scholarly journals All Structures Great and Small: Nanoscale Modulations in Nematic Liquid Crystals

Nanomaterials ◽  
2021 ◽  
Vol 12 (1) ◽  
pp. 93
Author(s):  
Edward T. Samulski ◽  
Denisse Reyes-Arango ◽  
Alexandros G. Vanakaras ◽  
Demetri J. Photinos

The nature of the nanoscale structural organization in modulated nematic phases formed by molecules having a nonlinear molecular architecture is a central issue in contemporary liquid crystal research. Nevertheless, the elucidation of the molecular organization is incomplete and poorly understood. One attempt to explain nanoscale phenomena merely “shrinks down” established macroscopic continuum elasticity modeling. That explanation initially (and mistakenly) identified the low temperature nematic phase (NX), first observed in symmetric mesogenic dimers of the CB-n-CB series with an odd number of methylene spacers (n), as a twist–bend nematic (NTB). We show that the NX is unrelated to any of the elastic deformations (bend, splay, twist) stipulated by the continuum elasticity theory of nematics. Results from molecular theory and computer simulations are used to illuminate the local symmetry and physical origins of the nanoscale modulations in the NX phase, a spontaneously chiral and locally polar nematic. We emphasize and contrast the differences between the NX and theoretically conceivable nematics exhibiting spontaneous modulations of the elastic modes by presenting a coherent formulation of one-dimensionally modulated nematics based on the Frank–Oseen elasticity theory. The conditions for the appearance of nematic phases presenting true elastic modulations of the twist–bend, splay–bend, etc., combinations are discussed and shown to clearly exclude identifications with the nanoscale-modulated nematics observed experimentally, e.g., the NX phase. The latter modulation derives from packing constraints associated with nonlinear molecules—a chiral, locally-polar structural organization indicative of a new type of nematic phase.

Author(s):  
U. Aebi ◽  
P. Rew ◽  
T.-T. Sun

Various types of intermediate-sized (10-nm) filaments have been found and described in many different cell types during the past few years. Despite the differences in the chemical composition among the different types of filaments, they all yield common structural features: they are usually up to several microns long and have a diameter of 7 to 10 nm; there is evidence that they are made of several 2 to 3.5 nm wide protofilaments which are helically wound around each other; the secondary structure of the polypeptides constituting the filaments is rich in ∞-helix. However a detailed description of their structural organization is lacking to date.


1976 ◽  
Vol 31 (12) ◽  
pp. 1700-1706 ◽  
Author(s):  
G. Heppke ◽  
F. Schneider ◽  
A. Sterzl

Abstract Investigation The electrical conductivity of the first eight homologous N-(4-n-alkoxybenzylidene)-4'-n-butylanilines is investigated in the isotropic, nematic and smectic phases using the electrolyte tetra-butylammonium picrate. The anisotropy ratio of the electrical conductivity shows pretransition behaviour in the nematic phases close to the transition to the smectic phases. This behaviour be-comes more distinct with increasing alkyl chain length of the liquid crystals. In the nematic phase of 40·4 an anisotropy ratio of V = 0.55 is determined at the lowest electrolyte concentration. Values of about V=0.05 were found in the smectic phases of several homologues. In the nematic phase, an increase of the electrolyte concentration effects an increase of the anisotropy ratio. This effect is systematically enhanced with increasing alkyl chain length.


2020 ◽  
Author(s):  
Jinying Lu ◽  
Zelong Zhang ◽  
Daoren Yan ◽  
Zhiyong Zhang ◽  
Jintao Guan ◽  
...  

<p></p><p>Azobenzene-based bent-core liquid crystals demonstrate a variety of mesomorphic behaviors and photochromic properties which are desirable for optical switching. Nowadays azobenzene-based bent-core liquid crystal (ABLC) compounds usually exhibit at least one of the following traits which are unfavorable for practical applications: (1) narrow temperature windows of nematic phases, (2) high phase transition temperature, and (3) long period of light stimulation to reach photostationary states. In this study, a series of ABLC compounds <b>4a–4g</b> were synthesized by adding azo functional groups and chlorine substituent to the central bent-cores to form 4-chloro-1,3-dizaophenylene bent-cores. These ABLC compounds were characterized by i. fourier-transform infrared spectroscopy (FTIR), <sup>1</sup>H and <sup>13</sup>C nuclear magnetic resonance (NMR), and mass spectrometry (MS) for their structures, ii. differential scanning calorimetry (DSC) and polarized optical microscopy (POM) for their mesogenic properties, and iii. ultraviolet–visible spectroscopy (UV-Vis) and POM for their photosensitivity. The experimental results show that all compounds exhibited broad temperature windows of mesogenic phases. In particular, compound <b>4c</b> showed a broad temperature window of 63.8 °C for nematic phase. Molecular simulations indicate that the molecular dipole moments of compounds <b>4a–4g</b> are closely associated with the temperatures of Sm – N phase transition and temperature ranges of nematic phases. In addition, simulation results reveal that the terminal alkyl chains exhibit a diphasic effect on the molecular polarity: extending the terminal chain can initially reduce and then increase the molecular dipole moments due to the severe structural disorder of overly extended terminal chain. These findings indicate that the intermolecular forces play a vital role in shaping the mesogenic behavior of ABLCs. Comprehensive characterizations of photochromatic properties show that <b>4c</b> was highly photosensitive and displayed rapid photoisomerization processes. At room temperature, compound <b>4c </b>dissolved in ethyl acetate solution can reach photostationary state in 10 seconds. At 95 °C, compound <b>4c</b> in nematic phase became isotropic liquid under UV-irradiation in 3 seconds due to the forward <i>trans – cis</i> photoisomerization and can be restored to be nematic under natural visible light in 5 seconds because of the backward <i>cis – trans</i> photoisomerization. This study linking the mechanistic details with mesogenic properties provides valuable insights to improve future design of azobenzene bent-core liquid crystals for practical applications especially in photonic applications.</p><p><br></p><p>Pertinent molecular structure files (mol2) can be downloaded from</p><p><a href="https://github.com/er1czz/ABLC">https://github.com/er1czz/ABLC</a><br></p><p></p>


2020 ◽  
Vol 35 (06) ◽  
pp. 2030003 ◽  
Author(s):  
Michael Pretko ◽  
Xie Chen ◽  
Yizhi You

Fractons are a new type of quasiparticle which are immobile in isolation, but can often move by forming bound states. Fractons are found in a variety of physical settings, such as spin liquids and elasticity theory, and exhibit unusual phenomenology, such as gravitational physics and localization. The past several years have seen a surge of interest in these exotic particles, which have come to the forefront of modern condensed matter theory. In this review, we provide a broad treatment of fractons, ranging from pedagogical introductory material to discussions of recent advances in the field. We begin by demonstrating how the fracton phenomenon naturally arises as a consequence of higher moment conservation laws, often accompanied by the emergence of tensor gauge theories. We then provide a survey of fracton phases in spin models, along with the various tools used to characterize them, such as the foliation framework. We discuss in detail the manifestation of fracton physics in elasticity theory, as well as the connections of fractons with localization and gravitation. Finally, we provide an overview of some recently proposed platforms for fracton physics, such as Majorana islands and hole-doped antiferromagnets. We conclude with some open questions and an outlook on the field.


Soft Matter ◽  
2020 ◽  
Vol 16 (18) ◽  
pp. 4350-4357 ◽  
Author(s):  
Wojciech Tomczyk ◽  
Lech Longa

Within mean-field theory for V-shaped molecules, we have investigated how the alteration of a molecule's structural features influence the stabilization of modulated and non-modulated nematic phases.


1983 ◽  
Vol 38 (12) ◽  
pp. 1362-1364 ◽  
Author(s):  
I. H. Ibrahim ◽  
W. Haase

Abstract Three mesogenic compounds of the general formula have been investigated by differential scanning calorimetry. thermal optical microscopy and X-ray diffraction methods/Enthalpy changes of The different phase transitions for these compounds have been determined. H33 exhibits smectic B and nematic phases, whereas H34 and H75 exhibit only smectic B phases. The thickness of the smectic layers and the average intermolecular distance have been evaluated, as well as the corresponding molecular parameters in the nematic phase.


2002 ◽  
Vol 56 (6) ◽  
pp. 744-750 ◽  
Author(s):  
T. Del Caño ◽  
J. Duff ◽  
R. Aroca

The synthesis, molecular spectra, and thin solid film properties of a perylene dye, bis(neopentylimido) perylene (BNPTCD),§ are reported. Absorption and emission molecular spectra are used to probe the structure of vacuum-evaporated thin films and Langmuir–Blodgett monolayers of BNPTCD. The experimental assignment of electronic, infrared, and Raman spectra are aided with calculated molecular structure and spectra using density functional theory (DFT) and Hartree–Fock (HF) computational methods at 6–31G level of theory. Characteristic vibrational modes and local symmetry of the planar perylene tetracarboxylic chromophore (PTCD) are used to extract molecular organization and film packing properties from spectral data collected using transmission and reflection-absorption (RAIRS) infrared spectroscopy. From the infrared spectra, using surface selection rules and polarization properties, it was extracted that the PTCD plane in BNPTCD is organized in the evaporated film with the plane head-on tilted over the substrate. After the molecular organization was identified, the effect of thermal annealing and solvent vapors on the film structure was examined. It is found that the PTCD plane turns to a flat-on orientation when the film is thermal annealed or solvent treated. The aggregation and molecular stacking in films was also inspected using the electronic absorption and fluorescence of monomers and aggregates.


1975 ◽  
Vol 53 (2) ◽  
pp. 161-166 ◽  
Author(s):  
Yunko Lee ◽  
Leonard W. Reeves

The proton magnetic resonance spectra of dimethylthallium ion has been studied in both anionic and cationic detergent nematic phases. The spectra were analyzed to derive the values of inter- and intramethyl proton–proton dipolar coupling and the intermethyl scalar coupling constants. The degree of orientation of the dimethylthallium cation is much higher in the anionic nematic phase than in the cationic phase. It is proposed that electrostatic interactions play a significant role in the orientation of complex ions in ionic detergent nematic phases.


Genome ◽  
2004 ◽  
Vol 47 (3) ◽  
pp. 535-545 ◽  
Author(s):  
Sergio L Pereira ◽  
Erwin T Grau ◽  
Anita Wajntal

The control region (CR) plays an important role in replication and transcription in the mitochondrial genome. Its supposedly high rate of DNA substitution makes it a suitable marker for studies of population and closely related species. Three domains can be identified in CR, each having its own characteristics regarding base composition, pattern of variability and rate of DNA substitution. We sequenced the complete CR for 27 cracids, including all 11 genera to characterize its molecular organization, describe patterns of DNA substitution along the gene, and estimate absolute rates of DNA substitution. Our results show that cracid CR organization and patterns of DNA substitution are typical of other birds. Conserved blocks C and B, fused CSB2/3, and a putative segment for bidirectional replication not usually present in birds were identified in cracids. We also suggest a new delimitation for domains to allow better comparisons among CRs at different taxonomic ranking. Our estimates of absolute rates of DNA substitution show that, in general, CR evolves at a rate slower than that of two protein-coding genes (CR, 0.14%–0.3%; ND2, 0.37%–0.47%; and cytochrome b, 0.29%–0.35% per site per million years within genera). Within CR domains, rates within genera ranged from 0.05% to 0.8% per site per million years.Key words: mitochondrial control region, molecular evolution, Cracidae, Aves.


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