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Crystals
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Photosensitive Liquid Crystals
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Photosensitive Liquid Crystals

A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Liquid Crystals".

Deadline for manuscript submissions: closed (28 February 2021) | Viewed by 13923

Special Issue Editors

Dr. Michal Kohout

E-Mail Website
Guest Editor
University of Chemistry and Technology, Prague, Department of Organic Chemistry, Prague, Czech Republic
Interests: chiral separation; chiral stationary phases; ion exchangers; smart organic materials; organocatalysis; liquid crystals
Dr. Mohamed Alaasar

E-Mail Website
Guest Editor
Institute of Chemistry, Martin-Luther University Halle-Wittenberg, Kurt-Mothes Str. 2, 06120 Halle/Saale, Germany
Interests: liquid crystals; design and synthesis of new liquid crystalline materials; bent-core liquid crystals; mirror-symmetry breaking; chirality; supramolecular chemistry; photosensitivity; azobenzene; hydrogen bonding; halogen bonding

Special Issue Information

Dear Colleagues,

Photosensitive materials capable of structural changes upon light irradiation, leading to the modification of their optical properties, have recently been the subject of intensive study due to their immense application potential in non-linear optics and photonics. Particularly, soft self-assembling materials (liquid crystals, combining the fluidity of liquids and the orientational ordering of solid crystals) can be utilized in the fabrication of various devices. However, photosensitive liquid crystals represent not only important materials of the current information age, but also a fascinating state of matter which continues to surprise scientists worldwide. From the pool of photosensitive units applied in the design of liquid crystals, the azo group stands out for the distinct conditions of its photoisomerization. The unique features of the azo group gave rise to materials and photosensitive dopants with intriguing properties, such as the photo-switching of chirality. Nevertheless, other photosensitive groups can be utilized to design and synthesize functional materials capable of self-assembly modulation on light stimulus.

The aim of this Special Issue is to provide an open platform for fundamental new research on the chemistry, physics, and possible applications of photosensitive liquid crystals.

Dr. Michal Kohout
Dr. Mohamed Alaasar
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Crystals is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2100 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • photosensitive liquid crystals
  • synthesis and characterization of new materials
  • light-induced mesophase modulation
  • light-sensitive materials
  • soft materials
  • photosensitive liquid crystalline mixtures
  • mirror-symmetry breaking
  • smart organic crystals

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Published Papers (5 papers)

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Research

14 pages, 3843 KiB  
Article
Photosensitive Bent-Core Liquid Crystals with Laterally Substituted Azobenzene Unit
by Diana Jágerová, Michal Šmahel, Anna Poryvai, Jan Macháček, Vladimíra Novotná and Michal Kohout
Crystals 2021, 11(10), 1265; https://doi.org/10.3390/cryst11101265 - 18 Oct 2021
Cited by 6 | Viewed by 2919
Abstract
Photosensitive liquid crystals represent an important class of functional materials that experience rapid development. Hereby, we present novel bent-core liquid crystals bearing a lateral substitution on the central core and in the vicinity of the photosensitive unit—an azo group. The azo group enables [...] Read more.
Photosensitive liquid crystals represent an important class of functional materials that experience rapid development. Hereby, we present novel bent-core liquid crystals bearing a lateral substitution on the central core and in the vicinity of the photosensitive unit—an azo group. The azo group enables fast (E)-to-(Z)-isomerization upon irradiation with UV-light and visible light, while the substitution facilitates the high stability of the photochemically formed (Z)-isomer. The effectiveness of the irradiation and the composition of photostationary states was determined by UV/Vis and 1H NMR spectroscopy. A nematic phase formed by the materials was characterized by differential scanning calorimetry and optical polarizing microscopy. We show that the materials easily change their relative configuration of the N=N double bond not only in solution, but also in the mesophase, which leads to fast isothermal phase transition from the nematic phase to isotropic liquid. Full article
(This article belongs to the Special Issue Photosensitive Liquid Crystals)
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15 pages, 5999 KiB  
Article
Chiral Photoresponsive Liquid Crystalline Materials Derived from Cyanoazobenzene Central Core: Effect of UV Light Illumination on Mesomorphic Behavior
by Anna Poryvai, Alexej Bubnov and Michal Kohout
Crystals 2020, 10(12), 1161; https://doi.org/10.3390/cryst10121161 - 21 Dec 2020
Cited by 6 | Viewed by 2687
Abstract
One of the most frequently utilized liquid crystalline (LC) materials is a rod-like (calamitic) compound 4-cyano-4′-pentylbiphenyl (5-CB). The main objective of this work is to enhance its functionality by introducing a photoresponsive diazenyl spacer in the aromatic core and replace the non-chiral pentyl [...] Read more.
One of the most frequently utilized liquid crystalline (LC) materials is a rod-like (calamitic) compound 4-cyano-4′-pentylbiphenyl (5-CB). The main objective of this work is to enhance its functionality by introducing a photoresponsive diazenyl spacer in the aromatic core and replace the non-chiral pentyl chain with various chiral alkyl carboxylate units. The mesomorphic properties of the prepared materials have been studied using polarizing optical microscopy and differential scanning calorimetry. It has been found that materials with an extended aromatic system possess the liquid crystalline behavior. The studied LC materials have shown mesophases at lower temperatures than previously reported analogous substances. Furthermore, one of them exhibits a chiral orthogonal frustrated twist grain boundary smectic phase, which has not been previously observed for this structural type of materials. We also investigated photoresponse of the mesophases under illumination with UV-light (365 nm) using a polarizing optical microscope. A non-conventional photoresponse of the prepared materials in a crystalline phase is presented and discussed. Full article
(This article belongs to the Special Issue Photosensitive Liquid Crystals)
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14 pages, 4302 KiB  
Article
Photosensitive Bent-Core Compounds with Azo-Group Attached to the Central Ring
by Martin Cigl, Věra Hamplová, Damian Pociecha and Vladimíra Novotná
Crystals 2020, 10(11), 1030; https://doi.org/10.3390/cryst10111030 - 11 Nov 2020
Cited by 3 | Viewed by 2473
Abstract
We prepared and studied bent-core liquid crystalline (LC) compounds based on 1,3-disubstituted benzene in a central part and azo-linkage attached directly to this bent core. We designed three structures and checked their mesogenic properties, as well as photosensitivity. We found that two studied [...] Read more.
We prepared and studied bent-core liquid crystalline (LC) compounds based on 1,3-disubstituted benzene in a central part and azo-linkage attached directly to this bent core. We designed three structures and checked their mesogenic properties, as well as photosensitivity. We found that two studied compounds revealed columnar LC mesophases, which we transformed to the isotropic phase under the illumination of UV light. We concluded that only one type of structural motif was not mesogenic. For LC compounds, we established phases and phase transition temperatures based on differential scanning calorimetry (DSC) measurements and observations in a polarizing microscope. To confirm phase identification, X-ray studies were performed and structural parameters describing the columnar phases supplied. Full article
(This article belongs to the Special Issue Photosensitive Liquid Crystals)
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Graphical abstract

11 pages, 2805 KiB  
Article
Competitive Dye Adsorption-Desorption on the Isotropic Surface at the Early Stage of the Photoexcitation of Azo Dye-Doped Liquid Crystals
by Wei-Ting Chen, Sheng-Chung Ji, Shih-Hsuan Chen, Chia-Yi Huang and Kuang Yao Lo
Crystals 2020, 10(9), 802; https://doi.org/10.3390/cryst10090802 - 10 Sep 2020
Cited by 1 | Viewed by 1922
Abstract
The instantaneous response of liquid crystal (LC) molecules near the dye-adsorbed layer of azo dye-doped LCs (ADDLCs) in early-stage photoexcitation was observed through the pumped-attenuated total reflection (PATR) method. As the sensing depth of the attenuated total reflection (ATR) method is in the [...] Read more.
The instantaneous response of liquid crystal (LC) molecules near the dye-adsorbed layer of azo dye-doped LCs (ADDLCs) in early-stage photoexcitation was observed through the pumped-attenuated total reflection (PATR) method. As the sensing depth of the attenuated total reflection (ATR) method is in the order of hundreds of nm, in situ observation using the PATR method directly inspected the restructuring of the adsorbed layer in the early-stage photoexcitation, which demonstrated the competition between adsorption and desorption on the isotropic surface without the interference of the elastic continuum in the bulk of a cell. Analyzing the frequency of the oscillation-like PATR signal, which reflects the evolution of the orientation of the LC molecules, revealed that, depending on the dose of the pump light, the structural evolution of the adsorbed layer is fast at the beginning of photoalignment in ADDLCs and slows down with the accumulation of the adsorbed layer. A suitable intensity of the pump light in the early stage of the photoalignment process can improve the reform of the adsorbed layer to strengthen its anchoring. Full article
(This article belongs to the Special Issue Photosensitive Liquid Crystals)
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14 pages, 2307 KiB  
Article
Characterization of New H-Bonded Liquid Crystalline Complexes Based on Iminophenyl Nicotinate
by Rua B. Alnoman, Mohamed Hagar, Hoda A. Ahmed, Khulood A. Abu Al-Ola, Magdi M. Naoum, Fahad Al-Elati, Yousef Abdullah Zaid, Abdulrahman Alsharif, Yazeed Al-Juhani and Abdulmjeed Abulrhelh
Crystals 2020, 10(6), 499; https://doi.org/10.3390/cryst10060499 - 9 Jun 2020
Cited by 8 | Viewed by 2533
Abstract
Two new 1:2 supramolecular H-bonded liquid crystalline complexes (SMHBCs) were prepared through double H-bond interactions between either isophthalic acid (A) or terephthalic acid (B) and a nicotinate Schiff base (I6). The formed complexes were thermally investigated by differential scanning calorimetry (DSC), and their [...] Read more.
Two new 1:2 supramolecular H-bonded liquid crystalline complexes (SMHBCs) were prepared through double H-bond interactions between either isophthalic acid (A) or terephthalic acid (B) and a nicotinate Schiff base (I6). The formed complexes were thermally investigated by differential scanning calorimetry (DSC), and their phases were identified by polarized optical microscopy (POM) and UV-spectroscopy. The formation of 1:2 hydrogen-bonded complexes was confirmed through their Fermi-bands observed by FT-IR spectroscopy. The first system (I6/A) was found to possess enantiotropic smectic A and nematic mesophases. Induced broad range of smectic A phase was observed in I6/A complex which is not shown by their individual components. The second complex system (I6/B) was purely nematogenic. Density functional theory (DFT) calculations were applied to predict their geometrical parameters. Theoretical studies revealed that the isophthalic complex adopted the W-shape; while the U-shape was adopted by the terephthalic acid complex (I6/B). The linear geometry of the complex based on the isophthalic acid enhanced the mesomorphic behavior observed by the terephthalic complex; I6/B. Actually; the orientation of the two carboxylic groups was shown to highly affect the softness of the derived complex. On the other hand; the mapping orientation of the charge distribution can be used to explain the mesophase behavior. The photophysical characterization of isophthalic complex (I6/A) is also discussed Full article
(This article belongs to the Special Issue Photosensitive Liquid Crystals)
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