The effect of olefinic terminal chains on the mesomorphic properties of 4,4'-disubstituted diphenyldiacetylenes

New diphenyldiacetylenes of the type with A, B = H and/or F; m = 0, 1; n = 1-4; and X = C n H 2n + 1 , F, CF 3 or CN were synthesized and their mesomorphic properties determined by hot stage polarizing microscopy and DSC. When m = 0, all of these compounds showed only a nematic phase except when X = CF 3 when both nematic and smectic A phases were seen. Both clearing and melting temperatures were higher than those reported for substitution with the corresponding alkyl chains but the much larger increase in clearing temperatures produced considerably wider nematic phases. Eutectic mixtures of a few of these olefins yielded nematic materials also having much wider temperature ranges and higher clearing temperatures than the eutectic mixtures of the alkyl compounds, while retaining their high birefringence and low viscosities. Such materials are of interest for beam-steering devices.

Four of the diacetylenes with m = 1 ( A, B = H) were also prepared ( X = C 6 H 13 , F, n = 2, 3). When X was C 6 H 13 ( n = 2), the nematic range was smaller in the 2- than in the 1-olefin but wider than in the alkyl series. When X = F, either no nematic phase or a monotropic one was observed, whereas the 1-olefins gave a much wider nematic phase. Both transition temperatures were lower than those for the corresponding 1-olefin and alkyl analogues. The compound with X = C 6 H 13 and n = 2 had a melting temperature below room temperature.     

Hydrogen-bonded block mesogens derived from semiperfluorinated benzoic acids and the non-mesogenic 1,2-bis(4-pyridyl)ethylene

Thermal properties of benzoic acids carrying one or two semiperfluorinated alkoxy tails on the aromatic core have been investigated in binary mixtures with the non-liquid crystalline bidirectional trans-1,2-bis(4-pyridyl)ethylene. The hydrogen bonded complexes built from the complementary molecular species show a significantly enhanced mesophase stability compared with the fluorinated acids in their pure states. The mesophase morphologies of the complexes are governed mainly by the number of the partially fluorinated chains grafted to the acid component. Mixed systems comprising the one-chain acids exhibit a smectic C phase followed by a smectic A phase at more elevated temperatures. Incorporation of a second semiperfluorinated chain into the acid leads to the formation of columnar mesophases. These columnar phases of the H-bonded complexes should represent ribbon phases resulting from the collapse of the smectic layers.     

Synthesis and studies of some 4-substituted phenyl-4'-azopyridine-containing hydrogen-bonded supramolecular mesogens

Novel azopyridine-containing supramolecular liquid crystalline (LC) materials built via 1 : 1-heterointermolecular hydrogen bonding between some 4-substituted phenyl-4'-azopyridines and 4- n -alkyloxybenzoic acids are reported. These hydrogen-bonded LC complexes exhibit well defined nematic, smectic A and smectic C phases over wide ranges of temperature depending upon the number of carbon atoms present in the alkyl chains. The formation of pure LC materials on 1 : 1-complexation could be confirmed from the phase diagrams of the binary mixtures, which clearly indicated a melting maximum for the 50 mol % mixture as well as the presence of two eutectic points on either side of this mixture.     

Bent-core V-shaped mesogens consisting of salicylaldimine mesogenic segments: synthesis and characterization of mesomorphic behaviour

A series of bent-core V-shaped mesogens consisting of salicylaldimine mesogenic segments have been synthesized and their mesomorphic behaviour characterized. In an attempt to understand structure-property relationships, the lengths of the terminal alkoxy chains have been varied from C₂ to C₁₂, C₁₆ and C₁₈, resulting in 13 new bent-core V-shaped molecules. The thermal behaviour of these new compounds has been investigated by optical microscopy, calorimetry and X-ray diffraction studies. In general the compounds show conventional mesophases similar to those shown by calamitic LCs. The materials exhibit good thermal stability, even though their melting and clearing transition temperatures are high as a result of the presence of intramolecular hydrogen bonding between the H-atom of the hydroxyl group and the N-atom of the imine functionality. The phase appearing in the first member of the series with ethoxy chains is a nematic, while the C₃ to C₆ derivatives exhibit a smectic A phase as well as a nematic phase. The higher homologues, C₇ to C₁₂, C₁₆ and C₁₈, show only the smectic A phase. X-ray studies reveal that the SmA phase has a partially bilayer (interdigitated) structure. Remarkably, in some cases, the smectic A phase supercools well below room temperature. It is apparent from our studies that increasing the length of the alkoxy chains promotes smectic behaviour, in agreement with the general observation made for such bent-core molecules.     

Synthesis and properties of fluorine tail-terminated cyanobiphenyls and terphenyls for chemoresponsive liquid crystals

ABSTRACT

A series of fluorine tail-terminated alkoxy and alkyl cyanobiphenyl compounds and some cyano-p-terphenyl derivatives were synthesised and mesogenic properties described. Comparison with the non-fluorinated K series and M series indicates that the terminal fluorine atom generally decreases the transition temperatures and, more interestingly, depresses the formation of a smectic phase. Several binary LC mixtures formed by the fluorine tail-terminated compounds were found exhibiting promising room temperature nematic phases with wide ranges. The mixture F7OCB and F8OCB shows homeotropic ordering at the metal salts-decorated surfaces and planar ordering at the free surface, which may have potential application in designing a more sensitive and faster LC system to targeted analytes.     

The effect of olefinic terminal chains on the mesomorphic properties of 4,4′-disubstituted diphenyldiacetylenes

New diphenyldiacetylenes of the type with A, B = H and/or F; m = 0, 1; n = 1–4; and X = C _n H_2n+1, F, CF₃ or CN were synthesized and their mesomorphic properties determined by hot stage polarizing microscopy and DSC. When m = 0, all of these compounds showed only a nematic phase except when X = CF₃ when both nematic and smectic A phases were seen. Both clearing and melting temperatures were higher than those reported for substitution with the corresponding alkyl chains but the much larger increase in clearing temperatures produced considerably wider nematic phases. Eutectic mixtures of a few of these olefins yielded nematic materials also having much wider temperature ranges and higher clearing temperatures than the eutectic mixtures of the alkyl compounds, while retaining their high birefringence and low viscosities. Such materials are of interest for beam-steering devices. Four of the diacetylenes with m = 1 (A, B = H) were also prepared (X = C₆H₁₃, F, n= 2, 3). When X was C₆H₁₃ (n=2), the nematic range was smaller in the 2- than in the 1-olefin but wider than in the alkyl series. When X=F, either no nematic phase or a monotropic one was observed, whereas the 1-olefins gave a much wider nematic phase. Both transition temperatures were lower than those for the corresponding 1-olefin and alkyl analogues. The compound with X=C₆H₁₃ and n=2 had a melting temperature below room temperature.     

Nematic and smectic mesophases in a new series of Cu(II) organometallic complexes

The synthesis and the phase behaviour of a homologous series of copper, bis[N-[[4-[4-(alkoxy)benzoyloxy],2-hydroxyphenyl]methylene]-methanamino] complexes is reported. They all exhibit thermotropic mesomorphism. The thermal stability range of the mesophase decreases slightly with increasing length of the alkoxy terminals. With the onset of smectic mesomorphism, for alkoxy groups containing ten carbon atoms, the stability interval of the nematic phase decreases progressively so that the compound containing 14 carbon atoms in the alkoxy terminal chains exhibits only smectic mesomorphism. Optical texture analysis suggests the smectic C nature of the phase in all cases. The enthalpies for the liquid crystal-isotropic transition have been measured and they are comparatively low. Remarkably low values have also been measured for the melting enthalpies of the smectogenic members of the series. These are associated with solid phase polymorphism which is discussed briefly.     

Synthesis and studies of some 4-substituted phenyl-4′-azopyridine-containing hydrogen-bonded supramolecular mesogens

Novel azopyridine-containing supramolecular liquid crystalline (LC) materials built via 1 : 1-heterointermolecular hydrogen bonding between some 4-substituted phenyl-4′-azopyridines and 4-n-alkyloxybenzoic acids are reported. These hydrogen-bonded LC complexes exhibit well defined nematic, smectic A and smectic C phases over wide ranges of temperature depending upon the number of carbon atoms present in the alkyl chains. The formation of pure LC materials on 1 : 1-complexation could be confirmed from the phase diagrams of the binary mixtures, which clearly indicated a melting maximum for the 50 mol% mixture as well as the presence of two eutectic points on either side of this mixture.     

SYNTHESIS OF A GROUP OF MESOGEN JACKETED LIQUID CRYSTAL POLYMERS

A group of the mesogen jacketed liquid crystal polymers based on the monomers 2,5-bis (4-substituted benzoyl)oxystyrenes are synthesized. The substituents include alkoxy, alkyl, and cyano groups. The synthesis and the primary characterization of the liquid crystal phase of the monomers and the polymers are described. While some of the monomers give smectic textures the polymers are found to be nematic above their melting or glass transition temperatures. Interestingly the unsubstituted monomer and its polymer poly 2,5-di( benzoyloxy ) styrene are also liquid crystalline. The single crystal structure of one of the monomers is also discussed.     

The effect of fluorinated terminal chains on the mesomorphic properties of 4,4'-disubstituted phenyl benzoates

A variety of 4,4'-disubstituted phenyl benzoates having a terminal chain containing multifluorine atoms, attached directly to the benzene ring or through an ester group, have been synthesized and their mesomorphic properties determined by hot stage polarizing optical microscopy. These properties were compared to those of the corresponding hydrogenated esters and to other esters containing rigid terminal chains. Usually transition temperatures were higher and mesophase ranges wider than those observed for the parent compounds but no nematic phases were found. Any mesophase seen was usually a smectic A phase sometimes accompanied by a smectic C phase. Crystal E phases were found along with the smectic A phase in alkyl or alkoxy esters having a C₉F₁₉CO₂ chain on the acid side. A first order smectic A-smectic C transition was observed in the ester with CN on the acid side and O₂CC₇F₁₅ on the phenol side. A comparison of the effect of a terminal fluorinated chain and a lateral fluorine group on one set of esters is also included.     

Ferroelectric liquid crystals V. Chiral alkenyloxyphenyl 4-biphenyl-1-carboxylates

Abstract

Approximately thirty 4-(alkenyloxy)phenyl 4′-alkyl/alkoxy-4-biphenyl-1-carboxylates incorporating a chiral centre at the point of branching of the terminal alkyl/alkoxy chain have been synthesized. The three carbon chains investigated were 2-methylbutyl, 3-methylpentyl and 2-methylbutoxy. The configuration of the chiral centre is (S) in each case. The effect of the alkenyloxy chain length on the liquid crystal transition temperatures of the three homologous series prepared has been studied systematically. Chiral smectic C and chiral nematic mesophases, as well as a smectic A mesophase in several cases, were observed over a wide temperature range for most of the esters prepared.     

Synthesis and characterisation of novel isoxazole-based banana liquid crystals from naturally occurring curcumin

Isoxazole-based bent-core liquid crystals (LCs) derived from naturally occurring curcumin were synthesised and their LC properties were investigated by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction techniques. Five compounds, including a branched alkyl chain derivative, were prepared and characterised. These derivatives exhibit enantiotropic mesophases. While lower homologues display wide-temperature-range nematic phase, a longer-chain derivative 3d shows smectic C phase in addition to the nematic phase. The bent angle in these compounds is in between calamitic LCs and banana LCs. Therefore, the molecules escape from polar order packing observed in typical bent-core LCs. Increasing the length of alkyl chain reduces both melting and isotropic temperatures in the series. However, the compound with branched alkyl chains exhibits significant reduction in the nematic-isotropic temperature only. Detailed XRD experiments confirm the presence of the N phase in the lower homologues and SmC phase in a higher homologue.     

Liquid crystalline behaviour of benzoic acid derivatives containing alkoxyazobenzene

Liquid crystal trimers based on the hydrogen bonding dimerization of 4‐{n‐[4‐(4‐m‐alkoxy‐phenylazo)phenoxy]alkoxy}benzoic acid (BAm‐n) have been synthesized and characterized. Temperature‐dependent FTIR spectroscopic studies showed that the carboxylic acid groups in BAm‐n are associated to form H‐bonded cyclic dimers both in their crystalline and liquid crystalline phases. The trimers exhibited enantiotropic liquid crystalline behaviour except for BA1‐3 which showed monotropic behaviour, and the mesophases changed from nematic to smectic phase, with the increase of length of the spacer and the terminal substituents. Pronounced odd–even effects in the melting temperatures, clearing temperatures and nematic–isotropic enthalpy changes were observed.     

The Effect Of The Position And Configuration Of Carbon-Carbon Double Bonds On The Mesomorphism Of Thermotropic, Non-Amphiphilic Liquid Crystals

The influence on their mesomorphic behaviour of introducing a carbon-carbon double bond into the chain, central linkage and alicyclic rings in the core of nematic and smectic liquid crystals (LCs) is discussed. Mesogens incorporating a trans-carbon-carbon double bond conjugated with an aromatic ring exhibit high mesophase-isotropic transition temperatures (T_c). However, they are photo-sensitive and can convert to the non-linear, non-mesogenic cis-isomers under the action of light. Non-conjugated double bonds in the terminal chain of mesogens can also lead to higher nematic and smectic C transition temperatures than those of the corresponding materials without a double bond, although the effect is not nearly as great. The position and trans-cis-configuration (E/Z) of the double bond are seen to be decisive. The combination of a hetero-atom (dipole effect) and the added rigidity imposed by the carbon-carbon double bond (steric effect) with a trans-configuration (E) in the terminal alkyl chain attached to the core of a liquid crystal molecule can give rise to a broad nematic phase. The double bond in the terminal chain of nematogens advantageously modifies the elastic constant ratios, as well as other properties of relevance to LCDs, especially for supertwisted TN-LCDs. The double bond in a central linkage gives rise to a broad spectrum of effects, sometimes suppressing undesired smectic phases and widening the nematic phase temperature range. Non-conjugated double bonds in the molecular core in the form of cyclohexene rings generally lead to lower transition temperatures, although smectic phases are sometimes suppressed and a nematic phase is observed. A conjugated double bond in a cyclohexene ring gives rise to a slightly higher T_NI. The effect on the transition temperatures of the double bond in steroid systems is complex.     

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.     

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.     

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).     

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.     

Creation of nematic and reentrant nematic phases in bi- and multi-component smectic mixtures

Abstract

Experimental results referring to the transformation of smectic phases, mainly smectic A, into nematic and reentrant nematic phases are reviewed. A new explanation of some experimental results is proposed. Factors which are responsible for the depression of smectic phases in mixtures of polar mesogens are discussed and the possibility of forming mixtures with a broad temperature range of nematic phase from smectic compounds, which can be useful for liquid crystal displays (LCDs), is shown. A nematic gap observed in some cases between monolayer (S_A1) or monolayer and partially bilayer (S_Ad) smectics results from the differences in the organization of the molecules in the smectic layers. It is concluded that polar phase from smectic A₁ phases can be divided into two groups: (a) the first one is characteristic for compounds with the -NCS, -F, -CI, -I or ?COC _m H_{2m + 1} terminal group. The spacing of the smectic layer slowly expands with the increase in alkyl chain length and the structure of the smectic A₁ phase slowly changes to be more like the smectic A_d phase (d/1 > 1). It is proposed that such a smectic is called an enhanced monolayer smectic (S_A1e (b) the second one is typical for compounds with the -CN terminal group. This kind of smectic A₁ phase is rapidly transformed into the smectic A_d phase with increasing alkyl chain length. These latter monolayer mesogens easily form the reentrant nematic phase when they are mixed with other polar smectic mesogens.