arXiv subjects

Katarzyna Merkel

Publications and source records attributed to Katarzyna Merkel.

5 recordsLinked to original sources

arXiv2026

Structure of the twist-bend nematic phase with respect to the orientational molecular order of the thioether-linked dimers

An analysis of the IR absorbance for the segmented functional groups of liquid crystal dimers: mesogen and linker, enabled the orientation order to be determined and information about the dipole interactions in the nematic and twist-bend nematic phases to be obtained. The long axis orientational order increases as the temperature decreases in the nematic phase, although much more slowly than for the classical nematics, and then reverses this trend in the twist-bend nematic phase due to the tilt of the molecules. In the nematic phase, the short axis of the molecule performs an isotropic uniform rotation and has a uniaxial alignment. In the twist-bend nematic phase, however, biaxial ordering occurs and grows significantly in accordance with the helical deformation of the director. Changes in the mean absorbance in the twist-bend nematic phase were observed: a decrease for the longitudinal dipole at the nematic-twist-bend nematic phase transition, thus emphasizing the antiparallel axial interaction of the dipoles, while the absorbance of the transverse dipoles remains unchanged up to 340 K, and then the latter become parallelly correlated.

cond-mat.soft↗

arXiv2022

Increased intermolecular interactions and cluster formation at the onset of the twist-bend nematic phase in thioether cyanobiphenyl-based liquid crystal dimers

Infrared spectroscopy (IR) and quantum chemistry calculations, based on density functional theory (DFT) were used to study the structure and the molecular interactions in the nematic (N) and twist-bend (NTB) phases of thioether-linked dimers. Infrared absorbance measurements were conducted in a polarized beam for homogeneously aligned samples to measure and understand the orientation of the vibrational transition dipole moments in the liquid crystal states. Revealing temperature-dependent changes in the mean IR absorbance in the twist-bend nematic phase were observed. In the transition from the N to the NTB phase, is was found to be associated with a decrease in absorbance for the longitudinal dipoles. This is a result of the antiparallel axial interactions of the dipoles, while the absorbance of the transverse dipoles remained unchanged up to 340 K and then increased and the dipoles correlated in parallel. In order to account for the molecular arrangement in the nematic phase, DFT calculations were conducted for the system nearest to the lateral neighborhoods. Changes in the square of transition dipoles were found to be quite similar to the mean absorbance that was observed in the IR spectra. Interactions of molecules that were dominated by pair formation was observed as well as a shift of the long axes of the molecules relative to each other. The most important observation from the spectroscopic measurements was the sudden increase in the intermolecular interactions as the temperature decreased from the N to the NTB phase. This was evidenced by a significant increase in the correlation of the dipoles that were induced in the dimer cores.

cond-mat.soft↗

arXiv2022

Study of the experimental and simulated vibrational spectra together with conformational analysis for thioether cyanobiphenyl-based liquid crystal dimers

Infrared spectroscopy (IR) and quantum chemistry calculations that are based on the density functional theory (DFT) have been used to study the structure, molecular interactions of the nematic and twist-bend phases for thioether-linked dimers. Infrared absorbance measurements were conducted in a polarized beam for a homogeneously aligned sample in order to obtain more details about the orientation of the vibrational transition dipole moments. The distributions to investigate the structure and conformation of the molecule dihedral angle were calculated. The calculated spectrum was compared with the experimental infrared spectra and as a result, detailed vibrational assignments are reported.

cond-mat.soft↗

arXiv2020

The role of intermolecular interactions in stabilizing the structure of the nematic twist-bend phase

The relationship between the molecular structure and the formation of the NTB phase is still at an early stage of development. This is mainly related to molecular geometry, while the correlation between the NTB phase and the electronic structure is ambiguous. To explore the electronic effect on properties and stabilization of the NTB phase we investigated 2,3-difluoro-4,4-dipentyl-p-terphenyl dimers (DTC5Cn). We used IR polarized spectroscopy, which can at least in principle, bring information about the ordering in NTB phase. All dimers show a significant drop of the average value of the transition dipole moment dμ/dQ for parallel dipoles at the transition to the NTB phase, and an increase for perpendicular dipoles, despite its remaining unchanged for the monomer. These results coincide well with DFT simulations of vibrational dipole derivatives for molecules assembled in pseudo-layers of the NTB phase. The DFT calculations were used to determine the geometric and electronic properties of the hydrogen bonded complexes. We have provided experimental and theoretical evidence of stabilization of the NTB phase by arrays of multiple hydrogen bonds (XF...HX, X-benzene ring).

physics.chem-ph↗

arXiv2020

Molecular biaxiality determines the helical structure -- Infrared measurements of the molecular order in the NTB phase of difluoro terphenyl dimers

IR polarized spectroscopy was employed, to obtain the three components of IR absorbance for a series DTC5Cn (n=5,7,9,11) of bent-shaped dimers containing double fluorinated terphenyl core. Then, they were used to calculate both uniaxial (S) and biaxial (D) order parameters, for various molecular groups of dimers. The molecule bend was estimated due to observed difference between S parameters for the terphenyl core and central hydrocarbon linker. It is found to be significant in N phase despite almost no macroscopic director bend. The temperature dependence of the order parameter, S, distinctly reverses its monotonic trend of increase to decrease at the transition temperature, TN-NTB to the NTB phase as result of the director tilt in the NTB phase. The biaxial order parameter, D, is negligible in the N phase, and then starts increasing on entering the NTB temperature, similarly to sin2q. The local director curvature is found to be controlled by the molecular biaxiality parameter.

cond-mat.soft↗