Laterally dibenzyloxy-branched nematogens with large nematic range and rich solid polymorphism

Mesogenic compounds containing four rings in the core usually have very high melting points. However, when two identical lateral benzyloxy groups are introduced on the same side of one of the central rings, the melting point is lowered dramatically and a large nematic range is retained. This range is affected by the bulkiness of the para-substituents in the lateral rings. Methyl groups can be introduced in the ortho- or meta-positions with a consequent decrease in the melting temperature without much affecting the nematic range. These compounds exhibit a rich solid polymorphism which is certainly related to the effect of the conformations of the lateral substituent on the molecular arrangment in the solid phase. Some preliminary NMR experiments on the nematic phase indicate that the molecular long axis coincides with the core axis, whereas the para-axis of the lateral fragment makes an angle close to the magic angle with respect to the molecular long axis.     

The effect of a lateral aromatic branch on the orientational ordering of laterally alkoxy substituted nematics

Two new mesogenic series have been synthesized. They have the same main core which contains four aromatic rings, with two lateral substituents on the same side of one of the inner rings. One of the substituents is a 4-chlorobenzyloxy group and the second is an alkoxy chain. The positions of the lateral substituents are different in the two series. Despite the large aromatic branch, an enantiotropic nematic phase is obtained for most of the compounds. The orientational ordering behaviour of these new compounds has been investigated by C-13 NMR, and two isomeric compounds containing a lateral dodecyloxy chain were chosen for the study. The order parameters of the chain were obtained by a 2D C-13 NMR technique with variable angle spinning. The temperature dependence of the order parameters was determined using the C-13 chemical shifts with slow spinning of the sample parallel to the magnetic field. The first methylene fragment in each lateral substituent of each compound has a positive C-H bond order parameter, implying that this fragment adopts a cis conformation in the nematic phase. The two lateral substituents are folded back along the mesogenic core in opposite directions. The lateral chain is found to be roughly aligned along the molecular long axis imposed by the core, whereas the para -axis of the phenyl ring in the lateral aromatic branch makes a considerable angle with the molecular long axis. It was also found that the different patterns of substitution do not affect the position of the molecular long axis to any large extent.     

Orientational ordering in some nematogens deviating from the classical rod-shape

Several new laterally substituted liquid crystalline compounds have been synthesized. They have the same main core which contains four rings (two aromatic, two alicyclic) with two lateral substituents introduced on the same side of one of the inner rings. One of the substituents is a 4-X-benzyloxy group (X=CH3,CN,Cl) and the other is a hexyloxy chain. The presence of the lateral aromatic substituent makes these compounds deviate markedly from the classical rod-shape. However, a wide enantiotropic nematic phase is present for all the compounds. The order parameters of the chain and the para -disubstituted aromatic rings were obtained by using a 2D 13C NMR technique with variable angle spinning. The temperature dependence of the order parameters was estimated using 13C chemical shifts with slow spinning of the sample parallel to the magnetic field. The results indicate that the two lateral substituents are more or less folded back along the mesogenic core. Thus, the flexible lateral chain is found to be roughly aligned with the molecular long axis, whereas the para axis of the less flexible aromatic branch makes a considerable angle with the molecular long axis imposed by the core, substantially increasing the mean width of the molecule. The core ordering does not seem to be influenced by the type and position of the substituents. The folding back of the lateral chain and the substantial tilt of the lateral aromatic branch with respect to the core main axis are confirmed by the X-ray structure of a parent compound.     

Orientational ordering in some nematogens deviating from the classical rod-shape

Several new laterally substituted liquid crystalline compounds have been synthesized. They have the same main core which contains four rings (two aromatic, two alicyclic) with two lateral substituents introduced on the same side of one of the inner rings. One of the substituents is a 4-X-benzyloxy group (X=CH3,CN,Cl) and the other is a hexyloxy chain. The presence of the lateral aromatic substituent makes these compounds deviate markedly from the classical rod-shape. However, a wide enantiotropic nematic phase is present for all the compounds. The order parameters of the chain and the para -disubstituted aromatic rings were obtained by using a 2D 13C NMR technique with variable angle spinning. The temperature dependence of the order parameters was estimated using 13C chemical shifts with slow spinning of the sample parallel to the magnetic field. The results indicate that the two lateral substituents are more or less folded back along the mesogenic core. Thus, the flexible lateral chain is found to be roughly aligned with the molecular long axis, whereas the para axis of the less flexible aromatic branch makes a considerable angle with the molecular long axis imposed by the core, substantially increasing the mean width of the molecule. The core ordering does not seem to be influenced by the type and position of the substituents. The folding back of the lateral chain and the substantial tilt of the lateral aromatic branch with respect to the core main axis are confirmed by the X-ray structure of a parent compound.     

Unusual thermal behaviour of mesogens containing an N,N-dialkylaminomethylene lateral substituent

An N,N-dialkylaminomethylene lateral substituent can be used to reduce the melting temperatures of nematogens having a long rigid core. However, compared with a single lateral alkoxy chain, this lateral substituent does not enhance the thermodynamic stability of the mesophase. DSC curves and optical microscopy show that after melting, a new solid appears in the liquid crystal phase or in the isotropic phase of these compounds. NMR spectra of the nematic phase and of the solid phase show that a thermal reaction releases dialkylamine into the medium and produces a disubstituted 2 H -indazole in rather good yield. Further heating of each individual mixture leads to a second wide range enantiotropic nematic phase. The purified disubstituted 2 H-indazole has a larger nematic range and the nematic phase is stable at high temperature, indicating that the 2 H -indazole motif can be used to build new mesogenic structures. The temperature dependence of the 13C chemical shifts of one of the initial compounds was obtained. The results indicate that the two chains on the nitrogen atom are equivalent and more or less folded back in the same way along the mesogenic core.     

Ordering of a lateral crown ether and terminal short POE chains in some symmetrical nematogens by 13 C NMR

A lateral crown ether fragment can be introduced on symmetrical mesogens containing only three aromatic rings. The replacement of the terminal alkoxy chains by chains containing oxyethylene units decreases the melting and clearing temperatures allowing one to obtain nematic compounds near room temperature. These compounds dissolve LiBF 4 salt only to a small extent, but the nematic arrangement, is thereby destroyed. The carbon chemical shift anisotropy and the local C-H bond order parameters were obtained for the nematic phase for the crown ether fragment and the terminal chains. This study indicates that the crown ether average conformation changes insignificantly on decreasing the temperature. The lateral crown ether protrudes markedly from the core with the consequence that the molecular shape is far from rod-like in geometry.     

Ordering of a lateral crown ether and terminal short POE chains in some symmetrical nematogens by 13 C NMR

A lateral crown ether fragment can be introduced on symmetrical mesogens containing only three aromatic rings. The replacement of the terminal alkoxy chains by chains containing oxyethylene units decreases the melting and clearing temperatures allowing one to obtain nematic compounds near room temperature. These compounds dissolve LiBF₄ salt only to a small extent, but the nematic arrangement, is thereby destroyed. The carbon chemical shift anisotropy and the local C-H bond order parameters were obtained for the nematic phase for the crown ether fragment and the terminal chains. This study indicates that the crown ether average conformation changes insignificantly on decreasing the temperature. The lateral crown ether protrudes markedly from the core with the consequence that the molecular shape is far from rod-like in geometry.     

Apolar 2-alkoxyalkoxy-substituted nematic liquid crystals

The synthesis is reported of a series of apolar nematic liquid crystals incorporating an alkoxyalkoxy chain attached in a terminal position to the molecular core. The dependence of the mesomorphism and phase transition temperatures on the nature of the terminal chain with one, two or three oxygen atoms was studied. Some compounds with a completely alicyclic molecular core and a terminal 2-methoxyethoxy chain exhibit a nematic phase with a low melting point, birefringence and viscosity and a relatively high clearing point in the absence of smectic phases. They do not to absorb light in the near UV and may be useful as components of UV-stable nematic mixtures for use in photoluminescent liquid crystal displays. The presence of the methoxyethoxy chain leads to a relatively large dipole moment perpendicular to the molecular long axis.     

Apolar 2‐alkoxyalkoxy‐substituted nematic liquid crystals

The synthesis is reported of a series of apolar nematic liquid crystals incorporating an alkoxyalkoxy chain attached in a terminal position to the molecular core. The dependence of the mesomorphism and phase transition temperatures on the nature of the terminal chain with one, two or three oxygen atoms was studied. Some compounds with a completely alicyclic molecular core and a terminal 2‐methoxyethoxy chain exhibit a nematic phase with a low melting point, birefringence and viscosity and a relatively high clearing point in the absence of smectic phases. They do not to absorb light in the near UV and may be useful as components of UV‐stable nematic mixtures for use in photoluminescent liquid crystal displays. The presence of the methoxyethoxy chain leads to a relatively large dipole moment perpendicular to the molecular long axis.     

Improved nematic mesophase stability of benzoxazole-liquid crystals via modification of inter-ring twist angle of biphenyl unit

A series of nematic heterocyclic liquid crystals, 2-(2?-fluoro-4?-alkoxy-1,1?-biphenyl-4-yl)-benzoxazole derivatives (coded as nPF(2)PBx) bearing hydrogen (nPF(2)PBH), methyl (nPF(2)PBM) and nitro (nPF(2)PBN) terminal groups, respectively, are developped and investigated. The results show that the nematic mesophase stability is obviously improved via a slightly increase in the inter-ring twist angle between the planes of two phenyl rings in the biphenyl unit, where modification of the twist angle can be achieved by adjusting the position of the lateral monofluorine substituent from the outer to the inter ring of the biphenyl mesogenic core unit. Meanwhile, the inter-ring lateral fluorine substituent results in a decrease in both melting and clearing points but only a nematic mesophase with an acceptably wide mesophase range for nPF(2)PBx, and thus a potential application in liquid crystal display mixtures is expected, especially for the compounds nPF(2)PBM and nPF(2)PBN. The reduced π–π interaction/conjugation caused by the increased twist angle is considered as one of the possible reasons for the formation of a nematic mesophase.     

Order parameters of the mesogenic core in two nematic liquid crystals containing a four-unit link

A series of compounds containing a four-unit link, 4'-n-alkylphenyl-4-n-hexyloxycinnamates, has been synthesized, and their mesomorphic behaviour has been determined. The orientational ordering of two compounds in this series has been studied in detail by 2D carbon-13 nuclear magnetic resonance with variable angle sample spinning. The carbon-proton dipolar coupling constants were obtained and the order parameters were calculated for different molecular segments (two phenyl rings, the central link containing an ethylenic motif, and the methylene carbons of the alkoxy chain). The temperature dependence of the local order parameters of each fragment were determined using these data and C-13 chemical shifts which were measured as a function of temperature. In these non-rigid molecules, the ratio between the order parameters of the two phenyl rings was found to be temperature dependent. The ring bearing the double bond is more ordered at low temperatures compared to the ring bearing the short alkyl chain, but this behaviour is reversed near the clearing point. The position of the major axis of the rigid core was calculated from the order parameter of one ring and the order parameter of the ethylenic link. It was found that this major axis forms a constant angle with respect to the para axis of the phenyl ring connected to the ethylenic group. On the other hand, the angle of this major axis with respect to the para axis of the phenyl ring connected to the carboxylic group is temperature dependent, varying from 11° at T/T_NI = 0·92 to 7° at T/T_NI = 0·98.     

Terminal end‐group efficiency for the nematic phase using model bicyclo[2.2.2]octanes

The synthesis is reported of new liquid crystals incorporating the 1,4‐disubstituted bicyclo[2.2.2]octane ring and a series of substituents in a terminal position on the molecular core. The nature of the terminal substituent is varied from apolar with a small dipole moment to polar with a strong dipole moment. The angle of the dipole moment with respect to the molecular axis is also varied. An updated order of terminal group efficiency for substituents in a terminal position for the nematic phase is provided. The bicyclo[2.2.2]octane ring shields halogen substituents in a lateral position on phenyl rings attached to the bicyclooctane ring to a small degree and reduces the steric efects of these substituents, giving rise to high relative nematic–isotropic transition temperatures.     

Polar 2-alkoxyethoxy-substituted nematic liquid crystals

The synthesis is reported of a series of polar nematic liquid crystals incorporating an alkoxyalkoxy chain attached in a terminal position to a molecular core consisting of two aromatic rings. The dependence of the mesomorphism and phase transition temperatures on the nature of this terminal chain was studied. The shape anisotropy of the core is increased by the presence of an additional 1,4-disubstituted phenyl unit in a number of p-terphenyls. A number of highly polar esters with a terminal methoxyethoxy chain have been prepared as dopants to lower the threshold voltage of LCDs. The presence of the methoxyethoxy chain leads to an increase in the molecular dipole moment, the dielectric anisotropy and the dielectric constant perpendicular to the molecular long axis. Several compounds with a four-unit diether central linkage were also prepared.     

Evidence for lateral chain conformation change in the solid phase of substituted nematogens

A new homologous series of 4-(4-chlorobenzoyloxy)-2-alkoxy-3-methyl-4'-(4-trans-pentyl-cyclohexanoyloxy) azobenzenes has been synthesized. These compounds contain four rings in the main core, a lateral alkoxy branch and an adjacent lateral methyl group on one inner ring. They present a large nematic range and a solid-solid phase transition for the first members of the series. The crystal structures of two similar compounds containing a lateral butoxy chain have been solved. The HM4 compound (C₃₁H₃₃N₂O₅Cl) is derived from the series, but does not have the terminal pentyl chain while the PM4 compound (C₃₆H₄₃N₂O₅Cl) possesses this terminal chain. HM4 and PM4 crystallize, respectively, in P2₁/c (Z = 4) and P1 (Z = 2) space groups. The final reliability factors are R = 0.138 for HM4 (the terminal cyclohexyl ring is largely disordered) and R = 0.041 for PM4, for 2204 and 1963 observed reflections, respectively. The four ring central core is linear and rigid for both compounds. The conformation of the butyloxy side chain branched on one of the central phenyl rings is very dependent on the terminal substituent, as it is found in a largely bent conformation in HM4 and in an extended conformation in PM4. To monitor the lateral chain conformation in the solid and nematic phase, we have used the -OCH₂- resonance in the ¹³C MAS NMR spectra. The temperature dependence of this resonance has indicated that the solid-solid transition encountered for the first members of the series is associated with a change of the lateral chain conformation within the solid phase. At the solid-nematic transition, the change of the chain conformation involving the first segment of the lateral chain is far less pronounced. When this solid-solid phase transition disappears for the last members of the series, this change in the conformation occurs at the solid-nematic transition.     

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 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.     

Terminal end-group efficiency for the nematic phase using model bicyclo[2.2.2]octanes

The synthesis is reported of new liquid crystals incorporating the 1,4-disubstituted bicyclo[2.2.2]octane ring and a series of substituents in a terminal position on the molecular core. The nature of the terminal substituent is varied from apolar with a small dipole moment to polar with a strong dipole moment. The angle of the dipole moment with respect to the molecular axis is also varied. An updated order of terminal group efficiency for substituents in a terminal position for the nematic phase is provided. The bicyclo[2.2.2]octane ring shields halogen substituents in a lateral position on phenyl rings attached to the bicyclooctane ring to a small degree and reduces the steric efects of these substituents, giving rise to high relative nematic-isotropic transition temperatures.     

Ordering of some liquid crystals containing the 2-phenylindazole core

Molecules containing the 2-phenylindazole core present liquid crystalline properties even if the two terminal chains do not point along the same axis. ¹³CNMR in the liquid crystalline phase shows that the molecular long axis is nearly aligned with the para-axis of the phenyl moiety of the 2-phenylindazole core. This implies that the first fragments of the chain belonging to the indazole moiety do not lie along the molecular long axis. To promote liquid crystal properties, this chain needs to possess at least six carbon atoms.     

Nematogens incorporating a lateral flexible ring

A lateral flexible ring can be introduced on a mesogenic core containing four rings. Lateral aliphatic rings containing up to 12 atoms have been synthesized. Despite this large lateral protruding substituent, a large nematic range is obtained. A compound with a benzo15-crown-5 lateral ring has been successfully obtained and shows the possibility of designing nematic liquid crystals with a crown ether moiety at a lateral position.     

Liquid crystalline behaviour from binary mixtures of non-mesogenic 4-alkyl-N-(4-cyanophenyl) piperidines [1]

A relatively rare phenomenon, the induction of thermotropic mesophases from combinations of homologues of non-mesogenic compounds, is examined in some detail. Thus, monotropic nematic phases have been obtained from binary mixtures of 4-alkyl-N-(4-cyanophenyl)piperidines (I). Except for the hexyl homologue, the compounds I with butyl to nonyl as alkyl groups are non-mesomorphic as neat materials. From phase diagrams, the nematic clearing temperatures of the binary mixtures are found to vary linearly with composition and to increase as the molecular lengths of the components are increased. By introducing a methyl group at the 1-position of the alkyl chain, the melting points of the neat components are depressed, and the nematic clearing temperatures of the binary mixtures are lowered. Factors associated with the induction of this mesomorphism and with the 'kinetic' and thermodynamic stabilities of the monotropic nematic phases are discussed. Among these is the orientation between the rings of the core groups in I and in the more common mesogens, the trans-4-alkyl-1-(4-cyanophenyl) cyclohexanes, II.