X-ray diffraction studies of the liquid crystal phases formed by certain 4'-n-alkylbiphenyl-4-yl 5-n-alkylthiophene-2-carboxylates

Twenty esters selected from the first ten homologues of each of the 4'-n-octylbiphenyl-, 4'-n-nonylbiphenyl- and 4'-n-decylbiphenyl-4-yl 5-n-alkylthiophene-2-carboxylates have been examined by X-ray diffraction techniques. These esters show extensive smectic polymorphism, but only tilted phases are observed, all with tilt angles of around 45°     

A study of homologation and the occurrence of an SA-SC-SA sequence of phases in the 4-n-alkoxy-3-fluorophenyl 4-(5-n-alkyl-2-thienyl)benzoates

Seventeen esters, derived from 4-n-alkoxy-3-fluorophenols and 4-(5-n-alkyl-2-thienyl)benzoic acids, have been prepared and their liquid crystal transition temperatures determined by thermal optical microscopy. On cooling the isotropic liquid, the S_A-S_C-S_A sequence of phases reported for the octyloxy-octyl and octyloxy-nonyl esters has been observed for certain other homologues, principally members of the 4-(5-n-nonyl-2-thienyl)benzoates. For these compounds, the temperature range of occurrence of the intermediate S_C phase decreases as the length of the alkoxy chain increases (for the hexyloxy-nonyl to dodecyloxy-nonyl esters) and the S_c phase is absent for the tetradecyloxy-nonyl homologue.     

The synthesis and liquid crystal properties of some 4-n-nonylphenyl esters of 3-(4'-n-alkoxybiphenylyl)- and 3-(4'-n-alkoxybiphenylyl)-3-methyl-propanoic acids and their laterally fluorinated analogues

A number of 4-n-nonylphenyl esters and mono- and di-fluorinated 4-n-nonylphenyl esters derived from 3-(4'-n-alkoxybiphenylyl)propanoic acids (II) have been synthesized and their thermotropic liquid crystal properties assessed with a view to obtaining tilted smectic phases for possible use in ferroelectric display devices. Many of these compounds exhibit wide temperature range S_C and S_I/F phases, but none gave the ideal phase sequence S_C-S_A-N-I mainly because the structure of these compounds was not conducive to the formation of the nematic phase. To try to alleviate this problem a series of 4-n-nonylphenyl esters based on 3-(4'-n-alkoxybiphenylyl)-3-methylpropanoic acid was prepared, where a lateral methyl group was incorporated in the β-position of the -CH₂CH₂CO₂- linkage. Incorporation of the lateral methyl group encouraged the formation of a nematic phase at the expense of both the smectic A phase and tilted smectic phases.     

Four unit linking groups II. Some novel smectic C materials

About 40 diverse phenyl benzoate esters incorporating a trans-1-4-disubstituted cyclohexane ring joined to the central aromatic core by new four unit linking groups (C₄H₈, C₄H₆, C₃H₆O and C₃H₄O) have been synthesized. The effect of various lateral substituents (F, Cl, CN and Br) and especially two fluorine atoms in the 2,3-positions of the 4-n-alkoxybenzoate part of the esters has been investigated. Three homologous series of 5-n-alkyl-2-(4-phenyl)pyrimidines incorporating the same four unit linking groups have also been prepared. Many members of the ester and pyrimidine series exhibit enantiotropic smectic C mesophases at moderately elevated temperatures. Several esters and pyrimidines have been found to improve the surface alignment and temperature range of chiral smectic C mixtures for surface stabilized ferroelectric liquid crystal displays. The effect of the new four unit linking groups on the liquid crystal transition temperatures, rotational viscosity and spontaneous polarization of their host structures in a standard chiral C mixture has been studied and compared to that of analogous materials containing no central linkage and standard central linkages (-, C₂H₄, CH₂O and COO). Several lactate ester derivatives incorporating the four unit linking groups have also been prepared and been found to exhibit a moderately high spontaneous polarization.     

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.     

Phase-transition studies of 5O.5 and 9O.4

As part of our detailed comparative studies of groups of liquid-crystalline compounds that belong to a homologous series, we present phase-transition studies of the compounds N-(4-n-pentyloxybenzylidene)4'-n-pentylaniline (5O.5) and N-(4-n-nonyloxybenzylidene)4'-n-butylaniline (9O.4) using different experimental techniques. The compound 5O.5 is reported to exhibit a phase sequence N, S_A, S_C, S_B and S_G, while 90.4 shows the sequence S_A, S_F and S_G. The salient features of our work on 5O.5 are (i) a new smectic F phase is found in place of the reported smectic B phase, which is confirmed by both miscibility and X-ray studies; (ii) the formation of smectic-C-like short-range order in the nematic phase well above the S_A-N transition; and (iii) a large tilt-angle variation in the smectic C phase (0-23·5°C) in a small temperature range (4·1°C). The phase changes across the S_A-I transition, and for the first time across S_F-S_A transition, are carried out by volumetric studies. The detailed inferences of these are also presented.     

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.     

The intercalated smectic A phase. The liquid crystal properties of the α-(4-cyanobiphenyl-4'-yloxy)-ω)-(4-alkyloxycinnamoate)alkanes

A novel system of non-symmetric dimers containing 4-n-alkyloxy-substituted cinnamic acid and cyanobiphenyl groups has been studied. Two series were prepared: in one the flexible spacer was varied in length while the spacer was fixed. The spacer length has a profound influence on the nematic-isotropic transition temperature of these materials and a large odd-even effect is observed for the series. The terminal chain also plays a significant role in determining the liquid crystal phase behaviour: a smectic A phase is exhibited for the ethyl and propyl homologues, in addition to a nematic phase; this smectic phase vanishes for intermediate chain lengths but then reappears for the nonyl and decyl members of the series. X-ray diffraction has revealed the structure of the smectic A phase for the ethyl homologue to be intercalated, whereas that for the decyl compound is interdigitated. The existence of the intercalated smectic A phase has previously been explained in terms of a charge-transfer interaction between unlike mesogenic groups. However, for the non-symmetric liquid crystal dimers described here this specific interaction appears unlikely and we discuss, therefore, other possible mechanisms for the formation of intercalated smectic phases.     

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.     

High precision density studies near the smectic A-nematic tricritical point

We have carried out very accurate density measurements (with a precision of ±5 × 10^-5g cm^-3) near the smectic A-nematic transition in binary mixtures of 4-n-nonyl-4'-cyanobiphenyl (9CB) and 4-n-decyl-4'-cyanobiphenyl (10CB). The transition crosses over from second to first order as the temperature range of the nematic phase decreases. For mixtures with the shortest nematic range the data deviate noticeably from a single power law behaviour. Such a deviation is an indication of the first order nature of the transition. Very good fits to a single power law have been obtained for pure 9CB and the x = 0·04 mixture where x is the mole fraction of 10CB in 9CB. The critical exponent obtained from the power law fitting has enabled us to locate the tricritical point to be very close to x=0·04, which is in agreement with the results obtained previously by high resolution calorimetric [1] and X-ray scattering studies [2].     

Laterally fluorinated liquid crystals containing the 2,2'-bithiophene moiety

The synthesis and systematic evaluation of the influence of lateral mono-, di- and tetra-fluorination of the terminal phenyl ring on mesomorphic properties in seven novel series of suitably fluoro-substituted 5-n-alkyl-5'-(4-n-decyloxyphenyl)-2,2'-bithienyls is reported. Compared with their non-fluorinated parent counterparts, lateral fluorination eliminates high order smectic phases and reduces thermal stability, to reveal compounds exhibiting a selection of nematic, smectic A and smectic C phase types. As the number of fluoro-substituents increases from one to two, mesophase thermal stability drops drastically; the disposition of the second fluoro-substituent is important. Across-axis disposition is more detrimental than along-axis. However, complete fluorination does not destroy mesophase formation. Indeed, tetrafluorophenyl compounds are more stable than certain 3,5- and 2,6-difluoro compunds. The extrapolated birefringence for members of a series of 5-n-alkyl-5'-(2,3-difluoro-4-n-decyloxyphenyl)-2,2'-bithienyls is approximately 0.21.     

Influence of the order parameter and of the director orientation on the surface tension of free nematic films

A radial hydrodynamic flow in the nematic phase of free, suspended cylindrical films of 4-n-heptyl- and decyloxybenzoic acid and in 4,4'-di-n-heptyl-oxyazoxybenzene has been observed. The flow starts about 7°C before the phase transition into the smectic C phase. Under the same experimental conditions such a hydrodynamic flow is not established in free nematic films of 4-n-heptyl-and octyloxy-4'-cyanobiphenyl, 4,4'-dimethoxyazoxybenzene and N-(4-ethoxybenzylidene)-4'-n-butylaniline after the completion of the transition from the isotropic liquid to the nematic phase. The observed hydrodynamic flow is explained by a non-linear temperature dependence of the surface tension.     

Some novel liquid crystalline derivatives of 4-carboxyphthalimide

A series of novel esters of 4-carboxyphthalimides has been prepared. Differential scanning calorimetry and polarizing optical microscopy studies reveal that N-substituted 4-cholesteryloxycarboxyphthalimides exhibit a chiral nematic (cholesteric) phase in the range 120-205°C. Unexpectedly, esters containing well-known mesogenic groups such as 4-cyano- or 4-methoxy-phenyl do not form liquid crystal phases.     

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.     

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 properties of certain 1-(4'-n-alkoxy-2,2',3,3',5,5',6,6'-octafluorobiphenyl-4-yl)- 2-(trans-4-n-pentylcyclohexyl)-ethanes and -ethenes

The synthesis and liquid crystal properties of eight members of the homologous series of 1-(4'-n-alkoxy-2,2',3,3',5,5',6,6'-octafluorobiphenyl-4-yl)-2-(trans-4-n-pentylcyclohexyl)-ethanes and eight members of the corresponding -ethanes, which have the E-configuration, are reported. These compounds are nematic, the ethenes having a considerably mesophase higher thermal stability than the analogous ethanes.     

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.     

Molecular dynamics of a ferroelectric smectogen in its smectic phases by means of 2H NMR spectroscopy

In this work the dynamic behaviour of the ferroelectric liquid crystal (-)-(S )-[4-(2-methylbutyloxycarbonyl)phenyl] 4-n-heptylbiphenylcarboxylate (MBHB) in its smectic A (SmA), unwound chiral smectic C (uSmC*) and chiral smectic C (SmC*) phases has been studied by means of ²H NMR spectroscopy. Zeeman (T _1Z) and quadrupolar (T _1Q) spin-lattice relaxation times have been analysed to extract dynamic parameters (diffusion coefficients and activation energies). The small step rotation diffusion model in the uniaxial approximation has been used to describe overall spinning and tumbling motions, and the strong collision model to describe the internal reorientations of the aromatic fragment. Relaxation data in the SmC* phase have been analysed by using a theoretical approach. The dynamic features obtained in the smectic phases of this mesogen are here presented and discussed in comparison with the results obtained in other ferroelectric liquid crystals, focusing on the fast regime of motions.     

A study of homologation and the occurrence of an SA-SC-SA sequence of phases in the 4-n-alkoxy-3-fluorophenyl 4-(5-n-alkyl-2-thienyl)benzoates

Abstract

Seventeen esters, derived from 4-n-alkoxy-3-fluorophenols and 4-(5-n-alkyl-2-thienyl)benzoic acids, have been prepared and their liquid crystal transition temperatures determined by thermal optical microscopy. On cooling the isotropic liquid, the S_A-S_C-S_A sequence of phases reported for the octyloxy-octyl and octyloxy-nonyl esters has been observed for certain other homologues, principally members of the 4-(5-n-nonyl-2-thienyl)benzoates. For these compounds, the temperature range of occurrence of the intermediate S_C phase decreases as the length of the alkoxy chain increases (for the hexyloxy-nonyl to dodecyloxy-nonyl esters) and the S_c phase is absent for the tetradecyloxy-nonyl homologue.     

Scanning tunnelling microscopic images of the surface ordering of the liquid crystal 4-n-octyl-4'-cyanobiphenyl

Images of the surface ordering of 4-n-octyl-4'-cyanobiphenyl, a room temperature smectic liquid crystal deposited upon graphite, have been obtained by scanning tunnelling microscopy. The microscope was operated in air using the constant-current mode. Under certain tunnelling conditions it has been possible to resolve both the aliphatic and aromatic parts of the molecule, and to observe individual benzene rings. Two previously unreported conformations have been observed: an overlapping bilayer structure with spacing 3.7 nm, and a monolayer structure with spacing 2.4 nm. The latter structure may represent the first visual evidence for a surface polar ordered structure.