Hydrogen-bonded polycatenar mesogens

The tricatenar, hydrogen-bonded complexes formed between alkoxystilbazoles and an imine were found to be mesomorphic. Subsequently, related tetracatenar complexes were also investigated and found to be non-mesomorphic. We report the transition temperatures of these complexes and discuss structural modifications.     

Hydrogen-bonded polycatenar mesogens

The tricatenar, hydrogen-bonded complexes formed between alkoxystilbazoles and an imine were found to be mesomorphic. Subsequently, related tetracatenar complexes were also investigated and found to be non-mesomorphic. We report the transition temperatures of these complexes and discuss structural modifications.     

Asymmetric oxadiazole mesogens as candidates for low-temperature biaxial nematics

We report the synthesis and properties of a homologous series of asymmetric, mesogenic derivatives of 2,5-bis-(p-hydroxyphenyl)-1,3,4-oxadiazole (ODBP). Benzyloxy (BnO-), alkoxy benzoate ester derivatives BnO-ODBP-Ph-O-C _n , where n?=?4, 5, 6, 7, 8, 9, 12, were studied by ²H-NMR, X-ray diffraction and polarising microscopy in order to ascertain if the lower temperature nematic phases exhibited biaxiality. Deuterium nuclear magnetic resonance (NMR) of labelled probes in these asymmetric mesogens does not show evidence of biaxiality. The absence of biaxiality is discussed in terms of the statistical non-linearity of these asymmetric ODBP mesogens.     

Chiral and orientationally ordered fluid mesophases formed by oxadiazole bisaniline based achiral bent mesogens

Development of new liquid crystalline materials exhibiting interesting properties and phases continues to be an enabling enterprise in the forward march of their successful display and non-display applications. The design and synthesis of a homologous series of liquid crystalline bent-core compounds derived from the oxadiazole bisaniline moiety and the phase behavior of three members of the series that exhibit nematic, smectic C, and dark conglomerate phases is reported. The liquid crystalline phases exhibited by these mesogens are characterized using polarized optical microscopy, differential scanning calorimetry and x-ray scattering techniques. All three homologs prepared exhibit the nematic phase. Interestingly, the homolog with short hexyl terminal chains exhibits only the nematic phase that is stable over a very broad, nearly 100 K wide, temperature range. The compound with terminal octyl chains shows the chiral dark conglomerate phase below the nematic phase despite the bent molecules being achiral. The homolog with dodecyl alkyl chains is found to possess the smectic-C and two additional lamellar phases besides the nematic phase. These compounds enrich the library of achiral bent-core materials capable of exhibiting chiral and nematic phases.     

Hydrogen-bonded block mesogens derived from semiperfluorinated benzoic acids and the non-mesogenic 1,2-bis(4-pyridyl)ethylene

Thermal properties of benzoic acids carrying one or two semiperfluorinated alkoxy tails on the aromatic core have been investigated in binary mixtures with the non-liquid crystalline bidirectional trans-1,2-bis(4-pyridyl)ethylene. The hydrogen bonded complexes built from the complementary molecular species show a significantly enhanced mesophase stability compared with the fluorinated acids in their pure states. The mesophase morphologies of the complexes are governed mainly by the number of the partially fluorinated chains grafted to the acid component. Mixed systems comprising the one-chain acids exhibit a smectic C phase followed by a smectic A phase at more elevated temperatures. Incorporation of a second semiperfluorinated chain into the acid leads to the formation of columnar mesophases. These columnar phases of the H-bonded complexes should represent ribbon phases resulting from the collapse of the smectic layers.     

Hydrogen-bonded dihydrotetraazapentacenes

Three new members of N-heteropentacenes explored herein have adjacent pyrazine and dihydropyrazine rings at one end of the pentacene backbone. Interesting findings from this study include self-complementary N-H···N H-bonds in the solid state, solvent-dependent UV-vis absorption caused by H-bonding, and new p-type organic semiconductors with field effect mobility up to 0.7 cm(2) V(-1) s(-1).     

Hydrogen-bonded complexes of lumichrome

Hydrogen bonds were shown to play an important role in the lumichrome photophysics and photochemistry both in solutions and in the solid state. In solutions, lumichrome can form hydrogen-bonded complexes with a variety of molecules, such as acetic acid or methanol, as supported by spectral and equilibrium studies. Photoexcitation of some hydrogen-bonded complexes, having appropriate configuration, as in the case of acetic acid, may lead to excited-state proton transfer, resulting in formation of the isoalloxazinic structure, detectable by its characteristic emission, distinct from that of the intrinsically alloxazinic lumichrome. Theoretical calculations confirmed the role of the hydrogen-bonded complexes, yielding several stable eight-membered cyclic structures of such complexes characterized by spectral changes similar to those observed experimentally. Hydrogen bonds play an essential role in the formation of the lumichrome crystal structure, as follows from the X-ray diffraction results. Interestingly, the crystals studied included molecules of methanol used as solvent in crystal growth. The emission studies of polycrystalline samples, similar to the processes occurring in solutions, point to the importance of hydrogen-bonding interactions in crystal packing allowed by the symmetry of the hydrogen-bonded dimers.     

Hydrogen-bonded supramolecular polymer networks

The strong dimerizing, quadruple hydrogen-bonding ureido-pyrimidone unit is used to obtain reversible polymer networks. A new synthetic route from commercially available starting materials is described. The hydrogen-bonding ureido-pyrimidone network is prepared using 3(4)-isocyanatomethyl-1-methylcyclohexyl-isocyanate (IMCI) in the regioselective coupling reaction of multi-hydroxy functionalized polymers with isocytosines. ¹H- and ¹³C-NMR, IR, MS, and ES-MS analysis, performed on a model reaction using butanol, demonstrated the formation of the hydrogen-bonding ureido-pyrimidone unit in a yield of more than 95%. The well-defined, strong hydrogen-bonding ureido-pyrimidone network is compared with a traditional covalently bonded polymer network, a multi-directional hydrogen-bonded polymer network based on urea units, and a reference compound. The advantage of the reversible, hydrogen-bonded polymer networks is the formation of the thermodynamically most favorable products, which show a higher “virtual” molecular weight and shear modulus, compared to the irreversible, covalently bonded polymer network. The properties of the ureido-pyrimidone network are unique; the well-defined and strong dimerization of the ureido-pyrimidone unit does not require any additional stabilization such as crystallization or other kinds of phase separation, and displays a well-defined viscoelastic transition. The ureido-pyrimidone network represents the first example of a truly reversible polymer network showing these features. Furthermore, the ureido-pyrimidone dimerization is strong enough to construct supramolecular materials possessing acceptable mechanical properties. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3657–3670, 1999     

Laterally aryl-substituted bent-core mesogens

Laterally aryl-substituted bent-core mesogens, prepared from 2-methyl-isophthalic acid, were studied. Optical polarising microscopy gave hints for nematic and smectic A phases which were supported by X-ray measurements. Two-dimensional patterns of samples aligned in the magnetic field prove clusters of the smectic C type within the nematic phases. The layer spacings point to intercalated structures because they are significantly shorter than the molecular lengths derived from molecular models. The shape of the molecules seems to be neither rod-like, nor really bent, nor discotic due to the extension of the bending angle caused by the lateral methyl group in a bay position and the different cross-sections of different molecular parts.     

Hydrogen-bonded networks through second-sphere coordination

The reaction of 4, 7-phenanthroline (1) with aqueous transitionmetal complexes [Mn(H2O)6][NO3]2, [Co(H2O)6][NO3]2, [Ni(H2O)6[NO3]2, [Mn(H2O)6][ClO4]2, and [Co(H2O)6][ClO4]2 does not produce coordination complexes between these metal cations and the N-donor ligand as expected. Instead, supramolecular hydrogenbonded networks are formed between the nitrogen donor atoms of 4, 7-phenanthroline and the OH groups of coordinated water molecules: M-O-H...N interactions. This motif of second-sphere coordination for 1 can be exploited as a tool for crystal engineering. As a demonstration of the generality of this new interaction as a supramolecular building block, five X-ray crystal structures are reported that utilise this hydrogen bonding scheme; [Co(H2O)4(NO3)2].(1)2 (2a), [Co(MeCN)2(H2O)4][ClO4]2.(1)2 (2b), [Ni(H2O)4(NO3)2].(1)2 (3a), [Mn(H2O)4(NO3)2].(1)2 (4a), and [Mn(H2O)6][ClO4]2.(1)(4).4H2O (4b). Each network involves complete saturation of the hydrogen-bond donor sets between the aqua complex and 1 using primarily M-O-H...N(1) and M-O-H...O(anion), interactions. Thermogravimteric analysis shows these materials to have stabililities similar to coordination polymers involving metal-ligand bonds; this demonstrates that second-sphere hydrogen bonding has potential for the construction of polymeric metal-containing materials.     

Monofluorinated unsymmetrical bent-core mesogens

The synthesis and mesomorphic properties of 30 bent-core compounds with a fluorine substituent in one of the outer rings are reported. The banana-shaped compounds are all derived from resorcinol and contain esters as linking groups between the five aromatic rings. The different mesophases have been characterized by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction studies and electro-optical investigations. The compounds with the longer terminal chains exhibit an antiferroelectric SmCP phase. Upon introduction of a fluorine substituent the layer spacing increases, as compared with the corresponding unsubstituted compound. The introduction of one terminal vinyl group in the mono-substituted bent-core mesogens has no significant influence on the liquid crystalline properties and the switching remains antiferroelectric. Due to the introduction of the terminal double bond, these banana-shaped compounds are suitable for the preparation of siloxane polymers or attachment to a hydrogen-terminated silicon surface.     

Cyano substituted triphenylene-based discotic mesogens

Cyano groups have been introduced into alkoxytriphenylene-based mesogens by bromination followed by treatment with copper(I) cyanide. The introduction of cyano substitutents enhances the mesophases ranges.     

Novel energetic oxadiazole assemblies

A series of novel, structurally diverse 1,2,4- and 1,2,5-oxadi- azole assemblies was synthesized from readily available furoxanyl precursors. Experimentally determined physico-chemical properties and calculated detonation parameters showed an application potential of the prepared nitrogen-oxygen molecular systems as promising energetic materials.     

Preliminary communication Hydrogen-bonded ionic liquid crystals

Ionic and non-ionic building units were connected via hydrogen bonds. Stable liquid crystalline phases were obtained by a suitable choice of the units. The ionic associates are not miscible with the respective non-ionic associates.     

Proton conduction in discotic mesogens

In this communication, we show that liquid crystalline phases lower the activation energy barrier for proton transport. The liquid crystalline phases were obtained using a triphenylene core with alkyl chains bearing a triazole moiety at their termini.     

Dendrimers with laterally grafted mesogens

Nematic liquid crystalline phases have been obtained by attaching mesogenic units in a lateral manner to the periphery of dendrimer generations 1, 2, 3, 4 and 5 of poly(propyleneimine). Powder XRD studies of the compounds have been performed in order to confirm the natures of the mesophases. Patterns obtained from oriented samples show a nematic phase in which the molecules are preferentially aligned in the stretching direction and also indicate the presence of local smectic C fluctuations. All compounds display an additional low temperature mesophase. Careful study of the XRD patterns of these low temperature phases has led to their assignment as a modulated Sm[Ctilde] -tilde mesophase.     

Electrostatic properties of cyano-containing mesogens

We analyse the electrostatic properties of a set of cyano-containing mesogen molecules with different rigid cores and variable alkyl chain lengths, computing the molecular charge distributions. Using the simple prototype benzonitrile, we analyse the reliability of the quantum chemical methods used to estimate the electrostatic dipole moments of polar conjugated molecules. We show that the electronic properties of the long mesogenic molecules can be treated by combining HF geometry optimization procedures with single point MP2 calculations. We compare the results of these computations with the available experimental phase transition data of mesogens and discuss some examples of how the non-trivial mesomorphic behaviour, which is usually observed in these cyano compounds, can be qualitatively explained by the molecular electrostatic interaction potential.     

Sulphur ligated siamese twin mesogens

The synthesis of 3,3'-sulphinyl-bis-[alkyl 4-(4-substituted benzoyloxy)-benzoates] and 3,3'-sulphonyl-bis-[alkyl 4-(4-substituent-benzoyloxy)-benzoates] is described. These new nematic compounds are of the ligated siamese twin type. By microscopy and DSC calorimetry the thermal behaviour was investigated. Many of the new compounds transform on quenching to the glassy nematic state with glass temperatures above room temperature. Above the glass temperature the substances behave like highly viscous but ordinary nematics. The substances may be used for the construction of new thermo-electrooptic displays. By means of dielectric and X-ray investigations we were able to show that the two halves of the twin molecules are in an antiparallel orientation in the nematic state. In mixtures of the new compounds with rod-like nematics reentrant nematic behaviour was observed.