Twist-bend heliconical chiral nematic liquid crystal phase of an achiral rigid bent-core mesogen.

The chiral, heliconical (twist-bend) nematic ground state is reported in an achiral, rigid, bent-core mesogen (UD68). Similar to the nematic twist-bend (N(TB)) phase observed in bent molecular dimers, the N(TB) phase of UD68 forms macroscopic, smecticlike focal-conic textures and exhibits nanoscale, periodic modulation with no associated modulation of the electron density, i.e., without a detectable lamellar x-ray reflection peak. The N(TB) helical pitch is p(TB) ∼ 14 nm. When an electric field is applied normal to the helix axis, a weak electroclinic effect is observed, revealing 50-µm-scale left- and right-handed domains in a chiral conglomerate.

Electro-optic response of the anticlinic, antiferroelectric liquid-crystal phase of a biaxial bent-core molecule with tilt angle near 45∘.

We describe the unusual electro-optic response of a biaxial bent-core liquid crystal molecule that exhibits an anticlinic, antiferroelectric smectic phase (Sm-C(A)P(A)) with a molecular tilt angle close to 45°. In the ground state, the sample shows very low birefringence. A weak applied electric field distorts the antiferroelectric ground state, inducing a small azimuthal reorientation of the molecules on the tilt cone. This results in only a modest increase in the birefringence but an anomalously large (∼40°) analog rotation of the extinction direction. This unusual electro-optic response is shown to be a consequence of the molecular biaxiality.

Structure of the B4 liquid crystal phase near a glass surface.

The B4 liquid crystal phase of bent-core molecules, a smectic phase of helical nanofilaments, is one of the most complex hierarchical self-assemblies in soft materials. We describe the layer topology of the B4 phase of mesogens in the P-n-OPIMB homologous series near the liquid crystal/glass interface. Freeze-fracture transmission electron microscopy reveals that the twisted layer structure of the bulk is suppressed, the layers instead forming a structure with periodic layer undulations, with the topography depending on the distance from the glass. The surface layer structure is modeled as parabolic focal conic arrays generated by equidistant parabolas whose foci are defect lines along the glass surface. Nucleation and growth of toric focal conics near the glass substrate is also observed. Although the growth of twisted nanofilaments, the usual manifestation of structural chirality in the B4 phase, is suppressed near the surface, the smectic layers are intrinsically chiral, and the helical filaments that form on top of them grow with specific handedness.

Right-handed double-helix ultrashort DNA yields chiral nematic phases with both right- and left-handed director twist.

Concentrated solutions of duplex-forming DNA oligomers organize into various mesophases among which is the nematic (N(∗)), which exhibits a macroscopic chiral helical precession of molecular orientation because of the chirality of the DNA molecule. Using a quantitative analysis of the transmission spectra in polarized optical microscopy, we have determined the handedness and pitch of this chiral nematic helix for a large number of sequences ranging from 8 to 20 bases. The B-DNA molecule exhibits a right-handed molecular double-helix structure that, for long molecules, always yields N(∗) phases with left-handed pitch in the µm range. We report here that ultrashort oligomeric duplexes show an extremely diverse behavior, with both left- and right-handed N(∗) helices and pitches ranging from macroscopic down to 0.3 µm. The behavior depends on the length and the sequence of the oligomers, and on the nature of the end-to-end interactions between helices. In particular, the N(∗) handedness strongly correlates with the oligomer length and concentration. Right-handed phases are found only for oligomers shorter than 14 base pairs, and for the sequences having the transition to the N(∗) phase at concentration larger than 620 mg/mL. Our findings indicate that in short DNA, the intermolecular double-helical interactions switch the preferred liquid crystal handedness when the columns of stacked duplexes are forced at high concentrations to separations comparable to the DNA double-helix pitch, a regime still to be theoretically described.

Physical polymerization and liquid crystallization of RNA oligomers.

Ultrashort complementary RNA oligomers, as short as six base pairs in length, are found to exhibit chiral nematic and columnar liquid crystal phases in aqueous solution, through end-to-end adhesion into physically bound, but chemically segmented, polymers. Geometrical constraints indicate that the phosphate helix is continuous along the aggregated chain. The end-to-end adhesion is due to a base-staking type interaction, whose energy and temperature dependence are determined.

Phase separation and liquid crystallization of complementary sequences in mixtures of nanoDNA oligomers.

Using optical microscopy, we have studied the phase behavior of mixtures of 12- to 22-bp-long nanoDNA oligomers. The mixtures are chosen such that only a fraction of the sample is composed of mutually complementary sequences, and hence the solutions are effectively mixtures of single-stranded and double-stranded (duplex) oligomers. When the concentrations are large enough, such mixtures phase-separate via the nucleation of duplex-rich liquid crystalline domains from an isotropic background rich in single strands. We find that the phase separation is approximately complete, thus corresponding to a spontaneous purification of duplexes from the single-strand oligos. We interpret this behavior as the combined result of the energy gain from the end-to-end stacking of duplexes and of depletion-type attractive interactions favoring the segregation of the more rigid duplexes from the flexible single strands. This form of spontaneous partitioning of complementary nDNA offers a route to purification of short duplex oligomers and, if in the presence of ligation, could provide a mode of positive feedback for the preferential synthesis of longer complementary oligomers, a mechanism of possible relevance in prebiotic environments.

End-to-end stacking and liquid crystal condensation of 6 to 20 base pair DNA duplexes.

Short complementary B-form DNA oligomers, 6 to 20 base pairs in length, are found to exhibit nematic and columnar liquid crystal phases, even though such duplexes lack the shape anisotropy required for liquid crystal ordering. Structural study shows that these phases are produced by the end-to-end adhesion and consequent stacking of the duplex oligomers into polydisperse anisotropic rod-shaped aggregates, which can order into liquid crystals. Upon cooling mixed solutions of short DNA oligomers, in which only a small fraction of the DNA present is complementary, the duplex-forming oligomers phase-separate into liquid crystal droplets, leaving the unpaired single strands in isotropic solution. In a chemical environment where oligomer ligation is possible, such ordering and condensation would provide an autocatalytic link whereby complementarity promotes the extended polymerization of complementary oligomers.

Optical activity produced by layer chirality in bent-core liquid crystals.

Large optical activity has recently been observed in chiral smectic liquid crystalline phases formed from achiral bent-core molecules. The origin of the optical activity remains unclear and has been attributed to both a helical superstructure and the layer chirality produced by simultaneous tilt and polar ordering of bent-core mesogens in the B2 phase. Here, we directly demonstrate that layer chirality produces optical activity in the well ordered SmC(A)P(A) subphase.

Reflection symmetry breaking in achiral rod-shaped smectic liquid crystals?

The SmC phase of 4'-octyloxyphenyl-4-octyloxybenzoate has been examined in light of recent reports that this phase is chiral. The results suggest that two varieties of chiral domains in LC cells of the phenylbenzoate are indeed formed, driven by interactions with surfaces. Application of sensitive probes for chirality and polarity in the absence of such interfacial influences failed to find any. Currently, there is no evidence that the subject SmC phase is chiral.

Frustrated smectic layer structures in bent-shaped dimer liquid crystals studied by x-ray microbeam diffraction.

The layer structures in bent-shaped liquid crystal dimers mOAM5AMOm (m=6-16) have been investigated by x-ray microbeam diffraction. These liquid crystal molecules have two rodlike mesogens connected with an odd-numbered alkylene spacer and form a bent shape. In these compounds it is found that the structure changes from the single (m=6) to frustrated-layer structures (m=8, 10, and 12) and switchable frustrated-layer structures (m=14 and 16) with increasing terminal chain length. An anticlinic antiferroelectric structure is suggested in the compound with m=16, based on the different electric-field-induced reorientation behavior from those in the other dimers.

Electric-field-induced transition between the polarization-modulated and ferroelectric smectic- C(S) P(F)* liquid crystalline states studied using microbeam x-ray diffraction.

We report x-ray microbeam studies of a bent-core liquid crystalline material with chiral citronellyl tails. This material has an equilibrium polarization-modulated smectic- CP (PM-SmCP) state exhibiting the B7 texture upon slow cooing from the isotropic while a metastable chiral synclinic ferroelectric Sm-CP state (Sm- C(S) P(*)(F) ) is obtained on quenching from the isotropic. The polarization modulated phase PM-Sm C(S) P(*)(F) shows typical x-ray patterns having multiple satellite peaks around the first-order layer reflection, indicating undulated layers, while the metastable Sm- C(S) P(*)(F) state exhibits a single layering peak indicating flat layers. The Sm- C(S) P(*)(F) state is also induced by the application of an electric field larger than the threshold field ( E(th) ) and thermally returns to the polarization modulated PM-Sm C(S) P(*)(F) structure.

Blue phases induced by doping chiral nematic liquid crystals with nonchiral molecules.

The emergence of the blue phases I and II that are thought to exist in highly chiral systems has been found in chiral nematic liquid crystals (N* LCs) when they are doped with achiral bent-core liquid crystals (banana mesogens). The same effect was also observed by adding racemic 4-(1-methylheptyl oxycarbonyl)phenyl 4'-octyloxybiphenyl-4-carboxylate (MHPOBC), while achiral terephthal-bis(p-butylaniline) (TBBA) was not effective. The difference was attributed to the different conformations, i.e., MHPOBC is known to have a bent shape, but not TBBA. The doping effect is more remarkable in N* LCs with higher chiral content for both banana mesogens and MHPOBC. This unusual doping effect was attributed to selective chiral interaction between the chiral conformers of guest molecules and chiral host molecules and/or the decrease of the surface elastic constant on adding bent-shaped guest molecules.

Dynamic local-layer response of surface-stabilized ferroelectric liquid crystals to a high electric field by time-resolved x-ray microdiffraction.

Time-resolved synchrotron x-ray microdiffraction measurements have directly revealed the dynamic local-layer response to the high electric field in a surface-stabilized ferroelectric liquid crystal. The irreversible layer transformation under the increasing electric field is found to consist of two stages; the initial vertical chevron structure transforms to the alternate vertical and horizontal chevrons and, with increasing field, the chevron angle decreases and the horizontal chevron structure develops. The time-resolved microdiffraction measurement has clarified the detailed reversible layer transformation between the mostly horizontal chevron at the high field and the so-called quasibookshelf at the low field during the ac field application. The transient layer response time is about 0.1-0.2 and 0.3-0.4 ms for falling and rising edges, respectively, and is slightly longer than the optical response time. The layer transformation is discussed in terms of electric torque and surface anchoring. The local-layer response in the antiferroelectric liquid crystal is also discussed for comparison.

Flexoelectric polarization in hybrid nematic films.

Hybrid films of the nematic liquid crystal penthylcyanobiphenyl (5CB) on the surface of glycerol were studied in the presence of an in-plane electric field. In this geometry there is a macroscopic flexoelectric polarization P(f) with a component in the film plane that couples linearly to the field. Here we report the electric field and film thickness dependence of the width of the 2pi reorientations (2pi walls) in the c director that are trapped by the field and interpret these observations based on continuum elastic theory. We find the difference between the flexoelectric coefficients to be e(1)-e(3)=+11 pC/m.