Synthesis and Mesomorphic Properties of the Homologous Series of Di-(4′- n-alkylphenyl)- trans-cyclohexane-1,4-dicarboxylates

The homologous series of di-(4′- n-alkylphenyl)- trans-cyclohexane-1,4-dicarboxylates in which the alkyl group is C₁-C₁₂ has been prepared by esterification of trans-1,4-cyclo-hexanedicarboxylic acid or dicarbonyl chloride with 4-alkylphenols. Mesomorphic properties and phase transitions as determined by hot-stage polarizing microscopy and DSC are discussed in relationship to various structural features. Comparisons of these properties with those for the corresponding alkoxy series and the analogous terephthalic acid diesters are also presented and discussed.     

The Liquid Crystal Properties of Some Aromatic Esters Derived From Naphthalene

Abstract The 2,6-naphthylene ring system has been used to replace each of the 1,4-phenylene rings in various 4-substituted-phenyl 4-substituted-benzoates, where the substituents were alkyl and/or alkoxy. For the dialkyl esters, the N-I temperatures were increased by 60–80° compared with the simpler, analogous phenyl benzoates, irrespective of the position of the 2,6-naphthylene ring. For the esters with one alkyl and one alkoxy substituent, the N-I temperatures were again higher than those of the corresponding phenyl benzoates (～60°); S_A properties were quite pronounced in the 6-alkyl-2-naphthyl 4-alkoxybenzoates. When both substituents were alkoxy groups, S_C phases of quite wide thermal range were exhibited, usually preceded on cooling by a nematic and a S_A phase.     

Mesomorphic Behavior of Some New Fluorene Compounds

Several new fluorene derivatives have been synthesized and their liquid crystalline polymorphic behavior examined. Specifically, 2-fluorenylmethylidene-4′-n-alkylanilines with different alkyl groups are compared with corresponding compounds wherein the alkyl groups have been replaced either by the alkoxy or other different groups. In addition, some of the 4-alkyl and alkoxybenzylidene-2-aminofluorenes have also been prepared for comparison. Many of these compounds show not only the nematic phase but also two smectic phases. The presence of a lateral substituent in analogous compounds decreases the nematic-isotropic transition by about 60[ddot]C.     

An X – Ray Study of the p-n-Alkoxycinnamic Acids. Part III.1

The effect upon molecular packing of increasing chain length in a homologous series of mesogens has been examined in the case of the p-n-alkoxycinnamic acids by determination of the crystal structures of the four nematogenic acids having two, four, six, and eight carbon atoms in the alkyl chain. The chains have the all-trans extended conformation, and the molecules exist in the crystal as planar hydrogen-bonded dimers. The dimers are arranged in end-to end fashion in parallel rows. Similar side-to-side packing of pairs of dimmers is found in each crystal structure giving a good fit between adjacent aromatic cores, but the amplitudes of thermal vibration of the chain carbon atoms increase markedly with increasing chain length, indicating a low packing efficiency for these moieties.     

Synthesis,characterization of new carboxylic acid derivatives bearing 1,3,4-thiadiazole moiety and study their liquid crystalline behaviors

Thirteen carboxylic acid derivatives containing 1,3,4-thiadiazole ring in their core and swinging alkoxy terminal were synthesized. They were characterized by ¹H, ¹³C NMR spectroscopy, FTIR, and mass spectrometry. Their liquid crystalline behaviors have been studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The 1,3,4-thiadiazole compounds in this study were 2,5 di-substituted asymmetrical, alkoxy, and carboxy linkages. The compounds with alkoxy of long carbon chains (n > 7) displayed Smectic C phase. The liquid crystal properties were found to be affected by the length of alkoxy chain attached to the phenyl moiety and the two types of the dimeric form were resulted from the hydrogen bonding interaction between carboxylic acid molecules.     

New Mesogenic Compounds Exhibiting a Thermotropic Optically Isotropic Phase

Abstract

The only two series of carboxylic acids, 3-nitro- or 3-cyano-4-alkoxybiphenyl-4′-carboxylic acid, are known to exhibit the optically isotropic phase (S_D). Now we synthesised 4-(branched alkoxy)-anilinebenzylidene-4′-carboxylic acid series, all being racemic, and investigated their mesophase properties by means of DSC, optical microscopy and X-ray measurement. As a result, some branched azomethine derivatives were found to form a S_D phase. We also synthesised cinnamic acid derivatives with branched alkoxy tail that were turned out to exhibit either S_D phase or hexagonal columnar phase. We tried biphenyl or azobenzene core analogues which were found to produce a highly ordered smectic phase or no mesophase, respectively. At this stage, we suppose it plays an important role that the branch is introduced into the terminal chain and a certain dipole moment is located within the mesogenic core. These factors may allow the peculiar molecular packing that causes an optically isotropic property.     

Terminal Effect of the Semi-fluorocarbon Chain on the Liquid Crystalline Behaviors of Some Compounds

Two series of chiral compounds with semi-perfluorocarbon chains and alkoxy ones, nBP4F and nBP6F, are synthesized. Enantiotropic SmA phase is identified in all of these compounds. When the alkoxy chain length is short, enantiotropic SmB is also identified. When the alkoxy chain length is longer than 6 carbons, enantiotropic SmC* phase is formed. Their clearing points increase with increasing the perfluorocarbon chain length. Comparison of compounds with -CF₃, -CHF₂ and -CF₂Cl groups at the terminals of the fluorocarbon chains, the clearing point sequence is -CHF₂ < -CF₂Cl < -CF₃; and the melting point sequence is -CF₃ < -CHF₂ < -CF₂Cl. Therefore, the compounds with semi-perfluorocarbon chains show the widest liquid crystalline temperature range. Moreover, the SmC* phase of the compounds with -CF₃ at the terminals shows the highest thermostability.     

Thermal Studies on the Phase Transitions of p-n-Dodecyloxy and p-n-Tetradecyloxy Benzoic Acids.: (Part III)

Thermal properties of p-n-dodecyloxy and p-n-tetradecyloxy benzoic acids have been investigated using a differential scanning calorimeter. Heat capacities of thermally stable and metastable crystals of these acids were estimated and the effects of the paraffinic chain attached to the benzoic acid on the formation of a metastable crystalline state are discussed. It is suggested that the disorder of hydrogen bond configuration in associating carboxylic acids in pairs brings about the formation of a metastable crystal state for the alkoxy benzoic acids in general. Under the same conditions the mobile paraffinic chains control the modification of the hydrogen bond during the course of crystallization.     

The relationship between the second and fourth rank order parameters for liquid crystals composed of non-rigid molecules

The values of the fourth rank order parameter P₄ determined by Raman scattering experiments are invariably lower than those predicted by the Maier-Saupe theory of nematic liquid crystals. However this theory is only applicable to rigid particles whereas the nematogenic molecules studies contain flexible alkyl chains. We have estimated the influence of the alkyl chain on P₄ with the aid of a model which account successfully for the variation of the second rank order parameter P₂ along the chain. It is found that the values of P₂ and P₄ calculated for the CN direction in the 4-n-alkyl-4′-cyanobiphenyls are indistinguishable from those predicted by the Maier-Saupe theory. It would appear therefore that the low values observed for P₄ cannot be attributed to the non-rigidity of the alkyl chain.     

Mesogenic 3,6-bis(4-hydroxyphenyl)-1,2,4,5-tetrazine alkanoate esters

A novel series of 3,6-bis(4-hdroxyphenyl)-1,2,4,5-tetrazine alkanoate esters were synthesized and their mesogenic properties were studied using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The impact of changing the tail-core linkage from alkyl or alkoxy to ester is profound. Compared to the alkyl or alkoxy linkages, the ester linkage reduced mesogenic properties. Short-tailed compounds are non mesogenic (4a-4e), while long-tailed compounds (4f-4r) exhibit nematic phases. Unlike the alkyl or alkoxy tail series, none of the 18 presented esters in this series exhibits a smectic phase.     

Calorimetric Study of Crystalline and Liquid-Crystalline Polymorphism in the Homologues of Terephthal-bis-n-alkylaniline

The phase sequences of the homologues with n (the number of carbon atoms in the terminal alkyl chains) from 2 to 8 are obtained. At low temperatures, i) several crystalline modifications are found in each of the members with n = 4, 5, and 6 and ii) the monotropic transitions between crystal and smectic phase are observed in almost all the members. At intermediate temperature, iii) between the smectic H and C phases, the smectic F phase is found in the homologues with n ≥ = 5 by morphological observation or by a rheological method. The constancy of the temperature of the S_H(or S_F) – S_c transition with respect to “and broadening of the temperature range of S_H with increasing n in the expense of nematic phase are worthy of note. The even-odd effect of the end chain length n is observed very distinctively on the crystal melting and clearing point. Proton NMR measurement reveals a large mobility in the terminal alkyl chains in the crystal of the member with n = 6.     

A Natural Progression from Smectic C to Tilted Smectic B Properties in the n-Alkyl 4′-n-Alkoxybiphenyl-4-carboxylates

Recently^1,2 the n-alkyl esters of the 4′-n-heptyloxy- to 4′-n-decyloxy-biphenyl-4-carboxylic acids (inclusive) were prepared. These homologous series all showed a sudden injection of smectic C properties at the n-butyl ester. In the current work, a number of esters of the 4′-n-dodecyloxy-4′-n-tetradecyloxy-, 4′-n-hexadecyloxy-, and 4′-n-octadecyloxy-biphenyl-4-carboxylic acids have been prepared in order to establish the smectic phase sequences at longer, terminal alkoxy chain lengths. It was found that, as the terminal alkoxy chain length was increased there was a natural, smooth progression to esters which exhibit tilted smectic B phases rather than tilted smectic C phases.     

Smectic phases of liquid crystals based on dinuclear palladium(II) complexes with carboxylato bridge

In this paper we present the preparation and the investigation of the liquid crystal properties of a series of dinuclear carboxylato bridge Pd(II) complexes bearing six alkoxy peripheral chains in the molecule. Their structures were assigned based on elemental analysis, IR and ¹H NMR spectroscopy whereas the thermal behaviour was investigated by polarizing optical microscopy and differential scanning calorimetry. The monotropic smectic A phase displayed by these materials was identified by miscibility studies with a previously reported mesogen. It was found that the transition temperatures and the SmA mesophase stability depend on the alkyl chain length of the carboxylato bridge. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Molecular Arrangement in Mesophases of Some p-n-Alkoxybenzoic Acids

The properties of an even homologous series of alkoxy benzoic acids have been investigated in the crystalline and mesomorphic states by means of thermal microscopy, x-ray scattering and dilatometry.

Structural parameters such as inter-lamellar distance d, and the molar volume V_m were studied as a function of the number n of methylene units of the alcoxy radical.

In the crystalline state and the smectic C state the variation of d and of V_m with n is perfectly linear. The angle of tilt of the paraffinic chains α′ and of the aromatic stems φ was determined in the crystalline state. It was found that both moieties have the same magnitude of the tilt. In the smectic C state the value of α′ was found to be 25° as compared to 60.7° in the crystalline state indicating a “pulling up” of hydrocarbon tails when passing from the crystalline state to the smectic state. The passage from crystalline into the smectic state is also accompanied by a strong increase in V_m. Analysis of data indicates that this increase is entirely due to the aromatic part of the molecule while the aliphatic chains have the same V_m as in the crystalline state. It is concluded that while in the crystalline state the cohesion of the phase is insured to a large extent by dimerized, hydrogen bonded molecules, in the smectic C state the cohesion is insured by attraction between extended aliphatic chains. Evidence is also given for the formation of cybotactic groups in the nematic phase of octyloxy benzoic acid.     

On Packing in Smectics: X-Ray Diffraction Study on the Homologous Series 4, 4′-di-n-alkyl-azoxybenzenes

The smectic layer spacings of the homologous series 4,4′-di-n-aLkyl-azoxybenzenes with alkyl chain length from m=6 to m=10 were determined and a mean layer spacing increment of 0.109 nm per methylen group was found.

The results are discussed in relation to two different models:

(I) The diffuse cone model of A. de Vries. Differences between layer spacing and molecular length are attributed to orientational disorder leading to mean tilt angles up to 18° of the molecules.

(II) The second model differentiates between the directions of the rigid aromatic core and that of the alkyl chains. Orthogonality of the rigid core long axis to the smectic layer planes and therefore tilting of the alkyl chains results in coincidence between calculated layer thickness and measured smectic layer spacings.

X-ray film erposures of oriented samples clearly demonstrate the S_A -type of the phases and confirm a slight odd-even effect of layer spacing increments.     

Comments on Some Recent Developments in the Field of Liquid Crystals

Three areas of research into liquid crystals which have seen considerable development since the last International Liquid Crystal Conference at Bordeaux in 1978 have been selected for a brief over-view.

These areas concern: 1. New Liquid Crystal Materials containing non-aromatic ring systems and the developments which have followed from the earlier studies of esters of trans-4-n-alkylcyclohexane-1-carboxylic acids and the trans-4-n-alkyl-l-(4′-cyanopheny1)cyclohexanes (PCH's).

2. Agreement over a Nomenclature Problem Concerning Smectic Polymorphs and some interesting features of the smectic phase sequences for the 4-n-alkoxybenzylidene-4′-n-alkylanilines.

3. Discotic Phases—the discovery that the historically well known mesophase of di-isobutyl-silandiol is a discotic liquid crystal phase and the progress being made in studies of other discotic systems.

     

Sels de Pyrylium et de Thiopyrylium Mésomorphes

Recently we described the redox and liquid-crystal properties of 2,2′, 6,6′-tetraaryl-4,4′-bipyrannylidenes and their sulfur analogs, in which aryls are substituted with long alkyl or alkoxy chains.

Some of the synthesis intermediate perchlorate salts of 2,6-λ diaryl pyrylium and thiopyrylium also show a mesomorphic behavior. So the introduction of long alkyl or alkoxy chain substituants in para position of the phenyls leads to liquid-crystals for both pyrylium and thiopyrylium perchlorates, but the mesomorphic phase is larger for the thiopyrylium ones.

The systematic study of the position of the substituant in the phenyl cycle of pyrylium salts demonstrates that the meta-substituted compounds have a lower transition temperature (Cr-M) than the para. Finally the ortho ones do not exhibit a liquid-crystal behavior.

Between the dialcoxy-substituted pyrylium salts, only the ortho, meta ones show mesomorphic phase, and the effect of the introduction of the second alcoxy-chain is the decrease of the transition temperature (Cr-M). The latter leads to liquid crystals at room temperature. Des séries homologues de perchlorates de pyrylium et de thiopyrylium-précurseurs de bipyranylidenes et de bithiopyranylidenes discotiques-ont été préparées. L'impact de divers paramètres sur le mésomorphisme de ces sels a été étudié (la nature des substituants, la longueur de la chaǐne, la position et le nombre des substituants dans le cycle, la nature de l'hétéroatome). Les composés obtenus présentent des mésophases themiquement stables (Tableaux I-III). Les températures de transition Cr-M sont fonction de la nature et du nombre de substituants et sont peu influencées par la longueur de la chaǐne.     

The Effect of Conjugation Length and Electron Donor Groups on the Second Order Nonlinear Polarizability of Cyano Substituted Aromatic Molecules

Second order nonlinear polarizabilities, α⁽²⁾, were measured for various cyano-substituted aromatic (phenyl, biphenyl, terphenyl and phenylpyrimidine) molecules. The largest nonlinearity was found in cyano-biphenyl compounds, indicating that ring twisting may be causing a decrease in terphenyl nonlinear polarizability. Electron donating substituent effects were demonstrated by comparison of the nonlinearity of alkyl and alkoxy cyanobiphenyls. In all cases α⁽²⁾ was determined from analysis of the optical second harmonic generation from a monolayer of amphiphilic molecules spread at the air/water interface.     

Vibrational spectra,electronic properties and effect of alkyl chain length on chemical stability of nematogens – A comparision using DFT and HF methods

The vibrational spectra of lower homologous series of nematogenic p-n-alkylbenzoic acids (nBAC) with 4 (4BAC) and 5 (5BAC) carbon atoms in the alkyl chain have been investigated using the Density Functional Becke3-Lee-Yang-Parr (B3LYP) level with the basis set 6–31++G(d.p) and Hartree Fock (HF) with the same basis set. The observed vibrational spectra has been resolved and assigned in detail for comparision with both the molecules. These results indicate that DFT and HF values are slightly different at both the level. A comparision of electronic properties such as HOMO (E_HOMO), LUMO (E_LUMO) energies, energy gap (E_g), ionization potential (I), electron affinity (A), electro negativity (χ), chemical hardness (η), electronic chemical potential (μ), electrophilicity index (ω), and softness (S) has been made. It has been observed that decrement occurred in the energy band gap value of isolated molecule with increment in alkyl chain length. This provides valuable information regarding the stability of liquid crystal materials.