M. F. Achard

Color-Tuned Electroluminescence from Columnar Liquid Crystalline Alkyl Arenecarboxylates

Simple ester derivatives of polycyclic arenes offer access to light-emitting diodes of nearly any visible color by making use of the good charge-transport properties of the columnar liquid crystals of these derivatives. The picture shows the orange-red electroluminescence of a light-emitting diode containing the perylene 3,4,9,10-tetracarboxylic acid ethyl ester (structure shown, R=Et). Through use of multiple layers of different esters light-emitting diodes with almost white luminescence can be obtained.

Inorganic monoliths hierarchically textured via concentrated direct emulsion and micellar templatesElectronic supplementary information (ESI) available: XRD profiles, nitrogen physisorption data and pore size distribution calculated from density functional theory, for the xSi-HIPE0.035 series. See http://www.rsc.org/suppdata/jm/b4/b400984c/

Hierarchical inorganic porous monoliths have been prepared using concentrated emulsion and micellar templates. The texture of these monoliths has to be tuned varying either the pH of the continuous aqueous phase, the emulsification process or the oil volumic fraction. These materials show interconnected macroporosity associated with vermicular-type mesostructuration. They possess an average mesoporosity of 800 m2 g−1 associated with bulk density as low as 0.08 g cm−3, which is comparable to values obtained for silica aerogel.

Symmetric and non-symmetric chiral liquid crystal dimers

The synthesis and characterisation of three sets of non-symmetric liquid crystal dimers consisting of a cholesteryl-based unit and either 4-methoxybiphenyl, 4-cyanobiphenyl or (S)-2-methylbutyl 4′oxybiphenyl-4-carboxylate are described. The transitional properties of these non-symmetric dimers are compared to those of the corresponding parent symmetric dimers. The symmetric dimers exhibit only chiral nematic or nematic phase behaviour. By contrast, members of the non-symmetric dimer series containing either 4-cyanobiphenyl or (S)-2-methylbutyl 4′-oxybiphenyl-4-carboxylate groups exhibit smectic behaviour. The former series show an interdigitated smectic A phase while for the latter the structure of the smectic A phase depends on the length of the flexible spacer. Specifically, for short spacer lengths a monolayer arrangement is seen while for long spacers an intercalated smectic A phase is formed. For an intermediate spacer length, the small-angle X-ray scattering pattern for the intercalated smectic A phase reveals short-range incommensurate structural fluctuations associated with the monolayer smectic A phase. The driving force for the formation of the intercalated phase may be an electrostatic interaction between the electron rich carbonyl linking group attached to the cholesteryl unit and the electron deficient aromatic unit while the monolayer arrangement may be stabilised by the van der Waals interactions between the cholesteryl unit and the alkyl chains. Blue phases are observed only for a small number of these non-symmetric dimers and these all contain an odd-membered spacer. This is in accord with the rather general observation that blue phases are observed for odd-membered non-symmetric dimers and not their even-membered counterparts.

Fluorescent columnar liquid crystalline 3,4,9,10-tetra-(n-alkoxycarbonyl)-perylenes

The title compounds were recently demonstrated to be useful as luminescent electron transport materials in organic light emitting diodes. Here, we present studies of the fluorescence of the homologues with chain lengths between 2 and 10 carbon atoms. Dilute solutions of the investigated compounds show absorption in the range between 410 and 490 nm and fluorescence between 475 and 555 nm. However, the columnar phase of the pure compounds exhibits fluorescence at considerably larger wavelength (550-650 nm) and gives larger fluorescence lifetimes. This behaviour can be explained by the formation of excimers in the mesophase. The phase transitions to the liquid crystal state are associated with a distinct change of the fluorescence intensity.

Rod-like mesogens with antipathetic fluorocarbon and hydrocarbon tails

Abstract A perfluorinated (or semi-perfluorinated) chain is affixed to various classical mesogenic moieties in lieu of the usual hydrocarbon tail. The consequences of this modification on the mesogenic behaviour are discussed by comparison with the perhydrogenated homologues. Smectic A phases are highly favoured. Their layer spacings show a trend to bilayer stacking in many cases likely to result from microsegregation among incompatible portions of the molecules.

Effect of the chemical constitution of the side-chain on the formation and the structure of mesophases in some polysiloxanes☆

Abstract The synthesis and properties are described for a series of new liquid crystal side-chain polymethylsiloxanes with the same mesogenic rigid core but different substitutents or flexible spacers. The effect of the chemical structure of these polymers upon the polymorphism is studied by means of optical, DSC and X-ray experiments. By comparison with conventional mesogens, the partially bilayered structure of the smectic A phases (i.e. when the layer spacing is between once and twice the length of the side-chain) could be explained, taking into account the asymmetric character of the side-chains.

Enantiotropic nematic reentrant behaviour at atmospheric pressure in a pure compound

Abstract By the way of microscopic observations (contact method) we conclude for the first time to the existence of a reentrant nematic at atmospheric pressure in a pure compound.

Unusual Property in a Novel L-C Side Chain Polysiloxane Series

Abstract A new series of polymethylsiloxanes with laterally grafted mesogens has been synthesized. For compounds with short spacers, the diamagnetic anisotropy variations in the nematic state present an anomaly which suggests a nematic-nematic transition.

New TGBA series exhibiting a SC∗SASA∗N∗ phase sequence

Abstract A new series with a chiral tolane core has been synthesized. These materials belong to the optically active series: 3-fluoro-4-[(R) or (S)-1-methylheptyloxy]4′-(4″-alkyloxy-3″-fluorobenzoyloxy)tolanes (nFBTFO1M7). For the first time, the helical SA∗ phase or TGBA phase is found in all of the derivatives from heptyloxy to octadecyloxy. The SA-SA∗-N∗ phase sequence is observed in several compounds with short chains and the SC∗-SA-SA∗-N∗ phase sequence is obtained with the decyloxy derivative. The TGBA phase has filament or cholesteric textures. The helical pitch of the TGBA phase is short for the octadecyloxy derivative and is compared with that for different chain lengths as a function of temperature. The layer spacing in the TGBA phase shows that it is incommensurate with the fully elongated molecular length.

Dimesogenic compounds consisting of a cholesteryl moiety and an aromatic mesogen interconnected through a central pentamethylene spacer

New non-symmetric dimesogens consisting of a cholesteryl moiety and an aromatic mesogenic unit interconnected through a pentamethylene spacer have been prepared. The aromatic units consist of two phenyl rings linked by carboxy, oxycarbonyl, ethylene, ethynylene, azo or Schiffs base groups. The mesomorphic and transitional properties have been investigated by using optical microscopy, differential scanning calorimetry, helical pitch measurements, and electric field and miscibility studies. The structures of the mesophases have been studied using X-ray diffraction. Several kinds of smectic packings are observed depending on the nature of the linking group and an attempt to understand the origin of the different phase structures at a molecular level is made.