The effect of lateral fluoro-substitution on the liquid crystal behaviour of some 4'-alkylphenylethyl- and trans-4'-alkylcyclohexylethyl-4,4'-disubstituted biphenyls

Abstract

The preparation of several 4'-alkylphenylethyl- and trans-4'-alkylcyclohexyl-ethyl-4,4'-disubstituted biphenyls and their 2-fiuoro derivatives is described and their transition temperatures are reported. The effect on the transition temperatures caused by the replacement of a cyclohexane ring by a benzene ring and the effect of fluoro-substitution on nematic, smectic A and smectic B thermal stabilities is discussed. In these systems, and in others, when the lateral fluoro-substituent points towards the longer terminal aliphatic unit (alkyl or alkylcyclo-hexylethyl) it causes a greater depression of smectic properties.     

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.     

The effect of terminal chain modifications on the mesomorphic properties of 4,4'-disubstituted diphenyldiacetylenes

A variety of terminal chain modifications (Y) were made on the diacetylenes in which X=C_nH_2n+1, C₁₂H₂₅O and F, and Y=CH₂CH(Me)C₂H₅, COCH₃, C≡CC₅H₁₁, C_nF_2n+1C_nH_2n+1 and CH=CHCO₂C₃H₇. Mesomorphic properties were determined by hot stage polarizing microscopy and DSC. These were compared with those for the dialkyl analogues (X=C_mH_2m+1, Y=C_nH_2n+1) and a series of 1- and 2-olefins (Y=CH=CHC_nH_2n+1 and CH₂CH=CHC_nH_2n+1). The 1-olefin series showed wider range nematics than the dialkyl compounds, whereas the above modifications showed either narrow range nematic phases, no mesophase or higher melting temperatures. New transition temperature and enthalpy data are provided for some of the dialkyl and F-alkyl compounds previously reported, for comparisons. Preliminary birefringence data are also included along with the results of some heat and UV stability studies.     

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.     

Polarity and steric effect of the lateral substituent on the mesophase behaviour of some newly prepared liquid crystals

Eight homologous series of 2- (or 3-) substituted phenyl 4?-(4?-alkoxy phenylazo) benzoates (In_a–h) were prepared in which, within each homologous series, the length of the terminal alkoxy group varies between 6, 8, 10 and 12 carbons, while the other substituent, X, is a laterally attached polar group that alternatively changed from CH₃, H, F, Br and CN. Compounds prepared were characterised by infrared and ¹H-NMR spectroscopy, and their mesophase behaviour investigated by differential scanning calorimetry and identified by polarised light microscopy. The results were discussed in terms of polarity and steric effects. The stability of the mesophase was correlated once with the dipolar anisotropy of the whole molecule and another with the dipolar anisotropy of the substituent, X. A comparative study was made between the investigated compounds and their previously prepared linear 4-substituted isomers, namely 4-substituted phenyl 4?-(4?-alkoxy phenylazo) benzoates (In_i–k).     

Synthesis and mesomorphic properties of 4-alkyl-3,4'-disubstituted biphenyls and terphenyls

Liquid-crystalline 4-alkyl-3-chloro(methyl)-4'-substituted biphenyls and terphenyls have been synthesized. Investigation of the mesomorphic properties of these compounds shows that unlike their unsubstituted analogues they form a liquid-crystalline phase at a lower temperature and over a narrower temperature range.     

Synthesis and mesomorphic properties of some bisegmented compounds derived from 4,4'-disubstituted biphenyl

New unsymmetrical disubstituted biphenyls have been synthesized. Two different chains (one hydrocarbon and the other fluorinated) are linked through an acetamide bond to 4-methoxy-4'-hydroxybiphenyl. Their mesomorphic properties have been characterized by light microscopy and by differential thermal analysis showing the peculiar contribution of the hydrocarbon chain and the fluorinated tail. The enhancement of the hydrocarbon moiety from one to twelve methylene groups leads to a decrease of the clearing temperatures; furthermore the enantiotropic behaviour is changed to monotropic. The variation of the fluorinated moiety generates strong effects on the transition temperatures. These mesomorphic properties are compared to those of their monocatenar perfluorinated analogues incorporating a connector of the ester or monosubstituted amide type.     

The effect of molecular broadening on liquid crystal behaviour in oligoaryl mesogens

The synthesis and phase behaviour of a series of broadened oligoaryls, in which replacement of one or two of the phenyl groups of bi- and ter-phenyl mesogens has been made by 1,4-substituted naphthalene, is reported. The novel materials, which are not mesogenic, can be compared with the liquid crystalline 2,6-substituted isomers.     

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 fluorocarbon tails on the phase behaviour of some Schiff's base liquid crystals

Several new liquid crystalline compounds comprising Schiff's-base cinnamate cores and fluorocarbon alkoxy tails of varying lengths have been synthesized and characterized. The thermal properties of these materials are compared to those of their hydrocarbon analogues. The results show that fluorination stabilizes the crystalline phase, quite unexpectedly destabilizes the S_c phase, and for two of the compounds produces extraordinarily stable S_A phases.     

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 effect of replacing a benzene ring with a saturated six-membered ring on the mesomorphic properties of 4,4'-disubstituted diphenyldiacetylenes

A limited selection of ring modified diphenyldiacetylenes of the type where A=, Y=C_nH_2n+1, CF₃, F, COMe, NH₂, and NMe₂, and A=, trans and cis with Y=F and trans with Y=C₃H₇, were synthesized. Mesomorphic properties were determined by hot stage polarizing microscopy and DSC. These properties were generally poorer than those found in the parent benzene compounds. This was also true of some pyrimidine analogues reported earlier. Birefringence values also decreased as expected.     

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).     

Influence of lateral methyl and terminal substituents on the mesophase behaviour of four rings azo-ester liquid crystal compounds

ABSTRACT

The effect of introducing a lateral methyl substitution into the previously investigated laterally neat four-ring analogues, 4-substituted phenylazo phenyl 4?-(4?-alkoxyphenylazo) benzoates (In_a–e), on their mesophase behaviour was investigated for the newly prepared five homologous series of 4-substituted phenylazo phenyl 4?-(3?-methyl-4?-alkoxyphenylazo) benzoates (IIn_a–e). Within each homologous series, the alkoxy group varies between 6, 8, 10, 12, 14, and 16 carbons, while the substituent, X, is a polar group that alternatively changes between the electron-donating (CH₃O and CH₃) groups, and the electron-withdrawing (Br and NO₂) groups, including the unsubstituted homologues (IIn_c). Their mesophase stabilities were determined by DSC and phases identified by PLM. The results showed that independent of the alkoxy-chain length or the polarity of the substituent X, the nematic phase is predominant with relatively high stability and wide temperature ranges. All compounds show a good thermal stability in the mesophases domain, except the nitro and Br substituted derivatives bearing short alkoxy chain length. Comparison of the mesophase behaviour was also made between the present series and corresponding three-ring laterally CH_3-substituted azo/ester analogues. UV-vis absorption spectra revealed that derivatives with electron donating or an electron withdrawing groups exhibited redshifts of the π→π* transition compared with unsubstituded derivative.     

Computational studies of inclusion phenomena and synthesis of a novel and selective molecular receptor for 1,4-disubstituted benzenes and 4,4'-disubstituted biphenyls

A new tetracationic molecular receptor has been synthesized and studied by semiempirical molecular orbital theory. This novel macrocycle, 1(4+), derived from pentacyclo[5.0.0.0.(2,6)0.(3,10)0(5,9)]undecane-8,11-dione (PCU-8,11-dione), structurally resembles cyclobis(paraquat-p-phenylene), 2(4+), in which a xylyl group has been replaced by a PCU unit. This derivatization effectively increases the size and flexibility of 1(4+) and changes its electronic, dynamical, and binding properties. A conformational search using Osawa's corner flapping technique and the PM3 semiempirical method identifies eight unique and low-energy 1(4+) conformers. The principal regions of structural variation occur in the bipyridinium torsion and in the ethylene bridges between PCU and the tetracationic unit. The inclusion complexes of 1(4+) with 1,4-disubstituted benzenes and 4,4'-disubstituted biphenyls have been studied by PM3. The first shell of solvation is approximated by the explicit inclusion of 12 acetonitriles in the computed supramolecular complexes. Binding of 1,4-disubstituted benzenes and 4,4'-biphenol is shifted from the 1(4+) geometric center. From the computations, host 1(4+) is predicted to have an enhanced binding preference for benzidine over 4,4'-biphenol, as compared to 2(4+). For all guests computed, 1(4+) binds more strongly than 2(4+). These properties can be exploited in the future design of supramolecular systems with potential applications as nanoscale devices.     

The synthesis and liquid crystal behaviour of monosubstituted ferrocenomesogens

The synthesis and characterization of several new series of monosubstituted ferrocenyl-containing liquid crystals has been achieved. The results indicate that a terminal ferrocenyl group can promote stabilization of a nematic liquid crystal state. At least three phenyl rings are required in the molecular core in order to provide nematic properties and addition of a fourth ring substantially enhances it. Bulky lateral substituents or the introduction of linker groups that introduce kinks inhibit liquid crystal phase formation. Short highly polarising terminal groups or terminal groups that support hydrogen bonding support liquid crystal behaviour.     

The influence of lateral substituents on the phase behaviour of nematogenic liquid crystals

The influence of lateral substituents or side chains attached to thermotropic nematogens is considered on the basis of the Flory lattice model. The rigid core of the nematogen is characterized by its axial ratio x and the characteristic temperature T* which measures the strength of the anisotropic dispersion forces operative between the cores. For the side chain an additional parameter zm which is proportional to its hard core volume enters the partition function. Given these quantities and the free volume as obtained from the thermal expansion coefficient, the nematic-isotropic transition temperature T_NI and the temperature dependence of the order parameter may be calculated. The results show that the lowering of T_NI by lateral substituents is caused by (1) their interference with molecules packing and (2) the dilution of the anisotropic dispersion forces between the central nematogenic cores. As an additional consequence of this the order parameter is lowered considerably. A survey of recent experimental data shows that the decrease of T_NI is correlated solely with the hard core volume of the substituent regardless of its chemical structure or polarity, thus corroborating the theoretical predictions.     

Synthesis and properties of some novel high birefringence phenylacetylene liquid crystal materials with lateral substituents

We have synthesized and investigated new 3-ring phenylacetylene liquid crystals with high birefringence values (Δn) to improve the nematic temperature range while retaining a high optical anisotropy. In the case of modifying the terminal ring, the introduction of branched alkoxy chains, fluorine or methyl groups shifted the nematic phase to lower temperatures. In order to minimize the influence of the substituent on Δn, the incorporation of lateral methyl groups was chosen as the most profitable method for obtaining wide and low temperature nematic phases. With these results in mind, we studied the effect of the position of the methyl group on the physical properties. The most effective methyl position for improving the nematic temperature range was on the central ring. From the standpoint of obtaining high Δn values and low viscosity, the central ring was also the best position.     

Influence of the liquid crystal behaviour on the Langmuir and Langmuir-Blodgett film supramolecular architecture of an ionic liquid crystal

Novel fluorescent poly(2-(acetoacetoxy)ethyl methacrylate)(PAAEMA) latexes have been synthesized by miniemulsion polymerization employing a polymeric costabilizer. Nanoscale aggregates of macromolecules bearing β-dicarbonyl are formed in the prepared latex particles. Ammonia and the β-dicarbonyl aggregates assemble a supramolecular complex, which exhibits strong visible fluorescence under UV light. The formation of the complex is confirmed by the characteristic absorption peak located at about 275 nm in UV-Vis spectra. The absorption spectrum has been found to be applicable for ammonia detection. Atomic Force Microscopy (AFM) studies of surface morphology reveal that gas-sensing properties of the PAAEMA thin films involve the reversible absorption and desorption of ammonia. PAAEMA thin films are sensitive to ammonia gas and have a short response time of 80s when exposed to 54 ppm of ammonia gas concentration.