EPR study of the polydomain structure of the twist-bend nematic phase of CB9CB in the bulk

Bacchiocchi, Corrado; Tamba, Maria-Gabriela; Mehl, Georg H.; Arcioni, Alberto; Miglioli, Isabella; Zannoni, Claudio

doi:10.1080/02678292.2018.1501110

Cited by 3 publications

(3 citation statements)

References 33 publications

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“…It is assumed that in each of the local ordered regions, the probe is oriented in accordance with the orientational potential of the mean field where (0, β m , γ m ) are Euler angles that transform the director reference frame into the molecular reference frame, k B is Boltzmann constant, and c n l are coefficients of the potential decomposition. It has been shown in the literature that the EPR spectra of spin probes in deformed liquid crystals can be well approximated by the superposition of spectra of individual local ordered regions described by a simple molecular field potential such as in eq . ,− This approach would fail if the spin probe molecules would predominantly be localized in the regions with a low orientation order, such as defects and interdomain boundaries. The spin probe chosen in the present work is embedded well in the bulk structure of the smectic mesophase, which is corroborated by the experimentally observed strong angular dependences of EPR spectra, and the absence of dipolar and exchange broadening in the recorded spectra.…”

Section: Simulation Of Epr Spectramentioning

confidence: 99%

Orientation Distribution of Molecules in a Smectic Liquid Crystal with a Distorted Director Geometry

Bogdanov

Vorobiev

2022

J. Phys. Chem. B

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Quantitative determination of the molecular orientation distribution function in samples of liquid crystals with a complex director geometry was performed using the numerical simulation of electron paramagnetic resonance (EPR) spectra of the spin probes in a liquid-crystalline medium. To achieve the quantitative agreement of experimental and simulated EPR spectra, the hierarchy of the orientation order was explicitly taken into account, namely, the local ordering of liquid crystal molecules by the mean-field potential of surrounding molecules, and the partial disordering of local directors within the sample. The samples under study are planar and twist LC cells with liquid crystal 8CB cooled from the nematic into the smectic A phase in the magnetic field. The presence of the magnetic field perpendicular to the cell director leads to distortion of the orientation of the liquid crystal in the cell. The spin probe technique was successfully employed for the reliable measurement of orientation distribution functions of the low nonorthorhombic symmetry. Orientation order parameters up to 12th rank were measured, including nonaxial and nonorthorhombic order parameters. It is shown that the presence of several contradicting aligning forces leads to the tilt of the preferential director toward the direction, which is a compromise between the orienting forces.

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Section: Simulation Of Epr Spectramentioning

confidence: 99%

Orientation Distribution of Molecules in a Smectic Liquid Crystal with a Distorted Director Geometry

Bogdanov

Vorobiev

2022

J. Phys. Chem. B

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“…For the flexoelectro-optic effect, we first investigate the switching behavior of a mixture that consists of a base chiral nematic mixture, comprising E7 + 3 wt % BDH1281 (Merck) doped with 20 wt % of the bimesogenic material 4 ,4 -(nonane-1,7-diyl)bis[(1 ,1 -biphenyl)-4 -carbonitrile] (CB9CB) [35,36]. The chiral nematic mixture, which forms a right-handed helical structure with a pitch of about 520 nm, is capillary filled into a nominally 5-µm-thick device that consists of glass substrates that are coated with ITO electrodes (to facilitate the application of an electric field along the normal to the device substrates) and antiparallel rubbed polyimide alignment layers.…”

Section: B Phase Modulation With Multipass Flexoelectro-optic Switchingmentioning

confidence: 99%

Millisecond Optical Phase Modulation Using Multipass Configurations with Liquid-Crystal Devices

et al. 2020

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We present two configurations for analog 0 to 2π optical phase modulation using liquid crystals (LCs), each of which achieve switching times that are 1 ms or less. One configuration is based on the switching behavior of a so-called nematic pi cell, and the other is based on the flexoelectro-optic effect in chiral nematic LCs when operated in the uniform lying helix geometry. Both configurations exploit a multipass optical arrangement to enhance the available optical phase range, while maintaining a fast switching speed. Moreover, these devices can be operated at or close to room temperature. Experimental data are found to be in good agreement with results predicted from theory for these multipass phase-modulation configurations.

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“…Additionally, the polarity and the chirality of the NTB phase formed by achiral molecules have been investigated by electro-optical [4,5,6,Error! Bookmark not defined.,11,17], nuclear magnetic resonance (NMR) [12,23,23,25,26,27,28,29], EPR [30], magneto-optical [31], dielectric [17,18,18,23,27,32] and Raman spectroscopy [33,34]. The most recent reports on NTB focus on to the study of the nanoscale pseudo-layered helical structures of the NTB phase using Resonant X-Ray Scattering (RXS) [35,36,37].…”

Section: Introductionmentioning

confidence: 99%

Dielectric response of electric-field distortions of the twist-bend nematic phase for LC dimers

Merkel

Welch

Ahmed

et al. 2019

The Journal of Chemical Physics

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Wide band dielectric spectroscopy of bent-shaped achiral liquid-crystal dimers 1-n-bis(4cyanobiphenyl-4-yl) n-alkanes (CBnCB n=7,9,11) have been investigated in a frequency range 0.1 Hz to 100 MHz using planar-aligned cells of sample thicknesses ranging from 2 to 10 (m) over a temperature range that covers both nematic and twist bend nematic phases.Two peaks in the dielectric spectrum in the higher frequency range are assigned to the molecular relaxation processes. The peak at the highest frequency, ~ 40 to 80 MHz, is assigned to an internal precessional rotation of a single unit of the dimer around the director.The mode in the next lower frequency range of 2 to 10 MHz is assigned to the spinning rotation of the dimer around its long axis. This involves fluctuations of the dipole moment of the bent-shaped conformation that is directed along its arrow direction of the bow shape formed by the dimer. The peak in the frequency range 100 kHz to 1 MHz can be assigned to the collective fluctuations of the local director with reference to the helical axis of the NTB structure. The dependence of its frequency on temperature is reminiscent of the soft mode observed at the SmA* to SmC* phase transition. This result clearly corresponds to the electroclinic effectthe response of the director to the applied electric field in an electro-optic experiment. The lowest frequency mode, observed in frequency range of 0.1 Hz -100 Hz, is identified with the Goldstone mode. This mode is concerned with the long range azimuthal angle fluctuations of the local director. This leads to an alternating compression and expansion of the periodic structure of the NTB phase.

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EPR study of the polydomain structure of the twist-bend nematic phase of CB9CB in the bulk

Cited by 3 publications

References 33 publications

Orientation Distribution of Molecules in a Smectic Liquid Crystal with a Distorted Director Geometry

Orientation Distribution of Molecules in a Smectic Liquid Crystal with a Distorted Director Geometry

Millisecond Optical Phase Modulation Using Multipass Configurations with Liquid-Crystal Devices

Dielectric response of electric-field distortions of the twist-bend nematic phase for LC dimers

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