Ferroelectric liquid crystal materials synthesized from 2(S)-[2(S)-methylbutyloxy]propanol

A new chiral alcohol, 2(S)-[2(S)-methylbutyloxy]propanol (3), containing two chiral centres has been synthesized from ethyl lactate and (S)-1-iodo-2-methylbutane. It was used as a chiral building block for the preparation of ferroelectric liquid crystal materials. Several of the new materials exhibit an enantiotropic S*_c phase with a wide temperature range. The results indicate that the molecular structure of 3 is useful for synthesizing ferroelectric liquid crystal materials.     

Ferroelectric liquid crystals induced by atropisomeric biphenyl dopants: the effect of chiral perturbations on achiral dopants

The addition of the achiral biphenyl dopant 2,2',6,6'-tetramethyl-4,4'-bis(4-n-nonyloxybenzoyloxy)biphenyl (3) or its dithionoester or dithioester analogue (4, 5) to a 4 mol % mixture of the atropisomeric biphenyl dopant (R)-2,2',6,6'-tetramethyl-3,3'-dinitro-4,4'-bis(4-n-nonyloxybenzoyloxy)biphenyl, (R)-1, in the phenylpyrimidine SmC host PhP1 produces a significant amplification of the spontaneous polarization induced by (R)-1. This amplification may be due to a chiral perturbation by (R)-1 which causes a shift in the equilibrium between enantiomeric conformations of the achiral dopant. The degree of polarization amplification afforded by the achiral dopant, as expressed by the polarization amplification factor PAF, varies with the nature of the linking group. This may be ascribed to different rotational distributions of the core transverse dipole moments relative to the polar axis of the SmC* phase and/or to differences in lateral bulk of the polar linking groups. The latter may affect the degree of chiral molecular recognition achieved by 3-5 in the binding site of the SmC* phase.     

Liquid crystalline properties of dissymmetric molecules IV. The substituent effect on thermal properties of nematic and smectic A phases in three aromatic ring systems with ester linkages

This paper describes the effect of substituent and ester linkage on smectic properties for some derivatives of 4-R -phenyl 4-(4-octyloxybenzoyloxy)benzoates (1 ), 4-octyloxyphenyl 4-(4-R-benzyloxy)benzoates (2), 4-(4-octyloxybenzoyloxy)phenyl 4- R -benzoates (3), and 4-R-phenyl 4-octyloxyphenyl terephthalates (4) where R = OCH₃, CH₃, OC₈H₁₇, C8H17, halogens, CF₃, OCF₃, CN, NO2, etc. The thermal properties are discussed in terms of the electrostatic nature of the substituents and the relative orientation of the ester groups with respect to both terminal substituents. The substituent effect on the layer structure of the smectic A phase is also examined by means of a small angle X-ray analysis.     

Induction of nematic cholesteric (N*) phases by charge transfer complexation of disc-shaped multiynes [1]

Based on charge transfer interactions with the (chiral) electron acceptor (-)-2-(2,4,5,7-tetranitro-9-fluorenylidenaminooxy)propionic acid ((-)-TAPA), our easily accessible disc-shaped electron rich multiynes (for example, 1 and 2) give rise to two types of cholesteric nematic phases. Thus, the binary mixture of the nematic discotic (N_D) pentakis-(4-methylphenylethynyl)phenyl hexadecyl ether (2) with (-)-TAPA exhibits its twisted variant (N*_D). Likewise, but now in a ternary mixture, the homologue 1—peripherically unsubstituted and non-mesogenic—with the two electron acceptors (-)- TAPA and 2,4,7-trinitrofluorenone—both also non-liquid crystalline—shows cholesteric nematic properties, most probably however of a columnar type (N*_C). First results concerning these charge transfer induced cholesteric-nematic properties, including phase diagrams and the helical twist of the two systems presented here, are discussed on the basis of data obtained by polarizing microscopy and differential scanning calorimetry.     

The properties of phenyl cyclohexyl ferroelectric liquid crystals

To continue the search for novel series of ferroelectric liquid crystals, an additional two series of compounds, C S(+)-4-[(2-methylbutyl)phenyl]4'-trans-n-alkylcyclohexyl) benzoates and D S(+)-4-[(4-methylhexanoyl)phenyl]4'(4'-trans-n-alkylcyclohexyl) benzoates, where alkyl = C₅-C₆ are reported in this paper. Their mesomorphic properties have been characterized by optical textural observations and DSC. Some of these compounds show a C-S*_C-S_A-N*-I phase transition sequence. The effects of the alkyl chains on transition temperatures and optically active terminal groups on phase behaviour are discussed.     

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.     

Heterocyclic benzoxazole-based liquid crystals

The synthesis, characterization, and mesomorphic properties of a new type of heteronuclear compounds 1a-c and a Pd complex 1d derived from benzoxazole as the core group are reported. These compounds were prepared by the ring closure reaction of 4-alkoxybenzoic acid 4-[(4-alkoxy-2-hydroxyphenylimino)methyl]phenyl esters 6 in the presence of lead(IV) acetate. All the compounds were characterized by ¹H and ¹³C NMR spectroscopies and elemental analysis. The phase behaviour of these mesogenic compounds was characterized and studied by differential scanning calorimetry and polarizing optical microscopy. All the compounds 1a exhibited nematic (N) and/or smectic C (SmC) phases, as expected for rod-like molecules; however, the compounds 1b and 1c exhibited crystal phases. For those compounds 1a having shorter carbon chains (n = 1, 3, 4) nematic phases were observed, whereas for compounds having longer carbon chains (n = 6, 7, 8, 10, 12, 14) smectic C behaviour was also observed at lower temperatures. The greater aspect ratio (l/d) of compounds 1a compared with 1b and 1c was found to be required for the observation of liquid crystallinity. The fluorescent properties of these compounds were also examined. All λ_max peaks of the absorption and photoluminescence spectra of compounds 1a-1c occurred at c. 316-322 nm and 371-382 nm, respectively. The quantum yields of some compounds were relatively low, and also slightly solvent-dependent.     

The synthesis and liquid crystalline behaviour of 2-(4-n-alkoxyphenyl)-5-methylpyridines

We have synthesized a new series of polyphenylene compounds with a pyridine ring, 2-(4-n-alkoxyphenyl)-5-methylpyridines (CH₃C₅H₃NC₆H₄OC_iH_2i1) (1a-i; the carbon number, i, of the alkoxy group is 1-9), and studied their phase transitions and mesogenicity using DSC and polarizing microscopy. In the homologous series of 1a-i, no mesomorphic phase appeared for i=1-5 and only a monotropic nematic phase was observed for i=6-9. The mesomorphic potential is relatively weaker for the 1a-i than for the 2a-i and 3a-i homologues, which are pyridine-containing three- and four-ring similar systems. This lowering of the mesogenicity may be due to the lack of a phenyl ring between the pyridine ring and the methyl group in the 1a-i homologues.     

Synthesis and transition temperatures of novel fluorinated chiral liquid crystals containing 1,4-tetrafluorophenylene units

To continue the search for novel series of fluorinated ferroelectric liquid crystals, an additional two series of 4-[(S)-2-methylbutoxy]phenyl 4-[(4-n-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] benzoates (C) and 4-(n-alkoxy)phenyl 4-[(4-(S)-2'-methylbutoxy-2,3,5,6-tetrafluorophenyl)] benzoates (D) have been synthesized. Polarizing microscopic textural observations and DSC measurements of the phase transitions of these novel compounds showed that compounds C were liquid crystals with a chiral nematic (N) phase and a monotropic chiral smectic C phase (S_c), and compounds D exhibited a chiral nematic (N) phase.     

Synthesis and properties of optically active 1,3-diols and their derivatives as chiral dopants for ferroelectric liquid crystals

New chiral dopants β-hydroxy ketones 1, 1,3-diols 2, 1,3-dioxanes 3 and 1,3-dioxan-2-ones 4 were designed and synthesized. Reaction of (R)-1,2-epoxyoctane with carbanions derived from 2-(4-substituted phenyl)-1,3-dithianes followed by hydrolysis of the resulting hydroxy dithianes afforded 1. Reduction of 1 gave the diols 2, which yielded 3 upon acetalization and 4 upon carbonation. The syn isomers of 2, 3, and 4 exhibited larger spontaneous polarizations, when applied as chiral dopants, than the anti isomers or hydroxy ketones 1.     

Phase transitions and conformational changes in an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl)phenyl 4'-octyloxybiphenyl-4-carboxylate (MHPOBC)

The thermodynamic properties and conformational structures in the crystalline and liquid crystalline phases of 4-(1-methylheptyloxycarbonyl)phenyl 4'-octyloxybiphenyl-4-carboxylate (MHPOBC) have been investigated by Raman scattering, X-ray diffraction and differential scanning calorimetry (DSC). MHPOBC can assume two different crystalline states at room temperature depending on crystallization conditions, and heating transforms the metastable crystal to the stable one before melting. In the liquid crystalline phases, Raman scattering spectra have revealed not only the flip-flop twisting vibration of the biphenyl group but also the internal rotation around C-C bonds between the carbonyl groups and the corresponding benzene rings.     

Structural characterization of the smectic polymorphism of chiral liquid crystalline polysiloxanes containing biphenyl mesogens

A structural characterization of chiral side chain siloxanes with different average degrees of polymerization, DP_n, was performed by X-ray diffraction experiments on powder and oriented fibre specimens. Polymers (DP_n = 35) and oligomers (DP_n = 4) contained the 4,4'-biphenylene unit with either an (S)-2-methylbutoxy (An, Bn) or an (S)-2-chloro-3-methylbutanoyloxy substituent (Cll). The spacer segment connected to the siloxane backbone had a variable number, n, of methylene groups (n = 5, 8, or 11). Independent of the spacer length and the chiral tail nature, the polysiloxanes underwent the same sequence of phases: C-S_F1 (or S_I1)-S_C1-S_A1-I, whereas in the oligosiloxanes the sequence C-S_B1-S_A1-I (B11) or C-S_F1-S_C1-I (B5) occurred. The influence of the structure of the polysiloxanes on the formation of the smectic (tilted or orthogonal) mesophases was elucidated. The rather large number of reflections (three or four) detected in the X-ray patterns at low angles, allowed a drawing of the projection of the electron density profiles along the layer normal, p(z), and deduction of the most physically acceptable electron density profiles from among the numerous possibilities for each smectic phase. The electron density profiles were in agreement with monolayer smectic phases presenting a microphase separation between the siloxane backbones and the side chains, so constraining the polymer backbones within a thin layer.     

First Total Synthesis of (±)-Ponganone Ⅲ

In recent years, the synthesis and pharmacology of pyranoflavanoids have been extensively investigated due to their wide range of pharmacological properties^[1-3]. Ponganone Ⅲ^[4],a new natural pyranoflavanone isolated from the Pongamia pinnala, was identified as (2S)-3',4'-dimethoxy-6',6"-dimethylpyrano-[2",3":7,8]-flavanone (2) on the basis of spectra data. Its precursor,3,4-dimethoxy-2'-hydroxy-6",6"-dimethylchromeno-[2",3":4',3']-chalcone (1) is also a new natural product^[5] isolated from the roots of Lonchocarpus subglaucescent. Their total synthesis have not been reported. Herein, we wish to report the first total synthesis of compounds 1 and 2 in order to confirm the proposed structure and further more to evaluate its biological activities.     

Liquid crystalline hexa-amides and side chain polysiloxanes of azacrown [18]-N6

The hexa-4-dodecyloxybenzoyl derivative 1 of azacrown [18]-N₆ was originally reported to have a 'tubular' mesophase on the basis of its large central ring and 6-fold symmetry. Starting in the mesophase, annealing of 1 under a cover slip results in formation of a new crystalline phase that melts directly to an isotropic liquid at the temperature previously observed for the mesophase to isotropic transition. Thus the phase behaviour of 1 is kinetically controlled. The analogous hexa-3,4-bisdodecyloxybenzoyl derivative 2 of azacrown [18]-N₆ has no kinetic limitations to its phase changes and has an enantiotropic columnar liquid crystalline phase. We have synthesized side chain copolysiloxanes with a (CH₂)₁₁ spacer and 75-84 per cent by weight of the same 4-dodecyloxybenzoyl-[18]-N₆ mesogen. The polysiloxanes also display a liquid crystalline phase.     

The substituent effect on mesomorphic properties of 4-octyloxyphenyl 4-(4-R-3-nitrobenzoyloxy)benzoates and 4-(4-octyloxybenzoyloxy)phenyl 4-R-3-nitrobenzoates

The thermal properties of 4-octyloxyphenyl 4-(4-R-3-nitrobenzoyloxy) benzo-ates (1) and 4-(4-octyloxybenzoyloxy)phenyl 4-R-3-nitrobenzoates (2) have been examined, where R = hydrogen, halogens, alkyl and alkoxy groups. The derivatives of compound 1 incorporating hydrogen, halogens, methoxy and nitro groups show a smectic A phase having a bilayer arrangement, and the others with a long alkoxy group show the S_A phase with the monolayer arrangement. The derivatives of compound 2 incorporating halogens, and the nitro group show the S_A phase with the monolayer arrangement. The alkoxy derivatives show a smectic C phase as well as the nematic phase. The nitro group at the lateral position tends to increase the ratio of the S_A-N transition temperature to the N-I. The effect of the nitro group on the smectic properties has been discussed in terms of the structural and electrostatic nature of the nitro group.     

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.     

Recent Progress in the Synthetic Applications of Substituted Furans and Pyrroles

In this lecture, I would like to present efficient synthetic routes towards the total synthesis of syringolide 1 (2)^1,2 and secosyrin 1 (3),¹ employing 2,4-disubstituted furan 1 as a common precursor. A modified total synthesis of prehispanolone (6)³ from (S)-(+)-Wieland-Miescher ketone (4) via hispanolone (5) will also be discussed. Our methodology for the synthesis of 3,4-disubstituted-lH-pyrroles⁴ and for the generation of 3,4-didehydrothiophene⁵ has also guided us to the successful identification of 1-tert-butoxycarbonyl-3,4-didehydro-1H-pyrrole (7),which is presumably the most strained heterocyclic cumulene ever recorded.     

Ferroelectric liquid crystal dopants with a chiral (R,R)-2,3-difluorooctyloxy side-chain: host dependence of the polarization power

The polarization powers δ_p of four chiral dopants with (R,R)-2,3-difluorooctyloxy side-chains were measured in four liquid crystal hosts with isotropic (I)-nematic (N)-smectic A (SmA)-smectic C (SmC) phase sequences. The four chiral dopants differ in terms of their core structures: 2-phenylpyridine (MDW950), biphenyl (5), 2-phenylpyrimidine (6) and 2-(3-nitrophenyl)pyrimidine (7). In each case, δ_p varies with the structure of the liquid crystal host, which is consistent with the behaviour of so-called Type II dopants that normally feature a chiral core structure. The δ_p(host) profile was found to depend on the degree of biaxiality of the dopant core structure, and on the degree of steric coupling between the chiral 2,3-difluorooctyloxy side-chain and the core. Conformational analyses at the B3LYP/6-31G* level suggest that the 2,3-difluorooctyloxy side-chain is conformationally more rigid than conventional chiral side-chains due to the added electrostatic repulsion of the two adjacent fluoro groups combined with the hyperconjugative 'gauche effect', and may therefore have a higher degree of biaxiality on the time average. This biaxial character should make the chiral side-chain more sensitive to variations in quadrupolar ordering imposed by the SmC phase of the liquid crystal host, and may therefore explain the dependence of δ_p on the host structure reported herein.     

V-shaped switching in binary mixtures of an achiral swallow-tailed material with the antiferroelectric liquid crystal (S)-MHPOBC

An achiral swallow-tailed material, 2-propylpentyl 4-(4'-decyloxybiphenyl-4-carbonyloxy)benzoate, p, showing SmA and SmC_alt phases was prepared for mixing (by weight percentage) with an antiferroelectric liquid crystal, (S)-MHPOBC, m, for the study. The binary mixture p15/m85 using (S)-MHPOBC (85%) as a host doped with achiral material (15%) resulted in a phase sequence SmA-SmC*-SmC*_A. The electro-optic response of this mixture in the ferroelectric SmC* phase displayed V-shaped switching, while that in the antiferroelectric SmC*_A phase displayed a double hysteresis switching. The mixture p85/m15 possessed SmA* and SmC*_A phases; V-shaped switching was found in the antiferroelectric SmC*_A phase of this mixture. These optical phenomena implied that a binary mixture containing a larger amount of achiral swallow-tailed material and/or possessing relatively lower polarization favours the occurrence of V-shaped switching in the antiferroelectric phase. The results of this work also suggested that thresholdless V-shaped switching in chiral smectic liquid crystals can be achieved by mixing an achiral swallow-tailed material with an antiferroelectric liquid crystal.     

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.