectric switching (FE) smectic and modulated smectic phases were induced. The effects of chain branching, spacer length and molecular chirality on the stability of the DCFE[*] and DEFE* phases and on the field-induced smectic and modulated smectic phases were investigated.

Effects of chain branching and chirality on liquid crystalline phases of bent-core molecules: blue phases, de Vries transitions and switching of diastereomeric states

Bent-core molecules based on a resorcinol bisbenzoate core with a series of distinct substituents in different positions at the central resorcinol core have been synthesized and characterized. The focus is on the effect of branched terminal groups in the racemic and chiral forms on the mesomorphic properties. These were investigated by differential scanning calorimetry, optical polarizing microscopy, X-ray diffraction, electro-optic and dielectric methods. Only bent-core mesogens derived from 4-cyanoresorcinol exhibit liquid crystalline phases and the mesophases of these compounds are strongly influenced by the branching and enantiomeric composition of the terminal chains. Depending on the structure of the rod-like wings and the enantiomeric composition, cybotactic nematic phases (NcybC), BPIII-like isotropic mesophases (BPIIIcybC*) and various polar and apolar smectic phases (SmA, SmC, SmC*, SmCsPA, SmCsPA*) are formed. For one compound, a de Vries type smectic phase is observed and it appears that with decreasing temperature, order develops in two steps. First, at the SmA to SmC transition, the tilt direction becomes long range ordered and in a second step a long range ordering in bend direction takes place. Hence, for the optically active compound a transition from chirality induced polar switching to bend induced (shape induced) antiferroelectricity takes place. In this SmCsPA* phase a homogeneous layer chirality is induced under an applied electric field which interacts with the fixed molecular chirality leading to the energetically favoured diastereomeric state and giving rise to a field direction dependent uniform tilt director orientation. Field reversal induces a flipping of the layer chirality, which first leads to the less favorable diastereomeric state, and then this slowly relaxes to the more stable one by a spontaneous reversal of the tilt direction.

Bent-core compounds with two branched chains: evidence of a new dark conglomerate mesophase

Biphenyl-3,4′-diol derived bent-core compounds with branched side chains have been synthesized and characterized to study the influence of terminal branched groups on mesophase properties and transition temperatures of bent-core mesogens. The liquid crystalline properties of the compounds were investigated by differential scanning calorimetry, optical polarizing microscopy and X-ray scattering. Depending on the length of the chain, the size of molecular branches and the position of branched chains, either enantiotropic or monotropic rectangular columnar phases were observed. For one of the compounds combining two different branched side chains, a new type of optically isotropic dark conglomerate phase (MIso[*]) was found. It is proposed to be a distorted version of a non-tilted or only slightly tilted B6-like intercalated smectic phase with additional in plane order.

Synthesis and mesomorphism of fluoroalkylated platinum complexes

First examples of liquid crystalline chloro-bridged dinuclear platinum organyls carrying semiperfluorinated alkyl chains at their peripheries have been synthesised. The liquid crystalline properties were investigated using polarised optical microscopy, differential scanning calorimetry and X-ray scattering. Compounds with six semifluorinated chains and two alkyl chains show hexagonal columnar liquid crystalline phases with dramatically enhanced mesophase stabilities compared to the related hydrocarbon derivatives, which have only monotropic discotic nematic phases.

Investigation of thermodynamic and surface characterisation of 4-[4-(2-ethylhexyloxy)benzoyloxy]benzoic acid thermotropic liquid crystal by inverse gas chromatography

The inverse gas chromatography method was used to obtain thermodynamic and surface properties of the thermotropic liquid crystalline material, 4-[4-(2-ethylhexyloxy)benzoyloxy]benzoic acid (EBBA). The retention diagrams of undecane, dodecane, tridecane, ethyl acetate, n-butyl acetate, iso-butyl acetate, toluene, ethylbenzene, iso-propylbenzene, n-propylbenzene and chlorobenzene on EBBA were plotted at temperatures between 443.2 and 468.2 K. The specific retention volume, , Flory–Huggins interaction parameter, , equation-of-state interaction parameter, , and effective exchange energy parameter, , were determined. The dispersive component of the surface free energy, , of the studied adsorbent surface was estimated using retention times of different nonpolar organic solvents in the infinite dilution region. The specific free energy of adsorption, the enthalpy of adsorption, , and the entropy of adsorption, , of solvents on liquid crystal were determined. The values of were correlated with the donor and the acceptor numbers of the probes to quantify the acidic and the basic parameters of the liquid crystal surface. The values obtained for and parameters indicated an acidic character for the surface of EBBA.

Effect of fluorocarbon chains on the mesomorphic properties of chiral imines and their complexes with copper(II)

The synthesis and mesomorphism of new chiral imine compounds incorporating a fluorocarbon chain which have calamitic molecular shape and their complexes with copper(II) are reported. The liquid crystalline properties were investigated using polarised optical microscopy, differential scanning calorimetry and X-ray diffraction. Imine compounds with a semifluorinated chain and a short chiral chain show chiral smectic mesophases with dramatically enhanced mesophase stabilities compared to the related hydrocarbon derivatives. In addition, the first examples of mesogenic chiral salicylaldiminate copper(II) complexes carrying semifluorinated alkyl chains at their peripheries have been synthesized and their liquid crystalline properties were investigated.

Use of inverse gas chromatography for the physicochemical characterisation of a new synthesised liquid crystal: (S)‐5‐(2‐methylbutoxy)‐2‐{[(4‐dodecyloxyphenyl)imino]methyl}phenol

A new liquid crystalline compound, i.e. (S)‐5‐(2‐methylbutoxy)‐2‐{[(4‐dodecyloxyphenyl)imino]methyl}phenol (MBDOPIMP), was prepared and characterised. Phase transition temperatures of MBDOPIMP were determined by polarising optical microscopy and differential scanning calorimetry. Trace amounts of ethyl acetate, butyl acetate, isobutyl acetate, hexane, heptane, octane, nonane and decane were passed through a chromatographic column loaded with MBDOPIMP coated on Chromosorb W. The retention diagrams of the solvents on MBDOPIMP were plotted using specific retention volumes at temperatures between 105 and 125°C. Thermodynamic parameters, such as Flory–Huggins liquid–solvent interaction, , equation‐of‐state liquid–solvent interaction, , effective exchange energy, X eff, the exchange enthalpy, X 12 and exchange entropy, Q 12, were determined for the liquid crystal–solvent systems. Subsequently, the partial molar heats of sorption, ΔH¯1,sorp and mixing, , were determined from the slopes of the plots of the logarithm of the specific retention volume, ln , versus 1/T and logarithm of the weight fraction activity coefficients, ln , versus 1/T, respectively.

Investigation of acid–base and surface characteristics of a thermotropic semifluorinated salicylaldimine liquid crystal by inverse gas chromatography

The surface energy of a semifluorinated salicylaldimine liquid crystal has been characterised by inverse gas chromatography over the temperature range 303 to 323 K using n-alkanes, tetrahydrofurane, dichloromethane, chloroform, acetone and ethyl acetate molecular probes. The dispersive component of the surface free energy of the adsorbent surface studied was calculated according to the approaches of Fowkes and Dorris–Gray in the infinite dilution region. The specific free energy, enthalpy and entropy of adsorption of polar probes on the liquid crystal were determined. The values of the specific enthalpy of adsorption were correlated with both the donor and the acceptor numbers of the probes to quantify the acidic and the basic parameters of the liquid crystal surface. The surface of the semifluorinated salicylaldimine liquid crystal was found to show a basic nature, which determined the nature of its interaction with the polar probes.

Development of tilt, biaxiality and polar order in bent-core liquid crystals derived from 4′-hydroxybiphenyl-3-carboxylic acid

A homologous series of unsymmetric bent-core compounds derived from 4′-hydroxybiphenyl-3-carboxylic acid and having strongly distinct chains at both ends has been synthesized and characterized. The liquid crystalline self-assembly of the compounds was investigated by differential scanning calorimetry, optical polarizing microscopy, X-ray scattering and electro-optic methods. With increasing chain length and upon cooling a series of smectic phases was observed, starting from SmA phases having an unusual structure composed of domains with a randomized tilt direction. This is followed by a series of biaxial smectic phases, involving a smectic phase composed of tilt domains with increased size (SmCr), anticlinic and synclinic tilted smectic phases (SmCs, SmCa) and finally columnar phases with an oblique (Colobl) or pseudo-rectangular (Colrec) lattice. This phase sequence is the result of the competition between an emerging tilt, the dense packing of the bent aromatic cores leading to restricted rotation and contributing to phase biaxiality, and the steric distortion of the dense packing by the bulky 3,7-dimethyloctyloxy chains, inhibiting the formation of long range polar order. In all smectic phases the tilt is relatively small, thus providing only weak layer coupling and this provides the basis for sensitive regulatory networks determining the mode of molecular self-assembly in the resulting LC phases. The columnar phases have a tilted B1rev-like ribbon structure with a temperature and chain length dependent inversion of the sign of birefringence. Though none of the racemic compounds show polar switching, the (S)-enantiomer of one compound shows ferroelectric-like switching in an anticlinic tilted smectic phase (SmCaPR*).

Thermodynamic and surface characterisation of (S)-5-dodecycloxy-2-[[[4-(2-methylbutoxy) phenyl]imino]metheyl]phenol thermotropic liquid crystal by inverse gas chromatography

The inverse gas chromatography method was used to obtain thermodynamic and surface properties of the calamitic thermotropic liquid crystalline material, (S)-5-dodecyloxy-2-[[[4-(2-methylbutoxy)phenyl]imino]methyl]phenol, which is a Schiff base. The retention diagrams of the infinitely diluted solutions of acetate and alcohol isomers of (S)-5-dodecyloxy-2-[[[4-(2-methylbutoxy)phenyl]imino]methyl]phenol were plotted between 313 and 383 K using the inverse gas chromatography method. The isomer seperation of (S)-5-dodecyloxy-2-[[[4-(2-methylbutoxy)phenyl]imino]methyl]phenol isomers as chromatographic probes was represented by retention diagrams. The thermodynamic interaction parameters such as the Flory–Huggins liquid crystal–solvent parameter and the hard-core liquid crystal–solvent parameter, the weight fraction activity coefficient and the exchange parameters such as the effective energy, enthalpy and entropy were determined for the studied solvents at infinite dilution. Then the partial molar heat of sorption, the partial molar heat of mixing at infinite dilution and as well as the molar heat of vaporisation of the studied solvents were determined in the isotropic phase of (S)-5-dodecyloxy-2-[[[4-(2-methylbutoxy)phenyl]imino]methyl]phenol. To characterise the surface properties of (S)-5-dodecyloxy-2-[[[4-(2-methylbutoxy)phenyl]imino]methyl]phenol, retention diagrams of several non-polar and polar solvents with (S)-5-dodecyloxy-2-[[[4-(2-methylbutoxy)phenyl]imino]methyl]phenol were measured in the temperature range from 313 to 333 K by inverse gas chromatography. The surface energy, the thermodynamic adsorption parameters and the acid–base constants of (S)-5-dodecyloxy-2-[[[4-(2-methylbutoxy)phenyl]imino]methyl]phenol in the crystalline phase were determined.