Synthesis, characterisation and liquid crystal properties of 2,5-bis[5-alkyl(alkoxy)phenyl-1,3,4-oxadiazole]bromobenzenes

A series of new thermal bilateral liquid crystal compounds with the phenylenebis-1,3,4-oxadiazole structure was synthesised. The molecular structures of the oxadiazole compounds were confirmed by FT-IR and ¹H NMR spectroscopy, elemental analysis and mass spectrometry. Thermogravimetric analysis indicates that the compounds in an atmosphere of nitrogen have good thermal stability. Measurements using differential scanning calorimetry, polarising optical microscopy and temperature-dependent wide-angle X-ray diffraction indicated that the liquid crystalline properties are related to the length of the end groups. When the end group was tert-butyl or alkoxy with four and six carbons, the compounds exhibit no liquid crystal phase. However, compounds with end groups containing more than eight carbons show significant bidirectional thermally-induced liquid crystal properties and the structure of the liquid crystal phase is the lamellar smectic A phase. All the compounds exhibit blue fluorescence.     

Synthesis,characterisation and liquid crystal properties of 2,5‐bis[5‐alkyl(alkoxy)phenyl‐1,3,4‐oxadiazole]bromobenzenes

A series of new thermal bilateral liquid crystal compounds with the phenylenebis‐1,3,4‐oxadiazole structure was synthesised. The molecular structures of the oxadiazole compounds were confirmed by FT‐IR and ¹H NMR spectroscopy, elemental analysis and mass spectrometry. Thermogravimetric analysis indicates that the compounds in an atmosphere of nitrogen have good thermal stability. Measurements using differential scanning calorimetry, polarising optical microscopy and temperature‐dependent wide‐angle X‐ray diffraction indicated that the liquid crystalline properties are related to the length of the end groups. When the end group was tert‐butyl or alkoxy with four and six carbons, the compounds exhibit no liquid crystal phase. However, compounds with end groups containing more than eight carbons show significant bidirectional thermally‐induced liquid crystal properties and the structure of the liquid crystal phase is the lamellar smectic A phase. All the compounds exhibit blue fluorescence.     

Synthesis of novel liquid crystal compounds with aromatic amide mesogenic cores

The synthesis and properties of a new series of compounds having aromatic amide mesogenic cores are reported. Most of these new compounds are thermotropic mesogens. In consideration of the fact that aromatic amides form crystals of high melting point which is unfavourable for the formation of thermotropic liquid crystals, we make use of lateral substitution to decrease both the packing efficiency and the hydrogen bonding, so that the melting temperature of the aromatic amides is sufficiently depressed. The lateral substituent used in these new compounds is bromine. In order to investigate the influence on properties of the end groups, different alkoxy, alkyl and other groups are used at the two ends of the rod-like molecules. The two ends are either identical or different, with an electron-donating alkoxy as one end and the electron-accepting cyano group as the other. The results indicate that appropriate lateral and terminal substitution is essential for the aromatic amides to form thermotropic liquid crystals. The peculiar mesophase characterized by an X-ray diffraction pattern of a SmC phase, but a texture of a nematic phase is also noted.     

How Nucleophilic Are Diazo Compounds?

The kinetics of the reactions of benzhydryl cations with eight diazo compounds 1 a-g were investigated photometrically in dichloromethane. The nucleophilicity parameters N and slope parameters s of these diazo compounds were derived from the equation log k (20 degrees C)=s (E+N) and compared with the nucleophilicities of other pi systems (alkenes, arenes, silyl enol ethers, silyl ketene acetals). It is shown that the nucleophilic reactivities of diazo compounds cover more than ten orders of magnitude, being comparable to that of styrene on the low reactivity end and to that of enamines on the high reactivity end. The rate-determining step of these reactions is the electrophilic attack at the diazo-carbon atom to yield diazonium ions, which rapidly lose nitrogen.     

Surface, core, and structure modifications of phosphorus-containing dendrimers. Influence on the thermal stability

Three new series of phosphorus-containing dendrimers are described. Their solubility depends on the type of end groups they bear. Perfluoroalkyl chains give dendrimers soluble in chlorofluorocarbons, whereas guanidinium and pyridinium derivatives give water-soluble compounds. The thermal stability of these compounds, as well as of 19 other dendrimers of various generations, having various cores, or various end groups, or branching points is studied. The main feature of this study is that the internal structure of these dendrimers is thermally stable at least up to 376°C. The number of the generation has practically no influence, whereas the principal criterion influencing the thermal stability is the type of end groups. The water-soluble cationic dendrimers are the least stable, but even those are stable up to 225°C. For most of these dendrimers, an important percentage of mass (around 50%) is retained even at a temperature as high as 1000°C. In the best case, up to 70% of the initial mass is retained at 1000°C.     

Conjugated oligomers with terminal donor-acceptor substitution

Conjugated oligomers represent a prominent class of compounds from a viewpoint of their theory, synthesis, and applications in materials science. Push-pull substitution with an electron donor D at one end of the conjugation and an electron acceptor A at the other end results in them having outstanding optical and electronical properties. This Review highlights fundamental synthetic strategies for the preparation of such oligomers with n repeat units (n=1, 2, 3, 4, ..) and the rules that govern their linear and nonlinear optical properties (absorption, frequency doubling and tripling). The unification of chemical, physical, and theoretical aspects with an interdisciplinary image of this class of compounds is attempted herein.     

P NMR spectroscopy in benzoxazine model compounds and benzoxazine chemistry - main chain and end group studies

Using ³¹P NMR spectroscopy after phosphorylation, different phenols are easily discriminated. Proposed phenolic model compounds from benzoxazine reaction/polymerization are phosphorylated with 2-chloro-1,3,2-dioxaphospholane directly in an NMR tube. The dimer and oligomer structure of benzoxazine is different from that of monomer, that is, the breaking of the oxazine ring upon polymerization results in the formation of Mannich bridge structure linking phenolic groups together. Different electronic environment in the phosphorus derivatives of the methylamine-based benzoxazine model dimer and oligomers allows comparison of these ³¹P NMR spectra with the ³¹P-NMR spectra of the compounds from traditional benzoxazine polymerization provides useful end group information. Phenolic functional groups in benzoxazine dimer and oligomers, e.g. phenolic end groups and phenolic groups on the backbone, are studied.     

T-shaped non-symmetrical twin liquid crystalline compounds

New unconventional T-shaped non-symmetrical dimeric liquid crystalline compounds have been synthesised and their thermotropic properties studied on the hot-stage of a polarising microscope. These compounds consist of an azo-ester mesogenic unit with a range of terminal substituents (–CH₃, –OCH₃, –NO₂ or –Cl) at one end, interconnected by a flexible spacer (n?=?4, 5 or 10) via ether and ester linking units to a biphenyl moiety at the lateral hydroxyl position of the azo-ester. All the compounds were characterised using a combination of elemental analysis and standard spectroscopic methods. The compounds were found to exhibit enantiotropic nematic and smectic mesophases. The effect of different terminal substituents on mesomorphism is discussed.     

Halogen bonding-driven formation of supramolecular macrocycles and double helix

Halogen bonding has been used to hold two hydrogen bonded aromatic amide foldamers to form supramolecular macrocycles.     

Novel chiral liquid crystals based on amides and azo compounds derived from 2-amino-1,3,4-thiadiazoles: synthesis and mesomorphic properties

Novel chiral amides (Ia-Ie, II) and azo compounds (III, IV) with a 1,3,4-thiadiazole unit in the rigid core were synthesized and their liquid crystalline properties investigated by polarizing optical microscopy and differential scanning calorimetry. The amides Ia-Ie contain a chiral alkoxy chain derived from (R)-2-octanol and an achiral chain varying from 6 to 10 carbon atoms at the end of the rigid core. In amide II one of the terminal group is a chiral alkoxy chain derived from (S)-isoleucine and the other terminal substituent is an achiral n-decyloxy chain. Azo compounds III and IV contain an achiral n-decyloxy chain and a chiral alkoxy chain derived from (R)-2-octanol and (S)-isoleucine, respectively, at the end of the rigid core. The first homologue in the series of amides (Ia) exhibits enantiotropic smectic X (SmX)-chiral nematic (N*) dimorphism and the homologues Ib-Ie display enantiotropic SmX-chiral smectic C (SmC*)-N* mesomorphism. Amide II displays an enantiotropic smectic A phase. The azo compounds III and IV do not show smectic order and only an enantiotropic N* phase was observed. Thus, the mesomorphic behaviour depends on the nature of the central linkage and on the nature of the chiral alkoxy chain.     

Diazo liquid crystals for potential infrared applications

Nine series of liquid crystals containing an electron-donating group at one end of a conjugated system and an electron-withdrawing group at the other end have been synthesized. The electron-donating group is 4-n-alkylpiperazine and the electron-withdrawing group is the nitro group; and the conjugated system is diphenyldiazene with zero, one or two substituents on the phenyl ring not containing the nitro group. The substituents are-F,-Cl, and-CH₃. Most of the compounds synthesized are nematogenic and exhibit rather broad liquid crystalline ranges. The effects of the lateral substituents on the optical absorption and phase transition temperatures are correlated with their nature and position of substitution. Dielectric anisotropy and birefringence measurements were made using mixtures containing 10 wt % of synthesized liquid crystals dissolved in commercial mixture E7.     

Vacuum Ultraviolet Free-Electron Laser Photoionization Mass Spectrometry of Alpha-pinene Ozonolysis

\begin{document}$\alpha$\end{document}-pinene is the most abundant monoterpene that represents an important family of volatile organic compounds. Molecular identification of key transient compounds during the \begin{document}$\alpha$\end{document}-pinene ozonolysis has been proven to be a challenging experimental target because of a large number of intermediates and products involved. Here we exploit the recently developed hybrid instruments that integrate aerosol mass spectrometry with a vacuum ultraviolet free-electron laser to study the \begin{document}$\alpha$\end{document}-pinene ozonolysis. The experiments of \begin{document}$\alpha$\end{document}-pinene ozonolysis are performed in an indoor smog chamber, with reactor having a volume of 2 m\begin{document}$^3$\end{document} which is made of fluorinated ethylene propylene film. Distinct mass spectral peaks provide direct experimental signatures of previously unseen compounds produced from the reaction of \begin{document}$\alpha$\end{document}-pinene with O\begin{document}$_3$\end{document}. With the aid of quantum chemical calculations, plausible mechanisms for the formation of these new compounds are proposed. These findings provide crucial information on fundamental understanding of the initial steps of \begin{document}$\alpha$\end{document}-pinene oxidation and the subsequent processes of new particle formation.     

Investigation of artificial biomembrane systems in biopartitioning micellar chromatography by method of mathematical design

An attempt to create and study an artificial membrane system was realized via biopartitioning micellar liquid chromatography. Towards this end the known formula of membrane permeability (on the basis of Fick's diffusion equation) was modified so that membrane permeability may be estimated in terms of chromatographic characteristics. The two-factoral experiments on the basis of mathematical design of second order were carried out. The regression equations are derived which describe the dependence of membrane permeability on the concentration of polyoxyethylene (23) lauryl ether in the mobile phase and its flow-rate for compounds with biomedical significance. Some regularities were revealed, which characterize the permeability of compounds of the different nature through membranes. The extremal dependence (with passing through minimum) of permeability on the concentration of non-ionic surfactant was observed for anionic compounds. The increasing character of permeability in relation with flow-rate of mobile phase was recognized for cationic samples. Both dependences were basically fulfilled for zwitterionic compounds.     

Studying Stabilization of PVC by Differential Thermal Analysis

A method for studying the thermal stability of PVC compounds by using differential thermal analysis (DTA) is presented. At isothermal conditions and in an oxygen-free nitrogen atmosphere, an exothermal maximum is observed at the moment when all stabilizer is consumed. The time until this maximum appears is taken as a measure of stability of the tested sample. Earlier DTA investigations on the thermal stability of PVC were done by Dunn and Ennis. They needed the addition of a destabilizer, such as ferric or zinc oxide to the PVC compound to get an easily determinable exothermal maximum. This means that applications of their method to industrial PVC compounds or end products were impossible, as neither should contain any of these oxides. Probably due to more sensitive equipment it is now possible to obtain an exothermal maximum without any destabilizer. A stabilizer such as tribasic lead sulfate (TBLS), for example, is still needed though. Plasticized or rigid compounds may be investigated. High amounts of some fillers cause the maximum to broaden, and the evaluation becomes more difficult. The method is easy and time saving. The very small amount of sample required (a few milligrams only) makes the sample preparation possible for all kinds of products.     

Effect of N-methyl substitution on the thermal decomposition processes in aliphatic–aromatic polyamides

The thermal decomposition processes of two polyamides, derived from succinic acid and two aromatic diamines, were studied by direct pyrolysis mass spectrometry. Fast atom bombardment (FAB) mass spectrometry has been also used in order to provide additional information for the elucidation of the thermal degradation mechanism of the polymers investigated. FAB mass spectra, obtained by introducing in the FAB ion source the solid residues from polymer pyrolysis performed in thermogravimetric experiments, allowed the detection of diagnostic compounds up to about 1600 amu. Our results indicate that the thermal stability of the N-methyl-substituted polyamide is higher than that of the unsubstituted polyamide. The difference in the thermal degradation mechanism accounts for the difference in the thermal stability of the two polyamides. In fact, the unsubstituted polyamide decomposes via an intramolecular exchange and a concomitant N? H hydrogen transfer process with formation of compounds with amine and/or succinimide end groups. Instead, the N-methyl-substituted polyamide decomposes via an α C? H hydrogen transfer process from the methyl group to the nitrogen atom with formation of compounds with amine and/or 2,5-piperidinedione end groups.     

Diazo liquid crystals for potential infrared applications

Nine series of liquid crystals containing an electron-donating group at one end of a conjugated system and an electron-withdrawing group at the other end have been synthesized. The electron-donating group is 4-n-alkylpiperazine and the electron-withdrawing group is the nitro group; and the conjugated system is diphenyldiazene with zero, one or two substituents on the phenyl ring not containing the nitro group. The substituents are-F,-Cl, and-CH₃. Most of the compounds synthesized are nematogenic and exhibit rather broad liquid crystalline ranges. The effects of the lateral substituents on the optical absorption and phase transition temperatures are correlated with their nature and position of substitution. Dielectric anisotropy and birefringence measurements were made using mixtures containing 10 wt % of synthesized liquid crystals dissolved in commercial mixture E7.     

Highly conjugated 4-ring nematic liquid crystals with a lateral alkoxy branch

Three types of liquid crystalline compound containing a 4-ring mesogenic core with a lateral alkoxy chain on one of the inner rings were synthesized, and their mesogenic properties studied. The 4-ring core of these compounds bears an electron-accepting nitro group at one end and an electron-donating alkylamino moiety at or near the other end. Therefore, they are highly coloured and have lambda max 473 nm. One of these three types of compound has a wide enantiotropic nematic range. Twelve homologous analogues in this series with different lengths for the terminal alkyl chain and the lateral alkoxy chain were synthesized and compared.     

Synthesis and fluorescence of a series of multichromophoric acenaphthenyl compounds

A novel free radical trapping reaction based on a stepwise radical reversible addition-fragmentation mechanism has been utilized to synthesize a series of acenaphthenyl dimers and trimers. The synthetic procedure involves the reaction of acenaphthylene with dithiobenzoate compounds (S=C(Ph)-SR) in the presence of a free radical initiator followed by reduction of the dithiobenzoyl end group with tributyltin hydride. Stereoisomers of the compounds have been isolated and their structures determined by proton NMR and X-ray crystallography. The solution fluorescence of the compounds has been characterized to reveal the requirements for intramolecular excimer (excited-state dimer) formation. Only in compounds containing identical stereochemical arrangements of adjacent acenaphthenyl groups is excimer fluorescence observed following photoexcitation.     

Trends in structure-toxicity relationships for carbonyl-containing alpha,beta-unsaturated compounds

Using toxicity data for 30 aliphatic polarized alpha,beta-unsaturated derivatives of esters, aldehydes, and ketones, a series of six structure-toxicity relationships were evaluated. The structure feature of all assessed compounds, an acetylenic or olefinic moiety conjugated to a carbonyl group, is inherently electrophilic and conveys the capacity to exhibit enhanced toxicity. However, the toxic potency of alpha,beta-unsaturated carbonyl compounds is dependent on the specific molecular structure with several trends being observed. Specific observations include: (1) between homologues, the acetylenic-substituted derivative was more toxic than the corresponding olefinic-substituted one, respectively; (2) between olefinic-homologues, terminal vinyl-substituted derivative was more toxic than the internal vinylene-substituted one; (3) within alpha,beta-unsaturated ketones, methyl substitution on the vinyl carbon atoms reduces toxicity with methyl-substitution on the carbon atom farthest from the carbonyl group exhibiting the greater inhibition; (4) between alpha,beta-unsaturated carbonyl compounds with the carbon-carbon double bond on the end of the molecule (vinyl ketones) and those with carbon-oxygen double bonds on the end of the molecule (aldehydes), the ketones are more toxic than the aldehydes; (5) between homologues of alpha,beta-unsaturated esters, those with additional unsaturated moieties (allyl, propargyl, or vinyl groups) were more toxic than homologues having relevant unsaturated moieties (propyl or ethyl groups); (6) between alpha,beta-unsaturated carbonyl compounds with different shaped alkyl-groups (i.e. different degrees of branching), homologues with straight-chain hydrocarbon moieties were more toxic than those with branched groups.