Molecular alignment and field-induced reorientation in a twisted nematic liquid crystal pi-cell

A twisted nematic pi-cell has been studied by optical transmission measurement, polarized Fourier-transform infrared (pFTIR) absorption spectroscopy, and Raman spectroscopy. Our pFTIR results suggest that the LC molecules undergo a restricted rotation about the molecular long axis. The rise and decay times of the optical response were found to be 6 ms and 1.6 ms, respectively. The switching dynamics of the twisted pi-cell was also studied using time-resolved Raman spectroscopy. A normal mode associated with the C-H out-of-plane wag on the LC core was found to be enhanced after the electric field was switched off. Our data show that LC molecules in the twisted pi-cell do not rotate like a rigid molecule during the field-induced reorientation process. The methods employed in this study have yielded valuable information about LC alignment and field-induced reorientation with respect to functional group specificity.     

Peculiarities of electric field alignment of nonlinear optical chromophores incorporated into thin film polymer matrix

The electric field poling process of free-standing films of poly(methyl methacrylate) (PMMA) matrix doped with the nonlinear optical compound 4-(dimethylamino)-4'-nitrostilbene (DMANS) was investigated by molecular simulation methods. The influence of the vacuum/bulk interfacial regions on static and dynamic properties, including the glass transition temperature T_g and the field-induced chromophore reorientation, was studied by employing films of three different thicknesses and by comparison with previous work on the bulk system. The interfacial region, defined as the region, where the local density increases from zero to the bulk density, is about 2 nm wide, independent of the film thickness. T_g decreases with decreasing film thickness, in accord with previous experimental work and theoretical predictions. The resistance against field-induced chromophore reorientation in the liquid state is found to increase strongly with the film thickness.     

A dual frequency addressed polymer stabilized pi-cell liquid crystal device

Dual frequency addressing of nematic pi-cell devices produces submillisecond switching times since the liquid crystal can be driven both parallel and perpendicular to the applied field and there is no kick-back of the director during switching. The nucleation of the V state in devices containing dual frequency liquid crystal materials is much slower than that in conventional pi-cells, however. Polymer stabilization of the V state eliminates the need for nucleation each time the device is used. In this paper we present a polymer stabilized pi-cell containing a dual frequency liquid crystal material, and show that the presence of the polymer network significantly influences the switching of the device. Some optimization of the addressing scheme is required when switching the polymer stabilized device in order to avoid transient formation of the twisted state. Using this optimization, the switching time is under 3 ms across a wide range of addressing voltages.     

Dynamic evolution of light-induced orientation of dye-doped liquid crystals in liquid phase studied by time-resolved optically heterodyned optical Kerr effect technique

Transient evolution of light-induced molecular reorientation both in 1-amino-anthraquinone (1AAQ) dye and azobenzene doped isotropic liquid crystals (LCs) were studied by time-resolved optically heterodyned optical Kerr effect method. The results give clear direct experimental proof that under short pulse (30 ps) excitation, LC molecules orientate toward the excitation light polarization direction in the 1AAQ/LC system. However, LC molecular orientation becomes orthogonal to the light polarization in azobenzene/LC system. Time-resolved excited-state absorption of 1AAQ and wavelength dependent excited-state absorption of azobenzene were also observed and their contributions to the early dynamics of the third order optical responses of the two systems were confirmed. A simplified two-level mean-field theory was derived to reveal the intensity dependence of orientation enhancement factor in azobenzene/LC system considering the photoisomerization process.     

Memory effects in chiral nematic liquid crystals doped with functionalised single-walled carbon nanotubes

Octadecylamine-functionalised single-walled carbon nanotubes (SWCNTs) were dispersed into nematic liquid crystals (LCs) doped with chiral molecules. The collective orientation of nematic LC molecules in helical layers was manipulated by varying dopant concentration. Highly anisotropic nature of SWCNTs enhanced the anisotropy of the LC as confirmed by polarised fluorescence spectroscopy. The π–π interaction of SWCNTs present in the planar alignment layers and twisted nematic LC molecules affects the molecular relaxation process. An irreversible electro-optic memory in the material has been observed.     

Helical twisting power and compatibility in twisted nematic phase of new chiral liquid crystalline dopants with various liquid crystalline matrices

ABSTRACT

Four chiral dopants exhibiting smectic LC phases themselves were prepared and their helical twisting power (HTP) and thermal phase behaviour in mixtures with four various LC hosts were studied. The influence of host liquid crystal on HTP was evaluated and generally higher values were found for hosts with high birefringence. Unexpectedly, high enhancement was found for an LC-chiral dopant pair, both having a similar aromatic core – biphenyl ring substituted with polar group. All studied chiral dopants exhibited limited compatibility with the LC hosts in twisted nematic phase at room temperature. For one of the studied mixtures, it was able to obtain single twisted nematic phase with selective light reflection band with maximum at wavelength about 1.0 µm. Carboxylic acid-type dopants exhibited total compatibility with the studied host in single twisted nematic phase at elevated temperatures, allowing preparation of mixtures with reflection band in the visible range. In case of the carboxylic acid dopants, blue phases for optimised compositions were observed. Intermolecular hydrogen bonding between carboxylic acid proton and pyridine nitrogen of chiral dopants was found. Doping the LC host with these dopants led to slight enhancement of HTP value and higher solubility in the LC host.     

Field-induced reorientation dynamics of a surface stabilized ferroelectric liquid crystal mixture

Time-resolved measurements of electro-optical response and Fourier-transform infrared absorption have been employed to study the submolecular motion of a surface stabilized ferroelectric liquid crystal (SSFLC) mixture during a field-induced reorientation process. All observed submolecular groups were found to rotate in unison about the layer normal in a steady d.c. field. In a transient situation, the FLC director can respond rapidly to a pulsed driving field. However, the orientation distribution was found to spread at first and then slowly converge to the new direction of the FLC director. When the field is switched off, all the core groups relax to a steady-state direction with varying relaxation times. The results reveal that submolecular fragments of different molecular species in the FLC mixture move correlatively in a steady d.c. field and during a field-induced reorientation process.     

Optical anisotropy of a photoreacted side-chain liquid-crystalline polymer induced by linearly polarized UV light

Linearly polarized (LP) UV photoreaction of a photo-crosslinkable side-chain liquid-crystalline polymer (SLCP) containing photoreactive cinnamoyl and biphenyl mesogenic groups ( 1 ) was studied. The optical anisotropy of the polymer film was induced by the LP-UV photoreaction and was investigated by the temperature-controlled polarized UV absorption spectroscopy and polarized FT-IR measurements. The reorientation of the nonreacted mesogenic groups along to the Ê direction of the incident LP-UV light during the photoreaction occurred at the LC temperature range of the polymer, and the induced birefringence Δn was about 0.02. Because of the high-density photo-crosslinking, the LP-UV photoreacted film showed orientational stability up to 160°C. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1521–1526, 1998     

Molecular reorientational dynamics of d-camphor systems under high pressure

The molecular dynamics of the plastic phase of d-camphor and d-camphor in n-hexane were probed using Raman spectroscopy. The orientational correlation functions of the various systems were found to decay at equal rates in the short time. In the time domain 0 to 1.5 ps the molecular reorientation of d-camphor molecules in n-hexane solution seems to be unaffected by pressures up to 5 kbar.     

Tetrathiafulvalene derivatives as NLO-phores: synthesis, electrochemistry, Raman spectroscopy, theoretical calculations, and NLO properties of novel TTF-derived donor-pi-acceptor dyads.

Novel pi-conjugated donor-acceptor chromophores, based on the strong electron-donating tetrathiafulvalene moiety and different electron-withdrawing acceptors, exhibit large second-order optical nonlinearities. The effect of increasing the length of the polyenic spacer and the influence of the nature of the acceptor moiety on the NLO properties have been studied by using the electric field-induced second-harmonic generation (EFISH) technique as well as by semiempirical and ab initio theoretical calculations. A charge-transfer band has been observed in the absorption spectra of these D-pi-A compounds that undergoes an hypsochromic shift when increasing the number of vinylenic spacer units connecting both donor and acceptor moieties. The degree of the intramolecular charge transfer from the donor to the acceptor has also been analyzed by means of Raman spectroscopy.     

银表面罗丹明6G的电化学表面增强拉曼光谱研究

罗丹明6G（Rhodamine 6G,R6G）是单分子表面增强拉曼光谱（SM-SERS）研究中最常用的探针分子之一,对R6G分子在表面吸附行为的研究有助于了解R6G分子和表面的相互作用. 本文应用电化学和电化学表面增强拉曼光谱技术,研究不同电位下R6G的银电极表面的吸附行为. 结果表明,随着电位负移罗丹明6G在银表面上从垂直吸附转为倾斜吸附,该变化和碱性条件下吸附于金纳米粒子上R6G的吸附构象一致. 这说明,在部分单分子实验中所发现的R6G反常光谱其来源是单个R6G分子在表面吸附取向变化. 本研究对后续详细分析SM-SERS研究中单分子SERS谱峰变化的机制有一定的参考价值.     

Two-dimensional Raman study of the submolecular, electric field-induced reorientation of a nematic liquid crystal

Two-dimensional Raman scattering is presented as a technique for the monitoring of electric field-induced, submolecular reorientation in liquid crystals. The motions of the flexible part and the rigid core of 4-pentyl-(4-cyanophenyl)cyclohexane (PCH5) are independently monitored in response to both step and oscillatory electric fields. Step voltage experiments show that the flexible group reorients before the rigid core. Also, oscillatory electric field experiments demonstrate that the flexible and rigid groups reorient asynchronously. In fact, at periodicities that are shorter than the bulk reorientation times, it is observed that the reorientation of the flexible part is amplified, while the motion of the rigid core is inhibited. The data suggest that the flexible group possesses a small, local dielectric anisotropy that can couple with the electric field to induce an independent, cooperative reorientation when the mobility of the rigid core is restricted.     

Cascading third-order Raman process and local structure formation in binary liquid mixtures of benzene and n-hexane

The cascading third-order Raman process in binary mixtures of benzene and n-hexane was studied by six-wave mixing coherent anti-Stokes Raman scattering spectroscopy. By examining the concentration dependence of the cascading third-order signal intensity, we investigated the formation of local structures of benzene in the binary mixtures. A significant deviation from the dependence expected for homogeneous mixtures was observed at benzene concentrations above 7 mol dm(-3). This deviation can be interpreted in terms of optical inhomogeneity caused by the formation of domain structures of benzene molecules. We discuss the feasibility of the cascading third-order process as a sensitive probe for the microscopic structures that are formed in liquids and solutions.     

Correlation between Ferromagnetic Layer Easy Axis and the Tilt Angle of Self Assembled Chiral Molecules

The spin–spin interactions between chiral molecules and ferromagnetic metals were found to be strongly affected by the chiral induced spin selectivity effect. Previous works unraveled two complementary phenomena: magnetization reorientation of ferromagnetic thin film upon adsorption of chiral molecules and different interaction rate of opposite enantiomers with a magnetic substrate. These phenomena were all observed when the easy axis of the ferromagnet was out of plane. In this work, the effects of the ferromagnetic easy axis direction, on both the chiral molecular monolayer tilt angle and the magnetization reorientation of the magnetic substrate, are studied using magnetic force microscopy. We have also studied the effect of an applied external magnetic field during the adsorption process. Our results show a clear correlation between the ferromagnetic layer easy axis direction and the tilt angle of the bonded molecules. This tilt angle was found to be larger for an in plane easy axis as compared to an out of plane easy axis. Adsorption under external magnetic field shows that magnetization reorientation occurs also after the adsorption event. These findings show that the interaction between chiral molecules and ferromagnetic layers stabilizes the magnetic reorientation, even after the adsorption, and strongly depends on the anisotropy of the magnetic substrate. This unique behavior is important for developing enantiomer separation techniques using magnetic substrates.     

Synthesis, spectroscopic characterization and alignment of novel azobenzene-containing monomers

A series of novel bifunctionalized photochromic monomers were synthesized, focusing on those with polymerizable acrylic/methacrylic groups attached to both ends of an azobenzene core via flexible spacers. The phase behaviour of the monomers was investigated using DSC, polarizing optical microscopy and X-ray diffraction. The change in UV-vis absorbance of the monomers under illumination with non-polarized/polarized UV light was studied for both solutions and thin films; also studied was its relaxation in the dark. On illumination with LPUV light, in-plane reorientation of the molecules normal to the polarization of the exciting UV light, and aggregation of the molecules in the films, were found.     

Banana-shaped liquid crystals with two oligosiloxane end-groups: field-induced switching of supramolecular chirality

The first bent core mesogen carrying branched oligosiloxane units at both ends was synthesized and investigated with polarized light microscopy, DSC, X-ray diffraction of well aligned samples, and electrooptical methods. Two different antiferroelectric switching liquid crystalline phases were found. Both can be regarded as modulated smectic phases in which the molecules have extremely large tilt angles. In both mesophases, the field-induced switching takes place by rotation around the molecular long axes. This field-induced reorientation switches the layer chirality. The transition between the two mesophases is associated with a change of the tilt direction from synclinic to anticlinic. The synclinic high-temperature phase is an obliquelike ribbon phase whereas the anticlinic low-temperature phase represents a sinusoidal undulated layer structure (SmCAPA). It is proposed that the phase transition from the ribbon phase to the undulated layer structure is mainly driven by the change of the interlayer correlation.     

Alignment of liquid crystals using Langmuir‒Blodgett films of unsymmetrical bent-core liquid crystals

ABSTRACT

The properties of the thin films of liquid crystal (LC) molecules can be governed easily by external fields. The anisotropic structure of the LC molecules has a large impact on the electrical and optical properties of the film. The Langmuir monolayer (LM) of LC molecules at the air–water interface is known to exhibit a variety of surface phases which can be transferred onto a solid substrate using the Langmuir?Blodgett (LB) technique. Here, we have studied the LM and LB films of asymmetrically substituted bent-core LC molecules. The morphology of LB film of the molecules is found to be a controlling parameter for aligning bulk LC in the nematic phase. It was found that the LB films of the bent-core molecules possessing defects favour the planar orientation of nematic LC, whereas the LB films with fewer defects show homeotropic alignment. The defect in LB films may introduce splay or bend distortions in the nematic near the alignment layer which can govern the planar alignment of the bulk LC. The uniform layer of LB film facilitates the molecules of nematic to anchor vertically due to a strong van der Waals interaction between the aliphatic chains leading to a homeotropic alignment.     

Fast response in TN liquid-crystal cells: effect of functionalised carbon nanotubes

The optical response time of a nematic liquid crystal (NLC) decreases due to incorporation of carbon nanotubes (CNTs) in the liquid-crystal host. Such reduction is believed to be due to an increase in the elastic constant of the nanotube-doped LC system. In this paper, we present the effect on optical response due to doping an NLC with octadecylamine functionalised single-walled carbon nanotubes (ODA-SWCNT) in a twisted alignment mode. The electro-optic switching amplitude of ODA-SWCNT nanocomposites of NLC decreases compared to pure NLC. A fast response time is observed with an increase in the concentration of ODA-SWCNT in NLC host. Additionally, optical response of pure NLC in a twisted nematic (TN) cell fabricated using mixtures of polyimide (PI) and ODA-SWCNT as an alignment layer is investigated. The optical response time decreased by ~75% in a TN cell fabricated with a mixture of PI and ODA-SWCNT compared to that of a TN cell prepared using a pure PI alignment layer. The presence of ODA-SWCNT in the alignment layer enhances the surface anchoring of the NLC molecules leading to an increase in the elastic constant and a decrease in the optical response time of NLC.     

The effect of surface polarity of glass on liquid crystal alignment

The effects of the surface polarity of a glass substrate on the orientation of nematic liquid crystals (LCs) were studied using the polarised optical microscope and Fourier-transform infrared spectroscopy. On the surface of oxygen plasma treated glass, a homeotropic alignment of LCs was induced for LCs with negative dielectric anisotropy. This suggests that vertical orientation of LCs could be induced on a polar glass substrate without using an LC alignment layer. Upon cooling towards the isotropic–nematic transition, E7 with positive dielectric anisotropy changes its LC arrangement to isotropic, homeotropic, planar orientations in order. The nematic LC anchoring transition of E7 was interpreted by considering the competition between van der Waals forces and dipole interactions that control the alignment of LC molecules on a polar glass surface.     

Dielectric studies of polymer-water interactions and water organization in PEO/water systems

We report the results of detailed investigations of polymer-water interactions and of the organization of water in the poly(ethylene oxide) (PEO)/water system by dielectric techniques. They include thermally stimulated depolarization currents (TSDC) techniques in the temperature range of 77-300 K and broadband dielectric relaxation spectroscopy (DRS) techniques of frequencies, 5 Hz-10 GHz, and temperatures, 173-300 K. The water content h was varied between 0 and 1.21 (grams of water per gram of dry PEO). The secondary γ mechanism of PEO and the reorientation of absorbed water molecules were extensively studied. The γ mechanism was found to be plasticized up to water contents of about 0.20. The reorientation of water molecules was studied in three different experiments and frequency/temperature regions. The results suggest strong interactions in the PEO/water system and indicate the presence of a separate water phase at high water contents. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1545–1560, 1997