Chiral spiro-connected 2,3,3-trisubstituted oxiranes New dopants for induced ferroelectric phases

The synthesis, phase behaviour and spontaneous polarization of a new class of chiral dopants for induced ferroelectric phases of general structure A and B, possessing a 2,3,3-trisubstituted oxirane ring spiro-connected with four-, five- or six-membered carbocycles are described. In a series of these compounds with the same mesogenic building blocks the one with the five-membered carbocycle exhibits the highest induced spontaneous polarization. Introduction of a carbonyl group adjacent to the oxirane ring leads to an increased induced spontaneous polarization. The sign of the helical twisting power is positive for compounds with a cyclobutane ring and negative for those with a cyclopentane or a cyclohexane ring, although the absolute configuration at the chiral carbon is the same for all compounds.     

Chiral 2,2-disubstituted oxirane ethers and carboxylic esters. New ferroelectric liquid crystals and dopants for induced ferroelectric phases

Abstract

The synthesis, phase behaviour and spontaneous polarization of a new class of chiral LCs and chiral dopants for induced ferroelectric phases of general structure A, possessing a (2S)-2-hydroxymethyloxirane unit, and B, possessing a (2R)-2-oxirane carboxylic acid unit connected to mesogenic building blocks are described. One of these new compounds exhibits a S*_c phase. A carbonyl group adjacent to the oxirane ring does not increase the spontaneous polarization which is in contrast to the results obtained for 2,3-disubstituted oxiranes. A comparison with analogous species containing a (2S)-2-hydroxymethyloxetane, a (2R)-2-hydroxymethylthiirane or a (2S,3R)-2-hydroxymethyloxirane unit is given.     

New dopants for induced ferroelectric SC* phases containing 1,3-dioxolan-2-one as a chiral heterocycle

Abstract

The synthesis, phase behaviour and spontaneous polarization of a new class of chiral dopants for induced ferroelectric phases of general structure 6 and 9, possessing a 1,3-dioxolan-2-one ring, are described. One of the new compounds exhibits a S_A phase. When the chiral ring system is positioned terminally to the mesogen (6) the cis disubstituted compounds show higher induced spontaneous polarization than those with an analogous trans configuration.     

Fast-switching ferrolectric liquid crystalline polymers containing one or two centres of chirality in the side chain

In a liquid crystalline side chain polyacrylate containing one center of chirality in the terminal alkyl chain of the mesogenic part, switching times of 200–400 μs were measured in the SmC^* phase. Below this phase an unidentified phase exists, which shows electroclinic-like switching. The phase transition between those two phases can be shifted by applying an electric field. At higher molecular weights three subphases emerge in the SmC^* region. Variation of the spacer length revealed, for the first time, ferroelectric switching even at a spacer length of only two CH₂ groups. By shifting the centre of chirality into the spacer of the side group a polymer resulted, which shows electroclinic switching in the SmC^* phase, changing to ferroelectric switching when the voltage is increased. Incorporation of an oxirane ring as chiral building block into the spacer yielded a polymer that shows a sign inversion of the spontaneous polarization in the SmC^* phase. A polymer containing a dioxolane carbonic ester as chiral unit exhibits three switching states, with the third state existing at a low or zero electric field. This phenomenon is known for antiferroelectric liquid crystals. By doping a racemic LC polymer with a chiral monomeric LC we induced a spontaneous polarization. Colored FLC polymers were obtained by two different approaches. In an FLC–dye copolymer, increasing switching speed in three different chiral smectic phases was observed when increasing the dye concentration.     

Effect of lateral substitution by fluorine and bromine atoms in ferroelectric liquid crystalline materials containing a 2‐alkoxypropanoate unit

Two homologous series of ferroelectric liquid crystalline compounds with 2‐alkoxypropanoate chiral unit containing biphenyl benzoate core laterally substituted by fluorine and bromine have been synthesized and studied. All compounds possess the ferroelectric smectic C* phase over a broad temperature range. For bromine‐substituted compounds values of spontaneous polarization reach high values up to 250 nC cm^?2. The effects of the lateral substitution on the phenyl ring far from the chiral centre by methyl and methoxy groups, fluorine, chlorine and bromine atoms on mesomorphic properties and on values of the spontaneous polarization are discussed.     

New ferroelectric liquid crystals with a trifluoromethyl group

Abstract

New chiral dopants, possessing a trifluoromethyl group, have been synthesized. These dopants have different polar groups, located between the chiral centre and the diphenylacetylene core. The polar part is chosen from ester, methylene ether and ether linkages. Each new chiral dopant was mixed with a non-chiral liquid crystal mixture or a ferroelectric liquid crystal mixture, and their transition temperatures and electrooptic properties measured. The largest spontaneous polarization, 4·1 nC cm^?2, was obtained at 25°C for the ferroelectric liquid crystal mixture in which the dopant, with a methylene ether linkage, was incorporated. Molecular orbital calculations (MNDO method) for the dopants show that the calculated dipole moments are correlated with the measured spontaneous polarization, and the calculated results explain the experimental results quite well. In addition, it has been found that the conformation of the chiral part is an important factor which affects the magnitude of spontaneous polarization.     

Synthesis and properties of new ferroelectric liquid-crystalline compounds having a tolane rigid core and an optically active alkyl sulphinate group

Abstract

The synthesis and the properties of new ferroelectric liquid-crystalline compounds with a tolane rigid core and an optically active alkyl sulphinate group are reported. These compounds contain two chiral centres: a sulphinate group and a branched aliphatic chain. The two pure diastereomers having opposite configurations on the sulphur have been isolated. The mesomorphic behaviour of these two compounds is different, only one exhibiting an enantiotropic chiral smectic C phase; this clearly demonstrates the role of the sulphur configuration in determining the thermotropic properties. The mesomorphic transition temperatures, the structural behaviour as a function of temperature and preliminary measurements of the spontaneous polarization are presented.     

Effect of lateral substitution by fluorine and bromine atoms in ferroelectric liquid crystalline materials containing a 2-alkoxypropanoate unit

Two homologous series of ferroelectric liquid crystalline compounds with 2-alkoxypropanoate chiral unit containing biphenyl benzoate core laterally substituted by fluorine and bromine have been synthesized and studied. All compounds possess the ferroelectric smectic C* phase over a broad temperature range. For bromine-substituted compounds values of spontaneous polarization reach high values up to 250 nC cm^-2. The effects of the lateral substitution on the phenyl ring far from the chiral centre by methyl and methoxy groups, fluorine, chlorine and bromine atoms on mesomorphic properties and on values of the spontaneous polarization are discussed.     

Ferroelectric properties of smectic C liquid crystals with induced helical structure

Ferroelectricity has been observed in liquid crystalline mixtures of chiral smectic C type with a helical structure induced by chiral guest molecules (pitch p ≈ 100 μm). Samples of thickness d?p form monodomains with finite zero-field electric polarization because the spontaneous polarization of the smectic layers is not compensated by the helical twist. The temperature dependence of the spontaneous polarization has been studied near the phase transition smectic C→ smectic A.     

Magnitude and direction of the spontaneous polarization of ferroelectric liquid crystals with several bond moments

The magnitude and direction of the spontaneous polarization in most ferroelectric liquid crystals have been confirmed to be determined by the location and magnitude of the bond moment around the chiral carbon and the core. In compounds with several bond moments their relative orientation is very important for obtaining a large spontaneous polarization. Compounds with benzene rings in the core substituted with OH also have a large spontaneous polarization, perhaps due to the formation of hydrogen bonds. Reversal of the direction of the spontaneous polarization with temperature has been found for EFPPOPB. This anomalous behaviour has been explained tentatively in terms of a conformation change due to the existence of a flexible -CH₂- unit between the chiral carbon and the dipole moment.     

Stereo- and regioselectivity in asymmetric synthesis of α-amino substituted benzocyclic compounds

The enantiomerically pure chiral benzocyclic amines 6–8 were obtained by asymmetric transamination of the corresponding prochiral ketones 9a–c. The method involves: (a) formation of chiral imines 10a–c from the prochiral ketones 9a–c and the inexpensive chiral auxiliary (R)- or (S)-phenylethylamine (PEA); (b) asymmetrically induced reduction of these imines to the diastereomeric amines 11a–c and 12a–c; (c) catalytic hydrogenation to remove the benzylic fragment of the chiral PEA auxiliary. The stereoselectivity of the imine reduction, as well as the regioselectivity of the catalytic hydrogenation, are strongly dependent on the size of the saturated ring condensed with the benzene ring. This approach was used to develop a convenient, high yielding, and stereoselective route to several practically important optically active α-amino substituted benzocyclic compounds.     

Synthesis and Reactions of 4-Oxiranylmethylfuro[3,2-b]pyrroles and Their Benzo Derivatives

Methyl 4-oxiranylmethyl-4H-furo[3,2-b]pyrrole-5-carboxylates 2a-c and methyl 1-oxiranylmethyl-1H-benzo[4,5]furo[3,2-b]pyrrole-2-carboxylate (2d) were prepared by reaction of the appropriate starting compounds 1a-d with excess chloromethyloxirane. The compounds 2a-d undergo oxirane ring opening by heterocyclic amines (morpholine, pyrrolidine, piperidine or 4-methylpiperazine) giving N-2-hydroxy-3-heteroaminopropyl-substituted compounds 3a-f or substituted 4,5-dihydrofuro[2',3':4,5]pyrrolo[2,1-c][1,4]oxazin-8-ones 4a-e.     

Tilt and temperature dependence of the pitch in the chiral smectic C phase

Abstract

The chirality of the constituent molecules in the chiral smectic phase induces a helical structure with a pitch, p ₀. Because of the tilt and chirality there is a spontaneous polarization and a bend deformation which act upon the induced helix. The resulting pitch is described as a function of p ₀ using the phenomenological theory of a chiral smectic C phase. The pitch, p ₀, is then calculated using a molecular theory of the cholesteric phase. The results obtained explain the experimental observations, at least qualitatively.     

Low frequency dielectric spectroscopy on mixtures containing ferroelectric liquid crystals

Abstract

Dielectric spectroscopy (frequency range 10 ^?1 Hz to 10⁵ Hz, temperature range 300 K to 370 K) was employed to investigate the Goldstone mode in ferroelectric liquid-crystalline mixtures of chiral and achiral compounds. While the spontaneous polarization decreases with increasing fraction of the achiral molecules the Goldstone mode remains constant in its oscillatory strength. As expected the Goldstone mode frequency shifts to lower values with increasing amount of achiral molecules in the ferroelectric liquid-crystalline mixture.     

Ferroelectricity of induced S*C phases with novel chiral dopants

Abstract

Induced S*_C phases can be obtained by dissolving chiral dopants in achiral S_C host phases. If the chiral guest molecules bear a transverse dipole, ferroelectricity will occur. The novel dopants under discussion are characterized by chiral centres and the transverse dipole situated not in the alkyl end groups of the mesogenic molecules, but directly in their rigid cores. As a rigid core, analogues of decalin were used. In those dopants, rotation around the molecular long axis is sterically restricted. According to the microscopic model of Zeks, this leads to enhanced values of the spontaneous polarization P _s. The magnitude as well as the sign of the spontaneous polarization P _s of the S*_C phases induced by the novel dopants in different host phases has been investigated. It has been found for the first time that for a given dopant, the polarization as well as the sign of P _s depends on the structure of the host phase. The results are discussed in terms of two microscopic models. They can be understood taking into account the situation that the potential of the restricted long axial rotation is determined by the hard core interactions of the molecules involved or that an orientation of the host dipoles by a guest/host interaction takes place.     

Strongly non-linear optical ferroelectric liquid crystals for frequency doubling

Abstract

From our research for novel non-linear optical (NLO) materials for frequency doublers and optical modulators we report on new ferroelectric liquid crystals, which for the first time, exhibit second order NLO coefficients (for example d ₂₂ = 5 pm V^?1, which are comparable to those of state of the art inorganic NLO materials. The novel compounds contain 5-amino-2-nitrophenyl groups attached close to the chiral centres. The switching behaviour of the new compounds, their spontaneous polarization, as well as their frequency doubling of Nd:YAG laser pulses in the S*_c and in the glass state, are reported. Moreover their waveguiding properties are presented.     

Ferroelectric liquid crystals and dopants containing the chiral thiirane unit. A comparison with analogous oxiranes

The synthesis and mesomorphic properties of a new type of ferroelectric liquid crystals and dopants (for induced ferroelectric phases) containing cis or trans disub-stituted thiiranes as the chiral unit are described. A comparison of the thiiranes with the corresponding oxiranes is given. In the thiirane series the tendency to exhibit mesophases is less pronounced. S*_C phases occur for three aromatic cores in the mesogenic part of the molecule. The temperature ranges of the liquid crystal phases are broader and the transition temperatures are higher in the trans compared with cis configured thiiranes and oxiranes. Cis configured thiiranes exhibit a higher spontaneous polarization than the corresponding trans compounds.     

Stereo- and regiocontrolled synthesis of highly functionalized cyclopentanes with multiple chiral centers

Abstract

The synthesis of some highly substituted three-dimensional cyclopentanes with multiple chiral centers and with high regiochemical and stereochemical diversity has been accomplished starting from cyclopentadiene-derived aminocyclopentenecarboxylic acids. The small-molecular design consisted of stereo- and regiocontrolled functionalization of the starting cyclopentene β- and γ-amino acids through oxirane formation/oxirane opening and afforded regio- and diastereoisomers of orthogonally protected aminocyclopentanecarboxylates.     

Synthesis and ferroelectric properties of laterally substituted fluoro liquid crystals derived from 4-mercaptobenzoic acid

Several new ferroelectric liquid crystals with thiobenzoate mesogenic cores have been synthesized and the influence of a single fluorine atom ortho to the alkyloxy tail on the mesomorphic properties of some two and three benzene ring core compounds has been studied. The two benzene ring derivatives do not display the ferroelectric smectic C phase but the three phenyl ring compounds exhibit a wide temperature range S_c* phase. The transition temperatures and enthalpies of transition for these compounds have been determined; the spontaneous polarization, response time and tilt angle have also been measured and are discussed as a function of the structure near to the chiral centre.     

Production of Enantiopure Chiral Epoxides with E. coli Expressing Styrene Monooxygenase

Styrene monooxygenases are a group of highly selective enzymes able to catalyse the epoxidation of alkenes to corresponding chiral epoxides in excellent enantiopurity. Chiral compounds containing oxirane ring or products of their hydrolysis represent key building blocks and precursors in organic synthesis in the pharmaceutical industry, and many of them are produced on an industrial scale. Two-component recombinant styrene monooxygenase (SMO) from Marinobacterium litorale was expressed as a fused protein (StyAL2StyB) in Escherichia coli BL21(DE3). By high cell density fermentation, 35 g_DCW/L of biomass with overexpressed SMO was produced. SMO exhibited excellent stability, broad substrate specificity, and enantioselectivity, as it remained active for months and converted a group of alkenes to corresponding chiral epoxides in high enantiomeric excess (˃95–99% ee). Optically pure (S)-4-chlorostyrene oxide, (S)-allylbenzene oxide, (2R,5R)-1,2:5,6-diepoxyhexane, 2-(3-bromopropyl)oxirane, and (S)-4-(oxiran-2-yl)butan-1-ol were prepared by whole-cell SMO.