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.     

A new series of nematic and smectic liquid crystals with negative dielectric anisotropy: the effect of terminal chain substitution on thermal and electro-optical properties

Two alkyl (1b and 1c) and four fluoroalkyl derivatives (1d-1f) of 4-arylbutyric acid (1c, 1d and 1e) and 4-arylbutanol (1b, 1f and 1g) [aryl = 2',3'-difluoro-4'-(2-(E-4-pentylcyclohexyl)ethyl)-biphenyl-1-yl] were prepared and investigated in the pure form as nematic materials (1b and 1c) and as additives to a ferroelectric liquid crystal (FLC) host (1d-1f). A comparison of 1b and 1c with the decyl analogue 1a demonstrates the effect of terminal chain modification on thermal and electro-optical properties. The substitution of the -CH₂O- (1b) or -COO- (1c) for -CH₂CH₂- in 1a destabilized the N and SmA phases or completely eliminated the smectic behaviour (1c). Dielectric analysis revealed that the chain modification increased the negative Δε, reduced elastic constant K ₁₁ and moderately decreased rotational viscosity γ₁. The temperature dependence of the key electro-optical parameters was analysed for ester 2, the methyl analogue of 1c, which exhibits a 45 K wide N phase. All four fluoroalkyl derivatives 1d-1f showed enantiotropic SmA phases and 1e also exhibited a monotropic SmC phase. Solutions of 1d-1f in a FLC host (0.2 mmol g^-1) increased the tilt angle Θ (up to 45° for 1g), reduced rotational viscosity γ₁ and the risetime τ. The most dramatic changes were observed for 1g, which contains 15 fluorine atoms.     

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.     

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.     

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.     

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.     

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.     

Photoswitching of ferroelectric liquid crystals using chiral thioindigo dopants: The development of a photochemical switch hitter

By virtue of its spontaneous polarization (PS), a ferroelectric SmC* liquid crystal can be switched between two states corresponding to opposite molecular tilt orientations using an electric field, thus producing an ON-OFF light shutter between crossed polarizers. Considerable efforts have been made over the past decade to develop photonic FLC light shutters because of their potential uses in dynamic holography and optical data storage. The ON-OFF switching of a FLC light shutter can be triggered by light via a photoinversion of PS using a photochromic dopant. The spontaneous polarization is a chiral bulk property that can be left-handed (negative) or right-handed (positive), depending on the absolute configuration of the chiral component of the SmC* phase. In the approach described herein, the magnitude of PS is modulated via the photoisomerization of a chiral thioindigo dopant that undergoes a large increase in transverse dipole moment upon trans-cis photoisomerization. The sign of PS is photoinverted using an "ambidextrous" thioindigo dopant containing a chiral 2-octyloxy side chain that is coupled to the thioindigo core and induces a positive PS, and a chiral 2,3-difluorooctyloxy side chain that is decoupled from the core and induces a negative PS. In the trans form, the 2,3-difluorooctyloxy side chain predominates and the net PS induced by the dopant is negative. However, upon trans-cis-photoisomerization, the increase in transverse dipole moment of the 2-octyloxy/thioindigo unit raises its induced PS over that of the decoupled 2,3-difluorooctyloxy side chain, and thus inverts the net sign of PS induced by the dopant from negative to positive.     

Photoinduced polarization inversion in a ferroelectric liquid crystal using an ambidextrous chiral thioindigo dopant

In this Communication, we report the first example of photoswitching of a ferroelectric SmC* liquid crystal based on a photoinduced sign inversion of the spontaneous polarization (PS) induced by a single chiral dopant. This is achieved without concomitant destabilization of the SmC* phase using the "ambidextrous" dopant 6-((R,R)-2,3-difluorooct-1-yloxy)-5'-nitro-6'-((R)-2-octyloxy)thioindigo. The (R)-2-octyloxy side chain is sterically coupled to the thioindigo core via the nitro substituent and induces a positive polarization, whereas the (R,R)-2,3-difluorooct-1-yloxy side chain is decoupled from the core and induces a negative polarization. With this new design, the increase in transverse dipole moment of the thioindigo core upon trans-cis photoisomerization raises the polarization power of the coupled 2-octyloxy/thioindigo unit above that of the 2,3-difluorooctyloxy unit and inverts the net sign of PS.     

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.     

Ferroelectric liquid crystals induced by dopants with axially chiral 2,2'-spirobiindan-1,1'-dione cores

The axially chiral dopants (R)-5,5'-, 5,6'-, and 6,6'-diheptyloxy-2,2'-spirobiindan-1,1'-dione ((R)-2, -3, and -4) were synthesized in optically pure form, and their absolute configurations were assigned by the exciton chirality method using circular dichroism spectroscopy. These new compounds were doped in four achiral liquid crystal hosts to give chiral smectic C* (SmC*) phases with spontaneous polarizations (Ps) that vary with the core structure of the host. The spontaneous polarization induced by the 5,5'-dialkoxy derivative (R)-2 is uniformly positive, whereas that induced by the 6,6'-dialkoxy derivative (R)-4 is uniformly negative and shows a different trend in host dependence. Polarization power (delta(p)) values range from +21 nC/cm2 for (R)-2 in 2',3'-difluoro-4-heptyl-4' '-nonyl-p-terphenyl (DFT) to -1037 nC/cm2 for (R)-4 in 4-(4'-heptyl[1,1'-biphen]-4-yl)-1-hexylcyclohexanecarbonitrile (NCB76). The unsymmetrical dopant (R)-3 behaves like a hybrid of the two symmetrical isomers, with lower absolute values of delta(p), on average, and varying signs of Ps. 2H NMR spectra of the doped mixtures using racemic mixtures of 2-4 with -OCD2C6H13 side-chains, in combination with phase diagrams, show that relatively minor changes in the dopant structure, that is, moving the alkoxy side-chains from the 5,5' to the 6,6' positions of the spirobiindandione core, have profound effects on dopant-host compatibility, and on the propensity of the dopant to exert chiral perturbations in the host environment. The variations in sign and magnitude of delta(p) as a function of alkoxy group positions are rationalized based on an analysis of zigzag conformations that conform to the binding site of the SmC host according to the Boulder model.     

Synthesis and thermal properties of mesomorphic 1,1'-bis[ω-(4'-cyano-4-biphenyloxy)alkyl] ferrocenes

A new series of 1,1'-disubstituted ferrocene compounds of the type [(η⁵-C₅H₄(CH₂)_nOC₆H₄C₆H₄CN]₂Fe (3a-d, n = 5, 6, 8, 11) incorporating a variable length alkyloxy cyanobiphenyl unit has been prepared and their mesomorphic properties have been investigated. Compounds 3b, c and d exhibit a thermotropic smectic C phase and 3c also exhibits a monotropic smectic A phase over a fairly wide range near ambient temperature.     

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.     

Thermotropic biaxial nematic phases with negative optical character [1]

Two disc-shaped multialkynyl arene ethers (1 and 2) with unusual thermo-mesomorphic properties are presented. Conoscopic studies show that the nematic phases of these new low molecular weight liquid crystals are biaxial and that the sign of their biaxiality is negative. The diether 2 is the first discotic twin liquid crystal which exhibits a nematic phase.     

Induced smectic C* phases

Induced ferroelectric S*_C phases are formed by non-chiral S_C host phases doped with chiral dipolar guest molecules. In those mixtures the spontaneous polarization P_s and the tilt angle Θ has been investigated as a function of the mole fraction x_G of the chiral dopant. In most cases the reduced polarization P₀ = P_S/ sin Θ has been found to depend linearly on x_G. The polarization power which is defined by δ_P=(∂P₀/∂x_G)ΔT is discussed in terms of the molecular structure of the chiral dopants. There are systems in which P₀(x_G) deviates positively from linearity. This behaviour can be understood by considering a local field correction to P₀. By assuming a local field of Lorentz type a theoretical relation for P₀(x_G) has been derived which explains the experimental results. The effect of a local field is considerable if the transverse dipole moment and the polarizability of the chiral dopant are large.     

Viscoelastic behaviour of the SmC* phase

Using a light scattering technique at a fixed temperature, we have investigated viscoelastic behaviour exhibited by the ferroelectric smectic phase (SmC*) of C₈tolane in a homeotropic orientation. Experiments were performed in backward and forward scattering geometries that allowed us to deduce separately orientational diffusivities k₃/η and k₊/η corresponding to the Goldstone mode. The k₃/η value measured in the SmC* phase is about 100 times higher than in the SmC_A* phase exhibited by the same liquid crystal compound. The factor 100 may be attributed in great part to the molecular arrangement mode in adjacent smectic layers. However k₊/η measured in the SmC* phase is in the same order of magnitude as those measured previously in SmC_A* phases.     

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.     

A study on the effect of carboxylate ester and thioester linking groups in ferroelectric liquid crystal dimers

Two series of symmetrical liquid crystal twins possessing a chiral centre at the inner side of the molecules were synthesized and characterized. Structural effects on the mesomorphic and physical properties were investigated in terms of (a) variation of carboxylate and thioester groups linking the spacer and core, (b) variation in achiral chain length between two mesogenic units. The mesomorphic investigation reveals that these compounds exhibit a BP, N*, SmA* and SmC* mesophase sequence for the BDPNP series, and a SmA and SmC* sequence for the ABPNTP-n series. The P_s values were measured and a largest of 38.5 nC cm^-2 was observed for ABPNTP-12.     

Novel mesogenic asymmetric diacylhydrazines: synthesis, mesomorphic behaviour and X-ray study

The synthesis and liquid crystalline properties of a new series of diacylhydrazine derivatives (2a-g) are reported. All compounds of series 2 exhibit a smectic C (SmC) phase. The first homologues (2a, 2b) display a monotropic SmC mesophase, whereas the highest homologues (2c-g) exhibit an enantiotropic SmC phase. The liquid crystalline properties were investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray measurements.     

Crystal-smectic E mesophases in a series of 2-(4-n-alkylphenyl)indenes

A series of 2-(4-n-alkylphenyl)indenes (3) with different alkyl substituents (CH₃ to C₁₀H₂₁) were synthesized and systematically characterized using differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction compared with 2-phenylindene (3a). Depending on the alkyl chain length, highly ordered crystal-smectic E mesophases were observed and confirmed by X-ray diffraction for the derivatives 3h-3k with heptyl to decyl chains (n = 6-9). For 3f with a pentyl side chain (n = 4), an X-ray crystal structure analysis was carried out.