Search

Although the impact of individual functional groups on the self-assembly of columnar liquid crystal phases has been widely studied, the effect of varying multiple substituents has received much less attention. Herein, we report a series of dibenzo[a,c]phenazines containing an alcohol or ether adjacent to an electron-withdrawing ester or acid. With one exception, these difunctional mesogens form columnar phases. The phase behavior appeared to be dominated by the electron-withdrawing substituent; transition temperatures were similar to derivatives with these groups in isolation. In most instances, the addition of an electron-donating group ortho to an ester or acid suppressed the melting temperature and elevated the clearing temperature, leading to broader liquid crystal thermal ranges. This effect was more pronounced for derivatives functionalized with longer chain hexyloxy groups. These results suggest a potential strategy for controlling the phase ranges of columnar liquid crystals and achieving room temperature mesophases. Key words: supramolecular chemistry, liquid crystal, physical organic chemistry, self-assembly. Bien que l'effet des groupes fonctionnels, pris individuellement, sur l'autoassemblage des phases colonnaires de cristaux liquides ait ete largement etudie, l'effet d'une variation de plusieurs substituants a la fois a beaucoup moins retenu l'attention. Dans le present article, nous presentons une serie de dibenzo[a,c]phenazines comportant un substituant alcool ou ether adjacent a un groupe electroattracteur ester ou acide carboxylique. Tous ces mesogenes bifonctionnalises, sauf un, forment des phases colonnaires. Le comportement des phases semble etre domine par le substituant electrodonneur; les temperatures de transition etant comparables a celles de derives dans lesquels ces groupes sont isoles. Dans la plupart des cas, l'ajout d'un substituant electrodonneur en position ortho par rapport a un groupe ester ou acide a elimine la temperature de fusion et a eleve la temperature de clarification, ce qui a donne lieu a des plages de temperatures elargies de l'etat cristal liquide. Cet effet etait plus marque pour les derives portant des groupes hexylhydroxy, dont la chaine est plus longue. Ces resultats laissent entrevoir une strategie pour moduler les plages thermiques des phases colonnaires des cristaux liquides et acceder a des mesophases a temperature ambiante. [Traduit par la Redaction] Mots-cles: chimie supramoleculaire, cristal liquide, chimie organique physique, autoassemblage., Introduction James Wuest has asserted that understanding molecular crystallization is one of the 'great unmet challenges of contemporary science.' (1) Although posed in the context of solids, this statement applies [...]

A series of [PcMnL2]SbF6 complexes (Pc = phthalocyanine) was synthesized and structurally characterized by stripping the chloride from PcMnCl with AgSbF6 in o-dichlorobenzene and adding a range of donor ligands (L = THF, pyridine, p-dimethylaminopyridine (DMAP), Ph3PO, N-methylimidazole (MeIm), MeCN) to the resulting solution. Addition of or exposure to water where L = heterocyclic amines yielded μ-oxo complexes of the form [PcMnL]2O, which were structurally characterized for L = DMAP and MeIm. The [PcMnL2]SbF6 complexes have an increased solubility in organic solvents, where the axial ligands inhibit the characteristic ring π−π aggregation of PcM complexes. A variety of colours were observed (blue/green to red/purple), with Q-band absorptions (excluding the μ-oxo species) spanning from 715–761 nm and LMCT-bands from 497–574 nm. The combination of the ligand-induced absorption shifts coupled with their relative intensities in the visible region is responsible for the observed colour range and illustrates that facile ligand exchange is a useful tool in producing materials with a variety of colours from PcMnCl. [ABSTRACT FROM AUTHOR]

Three series of dibenzo[a,c]phenazines were prepared in an order to assess the impact of side chain position on the phase stability of columnar liquid crystals. Each series was composed of four isomeric compounds differing only in the disposition of hexyloxy and decyloxy chains around the central aromatic core, giving rise to electronically similar compounds with varying shapes and symmetries. The substitution pattern was found to have a moderate effect on the clearing transition of the liquid crystal but a larger impact on the melting temperatures. These observations suggest a viable strategy for controlling the phase range of liquid crystals via judicious choice of peripheral chain structure and location. Key words: liquid crystals, self-assembly, symmetry, organic synthesis. Nous avons prepare trois series de dibenzo[a,c]phenazines en vue d'evaluer les effets de la position de la chaine laterale sur la stabilite de phase de cristaux liquides colonnaires. Chaque serie comptait quatre composes isomeriques qui se distinguaient uniquement par la disposition des chaines hexyloxy et decyloxy autour du noyau aromatique central, ce qui donnait lieu a des composes similaires sur le plan electronique et dont la forme et la symetrie variaient. Nous avons observe que l'arrangement des substituants avait un effet modere sur la transition nematique-isotrope du cristal liquide, mais un effet plus important sur les temperatures de fusion. Ces observations permettent d'envisager une strategie pratique permettant de moduler l'intervalle de transition de phase des cristaux liquides par un choix judicieux de la structure et de la position des chaines peripheriques. [Traduit par la Redaction] Mots-cles : cristaux liquides, autoassemblage, symetrie, synthese organique., Introduction The unique ability of liquid crystals (LCs) to simultaneously exhibit both order and fluidity has been exploited in an enormous variety of applications, including display devices, (1,2) holography, (3) [...]

Cyclodextrins (CDs) constitute a unique class of D -glucose-based macrocycles that possess a truncated cone-shaped central cavity. They are non-toxic and biodegradable. The presence of pseudo face-to-face symmetry in the native CDs represents a distinctive advantage to design amphiphilic materials capable of self-assembly into liquid crystals. In this work, a new family of amphiphilic β-CD derivatives possessing 14 stearoyl chains (non-polar) and 7 functionalized tetraethylene glycols were synthesized using an improved design and more efficient chemistry, and the synthetic targets showed excellent ability to form stable hexagonal columnar mesophases over wide temperature ranges. We demonstrated that the polarity and sizes of introduced functional groups can further affect the temperature ranges of formed LC phases. Moreover, variable temperature solid-state nuclear magnetic resonance spectroscopy studies of a lithium composite with compound 2 revealed fast local Li-ion exchange processes with very low activation energies, suggesting the benefit of using these materials as potential electrolytes for high ionic conductions. The results from this work can provide guidance for the design of future generations of CD-based LC materials for ion conduction. [ABSTRACT FROM AUTHOR]

The photochromism of two 2,3-diarylbenzo[g]quinoxaline derivatives was explored. Irradiating 2,3-diphenylbenzo[ g]quinoxaline with 350 nm light led to the formation of its photodimer, which could be reverted to its monomers either thermally or photochemically. The resulting photodimer absorbs at a longer wavelength than analogous dianthracene derivatives. 2,3-Dipyridylbenzo[g]quinoxaline was also observed to undergo photodimerization. The ability of these readily prepared benzoquinoxaline derivatives to undergo [4 + 4]-photocycloaddition reactions makes them attractive alternatives to anthracene derivatives. Key words: cycloaddition, photochromism, benzoquinoxaline. On a etudie la photochromie de deux derives de 2,3-diarylbenzo[g]quinoxaline. L'irradiation de la 2,3-diphenylbenzo[ g]quinoxaline avec de la lumiere a 350 nm conduit a la formation de son photodimere qui peut revenir a sa forme monomere d'une facon soit thermique ou photochimique. Le photodimere qui en resulte absorbe a une longueur d'onde plus grande que celle des derives dianthracenes analogues. On a observe que la 2,3-dipyridylbenzo[g]quinoxaline subit aussi une photodimerisation. L'habilite de ces derives de la benzoquinoxaline facilement accessibles a subir des reactions de [4 + 4]-photocycloaddition en font des alternatives attrayantes aux derives de l'anthracene. Mots-cles: cycloaddition, photochromie, benzoquinoxaline. [Traduit par la Redaction], Introduction Since the discovery of anthracene photochromism in the 19th century, (1) extensive studies have been carried out on the photodimerization of this and related molecules. (2,3) Although these reversible [...]

A large number of disc-shaped molecules are examined to elucidate how changes in the core structure alter the propensity of such compounds to self-assemble into columnar liquid crystalline phases. The results reveal that molecules with larger aromatic cores have a greater propensity to form columnar phases, as do compounds substituted with electron-withdrawing groups.

A series of highly reactive cyclopentadienones were prepared in situ from the corresponding hydroxycyclopent-2-enones and trapped with a variety of quinones. Reaction of 1,4-naphthoquinone with 4-hydroxy-3,4-diphenyl-cyclopent-2-enone afforded 2,3-diphenylanthraquinone, whereas reaction of benzoquinone with this same cyclopentadienone precursor yielded a mixture of 6,7-diphenyl-1,4-naphthoquinone and 2,3,6,7-tetraphenylanthraquinone. A number of other 2,3-diarylanthraquinones were likewise prepared in moderate yields from the reaction of 1,4-naphthoquinone with the appropriate 4-hydroxy-3,4-diarylcyclopent-2-enones. This method appears to be generally applicable to the synthesis of anthraquinone derivatives substituted at the 2- and 3-positions from inexpensive starting materials. Key words: anthraquinone, Diels-Alder, cyclopentadienone, in situ. On a prepare in situ une serie de cyclopentadienones tres reactives a partir des hydroxycyclopent-2-enones correspondantes et on les a piegees a l'aide d'une variete de quinones. La reaction de la 1,4-naphtoquinone avec la 4-hydroxy-3,4-diphenylcyclopent-2-enone conduit a la formation de 2,3-diphenylanthraquinone alors que la reaction de la benzoquinone avec la meme cyclopentadienone conduit a un melange de 6,7-diphenyl-1,4-naphtoquinone et de 2,3,6,7-tetraphenylanthraquinone. On a prepare un certain nombre d'autres 2,3-diarylanthraquinones avec des rendements moderes par reaction de la 1,4-naphtoquinone avec des 4-hydroxy-3,4-diarylcyclopent-2-enones appropriees. Le domaine d'application de cette methode semble general pour la synthese de derives de l'anthraquinone substitues dans les positions 2 et 3, a partir de produits de departs peu couteux. Mots cles : anthraquinone, Diels-Alder, cyclopentadienone, in situ. [Traduit par la Redaction], Introduction If you learn nothing else from this class, I want you to remember: 'Enolates are nucleophiles.'--Walter Szarek, speaking to his 3rd year organic chemistry class in 1991. Anthraquinones are [...]

Research was conducted to examine the role of dispersion and electrostatic forces on solute-solvent interactions in a nematic liquid crystal phase. A series of 23 substituted biphenyl, phenylpyridine and bipyridine derivatives were doped in the nematic liquid crystal host. Results indicated that charge-transfer complexation and molecular dipole-dipole interactions do not have a significant effect on the stability of arene-arene interactions in the nematic phase.

A novel chiral dopant is synthesized in 11 steps from 4,4'-dihydroxy-2,2'-dimethyl-6,6'-dinitrobiphenyl to give 9% yield. It induces a ferroelectric Sc* phase in two Sc liquid crystal hosts via spontaneous polarization. It induces positive polarization in the presence of cyclohexanecarbonitriles and negative polarization in the presence of phenyl benzoate. The polarization power of the dopant is dependent on the nature of the Sc host.

A molecular orbital computational study was carried out on cis-1,3-diarylcyclohexanes to quantify pi-stacking of the aromatic rings from the corformational equilibria shifts. The calculations from Hammett correlations, electrostatic potentials, and molecular orbital energy gaps conformed with experimental results. Arene-arene pi-stacking is governed more by electrostatic than charge-transfer interactions. The closest correlation is detected on the pi-face of individual component arenes by electrostatic potentials measured from the electron density surface.

The effects of functional group position on the phase behavior of discotic mesogens are examined for a series of dibenzophenazine derivatives bearing a carboxylic acid, methyl carboxylate or nitro group. The derivatives have formed [pi]-stacked dimers in solution and the structure of these dimers are consistent with the proposed structure of the columnar phases.

The viability of a new strategy for enforcing formation of the cross-cyclomers in the [4 + 4]-photocycloaddition reaction of anthracene derivatives by exciting only one component of a binary mixture is demonstrated. The strategy is much less restrictive than previous examples of selective cross-cycloadditions, and works either when the excitation wavelengths is blue- or red-shifted relative to S0 -> S1 absorption bands of anthracene.

In an effort to control the phase ranges of highly ordered smectic phases, we examined the impact of molecular symmetry on phase behaviour of a series of 12 symmetrical and unsymmetrical 4,4′-dialkanoyloxybiphenyl derivatives. Combined differential scanning calorimetry, polarised optical microscopy, and X-ray diffraction studies indicated that the compounds studied formed smectic F liquid crystals, and in some cases, G phases at lower temperatures. Although the clearing temperatures were largely unaffected by molecular symmetry, the transitions from the SmF liquid crystals to more ordered phases were consistently lowered upon reducing the molecular symmetry. As a result, unsymmetrical molecules had broader mesophases than their higher symmetry isomers, suggesting a strategy for tuning the phase behaviour of these highly ordered lamellar phases, which have been widely targeted for organic semiconductors. [ABSTRACT FROM AUTHOR]

A novel family of amphiphilic cyclodextrin(CD)-based liquid crystals that bear O-acetylated oligoethylene glycol chains at the secondary face is reported. Unlike most of the previously reported liquid crystals (LC) based on chemically modified CDs, which depend on H-bonding as the primary intermolecular forces, the present CD derivatives self-assemble into highly ordered smectic liquid crystal phases via the weaker dipole- dipole intermolecular interactions. The obtained materials are found to display much improved properties such as improved thermostability, reduced clearing temperatures, and better fluidity. The present work opens up new possibilities to design CD-based LC materials. [ABSTRACT FROM AUTHOR]

A series of PbII materials containing the highly anisotropic tridentate ligand 2,6-bis(benzimidazol-2-yl)pyridine (bbp) and [Au(CN)2]-, Hg(CN)2 or halide bridging units, [Pb(bbp)- Br2]·2H2O, Pb(bbp)I2, [Pb(bbp)I]2(μ-I)[Au(CN)2], [Pb(bbp)Br]2 (μ-Br)[Au(CN)2], [Pb(bbp)Br2][Hg(CN)2], Pb(bbp)Cl[Au(CN)2]· 0.25H2O and Pb(H2O)(bbp)2[Au(CN)2]2, have been prepared, structurally characterised and investigated spectroscopically by 207Pb solid-state NMR spectroscopy. Of these, five possess similar coordination geometries, providing an ideal series to investigate structural parameters as indicators for stereochemical activity by correlation with the 207Pb chemical shift span parameter (Ω). The bond length of the substituent opposite to the stereochemically active lone pair and the angle between axial ligands were found to correlate weakly with Ω for these compounds (R² = 0.85 and 0.66, respectively) and an expanded set of compounds with varying components and structures (R² = 0.60 and 0.67, respectively), which indicates that these (or similar) structural parameters or Ω can be used to estimate the degree of lone-pair activity. Compounds were also found to be highly birefringent with values ranging from 0.31(5) for Pb(H2O)(bbp)2[Au(CN)2]2 to 0.660(4) for [Pb(bbp)Br2]·2H2O, the latter being amongst the most birefringent compounds reported. [ABSTRACT FROM AUTHOR]