Research

14 pages, 3325 KiB

Open AccessEditor’s ChoiceArticle

Negative Thermal Quenching of Photoluminescence from Liquid-Crystalline Molecules in Condensed Phases

by Hussain Sami, Osama Younis, Yui Maruoka, Kenta Yamaguchi, Kumar Siddhant, Kyohei Hisano and Osamu Tsutsumi

Crystals 2021, 11(12), 1555; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst11121555 - 13 Dec 2021

Cited by 6 | Viewed by 3118

Abstract

The luminescence of materials in condensed phases is affected by not only their molecular structures but also their aggregated structures. In this study, we designed new liquid-crystalline luminescent materials based on biphenylacetylene with a bulky trimethylsilyl terminal group and a flexible alkoxy chain. [...] Read more.

The luminescence of materials in condensed phases is affected by not only their molecular structures but also their aggregated structures. In this study, we designed new liquid-crystalline luminescent materials based on biphenylacetylene with a bulky trimethylsilyl terminal group and a flexible alkoxy chain. The luminescence properties of the prepared materials were evaluated, with a particular focus on the effects of phase transitions, which cause changes in the aggregated structures. The length of the flexible chain had no effect on the luminescence in solution. However, in crystals, the luminescence spectral shape depended on the chain length because varying the chain length altered the crystal structure. Interestingly, negative thermal quenching of the luminescence from these materials was observed in condensed phases, with the isotropic phase obtained at high temperatures exhibiting a considerable increase in luminescence intensity. This thermal enhancement of the luminescence suggests that the less- or nonemissive aggregates formed in crystals are dissociated in the isotropic phase. These findings can contribute toward the development of new material design concepts for useful luminescent materials at high temperatures. Full article

(This article belongs to the Special Issue Synthesis and Properties of Light-Emitting Liquid Crystals (Volume II))

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15 pages, 7375 KiB

Open AccessEditor’s ChoiceArticle

Mesomorphic and Thermal Behavior of Symmetric Bent-Core Liquid Crystal Compounds Derived from Resorcinol and Isophthalic Acid

by Catalina Ionica Ciobanu, Iulian Berladean, Elena-Luiza Epure, Aurel Simion, Gabriela Lisa, Yahia Boussoualem and Irina Carlescu

Crystals 2021, 11(10), 1215; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst11101215 - 08 Oct 2021

Cited by 3 | Viewed by 1841

Abstract

Bent-core liquid crystals (BCLC) have been widely studied as a result of their unusual polar and chiral properties. Similar to calamitic and discotic molecules, BCLC molecules also exhibit nematic phases, besides other higher order mesophases. The aim of this work is to comparatively [...] Read more.

Bent-core liquid crystals (BCLC) have been widely studied as a result of their unusual polar and chiral properties. Similar to calamitic and discotic molecules, BCLC molecules also exhibit nematic phases, besides other higher order mesophases. The aim of this work is to comparatively analyze the mesomorphic behavior of some bent-core 1,3-disubstituted benzene core compounds derived from resorcinol and isophthalic acid. Thus, the two classes of compounds differ in the nature of the orientation of the ester bond between the benzene central core and the two branches attached to the core. The mesomorphic behavior was elucidated by polarized light optical microscopy and differential calorimetry. Given the relatively high isotropic points of the compounds, confirmation of the thermal stability in the domains manifesting liquid crystalline properties was performed by thermogravimetric studies. The theoretical explanation of the difference in mesomorphic behavior for the two classes was based on molecular modeling studies. Full article

(This article belongs to the Special Issue Synthesis and Properties of Light-Emitting Liquid Crystals (Volume II))

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13 pages, 2827 KiB

Open AccessEditor’s ChoiceArticle

Remarkable Increase of Fluorescence Quantum Efficiency by Cyano Substitution on an ESIPT Molecule 2-(2-Hydroxyphenyl)benzothiazole: A Highly Photoluminescent Liquid Crystal Dopant

by Tsuneaki Sakurai, Masaya Kobayashi, Hiroyuki Yoshida and Masaki Shimizu

Crystals 2021, 11(9), 1105; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst11091105 - 10 Sep 2021

Cited by 10 | Viewed by 3568

Abstract

Fluorescent molecules with excited-state intramolecular proton transfer (ESIPT) character allow the efficient solid-state luminescence with large Stokes shift that is important for various applications, such as organic electronics, photonics, and bio-imaging fields. However, the lower fluorescence quantum yields (Φ_FL) in [...] Read more.

Fluorescent molecules with excited-state intramolecular proton transfer (ESIPT) character allow the efficient solid-state luminescence with large Stokes shift that is important for various applications, such as organic electronics, photonics, and bio-imaging fields. However, the lower fluorescence quantum yields (Φ_FL) in the solution or viscous media, due to their structural relaxations in the excited state to reach the S₀/S₁ conical intersection, shackle further applications of ESIPT-active luminophores. Here we report that the introduction of a cyano group (-CN) into the phenyl group of 2-(2-hydroxyphenyl)benzothiazole (HBT), a representative ESIPT compound, remarkably increase its fluorescence quantum yield (Φ_FL) from 0.01 (without -CN) to 0.49 (with -CN) in CH₂Cl₂, without disturbing its high Φ_FL (=0.52) in the solid state. The large increase of the solution-state Φ_FL of the cyano-substituted HBT (CN-HBT) is remarkable, comparing with our previously reported Φ_FL values of 0.05 (with 4-pentylphenyl), 0.07 (with 1-hexynyl), and 0.15 (with 4-pentylphenylethynyl). Of interest, the newly-synthesized compound, CN-HBT, is miscible in a conventional room-temperature nematic liquid crystal (LC), 4-pentyl-4′-cyano biphenyl (5CB), up to 1 wt% (~1 mol%), and exhibits a large Φ_FL of 0.57 in the viscous LC medium. A similar Φ_FL value of Φ_FL = 0.53 was also recorded in another room-temperature LC, trans-4-(4-pentylcyclohexyl)benzonitrile (PCH5), with a doping ratio of 0.5 wt% (~0.5 mol%). These 5CB/CN-HBT and PCH5/CN-HBT mixtures serve as light-emitting room-temperature LCs, and show anisotropic fluorescence with the dichroic ratio of 3.1 upon polarized excitation, as well as electric field response of luminescence intensity changes. Full article

(This article belongs to the Special Issue Synthesis and Properties of Light-Emitting Liquid Crystals (Volume II))

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9 pages, 4025 KiB

Open AccessArticle

Blue-Shifting Mechanofluorochromic Luminescent Behavior of Polymer Composite Films Using Gelable Mechanoresponsive Compound

by Mizuho Kondo, Yuya Morita and Nobuhiro Kawatsuki

Crystals 2021, 11(8), 950; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst11080950 - 15 Aug 2021

Cited by 2 | Viewed by 1818

Abstract

Mechanochromic luminescent dyes change their luminescence color upon exposure to external mechanical stimuli. In this study, we synthesized a liquid crystalline mechanochromic luminescent dye containing a terminal cholesterol molecule. The dissolution of the dye in 1,4-dioxane resulted in the formation of a gel. [...] Read more.

Mechanochromic luminescent dyes change their luminescence color upon exposure to external mechanical stimuli. In this study, we synthesized a liquid crystalline mechanochromic luminescent dye containing a terminal cholesterol molecule. The dissolution of the dye in 1,4-dioxane resulted in the formation of a gel. The luminescence of the xerogel obtained from the dioxane solution changed from green to blue upon grinding, indicating mechanochromic luminescence behavior. The anisotropic patterning of short-wavelength-shifted luminescence color change by directional handwriting on surface layer of liquid crystal was successfully demonstrated. Furthermore, blue-shifting mechanoresponsive polymer composite surface was fabricated by using the luminophore. Full article

(This article belongs to the Special Issue Synthesis and Properties of Light-Emitting Liquid Crystals (Volume II))

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15 pages, 19513 KiB

Open AccessArticle

Effect of Fluoroalkyl-Substituent in Bistolane-Based Photoluminescent Liquid Crystals on Their Physical Behavior

by Shigeyuki Yamada, Yizhou Wang, Masato Morita, Qingzhi Zhang, David O’Hagan, Masakazu Nagata, Tomohiro Agou, Hiroki Fukumoto, Toshio Kubota, Mitsuo Hara and Tsutomu Konno

Crystals 2021, 11(4), 450; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst11040450 - 20 Apr 2021

Cited by 4 | Viewed by 2595

Abstract

Photoluminescent liquid crystals (PLLCs) have attracted significant attention owing to their broad applicability in thermosensing and PL switching. Extensive efforts have been made to develop bistolane-based PLLCs containing flexible units at both molecular terminals, and it has been revealed that their PL behavior [...] Read more.

Photoluminescent liquid crystals (PLLCs) have attracted significant attention owing to their broad applicability in thermosensing and PL switching. Extensive efforts have been made to develop bistolane-based PLLCs containing flexible units at both molecular terminals, and it has been revealed that their PL behavior can switch with the phase transition between the crystalline and LC phases. Although slight modulation of the flexible unit structure dramatically alters the LC and PL behaviors, few studies into the modification of the flexible units have been conducted. With the aim of achieving dynamic changes in their physical behaviors, we developed a family of bistolane derivatives containing a simple alkyl or a fluoroalkyl flexible chain and carried out a detailed systematic evaluation of their physical behaviors. Bistolanes containing a simple alkyl chain showed a nematic LC phase, whereas switching the flexible chain in the bistolane to a fluoroalkyl moiety significantly altered the LC phase to generate a smectic phase. The fluoroalkyl-containing bistolanes displayed a stronger deep blue PL than their corresponding non-fluorinated counterparts, even in the crystalline phase, which was attributed to the construction of rigid molecular aggregates through intermolecular F···H and F···F interactions to suppress non-radiative deactivation. Full article

(This article belongs to the Special Issue Synthesis and Properties of Light-Emitting Liquid Crystals (Volume II))

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11 pages, 1614 KiB

Open AccessArticle

Electrorheological Effect of Gold Nanoparticles Coated with Fluorescent Mesogenic Groups Dispersed in Nematic Liquid Crystal

by Kosuke Kaneko, Kosuke Yamashita, Daiki Fujioka, Kimiyoshi Kaneko, Kiyomi Fuchigami, Takeshi Hashishin and Tomonori Hanasaki

Crystals 2021, 11(2), 192; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst11020192 - 16 Feb 2021

Viewed by 1971

Abstract

The electrorheological (ER) properties of composite materials consisting of a nematic liquid crystal (LC) and gold nanoparticles (GNPs) coated with bistolane-based mesogenic groups were studied. The GNPs were coated by normal alkyl chains and the fluorescent LC compounds, of which the molecular structure [...] Read more.

The electrorheological (ER) properties of composite materials consisting of a nematic liquid crystal (LC) and gold nanoparticles (GNPs) coated with bistolane-based mesogenic groups were studied. The GNPs were coated by normal alkyl chains and the fluorescent LC compounds, of which the molecular structure was similar to that of the LC matrix. The dispersity of the GNPs in the nematic LC was investigated by polarizing optical microscopy (POM). In order to improve the ER effect of the composite, a simple strategy was investigated from the viewpoint of a material design in surface-modified GNPs by lateral substitution of the mesogenic groups. The presence of the GNPs in the nematic LC led to a slightly enhanced ER effect compared to that observed for only the nematic LC. This study demonstrates the potential of a hybrid system consisting of LCs and GNPs to yield a larger ER effect. Full article

(This article belongs to the Special Issue Synthesis and Properties of Light-Emitting Liquid Crystals (Volume II))

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15 pages, 4333 KiB

Open AccessArticle

Synthesis and Characterization of Photoluminescence Liquid Crystals Based on Flexible Chain-Bearing Pentafluorinated Bistolanes

by Shigeyuki Yamada, Masaya Sato and Tsutomu Konno

Crystals 2020, 10(7), 603; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst10070603 - 11 Jul 2020

Cited by 1 | Viewed by 2936

Abstract

The liquid-crystalline (LC) and photophysical properties of molecules are very sensitive to their electronic and molecular aggregate structures. Herein, to shed light on the structure–property relationships of pentafluorinated bistolane-based photoluminescence (PL) liquid crystals (PLLCs) previously reported by our group, we synthesized pentafluorinated bistolanes [...] Read more.

The liquid-crystalline (LC) and photophysical properties of molecules are very sensitive to their electronic and molecular aggregate structures. Herein, to shed light on the structure–property relationships of pentafluorinated bistolane-based photoluminescence (PL) liquid crystals (PLLCs) previously reported by our group, we synthesized pentafluorinated bistolanes with variable flexible chains and evaluated their LC and photophysical properties. The incorporation of an oxygen atom (to afford a 2-methoxyethoxy unit) or an oxygen atom and a methyl group (to afford a 1-methoxyprop-2-oxy unit) into the flexible butoxy chain significantly decreased the temperature of the crystalline-to-LC phase transition, and a chiral nematic phase comprising helical molecular aggregates was observed for the chiral 1-methoxyprop-2-oxy group–bearing bistolane. The synthesized bistolanes exhibited strong blue PL in both solution and crystalline phases; the featuring PL characteristics were maintained in the LC phase (produced by the crystalline-to-LC phase transition) except for a slight PL color change. Thus, it was concluded that the PL behavior of pentafluorinated bistolanes can be modulated by the choice of a suitable flexible chain, and the obtained insights are believed to facilitate the application of PLLCs in thermosensing PL materials. Full article

(This article belongs to the Special Issue Synthesis and Properties of Light-Emitting Liquid Crystals (Volume II))

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13 pages, 1935 KiB

Open AccessArticle

Stimuli-Sensitive Aggregation-Induced Emission of Organogelators Containing Mesogenic Au(I) Complexes

by Supattra Panthai, Ryota Fukuhara, Kyohei Hisano and Osamu Tsutsumi

Crystals 2020, 10(5), 388; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst10050388 - 09 May 2020

Cited by 4 | Viewed by 3181

Abstract

As the luminescence from conventional organic luminophores is typically quenched in constrained environments, the aggregation-induced emission (AIE) phenomenon is of interest for the development of materials that exhibit strong luminescence in condensed phases. Herein, new bismesogenic Au complexes were developed as organogelators and [...] Read more.

As the luminescence from conventional organic luminophores is typically quenched in constrained environments, the aggregation-induced emission (AIE) phenomenon is of interest for the development of materials that exhibit strong luminescence in condensed phases. Herein, new bismesogenic Au complexes were developed as organogelators and their photophysical properties, including their AIE characteristics, were investigated in organogels and crystals. The crystals of the gold complexes exhibited room-temperature phosphorescence with relatively high quantum yields. Moreover, the gold complexes also showed photoluminescence in the organogels and we demonstrated that the reversible switching of the luminescence intensity was induced by the sol-gel phase transition. The intense photoluminescence in the crystal and gel was induced by the restricted internal motion of the luminophore in the molecular aggregates. However, in the sol, the network structure of the organogel was destroyed and the nonradiative deactivation of the excited states was enhanced. As a result, we can conclude that the switching of the luminescence intensity was induced by changes in the aggregated structures of the molecules. The developed Au-complex-based gelators are excellent candidates for the realization of stimuli-responsive soft and smart luminescent materials. Full article

(This article belongs to the Special Issue Synthesis and Properties of Light-Emitting Liquid Crystals (Volume II))

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