Bent-shaped Molecules Research Articles

Liquid crystal blue phase induced by bent-shaped molecules with allylic end groups

A blue phase is induced by bent-shaped monomer with allylic end bonds when doped into chiral nematic liquid crystal. The mechanism of blue phase induction and stabilization is investigated. Its temperature range is further widened to 10.2 K by UV irradiation with a slowly cooling process. The widening principle is distinguishable from traditional polymer stabilization mechanism. The study provides some useful insights into the molecular design of suitable bent-shaped dopants towards wide range blue phase liquid crystals.

Effect of Lateral Fluorine Substituent on Mesophase Behavior of Bent-shaped Molecules with Asymmetric Central Naphthalene Core

Abstract Novel lateral fluorinated bent-shaped molecules based on 1,7- and 1,6-naphthalene central core are prepared. Their mesomorphic properties are discussed in comparison with those of conventional unsubstituted bent-shape molecules.

Synthesis and mesomorphic properties of bent-shaped molecule with low bent-angle central core and long alkylthio tail

Abstract Two homologous series of bent-shaped molecules composed of low bent-angle naphthalene central core and long alkoxy tail or alkylthio tail, N (1,7)- n -OPIMB and N (1,7)- n -SPIMB ( n = 20, 22), respectively, were synthesized. The mesomorphic properties were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD). Compared to the direct transition from the isotropic phase to the chiral B4 phase examined in N (1,7)- n -OPIMB, N (1,7)- n -SPIMB exhibited a rare Iso– N col –Col h –B4 phase sequence. The combination of the asymmetric low bent-angle 1,7-naphathalene central core and the flexible long alkylthio tail is considered to play an important role in the formation of such a novel phase sequence.

Calorimetric study of the effect of bent-shaped dopant molecules on the critical behavior at the nematic-smectic-Adphase transition

We report results of calorimetric studies for the binary mixture of rodlike host n-alkyloxy-cyanobiphenyl (nOCB, n=8,9) and bent-shaped guest 1,3-phenylene-bis[4-(3-methylbenzoyloxy)]-4'-n-dodecylbiphenyl-4'-carboxylate (BC12). The effect of bent-shaped dopant molecules on the critical behavior associated with the nematic-smectic-A(d) phase transition has been studied in detail. The transition temperature for the nematic-smectic-A(d) phase sharply decreases as the increase of the mole fraction of the dopant concentration (denoted X for the BC12/9OCB mixture and Y for the BC12/8OCB mixture). The dependence of the critical exponent α on X and Y is well explained in terms of the McMillan ratio. A nearly tricritical exponent has been obtained for the X=0.01 mixture. X=0.02-0.03 mixtures, pure 8OCB, and Y=0.01-0.03 mixtures exhibit nonuniversal behaviors with effective exponents lying between the 3D-XY and tricritical exponents. The heat capacity anomaly for Y=0.05 has been well described with the 3D-XY exponent. The critical amplitude ratio A(-)/A(+) is close to 1 and insensitive to the dopant concentration. No Fisher renormalization of the critical exponent has been observed even for nearly tricritical compositions, which indicates the smallness of the concentration plays a decisive role rather than the steepness of the N-SmA(d) phase boundary.

The liquid crystal blue phase induced by bent-shaped molecules with different terminal chain lengths

Several kinds of bent-shaped molecules with different lengths of the terminal chain are doped in chiral nematic liquid crystals (LCs), respectively, to induce the blue phase. The effects of the terminal chain length on the blue phase range are studied. The mechanisms of the phenomena are investigated through molecular dynamics, and the results indicate that molecules with a long terminal chain decrease the interfacial energy between the LCs and defects, whereas they also disturb the alignment of LCs near the interface and make an evident impact on the bent angle of molecules, which is not helpful in forming a stable blue phase. This work provides some useful insights into the molecular design of suitable bent-shaped dopants in order to obtain a wide range of LC blue phases.

Alkoxyphenyl-Substituted Symmetric Liquid Crystalline Diamantane Derivatives

Abstract Eleven novel thermotropic rod-shaped liquid crystals composed of alkoxyphenyl (methoxy to decyloxy) or hexanoyloxyphenyl groups on both ends of the diamantane core have been synthesized and their liquid crystalline properties investigated. All of them behaved as liquid crystals at considerably high temperatures. When the alkoxy side chains are propyloxy or shorter, the clearing points are over 300 °C. The clearing point of a liquid crystalline compound with two hexanoyloxy groups also exceeds 300 °C. When the alkoxy groups on both ends of the molecule are octyloxy or longer, they showed only smectic phases as the mesophase. When they are shorter than octyloxy, the compounds showed nematic phases just below their clearing points. A nematic phase was also observed for the hexanoyloxy compound as well as the smectic A and B phases. However, as the bent-shaped adamantane-derived hexyloxyphenyl molecule did not show any liquid crystalline properties, the linear structures of the diamantane-derived molecules were crucial in generating liquid crystalline phases.

Unusual Formation of Switchable Hexagonal Columnar Phase by Bent-Shaped Molecules with Low Bent-Angle Naphthalene Central Core and Alkylthio Tail

The low-angle bent-shaped molecules with 1,7-naphthalene central core and alkylthio tails can form a novel hexagonal columnar phase and a dark B4 phase. The columnar phase has a large two-dimensional hexagonal lattice with edges of 65–75 Å and exhibits polar switching with spontaneous polarization along the column axis. Calculated from the density (∼1 g·cm-3) and unit volume, the number of molecules that are necessary to fill a 4.6-Å stratum of each column were found to be ∼11. Such a large number of molecules can be accommodated only in the tube-like assembly, which may be the first example as formed by the usual bent-shaped molecule with a single alkylthio tail.

Wide blue phase range of chiral nematic liquid crystal doped with bent-shaped molecules

A wide blue phase range of 29 °C is induced by a bent-shaped molecule that is doped in chiral nematic liquid crystals. The physical mechanisms of this effect are investigated through experimental comparisons and molecular simulations. The results obtained indicate that the wide temperature range is closely related to the lower interfacial tension between liquid crystals and defects, and the shrinkage of the radius of the defect core after the bent-shaped molecules are doped in the systems. Apart from this, extending the rigid body of the bent-shaped molecules leads to an increase in the biaxiality and dipole moment, which are the two additional important factors for obtaining a stable blue phase. This study provides new insights into broadening the liquid crystal blue phase.

Spectral blue shift via intermolecular interactions in the B2 and B4 phases of a bent-shaped molecule

A set of precise measurements of UV-Vis absorption spectrum for the classical bent-shaped molecule, P-12-OPIMB, revealed that molecular exciton interactions exist even in the mesophases of this system, as manifested as an obvious and abrupt change in the positional and spectral shape at phase-transition points. Based on Kasha's molecular exciton model, we successfully conducted a quantitative analysis of the spectral shift and could determine the local structure of the B4 phase. To the best of our knowledge, such a spectroscopic analysis for the liquid crystals has not been connected with the local structure.

Superstructural chirality of a periodic system of domains of opposite chirality and π-walls in B2 phase composed of bent-shaped molecules

Principally, domains of opposite chirality can exist either in synclinic ferroelectric structures or in anticlinic antiferroelectric structures in the B2 phase of bent-shaped molecules. These domains are separated by π-walls with either right-handed or left-handed rotation of molecular polarizations. In this study, periodic systems of domains of opposite chirality connected by π-walls are discussed. A simple estimation of this periodicity is proposed. Systems of domains and π-walls create global structures which could have also the opposite chirality. The mentioned global structures are the result of so-called superstructural chirality of the system.

Calorimetric studies on isotropic-B 4 phase transition in the mixture of bent-shaped and rod-like molecules

Calorimetric studies have been performed on a mixture of achiral bent-shaped and rod-like molecules using a high-resolution ac calorimetry and a high-sensitivity differential scanning calorimetry (DSC). Both ac calorimetry and DSC yielded essentially the same result for B 4–B X transition, while a significant difference has been observed at the isotropic-B 4 transition during cooling. Particularly, DSC measurements revealed the characteristic heat capacity behavior accompanying distinct many peaks at the isotropic-B 4 transition. A comparison with recent reports strongly suggests that the obtained data reflect the growth processes of the helical nanofilaments in the isotropic 5CB molecules.

Antiferroelectric Switching between Optically Isotropic and Birefringent B2 Phases of Bent-Shaped Molecules with High Tilt Angle of 45°

We prepared a homologous series of bent-shaped molecules with a 1,3-naphthalene central core and long side wings in which one phenyl ring is attached to the classic one via ester linkage. These banana molecules form the B2 phase with a high tilt angle of approximately 45° in the wide temperature region and exhibit the field-induced transformation from racemic synclinic SmCSPA to homochiral anticlinic SmCAPA. The field induced anticlinic SmCAPA phase is optically isotropic because of the high tilt angle of around 45° and makes a high contrast in birefringence with the switched synclinic SmCSPF phase, suggesting potentiality as a new type of ferroelectric display mode.

Critical behavior at transitions from uniaxial to biaxial phases in a smectic liquid-crystal mixture

We report results of calorimetric and optical investigations of binary mixtures of rodlike and bent-shaped molecules. We find that the observed critical heat anomaly associated with the smectic-A2 to biaxial smectic-A2b transition is well described with a Fisher-renormalized form of the usual scaling expression. The effect of renormalization is large in this system in part because of the moderately steep slope of the phase boundary (dT(c)/ dX∼100 K, where X is the mole fraction of the bent-core molecules) and in part because of the proximity to the tricritical point. The magnitude of heat anomaly at the smectic-A2-smectic-A2b transition showed a drastic decrease as X becomes smaller. Moreover, the nematic-smectic-A2 transitions investigated turned out to be always first order and the transition enthalpy showed only weak dependence on the concentration X. The results imply that the energy fluctuations around the smectic-A2-smectic-A2b transition are very sensitive to the underlying ordering of the smectic-A2 background.

Formation of a homochiral antiferroelectric ground state in asymmetric bent-shaped molecules

New asymmetric bent-shaped molecules were synthesised and their mesomorphic properties and phase structures investigated by means of electro-optical measurements. All of the compounds formed fluid smectic phases and showed the homochiral antiferroelectric (SmCAPA) ground structure of the B2 phase. Comparing the new molecules with the previously reported classical symmetric bent-shaped molecules, we concluded that the modification of the chemical structure, especially the asymmetrical design of the bent-shaped molecules, is very significant for the interlayer interaction, influencing the formation of a specific polar order in banana phases.

A Model of π-Walls Between Domains of Opposite Chirality in B2 Phase of Bent-Shaped Molecules

Principally, domains of opposite chirality can exist either in anticlinic antiferroelectric structures in sample bulk or in synclinic ferroelectric structures induced near surfaces by strong polar surface anchoring or by the influence of an external electric field. Domains of opposite chirality are separated by π-walls. In the present contribution a simplified approximate theoretical model describing the molecular orientation in a π-wall is proposed. This model uses so called internal barrier energy introduced by M Nakata, et al. (Phys. Rev. Letters 2006;96:067802) and takes into account an external electric field acting along smectic layers. Electric field suppresses the component of spontaneous polarization perpendicular to field and layer normal and governs the wall width.

Is the General Angle Between the Tilt and the Polarization Possible in Rod-Like Systems?

Smectic structures with general orientation of polarizations with respect to the tilt are known from systems formed of bent-shaped molecules. However, such structures may exist in distorted clock systems of SmCFI1 and SmCFI2 phases in antiferroelectric liquid crystals formed of rod-like molecules. We present the analysis of circumstances in which the general angle between the polarization in the tilt is possible.

Electric-field-induced transition between ferro- and antiferroelectric ground states observed in the B7 phase of a bent-shaped molecule with alkylthio tails

We prepared a new bent-shaped molecule, in which dodecylthio tails and typical Schiff-based side wings are substituted on a conventional phenyl central core. This molecule forms the B7 phase in a temperature region from 117 to 142 °C, which shows characteristic banana leaf and circular domain textures. On application of an electric field, the B7 phase is easily transformed to the B2 phase with two different ground states, depending on the temperature. In the higher temperature region, the B2 phase possesses a homochiral ferroelectric (SmCSPF) ground state, whereas the lower temperature B2 phase has a homochiral antiferroelectric (SmCAPA) ground state. These two states transformed into each other reversibly at 124 °C under the applied field, while they are stable over the mesophase temperature on the field-off state.

Smectic A phase in a new bent-shaped mesogen based on a 2,3-naphthalene central core with an acute-subtended angle

A new bent-shaped molecule which consists of an acute-angled central core and all-ester linking groups has been prepared: 2,3-naphthylene bis[4-(4-n-dodecyloxybenzoyloxy)benzoate]. Its mesomorphic properties have been investigated by DSC, polarizing microscopy, and X-ray diffraction measurements. Moreover, we have carried out detailed electro-optical investigations including birefringence, switching current response, IR dichroism, and second-harmonic generation (SHG) measurements. On the basis of the experimental data a plausible model for the mesophase alignments has been proposed.

Channel Structures from Self-Assembled Hexameric Macrocycles in Laterally Grafted Bent Rod Molecules

Internally grafted bent rod molecules consisting of a bent-shaped nona-p-phenylene and different lengths of oligoether chains at the bay position were synthesized and characterized. All of the bent-shaped molecules showed ordered bulk-state structures as characterized by differential scanning calorimetry, X-ray scatterings, and transmission electron microscopy. The bent rod based on a short oligo(propylene oxide) chain self-assembles into a 2-D channel-like columnar structure, whereas the molecules with an intermediate length of flexible chains self-assemble into discrete channels that self-organize into honeycomb layers. A further increase in the length of the flexible chain induces a layered structure. In contrast to the bent-shaped molecules based on a linear chain, the molecules based on a branched chain self-assemble into an inverted 2-D columnar structure with an aromatic core surrounded by branched chains. We proposed the model of the channel structure on the basis of experimental data obtained from X-ray results and density measurements. Within the channels, six bent rods self-assemble into hexameric macrocycles that stack on one another to form channel-like columns where the interiors are filled by the flexible oligoether chains. Remarkably, the elongated channels break up into discrete channels of a well-defined length with increasing length of the oligoether chain. The resulting discrete channels self-organize into a hexagonally ordered honeycomb layer. The defined length of a channel is believed to be responsible for the formation of unique honeycomb layers.

Structural characteristics of the B6 phase for a bent-core molecular system observed through the B1-B6 transition

A bent-shaped molecule exhibiting the mesophase sequence of B6-B1 was studied to understand the structural characteristics of the B6 phase compared with those of the B1 phase. A well-oriented sample was prepared in a magnetic field and examined by wide-angle x-ray diffraction measurements through the B6-B1 phase transition. The B6 phase has been considered to show only the (002) reflection, but this B6 phase showed broad scattering just inside the (002) reflection. The broad scattering has an intensity maximum at a very similar position to that of the (101) reflection in B1 and evolves into the well-defined (101) reflection on cooling into B1. Thus, B6 has a similar frustrated structure to B1, but the size of the antidomain in B6 may be dynamically distributed while B1 possesses an antidomain of definite size.