Manipulation of Supramolecular Columnar Structures of H-Bonded Donor-Acceptor Units through Geometrical Nanoconfinement.

Ambipolar organic semiconductors are considered promising for organic electronics because of their interesting electric properties. Many hurdles remain yet to be overcome before they can be used for practical applications, especially because their orientation is hard to control. We demonstrate a method to control the orientation of columnar structures based on a hydrogen (H)-bonded donor-acceptor complex between a star-shaped tris(triazolyl)triazine and triphenylene-containing benzoic acid, using physicochemical nanoconfinement. The molecular configuration and supramolecular columnar assemblies in a one-dimensional porous anodic aluminium oxide (AAO) film were dramatically modulated by controlling the pore-size and by chemical modification of the inner surface of the porous AAO film. In situ experiments using grazing-incidence X-ray diffraction (GIXRD) were carried out to investigate the structural evolution produced at the nanometer scale by varying physicochemical conditions. The resulting highly ordered nanostructures may open a new pathway to effectively control the alignment of liquid crystal ambipolar semiconductors.

Electro-tunable liquid crystal laser based on high-Q Fabry-Pérot microcavity.

We demonstrate an electro-tunable laser based on a plano-concave (p-c) Fabry-Pérot (FP) microcavity filled with dye-doped liquid crystal (LC) gain material. Owing to the high Q-factor of the p-c FP microcavity, the lasing threshold of the LC laser is as low as 0.58 µJ/mm2. The single-mode emission wavelength can be easily tuned by varying applied voltage, which induces the reconfiguration of LC molecules. Furthermore, this lasing platform operates at room temperature, thanks to the wide temperature range of the nematic LC. We believe that our LC laser is widely applicable to light sources of various micro total analysis systems.

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.

Creation of liquid-crystal periodic zigzags by surface treatment and thermal annealing.

The orientation control of soft matter to create a large area single domain is one of the most exciting research topics in materials science. Recently, this effort has been extended to fabricate two- or three-dimensional structures for electro-optical applications. Here, we create periodic zigzag structures in liquid crystals (LCs) using a combination of surface treatment and thermal annealing. The LC molecules in the nematic (N) phase were initially guided by the alignment layer of rubbed polymers, which were quenched and subsequently annealed in the smectic A (SmA) phase to create periodic zigzag structures that represent modulated layer structures. Direct investigation of the zigzags was performed using microscopy and diffraction techniques, showing the alternately arranged focal conic domains (FCDs) formed. The resulting macroscopic periodic structures will be of interest in further studies of the physical properties of soft matters.

Nucleation and growth of a helical nanofilament (B4) liquid-crystal phase confined in nanobowls.

The B4 helical nanofilament (HNF) liquid crystal (LC) phase is a three-dimensional (3D) helical structure composed of 2D smectic layers. Because of the complex shape of the HNF phase, it is difficult to understand the generation mechanism of HNFs in the bulk as well as in the thin-film condition. Here, we directly investigated the nucleation and growth of HNFs in nanobowls. A combination of electron microscopy and X-ray diffraction was used to reveal the transitional surface structures, in which barrel-like structures as well as short HNFs with random handedness were observed, depending on the LC film thickness. These results will be useful in achieving a better understanding of thin film structures of complex chiral structures in soft matter.

Switchable Plasmonic Film Using Nanoconfined Liquid Crystals.

Structural coloration using plasmonic particles has received substantial attention due to its robust, permanent, and scalable characteristics across the full color range. In this study, a plasmonic structure based on a porous anodic aluminum oxide (AAO) film coated with a metallic film was fabricated. Colors were varied by changing the refractive index, which was achieved with a convolution with nanopores of AAO film and an infiltrated liquid crystal (LC) material. LC molecules confined in the porous AAO film were uniformly aligned, and they exhibited pore-size-dependent colors because of the specific refractive index. The thermal phase transition of the LC material in the nanopores changed the effective refractive index, switching the reflected colors, and the LC-infiltrated AAO remained stable over a month. We believe LC materials can extend the use of rigid conventional plasmonic structures from simple sensor applications to multifunctional uses such as color printing, writing pens, and displays.

Switchable Photonic Crystals Using One-Dimensional Confined Liquid Crystals for Photonic Device Application.

Photonic crystals (PCs) have recently attracted considerable attention, with much effort devoted to photonic bandgap (PBG) control for varying the reflected color. Here, fabrication of a modulated one-dimensional (1D) anodic aluminum oxide (AAO) PC with a periodic porous structure is reported. The PBG of the fabricated PC can be reversibly changed by switching the ultraviolet (UV) light on/off. The AAO nanopores contain a mixture of photoresponsive liquid crystals (LCs) with irradiation-activated cis/trans photoisomerizable azobenzene. The resultant mixture of LCs in the porous AAO film exhibits a reversible PBG, depending on the cis/trans configuration of azobenzene molecules. The PBG switching is reliable over many cycles, suggesting that the fabricated device can be used in optical and photonic applications such as light modulators, smart windows, and sensors.

Structural transitions and guest/host complexing of liquid crystal helical nanofilaments induced by nanoconfinement.

A lamellar liquid crystal (LC) phase of certain bent-core mesogenic molecules can be grown in a manner that generates a single chiral helical nanofilament in each of the cylindrical nanopores of an anodic aluminum oxide (AAO) membrane. By introducing guest molecules into the resulting composite chiral nanochannels, we explore the structures and functionality of the ordered guest/host LC complex, verifying the smectic-like positional order of the fluidic nematic LC phase, which is obtained by the combination of the LC organization and the nanoporous AAO superstructure. The guest nematic LC 4'-n-pentyl-4-cyanobiphenyl is found to form a distinctive fluid layered ordered LC complex at the nanofilament/guest interface with the host 1,3-phenylene bis[4-(4-nonyloxyphenyliminomethyl)benzoate], where this interface contacts the AAO cylinder wall. Filament growth form is strongly influenced by mixture parameters and pore dimensions.

Molecular Orientation of Liquid Crystals on Topographic Nanopatterns.

Controlling the orientation of building blocks in soft matter on the substrate has been a big challenge in material sciences. We have controlled the molecular orientation of liquid crystal (LC) materials on the porous anodic aluminum oxide (AAO) film having hexagonal pore arrays on the top surface. In our method, anchoring conditions can be varied by changing the pore size (Dp) and the porosity (P). As a proof-of-concept, the orientation of smectic A (SmA) structure at different anchoring conditions was successfully controlled in a sandwich cell consisting of AAO and a glass substrate, which has not been successfully controlled by conventional methods.

Cybotactic behavior in the de Vries smectic-A* liquid-crystal structure formed by a silicon-containing molecule.

We have identified a metastable liquid-crystal (LC) structure in the de Vries smectic-A* phase (de Vries Sm-A*) formed by silicon-containing molecules under certain boundary conditions. The phase transition with the metastable structure was observed in a LC droplet placed on a planar aligned substrate and LCs confined in the groove of a silicon microchannel. During the rapid cooling step, a batonnet structure was generated as an intermediate and metastable state prior to the transition that yielded the thermodynamically stable toric focal conic domains. This distinctive behavior was characterized using depolarized reflection light microscopy and grazing incidence x-ray diffraction techniques. We concluded that the silicon groups in the molecules that formed the de Vries phase induced the formation of layered clusters called cybotactic structures. This observation is relevant to an exploration of the physical properties of cybotactic de Vries phases and gives a hint as to their optoelectronic applications.