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.     

Liquid crystalline homopolyesters having main chain calamitic mesogens and one or two side chain azobenzene moieties in the repeat units

Combined semi-rigid homopolyesters, containing both main chain calamitic mesogens and one or two side chain azobenzene units separated by aliphatic (hexamethylene, octamethylene and decamethylene) chains in the polymer repeat units, were prepared and their liquid crystalline properties characterized. Polyesters having two side chain azobenzene units and a main chain biphenyl moiety showed a higher ordered smectic B or smectic F phase, whereas the other polymers containing a main chain 2,5-diphenyl-1,3,4-thiadiazole unit and one or two side chain methoxyazobenzene units formed a smectic C phase despite the presence of different mesogens in the main and side chains. This is probably due to the compact molecular chain-packing and intra- and intermolecular interactions between the polymer backbones and the two azobenzene units.     

Anomalies of periodicity in TGB structures in new liquid crystal dimers

Non-symmetric liquid crystal dimers consist of two different mesogenic units linked through a polymethylene flexible spacer. Our previous studies have shown that dimers containing a cholesteryl moiety as one of the mesogenic groups and a Schiff 's base unit as the second, exhibit a rich polymorphism and that several types of smectic packing are obtained depending on the molecular parameters: specifically, a smectic periodicity similar to the molecular length and an intercalated structure with a smectic parameter lower than half the molecular length can be obtained. The competition between these two incommensurate lengths can induce two-dimensional phases and/or an incommensurate smectic phase in which the two smectic periodicities coexist over a long range. Small modifications of the molecular structure can significantly influence the phase sequence. Here we have replaced the Schiff 's base by a tolan unit and the terminal alkyl chains by alkoxy chains. As a result, anomalies of periodicity are also observed in this new dimeric series, but they occur mainly in TGB structures.     

Liquid crystalline non-linear S-shaped oligomers consisting of azobenzene and biphenylene units: synthesis,characterisation and influence of central spacer

A series of non-linear S-shaped liquid crystal oligomers wherein the molecule consists of biphenylene moiety as a central core unit and two symmetrical side arms azobenzene moieties joined to catechol as a linkage group have been successfully synthesised and characterised. The members in this series possess different inner spacer with carbon number n ranging from 4 to 9 while the outer spacer length located in azobenzene moieties are remains unaltered. The members with even parity exhibit monotropic phase, whereas homologues with odd parity display enantiotropic phase. The appearance of nematic, smectic A and smectic B phases was validated with texture observation under polarised light and X-ray diffraction (XRD). The XRD study on the S-shaped oligomer indicates that the arrangement of smectic phase exists as an intercalated structure. The S-shaped oligomer shows photoisomerisation properties in solution whereby the trans to cis isomerisation for this molecule is accomplished 1140 s, whereas reverse process under thermal back reaction occurred in 4620 min.     

Liquid crystal molecules with an enyne rigid core

New mesogens are always a source of interest, especially when they possess a non-conventional architecture. In this article are presented the synthesis and polymorphism of a series of compounds possessing a 1,4-diaryl-1-buten-3-yne moiety as the rigid core with an alkoxy chain on each side. Such a core is termed an enyne core. The alkoxy chain is lengthened on each side of the enyne core according to two different fashions: symmetrically and asymmetrically. In this way a rich polymorphism is achieved in some compounds. At lower chain length, the compounds exhibit smectic H and nematic phases where cybotactic groups are observed in X-ray diffraction patterns. As the alkoxy chains extend, smectic C and smectic F phases appear. The non-cylindrical shape of these compounds involves a molecular packing that is preserved throughout the polymorphism. A comparison between symmetric and asymmetric compounds, from X-ray diffraction pattern analysis of their smectic H phases, reveals a parallel molecular stacking. It also discloses the importance of the moiety that is lengthened since different polymorphisms are obtained.     

Liquid crystal molecules with an enyne rigid core

New mesogens are always a source of interest, especially when they possess a non‐conventional architecture. In this article are presented the synthesis and polymorphism of a series of compounds possessing a 1,4‐diaryl‐1‐buten‐3‐yne moiety as the rigid core with an alkoxy chain on each side. Such a core is termed an enyne core. The alkoxy chain is lengthened on each side of the enyne core according to two different fashions: symmetrically and asymmetrically. In this way a rich polymorphism is achieved in some compounds. At lower chain length, the compounds exhibit smectic H and nematic phases where cybotactic groups are observed in X‐ray diffraction patterns. As the alkoxy chains extend, smectic C and smectic F phases appear. The non‐cylindrical shape of these compounds involves a molecular packing that is preserved throughout the polymorphism. A comparison between symmetric and asymmetric compounds, from X‐ray diffraction pattern analysis of their smectic H phases, reveals a parallel molecular stacking. It also discloses the importance of the moiety that is lengthened since different polymorphisms are obtained.     

Photochromic liquid-crystalline polymers. Main chain and side chain polymers containing azobenzene mesogens

Abstract

Two classes of thermotropic polymers were synthesized containing the trans-azobenzene unit as both a mesogenic and a photochromic group. In the former class (I) the azobenzene unit is incorporated into the main chain of substituted polymalonates, while in the latter class (II) it is appended as a side chain substituent to a polyacrylate backbone. The liquid-crystalline properties of the polymers were studied as a function of the chemical structure. All of the prepared polymers I have smectic phases. Polymers II are nematic and/or smectic, or cholesteric when including a chiral residue R'. Polymers I and II when radiated at 348 nm in chloroform solution undergo trans-to-cis isomerization of the azobenzene moiety. The calculated rate constants are comparable with those of low molar mass model compounds, and indicate that the macromolecular structure does not significantly affect the photoisomerization rate.     

Poly(L-lysine) containing azobenzene units in the side chains: influence of the degree of substitution on liquid crystalline structure and thermotropic behaviour

The structure of poly(L-lysine)s containing between 20% and 100% of azobenzene units in the side chains has been studied by X-ray diffraction, between room temperature and 250 C. Except for samples having very low contents of azobenzene, the polymers are found to exhibit mesomorphic structures of the smectic A1 type deriving from the beta -structure of polypeptides. For polymers in which all lysine residues were substituted, the polypeptide main chains are arranged in layers corresponding to the sheets of a polypeptide 'antiparallel' beta -structure, and the side chains are perpendicular to the smectic layers. For polymers containing both substituted and free lysine side chains, each smectic layer results from the superposition of two layers: one layer contains the free lysine side chains; the other contains the azobenzenemodified lysine side chains and the polypeptide main chains that are arranged in 'antiparallel' beta -structures. All polymers exhibit only one smectic A mesophase as a function of temperature. The thickness of the smectic layers increases with increasing temperature until a thickness is reached that corresponds to the maximum interaction between the azobenzene mesogens in their trans -configuration.     

Synthesis and thermotropic properties of new polyacrylates containing alkanoyl-substituted azobenzene mesogens

The synthesis and characterization of a series of new polyacrylates 1, containing the azobenzene mesogen spaced from the backbone by a hexamethylene segment and substituted in the 4-position by an alkanoyl chain of variable length, together with those of the relevant precursors are reported. Optical and X-ray analyses provided evidence for the presence of a smectic A mesophase in a fairly broad range of temperature in both polymers and low molar mass compounds. In polyacrylates 1, the side-chain substituents were fully interdigitated in a smectic A₁ structure. The comparison of the transition parameters of polymers 1 with those of another series of polyacrylates 7 containing the azobenzene unit substituted in the 4-position by an alkyloxy chain of variable length revealed that the replacement of an oxygen atom by a carbonyl group strongly enhanced the smectogenic character of the azobenzene group.     

Unique molecular orientation in a smectic liquid crystalline polymer film attained by surface-initiated graft polymerization

Liquid crystalline (LC) polymer brushes containing a mesogenic azobenzene (Az) moiety are synthesized on a quartz or silicon substrate by surface-initiated atom transfer radical polymerization. The molecular orientation of the Az units and the LC properties in the grafted chains are evaluated by UV-vis spectroscopy, polarized optical microscopy, and grazing incidence X-ray diffraction measurements. The Az side chains of the grafted chains exhibited a smectic LC phase in which the smectic layers are oriented perpendicular to the substrate with a parallel orientation of the mesogens. In contrast, a spincast film of the identical LC polymer without grafting to the surface shows layer structures parallel to the substrate. A drastic effect of tethering one end to the substrate on the LC orientation is demonstrated.     

Four ring achiral ferroelectric liquid crystals of 1,2,4-oxadiazoles: synthesis and characterisation

A novel series of four-ring achiral ferroelectric liquid crystals containing 1,2,4-oxadiazole cores with unsymmetrical substitutions at C-3 and C-5 positions are synthesised and characterised. A fluoro substituted biphenyl moiety is prepared by Suzuki coupling reaction and is directly attached to the oxadiazole core at the C-5 position for the first time in the literature. An octyl benzoate is attached to the oxadiazole core at the C-3 position of it. All the compounds exhibit polar smectic (B₂) mesophases with ferroelectric switching along with the orthogonal smectic-A mesophases. These compounds possess high mesomorphic thermal ranges of polar smectic phases and are towards the ambient temperatures. The influence of a more electronegative fluorine substituent on the electron rich biphenyl moiety (at the C-5 position) of the oxadiazole core is analysed for the prevalence and abundance of polar smectic (ferroelectric) mesophases.     

Dimesogenic compounds consisting of cholesterol and azobenzene-based moieties: dependence of liquid crystal properties on spacer length and fluorination of the terminal chain

The liquid crystalline (LC) properties of two series of non-symmetric dimesogenic compounds consisting of cholesterol and azobenzene-based moieties interconnected by ω-oxyalkanoyl spacers of varying length are compared: one series (AOC-n) has an octyloxy chain attached to the azobenzene mesogen unit while the other (AOCF-n) has a perfluoroheptylmethyloxy chain. In general, compounds bearing the fluorinated alkoxy chain exhibited LC properties over a much broader temperature range than those with the alkoxy chain. In addition, the AOC-n series exhibited the chiral smectic C (SmC*), smectic A (SmA) and cholesteric (N*) phases depending on the length of the central spacer, whereas the AOCF-n series favoured the formation of only the SmA phase with the N* phase completely suppressed. Both series showed an odd-even dependence of the isotropization temperature on spacer length.     

Liquid-crystalline structure of poly(L-lysine) containing azobenzene units in the side chain

The structure of poly(L -lysine) containing 44% azobenzene units in the side chain was studied by X-ray diffraction between room temperature and 150°C. The polymer exhibits a mesomorphic structure of the smectic A1 type. In this structure, stable at least until 150°C, each smectic layer of thickness d results from the superposition of two layers: one of thickness d_A contains the free lysine side chains, the other of thickness d_B contains the azobenzene modified lysine side chains and the polypeptide main chains, that in their planes are arranged as in the “antiparallel” β-structure classical for polypeptides.     

Synthesis and mesomorphic properties of compounds exhibiting the undulated twist grain boundary smectic C* phase

The synthesis and characterization of three homologous series of compounds exhibiting the undulated twist grain boundary smectic C* (UTGB_C*) phase are reported. The chiral mesophases have been obtained using cholesterol as the chiral moiety. Cholestanol and [S]-[+]-octan-2-ol have also been used as the chiral moiety for comparitive purposes. In addition to this novel phase, cholesteric, smectic A, smectic C* and TGB_A phases have also been observed. The mesophases were characterized using a combination of polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and measurement of helical pitch.     

Polar smectogens: polymorphism of some cyanoalkoxybiphenylalkoxybenzoates and related compounds

A series of new cyanoalkoxybiphenylalkoxybenzoates containing a highly dipolar end group electrically and mechanically decoupled from the rigid moiety has been synthesized. The thermal behaviour of these compounds and their precursors (cyanoalkoxyhydroxybiphenyls) has been investigated by means of differential scanning calorimetry and optical polarizing microscopy. It has been found that such compounds often exhibit double melting behaviour through isotropic or smectic phases. A rather rich polymorphism (N, S_A, S_B and S_E phases) is observed at high temperature (100-200°C).     

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.     

Synthesis and transition temperatures of liquid crystals incorporating a 1,4-tetrafluorophenylene unit

Two homologous series of mesogenic materials which incorporate a 1,4-tetrafluorophenylene moiety and a terminal cyano- or nitro-substituent have been synthesized by using palladiumcatalysed coupling procedures. The compounds exhibit nematic and/or smectic A phases.     

Dimesogenic compounds with an electron-attracting terminal group

Non-symmetric dimesogens composed of a classical aromatic mesogenic unit linked to a cholesteryl moiety by a flexible spacer form several types of smectic periodicities: one is connected to the cholesteryl length and the other to the length of the associated dimesogens. In some peculiar cases, anomalies of periodicity resulting from the competition between these incommensurate lengths are observed through the occurrence of two-dimensional modulated phases or incommensurate low ordered smectic phases (S_ic). As part of our continuing effort to understand the influence on the smectic arrangement of the molecular parameters of such non-symmetric dimesogens, new homologues with a cholesteryl unit linked by a pentamethylene spacer to an aromatic mesogenic moiety bearing different electron attracting terminal groups have been prepared. For these compounds, only the periodicity resulting from the associated dimesogens is observed. Nevertheless, an incommensurate smectic phase can be induced by mixing one of these compounds with another appropriate dimesogen. Molecular mechanics calculations suggest that the origin of the different smectic structures is strongly connected to the repartition of electrostatic potential along the dimesogen.     

Synthesis of new mesogenic compounds having the isoflavone core group

Two new series of calamitic liquid crystals containing the isoflavone moiety within the central core have been prepared and their liquid crystalline properties studied. The compounds exhibit typical nematic, smectic A and C mesophases. The influence of the linking group between a terminal alkyl chain and the central core has been proven to determine the variety of mesomorphism displayed by the compounds.