Dominant Factors Affecting Rheological Properties of Cellulose Derivatives Forming Thermotropic Cholesteric Liquid Crystals with Visible Reflection.

Hydroxypropyl cellulose (HPC) derivatives with alkanoyl side chains are known to form thermotropic cholesteric liquid crystals (CLCs) with visible reflection. Although the widely investigated CLCs are requisite for tedious syntheses of chiral and mesogenic compounds from precious petroleum resources, the HPC derivatives easily prepared from biomass resources would contribute to the realization of environment-friendly CLC devices. In this study, we report the linear rheological behavior of thermotropic CLCs of HPC derivatives possessing alkanoyl side chains of different lengths. In addition, the HPC derivatives have been synthesized by the complete esterification of hydroxy groups in HPC. The master curves of these HPC derivatives were almost identical at reference temperatures, with their light reflection at 405 nm. The relaxation peaks appeared at an angular frequency of ~102 rad/s, suggesting the motion of the CLC helical axis. Moreover, the dominant factors affecting the rheological properties of HPC derivatives were strongly dependent on the CLC helical structures. Further, this study provides one of the most promising fabrication strategies for the highly oriented CLC helix by shearing force, which is indispensable to the development of advanced photonic devices with eco-friendliness.

Chiral intertwined spirals and magnetic transition dipole moments dictated by cylinder helicity.

The presence of anomalous chirality in a roll of graphitic carbon sheets has been recognized since the discovery of carbon nanotubes, which are becoming available in higher quantities through the isolation of chiral single-wall congeners with high purity. Exploration of the properties arising from cylinder chirality is expected to expand the scope of tubular entities in the future. By studying molecular fragments of helical carbon nanotubes, we herein reveal interesting properties that arise from this chirality. The chirality of nanoscale cylinders resulted in chirality of larger dimensions in the form of a double-helix assembly. Cylinder chirality in solution gave rise to a large dissymmetry factor of metal-free entities in circular polarized luminescence. Theoretical investigations revealed the pivotal role of cylindrical shapes in enhancing magnetic dipole transition moments to yield extreme rotatory strength. Unique effects of cylinder chirality in this study may prompt the development of tubular entities, for instance, toward chiroptical applications.

Enantioselective synthesis and enhanced circularly polarized luminescence of S-shaped double azahelicenes.

The enantioselective synthesis of azahelicenes and S-shaped double azahelicenes has been achieved via the Au-catalyzed sequential intramolecular hydroarylation of alkynes. The use of excess AgOTf toward a Au(I) complex is crucial for this transformation. Interestingly, the circularly polarized luminescence activity of the S-shaped double azahelicenes was significantly higher than that of the azahelicenes.

Aligned 1-D nanorods of a π-gelator exhibit molecular orientation and excitation energy transport different from entangled fiber networks.

Linear π-gelators self-assemble into entangled fibers in which the molecules are arranged perpendicular to the fiber long axis. However, orientation of gelator molecules in a direction parallel to the long axes of the one-dimensional (1-D) structures remains challenging. Herein we demonstrate that, at the air-water interface, an oligo(p-phenylenevinylene)-derived π-gelator forms aligned nanorods of 340 ± 120 nm length and 34 ± 5 nm width, in which the gelator molecules are reoriented parallel to the long axis of the rods. The orientation change of the molecules results in distinct excited-state properties upon local photoexcitation, as evidenced by near-field scanning optical microscopy. A detailed understanding of the mechanism by which excitation energy migrates through these 1-D molecular assemblies might help in the design of supramolecular structures with improved charge-transport properties.

Cholesteric Liquid Crystals with Thermally Stable Reflection Color from Mixtures of Completely Etherified Ethyl Cellulose Derivative and Methacrylic Acid.

Cellulose derivatives have attracted attention as environmentally friendly materials that can exhibit a cholesteric liquid crystal (CLC) phase with visible light reflection. Previous reports have shown that the chemical structures and the degrees of substitution of cellulose derivatives have significant influence on their reflection properties. Although many studies have been reported on CLC using ethyl cellulose (EC) derivatives in which the hydroxy groups are esterified, there have been no studies on EC derivatives with etherified side chains. In this article, we optimized the Williamson ether synthesis to introduce pentyl ether groups in the EC side chain. The degree of substitution with pentyl ether group (DSPe), confirmed via 1H-NMR spectroscopic measurements, was controlled using the solvent and the base concentration in this synthesis. All the etherified EC derivatives were soluble in methacrylic acid (MAA), allowing for the preparation of lyotropic CLCs with visible reflection. Although the reflection peak of lyotropic CLCs generally varies with temperature, the reflection peak of lyotropic CLCs of completely etherified EC derivatives with MAA could almost be preserved in the temperature range from 30 to 110 °C even without the aid of any crosslinking. Such thermal stability of the reflection peak of CLCs may be greatly advantageous for fabricating new photonic devices with eco-friendliness.

Rhodium-catalyzed enantioselective synthesis, crystal structures, and photophysical properties of helically chiral 1,1'-bitriphenylenes.

The highly enantioselective synthesis of helically chiral 1,1'-bitriphenylenes has been achieved via rhodium-catalyzed double [2 + 2 + 2] cycloaddition of biaryl-linked tetraynes with 1,4-diynes (up to 93% ee). Crystal structures and photophysical properties of these helically chiral 1,1'-bitriphenylenes have also been studied.

Thermally assisted photonic inversion of supramolecular handedness.

Spiraling into control: A photoresponsive supramolecular assembly demonstrates that light, along with heating (Δ) and cooling (), can cause chiral communication between molecules. This effect leads to bias in the helicity of the complex, causing a reversible switching of macroscopic handedness, as shown by a reversal of sign of the circularly polarized luminescence (CPL) that is emitted.

Rheological Properties of Cholesteric Liquid Crystal with Visible Reflection from an Etherified Hydroxypropyl Cellulose Derivative.

Optical properties of hydroxypropyl cellulose (HPC) derivatives have been widely investigated for their ability to exhibit cholesteric liquid crystal (CLC) phase. However, there are only a limited number of studies on their rheological properties even though they are quite important for the applications of such HPC derivatives to the versatile CLC photonic devices. In this article, we report on the optical and rheological properties of an HPC derivative possessing pentyl ether groups in the side-chains. The etherified HPC derivative exhibited thermotropic CLC phase with light reflection in the temperature range between 25 °C and 120 °C. After the HPC derivative was heated once at isotropic phase, followed by being cooled to the CLC phase, the reflection peak could not be observed, even at the CLC phase. At this stage, the HPC derivative exhibited solid-like rheological responses compared to that of sheared at a constant shear rate of 1.0 s-1. Such differences in the optical and rheological properties of the HPC derivative can be ascribed to the difference in CLC orientation state. From the rheological results, the etherified HPC derivative showed liquid-like behavior rather than the esterified HPC derivatives. This evidence provides a promising clue for fabricating high-quality CLC devices by the facile CLC orientation.

Size-Controllable Synthesis of Monodisperse Magnetite Microparticles Leading to Magnetically Tunable Colloidal Crystals.

Colloidal crystals (CCs) are periodic arrays of monodisperse microparticles. Such CCs are very attractive as they can be potentially applicable as versatile photonic devices such as reflective displays, sensors, lasers, and so forth. In this article, we describe a promising methodology for synthesizing monodisperse magnetite microparticles whose diameters are controllable in the range of 100-200 nm only by adjusting the base concentration of the reaction solution. Moreover, monodisperse magnetite microparticles in aqueous suspensions spontaneously form the CC structures under an external magnetic field, leading to the appearance of Bragg reflection colors. The reflection peak can be blue-shifted from 730 nm to 570 nm by the increase in the external magnetic field from 28 mT to 220 mT. Moreover, the reflection properties of CCs in suspension depend on the microparticle concentration in suspension and the diameter of the magnetite microparticles. Both fine-control of microparticle diameter and investigation of magneto-optical properties of CCs would contribute to the technological developments in full-color reflective displays and sensors by utilizing these monodisperse magnetite microparticles.

Room-Temperature Cholesteric Liquid Crystals of Cellulose Derivatives with Visible Reflection.

Hydroxypropyl cellulose (HPC) derivatives with alkanoyl side chains have attracted attention as bio-based cholesteric liquid crystal (CLC) materials with reflection colors. By taking advantage of the ability to change the reflection color in response to external stimuli, the thermotropic CLCs can be applied to a wide variety of photonic devices for a sustainable society of future generations. However, the thermotropic CLCs of HPC derivatives substituted with only one kind of alkanoyl group are not suitable for such applications because they do not exhibit visible reflection at room temperature. In this report, we describe a promising strategy to control the reflection colors of HPC derivatives at room temperature by introducing two kinds of alkanoyl groups with different lengths into the side chains of HPCs, which also enables the fine control of temperature dependence on the reflection wavelength. By chemically optimizing the side chain, we successfully prepared room-temperature thermotropic CLCs of HPC derivatives with visible reflection. This report would contribute toward the development of versatile photonic applications by CLCs produced from biomass.

Colloidal Crystal Films with Narrow Reflection Bands by Hot-Pressing of Polymer-Grafted Silica Particles.

Previous reports have shown that colloidal crystal (CC) films with visible Bragg reflection characteristics can be fabricated by the surface modification of monodisperse silica particles (SiPs) with poly(methyl methacrylate) (PMMA) chains, followed by hot-pressing at 150 °C. However, the reflection bands of the CC films were very broad due to their relative disordering of SiPs. In this report, we attempted to fabricate the CC films using SiPs surface-modified with poly(n-octyl acrylate) (POA) chains by hot-pressing. When the cast films of POA-grafted SiPs were prepared by hot-pressing at 100 °C, the reflection bands were narrow rather than those of CC films of PMMA-grafted SiPs. This can be ascribed to easy disentanglement of POA chains during the hot-pressing process, thereby enabling the formation of well-ordered CC structures. Moreover, the reflection colors of CC films could be easily tuned by controlling the molecular weight of POA chains grafted on the SiP surface.

Highly sensitive photoalignment of calamitic and discotic liquid crystals assisted by axis-selective triplet energy transfer.

Highly sensitive photoalignment of liquid crystals (LCs) can be realized by axis-selective triplet energy transfer. Addition of a triplet photosensitizer (phosphorescent donor) into a photocrosslinkable polymer tethering E-cinnamate side chains ensures dramatic enhancement of photosensitivity to generate the optical anisotropy of polymer film and surface-assisted LC photoalignment. Photoirradiation of triplet photosensitizer-doped polymer films with linearly polarized 365 nm light for the selective excitation of triplet sensitizer gives rise to optical anisotropy of cinnamates as a result of axis-selective triplet energy transfer. By analyzing phosphorescence spectra with theoretical Perrin's formula, we find that triplet energy transfer is efficient within a radius of ∼0.3 nm from the triplet photosensitizer. Such photoaligned polymer films can be used for the surface-assisted orientation photocontrol of not only calamitic LC, but also discotic LC, even for extremely low exposure energies. The present procedure would be greatly advantageous for high-throughput fabrication of optical devices by photoalignment techniques.

Colloidal Photonic Crystals of Reusable Hydrogel Microparticles for Sensor and Laser Applications.

Although a wide variety of techniques have been developed to date for the fabrication of high-quality colloidal photonic crystals (CPCs) using monodisperse silica and polystyrene microparticles, poly(N-isopropylacrylamide) (PNIPA) hydrogel microparticles have rarely been utilized for the preparation of active CPCs despite the intriguing feature of temperature-responsive volume changes. This report describes the promising potential abilities of PNIPA hydrogel microparticles for sensor and laser applications. Monodisperse PNIPA hydrogel microparticles were synthesized by emulsion polymerization, and the microparticle diameter was finely controlled by adjusting the surfactant concentration. Such hydrogel microparticles spontaneously formed uniform CPCs with visible Bragg reflection even in fluid suspensions. The addition of small amounts of ionic substances into the centrifuged and deionized CPC suspensions enabled the on-demand color switching between Bragg reflection and white turbidity with temperature, leading to temperature- and ion-sensing applications. Moreover, our expanding experiments successfully demonstrated the optically excited laser action with a single and narrow peak from CPC suspensions with light-emitting dyes by the photonic band gap effect. After the light-emitting dyes were simply removed from the CPC suspensions by centrifugation, the purified PNIPA hydrogel microparticles were permanently reusable as the CPC laser microcavities to generate the laser action at other wavelengths using different dyes. This study contributes the circular economy concept using reusable hydrogel microparticles for the realization of a sustainable society.

Recent progress in chiral photonic band-gap liquid crystals for laser applications.

This article describes a brief review of recent research advances in chiral liquid crystals (CLCs) for laser applications. The CLC molecules have an intrinsic capability to spontaneously organize supramolecular helical assemblages consisting of liquid crystalline layers through their helical twisting power. Such CLC supramolecular helical structures can be regarded as one-dimensional photonic crystals (PhCs). Owing to their supramolecular helical structures, the CLCs show negative birefringence along the helical axis. Selective reflection of circularly polarized light is the most unique and important optical property in order to generate internal distributed feedback effect for optically-excited laser emission. When a fluorescent dye is embedded in the CLC medium, optical excitation gives rise to stimulated laser emission peak(s) at the band edge(s) and/or within the CLC selective reflection. Furthermore, the optically-excited laser emission peaks can be controlled by external stimuli through the self-organization of CLC molecules. This review introduces the research background of CLCs carried out on the PhC realm, and highlights intriguing precedents of various CLC materials for laser applications. It would be greatly advantageous to fabricate active CLC laser devices by controlling the supramolecular helical structures. Taking account of the peculiar features, we can envisage that a wide variety of supramolecular helical structures of CLC materials will play leading roles in next-generation optoelectronic molecular devices.

Aqueous dispersions of carbon nanotubes stabilized by zirconium acetate.

This report describes a simple strategy for preparing dispersion of multi-walled carbon nanotubes (MWCNTs) in water. Zirconium acetate (ZrAc3) was used as a dispersant for the MWCNTs in water. Interestingly, an aqueous dispersion of MWCNTs was stabilized by adding an extremely small amount of ZrAc3, followed by ultrasonic agitation. The resultant MWCNT dispersion was durably retained even after storing at room temperature over 6 months. The dispersion state of MWCNTs on a substrate was observed by means of reflection optical and scanning electron microscopes. Furthermore, we demonstrated the effect of external high magnetic field on the orientation of MWCNTs on the substrate surface from the aqueous dispersion.

Side Chain Effect of Hydroxypropyl Cellulose Derivatives on Reflection Properties.

Some cellulose derivatives are known to exhibit thermotropic and lyotropic cholesteric liquid crystal (CLC) phases with a visible reflection feature by changing the side chains and mixing with specific solvents, respectively. Although many studies have been reported so far, most of the derivatives have the side chains of linear alkyl groups, but not the bulky phenyl groups. In this report, we synthesized a series of hydroxypropyl cellulose (HPC) derivatives that possessed both linear propionyl esters and bulky (trifluoromethyl)phenyl carbamates in the side chains. The reflection peaks of HPC derivatives shifted to longer wavelengths upon heating due to an increase in the CLC helical pitch. Such thermally induced shifting behavior of the reflection peak was crucially dependent on not only the propionyl esterification degree, but also the substituents in the side chains of HPC derivatives. When the side chains of HPC were chemically modified with both propionyl esters and bulky substituents such as 3,5-bis(trifluoromethyl)phenyl carbamates, the reflection peaks emerged at longer wavelengths at the same temperature. This probably happened because of the steric hindrance of bulky side chains, as supported by the empirical molecular modeling calculation. Although the occupied volumes of (trifluoromethyl)phenyl groups were independent of the CLC phase temperature with visible Bragg reflection, the substituent position, i.e., substituent orientation of trifluoromethyl groups affected the CLC phase temperature. Moreover, we found that the hydrogen bonds between carbamate moieties in the HPC side chains play an important role in the thermally induced shift of reflection peaks.

Effective Mn-Doping in AgInS2/ZnS Core/Shell Nanocrystals for Dual Photoluminescent Peaks.

We developed the effective Mn-doping procedure for AgInS2(AIS)/ZnS core/shell nanocrystals (NCs) to exhibit dual photoluminescence (PL) peaks. Although the AIS/ZnS core/shell NCs showed solely a single PL peak at ~530 nm, incorporation of a small amount of Mn as a dopant within the AIS/ZnS NCs resulted in the simultaneous emergence of dual PL peaks at ~500 nm (green PL) arising from AIS/ZnS NCs and ~600 nm (orange PL) from the Mn dopants. Furthermore, we succeeded in significantly increasing the absolute PL quantum yield value of dual emissive AIS/ZnS NCs incorporated with Mn dopants from 10% to 34% after surface passivation with another ZnS shell for the formation of core/shell/shell structures.

Surface-assisted photoalignment of discotic liquid crystals by nonpolarized light irradiation of photo-cross-linkable polymer thin films.

In this article, we describe the surface-assisted photoalignment of discotic liquid crystals (DLCs) on thin films of photo-cross-linkable polymers with cinnamoyl moieties as the side chains. Oblique irradiation of the polymer thin films with nonpolarized UV light at 313 nm brought about inclined orientation of the cinnamoyl residues as a result of their direction-selective photoisomerization and photodimerization. The DLC molecules on the photoirradiated polymer films were aligned in a tilted hybrid manner. This means that the DLC directors are continuously altered from the substrate to the DLC film surface so as to minimize the elastic free energy. Interestingly, we found that the tilted direction of aligned DLC molecules is clearly influenced by the chemical structures of the cinnamate-containing polymers. When a poly(vinyl cinnamate) thin film was obliquely exposed to nonpolarized UV light, the DLCs were inclined to the direction opposite to the UV light propagation. In a keen contrast, the thin film of poly(methacrylate)s tethering cinnamoyl groups, which was obliquely exposed to nonpolarized UV light in advance, provided the tilting DLC direction in parallel with the light propagation. The results were supported by tilted orientation of calamitic (rod-shaped) liquid crystal on the obliquely irradiated polymer films. Such photoalignment behavior of the DLCs can be rationalized by anchoring balance between intermolecular interaction of the DLC molecules with the photodimers of polymer films and those with the remaining E-isomers of cinnamoyl side chains at the film interface. The present technique of DLC photoalignment opens promising ways not only to understand anisotropic physical properties of DLCs, but also to design and fabricate novel nanodevices for photonics and electronics applications.

Circularly polarized laser emission induced by supramolecular chirality in cholesteric liquid crystals.

This paper describes the circularly polarized spectroscopic studies on absorption and emission of an achiral fluorescent dye embedded in cholesteric liquid crystals (CLCs). Optical excitation of the dye-doped CLC cell with a linearly polarized laser brought about the two laser emission peaks at longer and shorter reflection band edges of the CLC host through the internal laser feedback effect of the one-dimensional CLC photonic band-gap. At this stage, the optically excited laser emissions showed circularly polarized characteristic, even though the excitation beam was linearly polarized. The circularly polarized direction of the laser emission was determined by molecular chirality of only few mol% of the enantiomeric chiral dopant in this molecular system.

One-dimensional self-assembly of alkoxy-capped silicon nanoparticles.

We demonstrate here a novel method for self-assembling in dimensional alignment the alkoxy-capped silicon nanoparticles synthesized through a room-temperature chemical route. The alkoxy-capped silicon nanoparticles were prepared via a reduction of silicon tetrachloride with sodium-naphthalide and subsequent surface capsulation with 1-octanol monolayers. In the present method, a sublimation process, which was employed as a final purification process for removing the residual naphthalene, influenced significantly on the final morphology of the resultant nanoparticles. Scanning transmission electron microscope (STEM) confirmed the spherical nanoparticles on a holey carbon grid after sublimation process, while only the fibril-like morphology just before sublimation process. In the former sample, the resultant particle size was measured by STEM to be about 9.5 nm +/- 3.4 nm. On the other hand, in the latter sample, the fibril-like structures were shaped by self-assembled silicon nanoparticles in dimensional alignment. The diameters and lengths of the fibril-like assemblies were approximately measured to be 10 to 20 nm and over 5 microm, respectively.