Dimeric liquid crystalline compounds having a central malonic acid moiety: effect of the length of the spacers and terminal chains

We have studied the liquid crystalline properties of two new series of dimeric compounds consisting of a central malonic acid moiety, polymethylene spacers, and Schiff's base mesogens. In the first series we changed the length of the spacers with the terminal chain fixed as the butoxy group; in the second series we changed the length of the terminal alkyl or alkoxy groups while keeping the spacers fixed as decamethylene groups. In the first series, as the length of the spacer increased, the tendency to form smectic phases grew. In the second series, changing the alkyl terminal chain to the alkoxy terminal group raised the melting and isotropization temperatures. Increasing the length of the terminal group diminished the temperature range for the higher temperature smectic phase. The smectic phases are assigned to be smectic A and crystal smectic E phases.     

Dimeric liquid crystalline compounds having a central malonic acid moiety: effect of the length of the spacers and terminal chains

We have studied the liquid crystalline properties of two new series of dimeric compounds consisting of a central malonic acid moiety, polymethylene spacers, and Schiff's base mesogens. In the first series we changed the length of the spacers with the terminal chain fixed as the butoxy group; in the second series we changed the length of the terminal alkyl or alkoxy groups while keeping the spacers fixed as decamethylene groups. In the first series, as the length of the spacer increased, the tendency to form smectic phases grew. In the second series, changing the alkyl terminal chain to the alkoxy terminal group raised the melting and isotropization temperatures. Increasing the length of the terminal group diminished the temperature range for the higher temperature smectic phase. The smectic phases are assigned to be smectic A and crystal smectic E phases.     

Phase characterization of LC (meth)acrylic monomers based on ω-hexyloxy- and ω-undecyloxy-salicylaldimine groups with different alkoxy tail substitutions

The synthesis and phase characterization of three homologous series of liquid crystalline acrylic and methacrylic monomers, consisting of 21 new compounds are presented. They are based on ω-hexyloxy- and ω-undecyloxysalicylaldimine groups with different alkoxy tail substitutions. The liquid crystalline materials were characterized by polarizing optical microscopy and differential thermal analysis. Smectic A and tilted smectic C phases were observed in the compounds. Near the transition to the isotropic, a narrow nematic phase, coexisting with the smectic A phase, was detected for the pentyloxy and hexyloxy derivatives in the M11 and A11 series. In case of M11R11 and M11R12 only a tilted smectic C phase was detected. The clearing point was comparable for all series, around 100°C.     

Phase characterization of LC (meth)acrylic monomers based on ω-hexyloxy- and ω-undecyloxy-salicylaldimine groups with different alkoxy tail substitutions

The synthesis and phase characterization of three homologous series of liquid crystalline acrylic and methacrylic monomers, consisting of 21 new compounds are presented. They are based on ω-hexyloxy- and ω-undecyloxysalicylaldimine groups with different alkoxy tail substitutions. The liquid crystalline materials were characterized by polarizing optical microscopy and differential thermal analysis. Smectic A and tilted smectic C phases were observed in the compounds. Near the transition to the isotropic, a narrow nematic phase, coexisting with the smectic A phase, was detected for the pentyloxy and hexyloxy derivatives in the M11 and A11 series. In case of M11R11 and M11R12 only a tilted smectic C phase was detected. The clearing point was comparable for all series, around 100°C.     

Liquid crystalline compounds having a tribranched structure: substituted malonic esters consisting of two Schiff's base units and a cholesteryloxyalkyl substituent

New tribranched thermotropic liquid crystal compounds were synthesized and their liquid crystalline properties studied by differential scanning calorimetry, X-ray diffraction and polarizing optical microscopy. The compounds are the bis-{10-[4-(4-alkylphenyliminomethynyl)phenoxy]decyl} 2-[6-(cholesteryloxy)hexyl]malonates and the corresponding alkoxy derivatives. These compounds contain three mesogenic units, two identical Schiff's base type mesogens and one cholesteryl either moiety, interconnected in a tribranched structure via spacers. The cholesterly moiety is attached to the malonic acid core through an oxyhexamethylene spacer while the two Schiff's-base moieties are attached through oxydecamethylene spacers. The terminal alkyl group of the Schiff's base unit is either a butyl or decyl group, and the alkoxy terminal group is either a butoxy or decyloxy chain. All the compounds form only an enantiotropic smectic phase, most probably of the smectic C type. The larger spacings determined by small angle X-ray diffraction range from 3.3 to 4.1 nm, which are much shorter than the end-to-end distance (5.9-7.4 nm) of the molecules estimated using molecular models assuming an all trans extended conformation for all the alkyl spacers.     

The mesomorphic anomaly of 4-[2-(4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-pentylcyclohexyl-1-carboxylates

Four homologous series of 4-[2-[4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-alkylcyclohexyl-1-carboxylates have been prepared. Their liquid crystalline behaviour was investigated by optical polarizing microscopy and DSC. Series A, B and C exhibit the nematic phase. The phase transition behaviour of series D is anomalous. The compounds of series D with a short alkoxy chain exhibit enantiotropic smectic A and nematic phases, while those with a long alkoxy chain exhibit only the nematic phase.     

The mesomorphic anomaly of 4-[2-(4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-pentylcyclohexyl-1-carboxylates

Four homologous series of 4-[2-[4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-alkylcyclohexyl-1-carboxylates have been prepared. Their liquid crystalline behaviour was investigated by optical polarizing microscopy and DSC. Series A, B and C exhibit the nematic phase. The phase transition behaviour of series D is anomalous. The compounds of series D with a short alkoxy chain exhibit enantiotropic smectic A and nematic phases, while those with a long alkoxy chain exhibit only the nematic phase.     

Investigation of thermal and luminescent properties in 4,7-diphenylethynyl-2,1,3-benzothiadiazole systems

A series of luminescent liquid crystals based on 4,7-diphenylethynyl-2,1,3-benzothiadiazole system was prepared and characterised. The molecules were planned varying number, length and position of the linear alkoxy/thioalkyl chains. Their mesomorphic properties were studied by differential scanning calorimetry, polarised optical microscopy and X-ray diffraction. The liquid crystalline properties were found to be dependent on the length, position and number of the peripheral alkoxy chains. It was observed preferentially smectic C and nematic phases whose stability increase with the elongation of the peripheral portion. Photophysical studies were performed in chloroform solution. These compounds displayed strong yellow photoluminescent properties in the visible region, with absorption patterns peaking around 400–450 nm and emission between 500–610 nm with good quantum yields. Also, solvatochromism studies were performed.     

Preorganised effects of a tetramesogenic supermolecule on supramolecular assembly in the liquid crystalline phases

After preparing a homologous series of tetrameric mesogenic compounds in which two U-shaped molecules were connected via a rigid benzene derivative or a flexible alkyl chain, we investigated their phase transition behaviour using optical microscopy, differential scanning calorimetry and X-ray diffraction analysis. The compounds possessing an alkyl spacer as the central group exhibited nematic and smectic A phases just as the corresponding U-shaped molecule did. The compound possessing a 1,2-benzene unit as the connecting group showed nematic and smectic A phases, although the compound possessing a 1,3-benzene unit exhibited only an anticlinic smectic C phase. Structure–property relations of the liquid crystalline tetramers are interpreted in terms of preorganised effects of the four mesogenic units.     

Phase characterization of two homologous series of LC methacrylic monomers based on ω-hexyl- and ω-butyl-oxysalicylaldimine groups with different alkoxy tail substitutions

The phase characterization of two homologous series of liquid crystalline methacrylic monomers based on χ -hexyl- and χ -butyloxysalicylaldimine groups, with different alkoxy tails, is presented. The liquid crystalline materials were characterized by polarising optical microscopy, differential scanning calorimetry, differential thermal analysis, and X-ray diffraction. All the monomers exhibit the simultaneous occurrence of smectic A and C phases. When the alkyl chain is short, a narrow nematic phase is observed, leading to an I-N-SmA-SmC phase sequence.     

Studies of first- and second-order phase transitions in LC methacrylic monomers based on the ω-hexyloxysalicylaldimine group

The synthesis and phase characterization of two liquid crystalline methacrylic monomers based on the ω-hexyloxysalicylaldimine group with octyl and decyloxy chain substitutions is presented. The liquid crystalline materials were characterized by polarizing optical microscopy, differential thermal analysis and X-ray diffractometry. Nematic and tilted smectic C phases were observed in the compounds. Their liquid crystalline properties where compared with previously studied samples of homologous compounds. In contrast to previous results, this structural modification induces pronounced enantiotropic first-order phase transitions between nematic and smectic C mesophases. A correlation between the phase transition behaviour and structural features of the sample is included.     

Liquid crystals derived from multi-cationic azamacrocyclic alkylsulphates

Two homologous series of azamacrocyclic n-alkylsulphates were synthesized and characterized. Their thermotropic liquid crystalline behaviour was studied by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. At room temperature both series exhibited lamellar crystalline phases. For the tetraazacyclotetradecane alkylsulphate salts a highly ordered smectic phase was observed following their melting. The triazacyclododecane derivatives however melted into disordered smectic A phases, apparently due to the less symmetric polar group which does not favour in-layer organization. Extensive hydrogen bonding was observed in the crystalline phases of both series of compounds as well as above their melting into smectic phases, albeit rather weak in the case of the triaza derivatives.     

Smectic polymorphism of the 4-butyl-4-alkoxyazobenzenes and 4-pentyl-4-alkoxyazobenzenes

Two families of liquid crystalline compounds have been synthesised, the 4-butyl-4- alkoxyazobenzenes and the 4-pentyl-4-alkoxyazobenzenes; for the second family results are presented for the first time for alkoxy chains longer than butyl. The results for both families have been obtained up to the octadecyl homologues. In both families, on the basis of DSC, polarizing microscopy and thermo-optical analysis, a rich polymorphism has been detected (maximum tetramorphism). The smectic properties start with the hexyl derivative (for the butyl family) and with the heptyl derivative (for the pentyl family). Strong odd-even effects for the temperatures of clearing in both groups of compounds were detected. Our results are compared with those of de Jeu et al. and of Adomenas et al. for the 4-butyl-4- alkoxyazobenzenes, for which only one smectic modification was described.     

T-shaped ionic liquid crystals based on the imidazolium motif: exploring substitution of the C-2 imidazolium carbon atom

In this contribution the first examples of so‐called rigid‐core, T‐shaped imidazolium ionic liquid crystals, in which the C‐2 atom of the imidazolium ring is substituted with an aryl moiety decorated with one or two alkoxy chains, are described. The length of the alkoxy chain(s) was varied from six to eighteen carbon atoms (n=6, 10, 14–18). Whereas the compounds with one long alkoxy chain display only smectic A phases, the salts containing two alkoxy chains exhibit smectic A, multicontinuous cubic, as well as hexagonal columnar phases, as evidenced by polarising optical microscopy, differential scanning calorimetry, and powder X‐ray diffraction. Structural models are proposed for the self‐assembly of the molecules within the mesophases. The imidazolium head groups and the iodide counterions were found to adopt a peculiar orientation in the central part of the columns of the hexagonal columnar phases. The enantiotropic cubic phase shown by the 1,3‐dimethyl‐2‐[3,4‐bis(pentadecyloxy)phenyl]imidazolium iodide salt has a multicontinuous Pm$\bar 3In this contribution the first examples of so-called rigid-core, T-shaped imidazolium ionic liquid crystals, in which the C-2 atom of the imidazolium ring is substituted with an aryl moiety decorated with one or two alkoxy chains, are described. The length of the alkoxy chain(s) was varied from six to eighteen carbon atoms (n=6, 10, 14-18). Whereas the compounds with one long alkoxy chain display only smectic A phases, the salts containing two alkoxy chains exhibit smectic A, multicontinuous cubic, as well as hexagonal columnar phases, as evidenced by polarising optical microscopy, differential scanning calorimetry, and powder X-ray diffraction. Structural models are proposed for the self-assembly of the molecules within the mesophases. The imidazolium head groups and the iodide counterions were found to adopt a peculiar orientation in the central part of the columns of the hexagonal columnar phases. The enantiotropic cubic phase shown by the 1,3-dimethyl-2-[3,4-bis(pentadecyloxy)phenyl]imidazolium iodide salt has a multicontinuous Pm ?3m structure. To the best of our knowledge, this is the first example of a thermotropic cubic mesophase of this symmetry.     

Liquid crystalline thiadiazole derivatives: new ferroelectric thiadiazole derivatives

A series of optically active thiadiazole Schiff's bases were prepared and their liquid crystalline properties were studied. Chirality was achieved by introducing an asymmetric carbon atom into the alkoxy group. Thus, 2-( p -active-alkoxyphenyl-imine)-5-( p - n -alkoxy)phenyl-1,3,4-thiadiazoles were synthesized. Their structures were characterized using elemental analysis, and IR, 1 H NMR and mass spectroscopies; their thermal properties were determined using DSC and polarized light microscopy. All the compounds are enantiotropic liquid crystals, and most of them exhibited both chiral smectic C* and cholesteric mesophases. Only one of them did not show a smectic C* phase.     

Synthesis and mesomorphic properties of fluoro-containing azobenzene liquid crystals

Four series of azobenzene liquid crystal compounds containing a fluorinated tolan unit have been synthesized. The mesomorphic behaviours of these compounds were characterized by differential scanning calorimetry and optical polarizing microscopy. The compounds of these series exhibit a nematic, a smectic C or a smectic B phase. This investigation revealed that the alkoxy chain at the side of the azobenzene moiety plays a more important role in the formation of smectic phases than the one at the side of the tolan moiety.     

Synthesis and mesomorphic properties of fluoro-containing azobenzene liquid crystals

Four series of azobenzene liquid crystal compounds containing a fluorinated tolan unit have been synthesized. The mesomorphic behaviours of these compounds were characterized by differential scanning calorimetry and optical polarizing microscopy. The compounds of these series exhibit a nematic, a smectic C or a smectic B phase. This investigation revealed that the alkoxy chain at the side of the azobenzene moiety plays a more important role in the formation of smectic phases than the one at the side of the tolan moiety.     

Thermal analysis of binary liquid crystalline mixtures

Two series of binary mixtures composed of bent shaped and rod like molecules are reported. The first star shaped bent core molecules were synthesized and used as a component of binary mixtures. The chiral rod like compounds having commensurable length with the arms of the bent core compounds have been chosen for these mixtures. The resulted compositions show various thermotropic liquid crystalline phases that are characteristic to both types of liquid crystalline materials. In case of mixing the rod like molecules to the bent core compound the B2, B7 and induced B1 phases have been observed. While using the star-shaped bent core and chiral rod like compounds in mixture, the paraelectric smectic A, ferroelectric smectic C* and orthogonal hexatic smectic B phases were preferred. The appearing mesophases were investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction methods.     

Liquid crystals of 4-octyloxy-N-(benzylidene)aniline derivatives bearing trifluoromethyl or trifluoromethoxy end groups

Abstract

Seven new derivatives of 4-octyloxy-N-(4-substituted benzylidene) aniline have been synthesized. 4-Trifluoromethyl and 4-trifluoromethoxy derivatives exhibit stable smectic B and A phases, respectively, while both the 4-methyl and 4-methoxy derivatives have monotropic nematic phases. Fundamental liquid crystalline properties such as entropies of the phase transitions, microscopic textures, smectic layer spacings, orientational order parameters, and molecular dipole moments were determined. It has been revealed that moderately polar nature of these mesogens act to stabilizing monolayer smectic states. The smectic A phase of 4-trifluoromethoxy derivative exhibit very high orientational order. None of the disubstituted compounds, 3,5-bis(trifluoromethyl), 3,5-dimethyl, and 3,5-dimethoxy derivatives were mesogenic. The effect of terminal trifluoromethylation on the liquid crystalline properties is discussed.     

Liquid crystalline behaviour of hydrogen bonded complexes of a non-mesogenic anil with p-n-alkoxybenzoic acids

Phase diagrams of binary mixtures of the non-mesogenic N -( p -methoxy- o -hydroxybenzylidbe ene)- p -aminopyridine with a series of p - n -alkoxybenzoic acids ranging from methoxy to hexadecyloxy were established using differential scanning calorimetry and polarising optical microscopy. The key results obtained are: (1) the formation of 1 1 hydrogen bonded complexes between the pyridine derivative and the alkoxybenzoic acids, (2) the stability of the alkoxybenzoic acid mesophases over a wide range of compositions (up to slightly over 50 mol% of the pyridine derivative), (3) the absence of additional mesophases corresponding specifically to the 1 1 complexes, and (4) the complete miscibility of the acids with the complexes in the mesomorphic state. With alkoxy chains from methoxy to heptyloxy, mixtures produce only nematic phases; they produce both nematic and smectic phases with chains from octyloxy to dodecyloxy, and only smectic phases with chains from tetradecyloxy to hexadecyloxy. The formation of hydrogen bonded complexes was investigated at various temperatures using FTIR spectroscopy. Molecular ordering was studied by X-ray diffraction as a function of temperature and composition both for the crystalline and the mesomorphic states.