A polar biaxial smectic A phase in new unsymmetrical compounds composed of bent-core molecules

The synthesis and liquid crystalline properties of several new compounds unsymmetrically substituted about two different central phenyl rings are reported. All the compounds contain a strongly polar terminal cyano/nitro substituent. Some of the nitro-substituted compounds derived from 3-hydroxybenzoic acid show a transition from the partial bilayer uniaxial smectic A (SmA_d) phase to a polar partial bilayer biaxial smectic A (SmA_dP_A) phase. The analogous cyano-substituted derivatives exhibit only a SmA_d phase. However, compounds containing a 4-cyanobiphenyl moiety in the terminal position and derived from 1,3-dihydroxybenzene show the SmA_d - SmA_dP_A transition. The mesophases were characterized using polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction studies, conoscopic examination and miscibility studies.     

Synthesis and liquid crystalline properties of several compounds with semi-fluorocarbon chains

Several molecules having long semi-fluoroalkyl chains have been synthesized, and their phase transition temperatures measured by texture observation in a polarizing microscope and DSC. The SmC phase is particularly favoured in this series, especially for compounds with a long semi-fluoroalkyl chain. It was found that when the terminal hydrogen atom of the semifluoroalkyl chain of one of the compounds (D) was substituted by a chlorine atom, the liquid crystalline sequence and ranges did not change; however, the clearing and melting points and T_I-SmA and T_SmA-SmC increased by nearly 10°C.     

Phenyl-cyclohexyl enaminoketone ligands and their Cu(II) complexes

The liquid crystalline properties of 1-(alkylamino)-3-[(4'-hexyl-trans-cyclohexyl-4'-phenyl]-prop-1-en-3-one-s, from methyl to octadecyl, and their copper (II) complexes have been examined by optical, DSC, X-ray and EPR methods. The compounds are enantiotropic nematogens except those having the shortest and the longest terminal chains. Short chains promote the S_A phase in both ligands and complexes, whereas long chains promote S_C and crystal H phases for the ligands or S_A and S_C phases for the complexes. A partly bilayer smectic A_d phase is observed from ligands terminated with short non-polar substituents. Direct isotropisation from the crystal H phase for some of the compounds, as well as other phase transitions have been studied. The molecular shape of the complexes and the organization of their mesomorphic phases are discussed, based on the refractive indices and X-ray data.     

Experimental Binary Phase Diagram of Bilayer Compounds [n-CnH2n+1N(CH3)3]2CoCl4

Compounds [n-C_nH_2n+1N(CH₃)₃]₂CoCl₄(n=16, C₁₆C₃Co; n=18, C₁₈C₃Co) containing lipid-like bilayers embedded in a crystalline matrix exist in solid-solid phase transition. The low-temperature bilayer structures of the two compounds were organized by neutralizing CoCl₄^2- with alkylammonium ions. Alkyl chains lay parallel to each other and slightly tilted with respect to the normal of the inorganic layers. The adjacent alkyl chains interacted with each other by van der Waals interaction. When the temperature increased, the two compounds underwent a reversible solid-solid phase transformation within 310―330 K. In such a case, the chains showed a large motional freedom, and a disordered phase appeared. The structures can alternatively be viewed as a double layer of alkylammonium ions between CoCl₄^2- sheets and be considered as crystalline models of lipid bilayers. The experimental subsolidus binary phase diagram of [n-C₁₆H₃₃N(CH₃)₃]₂CoCl₄-[n-C₁₈H₃₇N(CH₃)₃]₂CoCl₄ was constructed over the entire composition range by differential scanning calorimetry and X-ray diffraction technique. Experimental phase diagram indicates one stable intermediate phase [n-C₁₆H₃₃N(CH₃)₃][n-C₁₈H₃₇N(CH₃)₃]CoCl₄ at w_C16C3Co= 39.89% and two invariant three- phase equilibria, which shows two eutectoid temperatures: T_e1 at (316±1) K for w_C16C3Co= 27.35% and T_e2 at (313±1) K for w_C16C3Co=59.76%. These three noticeable solid-solution ranges are α-phase at the left, β-phase at the right, and γ-phase in the middle of the phase diagram.     

X-ray study of the highly ordered smectic phases of N-pentyl-N'-(p-pentyloxyphenyl)piperazine

The structures of the highly ordered liquid crystalline smectic phases of N-pentyl-N'-(p-pentyloxyphenyl)piperazine are identified using X-ray diffraction methods. For this compound a phase sequence hexagonal smectic B (S_B)-orthofrhombic crystalline smectic H (C_H)-monoclinic crystalline smectic H (C_H) is observed for the first time. The changes in structural symmetry at the phase transitions are discussed.     

Synthesis and liquid crystalline properties of novel laterally connected trimesogens and tetramesogens

Three types of laterally connected triplet mesogens and one quadruplet mesogen incorporating rigid p-terphenyl units have been synthesized. Their liquid crystalline behaviour was investigated by polarizing microscopy, differential scanning calorimetry and X-ray scattering. The lateral fixation of three rod-like 4,4'-didecyloxy-p-terphenyl units mostly gives liquid crystalline materials with considerably increased mesophase stabilities with respect to the parent 4,4'-didecyloxy-2'-methyl-p-terphenyl. The mesophase stability strongly depends on the type of connection. The highest clearing temperatures were observed for triplets which are connected in line with each other (type I) and triplets which are laterally connected in a peripheral manner. Only the oligomesogens of type III are not liquid crystalline. All compounds incorporating exclusively decyloxy chains exhibit smectic phases (S_A and S_c). For the ethoxy derivatives the nematic phase was found.     

Synthesis and characterization of new series of ferroelectric liquid crystals containing oligooxyethylene spacers

Four series of ferroelectric liquid crystals containing oligooxyethylene spacers have been synthesized. These obtained liquid crystal compounds were characterized by NMR, differential scanning calorimetry (DSC) and optical polarized microscopy (POM). The properties of the liquid crystalline phase were investigated as a function of spacer units, numbers of core aromatic rings, and different terminal asymmetric moieties. It was found that (i) the phase transition temperature decreased with the increasing oligooxyethylene spacer unit, (ii) the liquid crystalline phases were enhanced in three phenyl ring system than in two phenyl ring system, and (iii) ferroelectric liquid crystals containing different terminal asymmetric moieties exhibited novel mesophase phenomena. A twist grain boundary phase (TGB_A phase) was observed in some compounds of this study. Furthermore, a wide temperature chiral smectic Crange including room temperature was achieved.     

The ferro-, ferri- and antiferro-electric properties of a series of novel 2- or 3-substituted-alkyl 4-(4'-dodecyloxybiphenyl-4-carbonyloxy)-benzoate esters

This paper reports the synthesis of six chiral liquid crystalline 2- or 3-substituted-alkyl 4-(4'-dodecyloxybiphenyl-4-carbonyloxy)benzoates, their mesophase assignments and the phase transition temperatures. All six materials display S*_A followed by S*_C phases on cooling from their isotropic liquids. The magnitudes of switched tilt angle, spontaneous polarization, helical twist sense (L.H.) and direction of polarization (P_s(-)) are given for the S*_C phases. Close inspection of the S*_C phases revealed that for the six compounds, two showed a ferrielectric (S*_Cγ) and an antiferroelectric (S*_A) subphase, two showed only a ferrielectric subphase and one showed only an antiferroelectric subphase. The field-dependent behaviour of the switched tilt angle of the ferrielectric and antiferroelectric phases of some of the compounds is also presented.     

Phase behaviour of thermotropic banana-shaped compounds under pressure

The phase behaviour of two achiral bent core banana-shaped compounds, the hexyloxy (compound I) and decyloxy (compound II) members of the 1,3-phenylene bis[N-(2-hydroxy-4-n-alkoxybenzylidene)-4'-aminobenzoate] series was investigated under hydrostatic pressures up to 300 MPa using high pressure differential thermal analysis and light transmission methods. The reversible transition sequence crystal (Cr₁)-B₁ phase-isotropic liquid (I), observed at room pressure for compound I, remains in the pressure region up to c 70 MPa. At higher pressures a pressure-induced crystalline phase (Cr_i) appears between the Cr₁ and B₁ phases, its temperature region becoming wider with increasing pressure. The temperature vs. pressure phase diagram shows a triple point of 72.9 MPa and 160.3°C for the Cr₁, Cr_i and B₁ phases, indicating the lower limit of pressure for the Cr_i phase. In compound II the reversible transition sequence crystal (Cr₁)-B₂ phase-I is seen over the whole pressure region, and the temperature range of the B₂ phase remains unaltered. It is concluded that both the B₁ and B₂ banana phases are stable over the whole pressure region studied.     

Asymmetric banana-shaped liquid crystals with two different terminal alkoxy chains

Two series of asymmetric banana-shaped compounds have been synthesized and studied. In the 1,3-phenylene bis[4-(4'-alkoxybenzoyloxy)]benzoate series the lack of symmetry was derived solely from the difference in length of the two terminal alkoxy chains. In the 3,4'-biphenylene bis[4-(4'-alkoxybenzoyloxy)]benzoate series the asymmetric nature originates from the 3,4'-substitution of the central biphenyl group and from the difference in length of the two terminal chains. All the melting points of the asymmetrical compounds in the series with the central phenyl unit are lower than those of the symmetrical compounds. The liquid crystalline B₁ or B₂ phase was retained in all cases. In the series with the central biphenyl unit the compounds with the shortest chain attached to the para-position of the central biphenyl unit have the lowest melting points. A significant lowering of the melting points in comparison with the symmetrically substituted compounds, however, could not be achieved. All the compounds of both series show a layer spacing which is comparable to those of the symmetrically substituted parent compounds. The observed switching behaviour of both the symmetric and asymmetric compounds with a B₂ phase was antiferroelectric.     

Non-symmetric bent-core mesogens with one terminal vinyl group

Two series of non-symmetric banana-shaped compounds, both with one alkyl and one alkenyl terminal tail, have been synthesized and studied. Both series were compared with the corresponding series with two saturated terminal alkyl tails. All the compounds have a bent central 1,3-phenylene bis(4-benzoyloxy)benzoate core; their mesophases were characterized by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and switching current response experiments. In all four series one of the terminal tails is varied from OC₈H₁₇ to OC₁₆H₃₃. The other terminal tails are OC₁₁H₂₃, O(CH₂)₉CH = CH₂, OC₁₀H₂₁ and O(CH₂)₈CH = CH₂. The short-tailed compounds show monotropic or enantiotropic B₁ phases and the long-tailed compounds the B₂ phase. The introduction of one terminal vinyl group slightly lowers the transition temperatures. The introduction of a second terminal vinyl group further suppresses the liquid crystalline properties. All compounds with B₂ phases have layer spacings that suggest a tilt of ∼45° of the bent molecules in the layers, and their switching behaviour is antiferroelectric.     

The formation and stability of hydrated and anhydrous uranyl arsenates

Three hydrated uranyl arsenates, (UO₂)₃(AsO₄)₂ · 11 H₂O, UO₂HAsO₄ · 4 H₂O, and UO₂(H₂AsO₄)₂ · 1 H₂O, have been prepared. The dehydration of these compounds has been studied by thermal analysis. Three crystalline anhydrous uranyl arsenates, (UO₂)₃(AsO₄)₂, (UO₂(AsO₃)₂, have been found. These show melting phenomena and lose arsenic oxide vapour at high temperatures to result, finally, in U₃O₈ at 1500°C in air. The anhydrous compounds have been prepared under isothermal conditions and the strongest X-ray reflections are given. A tentative phase diagram in the composition range UO₃ to As₂O₅ has been constructed.     

Self-assembled diols: synthesis, structure and dielectric studies

In single-, double-, and triple-chain amphiphilic diols the CONH group was replaced by CON(CH₃) in order to reduce the number of proton donor groups available for intermolecular hydrogen bonding. The resulting three new liquid crystalline diols were studied by DSC, X-ray and dielectric measurements, and show the mesophases SmA, Col_H2 or Cub_I2, depending on the number of decyloxy groups in the hydrophobic part of the molecule. The process of self-assembly to different liquid crystalline phases is well seen in the dielectric spectrum and details of this process are discussed together with results from the X-ray measurements. All the compounds show a high frequency dielectric absorption caused by the dynamics of the network of hydrogen bonds. An additional low frequency process related to the internal dynamics of the columns is seen only in the columnar phase.     

Liquid crystalline hexa-amides and side chain polysiloxanes of azacrown [18]-N6

The hexa-4-dodecyloxybenzoyl derivative 1 of azacrown [18]-N₆ was originally reported to have a 'tubular' mesophase on the basis of its large central ring and 6-fold symmetry. Starting in the mesophase, annealing of 1 under a cover slip results in formation of a new crystalline phase that melts directly to an isotropic liquid at the temperature previously observed for the mesophase to isotropic transition. Thus the phase behaviour of 1 is kinetically controlled. The analogous hexa-3,4-bisdodecyloxybenzoyl derivative 2 of azacrown [18]-N₆ has no kinetic limitations to its phase changes and has an enantiotropic columnar liquid crystalline phase. We have synthesized side chain copolysiloxanes with a (CH₂)₁₁ spacer and 75-84 per cent by weight of the same 4-dodecyloxybenzoyl-[18]-N₆ mesogen. The polysiloxanes also display a liquid crystalline phase.     

New mesomorphic organocyclosiloxanes II. Thermal behaviour and mesophase structure of organocyclohexasiloxanes

Investigations of thermotropic phase transitions performed on organocyclosiloxanes [PhSi(O)OSiR]₆, where R is Me₃, Me₂(CH₂Cl) or Me₂(CH≃CH₂), have revealed that all these hexamers are mesomorphic compounds. The hexamers exhibit uncommon polymesomorphic behaviour forming two quite different mesomorphic structures. The molecular arrangement in the low temperature (LT) modification is characterized by two-dimensional (2D) long-range order with hexagonal packing. The X-ray diffraction pattern and peculiarities of molecular packing in the crystal lead us to suggest that the LT-mesophase is columnar, presumably of the Col_hd type. The LT-mesophase is formed by dimeric moieties, which associate with each other in column-like substructures, the ring planes not orthogonal to the stack axis. The high temperature (HT) mesophase is a plastic crystal (3D-order), where molecules take up positions in a face-centred cubic lattice. This is a very uncommon example of thermal behaviour for plastic crystals that provides a unique opportunity to bridge the gap between plastic crystalline and liquid crystalline mesomorphic behaviour. The thermal and structural properties of the mesophases depend upon the type of side groups of the hexamers. The size of the ring also affects the phase behaviour and the mesomorphic structure. This conclusion is consistent with data obtained by us earlier for cyclotetrasiloxanes.     

Liquid crystalline properties of salicylaldimine-based dimers: influence of terminal alkyl chain length and central part

The synthesis and thermotropic properties of four homologous series of salicylaldimine-based dimer liquid crystals are reported. Two 4-(4-alkoxy-2-hydroxybenzylideneamino)benzoyloxy groups are connected to a central part consisting of a 1,3-phenylene, 1,5-pentylene, 2,2-dimethyl-1,5-pentylene or 3,3-dimethyl-1,5-pentylene unit. The terminal alkoxy chains have been varied from 4 to 16 carbon atoms in length. All the compounds exhibit liquid crystalline phases whose behaviour depends on the nature of the central part and the length of the alkoxy terminal chains. All compounds of the series with the central phenyl part exhibit enantiotropic B-phases, and the sequence B₆-B₁-B₂ on increasing terminal chain length was observed. Replacement of the phenyl group with a pentyl central group partly suppresses the formation of B-phases. The longer homologues of this series show the B₁ phase, while the shorter exhibit an intercalated SmC_c mesophase. The introduction of methyl substituents to the pentyl spacer causes the melting points to fall dramatically and the formation of B-phases is totally suppressed. The compounds with the long tails show intercalated SmA_c phases and those with short tails show intercalated SmC_c phases.     

Mesogenic properties of 1,2,3-tri-[3'-(4'-alkoxyphenyl)-3'-oxo-1'-propenylamino]propane

The properties of compounds incorporating three mesogenic cores linked to a central 1,2,3-trisubstituted propane unit are reported. They form S_A and S_c calamitic liquid crystalline phases with a layer spacing in the orthogonal smectic phase significantly smaller than a single molecular length. The mesogenic properties of related molecules with two mesogenic cores depend on the position of the enaminoketone ring with respect to the joint. Intramolecular dipolar interactions of the enaminoketone rings are suggested as being responsible for the hindered frmation of the trans-conformational structure of the cores in the dimeric molecules.     

Mesomorphism of lanthanide-containing Schiff's base complexes with dodecyl sulphate counterions

Liquid crystalline complexes of the formula [Ln(LH)₃(DOS)₃] have been synthesized, where Ln is a trivalent rare earth-ion (Y, La-Lu, except Pm), LH is the ligand N-octadecyl-4-tetradecyloxysalicylaldimine and DOS is the dodecyl sulphate counterion. Although the Schiff 's base ligands do not exhibit mesomorphism, the complexes do (SmA phase). The mesophase behaviour of these compounds has been investigated by polarizing optical microscopy, differential scanning calorimetry, high temperature X-ray diffraction and thermogravimetric analysis. The stoichiometry of the complexes remains constant throughout the lanthanide series.     

A simple molecular model for N-SmAd-NdRe-N1Re-SmA1 phase sequence in highly polar compounds

Our earlier model of re-entrant liquid crystalline phases exhibited by highly polar compounds, in which the mutual orientation of near-neighbour molecules can change from an antiparallel to a parallel configuration, has been extended to include both nematic and smectic interactions. We show that, as the McMillan parameter alpha is decreased, the SmA_d-SmA₁ line goes over to the SmA_d-N_1Re line, finally becoming the N_dRe-N_1Re transition line, the latter ending in a critical point. This sequence is in agreement with the predictions of Prost's Landau model as well as with an experimental result. The phase sequence N-SmA_d-N_dRe-N_1Re-SmA₁ is obtained on cooling for a range of alpha_A values.     

Cholesterol-based dimeric liquid crystals: synthesis and mesomorphic behaviour

Chiral non-symmetric dimeric liquid crystals consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core, interconnected through n-butyl (C₄) or n-pentyl (C₅) parity alkylene spacers, have been synthesized and investigated for their liquid crystalline properties. All the dimers exhibit enantiotropic mesophases. The first member of the dimers having the C₄ central spacer exhibit only the chiral nematic (N*) mesophase, while the higher homologues also show smectic A (SmA) and twist grain boundary (TGB) mesophases. The dimers of the other series containing the C₅ central spacer also have stable SmA, TGB and N* mesophases, except for the first which does not show the TGB phase. Both series of compounds show a weak odd-even effect with terminal alkyl chain substitution, while the spacer length has a marked influence on the phase transition temperatures.