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

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.

Rotational Dynamics of Nitroxide Biradical in Room-Temperature Ionic Liquids Measured by Quantitative Simulation of EPR Spectra.

Temperature dependences of electron paramagnetic resonance (EPR) spectra of an imidazoline nitroxide biradical spin probe in a series of room-temperature ionic liquids in the temperature range 124-390 K have been quantitatively simulated. The unusual asymmetric EPR spectrum shape previously observed in these systems [Kokorin et al., Appl. Magn. Res. 48 (2016) 287] is shown to originate from anisotropic rotational diffusion of the probe molecule. All experimental spectra were quantitatively reproduced in simulation using a unified set of geometrical and magnetic parameters of the spin probe, which were found to be fully consistent with the biradical geometry obtained from density functional theory calculations. Temperature dependences of rotation diffusion coefficient of the probe characterize the molecular mobility of the ionic liquid, whereas the temperature dependences of the spin-exchange integral J and of the isotropic hyperfine interaction constant, aN, are shown to reflect the intramolecular conformation motions of the biradical probe.

Spin Probe Determination of Molecular Orientation Distribution and Rotational Mobility in Liquid Crystals: Model-Free Approach.

A model-free approach for simulation of EPR spectra of nitroxide spin probes in liquid-crystalline materials was suggested and used to obtain parameters of molecular orientation and rotational mobility. The developed method is based on experimental recording and numerical simulation of the angular dependence of EPR spectra, which is shown to be much more informative in comparison with a single EPR spectrum. Quantitative spectral simulations considering both local orientational ordering and distribution of local directors in the sample were used for discrimination of models of rotational mobility and orientational alignment. The method was applied for detailed quantitative characterization of axial, orthorhombic, and low-symmetry non-orthorhombic molecular orientation distributions. It is shown that the ordinarily used model of rotational diffusion in a mean-field potential is suitable for the description of molecular mobility and orientational ordering only for relatively low sample temperatures and low-mobility probe molecules with large sizes. In cases of high molecular mobility, the more realistic jump mechanism of molecular moves can be approximately described as quasi-librations. For ordered liquid crystals it was found that mostly the order parameters up to rank 12-14 are essential and easily determined. When well-aligned materials are described, the order parameters up to 18th rank or even higher become meaningful. Both molecular and sample biaxiality is analyzed and quantitatively characterized. The local molecular ordering and sample orientational alignment are quantitatively characterized separately.

Molecular orientational order of nitroxide radicals in liquid crystalline media.

The orientational distribution of a set of stable nitroxide radicals in aligned liquid crystals 5CB (nematic) and 8CB (smectic A) was studied in detail by numerical simulation of EPR spectra. The order parameters up to the 10th rank were measured. The directions of the principal orientation axes of the radicals were determined. It was shown that the ordering of the probe molecules is controlled by their interaction with the matrix molecules more than the inherent geometry of the probes themselves. The rigid fused phenanthrene-based (A5) and 2-azaphenalene (A4) nitroxides as well as the rigid core elongated C11 and 5α-cholestane (CLS) nitroxides were found to be most sensitive to the orientation of the liquid crystal matrixes.

Determination of structural characteristics of all-organic radical liquid crystals based on analysis of the dipole-dipole broadened EPR spectra.

The angular dependences of g-value and line width of EPR spectra of paramagnetic all-organic liquid crystalline (LC) materials were measured for the quantitative characterization of the nematic, cholesteric, and smectic C phases. The detailed molecular alignment in mesophases was determined by means of numerical spectra simulation focusing on spin exchange and dipole-dipole magnetic interactions of neighboring molecules. The obtained structural data indicate that the spin polarization mechanism between neighboring molecules rather than the direct through-space interactions between paramagnetic centers is responsible for the specific magnetic properties of the studied LC materials.

Order parameters , , and of aligned nematic liquid-crystalline polymer as determined by numerical simulation of electron paramagnetic resonance spectra.

High rank order parameters may comprise substantial information about molecular orientational distribution of liquid-crystalline materials. There are few experimental procedures targeted at the determination of high rank order parameters. We suggest a procedure for the determination of order parameters of a spin probe by numerical simulation of electron paramagnetic resonance (EPR) spectra. The procedure is based on revealing the molecular orientation axis of a spin probe. Order parameters , , and of the spin probe in aligned liquid-crystalline polymer have been measured by the suggested procedure. It was shown that the procedure gives more reliable and complete characteristics of molecular order than does polarized UV-visible spectroscopy.