Influence of meta-extended rigid-core,complementary hydrogen bonding and flexible chain on polymorphism in Schiff-base liquid crystals: (4)MeOBD(3)AmBA:nOBAs

ABSTRACT

Schiff-base complementary hydrogen-bonded liquid crystals (HBLC), viz., (4)_MeOBD(3)_AmBA:nOBAs with flexible chain length for n = 3,4,5,6,7,8,9,10,11 and 12 are reported. ¹H; ¹³C-NMR and Infra-Red spectroscopy used to confirm the formation of HBLCs. LC phases and transition temperatures (T_c) determined by polarised optical microscopy (POM) and differential scanning calorimetry (DSC). T_c and enthalpy (?H) determined by DSC also. Odd-Even effect observed at clearing and melting transitions. Influence of Schiff base and Oxygen as bridging atom promote smectic phase abundance. HBLCs exhibit tetra- or penta-phase variance. Maximum (penta) phase variance is exhibited by n = 8 and 12 with long flexible chain. Prevalent abundance of quasi-two-dimensional (2D) LC phases of SmF and SmI is observed. Nematic phase is quenched. Lower (n = 4) and intermediate (n = 6) members exhibited SmB_cryst phase. Predominant occurrence of enantiotropic LC phases is noticed. All members exhibited 3D tilted SmG phase. A-C transition exhibited by intermediate homologues (for n = 7, 8, 9 and 10) is found to be either second order or with very small enthalpy. Phase diagram reveals the abundance of multi-critical points with LC phasesinvolving exotic symmetries. Influence of meta-extended rigid core, complementary HB and flexibility is studied for the LC phase abundance with characteristic structural order. POM and DSC results are discussed in the wake of reports in other achiral calamitic LCs.     

Experimental investigation on the effect of mesogenic ratio in hydrogen-bonded liquid crystal complexes

Hydrogen-bonded liquid crystal complex (HBLC) is synthesized from mesogenic 4-hexyloxy benzoic acid (6OBA) and aliphatic non-mesogenic citric acid (CA). The presence of intermolecular hydrogen bond in the HBLC complex is confirmed by FTIR spectrum. Phase transition temperature and enthalpy values are identified using different techniques. Induced highly ordered smectic G phase has been noticed in the present HBLC complex. Thermal span width and thermal stability factor of the individual mesogenic phase are discussed. The complexes are synthesized in different mole ratios, and their corresponding influence on the phase transitions is also discussed.     

Conformational studies of intermolecular hydorgen bonding through induced crystal G phase in nBA:7HB

This paper exploits the physical investigation on liquid crystal complexes obtained by self-organisation of p-n-alkyl benzoic acid (nBA) mesogens with non-mesogenic materials heptyl p-hydroxy benzoate (7HB). Intermolecular interactions of the molecules result the hydrogen bond between the proton donor (COOH) of nBA and proton acceptor (OH) of 7HB. The formation of hydrogen bond is attributed to the quenching of the nematic phase and inducement of crystal G phase in liquid crystal complex. A comparative study of phase abundance is presented with respect to the pure nBAs and other hydrogen bonded liquid crystal complexes of nBAs. Thermal and phase behaviour of the complexes are determined by polarising optical microscope (POM), differential scanning calorimetry (DSC) and image moments approach. Intermolecular interactions which result the hydrogen bond in complexes are investigated using Fourier transform infrared (FTIR) spectroscopy. Molecular structure of the liquid crystal complexes in the solid phase was elucidated using powder X-ray diffraction and proton nuclear magnetic resonance (¹HNMR).     

A Twist‐Bend Nematic Phase Driven by Hydrogen Bonding

The liquid crystalline phase behavior of 4‐[6‐(4′‐cyanobiphenyl‐4‐yl)hexyloxy]benzoic acid (CB6OBA) and 4‐[5‐(4′‐cyanobiphenyl‐4‐yloxy)pentyloxy]benzoic acid (CBO5OBA) is described. Both acids show an enantiotropic nematic phase attributed to the formation of supramolecular complexes by hydrogen bonding between the benzoic acid units. In addition, CB6OBA provides the first example of hydrogen bonding driving the formation of the twist‐bend nematic phase. The observation of the twist‐bend nematic phase for CB6OBA, but not CBO5OBA, is attributed to the more bent molecular shape of the complexes formed by the former, reinforcing the view that shape is a key factor in stabilizing this new phase. Temperature‐dependent FTIR spectroscopy reveals differences in hydrogen bonding between the two nematic phases shown by CB6OBA which suggest that the open hydrogen‐bonded complexes may play an important role in stabilizing the helical arrangement found in the twist‐bend nematic phase.     

Fluorescence study of thermotropic liquid-crystalline polyesters

Photoluminescence behavior (polarization, lifetime) related to liquid-crystal (LC) formation was examined for the thermotropic liquid-crystalline polyesters poly [(ethylene terephthalate)-co-(p-oxybenzoate)] (PET40/OBA60) (OBA content: 60 mol %) and poly [(ethylene 2,6-naphthalene dicarboxylate)-co-(p-oxybenzoate)] (PEN50/OBA50) (OBA:50 mol %). The Growth of liquid-crystalline (LC) phases of PET40/OBA60 proceeded during annealing. even at low temperature (e.g., 138°C) and were promoted by an increase in annealing temperatures T_a in the experimental temperature range 138–260°C. The concentration dependence of fluorescence spectra of PET40/OBA60 in solution suggested that the fluorescences at 325 and 395 nm can be attributed to monomer and ground-state dimer, respectively. The increase in dimer fluorescence intensity and the decrease in the fluorescence anisotropy ratio r from 0.06 to –0.14 were observed with growth of LC phases. These effects are explained by an increase in the ground-state dimer population and a slight change in the dimer configuration, respectively. PEN50/OBA50 showed monomer fluorescence at 395 nm due to naphthalenedicarboxylate segments and excimer fluorescence at 430 nm. The r value for the excimer fluorescence decreased from zero to about ?0.14 with growth of the LC phase. Such an extraordinary phenomena, in comparison with the usual excimer fluorescences which occurs through energy migration, could be interpreted as the result of formation of high-concentration excimer sites induced by chain orientation in LC domains. © 1993 John Wiley & Sons, Inc.     

Morphological studies of a hydrogen-bonded LC polymer obtained by photopolymerization in LC solvents

Anisotropic morphologies and the phase behaviour of a hydrogen-bonded LC polymer obtained by photopolymerization in two kinds of LC solvent are discussed. The hydrogen-bonded LC monomer, 4-(6-acryloyloxyhexyloxy) benzoic acid (A6OBA), was photopolymerized in 4-cyano-4'-hexyloxybiphenyl (6OCB) and in 4-cyano-4'-undecyloxybiphenyl (11OCB), which show a nematic phase and a smectic A phase, respectively. After photo-polymerization, the LC media were removed by extraction and the pure polymer was observed by scanning electron microscopy. SEM images showed that the polymer possessed fibrous morphology with a fibre diameter of a few micrometers, based on polymerization-induced phase separation. The overall geometries reflected typical LC characteristics such as schlieren and focal-conic fan textures. It was found that the hydrogen bond between benzoic acid groups in the monomer was rigid enough to fix the anisotropic phase-separated structure forming during the early stage of phase separation; however, it could not permanently maintain the fibre structure due to dissociation at elevated temperature. X-ray measurements revealed that a well developed layer structure of the hydrogen-bonded mesogen existed in the polymer obtained from the smectic phase of 11OCB, whereas a polymer layer structure could develop only partially from the nematic phase of 6OCB.     

Morphological studies of a hydrogen-bonded LC polymer obtained by photopolymerization in LC solvents

Anisotropic morphologies and the phase behaviour of a hydrogen-bonded LC polymer obtained by photopolymerization in two kinds of LC solvent are discussed. The hydrogen-bonded LC monomer, 4-(6-acryloyloxyhexyloxy) benzoic acid (A6OBA), was photopolymerized in 4-cyano-4′-hexyloxybiphenyl (6OCB) and in 4-cyano-4′-undecyloxybiphenyl (11OCB), which show a nematic phase and a smectic A phase, respectively. After photo-polymerization, the LC media were removed by extraction and the pure polymer was observed by scanning electron microscopy. SEM images showed that the polymer possessed fibrous morphology with a fibre diameter of a few micrometers, based on polymerization-induced phase separation. The overall geometries reflected typical LC characteristics such as schlieren and focal-conic fan textures. It was found that the hydrogen bond between benzoic acid groups in the monomer was rigid enough to fix the anisotropic phase-separated structure forming during the early stage of phase separation; however, it could not permanently maintain the fibre structure due to dissociation at elevated temperature. X-ray measurements revealed that a well developed layer structure of the hydrogen-bonded mesogen existed in the polymer obtained from the smectic phase of 11OCB, whereas a polymer layer structure could develop only partially from the nematic phase of 6OCB.     

Determination of 2,6-dichlorobenzamide and its degradation products in water samples using solid-phase extraction followed by liquid chromatography–tandem mass spectrometry

An analytical method was developed for the determination of 2,6-dichlorobenzamide (BAM) and five degradation products thereof including 2-chlorobenzamide (OBAM), 2,6-dichlorobenzoic acid (DCBA), 2-chlorobenzoic acid (OBA), benzoic acid (BA) and benzamide (BAD) in water samples. Solid-phase extraction was combined with liquid chromatography coupled to tandem mass spectrometry using electrospray ionisation. Groundwater spiked at a concentration of 1.0 μg/L gave recoveries on day 1 between 91 and 102% (relative standard deviation: 2.2–26.5%) for OBAM, BAM, DCBA, BA and OBA, while BAD showed a somewhat lower recovery of 60% (relative standard deviation: 25%). Corresponding figures on day 3 gave recoveries of 97–110% (relative standard deviation: 3–22%) for OBAM, BAM, DCBA, BA and OBA, while BAD had a recovery of 51% (relative standard deviation: 4%). The final SPE-LC–MS/MS method had a LOD_Method of 0.009, 0.007, 0.010, 0.021, 0.253 and 0.170 μg/L groundwater for BAD, OBAM, BAM, DCBA, BA and OBA and a LOQ_Method of 0.030, 0.023, 0.035, 0.071, 0.842 and 0.565 μg/L groundwater in the same order of appearance. Analysis of three different Danish groundwaters confirmed the occurrence of BAM at levels exceeding the threshold value of 0.1 μg/L, while no degradation products were found above LOD_Method.     

Study of Field Induced Transition (FiT) and Analysis of Crystallization Kinetics in the Nematic Phase of an Interhydrogen Bonded Nanoliquid Crytsal

An interhydrogen bonded liquid crystal with complementary hydrogen bonding between succenic acid (SA) and pentyloxy benzoic acid (5OBA) referred as SA + 5OBA has been synthesized and characterized. Multiwalled carbon nanotubes (MWCNT) are dispersed in SA + 5OBA and the resultant complex is referred as SA + 5OBA+MWCNT. Both complexes exhibit liquid crystallinity with the presence of nematic phase. FTIR and NMR studies confirms the formation of the interhydrogen bonds. Transition temperatures and enthalpy values are obtained by DSC studies. Considerable hyteresis in dielectric permittivity has been observed in SA + 5OBA + MWCNT, which enable it to be used in device applications. An interesting observation in SA + 5OB A + MWCNT complexes is the field induced transition (FiT) in nematic phase, which is studied by conductance, permittivity, and helicoidal structure deformations. This complex can be used in light modulation applications. The liquid crystalline behavior together with the rate of crystallization in nematic phase of pure and MWCNT dispersed hydrogen bonded complex are discussed in relation to the kinetophase (which occurs prior to the crystallization). The molecular mechanism and dimensionality in the crystal growth are computed from the Avrami equation. The characteristic crystallization time (t*) at each crystallization temperature is deduced from the individual plots of log t and ΔH. The influence of MWCNT on crystallization kinetics in the nematic phase of an interhydrogen bonded liquid crystal is discussed.     

Cautions regarding the physical interpretation of statistically based separation of the ortho substituent effect into inductive,mesomeric, and steric components—II : Acid catalysed esterification of benzoic acids by methanol and cyclohexanol

In order to attain a better insight into the composition of Taft E_s^o, constants the rate data of hydrion catalysed esterification of both m,p-substituted and o-substiluted benzoic acids by cyclohexanol and methanol were submitted to a statistical analysis using inductive mesomeric and steric substituent constants and various dummy variables differently structured. Furthermore a principal component analysis with subsequent identification of the first principal component via multiple regression analysis was applied. It has been demonstrated that in the reactions of m,p substituted compounds some substituents capable of exerting strong mesomeric effects show peculiar characteristics deviating from the general trend. Since the same result was obtained in the correlations of ortho substituted benzoic acids this effect was taken into account using an appropriate dummy variable which in all cases improved the multiple coefficient of determination. It is concluded that the esterification rates of the ortho substituted compounds depend essentially upon inductive and steric effects (taken away OMe OEt and NO₂) as proposed by Taft. While generally the E_s^o values may be regarded as some measure of a steric effect, this is not true for the methoxy and ethoxy groups.     

Helical twisting power and compatibility in twisted nematic phase of new chiral liquid crystalline dopants with various liquid crystalline matrices

ABSTRACT

Four chiral dopants exhibiting smectic LC phases themselves were prepared and their helical twisting power (HTP) and thermal phase behaviour in mixtures with four various LC hosts were studied. The influence of host liquid crystal on HTP was evaluated and generally higher values were found for hosts with high birefringence. Unexpectedly, high enhancement was found for an LC-chiral dopant pair, both having a similar aromatic core – biphenyl ring substituted with polar group. All studied chiral dopants exhibited limited compatibility with the LC hosts in twisted nematic phase at room temperature. For one of the studied mixtures, it was able to obtain single twisted nematic phase with selective light reflection band with maximum at wavelength about 1.0 µm. Carboxylic acid-type dopants exhibited total compatibility with the studied host in single twisted nematic phase at elevated temperatures, allowing preparation of mixtures with reflection band in the visible range. In case of the carboxylic acid dopants, blue phases for optimised compositions were observed. Intermolecular hydrogen bonding between carboxylic acid proton and pyridine nitrogen of chiral dopants was found. Doping the LC host with these dopants led to slight enhancement of HTP value and higher solubility in the LC host.     

Fluorescence study on a thermotropic liquid-crystalline polyester at high temperatures

A thermotropic liquid-crystalline (LC) polyester, poly[(ethylene terephthalate)-co-(p-oxybenzoate)] (PET40/OBA60) (OBA content: 60 mol %), is investigated by fluorescence technique using two model compounds: dimethyl terephthalate (DMT) and methyl methoxybenzoate (MMB) and is demonstrated to form an intermolecular ground-state complex between the terephthalate and OBA moieties. The change in fluorescence of PET40/OBA60 film is studied from 25°C to 450°C. The peak wavelength change for fluorescence of the intermolecular ground-state complex from 394 to 430 nm was observed in the temperature range between T_g and the LC transition temperature (115～ 250°C). This is attributed to the electronic distribution change between terephthalate and OBA moieties in the excited state, which play roles of acceptor and donor, respectively. The increase in the fluorescence intensity from the temperature near the annealing temperature to the temperature near the isotropic temperature (287～370°C) is suggested to be the increase in LC configuration and the formation of a more stable excited state due to the electronic distribution change between terephthalate and OBA moieties. The lifetime of PET40/OBA60 film quenched from LC temperature (300°C) to room temperature is in agreement with that of the nonannealed one, which is due to the fact that the deactivation process of the sample quenched from LC temperature is in accord with that of the nonannealed one. © 1995 John Wiley & Sons, Inc.     

Linear and A2+B3‐type hyperbranched polyesters comprising phenylbenzothiazole unit: Preparation,liquid crystalline,and optical properties

Novel liquid crystalline (LC) hyperbranched (HB) polyesters comprising phenylbenzothiazole (PBT) unit as mesogen in the interiors were prepared at various feed mole ratios (A₂/B₃) by solution polycondensation of a dioxydiundecanol derivative of PBT (A₂ monomer) with trimesic acid trimethyl ester (B₃ monomer) via A₂+B₃ approach and their LC and optical properties were investigated. Analogous linear polyesters containing the PBT unit in the main chains were also prepared by the solution polycondensation of A₂ monomer with aromatic or aliphatic dimethyl esters. FTIR and ¹H‐NMR spectroscopies indicated that the HB polyesters are produced without gelation during the polycondensation and have degree of branching (DB) of 7–46%. The structures of HB polymers changed depending on the feed mole ratios and the polymer prepared in the mole ratio of A₂/B₃ = 3/2 had the highest inherent viscosity and DB. Acetylation of terminal OH group‐having HB polyesters prepared in excess mole ratios of A₂/B₃ afforded ones bearing acetoxy groups in the terminals. DSC measurements, polarizing microscope observations of textures, and X‐ray analyses suggested that only the terminal OH group‐having HB polymer prepared in the mole ratio of A₂/B₃ = 3/1 form smectic C phase. In the linear polymers, the polymers derived by using the aromatic dimethyl esters had no LC melt, but those from the aliphatic dimethyl esters formed LC smectic C phase. The acetoxy group‐bearing HB polymers showed more stable smectic A or C phase than those with the OH terminals. Solution UV‐vis and photoluminescent (PL) spectra indicated that the linear and the HB polymers have analogous optical properties and display maximum absorbances and blue‐light emission on the basis of the PBT unit, where the Stokes shifts were observed because of intermolecular aggregation effects, but there is a large difference between the optical behaviors of the linear and the HB polymers in film, whose E_g values of the linear polymers decreased and those of the HB polymers vice versa. Quantum efficiencies (Φ) had a tendency of increase in the linear polymers and the HB polymers forming LC phases. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6688–6702, 2008     

Raman spectra of magnetic field induced cholesteric → nematic phase transition in doped p-octoxybenzoic acid

Raman spectroscopy is employed to investigate helical twist formation in nematic and smectic C phases of p-n-octoxybenzoic acid (OBA) doped with a small amount of Cholesteric Nonanoate (CN). A cholesteric→ nematic phase transition in OBA/CN is induced by an external magnetic field in the temperature range 135–148°C. The threshold field (H_c) is equal to 8.0 kGauss for 0.33 % by weight CN/OBA, and no hysteresis is observed.     

Charge transfer interactions in liquid crystalline polymers probed by fluorescence spectroscopy

Rigid-rod aromatic LC polyester with long alkyl side chains and two thermotropic LC polyesters (PET40/OBA60 and PB-10) were studied by fluorescence spectroscopy to investigate their charge transfer interactions corresponding to LC configuration and changes during phase transition.     

Chromatography of substituted benzoic acids with methanol-water-carbon dioxide mixtures.

The impact of the proportion of CO2 concentration in methanol-water-CO2 mobile phases on the separation of several substituted benzoic acids was explored by studying the variation of retention with mobile phase pH in these mixtures. As the amount of CO2 in methanol-aqueous buffer-CO2 mixtures increased, a more basic buffer was needed to control the dissociation of these acids. Differences in terms of retention, separation efficiency and peak asymmetry were shown for substituted benzoic acids with methanol-water-CO2 and methanol-aqueous buffer-CO2 mixtures. Variations of these chromatographic parameters with mobile phase pH were related to the dissociation of these acids and their interaction with methanol-aqueous buffer-CO2 mobile phases and the stationary phase. The addition of a buffer into methanol-aqueous solution-CO2 was an effective means to optimize separations of acidic analytes with high fluidity liquid mobile phases. The substituted benzoic acids had baseline separation in the least amount of time using the high fluidity liquid mobile phases.     

Liquid crystalline behaviour of benzoic acid derivatives containing alkoxyazobenzene

Liquid crystal trimers based on the hydrogen bonding dimerization of 4‐{n‐[4‐(4‐m‐alkoxy‐phenylazo)phenoxy]alkoxy}benzoic acid (BAm‐n) have been synthesized and characterized. Temperature‐dependent FTIR spectroscopic studies showed that the carboxylic acid groups in BAm‐n are associated to form H‐bonded cyclic dimers both in their crystalline and liquid crystalline phases. The trimers exhibited enantiotropic liquid crystalline behaviour except for BA1‐3 which showed monotropic behaviour, and the mesophases changed from nematic to smectic phase, with the increase of length of the spacer and the terminal substituents. Pronounced odd–even effects in the melting temperatures, clearing temperatures and nematic–isotropic enthalpy changes were observed.     

Influence of Hydrogen Bond on Thermal and Phase Transitions of Binary Complex Liquid Crystals

A novel supramolecular liquid crystal (LC) is synthesized from the binary complex of 4-decyloxy benzoic acid and cholesteryl acetate. Fourier transform infrared (FTIR) spectroscopic study confirms the formation of intermolecular hydrogen bond between the mesogens. Various mesophases and corresponding textural changes in the complex are observed by comparing with its constituents through polarizing optical microscopic (POM) studies. The thermal stability factor of smectic phase for present complex is calculated. An interesting observation of present work is that investigation of extended thermal span of mesomorphic phases, decreased enthalpy, a nematic phase with a high clearing point and a low melting point. This is due to an arrangement of molecular reorientations and the development of new associations by hydrogen bonding. Optical tilt angle for smectic C phase is determined and the same is fitted to a power law.     

溶剂效应对金属有机骨架化合物结构的影响

以水和DMF为溶剂,在水热(溶剂热)条件下以"柱撑"方式利用相同的金属和配体合成了2个不同的新型金属有机骨架化合物[Cd(OBA)(bipy)]·DMF(1)和Cd(OBA)(bipy)(2)[OBA=4,4'-Oxybis(ben-zoic acid),bipy=4,4'-bipyridine].单晶X射线衍射分析数...     

Liquid crystals comprising hydrogen-bonded organic acids I. Mixtures of non-mesogenic acids

Stable nematic phases are reported for binary mixtures of p-alkylbenzoic acids (nBA; where n = 1,2,3 denotes methyl, ethyl and propyl), p-alkoxybenzoic acids (nOBA, n = 1,2), and p-fluorobenzoic acid, which separately do not exhibit liquid crystalline phases. The mesophase stability increases in equimolar mixtures of acids having incomensurate lengths; the larger the difference in the tail substituents in the mixed organic acids, the broader the nematic phase. The trends in excess nematic range exhibited by the H-bonded supramesogens in binary mixtures parallels the behaviour of covalent mesogens. This is indicative of an influence of heterodimers comprising H-bonded pairs of the two different acids in equimolar mixtures of the non-mesogenic acids. It would appear that dimerized organic acids constitute a viable molecular-structural 'bread-board' for rapid screening for potential mesogen constituents.