Synthesis and characterization of new ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligooxyethylene spacers

Two series of ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligooxyethylene spacers were synthesized. The mesomorphic behaviours of the compounds were characterized using differential scanning calorimetry (DSC), optical polarizing microscopy (POM) and powder X-ray diffraction measurements. These series all contain a smectic A (S_A), a chiral smectic C (S_C) and a crystal E (C_r E) phase sequence. The ordered smectic Cr E phase was observed for the short spacer chain (n = 0, 1, 2) homologues of the two series. Moreover, the mesomorphic properties are discussed as a function of the oligooxyethylene spacer length, and the position of the aromatic core rings. These results indicate that the structures with a rigid part (biphenyl group) closer to the flexible spacer chain (oxyethylene unit) tend to form the symmetrical smectic S_A and Cr E phases.     

Mesomorphic properties of a homologous series of chiral liquid crystals containing the α-chloroester group

The mesomorphic properties of the N*, S*_c and higher ordered smectic phases have been investigated for a homologous series of 4-[(S)-2-chlor-3-methylbutanoyloxy]-4'-(4-n-alkyloxy-benzoyloxy)biphenyls. They have been characterized by optical texture observation, differential scanning calorimetry (DSC), small angle X-ray scattering and electro-optic measurements. The compounds exhibit a strongly twisted cholesteric phase and smectic phases with large spontaneous polarization and tilt angle values. One or two higher ordered, monotropic smectic phases were found which significantly differ in their rotational viscosity.     

Synthesis and characterization of a series of side chain chiral smectic liquid crystalline elastomers

A series of new chiral smectic liquid crystalline elastomers was prepared by graft polymerization of a nematic monomer with a chiral and non-mesogenic crosslinking agent, using polymethylhydrosiloxane as backbone. The chemical structures of the monomers and polymers obtained were confirmed by FTIR and ¹H NMR. The mesomorphic properties were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy and X-ray diffraction. Monomer M₁ showed a nematic phase during heating and cooling. Polymer P₀ exhibited a smectic B phase; elastomers P₁-P₃ showed the smectic A phase, P₄-P₆ showed a chiral smectic C(SmC*), and P₇ displayed stress-induced birefringence. Elastomers containing less than 15 mol % M₂ displayed elasticity, reversible phase transitions with wide mesophase temperature ranges, and high thermal stability. With increasing content of the crosslinking unit, glass transition temperatures first increased, then fell, then increased again; isotropization temperatures and mesophase temperature ranges steadily decreased.     

Antiferroelectric liquid crystals with amide linking group positioned at chiral tail

A homologous series of chiral liquid crystal compounds, N-methyl-N-pentyl-(S)-2-(6-(4-(4-alkyloxyphenyl)benzoyloxy)-2-naphthyl)propionamide, with an amide linkage in a chiral tail was synthesized and their mesomorphic properties studied. All the materials possessed an antiferroelectric smectic C (SmC_A*) phase, which was confirmed by observations of microscopic texture, switching current behaviour and electro-optical responses. The spontaneous polarization, P _s, and apparent tilt angle, θ, were also measured. The maximum P _s values are in the range of 173-222 nC cm^-2, and the maximum θ values are in the range of 26-30°.     

Novel liquid crystals and dopants (for induced ferroelectricity) possessing a chiral 2-oxetanone unit. A comparison with corresponding dioxolanones

The mesomorphic properties, induced spontaneous polarizations, and response times of a new type of liquid crystal and ferroelectric liquid crystal mixtures containing di- or trisubstituted 2-oxetanones as a chiral unit are described. By heating above 140°C oxetanones, bearing a three core mesogenic part, suffer cycloelimination of carbon dioxide yielding liquid-crystalline olefines with nematic, smectic C and higher ordered smectic phases, which are best suited as host materials for induced S_c* phases using the corresponding oxetanones as dopants. Compared with 1,3-dioxolan-4-ones recently synthesized and investigated, the oxetanones show minor mesomorphism.     

Reinvestigation of liquid crystalline property of dimesogenic compound consisting of cholesterol and cyanobiphenyl

The dimesogenic compound consisting of cholesterol and cyanobiphenyl mesogens interlinked byω-oxyalkanoyl spacer was synthesized.The mesomorphic properties were investigated by differential scanning calorimetry(DSC),polarizing optical microscopy(POM),and X-ray diffraction(XRD).The experimental results indicated that this compound exhibited mesophase over a much wider temperature range and a new mesophase blue phase(BP).Focal conic domains(FCDs) and droplets texture to the smectic A phase(SA),oily streaks...     

Synthesis and characterization of new ferroelectric liquid crystals containing a (2S)-2-[6-(4-hydroxybiphenyl-4'-carbonyloxy)-2'- naphthyl] propionate mesogenic group and oligo(oxyethylene) spacers

Three series of ferroelectric liquid crystals containing a (2S)-2-(6-(4-hydroxybiphenyl-4'-carbonyloxy)-2'-naphthyl)propionate mesogenic group and oligo(oxyethylene) spacers were synthesized. These obtained liquid crystal compounds were characterized by NMR, differential scanning calorimetry (DSC), optical polarized microscopy (POM), and X-ray powder diffraction measurements. Some of these materials containing four phenyl rings of ester cores (i.e. -Ph-Ph-COO-naph-) and chiral heptyl tail exhibited a rich mesomorphic behaviour, a blue phase (BP), a cholesteric phase (Ch), a smectic A (S_A), a twist grain boundary A (TGB_A), and a chiral smectic C (S_C*) phase. Another series containing four phenyl rings of ester cores and chiral butyl and pentyl chain tails revealed only a S_A phase and a S_C* phase. Moreover, a crystal E phase was observed in the short spacer chain (n = 0 or 1) homologues of three series of compounds. Also, the mesomorphism properties were discussed as a function of spacer units, numbers of aromatic rings of core, and different terminal asymmetric moieties.     

Dimeric liquid crystals with 5-(4-alkoxybenzoyloxy)tropone or 4-(4-alkoxybenzoyloxy)phenyl cores: evaluation of the tilt angles of the cores, spacers, and side chains

Two types of symmetric dimers with 5-(4-alkoxybenzoyloxy)tropone cores or with 4-(4-alkoxybenzoyloxy)phenyl cores were synthesized to evaluate the effect of the core structure and the length of the spacer on the mesomorphic properties. The former had smectic C phases whereas the latter had smectic C and F phases. Both types of dimer showed a remarkable odd-even effect on varying the spacer on the mesomorphic properties. Comparison of the thermal stability between them demonstrated that benzenoid twins are more stable than troponoid ones. The layer spacings of the smectic C phases were measured to determine the tilt angles of the core part, the spacer, and the side chains on changing the length of the spacer and the side chains. Troponoid dimers had a larger tilt angle of the core part of the smectic C phase than benzenoid twins, which lowered the thermal stability of the troponoid. Entropy changes of the smectic C phase to the isotropic liquid showed a contrast between troponoids and benzenoids. The former had smaller values and odd-even effects than the latter, which indicated that the former troponoids had a limited number of conformers in mesophases.     

Enaminoketones as new hydrogen bonded liquid crystals

The relation between molecular structure of enaminoketone derivatives and their mesomorphic properties is discussed. It has been observed that the electron accepting terminal groups (F,Cl,Br,CN) promote formation of orthogonal phases while strongly electron donating groups (OC_nH_2n+1) favour tilted phases. Among the synthesised compounds, materials useful for structural studies of two dimensional (2D) melting in orthogonal, as well as the tilted systems, were found. It has been determined that the heat effects in the melting process from crystalline smectic into liquid-like smectic, through the hexatic state, are correlated to the density changes.     

Synthesis and thermal properties of mesomorphic 1,1'-bis[ω-(4'-cyano-4-biphenyloxy)alkyl] ferrocenes

A new series of 1,1'-disubstituted ferrocene compounds of the type [(η⁵-C₅H₄(CH₂)_nOC₆H₄C₆H₄CN]₂Fe (3a-d, n = 5, 6, 8, 11) incorporating a variable length alkyloxy cyanobiphenyl unit has been prepared and their mesomorphic properties have been investigated. Compounds 3b, c and d exhibit a thermotropic smectic C phase and 3c also exhibits a monotropic smectic A phase over a fairly wide range near ambient temperature.     

Synthesis and thermal behaviour of salicylaldimine-based liquid crystalline symmetrical dimers

A series of symmetrical dimers consisting of salicylaldimine moieties connected by flexible alkylene central spacer via ether linkages has been synthesized. In order to validate the empirical rule suggested by Date et al. to account for the smectic behaviour of such dimers, the chain length of the terminal alkoxy chain has been kept constant (C₈) while the number of methylene units in the central spacer was varied from C₃ to C₁₁. Another aim of the present investigation was to understand structure-property relationship in these dimers in which the salicylaldimine mesogenic segment has been used for the first time in dimers. The mesomorphic behaviour of these dimers was evaluated using optical microscopy and differential scanning calorimetry and the structure of some of the mesophases has been further investigated with the help of X-ray diffraction. Our studies reveal that the dimers consisting of 3 to 8 methylene units in the flexible spacer show only smectic (smectic C and smectic A) phases. For the dimers containing 4, 6 and 8 methylene units in the central spacer, a unique filament growth pattern has been observed in the smectic A phase while cooling from the isotropic phase. The dimers containing of C₉ to C₁₁ methylene groups exhibit the nematic phase in addition to smectic modifications. This observation indicates that when the terminal chains are shorter than the spacer, the tendency to form smectic phases is not fully extinguished but is perhaps reduced.     

Mesogenic Unsymmetric dimers containing cholesteryl ester and tolane moieties

Among unsymmetric oligomesogens, chiral dimers formed by connecting a cholesteryl ester fragment with various aromatic mesogenic cores through a polymethylene spacer have been attracting much attention due to their remarkable thermal behaviour. In particular, dimers containing a diphenylacetylene segment having an alkoxy chain have shown interesting mesomorphic behaviour. In view of this a new series of unsymmetric dimers consisting of a diphenylacetylene moiety having an alkyl chain and a cholesteryl ester unit joined through a paraffinic spacer have been synthesized and their liquid crystalline properties characterized. The lengths of the central methylene spacer (C₃, C₄, C₅ and C₇) as well as that of the alkyl chain (n-butyl, n-pentyl, n-hexyl and n-heptyl) have been varied to establish structure-property relationships. These investigations have revealed that all the dimers exhibit smectic A, twist grain boundary and chiral neamtic (N*) phases with the exception of one of the dimers for which only the N* phase was observed. Some differences in the mesomorphic properties of the unsymmetric dimers containing odd or even parity methylene spacers have been observed. The majority of dimers having an even (C₄) parity paraffinic spacer show a blue phase while the dimers with odd (C₃, C₅ and C₇) parity spacers exhibit the chiral smectic (SmC*) phase. In some cases, the SmC* phase exists well below (-60°C) and above room temperature.     

Heterocyclic benzoxazole-based liquid crystals

The synthesis, characterization, and mesomorphic properties of a new type of heteronuclear compounds 1a-c and a Pd complex 1d derived from benzoxazole as the core group are reported. These compounds were prepared by the ring closure reaction of 4-alkoxybenzoic acid 4-[(4-alkoxy-2-hydroxyphenylimino)methyl]phenyl esters 6 in the presence of lead(IV) acetate. All the compounds were characterized by ¹H and ¹³C NMR spectroscopies and elemental analysis. The phase behaviour of these mesogenic compounds was characterized and studied by differential scanning calorimetry and polarizing optical microscopy. All the compounds 1a exhibited nematic (N) and/or smectic C (SmC) phases, as expected for rod-like molecules; however, the compounds 1b and 1c exhibited crystal phases. For those compounds 1a having shorter carbon chains (n = 1, 3, 4) nematic phases were observed, whereas for compounds having longer carbon chains (n = 6, 7, 8, 10, 12, 14) smectic C behaviour was also observed at lower temperatures. The greater aspect ratio (l/d) of compounds 1a compared with 1b and 1c was found to be required for the observation of liquid crystallinity. The fluorescent properties of these compounds were also examined. All λ_max peaks of the absorption and photoluminescence spectra of compounds 1a-1c occurred at c. 316-322 nm and 371-382 nm, respectively. The quantum yields of some compounds were relatively low, and also slightly solvent-dependent.     

Synthesis and mesomorphic properties of 4-(4-bromopropyloxy)-4’-(4-alkyloxybenzylidene)anilines

This article describes the preparation and liquid crystalline properties of a new homologous series of 4-（4-bromopropyloxy）-4’- （4-alkyloxybenzylidene）anilines in which the phenyl ring is armed by a bromopropyloxy chain.The thermal behavior and mesomorphic properties of the synthesized compounds were studied with particular attention given to the correlation between their phase transition temperatures and anisotropic change influenced by molecular structure.All the members displayed enantiotropic smectic phase except for the homologue with the longest alkyloxy chain(R = C₁₈H₃₇),exhibiting only monotropic characteristic. The presence of bromine atom from the propyloxy side chain is found to be capable of altering and influencing the mesomorphic properties.     

Synthesis and mesomorphic properties of 6-n-decyloxy-2-[4'-N-alkoxyphenylimino)methyl]quinolines (IV)

The synthesis and mesomorphic properties of a new series of liquid crystals derived from quinoline with an imine central bond is described. All compounds exhibit mesophase ranges greater than 50°C. The lower homologues (n = 4-6) show trimorphism S_c-S_a-N (for n = 4, the S_c phase is monotropic). The higher homologues (n = 7-10) show nematic and smectic C phases. Compared to the styrylquinoline analogues the imine bond gives rise to similar liquid crystal phase ranges but lower melting points.     

Synthesis and mesomorphic properties of four-ring liquid crystals containing 2,3-difluorophenyl and 1,3-dioxane units

Two series of novel liquid crystalline materials were synthesized: 2-(2,3-difluoro-4-alkoxyphenyl)-5-[4-(4-alkylcyclohexyl)phenyl]-1,3-dioxanes (A_m-n) and 2-(2,3-difluoro-4-alkylphenyl)-5-[4-(4-alkylcyclohexyl)phenyl]-1,3-dioxanes (B_m-n). Their mesmorphic properties were characterized by polarizing optical microscopy and differential scanning calorimetry. All these compounds exhibit broad mesomorphic phases, and most of them show wide smectic C phases and the phase sequence Cr-SmC-SmA-N-I. The relationship between properties and chemical structures is discussed in detail.     

Synthesis and mesomorphic properties of four-ring liquid crystals containing cyclohexyl, phenyl and pyridyl units

Two series of chiral and achiral 4-[(3- n -alkoxycarbonylpyridyl)-6-ethynyl]phenyl 4-( trans -4- n -alkylcyclohexyl)benzoate liquid crystals have been synthesized. Their mesomorphic properties were observed and measured by polarizing optical microscopy and DSC. A broad chiral smectic C phase was observed when terminal chains contained a chiral centre.     

Chiral liquid crystalline m-nitrotolans and tolans: synthesis and mesomorphic properties

Two homologous series, 4'-(4-n-alkoxybenzoyloxy)-4-substituted 3-nitrotolans (Ia-d) and 4'-(4-n-alkoxybenzoyloxy)-4-substituted tolans (IIa-c), with substituents (S)-2-methyl-1-butyl and n-alkyl, were synthesized by Sonogashira's coupling. Their mesomorphic behaviour is reported. The thermal stability of the series II is higher than that of series I. Series I melting points and clearing points are lower than those of series II. None of the chiral tolans or m-nitrotolans have an enantiotropic smectic C phase. When the chiral chains are changed to n-alkyl groups in compounds Ia,b and IIa, an enantiotropic smectic C phase is seen. All compounds have a nematic or cholesteric phase, and two homologues of series II present a monotropic smectic C phase with mosaic texture. The mesophases were characterized using optical polarized light microscopy and differential scanning calorimetry.     

Rare-earth complexes of mesomorphic Schiff's base ligands

Rare-earth complexes of mesomorphic Schiff 's bases, 4-[(alkylimino)methyl]-3-hydroxyphenyl 4-alkyloxybenzoates, were synthesized. Whereas the ligands LH display a nematic and/or a smectic C phase, the metal complexes show a viscous smectic A phase and decompose at the clearing point. The mesophase was investigated by hot-stage polarizing optical microscopy, by differential scanning calorimetry and by high temperature X-ray diffraction. Two types of complex were found, [Ln(LH)₃ (NO₃)₃] and [Ln(LH)₂L(NO₃)₂], depending on the ligand or the central metal ion. The first coordination sphere of the rare-earth ion in these metallomesogens is comparable to that in the structure of complexes with 4-alkoxy-N-alkyl2-hydroxybenzaldimine ligands.