Synthesis and mesomorphic properties of chiral liquid crystals having thiolactate groups

A homologous series of chiral thiolactate liquid crystals, alkyl 2-[4-(4-decyloxyphenyl)benzoyloxyphenyloxy]thiopropionates, ADBPT-n (n=2-6), was prepared from (R)-2-(4-hydroxyphenoxy)propionic acid. Structural effects on the mesomorphic and physical properties were investigated in terms of variation in the length of alkyl chain attached to the thiolactate group. The mesophases and their corresponding transition temperatures were identified by polarizing optical microscopy and differential scanning calorimetry. Compound ADBPT-4 exhibited SmA* and SmC* phases, whereas other compounds in the series exhibited the SmA* phase along with unidentified SmX₁* and SmX₂* phases. The maximum P_s value measured for ADBPT-4 in the SmC* phase was 17.5 nC cm^-2, suggesting the polarization of the material is rather low. The tilt angles were also measured, reaching a maximum of 48 degree.     

Synthesis and frustrated phases of chiral liquid crystals derived from (R)-3-alkoxy-2-methylpropionic acids

A homologous series of chiral compounds, (R)-6-(3-alkoxy-2-methylpropionyloxy)-2-naphthyl 4-alkoxybenzoates, nMPNmB (n = 2-5, m = 6-13), derived from methyl (R)-3-hydroxy-2-methylpropionate, was prepared for the study of mesormorphic phases. The mesophases were determined mainly by their microscopic textures and identified by the racemic modification. The results show that most of these compounds displayed frustrated phases BP, TGB_A* and TGB_C*, and the N*phase. Moreover, the thermal stability of the frustrated phases is significantly affected by the length of the alkyl chain in both achiral and chiral groups of the compounds. The spontaneous polarization in the TGB_C* phase of compounds nMPN10B in a surface-stabilized geometry was measured, giving P_s values of about 20 nC cm^-2.     

TGBA phase in a series of propionates containing two stereocenters

A homologous series of chiral materials, (R)-2-pentyl (S)-2-(6-(4-(4'-alkoxyphenyl)benzoyloxy)-2-naphthyl)propionates (R,S)PmPBNP (m = 7-14), derived from a naphthalene ring as part of the core structure in conjunction with a chiral tail containing two stereocentres has been synthesized for the investigation. The twist grain boundary smectic A (TGB_A) and smectic C (TGB*_C) phases were characterized by the microscopic textures of compounds packed in two untreated glass slides, Cano wedge cell, or homogeneously aligned cell. It was found that the occurrence of these phases depend remarkably on the nature of alkyl chain length m; in the case of shorter alkyl chain length (m = 7-10), the TGB_A phase behaves as a metastable-like phase mediated between N and S_A phases in a short temperature range, whereas in the longer chain length (m' 11-14), both the TGB_A and TGB_C phases become thermodynamically stable phases with a wide temperature range. Consequently, this series of chiral materials resulted in two different mesophase sequences: N-TGB_A-S_A-S_Cand N-TGB_A-TGB_C. A kind of parquet texture displaying two types of domain with different relative directions of the smectic layer normal was found in the S phase from the materials (m = 9-14) packed in 2 μm homogeneously aligned cells and cooled down from the isotropic liquid without applying an electric field. The magnitudes of spontaneous polarization (P_s) in the S_C and TGB_C phases showed that the P_s values are nearly the same for all compounds at the same temperature below the Curie point. Dielectric measurements revealed no significant occurrence of soft mode switching in the TGB_A phase.     

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

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 S_A* phase or TGB_A phase is found in all of the derivatives from heptyloxy to octadecyloxy. The S_A-S_A*-N* phase sequence is observed in several compounds with short chains and the S_C*-S_A-S_A*-N* phase sequence is obtained with the decyloxy derivative. The TGB_A phase has filament or cholesteric textures. The helical pitch of the TGB_A 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 TGB_A phase shows that it is incommensurate with the fully elongated molecular length.     

Stereochemical studies of optically active 2-hydroxy-5-alkyl-δ-valerolactone derivatives as chiral dopant for ferroelectric liquid crystals

Both (2S, 5R)- and (2R, 5R)-2-hydroxy-5-alkyl-δ-valerolactone derivatives, cis and trans, respectively, show almost the same magnitude of spontaneous polarization (P_s) when added to a non-chiral smectic C mixture. The stereochemistry of these chiral dopants was studied using ¹H NMR. Trans derivatives seem to have a half-chair conformation with the 2, 5-diequatorial substituents and the cis derivatives have rather a flat conformation in solution. However in the liquid-crystalline phase, the cis and trans derivatives appear to change their conformation or the distribution of their conformations as the alkyl chain length is varied. The difference in the effect as a chiral dopant depends upon lateral interactions between chiral molecules through the solvent liquid crystal phase.     

Ferroelectric liquid crystals with a thioester linking group positioned at the chiral tail

A homologous series of chiral thioesters, hexyl (2S)-2-[6-(4-alkanoyloxyphenyl)benzoyloxy-2'-naphthyl]thiopropionates, HnPBNTP (n=7-11) was synthesized, and the mesophases and electro-optical properties were studied. All the materials possess a ferroelectric SmC* phase, which was confirmed by the switching current behaviour. It was found that compounds with a shorter alkyl chain length (n=7-9) displayed SmA* and SmC* phases, while, compounds with a longer alkyl chain length (n=10 and 11), displayed the SmC* phase exclusively. Spontaneous polarization and apparent tilt angle were also measured. The maximum P_s values are in the range 15-26 nC cm^-2, and the maximum θ values are in the range 23°-28°.     

Synthesis and mesomorphic properties of chiral liquid crystals derived from (S)-2-(6-hydroxy-2-naphthyl)propionic acid with a methyleneoxy linking group

Three homologous series of chiral materials derived from (S)-2-(6-hydroxy-2-naphthly)propionic acid with a methyleneoxy linking group were synthesized for the investigation of mesomorphic properties. All the materials displayed enantiotropic SmA* and SmC* phases. The spontaneous polarization (P _s) and optical tilt angles in the SmC* phase of the chiral materials were measured. The maximum spontaneous polarizations are in the range 12.3-19.1 nC cm^-2. No significant difference could be found in the P _s values with respect to the various alkyl chiral chain lengths, n. The maximum optical tilt angles are in the range 25-30°. The optical tilt angles decrease with the increase in alkyl chain length, n.     

A comparison of ferroelectric liquid crystals containing diastereomeric propionic acids derived from natural ethyl lactate

Two diastereomeric carboxylic acids, 2(S)-[2(R)-methylhexyloxy]propionic acid and 2(S)[2(S)-methylhexyloxy]propionic acid, were prepared from ethyl (S)-lactate and (R)-1-iodo2-methylhexane or (S)-1-iodo-2-methylhexane in the presence of Ag₂O. From these acids two liquid crystals, 2 and 3 , whose configurations are (S , R ) and (S, S) were synthesized and their liquid crystal properties investigated. Although both LCs have the same phase sequence Cr-SmC*-N*-I as well as a wide SmC* phase range, the influence of the relative stereochemistry on their physical properties is clear. The liquid crystal with (S, S)-configuration possesses better properties: lower SmC* phase transition temperature, wider SmC* phase range and higher P_s value. The P_s value difference between the ferroelectric LCs 2 and 3 (97 and 131nCcm^-2, respectively, at T_c - T = 10°C) is unexpectedly large. The consideration, alone, of a zigzag conformation at the chiral molecular part of 2 and 3 is insufficient to explain such a difference.     

Crystal structures of ferro- and antiferroelectric mesogens with a 1-methylalkoxycarbonyl group

Crystal structures have been determined for 4-[(S)-1-methylpentyloxycarbonyl]phenyl, 4-([S)-1-methylhexyloxycarbonyl]phenyl 4'-octyloxybiphenyl-4-carboxylates (1 and 2, respectively) and (R)-1-methylheptyl 4-(4'-octyloxybiphenyl-4-yloxymethylene)benzoate (3), which have S_CA*, S_C* and S_CA* phases, respectively. All the crystals have a common packing mode with the previously determined structure of 4-(1-methylheptyloxycarbonyl)phenyl 4'-octyloxybiphenyl-4-carboxylate (MHPOBC). Each crystal forms a smectic-like layer structure composed of largely bent molecules with the long alkyl chain of the chiral group almost perpendicular to the core moiety. Differential scanning calorimetry showed that the crystals of 1, 2, and MHPOBC, which have a carbonyloxy group as the central linkage between the biphenyl and phenylene moieties, transform to higher temperature solid phases in a similar temperature range (around 60°C). For 3, which has an oxymethylene linkage between the biphenyl and phenylene moieties, three kinds of solids were distinguished.     

Mesophase characterization of some long chain vicinal multihydroxy amphiphiles [1,2]

The mesophase characterization of three stereoisomeric multihydroxy compounds, the 1, 2(S), 3(R), 4(R)-, 1, 2(R), 3(S), 4(R)-, and 1, 2(S), 3(S), 4(R)-ico-sanetetrols, as well as of an octadecafuranose derivative has been carried out by means of polarizing microscopy, X-ray diffraction etc. The four multiols with their very long flexible alkyl chains are mono-thermomesomorphic; their single mesophases are miscible with each other, with a typical carbohydrate derivative, and are characterized by X-ray diffraction having a layer structure without order in it. The phase transition data of the three icosanetetrols are strongly dependent on their stereochemistry. The measurements of the refractive indices of one of the tetrols show a typical behaviour known for S_A phases or for lamellar phases, i.e. the sign of the birefringence is positive.     

Direct transition from the SmA phase to the tilted hexatic phase in liquid crystals with several lactate units

Compounds with differing numbers of lactate units in the chiral part were synthesized. For all materials, at least two smectic phases were found. In addition to the SmA, the SmC* and/or the tilted hexatic SmI*(F*) phase appear according to the length of the non-chiral alkyl chain. For the shortest non-chiral chain, a direct transition from the SmA phase to the SmI*(F*) phase has been discovered and studied. For compounds with the 2-(S)-methylbutyl alkyl chain and two lactate units in the chiral part the antiferroelectric SmC*_A phase occurs. The ferroelectric character of the hexatic phase has been confirmed even just below the SmC*_A phase.     

Synthesis and mesomorphic properties of chiral swallow-tailed thioester liquid crystals with lateral substituents

Two homologous series of chiral swallow-tailed thioester materials, 1-ethylpropyl (R)-2-[3-fluoro-4-(4'-alkyloxybiphenylcarbonylthio)phenoxy]propionates, I-6 (m = 8-12), and 1-ethylpropyl (R)-2-[3-methyl-4-(4'-alkyloxybiphenylcarbonylthio)phenoxy]propionates, II-6 (m = 8-12), have been synthesized and their mesomorphic phases studied. The results show that materials I-6 (m = 8-12) with a fluoro substituent, display the phase sequence I-BPII-N*-SmC*-Cr, whereas materials II-6 (m = 8-12) with a methyl substituent display the phase sequence I-N*-TGBA*-SmA*-Cr during cooling. The physical properties of the SmC* phase in materials I-6 were measured. The results show that the maximum P _s values are in the range 91-95 nC cm^-2 and the maximum tilt angles are about 34-37°.     

Synthesis and ferroelectric properties of chiral liquid crystals derived from (S)-1-propyloxy-2-propanol

A new optically active chiral moiety, (S)-1-propyloxy-2-propanol, was designed and synthesized by the treatment of 1-propanol with (S)-propylene oxide under basic conditions. Its derivatives, the (R)-1-propyloxy-2-propyl 4-[4-(4-alkoxyphenyl)phenoyloxy]benzoates, PPmPPB (m = 8-12), were prepared for the investigation of mesomorphic properties. All of the chiral materials displayed enantiotropic SmA* and SmC* phases, and the shorter alkyl chain members (m = 8-11) displayed an additional unidentified SmX* phase. The switching current, spontaneous polarization, tilt angle, dielectric constant and electro-optical response for the materials in the ferroelectric SmC* phase were measured. The electro-optic responses in polyimide film-coated homogeneously aligned cells exhibit thresholdless, V-shaped switching in the ferroelectric phase.     

Ferroelectric liquid crystals with fluoro- and aza-fluorenone cores: the effect of stereo-polar coupling

The chiral fluorenone mesogens (R)-1-fluoro-2-(2-octyloxy)-7-(4-undecyloxybenzoyloxy)fluoren-9-one (2) and (R)-2-(2-octyloxy)-7-(4-undecyloxybenzoyloxy)-3-azafluoren-9-one (3) were synthesized using a combined ortho-directed and remote metalation strategy, which also incorporates a Suzuki-Miyaura crosscoupling step. These compounds form chiral SmC* liquid crystal phases with reduced polarizations (P _o) of +475 and +332 nC cm^-2 at 10 K below the Curie point, respectively. These values are considerably larger than that previously reported for the unsubstituted fluorenone (R)-2-(2-octyloxy)-7-(4-undecyloxybenzoyloxy)fluoren-9-one (1), which is +111 nC cm^-2 at 10 K below the Curie point. Molecular modelling based on the Boulder model suggests that the larger polarizations result from a conformational bias of the fluorenone core dipole moment along the polar axis of the SmC* phase caused by stereo-polar coupling with the chiral 2-octyloxy side chain.     

(R)-Binaphthoxy diiodide lanthanides

The synthesis and characterization of novel enantiopure binaphthoxy-diiodo lanthanides [(R)-2-(1-naphthol)-1′-naphthoxide)LnI₂(THF)₃] (Ln=Sm (4a), Yb (4b), La (4c)) are described. These complexes have been prepared by reacting the mono potassium salt of (R)-binaphthol with the corresponding lanthanide triiodides and were characterized by elemental analysis, IR and NMR spectroscopies. Recrystallization of 4c from THF–hexane led to monocrystals of [(R)-2-(1-naphthol)-1′-naphthoxide)]-diiodolanthane-tetrakistetrahydrofurane] (4c*). Complex 4c* crystallizes in the orthorhombic space group, P2₁2₁2₁ with cell parameters a=13.086(1) Å, b=15.496(1) Å, c=18.854(1) Å, V=3823.2(6) ų, and Z=4.     

(R)-Oxynitrilase catalyzed synthesis of (R)-ketone cyanohydrins

(R)-Oxynitrilase from almonds (Prunus amygdalus) catalyzes the enantioselective addition of HCN to ethyl alkyl ketones 1 in diisopropyl ether yielding (R)-ethyl alkyl ketone cyanohydrins (R)-2, which are hydrolyzed under acid catalysis to give the -hydroxy acids (R)-3. This (R)-oxynitrilase also catalyzes the enantioselective addition in aqueous citrate buffer (50 mM, pH 4.0), as demonstrated for the preparation of (R)-methyl alkyl ketone cyanohydrins (R)-5 which are obtained in high enantiomeric excesses comparable to those in diisopropyl ether as solvent.     

Tris-chelate complexes with chiral ligands: In search of diastereoisomeric selectivity with remote stereogenic centres

The chiral ligands, 4,4′-bis{(1S,2R,4S)-(−)-bornyloxy}-2,2′-bipyridine, (1S,2R,4S)-1, and 4,4′-bis{(1R,2S,4R)-(+)-bornyloxy}-2,2′-bipyridine, (1R,2S,4R)-1, have been prepared and characterized by spectroscopic techniques and, for (1S,2R,4S)-1, by single crystal X-ray diffraction. Despite the use of enantiomerically pure ligands, the formation of the complexes [Fe((1S,2R,4S)-1)₃]²⁺, [Ru((1S,2R,4S)-1)₃]²⁺, [Ru((1S,2R,4S)-1)(bpy)₂]²⁺ and [Ru((1R,2S,4R)-1)(bpy)₂]²⁺ proceeds without preference for either the Δ or Λ-diastereoisomers.     

Cholesteric helix inversion: Novel nitro compounds showing unusual changes of the cholesteric helical pitch

We present a homologeous series of liquid crystalline trioxadecalin compounds having a terminal alkoxy chain and a nitro group. The (1S, 3R, 6R,8R)-3-(4'-nitrophenyl)-8-(4'-octoxyphenyl)-2,4, 7-trioxabicyclo [4.4.0]decane 2 c shows a temperature-dependent inversion of the cholesteric helix at lower temperatures. For higher temperatures, the reciprocal helical pitch reaches a minimum, then it increases, tending to a second inversion point just above the clearing point. An additional chiral centre in the side chain leads for nitro compounds 3 to non-mesomorphic behaviour. For the cyano compounds 4, the change in the cholesteric helix is suppressed, for the R configuration, but for the S configuration helical inversion occurs at high temperatures and selective reflection above the transition to the TGB_A phase.     

Ferroelectric liquid crystals induced by dopants with axially chiral 2,2'-spirobiindan-1,1'-dione cores: diether vs. diester side-chains

A series of axially chiral 5,5'- and 6,6'-dialkanoyloxy-2,2'-spirobiindan-1,1'-dione dopants, (R)-2 and (R)-4a-4c were synthesized in optically pure form and their ferroelectric polarization powers, δ_p, measured in four liquid crystal hosts with isotropic (I)-nematic (N)-smectic A (SmA)-smectic C (SmC) phase sequences. The results show that the sign of polarization P _S induced by (R)-2 and (R)-4a-4c follows the same trend as that previously reported for the 5,5'- and 6,6'-diheptyloxy-2,2'-spirobiindan-1,1'-dione dopants, (R)-1 and (R)-3. The polarization induced by (R)-2 in the host DFT is below detection limits, and the sign of P _S was found to invert as a function of temperature at mole fractions as low as 0.01. On the other hand, the polarization power of the 6,6'-diheptanoyloxy dopant (R)-4b in the host NCB76 is -1449 nC cm^-2, the fourth highest value reported so far, and more than three times the δ_p value of the 5,5'-diheptanoyloxy analogue (R)-2 in that host (+474 nC cm^-2). Results of ²H NMR experiments suggest that (R)-4b exerts stronger local perturbations in NCB76 than (R)-2, and that these perturbations may be chiral in nature.     

Collective and molecular processes in a new class of ferroelectric liquid crystals

Electro-optic and dielectric studies have been performed for a thioester (with a chiral chain incorporating the lactic acid ester moiety), which exhibits a monotropic room temperature ferroelectric S_c* phase. Moreover, there are some hints that this substance also shows the recently discovered helicoidal S_A* phase. The dielectric and electro-optic measurements have been carried out using ITO coated glass plates (d = 10 μm). Planar alignment was obtained by means of an AC electric field (v = 1Hz and V_p-p = 70V). The tilt angle and spontaneous polarization temperature dependences show that the S_A*-S_c* transition is of second order. Soft mode behaviour in the pretransitional region also confirms this result. The monotropic S_c* phase exhibits a rather strong Goldstone mode (GM) with pronounced temperature dependence of the dielectric parameters. The molecular reorientation about the long axis has been found for both principal alignments. The homeotropic effective alignment was obtained by applying a magnetic field of 1-2 T to a thick sample (c. 100μm) placed in a specially designed cell for anisotropic high frequency measurements. The reorientation about the long axis gives a broad spectrum centred at c. 100MHz. This spectrum shows minor changes at the S_A*-S_c* transition.