arXiv subjects

Damian Pociecha

Publications and source records attributed to Damian Pociecha.

27 records · Page 2Linked to original sources

arXiv2021

Complex chiral columns made of achiral quinoxaline derivatives with semi-flexible cores

Mesogenic materials, quinoxaline derivatives with semi-flexible cores, are reported to form new type of 3D columnar structure with large crystallographic unit cell and Fddd symmetry below columnar hexagonal phase. The 3D columnar structure is a result of frustration imposed by arrangement of helical columns of opposite chirality into triangular lattice. The studied materials exhibit fluorescent properties that could be easily tuned by modification of molecular structure, compounds with the extended π electron conjugated systems form aggregates and fluorescence is quenched. For molecules with flexible structure the fluorescence quantum yield reaches 25%. On the other hand, compounds with more rigid mesogenic core, for which fluorescence is suppressed show strong hole photocurrent. For some materials also bi-polar: hole and electron transfer was observed.

cond-mat.mtrl-sci↗

arXiv2019

Organic nanotubes created from mesogenic derivatives

A facile route to prepare the nanotubes from rod-like mesogens dissolved in typical organic solvents is reported. For selected types of chiral rod-like molecules, both enantiomers as well as the racemic mixtures formed the nanotubes by slow evaporation from a solution, regardless of the solvent, concentration or deposition type. Obtained supramolecular assemblies were studied using AFM and SEM techniques. Additional methods (IR, UV-Vis spectroscopy and x-ray diffraction) were applied to characterize the observed nanotubes. The difference in the surface tension at opposite crystallite surfaces is suggested as a possible mechanism for the nanotube nucleation.

cond-mat.mes-hall↗

arXiv2019

Multi-level Chirality in Liquid Crystals Formed by Achiral Molecules

In many biological materials with a hierarchical structure there is an intriguing and unique mechanism responsible for the 'propagation' of order from the molecular to the nano- or micro-scale level. Here we present a much simpler molecular system built of achiral mesogenic dimeric molecules that shows a similar complexity with four levels of structural chirality (i) layer chirality, (ii) helicity of a basic 4-layer repeating unit, (iii) a helix with a pitch of several layers and (iv) mesoscopic helical filaments. As seen in many biological systems, there is a coupling between chirality at different levels. The structures were identified by a combination of hard and soft x-ray diffraction measurements, optical studies and theoretical modelling.

cond-mat.soft↗

arXiv2018

Nature of nematic to twist bend nematic phase transition

The critical behavior at the transition from uniform nematic to twist-bend modulated nematic phase is revealed and shown to be well explained by the mean field approximation. The study was performed on a group of materials that exhibit an unusually broad temperature range of the nematic phase above the twist-bend modulated nematic phase, so the critical range in which the order parameter fluctuations are strong and can be experimentally observed is wide. The formation of instantaneous helices is observed already several degrees above the transition temperature, strongly influencing the birefringence of the system. The analysis of a critical part of the birefringence changes provides a set of critical exponents that are consistent with the mean field approximation, indicating a large correlation length of helical fluctuations in the nematic phase.

cond-mat.soft↗

arXiv2017

Heliconical smectic phases formed by achiral molecules

A series of asymmetric dimers with an odd number of atoms in the spacer were found to form different types of twisted structures despite being achiral. The formation of a variety of helical structures is accompanied by a gradual freezing of molecular rotation. The tight pitch heliconical nematic (NTB) phase and heliconical tilted smectic C (SmCTB) phase are formed. In the lowest temperature smectic phase, HexI, the twist is expressed through the formation of hierarchical structure: nano-scale helices and mesoscopic helical filaments. The short pitch helical structure in smectic phases is confirmed by resonant x-ray measurements.

cond-mat.soft↗

arXiv2017

Structure of nanoscale-pitch helical phases: blue phase and twist-bend nematic phase resolved by resonant soft X-ray scattering

Periodic structures of phases with orientational order of molecules, but homogenous electron density distribution: a short pitch cholesteric, blue phase and twist-bend nematic phase, were probed by a resonant soft x-ray scattering (RSoXS) at the carbon K-edge. The theoretical model shows that in case of a simple heliconical nematic structure two resonant signals corresponding to the full and half pitch band should be present, while only the full pitch band is observed in experiment. This suggests that the twist-bend nematic phase has complex structure with a double-helix, built of two interlocked, shifted helices. We confirm that the helical pitch in the twist-bend nematic phase is in a 10 nm range, for both, the chiral and achiral materials. We also show that the symmetry of a blue phase can unambiguously be determined through a resonant enhancement of x-ray diffraction signals, by including polarization effects, which are found to be an important indicator in phase structure determination.

cond-mat.soft↗

arXiv2016

Linkage-length dependent structuring behaviour of bent-core molecules in helical nanostructures

We studied the correlation between the molecular structure and the formation of helical nanofilaments (HNFs) of bent-core dimeric molecules with varying linkage lengths. To obtain precise structural data, a single domain of HNFs was prepared under physical confinement using porous 1D nanochannels, made up of anodic aluminium oxide films. Electron microscopy and grazing incidence X-ray diffraction were used to elucidate the linkage length-dependent formation of HNFs.

cond-mat.soft↗

arXiv2014

Do the short helices exist in the nematic TB phase?

Dimeric compounds forming twist-bend nematic, Ntb, phase show unusual optical texture related to the formation of arrays of focal conic defects. Some of the focal conics show submicron internal structure with 8 nm periodicity, which is very close to that found in the crystalline phase of the material, suggesting surface freezing.

cond-mat.soft↗

arXiv2004

Switching mechanism in polar columnar mesophases made of bent-core molecules

The behavior of polar columnar phases made of bent shaped molecules (B_1Rev and B_1RevTilted) was studied under applied electric field. There are two competing mechanisms of switching in polar B_1RevTilted columnar phase: the collective rotation around the long molecular axis and the collective rotation around the tilt cone. Respectively monostable and bistable optical switching is observed for antiferroelectric B_1RevTilted phase. The main factor discriminating the type of the switching is a width of the column cross-section.

cond-mat.soft↗