2-(4-Alkylphenyl)-5-(alkenyloxy)pyrimidines: Synthesis, liquid crystal transition temperatures and some physical properties

We have recently reported the introduction of a carbon-carbon double bond into a wide variety of 5-n-alkyl-2-(4-n-alkoxyphenyl)pyrimidines to produce the corresponding alkenyloxy derivatives. The position and nature (E/Z) of the double bond were varied systematically and the effect on the liquid crystal transition temperatures studied. The position and nature (E/Z) of the double bond changed the conformation of the alkenyloxy chain substantially. This resulted in higher smectic C and nematic transition temperatures for compounds with a trans-double bond (E) at an even number of carbon atoms from the molecular core. Significantly lower transition temperatures (including the melting point) were observed for materials with a cis-double bond (Z) at an odd number of carbon atoms from the molecular core. We have now performed the same operation on the related 2-(4-n-alkylphenyl)-5-n-alkoxypyrimidines to produce the corresponding alkenyloxy derivatives. An interesting feature of the new results is the high melting points of the trans-substituted materials and the low melting points of the terminally substituted compounds. The smectic C transition temperatures of both series are high. No nematic phases could be observed. However, in admixture with other smectic C components, the new compounds lead to surprisingly fast switching times, high smectic C transition temperatures and low melting points/crystallization temperatures in experimental mixtures designed for electro-optic display devices based on ferroelectric effects.     

Alkyl/alkenyloxy phenyl/biphenylpyrimidines: Dependence of the liquid crystal transition temperatures on the position of the nitrogen atoms and the position and configuration of carbon-carbon double bonds

The influence on the transition temperatures of a carbon-carbon double bond in the terminal alkenyloxy (ether) chain of four series of three-ring phenyl/biphenyl-pyrimidines has been systematically investigated. The position and configuration of the double bond has been varied systematically in order to determine the optimal configuration and conformation of the terminal chains for smectic C formation. Four positions of the two nitrogen atoms were chosen. This produced four isomeric series of pyrimidines and differences in the mesomorphic behaviour were observed. The dependence of the transition temperatures on chain length was studied for the same four isomeric pyrimidine series. Comparisons between the new three-ring phenyl/biphenyl-pyrimidines and the corresponding two-ring phenylpyrimidines revealed almost identical tendencies. These results are consistent with a linearly-extended conformation of the chain. Several of the new ethers exhibit remarkably low viscosity values (i.e. short response times) and wide switching angles in an optically active base mixture used for evaluation and comparison purposes. Hence they can be used to widen the temperature range of mixtures designed for electro-optic display devices based on ferroelectric effects (FLCDs) without leading to longer response times.     

Smectic C phenylpyridines with an alkenyloxy chain

Abstract

The known 5-n-alkoxy-2-[4-(n-alkoxy)phenyl]pyridines exhibit high smectic C transition temperatures as well as various highly ordered smectic mesophases. An unsaturated carbon-carbon double bond has now been introduced into the terminal alkoxy chain of these heterocyclic materials to produce the corresponding alkenyloxy substituted derivatives. The postion and nature (E/Z) of the double bond has been varied systematically and the effect on the liquid crystal transition temperatures determined. A number of homologous series of the most promising alkenyloxy substituted materials has been prepared and evaluated. The position and nature (E/Z) of the double bond changes the conformation of the alkenyloxy chain to a significant degree. This can lead to slightly higher smectic C transition temperatures for compounds with a trans-double bond (E) at an even number of carbon atoms from the molecular core. However, the highly ordered mesophase transition temperatures are increased to a greater degree leading to a reduction in the smectic C temperature range. Significantly lower transition temperatures (including the melting point) are observed for materials with a cis-double bond (Z) at an odd number of carbon atoms from the molecular core. Comparisons with the corresponding alkoxy substituted materials (i.e. without a double bond) are made. These new alkenyloxy materials can be used to increase the smectic C and nematic transition temperatures of chiral mixtures for electrooptical display devices based on ferroelectric effects.     

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.     

Ferroelectric liquid crystals IV. Chiral alkenyloxyphenyl benzoates

Over forty 4-alkoxyphenyl 4-alkoxybenzoates incorporating an olefinic double bond in one carbon chain and an optically active centre in the second (methyl branched) chain have been prepared. The effect of chain length and the position of the chiral centre on the liquid crystal transition temperatures of the esters has been investigated systematically. Apart from one important exception the double bond was maintained in a terminal position of the carbon chain. Several of the new esters exhibit an enantiotropic chiral smectic C mesophase at and just above room temperature.     

2-[4-Alkenyloxy)phenyl]-5-alkylpyrimidines The relationship between position and nature (E/Z) of the double bond and transition temperatures

Abstract

The 5-n-alkyl-2-[4-(n-alkoxy)phenyl]pyrimidmes are essential components of most commercial chiral smectic C mixtures for electrooptic display devices based on ferroelectric effects. This is due to their generally relatively low melting points, enantiotropic, relatively wide range smectic C mesophases, low viscosity and ease of preparation. An unsaturated carbon–carbon double bond has now been introduced into the terminal alkoxy chain of the 5-n-alkyl-2-[4-(alkoxy)phenyl]pyrimidines to produce the corresponding alkenyloxy substituted derivatives. The position and nature (E/Z) of the double bond has been varied systematically and the effect on the liquid crystal transition temperatures studied. A number of homologous series of the most interesting alkenyloxy substituted materials has been prepared and evaluated. The position and nature (E/Z) of the double bond changes the conformation of the alkenyloxy chain substantially. This can result in significantly higher smectic C transition temperatures for compounds with a trans double bond (E) at an even number of carbon atoms from the molecular core. Significantly lower transition temperatures (including the melting point) are observed for materials with a cis double bond (Z) at an odd number of carbon atoms from the molecular core. Comparisons with the corresponding alkoxy substituted materials (i.e. without a double bond) are made.     

2-(4-Alkylphenyl)-5-(alkenyloxy)pyrimidines: Synthesis,liquid crystal transition temperatures and some physical properties

Abstract

We have recently reported the introduction of a carbon-carbon double bond into a wide variety of 5-n-alkyl-2-(4-n-alkoxyphenyl)pyrimidines to produce the corresponding alkenyloxy derivatives. The position and nature (E/Z) of the double bond were varied systematically and the effect on the liquid crystal transition temperatures studied. The position and nature (E/Z) of the double bond changed the conformation of the alkenyloxy chain substantially. This resulted in higher smectic C and nematic transition temperatures for compounds with a trans-double bond (E) at an even number of carbon atoms from the molecular core. Significantly lower transition temperatures (including the melting point) were observed for materials with a cis-double bond (Z) at an odd number of carbon atoms from the molecular core. We have now performed the same operation on the related 2-(4-n-alkylphenyl)-5-n-alkoxypyrimidines to produce the corresponding alkenyloxy derivatives. An interesting feature of the new results is the high melting points of the trans-substituted materials and the low melting points of the terminally substituted compounds. The smectic C transition temperatures of both series are high. No nematic phases could be observed. However, in admixture with other smectic C components, the new compounds lead to surprisingly fast switching times, high smectic C transition temperatures and low melting points/crystallization temperatures in experimental mixtures designed for electro-optic display devices based on ferroelectric effects.     

Thermal behaviour of lanthanum(III) alkanoates

The mesophase behaviour of the lanthanum(III) alkanoates [La(C_xH_2x+1COO)₃] (x =3-19) has been investigated by hot-stage polarizing optical microscopy, differential scanning calorimetry and high-temperature X-ray diffraction. Lanthanum(III) butyrate monohydrate shows no mesomorphism, whereas for the remaining short chain homologues (x = 4-9) a highly viscous mesophase M and a smectic A phase were observed. The longer chain lanthanum(III) soaps (x = 10-19) exhibit only a smectic A phase. However, the chain length has a pronounced effect on the transition temperatures. The thermal behaviour of lanthanum(III) alkanoates is compared with that of other lanthanide(III) alkanoates.     

The synthesis and liquid crystal transition temperatures of some weakly polar nematic trans-4-substituted-cyclohexyl (E)-alk-2-enoates

The effect on the liquid crystal transition temperatures of introducing various groups (for example incorporating C=C, O, CO₂ and CO) into the terminal alkyl chain of a weakly polar model compound 1-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]pentane has been investigated systematically. Only the compound containing both an ester function and a trans-carbon-carbon double bond exhibited a wide-range nematic mesophase at elevated temperatures. Therefore, a wide variety of trans-4-substituted-cyclohexyl (E)-alk-2-enoates incorporating a carbon double bond with a trans-configuration (E) in the terminal alkyl chain has been synthesized. Nearly all the two-ring esters prepared exhibit a nematic phase over a wide temperature range (≤ 100°C) at elevated temperatures (≤ 200°C). The tendency to form smectic mesophases is often low. Comparisons with the corresponding derivatives incorporating either just a carboxy group (COO) or just a carbon-carbon double bond (C=C) in the same positions indicate that synergetic effects lead to broader nematic phases than would otherwise have been expected. The new compounds are easily prepared from known starting materials.     

Mesomorphic behavior of new azomethine liquid crystals having terminal iodo group

A series of azomethine esters,3-hydroxy-4-{[（4-iodophenyl）imino]methyl}phenyl alkanoates possessing even number of carbon atoms at the terminal alkanoyloxy chain(C_n-1H_2n-1COO-,n = 4,6,8,10,12,14) was synthesized.n-Butanoyloxy was found non-mesogenic,whilst n-hexanoyloxy to n-tetradecanoyloxy exhibited enantiotropic smectic A phase with fan-shaped texture.It was found that the length of terminal alkanoyloxy chain influenced the mesomorphic properties.     

4-(5-Alkyl-2-pyridinyl)phenyl alkanoates: Some novel smectic materials for display devices

Abstract

Three homologous series of 4-(5-alkyl-2-pyridinyl)phenyl alkanoates have been prepared. The synthesis and the liquid crystal transition temperatures of these esters are reported. Many members of these three ester series exhibit enantiotropic, wide range smectic F mesophases and some narrow range smectic C mesophases. However, in admixture with a chiral smectic C base mixture, the esters often induce a substantial increase in the chiral smectic C–smectic A transition temperature and, at the same time, decrease the temperature of crystallization. Thus, the temperature range of the chiral smectic C mesophase is substantially extended at both high and low temperatures. Ordered smectic mesophases are only observed at very low temperatures, if at all. These chiral smectic C mixtures are characterized by short switching times in surface-stabilized ferroelectric liquid crystal displays (SSFLCD). These novel esters are of especial interest for short-pitch chiral smectic C mixtures for short pitch bistable ferroelectric liquid crystal displays (SBFLCD) with short response times.     

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

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.     

Mesomorphic behaviour of praseodymium(III) alkanoates

In this paper, we describe the synthesis, characterization and thermal behaviour of praseodymium(III) alkanoates. The compounds have the stoichiometry [Pr(C_xH_2x+1COO)₃], where x = 5-19, and were characterized by elemental analysis and infrared spectroscopy. The thermal behaviour was investigated by hot-stage polarizing optical microscopy, DSC and high temperature X-ray diffraction. A highly viscous mesophase M and a smectic A phase were observed for the shorter chain compounds (x = 5-8), whereas only a smectic A phase was observed for the longer chain compounds of this type of metallomesogens. The chain length has a pronounced effect on the transition temperatures.     

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.     

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).     

Polar nematic methyl (E)-[trans-4-cyclohexyl-substituted]allyl ethers: Synthesis, liquid crystal transition temperatures and some physical properties

We have introduced an oxygen atom and a carbon-carbon double bond with a trans-configuration (E) into the terminal alkyl chain attached to the cyclohexyl ring of a variety of two- and three-ring nematic liquid crystals of positive dielectric anisotropy. The new polar two-ring methyl (E)-allyl ethers often possess low melting points, but are not mesomorphic in general. The related three-ring methyl (E)-allyl ethers exhibit high clearing points and wide nematic ranges. 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. This is partially due to the low smectic transition temperatures observed for the methyl (E)-allyl ethers. Selected physical properties of three binary mixtures of a weakly polar standard nematic liquid crystal and three difluoro-substituted (polar) liquid crystals (including the new ethers) differing only in the nature of the terminal chain show that, although some of the methyl (E)-allyl ethers exhibit longer switch-off times in TN cells than those of analogous liquid crystals incorporating either a methyl propyl ether or a 1-(E)-propenyl chain instead of the methyl (E)-allyl ether chain, they are still useful components for nematic mixtures, especially where a wide temperature range is required.     

Coexistence of two different side chain packing structures in a smectic phase of liquid-crystalline side chain polymers

Ten varieties of liquid-crystalline side chain polymers, poly(cholesteryl-ω-(methacryloyloxy)alkanoates) (pChMO-n, n = 1, 2, 3, 4, 5, 7, 9, 10, 11 and 15; the carbon number of the alkyl chain), were studied by differential scanning calorimetry and small angle X-ray scattering. On and after the first cooling run from the isotropic state, these polymethacrylates gave the same smectic phase. X-ray investigations showed that pChMO-n with short spacers (n = 1-7) has a two layer (bilayer) S_A packing structure, and pChMO-n with a longer spacer (n= 15) has a single layer (monolayer) S_A packing structure. However, these two types of packing structure appear simultaneously in pChMO-n (n = 9s-11) below their phase transition temperature. To clarify the manner of the coexistence of the two different structures the smectic layer spacing and X-ray diffraction patterns were examined by small angle X-ray scattering at various temperatures.     

Ferroelectric liquid crystals VI. Chiral 5-alkyl-2-phenylpyrimidines

Over 50 variously substituted 5-alkyl-2-phenylpyrimidines have been synthesized. The effect of the presence of an additional olefinic double bond in the terminal position of the alkoxy chain or of a trans-1,4-disubstituted cyclohexane ring on the liquid crystal transition temperatures of these systems has been studied in detail. All of the phenyl-pyrimidines studied possess an optically active centre at the point of methyl branching of one of the terminal carbon chains. The dependence of the transition temperatures of these systems on the position of the chiral centre has also been investigated. The effect of chain length has also been studied for various homologous series of mesogens. Almost all of the 5-alkyl-2-phenylpyrimides prepared exhibit enantiotropic chiral smectic C and cholesteric mesophases, some at and just above room temperature.     

Thermal behaviour of lanthanum(III) alkanoates

The mesophase behaviour of the lanthanum(III) alkanoates [La(C_xH_2x+1COO)₃] (x =3-19) has been investigated by hot-stage polarizing optical microscopy, differential scanning calorimetry and high-temperature X-ray diffraction. Lanthanum(III) butyrate monohydrate shows no mesomorphism, whereas for the remaining short chain homologues (x = 4-9) a highly viscous mesophase M and a smectic A phase were observed. The longer chain lanthanum(III) soaps (x = 10-19) exhibit only a smectic A phase. However, the chain length has a pronounced effect on the transition temperatures. The thermal behaviour of lanthanum(III) alkanoates is compared with that of other lanthanide(III) alkanoates.