Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their mixtures XIII. 4-(4-Substituted phenylazo)phenyl 4-alkoxybenzoates

Five homologous series of 4-(4-substituted phenylazo)phenyl 4-alkoxybenzoates (Ia-e) were prepared in which the substituent (X) was taken from CH₃O, CH₃, Cl, NO₂, and CN, while, within each homologous series, the length of the terminal alkoxy group varied between 6 and 16 carbon atoms. Compounds prepared were characterized by infrared spectroscopy, and their mesophase behaviour investigated by differential scanning calorimetry and polarizing optical microscopy. The results are discussed in terms of mesomeric and polarizability effects. In each group of compounds bearing the same alkoxy group, the mesophase - isotropic transition temperatures (T_c) were successfully correlated with the polarizability anisotropy of bonds to the substituent X.     

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their binary mixtures III. Binary mixtures of alkoxyphenyl arylates and arylalkoxybenzoates

4-Substituted phenyl-4-hexadecyloxybenzoates and 4-hexadecyloxyphenyl-4-substituted benzoates were prepared and thermally characterized by differential scanning calorimetry. Transition temperatures were identified by polarized-light microscopy. Substituents were chosen to cover a wide range of electronic and dipolar characteristics. Binary mixtures of different components were prepared and similarly characterized to construct their phase diagrams. In mixtures of the 4-CN and 4-NO2 analogues, enhanced SmA mesophases were found when either of these esters was mixed with one of the methoxy analogues. A weak molecular complex is proposed to account for this non-ideal solution behaviour. Linear or slightly enhanced mixture behaviours were found for the nitro or cyano derivatives when mixed with each other. The results are interpreted in terms of electronic, dipolar, and molecular interactions.     

Synthesis and thermal reaction of 2-[4-(4-ethynylphenoxy)phenylene]-3-phenylquinoxaline

2-[4-(4-Ethynylphenoxy)phenylene]-3-phenylquinoxaline was prepared by two routes and was thermally reacted at various temperatures. The intermediate reactants, the acetylene containing phenylquinoxaline, and its thermally induced reaction product were characterized by high pressure liquid chromatography and mass spectroscopy. The principal component in the 350° reaction product was a dimer from addition of an acetylene group to another.     

The synthesis of 1-biphenyl-4-alkyl-[1,2,3]-triazoles and their mesomorphic behaviour

A series of 1-biphenyl-4-alkyl-[1,2,3]-triazole-based liquid crystal compounds were synthesised via a fast and efficient microwave mediated ‘Click Reaction’ and their phase behaviour is presented. Most of the 1-(3-fluorobiphenyl)-4-alkyl-[1,2,3]-triazole compounds exhibited a smectic A phase with relatively low transition temperatures compared to previously examined aryl-triazole-based mesogens. Small angle X-ray scattering measurement indicated the existence of deep smectic phase for some cases. Structural variations gave rise to different phase behaviour in these triazole-based compounds.     

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their binary mixtures VI[1]. The effect of molecular length

The apparent solution dipole moments of compounds based on 4,4-di-substituted phenyl benzoate (ROC6H4COOC6H4X), I, where X is a cyano group and R a terminal linear alkyl chain ranging from C12 to C20, were determined in cyclohexane at 30 C. The compounds were also thermally characterized by polarized light microscopy and differential scanning calorimetry. Phase diagrams were constructed for various binary mixtures prepared from any two homologues of series I as well as for every one of them with the nitro analogue II, where X is a nitro group and R C16H33. The study was undertaken in order to investigate the effect of the alkyl chain length on the mesophase behaviour. In order to assess the influence of structural variation in the central mesogenic group on the mesophase stability of pure and mixed compounds, the investigation was extended to cover binary mixtures of any two of the three compounds: analogue II (X=OC16H33), and the symmetric dialkoxy substituted phenyl esters (C16H33OC6H4COO)2 A, where A is the 1,4-phenylene group (IV) or 4,4-biphenylene (V).     

Rigid rod molecules as liquid crystal thermosets. II. Rigid rod esters

Nine rigid rod ester monomers endcapped with maleimide, nadimide, and methylnadimide groups were prepared and studied by DSC and hot stage polarized light microscopy. All of the monomers showed thermotropic nematic liquid crystalline phases and could be thermally crosslinked in the nematic phase. The nematic texture was maintained in the crosslinked solid state. The nematic phase range was enlarged by B-staging the monomeric compounds. Heating the monomers for a short period of time in the nematic phase lowered the crystal to nematic transition temperatures and increased the nematic to isotropic transition temperatures. A nonequilibrium phase diagram was proposed to explain the melting behavior of these reactive liquid crystal thermoset materials.     

Synthesis and study of 4-(4'-n-alkoxybenzoyloxybenzoyl)-4'-n-butoxyanilides

Thirteen compounds with ester and amide linkages were synthesized and their mesogenic properties evaluated. Methyl to n-propyl derivatives exhibit nematic phases, n-butyl to n-decyl derivatives exhibit smectic and nematic mesophases, whereas n-dodecyl to n-octadecyl derivatives exhibit only smectic phases. All the smectic homologues exhibit smectic C phases. Middle members of the homologous series exhibit polymorphism of smectic mesophase. A plot of transition temperatures versus number of carbon atoms in the alkoxy chain reveals an odd-even effect for nematic-isotropic transition temperatures. Nematic-isotropic and smectic-cholesteric thermal stabilities of the prepared compounds (series I) are higher compared to those of previously reported compounds, series A, B and C. The results indicate that a simple reversal of a central linkage has a dramatic effect on the appearance of smectic mesophase in a homologous series. The structures of the synthesized compounds were characterized using elemental analysis, thin-layer chromatography and spectral data.     

The synthesis and mesomorphic behaviour of the 1,2,4,5-tetrasubstituted benzenes with (4-tridecyloxyphenyl)ethynyl and (4-tridecyloxyphenyl)carbonyloxy substituents

The synthesis, mesophase morphology and transition temperatures of a series of 1,2,4,5-tetrasubstituted benzenes are described. The compounds have (4-tridecyloxyphenyl)carbonyloxy or (4-tridecyloxyphenyl)ethynyl as the peripheral substituents in all possible combinations ranging from the tetra-ester to the tetra-alkyne. Both the substituents used have similar structural dimensions but the greater rigidity of the alkynyl unit ensures that it projects more directly from the central core than does the more flexible ester unit. All of the seven compounds are nematogenic and five show a smectic C phase; the differences observed in the transition temperatures of the compounds are discussed in terms of molecular shape and the relationship of the compounds to appropriate calamitic and discotic systems. Molecular modelling indicates that pi-pi interactions of peripheral phenyl groups can lead to molecules with calamitic shapes.     

Preparation and mesomorphic properties of a homologous series of N,N'-(2-chloroterephthalylidene)bis(4-n-alkylaniline)s

A series of N,N'-(2-chloroterephthalylidene)bis(4-n-alkylaniline)s (CTBAAs) were prepared and their mesomorphic behaviour characterized. None of the homologues showed smectic behaviour, but nematic behaviour with broad mesophases and rather low melting points was seen. The mesomorphic behaviour was compared with that of N,N'-(terephthalylidene) -bis(4-n-alkylaniline)s (TBAAs) and 4,4-bis-(n-alkylbenzal)-2-chloro-1,4-phenylenediamines (BACPDs). An X-ray diffraction analysis of N,N'-(2-chloro-terephthal)bis(4-n-butylaniline) (CTBBA) showed that the molecules were packed in crystals forming a staggered arrangement called an 'imbricated structure'.     

The synthesis and transition temperatures of 5-(4-alkyl- and 4-alkoxy-phenyl)-2-cyanobenzo[b]furans and a 5-(4'-alkylbiphenyl-4-yl)-2-cyanobenzo[b]furan: a comparison with their biphenyl and terphenyl analogues

The synthesis and transition temperatures of a series of 5-(4-alkyl- and 4-alkoxy-phenyl)2-cyanobenzo[b]furans and a 5-(4'-alkylbiphenyl-4-yl)-2-cyanobenzo[b]furan are presented. The 2-cyanobenzo[b]furans show similar mesophase types to the analogous biphenyl and terphenyl compounds, which are obtained by replacing the benzo[b]furan unit with a phenyl ring. The transition temperatures for the 2-cyanobenzo[b]furan compounds are always higher than for their biphenyl and terphenyl counterparts, but they are much lower than for the corresponding phenylnaphthalenes. Five mesogenic benzo[b]furans without a cyano group were prepared as intermediates and these compounds have lower clearing points than their biphenyl analogues.     

The mesomorphic anomaly of 4-[2-(4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-pentylcyclohexyl-1-carboxylates

Four homologous series of 4-[2-[4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-alkylcyclohexyl-1-carboxylates have been prepared. Their liquid crystalline behaviour was investigated by optical polarizing microscopy and DSC. Series A, B and C exhibit the nematic phase. The phase transition behaviour of series D is anomalous. The compounds of series D with a short alkoxy chain exhibit enantiotropic smectic A and nematic phases, while those with a long alkoxy chain exhibit only the nematic phase.     

The mesomorphic anomaly of 4-[2-(4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-pentylcyclohexyl-1-carboxylates

Four homologous series of 4-[2-[4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-alkylcyclohexyl-1-carboxylates have been prepared. Their liquid crystalline behaviour was investigated by optical polarizing microscopy and DSC. Series A, B and C exhibit the nematic phase. The phase transition behaviour of series D is anomalous. The compounds of series D with a short alkoxy chain exhibit enantiotropic smectic A and nematic phases, while those with a long alkoxy chain exhibit only the nematic phase.     

Manipulating Electron-Transfer Events in [Fe4Co4] Cubes via a Mixed-Ligand Approach: The Impact of Elastic Frustration

The engineering of intermolecular interaction is challenging but critical for magnetically switchable molecules. Here, we prepared two cyanide-bridged [Fe₄Co₄] cube complexes via the alkynyl- and alcohol-functionalized trispyrazoyl capping ligands. The alkynyl-functionalized complex 1 exhibited a thermally-induced incomplete metal-to-metal electron transfer (MMET) behaviour at around 220 K, while the mixed alkynyl/alcohol-functionalized cube of 2 showed a complete and abrupt MMET behaviour at 232 K. Remarkably, both compounds showed a long-lived photo-induced metastable state up to 200 K. The crystallographic study demonstrated that the incomplete transition of 1 was likely due to the possible elastic frustration originating from the competition between the anion-propagated elastic interactions and inter-cluster alkynyl-alkynyl & CH-alkynyl interactions, whereas the latter are eliminated in 2 as a result of the partial substitution by the alcohol-functionalized ligand. Additionally, the introduction of chemically distinguishable cobalt centers within the cube unit of 2 did not lead to a two-step but a one-step transition, possibly because of the strong ferroelastic intramolecular interaction through the cyanide bridges.     

Synthesis, characterization and thermal degradation of functional benzoxazine monomers and polymers containing phenylphosphine oxide

Three phosphorus-containing bisphenol compounds, bis(4-hydroxyphenyl)phenylphosphine oxide (BHPPO), bis(4-hydroxyphenoxyphenyl)phenylphosphine oxide (BPPPO), and bis(4-hydroxyphenoxy)phenylphosphine oxide (BPHPPO) have been synthesized as starting materials for the synthesis of benzoxazine monomers. Benzoxazine monomers containing phenylphosphine oxide have been prepared and subsequently characterized by FT-IR and ¹H NMR. The monomers are thermally initiated and polymerized via ring-opening polymerization. Thermogravimetric analysis indicates that phosphorylation can have a profound effect on increasing char yield and on thermal degradation temperatures.     

The synthesis and transition temperatures of 5-(4-alkyl- and 4-alkoxy-phenyl)-2-cyanobenzo[b]furans and a 5-(4'-alkylbiphenyl-4-yl)-2-cyanobenzo[b]furan: a comparison with their biphenyl and terphenyl analogues

The synthesis and transition temperatures of a series of 5-(4-alkyl- and 4-alkoxy-phenyl)2-cyanobenzo[b]furans and a 5-(4'-alkylbiphenyl-4-yl)-2-cyanobenzo[b]furan are presented. The 2-cyanobenzo[b]furans show similar mesophase types to the analogous biphenyl and terphenyl compounds, which are obtained by replacing the benzo[b]furan unit with a phenyl ring. The transition temperatures for the 2-cyanobenzo[b]furan compounds are always higher than for their biphenyl and terphenyl counterparts, but they are much lower than for the corresponding phenylnaphthalenes. Five mesogenic benzo[b]furans without a cyano group were prepared as intermediates and these compounds have lower clearing points than their biphenyl analogues.     

Synthesis and characterization of benzocyclobuten-4-yl acrylate monomer and its radical polymerization

<正>A benzocyclobuten-4-yl acrylate(1) monomer was prepared by esterification of 4-hydroxybenzocyclobutene with acryloyl chloride.The radical homopolymerization of 1 and copolymerization of 1 with styrene or n-butyl acrylate were carried out to produce linear polymers 2a,2b and 2c.Heating of these linear polymers under thermal initiation gave corresponding cross-linked polymers 3a,3b and 3c.The ring-opening reaction in the cross-linking process was confirmed by on-line infrared spectra.Differential scanning calorimetry showed that the glass transition temperatures of linear polymers 2a and 2b were 83.2℃and 68.1℃,respectively.Thermogravimetric analysis of the cross-linked polymers showed that they all exhibited good thermal stability.     

The synthesis and transition temperatures of some ferroelectric host materials based on 4- and 4'-(trans-4-alkylcyclohexylmethoxy)-2,3-difluorobiphenyls

A range of 2,3-difluoro-substituted biphenyls incorporating a cyclohexylmethoxy unit as one terminal chain has been prepared and these materials are compared to the analogous compounds already reported with a dimethylene linking group. Related, directly-linked cyclohexylbiphenyls and some open-chain analogues have also been prepared which add to the diversity of the compounds reported. These were prepared as host materials of low birefringence for ferroelectric (S*_c) mixtures and in general, the compounds have quite high S_c phase stability. Metallation procedures have been used to prepare arylboronic acids which were employed in palladium-catalysed cross-coupling reactions to provide good yields of liquid crystal materials. The synthesis and transition temperatures of these compounds are discussed and compared with each other and with other selected ortho-difluoro-substituted systems.     

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their mixtures XIV. Binary mixtures of 4-(4-substituted phenylazo) phenyl-4-alkoxybenzoates

Binary mixtures formed from components of the five homologous series of the 4-(4-substituted phenylazo)phenyl-4-alkoxybenzoates Ia-e were prepared and their mesophase behaviour characterized. Transition temperatures of the mixtures prepared were measured by differential scanning calorimetry and identified by polarizing optical microscopy. Each binary combination was made from components bearing a terminal alkoxy group with the same number of carbon atoms (kept constant at n = 6, 8, 10, 12, 14 or 16), while the other substituent (X) was different—CH₃O, CH₃, Cl, NO₂, or CN. Phase diagrams were constructed for the various systems in order to investigate the effect of the terminal substituent X, as well as of the alkoxy chain length, on the phase behaviour of mixed systems.     

Synthesis and photoluminescence of linear and hyperbranched polyethers containing phenylquinoxaline units and flexible aliphatic spacers

Ultrahigh‐molecular‐weight linear polyethers were prepared through a reaction between the phenylquinoxaline monomers 2,3‐bis(4‐hydroxyphenyl)‐6‐fluoroquinoxaline and 2,3‐bis(4‐hydroxyphenyl)‐6‐(α,α,α‐trifluoromethyl)quinoxaline and 1,12‐dibromododecane. A new hyperbranched polyether containing a phenylquinoxaline moiety was also prepared from a new self‐polymerizable AB₂ monomer, 2,3‐bis(6‐bromohexyloxyphenyl)‐6‐(4‐hydroxyphenyloxy)quinoxaline. All the polyethers were amorphous and soluble in polar aprotic solvents. Their solution‐cast thin films were light yellow, ductile, and optically transparent. The polymers were thermally stable up to 350 °C and had glass‐transition temperatures in the range of 25–83 °C, which depended on the architecture and monomer structure. The monomers and polymers displayed fluorescence maxima in the blue‐light region in the range of 431–449 nm with relatively narrow peak widths; this indicated that they had pure and intense fluorescence. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3587–3603, 2004     

Trimeric liquid crystals containing lateral alkyl chains

The first twelve members of the n-alkyl 2,5-bis-(4'-cyanobiphenyloxybutyloxy)-benozates have been prepared. The compounds are nematogenic; in addition the first three members exhibit a smectic A phase. The nematic-isotropic transition temperatures decrease with little or no alternation on increasing the number of methylene groups in the lateral chain; this is analogous to the behaviour found in monomeric and polymeric mesogens possessing lateral alkyl chains.