Synthesis and characterization of achiral banana‐shaped liquid crystalline molecules containing bisnaphthyl moieties

Three analogous series of symmetric banana‐shaped liquid crystalline molecules containing bisnaphthyl units were synthesized and characterized. The effects of linking groups (on the side wings) and lateral meta‐fluoro substitutions (on the middle outer rings) on the mesogenic properties were examined. The type of mesophase depends on the lengths of terminal alkoxy chains. Thus, achiral molecules with shorter flexible chains (n = 8) exhibit a rectangular columnar (B₁) phase, while analogous derivatives with longer flexible chains (n = 12) display the B₂ phase. All lateral meta‐fluoro substituted analogues (series II) possess the lowest isotropization temperatures and the narrowest mesophasic ranges of the B₁ and B₂ phases. The B₁ and B₂ phases were confirmed by X‐ray diffraction, polarizing optical microscopy (POM) and electro‐optical (EO) switching experiments. An electric field‐induced transition from an antiferroelectric (tristable) state to a ferroelectric (bistable) state was observed in the EO measurements. Spontaneous polarization (by switching current response), tilt angle of chiral domains (by POM), and transmittance–voltage measurements of the B₂ phase in related compounds have been surveyed in this study.     

Synthesis and structure of wholly aromatic liquid–crystalline polyesters containing meta- and ortholinkages

Wholly aromatic main-chain rigid-chain polymers containing meta- and orthokink linkages were synthesized. The thermal property, liquid crystallinity, and crystalline structure were studied using DSC, polarized light microscopy, and wide-angle X-ray scattering. These polymers exhibited liquid crystallinity up to 50 mol % of meta- and orthokink linkages. The existence of liquid crystallinity in these polymers may be attributed to the adoption of cis conformation in kink units in these polymers because the energy penalty for doing so can be compensated by the formation of the liquid–crystalline phase. The crystallinity of the polymers was low, and the crystal structure was quite similar to that of the pure polymers (without kink units). This can be explained by that fact that the crystal region mainly consists of the nonkink units, and the kink units disrupt the crystallization of the polymers and form defects. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1242–1248, 2001     

Synthesis and characterisation of main‐chain hydrogen‐bonded supramolecular liquid crystalline complexes formed by azo‐containing compounds

A series of azopyridine‐containing hydrogen bonding acceptors (4a–c) with flexible spacers of oligo(methylene) were synthesised. Hydrogen‐bonded polymeric complexes 4/5 and trimeric complexes 4/6₂ , where 5 and 6 are aromatic dicarboxylic acids and monocarboxylic acids, respectively, were prepared and their liquid crystallinity was examined using differential scanning calorimetry and polarising optical microscopy. The study showed that most of the complexes displayed reversible thermotropic nematic phase. The isotropic to nematic phase transition temperatures of polymeric complexes 4/5 and trimeric complexes 4/6₂ in general decreased with the increase in length of spacers and terminal groups in the corresponding proton acceptors 4 and the proton donors 5 and 6, respectively. Hydrogen bonding interactions in complexes 4/5 and 4/6₂ were studied by X‐ray photoelectron spectroscopy and Fourier transform infrared spectroscopy.     

Ferroelectric liquid crystals derived from (S)‐2‐(6‐methoxy‐2‐naphthyl)propionic acid with non‐fluorinated or semi‐fluorinated alkanes at a chiral terminal chain

Two series of ferroelectric liquid crystals derived from (S)‐2‐(6‐methoxy‐2‐naphthyl)propionic acid, with non‐fluorinated or semi‐perfluorinated alkanes positioned at a chiral terminal chain, have been synthesized and characterized by differential scanning calorimetry, polarizing optical microscopy and electro‐optical measurements. The non‐fluorinated compounds, 1‐hexyl (S)‐2‐{6‐[4‐(4‐alkanoyloxyphenyl)benzoyloxy]‐2‐napthyl}propionates exhibit rich mesomorphism—the BP_II, N*, TGB_A*, SmA* and SmC* phases. The fluorinated compounds display only the SmA* and SmC* phases, suggesting that the fluorination promotes the formation of smectic phases. In addition, the SmA* and SmC* phases of the fluorinated compounds have enhanced thermal stability as compared with the corresponding phases of the non‐fluorinated compounds. The spontaneous polarization (P _s values) for the non‐fluorinated compounds are higher than those of the fluorinated compounds at any reduced temperature below the SmA*–SmC* transition. The electro‐optical responses measured for these compounds in the ferroelectric phase displayed thresholdless, V‐shaped switching.     

1,4‐Diaryl and Schiff's base [1,2,3]‐triazole derivative liquid crystalline compounds

Regioselective synthesis of [1,2,3]‐triazoles based on the ‘click‐reaction’, involving the Cu(I)‐catalysed 1,3‐dipolar cycloaddition of aryl azide to terminal arylacetylenes, was successfully employed in the preparation of two series of non‐linear mesogens, based on 1,4‐diaryl‐[1,2,3]‐triazole (Ia–e) and with Schiff's base units (IIa–c). Their mesophases were characterized by polarizing optical microscopy and differential scanning calorimetry. All final compounds exhibited preferentially the SmC phase. In addition, compounds of series I showed weak blue fluorescence in solution (λ_{max. em.} = 380–386 nm) with poor quantum yields and a Stokes shift of around 90 nm.     

Synthesis and liquid crystalline properties of novel compounds containing a 3‐fluoro‐4‐cyanophenoxy group

A series of novel compounds containing a 3‐fluoro‐4‐cyanophenoxy group were synthesized and fully characterized by IR and ¹H NMR, and their mesomorphic properties were studied. Seven compounds exhibited enantiotropic nematic phases and three compounds exhibited monotropic nematic phases, as confirmed by differential scanning calorimetry and polarizing optical microscopy. Selected properties of the liquid crystalline compounds synthesized were calculated by ab initio methods at a HF/6‐31G level. The bond lengths, bond angles and dihedral angles of the fragments with the same structure change little between the compounds. All the compounds with a terminal alkoxy chain approached a planar structure.     

A new series of liquid‐crystalline bi‐1,3,4‐oxadiazole derivatives: synthesis and mesomorphic behaviour

Symmetrical bi‐1,3,4‐oxadiazole derivatives, namely 5,5′‐bis(phenyl 4‐alkoxybenzoate)‐2,2′‐bi‐1,3,4‐oxadiazole (BBOXD‐n, n = 6, 10, 14, 16), were synthesised. All BBOXD‐n exhibited remarkably stable SmC phases by virtue of the high transition enthalpies of SmC–I. In addition, BBOXD‐6 and BBOXD‐10 showed an enantiotropic nemetic phase with enthalpies of the N–I transition up to 5.16 kJ mol^?1. As confirmed by wide‐angle X‐ray diffraction analysis and MM2, molecules of BBOXD‐n showed high‐angle tilting (55–57°) within their smectic C phases.     

Novel photo‐crosslinkable liquid crystalline polymers containing vanillylidene cycloalkanones and azobenzene units

Two series of novel liquid crystalline photo‐crosslinkable bis(vanillylidene‐azobenzene) cycloalkanone containing polymers, namely poly(vanillylidene alkyloxy‐4,4′‐azobenzenedicarboxylic ester)s, have been synthesised from bis[m‐hydroxyalkyloxy(vanillylidene)cycloalkanone] (m = 6, 8, 10) with azobenzene dicarbonylchloride by solution polycondensation method at ambient temperature. Polymers with varying spacer lengths have been synthesised and characterised by spectroscopic techniques. These variations have been correlated with the thermal properties and transition temperatures. Thermal transitions were analysed by differential scanning calorimetry (DSC) and the mesophases were identified by hot stage optical polarised microscopy (HOPM). All of the polymers were found to exhibit liquid crystalline properties. Transition temperatures were observed to decrease with increasing spacer length. The thermogravimetric analysis reveals that all of the polymers were stable up to 280°C undergo two‐stage decomposition. Using the UV–visible photolysis studies we investigated the simultaneous behaviour of reactivity rates of crosslinking in the vanillylidene unit and isomerisation caused by the azobenzene unit in the photo‐crosslinkable main chain liquid crystalline polymers. The photolysis of liquid crystalline bis(vanillylidene)cycloalkanone‐based polymers reveals that there are two kinds of photoreactions in these systems: the EZ photoisomerisation of azobenzene unit and 2p+2p addition by vanillylidene units. The EZ photoisomerisation in the liquid crystal phase disrupts the parallel stacking of the mesogens, resulting in the transition from the liquid crystal phase to isotropic phase. The photoreaction involving 2p+2p addition of the bis(vanillylidene)cycloalkanone units in the polymers results in the conjoining of the chains. The cyclopentanone polymers exhibited a faster rate of photolysis than the cyclohexanone polymers.     

Tailoring of ionic supramolecular assemblies based on ammonium carboxylates toward liquid‐crystalline micellar cubic mesophases

Ionic liquid crystals based on ionic complexation of tris(2‐aminoethyl)amine (1) with 3,4,5‐tris(7,7,8,8,9,9,10,10,11,11,12,12,12‐tridecafluorododecyloxy)benzoic acid (2) and with 3,4,5‐tris(2‐octyldodecyloxy)benzoic acid (3) were investigated. The ionic complex with the partially fluorinated alkyl chains (1·2) exhibited a morphological transition from a hexagonal columnar mesophase to a Pm3n micellar cubic phase upon increasing the molar ratio of 2 to 1. For the complex with the branched alkyl chains (1·3) a micellar cubic mesophase was exclusively generated at appropriate composite ratios. The generation of the micellar cubic mesophases is attributed to the introduction of the laterally expanded volume of the alkyl chains compared with the corresponding normal dodecyl chains. Their thermal stabilities were most enhanced at a specific molar ratio of 1:5 for 1·2 and 1:4 for 1·3. This result corresponds to the most suitable chain volume for the stable micellar cubic mesophase.     

Synthesis and mesomorphic properties of tolane‐based liquid crystals with a fluorinated polar end group

Three series of tolane‐based liquid crystalline compounds with a fluorinated polar end group have been synthesized. Their phase transition temperatures were measured by texture observation in a polarizing microscope and confirmed by DSC. The all‐fluorinated trifluoromethyl and trifluoromethoxy group have a tendency to promote the smectic A phase, but the difluoromethoxy group has the tendency to promote a wide nematic phase.     

Synthesis and properties of novel 1,1‐difluoropropoxy‐linked liquid crystalline compounds of negative dielectric anisotropy

A series of liquid crystalline compounds having the difluoropropyleneoxy moiety (–OCF₂C₂H₄–) as a linking group has been synthesized. The physical properties, i.e. dielectric anisotropy, birefringence, viscosity and phase transition temperatures, have been measured. This novel class of compounds shows a larger negative dielectric anisotropy than the corresponding compounds having a propyleneoxy (–OC₃H₆–) linking group. Their dielectric properties may be explained by the combined effect of fluorine atoms with large electron negativities and the electron donating feature of an oxygen atom substituted on the same carbon atom. The semi‐empirical quantum calculation method (AM1) also confirmed this dielectric behaviour of the difluoropropyleneoxy linkage group.     

Synthesis and liquid crystalline properties of substituted 2,5‐diaryl 1,3,4‐oxadiazole derivatives without flexible chains

A series of new compounds based on aromatically 2,5‐disubstituted 1,3,4‐oxadiazoles without flexible chains, formulated as p‐R–C₆H₄–(OC₂N₂)–(p‐C₆H₄)₂–R′ with (i) R = CH₃O, R′ = CH₃O, CH₃S, F, H (Ia–Id), (ii) R = CH₃S, R′ = CH₃O, CH₃S, F, H (IIa–IId) and (iii) R = F, R′ = CH₃O, CH₃S, F, H (IIIa–IIId) (p‐C₆H₄ and OC₂N₂ represent a p‐phenylene spacer and a 1,3,4‐oxadiazole ring, respectively), were synthesised and characterised by ¹H and ¹³C NMR, MS and HRMS techniques. Mesomorphic properties were investigated using differential scanning calorimetry and polarizing optical microscopy. All of the target compounds (except Id, IId, IIIc and IIId) exhibited an enantiotropic nematic mesophase with high melting temperatures. The liquid crystalline properties of these compounds were influenced greatly by polarity, steric factors and positions of the terminal groups. The effect of the terminal groups on the liquid crystal properties is discussed.     

Synthesis and mesomorphic properties of new liquid crystalline compounds with ß-hydroxy-, ß-chloroketone and a,ß-unsaturated ketone moieties in the terminal chains

The syntheses of new liquid crystalline compounds containing β-hydroxy-, β-chloroketone and α,β-unsaturated ketone moieties are described. The key intermediate 1-(4-hydroxyphenyl)-3-hydroxyoctan-1-one was obtained by the hydrogenolysis of the heterocycle in 3-(4-hydroxyphenyl)-5-pentyl-2-isoxazoline. Dehydratation of the intermediate β-hydroxyketone led to 1-(4-hydroxyphenyl)-oct-2-en-1-one. Reaction of 1-(4-hydroxyphenyl)-3-hydroxyoctan-1-one with hydrochloric acid yielded 1-(4-hydroxyphenyl)-3-chlorooctan-1-one. The target liquid crystalline compounds were synthesized by the esterification of these phenols with corresponding acids. The relationships between the moiety type in the terminal chain and the liquid crystalline properties are discussed.     

Thermal switching characteristics based on smectic-A↔chiral nematic phase transitions of (liquid crystals/chiral dopant) composites,with and without side chain-type liquid crystalline polymer

The light switching characteristics induced by a thermal smectic A (SmA) ? chiral nematic (N*) phase transition were studied for homeotropically aligned [smectic A liquid crystal (SmA-LC)/nematic liquid crystal (N-LC)/chiral dopant] and [side chain type smectic A liquid crystalline polymer (SmA-LCP)/N-LC/chiral dopant] composites. A drastic change from a transparent SmA phase to a light-scattering N* phase occurred in both composites upon heating. In the case of the heat-induced N* phase for the (SmA-LC/N-LC/chiral dopant) composite, the N* phase exhibited weak light scattering due to formation of a scroll texture. On the other hand, in the case of the heat-induced N* phase for the (SmA-LCP/N-LC/chiral dopant) composite, the N* phase showed strong light scattering due to formation of a focalconic texture. The existence of a SmA-LCP was responsible for a higher contrast ratio between the transparent SmA phase and the light scattering N* phase for the (SmA-LCP/ N-LC/chiral dopant) composite than for the (SA-LCN/N-LC/chiral dopant) composite.     

Chiral β-hydroxycarbonyl compounds based on (−)-menthone: structure and behavior in liquid crystalline systems

It has been established by X-ray structural analysis that 2-(1-biphenyl-4-yl-1-hydroxy)methyl-p-menthan-3-one, one of the products of the reaction of (–)-menthone triisopropyloxytitanium enolate with 4-phenylbenzaldehyde, has a 1R,2S,4S,1S configuration. In crystals, this -hydroxyketone adopts a chair conformation with equatorial methyl and isopropyl groups and an axial 2-(1-biphenyl-4-yl-1-hydroxy)methyl substituent. Unlike the stereoisomeric compound with the 1R,2S,4S,1S configuration, the exocyclic fragment of which has an intramolecular >C=O...H-O- hydrogen bond in crystals and solutions, in the crystals of the 1R,2S,4S,1S ketol under study, molecules are linked by a network of cooperative -O-H...O-H...O-H... hydrogen bonds. Based on the results of molecular mechanics calculations and experimental data of¹H NMR and IR spectroscopy, conformations of molecules of this compound, which are in equilibrium in solution, have been characterized. Based on data on spatial structures of stereoisomeric -hydroxyketones and the character of H-bonds formed by these compounds, the characteristic features of the effect of these chiral alloying additives on the supramolecular structure and macroscopic properties of liquid crystalline systems have been interpreted.The results of a study of stereoselectivity of the reactions of different (–)-menthone enolates with aromatic aldehydes will be published later.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1247–1255, July, 1995.The work was supported by the Foundation for Basic Research of the State Science and Engineering Committee of Ukraine (Project No. 93-3.2/75).     

Liquid‐crystalline glycolipids: towards understanding the roles of liquid crystals in biological and life processes

A Commentary on the paper ”Liquid‐crystalline behaviour in the n‐alkyl gluconamides and other related carbohydrates?, by B. Pfannemüller, W. Welte, E. Chin and J.W. Goodby. First published in Liquid Crystals, 1, 357‐370 (1986).     

Side chain functionalized η-tetramethyl cyclopentadienyl complexes of Rh and Ir with a pendant primary amine group

Conversion of 2-(2,3,4,5-tetramethylcyclopentadien)ethylamine tautomere C₅Me₄H₂(CHCH₂)NH₂ (1) with MCl₃ · n H₂O (M = Rh, Ir) under acidic conditions gives the respective μ-chloro-bridged chelates [(η⁵-Me₄Cp(CH₂)₂NH₃)RhCl₂]₂Cl₂ (2) and [(η⁵-Me₄Cp(CH₂)₂NH₃)IrCl₂]₂Cl₂ (3). The dimeric complexes are received as ammonium salts and thus display good solubility in strong donor solvents such as water and DMSO. Addition of triphenyl phosphine converts Rh-dimer 2 into the mononuclear phosphine complex 4. Under basic conditions, no intramolecular coordination of the pendant NH₂ is observed and thus the primary amino group of 4 reacts selectively with succinic anhydride by formation of a peptide bond. Hence, the electrophilic metal center and the latent nucleophilic nitrogen, which represent complementary functionalities, can be addressed separately under the appropriate reaction conditions.     

Synthesis and liquid crystal properties of a new class of calamitic mesogens based on substituted 2,5‐diaryl‐1,3,4‐thiadiazole derivatives with wide mesomorphic temperature ranges

Liquid crystals based on substituted 2,5‐diaryl‐1,3,4‐thiadiazole derivatives (1a–1f, 3a and 3b) and 1,3,4‐oxadiazole analogues (2a–2f, 4a and 4b) were synthesised and characterised by ¹H, ¹³C nuclear magnetic resonance, Fourier transform infrared, mass spectrometry, high‐resolution mass spectrometry techniques and elemental analyses. The X‐ray crystal structure of 1e revealed that it contains tilted lamellar arrangement of molecules in the crystalline solid. The liquid crystal properties have been investigated by polarised‐light optical microscopy, differential scanning calorimetry and in‐situ variable‐temperature X‐ray diffraction. All compounds (except 2e and 2f) exhibited thermotropic liquid crystal behaviours with various mesophases (smectic A and C, nematic N or soft crystal E phases). Notably, the 1,3,4‐thiadiazole derivatives consistently have wider mesomorphic temperature ranges than those of the respective 1,3,4‐oxadiazole analogues. The solutions of all compounds in CH₂Cl₂ individually displayed one or two absorption bands with λ _max values at 297–355 nm and emitted with λ _max values at 363–545 nm and quantum yields of 0.12–0.73. Structure–property relationships of these compounds are discussed in the contexts of their molecular structures and weak intermolecular interactions.     

Ferroelectric liquid crystal dopants with a chiral (R,R)‐2,3‐difluorooctyloxy side‐chain: host dependence of the polarization power

The polarization powers δ_p of four chiral dopants with (R,R)‐2,3‐difluorooctyloxy side‐chains were measured in four liquid crystal hosts with isotropic (I)–nematic (N)–smectic A (SmA)–smectic C (SmC) phase sequences. The four chiral dopants differ in terms of their core structures: 2‐phenylpyridine (MDW950), biphenyl (5), 2‐phenylpyrimidine (6) and 2‐(3‐nitrophenyl)pyrimidine (7). In each case, δ_p varies with the structure of the liquid crystal host, which is consistent with the behaviour of so‐called Type II dopants that normally feature a chiral core structure. The δ_p(host) profile was found to depend on the degree of biaxiality of the dopant core structure, and on the degree of steric coupling between the chiral 2,3‐difluorooctyloxy side‐chain and the core. Conformational analyses at the B3LYP/6‐31G* level suggest that the 2,3‐difluorooctyloxy side‐chain is conformationally more rigid than conventional chiral side‐chains due to the added electrostatic repulsion of the two adjacent fluoro groups combined with the hyperconjugative ‘gauche effect’, and may therefore have a higher degree of biaxiality on the time average. This biaxial character should make the chiral side‐chain more sensitive to variations in quadrupolar ordering imposed by the SmC phase of the liquid crystal host, and may therefore explain the dependence of δ_p on the host structure reported herein.