Essais de réactions dissymétriques sur quartz optiquement actif

I. Three catalytical reactions are investigated (hydrogenation of methylethylketone, dehydrogenation and dehydration of 2-butanol) the catalyst being optically active quartz, either pure or metal covered. Emphasis is laid upon the most appropriate experimental conditions, generally neglected heretofore, when stereospecificity may be expected in a catalytic heterogeneous reaction. Numerous attempts under carefully controlled conditions have always led to negative results. Apparent optical rotations sometimes observed are shown to result from an extraneous effect due to minute quartz particles carried away with the reaction products. II. Attempts to separate at room temperature various racemic modifications (2-butanol, cobalt or chromium complexes, ammonium tartrate) by liquid-solid chromatography on optically active quartz have always led to negative results. The resolution of (±)-2-butanol at dry ice temperature was also unsuccessful. III. The failure to observe any asymmetric effect in catalysis as well as in adsorption on optically active quartz prompts to a critical analysis of previous work where small but positive effects have been claimed. It is shown that most of the small rotatory powers observed are within the limit of error of the measurements and that a number of results are inconsistent or unlike. These may have been vitiated by an extraneous effect which has been recognised and had been overlooked previously (dichroism or double refraction due to minute quartz particles suspended in the observed liquid).     

Chirality-induced phase transitions in some liquid crystalline binary mixtures

Novel binary mixtures have been prepared between an optically active antiferroelectric liquid crystal, (S)-4-(1-methylheptyloxycarbonyl)phenyl 4'-octyloxy-biphenyl-4-carboxylate, and an optically active twin liquid crystal, (R)-3-methyladipic acid bis[4-(5-octyl-2-(pyrimidinyl)phenyl] ester, and the liquid crystalline properties investigated. The stability of each liquid crystal phase was found to decrease by mixing these two liquid crystalline materials. Furthermore, a phase diagram between these compounds showed a clear discontinuity in phase sequences. These results indicate that the liquid crystal phases are different in nature between these materials. The mixture consisting of the antiferroelectric material (40 per cent) and the twin material (60 per cent) shows an unusual liquid crystal phase, where the texture is similar to that reported for the twist grain boundary (TGB) phase. Related binary mixtures have been prepared between optically active or racemic materials, where the chirality of the system is expected to be altered systematically. The TGB phase was found to be induced only in the mixture between the optically active materials. Two kinds of effect on the appearance of the TGB phase, i.e. a strong helical structure induced by the optically active twin liquid crystal and a decrease of the smectic layer strength achieved by mixing between two types of liquid crystalline materials, are discussed.     

Terpenoids—LVII Synthesis of some germacrane derivatives

Both liquid and solid hexahydrocostunolides obtained by the pressure hydrogenation of costunolide (V) and solid dihydrocostunolide (VI) respectively, give the same optically active hexahydrogermacol (IIIa), when subjected to controlled reduction with LiAlH₄, followed by Huang-Minlon reduction. The alcohol (IIIa) on oxidation with chromic oxide gives optically active hexahydrogermacrone (IIa). Dehydration of IIIa with KHSO₄ followed by catalytic hydrogenation gives the optically inactive saturated hydrocarbon, germacrane (IV). Since the total synthesis of solid dihydrocostunolide (VI) has already been achieved the preparation of IIIa, IIa and IV from VI constitutes a total synthesis of the above three compounds.     

Effects of liquid crystalline and shear alignment on the optical properties of cellulose nanocrystal films

Rheo-optics, microspectrophotometry, and optical contrast measurements were used to gain new insights into the interrelated effects of liquid crystalline phase behavior, flow alignment, and microstructural relaxation on cellulose nanocrystal (CNC) films’ alignment and optical properties. Optical contrast measurements were found to be an effective and facile way of determining changes in anisotropy directly from cross-polarized microscopy images. This method was used to continuously measure microstructural relaxation after the cessation of shear as well as the anisotropy of dried CNC films. Aqueous liquid crystalline CNC dispersions showed greater alignment after shear than isotropic or biphasic dispersions. However, CNC gels exhibited lower alignment at equivalent shear rates. The combination of greater initial alignment and slower relaxation of sheared liquid crystalline dispersions resulted in the most optically anisotropic films. Depending on their thickness, the CNC films were optically transparent in the visible regime or exhibited tunable interference colors. The results of this work highlight the tunability of CNC dispersion processing for producing color filters and other optical materials with controlled properties.     

Recent progress in chiral photonic band‐gap liquid crystals for laser applications

This article describes a brief review of recent research advances in chiral liquid crystals (CLCs) for laser applications. The CLC molecules have an intrinsic capability to spontaneously organize supramolecular helical assemblages consisting of liquid crystalline layers through their helical twisting power. Such CLC supramolecular helical structures can be regarded as one‐dimensional photonic crystals (PhCs). Owing to their supramolecular helical structures, the CLCs show negative birefringence along the helical axis. Selective reflection of circularly polarized light is the most unique and important optical property in order to generate internal distributed feedback effect for optically‐excited laser emission. When a fluorescent dye is embedded in the CLC medium, optical excitation gives rise to stimulated laser emission peak(s) at the band edge(s) and/or within the CLC selective reflection. Furthermore, the optically‐excited laser emission peaks can be controlled by external stimuli through the self‐organization of CLC molecules. This review introduces the research background of CLCs carried out on the PhC realm, and highlights intriguing precedents of various CLC materials for laser applications. It would be greatly advantageous to fabricate active CLC laser devices by controlling the supramolecular helical structures. Taking account of the peculiar features, we can envisage that a wide variety of supramolecular helical structures of CLC materials will play leading roles in next‐generation optoelectronic molecular devices. © 2010 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.201000013     

Temperature- and solvation-dependent dynamics of liquid sulfur dioxide studied through the ultrafast optical Kerr effect

The ultrafast dynamics of liquid sulphur dioxide have been studied over a wide temperature range and in solution. The optically heterodyne-detected and spatially masked optical Kerr effect (OKE) has been used to record the anisotropic and isotropic third-order responses, respectively. Analysis of the anisotropic response reveals two components, an ultrafast nonexponential relaxation and a slower exponential relaxation. The slower component is well described by the Stokes-Einstein-Debye equation for diffusive orientational relaxation. The simple form of the temperature dependence and the agreement between collective (OKE) and single molecule (e.g., NMR) measurements of the orientational relaxation time suggests that orientational pair correlation is not significant in this liquid. The relative contributions of intermolecular interaction-induced and single-molecule orientational dynamics to the ultrafast part of the spectral density are discussed. Single-molecule librational-orientational dynamics appear to dominate the ultrafast OKE response of liquid SO2. The temperature-dependent OKE data are transformed to the frequency domain to yield the Raman spectral density for the low-frequency intermolecular modes. These are bimodal with the lowest-frequency component arising from diffusive orientational relaxation and a higher-frequency component connected with the ultrafast time-domain response. This component is characterized by a shift to higher frequency at lower temperature. This result is analyzed in terms of a harmonic librational oscillator model, which describes the data accurately. The observed spectral shifts with temperature are ascribed to increasing intermolecular interactions with increasing liquid density. Overall, the dynamics of liquid SO2 are found to be well described in terms of molecular orientational relaxation which is controlled over every relevant time range by intermolecular interactions.     

Dynamic Photocontrol of the Coffee‐Ring Effect with Optically Tunable Particle Stickiness

When a colloidal drop dries on a surface, most of the particles accumulate at the drop periphery, yielding a characteristic ring‐shaped pattern. This so‐called coffee‐ring effect (CRE) is observed in any pinned evaporating drop containing non‐volatile solutes. Here, the CRE is dynamically controlled for the first time by using light, and an unprecedented reconfigurability of the deposit profile is demonstrated. This is achieved through a new mechanism where particle stickiness is optically tuned on demand, thus offering reliable modulation of the deposition pattern. The system consists of anionic nanoparticles and photosensitive cationic surfactants dispersed in water. It is shown that light‐dependent modulation of surfactant–particle interactions dictates particle attraction and trapping at the liquid–gas interface, which allows us to direct particle deposition into a wide range of patterns from rings to homogeneous disks. Patterning from single drops is photoreversible upon changing the wavelength whereas spatial control in multiple drop arrays is achieved using a photomask.     

Electrically tunable photo-aligned two-dimensional liquid crystal polarisation grating

Two-dimensional liquid crystal polarisation grating based on twist liquid crystal has been demonstrated and fabricated by polarisation holography in this paper. The maximal diffraction efficiency of the second order is up to 90%. And the two-dimensional liquid crystal polarisation grating has the properties of electrically tenability and polarisation tenability. The two-dimensional polarisation grating diffracts light into a spot array. Different direction diffraction light is with different polarisation states. The intensity of the different orders diffraction light is optically controlled.     

Selective Decrosslinking in Liquid Crystal Polymer Actuators for Optical Reconfiguration of Origami and Light‐Fueled Locomotion

The ability to optically reconfigure an existing actuator of a liquid crystal polymer network (LCN) so that it can display a new actuation behavior or function is highly desired in developing materials for soft robotics applications. Demonstrated here is a powerful approach relying on selective polymer chain decrosslinking in a LCN actuator with uniaxial LC alignment. Using an anthracene‐containing LCN, spatially controlled optical decrosslinking can be realized through photocleavage of anthracene dimers under 254 nm UV light, which alters the distribution of actuation (crosslinked) and non‐actuation (decrosslinked) domains and thus determines the actuation behavior upon order‐disorder phase transitions. Based on this mechanism, a single actuator having a flat shape can be reconfigured in an on‐demand manner to exhibit reversible shape transformation such as self‐folding into origami three‐dimensional structures. Moreover, using a dye‐doped LCN actuator, a light‐fueled microwalker can be optically reconfigured to adopt different locomotion behaviors, changing from moving in the laser scanning direction to moving in the opposite direction.     

Nonlinear optical materials by the sol-gel method

The sol-gel method, because of the use of homogeneous liquid solutions and the ability to form gels at room temperature, is uniquely suited for the preparation of many nonlinear optical (NLO) materials, especially films. The preparations of NLO crystalline solids, such as ferroelectrics, oxide glasses, and amorphous ferroelectrics, are described. The preparation of NLO nanocomposites can be made by a number of approaches. These include the mixing of optically active organics into the sol-gel liquid solution, the impregnation of organics into the interconnecting pores of a stabilized oxide gel, and the direct chemical bonding of optically active organics and inorganics to form new hybrid NLO nanocomposites.     

Liquid Crystals: A Tool for Studies on Chirality

Nematic liquid crystals are extremely sensitive to optically active molecules, which, at very low concentration, convert them into cholesteric liquid crystals. The purpose of this report is to review recent applications of liquid crystals to chirality studies. A brief discussion of the structure and properties of liquid crystals is followed by a survey of the methods currently being employed for the determination of the pitch and handedness of cholesteric liquid crystals. Of special interest in the formation of induced-cholestric phases are the twisting powers of optically active compounds and the structural relationships between dopants and nematics. Liquid crystals can be used, inter alia, for the detection of small optical activities and for the determination of thermal racemization barriers and absolute configurations.     

Structural properties of nonionic surfactant/glycerol/paraffin lyotropic liquid crystals

Lamellar lyotropic liquid-crystalline systems are thermodynamically stable, optically isotropic and are formed spontaneously. New possibilities for the development of controlled drug delivery systems are inherent in these systems in consequence of their stability and special, skin-friendly structure. The aim was to formulate and study two-component or multicomponent compositions with a relatively low Brij 96V content, liquid paraffin, glycerol and water for therapeutic purposes. The liquid crystals were examined by polarizing light microscopy and transmission electron microscopic observation of replicas produced by the freeze–fracture technique to demonstrate the presence of lamellar liquid-crystalline domains. The existence of a regular structure was determined by X-ray diffraction.     

Manifestation of chiral asymmetry of ferroelectric liquid crystals induced by optically active dipole dopants in a linear electrooptic effect

The existence of eight combinations of absolute spatial configuration, helix handedness and handedness of director tilt has been shown for ferroelectric liquid crystals induced by optically active dipole dopants (optically active diesters of 4,4'-terphenyl dicarboxylic acid). As in the case of individual ferroelectrics alternation of the helix handedness is observed depending on absolute configuration of the C*-atom and its position relative to the rigid core of the molecule. However for these induced ferroelectric liquid crystals the helix handedness does not depend on the inductive effect of the substituent adjacent to the C* atom, e.g. the helix handedness of all the (S)-2-chlorine substituted materials coincides with that of the (S)-2-methyl-butyl derivative. Substitution of a chlorine atom by a cyano group followed by conversion of absolute spatial configuration of the C* atom results in the opposite helical sense. Thus asymmetric parameters of the induced ferroelectric liquid crystals helix handedness and the handedness of director tilt (or the sign of P_s) do not depend directly on the absolute configuration of C* atom and its position in a molecule. For the substances investigated within all of the temperature range of the induced smetic C* phase no reversal of the tilt direction handedness was observed.     

Electroconvection characterization of guest-host nematic liquid crystals for dynamic scattering mode applications

ABSTRACT

The threshold voltage for electro-hydrodynamic convection (EHC) in nematic liquid crystals in the low-frequency convective regime is examined. The addition of dopant species is known to strongly effect the frequency response of this regime. In this work, we investigate the effect of adding optically anisotropic species on the low-frequency threshold, and, demonstrate that certain dichroic dyes have a marked effect. We have measured electrical transport properties of suitably doped liquid crystal mixtures in order to make a comparison with theoretical predictions based upon linear stability considerations. Our results show satisfactory agreement with these predictions. Our findings indicate future directions for new applications of dynamic scattering mode technologies with more accessible operating parameters previously known.     

Polymerization Processes in Ionic Liquids. Cationic Polymerization of Styrene

There is an increasing interest in using ionic liquids as solvents for polymerization processes. Most published data deals with controlled radical polymerization. It has been shown that ionic liquids offer several advantages for conducting Atom Transfer Radical Polymerization (ATRP), such as good solubility of catalyst and improved k_p/k_t ratio. Ionic liquids are highly polar therefore they seem to be suitable solvents for conducting also ionic polymerization processes. In our preliminary communication we reported on cationic polymerization of styrene initiated by R-Cl/TiCl₄ system in ionic liquid. To clarify the mechanism of this process, racemization of optically active 1-phenylethyl choride (initiator and the model of dormant species) was studied and it was shown that in ionic liquid racemization proceeds even in the absence of coinitiator (TiCl₄). Because racemization proceeds through ionization of C Cl bond, this explains the cationic polymerization of styrene initiated by R-Cl alone (in the absence of coinitiator). Chain transfer, however, cannot be eliminated, therefore polymerization is not controlled.     

Highly enantioselective separation using a supported liquid membrane encapsulating surfactant-enzyme complex

We developed a highly enantioselective separation system for the optically active compounds, (S)-ibuprofen and l-phenylalanine, from their racemic mixtures by employing a supported liquid membrane (SLM) encapsulating a surfactant-lipase complex (or a surfactant-alpha-chymotrypsin complex). In the present system, enzymes encapsulated in the liquid-membrane phase effectively drove the enantioselective transport of optically active compounds through the SLM. This novel SLM allowed high enantioselectivity (ee over 91%) in the optical resolution of racemic ibuprofen and phenylalanine.     

Ferroelectric liquid crystals IV. Chiral alkenyloxyphenyl benzoates

Over forty 4-alkoxyphenyl 4-alkoxybenzoates incorporating an olefinic double bond in one carbon chain and an optically active centre in the second (methyl branched) chain have been prepared. The effect of chain length and the position of the chiral centre on the liquid crystal transition temperatures of the esters has been investigated systematically. Apart from one important exception the double bond was maintained in a terminal position of the carbon chain. Several of the new esters exhibit an enantiotropic chiral smectic C mesophase at and just above room temperature.     

Insight into acid-mediated asymmetric spirocyclization in the presence of a chiral diol

Asymmetric spirocyclization based on intramolecular conjugate addition using a combination of a Lewis acid and an optically active cyclohexane-1,2-diol has been studied in connection with 1) the effect of substituents on the cyclohexane-1,2-diol and 2) the effect of substituents on the substrate. This reaction was found to be both thermodynamically and kinetically controlled under restricted conditions.     

Synthesis of Novel Chiral Ionic Liquid and Its Application in Reduction of Prochiral Ketones to the Corresponding Chiral Alcohols Using NaBH4

A novel chiral ionic liquid (CIL) based on nicotinium salt has been synthesized and used as an efficient asymmetric chiral catalyst for reduction of acetophenone derivatives with NaBH₄ in methanol at room temperature. The optically active alcohols were obtained in low to moderate enantiomeric excess in a short reaction time.     

Synthesis and properties of a diastereopure ionic liquid with planar chirality

An imidazolium-based ionic liquid with cyclophane-type planar chirality was synthesized in an optically pure form for the first time. The resultant ionic liquid existed as a liquid at room temperature (T_g = −35 °C), and was found to be applicable as an NMR chiral shift reagent for racemic anions. Excellent robustness of the ionic liquid to a highly elevated temperature (270 °C) was proved from the viewpoints of isomerization and thermal decomposition.