Thermotropic Liquid-Crystalline and Light-Emitting Properties of Bis(4-aalkoxyphenyl) Viologen Bis(triflimide) Salts.

A series of bis(4-alkoxyphenyl) viologen bis(triflimide) salts with alkoxy chains of different lengths were synthesized by the metathesis reaction of respective bis(4-alkoxyphenyl) viologen dichloride salts, which were in turn prepared from the reaction of Zincke salt with the corresponding 4-n-alkoxyanilines, with lithium triflimide in methanol. Their chemical structures were characterized by 1H and 13C nuclear magnetic resonance spectra and elemental analysis. Their thermotropic liquid-crystalline (LC) properties were examined by differential scanning calorimetry, polarizing optical microscopy, and variable temperature X-ray diffraction. Salts with short length alkoxy chains had crystal-to-liquid transitions. Salts of intermediate length alkoxy chains showed both crystal-to-smectic A (SmA) transitions, Tms, and SmA-to-isotropic transitions, Tis. Those with longer length of alkoxy chains had relatively low Tms at which they formed the SmA phases that persisted up to the decomposition at high temperatures. As expected, all of them had excellent thermal stabilities in the temperature range of 330-370 °C. Their light-emitting properties in methanol were also included.

Anomalous Nanoscale Optoacoustic Phonon Mixing in Nematic Mesogens.

Recent inelastic X-ray scattering (IXS) experiments on mesogens have revealed entirely new capabilities with regards to their nanoscale phonon-assisted heat management. Mesogens such as nematic liquid crystals (LCs) are appealing systems for study because their structure and morphology can easily be tuned. We report on Q-resolved ultra-high-resolution IXS, X-ray diffraction, and THz time-domain spectroscopy experiments combined with large-scale all-atom molecular dynamics simulations on the dynamic properties of 5CB LCs. For the first time, we observe a strong mixing of phonon excitations originating from independent in-phase and out-of-phase van-der-Waals-mediated displacement patterns. The coexistence of transverse acoustic and optical modes of 5CB LCs at near room temperature is revealed through the emergent transverse phonon gap and THz light-phonon coupling taking place within the same energy range. Furthermore, our experimental observations are supported by analysis showing correlations of spontaneous fluctuations of LCs on picosecond time scales. These findings are significant for the design of a new generation of soft molecular vibration-sensitive nanoacoustic and optomechanical applications.

Emergent Optical Phononic Modes upon Nanoscale Mesogenic Phase Transitions.

The investigation of phononic collective excitations in soft matter systems at the molecular scale has always been challenging due to limitations of experimental techniques in resolving low-energy modes. Recent advances in inelastic X-ray scattering (IXS) enabled the study of such systems with unprecedented spectral contrast at meV excitation energies. In particular, it has become possible to shed light on the low-energy collective motions in materials whose morphology and phase behavior can easily be manipulated, such as mesogenic systems. The understanding of collective mode behavior with a Q-dependence is the key to implement heat management based on the control of a sample structure. The latter has great potential for a large number of energy-inspired innovations. As a first step toward this goal, we carried out high contrast IXS measurements on a liquid crystal sample, D7AOB, which exhibits solid-like dynamic features, such as the coexistence of longitudinal and transverse phononic modes. For the first time, we found that these terahertz phononic excitations persist in the crystal, smectic A, and isotropic phases. Furthermore, the intermediate smectic A phase is shown to support a van der Waals-mediated nonhydrodynamic mode with an optical-like phononic behavior. The tunability of the collective excitations at nanometer-terahertz scales via selection of the sample mesogenic phase represents a new opportunity to manipulate optomechanical properties of soft metamaterials.

Reversible Isothermal Twist-Bend Nematic-Nematic Phase Transition Driven by the Photoisomerization of an Azobenzene-Based Nonsymmetric Liquid Crystal Dimer.

The liquid crystal nonsymmetric dimer, 1-(4-butoxyazobenzene-4'-yloxy)-6-(4-cyanobiphenyl-4'-yl) hexane (CB6OABOBu), shows enantiotropic twist-bend nematic, NTB, and nematic, N, phases. The NTB phase has been confirmed using polarized light microscopy, freeze fracture transmission electron microscopy, and X-ray diffraction. The helicoidal pitch in the NTB phase is 18 nm. The NTB-N (TNTBN) and N-I (TNI) transition temperatures are reduced upon UV light irradiation, with the reduction in TNTBN being much larger than that in TNI. An isothermal, reversible NTB-N transition may be driven photochemically. These observations are attributed to a trans-cis photoisomerization of the azobenzene fragment on UV irradiation, with the cis isomers stabilizing the standard nematic phase and the trans isomers stabilizing the NTB phase. The dramatic changes in TNTBN provide evidence that the transition between the normal nematic and twist-bend nematic with spontaneous breaking of chiral symmetry is crucially dependent on the shape of molecular dimers, which changes greatly during the trans-cis isomerization.

Smectic-A and Hexatic-B Liquid Crystal Phases of Sanidic Alkyl-Substituted Dibenzo[fg,op]naphthacenes.

Despite longstanding interest in liquid crystalline compounds with simple rod- or disc-like shapes (calamitics or discotics), very few examples of the analogous board-shaped, or "sanidic", liquid crystals exist. A new series of alkyl-substituted dibenzo[fg,op]naphthacenes have been prepared by planarization of o-phenylene precursors through dehydrohalogenation. Their photophysical properties have been studied in dichloromethane. Liquid crystal phase behavior was characterized by polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction. All of the compounds exhibit monotropic liquid crystal phases on cooling from the isotropic phase. The compounds with shorter alkyl (pentyl and heptyl) chains exhibit the uniaxial smectic-A phase analogous to that of simple calamitic mesogens. The compounds with longer alkyl (nonyl, undecyl, and tridecyl) chains exhibit a new smectic liquid crystal phase featuring short-range positional order with an apparent rectangular lattice in the smectic layers, that is, an orthogonal biaxial hexatic-B. The molecular arrangement in this phase likely corresponds to a distorted herringbone packing of the board-shaped structures. Further, the compound with nonyl chains exhibits an underlying smectic-B phase. DFT calculations show that the cores of the mesogens are twisted into C2-symmetric saddle-shaped geometries because of steric interactions along their rims. The liquid crystal phases and their structures are discussed in the context of the compounds' board-like shapes and intercore interactions.

X-ray and Raman scattering study of orientational order in nematic and heliconical nematic liquid crystals.

The temperature dependence of the orientational order parameters 〈P_{2}(cosß)〉 and 〈P_{4}(cosß)〉 in the nematic (N) and twist-bend nematic (N_{tb}) phases of the liquid crystal dimer CB7CB have been measured using x-ray and polarized Raman scattering. The 〈P_{2}(cosß)〉 obtained from both techniques are the same, while 〈P_{4}(cosß)〉, determined by Raman scattering is, as expected, systematically larger than its x-ray value. Both order parameters increase in the N phase with decreasing temperature, drop across the N-N_{tb} transition, and continue to decrease. In the N_{tb} phase, the x-ray value of 〈P_{4}(cosß)〉 eventually becomes negative, providing a direct and independent confirmation of a conical molecular orientational distribution. The heliconical tilt angle α, determined from orientational distribution functions in the N_{tb} phase, increases to ∼24^{∘} at ∼15 K below the transition. In the N_{tb} phase, α(T)∝(T^{*}-T)^{λ}, with λ=0.19±0.03. The transition supercools by 1.7 K, consistent with its weakly first-order nature. The value of λ is close to 0.25 indicating close proximity to a tricritical point.

Dual relaxation and structural changes under uniaxial strain in main-chain smectic-C liquid crystal elastomer.

The relationship between strain-dependent macroscopic elastic behavior and the changes in microscopic structure of the smectic-C liquid crystal elastomer (LCE), C11MeHQSi8 were investigated using synchrotron X-ray studies. At very low strains ε ≤ 0.2, the smectic layers are randomly oriented. As the strain increases beyond 0.2, the smectic layers reorient and become parallel to the direction of the applied strain. The polydomain to monodomain (P-M) transition accompanied by the formation of chevron structure ensues for ε > 0.2 and is nearly complete for ε = 0.7. The chevron structure relaxes after the applied strain changes, with a time constant τα ∼ 45 min while the orientation order parameters of the mesogenic and elastomeric components gradually increase and saturate at 0.83 and 0.4, respectively at ε = 1.7 which is near the end of the plateau region. Relaxation rates τα for the tilt angle and τd corresponding to the smectic layer spacing both become about 10 times faster when the strain exceeds 0.7. The LCE remains "locked" into the monodomain state and retains 90% and 80% values of α and S, respectively for 24 hours after the applied strain is removed. The viscoelastic properties of the liquid crystal appear to dominate the equilibration process at low strains while the elastomeric properties control the system's response at high strains.

Columnar molecular aggregation in the aqueous solutions of disodium cromoglycate.

Stack, chimneylike, and threadlike assemblies have previously been proposed for the structure of disodium cromoglycate (DSCG) aggregates in aqueous solutions. The results of the synchrotron x-ray scattering investigations reported here reveal the formation of simple columnar assemblies with π-π stacking at a separation of 3.4 Å between the DSCG molecules. Lateral separation between the assemblies is concentration and temperature dependent, varying from ∼35 to 42 Å in the orientationally ordered nematic (N) phase and from 27 to 32 Å in the columnar or middle (M) phase having long range lateral positional order. The assemblies' length depends on concentration and consists of ∼23 molecules in the N phase, becoming three to ten times larger in the M phase. The scission energy is concentration dependent in the N phase with values ∼7.19 ± 0.14 k_{B}T (15 wt %), 2.73 ± 0.4 k_{B}T (20 wt %), and 3.05 ± 0.2 k_{B}T (25 wt %). Solutions of all concentrations undergo a spinodal decomposition at temperatures above ∼40 °C, resulting in DSCG-rich regions with the M phase and water-rich regions in the N and isotropic phases.

Origin of weak layer contraction in de Vries smectic liquid crystals.

Structural investigations of the de Vries smectic-A (SmA) and smectic-C (SmC) phases of four mesogens containing a trisiloxane end segment reveal a linear molecular conformation in the SmA phase and a bent conformation resembling a hockey stick in the SmC phase. The siloxane and the hydrocarbon parts of the molecule tilt at different angles relative to the smectic layer normal and are oriented along different directions. For the compounds investigated, the shape of orientational distribution function (ODF) is found to be sugarloaf shaped and not the widely expected volcano like with positive orientational order parameters: ⟨P2⟩ = 0.53-0.78, ⟨P4⟩ = 0.14-0.45, and ⟨P6⟩∼0.10. The increase in the effective molecular length, and consequently in the smectic layer spacing caused by reduced fluctuations and the corresponding narrowing of the ODF, counteracts the effect of molecular tilt and significantly reduces the SmC layer contraction. Maximum tilt of the hydrocarbon part of the molecule lies between approximately 18° and 25° and between 6° and 12° for the siloxane part. The critical exponent of the tilt order parameter, ß∼0.25, is in agreement with tricritical behavior at the SmA-SmC transition for two compounds and has lower value for first-order transition in the other compounds with finite enthalpy of transition.

Bilayers in nanoparticle-doped polar mesogens.

Structures of the mesophases of five members of the 4-n-alkyl-4'-cyanobiphenyl homologous series (4-n-butyl-4'-cyanobiphenyl to 4-n-octyl-4'-cyanobiphenyl) doped with milled BaTiO_{3} nanoparticles were investigated by x-ray scattering. Clear solutions of each of the 4-n-alkyl-4'-cyanobiphenyls were first prepared in n-heptane and then doped with an n-heptane/nanoparticle dispersion, which led to gelation. The nanogels were found to be one-dimensional, multilayered, smectic nanostructures in each case. Surprisingly, a characteristic layer spacing of 4.5 nm was observed in all five homologues. Synchrotron x-ray scattering study of the multilayer structures of doped 4-n-pentyl-4'-cyanobiphenyl and 4-n-octyl-4'-cyanobiphenyl revealed nine orders of the primary Bragg reflection which were used to calculate the electron density profiles of the multilayers by Fourier analysis. The multilayers were found to consist of molecular bilayers wherein the mesogens were arranged in a head-to-head assembly of the polar head groups. The alkyl tails of the mesogenic molecules were freely movable and the tail-to-tail assembly was stabilized by heptane. The dissolved nanoparticles clearly induced a new self-assembled nanostructure in which the rigid aromatic part, and not the overall length, of the molecules defined the layer spacing.

Crystalline, liquid crystalline, and isotropic phases of sodium deoxycholate in water.

Sodium deoxycholate (NaDC) is an important example of bile salts, representing systems with complex phase behavior involving both crystalline and mesophase structures. In this study, properties of NaDC-water mixtures were evaluated as a function of composition and temperature via X-ray diffraction with synchrotron (sXRD) and laboratory radiation sources, water sorption, polarized light, hot-stage microscopy, and freezing-point osmometry. Several phases were detected depending on the composition and temperature, including isotropic solution phase, liquid crystalline (LC) phase, crystalline hydrate, and ice. The LC phase was identified as hexagonal structure by sXRD, with up to 14 high-order reflections detected. The crystalline phase was found to be nonstoichiometric hydrate, based on XRD and water sorption data. The phase diagram of NaDC-water system has been refined based on both results of this study and other reports in literature.

Direct observation of diffuse cone behavior in de Vries smectic-A and -C phases of organosiloxane mesogens.

Simultaneous and direct x-ray measurements of the smectic layer spacing, molecular tilt, and orientational order in the de Vries smectic A (SmA) and C (SmC) phases of two organosiloxane mesogens reveal that (i) the SmC (tilt) order parameter exponent ß=0.26±0.01 for 2nd order SmA-SmC transition--in excellent agreement with the tricritical behavior, (ii) the siloxane and hydrocarbon parts of the molecules are segregated and oriented parallel to the director with very different degree of orientational order, and (iii) thermal evolution of the effective molecular length is different in the two phases.