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.     

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.     

The influence of double bonds in the terminal chain of 2-ring compounds on the physical properties of S*c mixtures

We present the properties of S*_c mixtures containing new 2-ring 5-n-alkyl-2-(4-n-alkenyloxyphenyl)pyridines and pyrimidines with systematically varying positions and configurations of the double bond Trans configurations at odd positions (counting the number of atoms from the core including the oxygen and the first carbon atom of the double bond) suppress the S_A phase, increase the S*_c tilt angle, Θ, and the spontaneous polarization, P_s, and lead to long switching times τ. Cis configurations at even positions suppress the nematic phase in favour of smectic phases, decrease Θ and P_s, and shorten τ. Other positional configurational combinations strongly reduce the clearing point. Furthermore, our results indicate that the preferred conformation of the alkenyloxy chain consists of alternating cis and trans units.     

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 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).     

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.     

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.     

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.     

Novel 2-(4-octylphenyl)pyridin-5-yl alkanoates and alkenoates: Influence of dipoles and chain conformation on smectic C formation

A series of 2-(4-octylphenyl)pyridin-5-yl alkanoates has been synthesized and found to exhibit smectic mesomorphism including the smectic C phase. The influence on the transition temperatures of introducing a carbon-carbon double bond in the terminal alkanoyloxy (ester) chain of the alkanoates to produce the corresponding alkenoates has also been investigated. The position and configuration of the double bond has been changed systematically in order to determine the optimal configuration and conformation of the terminal chains for smectic C formation. The observed results are consistent with a linearly-extended (alternately cis and trans) conformation of the chain. The dependence of the transition temperatures on chain length was studied for one homologous series each of the alkanoates and the (E)-alk-2-enoates. The new esters are constitutional isomers of the 5-(4-octylphenyl)pyridin-2-yl alkanoates and alkenoates previously synthesized (differing only in the position of the nitrogen atom). Comparisons revealed consistently higher smectic C transition temperatures and lower ordered smectic tendencies for the new esters. The dependence of S_c formation on the position and number of dipoles associated with oxygen atoms, nitrogen atoms and carboxy groups was also investigated. Several of the new esters exhibit remarkably low viscosity values (i.e. short response times) in an optically active base mixture used for evaluation and comparison purposes.     

Synthesis and mesomorphic properties of some platinum(II) and oxovanadium(IV) complexes

The synthesis and mesomorphic properties of a homologous series of N-(2-hydroxy-4-n-alkoxybenzylidene)-4'-n-decylphenylanilines and their platinum(II) and oxovanadium(IV) complexes are reported. All the ligands and their metal chelates exhibit enantiotropic mesophases, predominantly smectic A and smectic C phases. The transition temperatures and enthalpies have been determined for most of the compounds. The platinum(II) complexes have higher melting points and mesophase thermal stabilities. However, the oxovanadium(IV) complexes have a wider thermal range for the mesophase. Both platinum(II) and oxovanadium(IV) complexes containing only a chain on the biphenyl moiety exhibit a nematic phase.     

Stereocontrolled synthesis of (RR)- and (RS)-5-hydroxy-2-methylhexanoic acid lactones : Pheromone of the carpenter bee via palladium-catalyzed reactions

(E,Z)-2,4-Hexadiene was transformed to the lactone cis-1 ( 93% cis)(pheromone of the carpenter bee) in a stereospedfic reaction sequence via a Pd-calalyzed 1,4-acetoxychlorination. The same reaction sequence applied to (E,E)-2,4-hexadiene afforded the isomeric lactone trans-1 (91% trans).     

Polar nematic trans-4-substituted-cyclohexyl (E)-alk-2-enoates The influence of dipoles and double bonds on the transition temperatures and other physical properties

As part of a systematic study of the factors affecting nematic phase formation, the influence of introducing dipoles (in the form of oxygen, carbonyl and carboxy groups) and steric restrictions (in the form of carbon-carbon double bonds) in various positions, configurations and combinations in a model system (4-[trans-4-pentylcyclohexyl]benzonitrile) has been investigated. On the basis of these results, we have introduced an ester group 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 mesogens of positive dielectric anisotropy. This is a new combination of a polar ester group (dipole effect) and the added rigidity imposed by the double bond (steric effect). Most of the new (E) alk 2-enoates containing two rings in the molecular core possess high melting points. Only a few two ring esters exhibit nematic phase, although the clearing point of those esters exhibiting mesomorphic behaviour was high. The corresponding three-ring (E)-alk-2-enoates incorporating an additional phenyl or cyclohexane ring also possess high melting and clearing points, as well as wide nematic ranges. No smectic mesophases could be observed for any of the (E)-alk-2-enoates synthesized. Comparisons with the corresponnding derivatives incorporating either just an ester group, or just a carbon-carbon double bond in the same position indicate that synergetic effects lead to higher clearing points than would otherwise have been expected. The new (E)-alk-2-enoates possess a surprisingly moderate viscosity for esters. The high value of the elastic constant ratio k₃₃/k₁₁ is of advantage for mixtures designed for supertwisted nematic LCDs.     

Synthesis and mesomorphic properties of 6-n-decyloxy-2-[4'-N-alkoxyphenylimino)methyl]quinolines (IV)

The synthesis and mesomorphic properties of a new series of liquid crystals derived from quinoline with an imine central bond is described. All compounds exhibit mesophase ranges greater than 50°C. The lower homologues (n = 4-6) show trimorphism S_c-S_a-N (for n = 4, the S_c phase is monotropic). The higher homologues (n = 7-10) show nematic and smectic C phases. Compared to the styrylquinoline analogues the imine bond gives rise to similar liquid crystal phase ranges but lower melting points.     

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.     

Structure of trans-4-(trans-4-n-pentylcyclohexyl)cyclohexylcarbonitrile (CCH5) in the isotropic and nematic phases: a computer simulation study

We have studied the single particle structural properties of the nematogen trans-4-(trans-4-n-pentylcyclohexyl)cyclohexylcarbonitrile (CCH5) by molecular dynamics simulation using realistic atom-atom potentials. On going from the isotropic phase at 390 K to the nematic phase at 350 K, the molecules become significantly longer and thinner, as indicated by the equivalent molecular moment-of-inertia spheroid and the distribution of trans and gauche bonds. This change is only partly accounted for by the lowering of the temperature, there being a significant quenching effect due to the change in the molecular environment. This quenching effect is also apparent in the distribution of molecular shapes seen in molecular width-breadth contour maps. In the nematic phase, at 350K, the distributions of alkyl tail bond orientations with respect to the director show a pronounced odd-even effect, with peaks in the distributions occurring alternately parallel to, and at an angle to, the director.     

Aromatic liquid crystals with a trifluoromethyl group in the terminal chain for use in nematic LC mixtures

A new class of compounds suitable for LCD applications has have synthesized, incorporating a trifluoromethyl group in the terminal alkoxy or alkenyloxy chain. These compounds fulfil many of the specifications for use in TN-LCDs. Compounds containing several aromatic rings were synthesized with a view to producing compounds of high birefringence. This aspect also included the synthesis of compounds containing a carbon-carbon triple bond. Materials with three phenyl rings were also prepared in an attempt to produce nematic liquid crystals with a high clearing point and a high birefringence. Molecules containing a lateral fluoro substituent were synthesized in order to generate a high positive value of the dielectric anisotropy, a low melting point and no smectic mesophases. A series of compounds incorporating a carbon-carbon double bond was prepared in an attempt to produce compounds with a high nematic clearing point and a high birefringence.     

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

Abstract

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[dbnd]C, OH, CO₂C₂H₅ and OCH₃ groups).     

Coordinatively saturated cyclometallated Pt(IV) azobenzene complexes: synthesis and mesomorphic behaviour

The mesomorphic 4,4-bis[4-n-octyloxybenzoyloxy]azobenzene dinuclear chloro-bridged cycloplatinated complex [(Azo)Pt(mu;-Cl)]2 (smectic C between 263 and 342 C) has been reacted with different chelating ligands, giving rise to a family of square-planar ortho-platinated derivatives, [(Azo)Pt(L)] (L = tropolonate, 8-hydroxyquinolinate and 1,1,1,5,5,5-hexafluoro2,4-pentanedionate). Thermotropic mesomorphism is preserved for these mononuclear complexes which exhibit at least a nematic mesophase and transition temperatures lower by over 100° C than that of the corresponding dimeric precursor. Oxidative addition to the Pt(II) [(Azo)Pt(L)] species of electrophilic substrates such as I₂ or CH₃I eventually led to the corresponding octahedral [(Azo)Pt(L)(I )(X)] products. The introduction of two further ligands leads to Pt(IV) derivatives showing smectic and nematic mesophases for all L ligands. For the hexacoordinated [(Azo)Pt(L)(I)(CH₃)] complexes it has been verified that the oxidative addition of methyl iodide is a thermally reversible process, indicating that these species have potential applications as switchable systems.     

Synthesis and characterization of homologous series with chiral (S)-1-methylpropyl terminal substituent

Homologous series of liquid crystalline azoesters and azomethine esters consisting of a (S)-1-methylpropyl group attached in one of the terminal positions have been synthesized and thermally characterized. All twenty-four derivatives from both series, namely, the 4-(4-n-alkoxybenzoyloxy)-4'-1-(S)-methylpropylazobenzenes and 4-(4-n-alkoxybenzoyloxy)benzylidene-4'-1-(S)-methylpropylanilines exhibit mesomorphism. The lower members of the homologous series show a chiral nematic phase while the higher members show smectic C*, smectic A as well as chiral nematic mesophases. The homologues have been characterized using IR, NMR and UV-Visible, spectroscopies, X-ray diffraction and DSC. Their mesomorphic properties are compared with those of structurally related homologous series.     

Synthesis and mesomorphic properties of the homologous series of 4'-bromophenyl 4'-[(4-n-alkoxy-2,3,5,6 tetrafluorophenyl)ethynyl]benzoates

A homologous series of 4'-bromophenyl 4'-[(4-n-alkoxy-2,3,5,6-tetrafluoro-phenyl)ethynyl]benzoates has been synthesized. The transition temperatures were studied by polarizing optical microscopy. The compounds exhibit nematic and smectic A phases.