Search

The surface electroclinic effect, which is a rotation of the molecular director in the substrate plane proportional to an electric field E applied normal to the substrate, requires both a chiral environment and C(2) (or lower) rotational symmetry about E. The two symmetries typically are created in tandem by manipulating the surface topography, a process that conflates their effects. Here we use a pair of rubbed polymer-coated substrates in a twist geometry to obtain our main result, viz., that the strengths of two symmetries, in this case the rub-induced breaking of C(∞) rotational symmetry and chiral symmetry, can be separated and quantified. Experimentally we observe that the strength of the reduced rotational symmetry arising from the rub-induced scratches, which is proportional to the electroclinic response, scales linearly with the induced topographical rms roughness and increases with increasing rubbing strength of the polymer. Our results also suggest that the azimuthal anchoring strength coefficient is relatively insensitive to the strength of the rubbing.

According to a new design strategy for "de Vries-like" liquid crystal materials, we report the synthesis and characterization of an organosiloxane mesogen with a 5-phenylpyrimidine core that forms SmA and SmC liquid crystal phases. This new material is characterized by a maximum layer contraction of 1.2% upon SmA-SmC phase transition and is comparable to the best "de Vries-like" materials reported heretofore.

Smectic liquid crystals with a low layer contraction of less than 1% upon an SmA to SmC transition are referred to as 'de Vries-like'. These materials have attracted considerable attention as possible alternative to problems with surface-stabilized ferroelectric liquid crystals in liquid crystal displays. The 'de Vries-like' properties of liquid crystals with nanosegregating carbosilane segments are enhanced by lengthening the carbosilane end-groups from mono- to tricarbosilane. The observed enhancement is thought to arise from an increase in the cross-section of the free volume in the hydrocarbon sub-layer. The hypothesis is tested by assuming that dimers with a tricarbosilane linking group have smaller time-averaged cross-sections. This paper reports the synthesis and characterization of two homologous series of liquid crystals and an investigation of their mesogenic properties as model systems employing liquid crystalline monomers (QL39-n) and dimers (QL40-n) comprising 2-phenylpyrimidine cores and tricarbosilane end-groups and spacers, respectively. We find that the QL39-n monomers form only a tilted SmC phase, whereas the QL40-n dimer forms an orthogonal SmA phase due to the tricarbosilane spacer will not have a spherically shape on the time average like that predicted for the tricarbosilane end-group. Results are discussed in the context of the timeaveraged cross-sections of the materials. [ABSTRACT FROM AUTHOR]

We investigate the origins of 'de Vries-like' liquid crystalline behaviour by introducing an ethynyl spacer in the core of the tricarbosilane-terminated 5-phenylpyrimidine mesogens QL16-6 and QL17-6. The rationale for this structural change is based on the assumption that an ethynyl spacer would create more free volume in the core sub-layer and therefore decrease the orientational order parameter S2 in the SmA phase. The tricarbosilane-terminated mesogens WL41-5 and WL42-6 with a 5-(phenylethynyl)pyrimidine core in either a normal or inverted orientation were synthesised, and their mesomorphic and 'de Vries-like' properties characterised using polarised optical microscopy, differential scanning calorimetry, birefringence measurements, small–angle (SAXS) and 2D X-ray scattering. Reduction factors R derived from SAXS and optical tilt angle measurements suggest that neither WL41-5 (R = 0.49) nor WL42-6 (R = 0.80) exhibit 'de Vries-like' properties. The S2(T) profiles show an increase in orientational order with decreasing temperature and a sharp discontinuity at the SmA-SmC transition, which is consistent with 'de Vries-like' behaviour. However, the Leff(T) profiles suggest an increase in interdigitation that reduces the positive effect of increasing S2 in compensating for the molecular tilt. [ABSTRACT FROM AUTHOR]

The chiral carbosilane-terminated liquid crystal 2-[(2S,3S)-2,3-difluorohexyloxy]-5-[4-(12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecyloxy)phenyl]pyrimidine (QL32-6) undergoes a smectic A*-smectic C* phase transition with a maximum layer contraction of only 0.2%. It exhibits an electroclinic effect (ECE) comparable to that reported for the ‘de Vries-like’ liquid crystal 8422[2F3] and shows no appreciable optical stripe defects due to horizontal chevron formation. [ABSTRACT FROM AUTHOR]

Two sterically equivalent series of phenoxy-terminated 5-alkoxy-2-(4-alkoxyphenyl)pyrimidine liquid crystals were synthesised, and their mesogenic properties were characterised by polarised optical microscopy and differential scanning calorimetry (DSC). The phenoxy end-group causes a significant increase in melting point and inhibits – at least partially – the mesomorphism of these materials relative to the parent isomers; in most cases, the broad enantiotropic SmC phase formed by the parent isomers is suppressed by the addition of the phenoxy end-group. However, detailed analyses by small-angle X-ray scattering and monodomain 2D X-ray scattering suggest that these compounds form a SmA phase with a partially intercalated bilayer structure in which the phenoxy end-groups are nanosegregated. Such an intercalated bilayer structure might enable the tuning of smectogenic properties by appropriate substitution of the phenoxy end-groups. [ABSTRACT FROM AUTHOR]

Multi-walled carbon nanotubes were dispersed at low concentrations into various achiral liquid crystals having either a thioester or oxoester linkage group in the core. The presence of the carbon nanotubes resulted in chiral signatures being observed in the liquid crystals, including an electroclinic effect (a rotation of the liquid crystal director perpendicular to, and linear in, an applied electric field) in both the nematic and smectic A phases, and a macroscopic helical twist of the liquid crystal director in the nematic phase. For both experiments the chiral signatures for the thioester liquid crystals were found to be an order of magnitude larger than those of the oxoesters. We speculate that the much larger strength of the thioester's chiral properties is a result of stronger non-covalent interactions between the liquid crystal molecule and carbon nanotube. [ABSTRACT FROM AUTHOR]

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, δp, 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 δp 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. [ABSTRACT FROM AUTHOR]

The polarization powers δp 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, δp 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 δp(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 δp on the host structure reported herein. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Two new ferroelectric liquid crystals, 4-[(S)-1-(ethoxycarbonyl)]ethyl 4-[4-(9-decenyloxy)biphenyl]azobenzoate (10) and its ester analogue 4-[(S)-1-(ethoxycarbonyl)]ethyl {[4-(9-decenyloxy)]biphenyl-4'-yl}carbonyloxybenzoate (17), and side chain polysiloxane copolymers and terpolymers containing various amounts of 10 and 17 as mesogenic side groups have been synthesized. Their mesomorphic and physical properties have been characterized. Both 10 and 17 exhibit isotropic, SmA*, SmC*, and Sm(I or F)* phases. The copolymer with 17 as the mesogenic group and terpolymers having low content of the azo compound 10 show a SmC* phase over a large temperature range down to room temperature. The copolymer with the azo compound 10 as the mesogenic group is amorphous. [ABSTRACT FROM AUTHOR]

Two series of mesogens derived from the non-activated spiropyran dyes 6-hydroxy-1 ,3 ,3- trimethylspiro\[2H -1-benzopyran-2,2-indoline] (series trimethylspiro\[2 H -1-benzopyran-2,2-indoline] (series 2 ) have been synthesized. Analysis by differential scanning calorimetry, polarizing microscopy and X-ray diffraction showed compounds in series 1 form monotropic nematic and SmA phases, while compounds in series 2 form only a monotropic nematic phase. [ABSTRACT FROM AUTHOR]

A nematic twist cell having a thickness gradient was filled with a mixture containing a configurationally achiral liquid crystal (LC) and chiral dopant. A chiral-based linear electrooptic effect was observed on application of an ac electric field. This 'electroclinic effect' varied monotonically with d, changing sign at d=d0 where the chiral dopant exactly compensated the imposed twist. The results indicate that a significant chiral electrooptic effect always exists near the surfaces of a twist cell containing molecules that can be conformationally deracemized. Additionally, this approach can be used to measure the helical twisting power (HTP) of a chiral dopant in a liquid crystal. [ABSTRACT FROM AUTHOR]

The solubility of two partially deuterated thioindigo dopants in a smectic liquid crystal host was evaluated by variable temperature [sup 2] H NMR spectroscopy and polarized microscopy. 2H NMR spectra showed that the dopant (±)-6,6'-bis(2-octyloxy)-5,5-dinitrothioindigo-d[sub 6] forms a homogeneous solution with the smectic phases of the liquid crystal host (±)-4-(4-methylhexyloxy)phenyl 4-decyloxybenzoate (PhB) up to its saturation point of 3 mol %. These results are consistent with polarized microscopy observations of the dopant crystallizing out of solution upon reaching a concentration of 3 mol %. On the other hand, [sup 2] H NMR spectra of (±)-5,5'-dichloro-6,6'-bis(2-octyloxy)thioindigo-d[sub 6] dissolved in PhBshowed evidence of a partitioning of the solution between smectic and isotropic microdomains, which increases with increasing dopant concentration-from 1.2 to 9.1 mol %. To a large extent, this smectic/isotropic microphase separation could not be detected by polarized microscopy. These results suggest that [sup 2] H NMR spectroscopy can provide a more accurate determination of the occurrence and extent of microphase separation in doped liquid crystal samples. [ABSTRACT FROM AUTHOR]

Chiral organosilica particles of size ∼200 nm were synthesized from an enantio-pure multi-armed chiral d -maltose organosilane precursor in the absence of co-condensation with an achiral monomer. Two distinct experiments were performed to demonstrate the particles’ ability to induce conformational deracemization of a host liquid crystal. The first involves an electric field-induced tilt of the liquid crystal director in the deracemized smectic-A phase. The other involves domain wall curvature separating left- and right-handed liquid crystal helical pitch domains imposed by the cells’ substrates. The results demonstrate unequivocally that enantio-pure organosilica nanoparticles can be synthesized and can induce chirality in a host. [ABSTRACT FROM AUTHOR]

Studies the photoinduced polarization inversion in a ferroelectric liquid crystal (FLC) using an ambidextrous chiral thioindigo dopant. Alkylation of the known compound (R)-6'-hydroxy-5-nitro-6-(2-octyloxy)thioindigo with (R,R)-2,3-difluorooct-1-y1 tosylate; Measurement of the spontaneous polarization of the FLC film.

According to a new design strategy for "de Vries-like" liquid crystal materials, which are characterized by a maximum layer contraction of ⩽ 1% upon transition from the SmA phase to the SmC phase, we report the synthesis and characterization of two homologous series of organosiloxane mesogens. The design of these new materials is based on a frustration between one structural element that promotes the formation of a SmC phase (a trisiloxane-terminated side-chain) and one that promotes the formation of a SmA phase (either a chloro-terminated side-chain or a 5-phenylpyrimidine core). Measurements of smectic layer spacing d as a function of temperature by small-angle X-ray scattering (SAXS) combined with optical tilt angle measurements revealed that the mesogeris 5-(4-(11 -(1,1,1 ,3,3,5,5-heptamethyltri- siloxanyl)-undecyloxy)phenyl)-2-(1 -alkyloxy)pyrimidine (3(n)) undergo SmA-SmC phase transitions with maximum layer contractions ranging from 0.5% to 1.4%. A comparison of reduction factors Rand I suggests that this behavior is due in part to a pronounced negative thermal expansion in the SmC phase that counterbalances the layer contraction caused by increasing tilt. SAXS measurements also revealed that compounds 3(n) are characterized by low orientational and high translational order, which is consistent with theoretical predictions that such materials should exhibit de Vries-like properties. The R values for series 3(n) are comparable to, and even lower than, those reported for established de Vries-like materials such as the perfluorinated 2-phenylpyrimidine material 3M 8422. [ABSTRACT FROM AUTHOR]

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 (δp) 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 δp, 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 δp 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. [ABSTRACT FROM AUTHOR]

A new series of smectic C* (SmC*) mesogens containing a chiral (R)-2-octyloxy side chain and either a fluorenone (2a-e) or chiral fluorenol (3a-e) core were synthesized using a combined directed ortho metalation-directed remote metalation strategy. The SmC* phase formed by the fluorenol mesogens is more stable and has a wider temperature range than that formed by the fluorenone mesogens, which may be ascribed to intermolecular hydrogen bonding according to variable-temperature FT-IR measurements. The C[Sub11] fluorenol mesogens (R,R)-3d and (S,R)-3d were obtained in diastereomerically pure form and gave reduced polarization (P[Subo]) values of +106 and +183 nC/cm[Sup2], respectively, at 10 K below the SmA*-SmC* phase transition temperature. The difference in P[Subo] values suggests that the chiral fluorenol core contributes to the spontaneous polarization of the SmC* phase. This is ascribed to the bent shape of the fluorenol core, which should restrict its rotation with respect to the side chains in the SmC* phase and favor one orientation of its transverse dipole moment along the polar axis, and to steric coupling of the core to the chiral 2-octyloxy side chain. [ABSTRACT FROM AUTHOR]