New liquid crystalline di- and tetra-acrylates for network formation

New liquid crystalline diacrylates and tetra-acrylates containing four to six aromatic rings were synthesized and characterized, and their mesophase behaviour was investigated. They are designed to be used in combination with chiral molecules to form cholesteric mesophases which can be crosslinked by photopolymerization. The acrylates presented exhibit broad mesophase ranges since mesogenic moieties longer than three are employed. Most diacrylates show no isotropization, due to premature thermal polymerization above 180°C. Additionally, liquid crystalline dipropionates were synthesized as reference compounds which cannot be crosslinked, and selected examples of these exhibit isotropization temperatures as high as 238°C prior to thermal degradation. Substituents at the mesogenic moiety have a great influence on the mesophase characteristics. Bulky substituents such as the tert-butyl group, induce a nematic mesophase, whereas compounds with small substituents (e.g. OCH3) or unsubstituted molecules also exhibit smectic phases. Tetra-acrylates with unsubstituted and substituted mesogenic units feature nematic mesophases only as a result of the additional spacers attached. Here isotropization was observed without polymerization at temperatures around 120-160°C.     

Thermally stable azo-substituted bent-core nematogens: observation of chiral domains in the nematic mesophases composed of smectic nano clusters

In an effort to synthesise thermally stable bent-core nematogens with fairly low transition temperatures and wide nematic mesophase range, we have synthesised five new series of azo-substituted bent-core compounds without Schiff’s base unit. Here, we studied the effect of different lateral substituents (–CH₃ and –Cl), at two different positions in one of the arms, on the mesogenic properties. We found that such variation in the molecular architecture has a clear-cut impact on the transition temperatures as well as the liquid crystalline properties. The mesogenic properties of these compounds were studied using polarising optical microscopy and differential scanning calorimetry. The exact nature of different mesophases is investigated using X-ray diffraction studies. The results obtained indicated the presence of smectic nano clusters in the nematic mesophases of these compounds. We observed domains of opposite handedness in these nematic phases. The unusual properties of these mesophases were investigated by electro-optical studies. The electro-convection pattern study showed that these nematic mesophases are of negative dielectric anisotropic in nature. The compounds synthesised here exhibit photo-switching both in solution and in their nematic mesophases, a property that can be exploited for practical applications.     

Comparison of liquid crystalline properties of dimeric compounds of different skeletal shapes

Series of dimeric compounds of different skeletal shapes consisting of two triad aromatic ester type mesogenic moieties connected via polymethylene spacers were synthesized and their liquid crystalline properties compared. The two mesogenic units are connected in either Hor T-shape or in linear fashion. In general, it was found that mesophase temperature ranges for the T- and linear-shaped compounds are much wider than for the H-shaped compounds. Moreover, the former are enantiotropic thermotropic materials, whereas the latter tend to be monotropic unless the spacer length is fairly long, i.e. longer than decamethylene. Among the three series, the linearly linked twin compounds had the highest melting and isotropization temperatures. All of the linear and T-shaped dimeric compounds reported in this article form only nematic mesophases.     

Comparison of liquid crystalline properties of dimeric compounds of different skeletal shapes

Series of dimeric compounds of different skeletal shapes consisting of two triad aromatic ester type mesogenic moieties connected via polymethylene spacers were synthesized and their liquid crystalline properties compared. The two mesogenic units are connected in either Hor T-shape or in linear fashion. In general, it was found that mesophase temperature ranges for the T- and linear-shaped compounds are much wider than for the H-shaped compounds. Moreover, the former are enantiotropic thermotropic materials, whereas the latter tend to be monotropic unless the spacer length is fairly long, i.e. longer than decamethylene. Among the three series, the linearly linked twin compounds had the highest melting and isotropization temperatures. All of the linear and T-shaped dimeric compounds reported in this article form only nematic mesophases.     

Side chain liquid crystalline diethanolamine-based polyesters with methoxy and/or nitroazobenzene mesogenic moieties

Side-chain polyesters based on diethanolamine and 5-hydroxyisophthalic acid with mesogenic methoxyazobenzene and nitroazobenzene groups were synthesized and their liquid crystalline properties were examined.

Four different types of polyesters were prepared by using various combinations of monomers differing in the mesogen group and the length of the spacer. The thermal and structural properties depend on the type of polyester as well as on the side chain length. The glass transition and isotropization temperatures increase when the methoxyazobenzene group is bound to a diol and decrease when it is bound to an isophthalic acid. Most of the polyesters are thermotropic liquid crystals and form a nematic phase on heating. Some polyesters exhibit shear induced isotropisation to nematic transition up to 25°C above the isotropization temperature.     

Synthesis and study of 4-(4'-n-alkoxybenzoyloxybenzoyl)-4'-n-butoxyanilides

Thirteen compounds with ester and amide linkages were synthesized and their mesogenic properties evaluated. Methyl to n-propyl derivatives exhibit nematic phases, n-butyl to n-decyl derivatives exhibit smectic and nematic mesophases, whereas n-dodecyl to n-octadecyl derivatives exhibit only smectic phases. All the smectic homologues exhibit smectic C phases. Middle members of the homologous series exhibit polymorphism of smectic mesophase. A plot of transition temperatures versus number of carbon atoms in the alkoxy chain reveals an odd-even effect for nematic-isotropic transition temperatures. Nematic-isotropic and smectic-cholesteric thermal stabilities of the prepared compounds (series I) are higher compared to those of previously reported compounds, series A, B and C. The results indicate that a simple reversal of a central linkage has a dramatic effect on the appearance of smectic mesophase in a homologous series. The structures of the synthesized compounds were characterized using elemental analysis, thin-layer chromatography and spectral data.     

Mesogenic cinnamates with a substituted ethyl terminal chain

Two new mesogenic homologous series of liquid crystalline cinnamates with substituted ethyl tails, β-methoxyethyl [4-(4'-n-alkoxycinnamoyloxy)benzoates (I) and β-chloroethyl [4-(4'-n-alkoxycinnamoyloxy)benzoates (II), have been synthesized and characterized by a combination of elemental analysis and standard spectroscopic methods. In series I, lower-chain members exhibit nematic mesophase, middle members exhibit enantiotropic nematic as well as smectic A (SmA) mesophases, whereas higher members exhibit only an enantiotropic SmA mesophase. In series II, methoxy to n-butyloxy derivatives exhibit a monotropic nematic mesophase. The SmA mesophase commences from n-propyloxy derivative as monotropic and persists up to the last member synthesized. The mesomorphic properties of present series were compared with each other and with a structurally related mesogenic homologous series to evaluate the effects of substituted ethyl tail and cinnamoyloxy central linkage on mesomorphism.     

Side-chain liquid crystalline polymers with silphenylene-siloxane main chains. III. Synthesis and characterization of polymers with phenyl benzoate mesogenic groups

Side-chain liquid crystalline (SCLC) silphenylene-siloxane polymers with a phenyl benzoate mesogenic group and polymethylene spacers were prepared and characterized, and their properties were compared with those of equivalent SCLC polymers, SCLCPs, with a biphenyl mesogenic group. With identical spacers and terminal substituents, the melting temperatures of the former were much lower, but the isotropization temperatures were lowered to a lesser extent, than those of the latter, and, consequently, a more thermally stable nematic phase was obtained for the former. Both types of SCLCPs formed nematic phases, while polymethylsiloxanes with the same side-chain mesogens exhibited smectic phases with wider temperature ranges. The lower thermal stability of the mesophases in the silphenylene-siloxane SCLCPs compared to those of the SCLC polymethylsiloxanes can be attributed to both the rigidity of the backbone and the greater separation of the side-chains along the main chains of the former.     

Synthesis, characterization and photolysis studies on liquid crystalline poly[4-(4′-x-biphenyl)yl-4″-(m-methacryloyloxyalkyloxy) cinnamate]’s

Three series of side chain liquid crystalline polymers containing terminally substituted biphenyl cinnamoyl esters were synthesized and characterized. The para position in the cinnamoyl group was connected with polymer backbone through various even numbers of methylene spacers. The terminal electron acceptor and donor substituted biphenyl groups were linked to cinnamoyl group through ester linkages. Polarizing microscopic and DSC studies confirm the formation of thermotropic mesophase up on heating. All the polymers exhibited nematic/grainy mesophases. The TGA and DSC studies showed that the nature of the terminal substituents have significant effect on mesophase temperatures as well as in thermal stability of these liquid crystalline polymers. UV light promoted photocrosslinking studies reveal that the clear involvement on photocrosslinking efficiency, although they are linked to the olefinic bond of cinnamoyl group through their ester linkage. It is observed that electron-donating group accelerate the photocrosslinking rate, while acceptor group retard the phase of the reaction on the other hand, the unsubstituted polymers show an intermediate rate.     

Side chain liquid crystalline diethanolamine‐based polyesters with methoxy and/or nitroazobenzene mesogenic moieties

Side‐chain polyesters based on diethanolamine and 5‐hydroxyisophthalic acid with mesogenic methoxyazobenzene and nitroazobenzene groups were synthesized and their liquid crystalline properties were examined.

Four different types of polyesters were prepared by using various combinations of monomers differing in the mesogen group and the length of the spacer. The thermal and structural properties depend on the type of polyester as well as on the side chain length. The glass transition and isotropization temperatures increase when the methoxyazobenzene group is bound to a diol and decrease when it is bound to an isophthalic acid. Most of the polyesters are thermotropic liquid crystals and form a nematic phase on heating. Some polyesters exhibit shear induced isotropisation to nematic transition up to 25°C above the isotropization temperature.     

Synthesis and study of 4‐(4′‐n‐alkoxybenzoyloxybenzoyl)‐4′′‐n‐butoxyanilides

Thirteen compounds with ester and amide linkages were synthesized and their mesogenic properties evaluated. Methyl to n‐propyl derivatives exhibit nematic phases, n‐butyl to n‐decyl derivatives exhibit smectic and nematic mesophases, whereas n‐dodecyl to n‐octadecyl derivatives exhibit only smectic phases. All the smectic homologues exhibit smectic C phases. Middle members of the homologous series exhibit polymorphism of smectic mesophase. A plot of transition temperatures versus number of carbon atoms in the alkoxy chain reveals an odd–even effect for nematic–isotropic transition temperatures. Nematic–isotropic and smectic–cholesteric thermal stabilities of the prepared compounds (series I) are higher compared to those of previously reported compounds, series A, B and C. The results indicate that a simple reversal of a central linkage has a dramatic effect on the appearance of smectic mesophase in a homologous series. The structures of the synthesized compounds were characterized using elemental analysis, thin‐layer chromatography and spectral data.     

The synthesis and liquid crystal transition temperatures of some weakly polar nematic methyl (E)-[trans-4-substituted-cyclohexyl]-allyl ethers

We have introduced an oxygen atom and a carbon-carbon double bond with a trans-configuration (E) into the terminal alkyl chain of a wide variety of liquid crystalline cyclohexane derivatives to produce a variety of new methyl (E)-allyl ethers. The melting points and tendency to form smectic mesophases are often low, while nearly all of the compounds prepared exhibit a nematic phase. Thus, even two-ring derivatives can exhibit nematic phases over a wide temperature range (≤80°C), sometimes starting below room temperature (T_m≈10°C). Comparisons with the corresponding derivatives incorporating either just an oxygen atom or just a carbon-carbon double bond in the same position indicate that synergetic effects lead to broader nematic phases than would otherwise have been expected. Thus many of the new methyl (E)-allyl ethers exhibit nematic phases over a wider temperature range than the corresponding materials with an unsubstituted alkyl chain attached to the cyclohexyl ring. The new compounds are easily prepared from known starting materials. Many intermediates are themselves liquid crystalline. This allows investigation of the relationship between liquid crystal transition temperatures and the nature of the terminally substituted alkyl chain (for example, incorporating C=C, OH, CO₂C₂H₅ and OCH₃ groups).     

Synthesis and characterization of thiophene-containing liquid crystals

Six series of liquid crystal materials containing a 2,5-disubstituted thiophene unit were synthesized. The liquid crystal compounds obtained were characterized by NMR, differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction techniques. The properties of liquid crystalline phases were investigated as a function of spacer units, number of aromatic core rings and different terminal moieties. Cyano, methoxy and iodo groups were used as terminal groups. It is found that: (i) compounds having one thiophene ring and one phenyl ring connected by an ester group, with a length/breadth value of 2.1, exhibit no mesophase, while other compounds containing two biphenyl rings, with a length/breadth ratio of 2.7, show mesophases; (ii) the polarity of terminal groups and the flexible spacer length significantly affect the thermal behaviour of these compounds; (iii) the nematic transition range of cyano-containing compounds decreases with increasing length of the flexible spacer, and long alkenyloxy chains tend to facilitate the formation of the smectic phase and suppress the nematic phase in all the mesogenic compounds synthesized.     

Synthesis and characterization of polymaleimides containing 4-cyanobiphenyl–based side groups for nonlinear optical applications

A new homologous series of SCLCPs containing the 4-cyanobiphenyl mesogenic group attached to the polymaleimide backbone through paraffinic spacers of two to eight methylene units have been prepared. All the polymers exhibit liquid crystalline behavior; specifically S_Ad- (or S_C-) like and nematic phases are observed. The glass transition temperature decreases from 150 to 43°C on increasing spacer length. The isotropization temperatures exhibit an odd–even effect on varying the length and parity of the spacer, in which the odd members exhibit the higher values. This is attributed to the change in the average shape of the side chain as the parity of spacer is varied. The isotropization temperatures (>300–120°C) and the mesophase thermal stabilities (190–60°C) are high. Comparison is made with polymers containing the same mesogenic group attached to backbones of decreasing rigidity. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2531–2546, 1998     

Synthesis and characterization of thiophene-containing liquid crystals

Six series of liquid crystal materials containing a 2,5-disubstituted thiophene unit were synthesized. The liquid crystal compounds obtained were characterized by NMR, differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction techniques. The properties of liquid crystalline phases were investigated as a function of spacer units, number of aromatic core rings and different terminal moieties. Cyano, methoxy and iodo groups were used as terminal groups. It is found that: (i) compounds having one thiophene ring and one phenyl ring connected by an ester group, with a length/breadth value of 2.1, exhibit no mesophase, while other compounds containing two biphenyl rings, with a length/breadth ratio of 2.7, show mesophases; (ii) the polarity of terminal groups and the flexible spacer length significantly affect the thermal behaviour of these compounds; (iii) the nematic transition range of cyano-containing compounds decreases with increasing length of the flexible spacer, and long alkenyloxy chains tend to facilitate the formation of the smectic phase and suppress the nematic phase in all the mesogenic compounds synthesized.     

Synthesis, micro structure, and thermal stability of side chain liquid crystalline polysiloxane polymers with an oligo (ethylene oxide) unit in the side chain

A series of new alkene monomers [MS3BDBEn, n=1-3] containing 4-oligo (ethylene oxide) monomethyl ether 4-biphenyl ether carboxyl benzoate as terminal groups were synthesized. These polymers were prepared by grafting these monomers onto the poly (methylhydrosilox-ane) (PMHS) backbone. The transition temperatures, liquid crystalline textures, and thermal stability of the polysiloxane polymers have been determined by thermal data, by optical texture, and by X-ray diffraction patterns. Polymers PS3BDBEn showed smectic or smectic and nematic phases which were not analogous to their precursor nematic monomers. The terminal length of the polymers affects not only the mesophase transition temperatures but also the layer-spacing length (d₁) and the side-chain distance (d₂). The long- and short-range orders can remain to some extent above the isotropization temperature and below the melting point. The polymer PS3BDBE3 decomposed in air 20°C above the isotropization temperature and lost its short range orders as detected by the X-ray diffraction analysis.     

Effect of electron donor-acceptor interaction on the thermal stability of supramolecular side chain liquid crystalline polymers based on poly(3-carboxypropylmethylsiloxane)

Supramolecular side chain liquid crystalline polymers were prepared from poly(3-carboxypropylmethylsiloxane) (PSI100) and azobenzene-based derivatives through intermolecular hydrogen bonding between the carboxylic acid groups of PSI100 and the imidazole rings in the azobenzene-based derivatives. The presence of H-bonding was confirmed using FTIR spectroscopy. The polymeric complexes behave as liquid crystalline polymers and exhibit nematic mesophases identified on the basis of the observation of Schlieren textures. The mesogenic behaviour of these complexes was studied by polarizing optical microscopy and X-ray diffraction. The thermal behaviour of the complexes was investigated by differential scanning calorimetry. On increasing the spacer length, the transition temperatures initially increase. A further increase in spacer length, however, leads to a decrease in the transition temperatures. The electron donor-acceptor interaction between unlike mesogenic units in supramolecular copolymeric complexes helps to stabilize the mesophase.     

Effect of electron donor-acceptor interaction on the thermal stability of supramolecular side chain liquid crystalline polymers based on poly(3-carboxypropylmethylsiloxane)

Supramolecular side chain liquid crystalline polymers were prepared from poly(3-carboxypropylmethylsiloxane) (PSI100) and azobenzene-based derivatives through intermolecular hydrogen bonding between the carboxylic acid groups of PSI100 and the imidazole rings in the azobenzene-based derivatives. The presence of H-bonding was confirmed using FTIR spectroscopy. The polymeric complexes behave as liquid crystalline polymers and exhibit nematic mesophases identified on the basis of the observation of Schlieren textures. The mesogenic behaviour of these complexes was studied by polarizing optical microscopy and X-ray diffraction. The thermal behaviour of the complexes was investigated by differential scanning calorimetry. On increasing the spacer length, the transition temperatures initially increase. A further increase in spacer length, however, leads to a decrease in the transition temperatures. The electron donor-acceptor interaction between unlike mesogenic units in supramolecular copolymeric complexes helps to stabilize the mesophase.     

Induction of smectic layering in nematic liquid crystals using immiscible components IV. The effect of bulky lateral carboxyl substituents on the thermotropic behaviour of 2,5-bis[4-(n-perfluoroheptyloctyloxy)benzoyloxy]toluene

Bulky lateral carboxylate substituents were introduced at the benzylic position of 2,5-bis[4-(n-perfluoroheptyloctyloxy)benzoyloxy]toluene by esterification of the corresponding benzyl bromide with potassium carboxylates. In spite of the bulky lateral substituents, none of the 2,5-bis[4-(n-perfluoroheptyloctyloxy)benzoyloxy]benzyl carboxylates exhibit a nematic mesophase in addition to, or instead of, the smectic mesophases. All of the crystalline and SmC-SmA transition temperatures and, with the exception of the 9-anthracene carboxylate derivative, all of the isotropization temperatures of the resulting 2,5-bis[4-(n-perfluoroheptyloctyloxy)benzoyloxy]benzyl carboxylates are lower than those of the parent toluene compound. The SmC-SmA transition decreases the most, thereby stabilizing the SmA mesophase. In most cases, the SmC mesophase is destabilized from an enantiotropic to a monotropic mesophase. There is no correlation between any of the transition temperatures and the size of the lateral substituent.     

Mesogenic cinnamates with a substituted ethyl terminal chain*

Two new mesogenic homologous series of liquid crystalline cinnamates with substituted ethyl tails, β‐methoxyethyl [4‐(4′‐n‐alkoxycinnamoyloxy)benzoates (I) and β‐chloroethyl [4‐(4′‐n‐alkoxycinnamoyloxy)benzoates (II), have been synthesized and characterized by a combination of elemental analysis and standard spectroscopic methods. In series I, lower‐chain members exhibit nematic mesophase, middle members exhibit enantiotropic nematic as well as smectic A (SmA) mesophases, whereas higher members exhibit only an enantiotropic SmA mesophase. In series II, methoxy to n‐butyloxy derivatives exhibit a monotropic nematic mesophase. The SmA mesophase commences from n‐propyloxy derivative as monotropic and persists up to the last member synthesized. The mesomorphic properties of present series were compared with each other and with a structurally related mesogenic homologous series to evaluate the effects of substituted ethyl tail and cinnamoyloxy central linkage on mesomorphism.