Shin-Woong Kang

Nature-inspired light-harvesting liquid crystalline porphyrins for organic photovoltaics

A new class of nanoscale light‐harvesting discotic liquid crystalline porphyrins, with the same basic structure of the best photoreceptor in nature (chlorophyll), was synthesized. These materials can be exceptionally aligned into a highly ordered architecture in which the columns formed by intermolecular π–π stacking are spontaneously perpendicular to the substrate. The homeotropic alignment, well confirmed by synchrotron X‐ray diffraction, could not only provide the most efficient pathway for hole conduction along the columnar axis crossing the device thickness, but also offer the largest area to the incident light for optimized light harvesting. Their preliminary photocurrent generation and photovoltaic performances were also demonstrated. The results provide new and efficient pathways to the development of organic photovoltaics by using homeotropically aligned liquid crystal thin films.

Shape-persistent V-shaped mesogens—formation of nematic phases with biaxial order

A homologous series of shape-persistent V-shaped molecules has been designed to form the biaxial nematic phase. Phenyleneethynylene moieties are attached to a bent fluorenone unit to create an apex angle of about 90°, which is determined from the single crystal structure. Two mesogens, one symmetric and another unsymmetric, have been synthesized by attaching a cyano group to one or both of the peripheral phenyl units, respectively. These groups introduce local dipoles essential for the formation of the nematic phases. The tendency to form a crystalline phase is reduced by laterally substituted hexyloxy chains which allow the nematic phase to be supercooled to a glassy state. Two of the three fluorenone derivatives exhibit a transition from the uniaxial nematic to the biaxial nematic phase. This transition has an undetectably small transition enthalpy, but the X-ray diffraction, polarizing optical microscopy, and conoscopy reveal the presence of the biaxial order in the low temperature nematic phase.

Condensation of Self-Assembled Lyotropic Chromonic Liquid Crystal Sunset Yellow in Aqueous Solutions Crowded with Polyethylene Glycol and Doped with Salt

We use optical and fluorescence microscopy, densitometry, cryo-transmission electron microscopy (cryo-TEM), spectroscopy, and synchrotron X-ray scattering to study the phase behavior of the reversible self-assembled chromonic aggregates of an anionic dye Sunset Yellow (SSY) in aqueous solutions crowded with an electrically neutral polymer polyethylene glycol (PEG) and doped with the salt NaCl. PEG causes the isotropic SSY solutions to condense into a liquid-crystalline region with a high concentration of SSY aggregates, coexisting with a PEG-rich isotropic (I) region. PEG added to the homogeneous nematic (N) phase causes separation into the coexisting N and I domains; the SSY concentration in the N domains is higher than the original concentration of PEG-free N phase. Finally, addition of PEG to the highly concentrated homogeneous N phase causes separation into the coexisting columnar hexagonal (C) phase and I phase. This behavior can be qualitatively explained by the depletion (excluded volume) effects that act at two different levels: at the level of aggregate assembly from monomers and short aggregates and at the level of interaggregate packing. We also show a strong effect of a monovalent salt NaCl on phase diagrams that is different for high and low concentrations of SSY. Upon the addition of salt, dilute I solutions of SSY show appearance of the condensed N domains, but the highly concentrated C phase transforms into a coexisting I and N domains. We suggest that the salt-induced screening of electric charges at the surface of chromonic aggregates leads to two different effects: (a) increase of the scission energy and the contour length of aggregates and (b) decrease of the persistence length of SSY aggregates.

A controllable viewing angle LCD with an optically isotropic liquid crystal

An optically isotropic liquid crystal (LC) such as a blue phase LC or an optically isotropic nano-structured LC exhibits a very wide viewing angle because the induced birefringence is along the in-plane electric field. Utilizing such a material, we propose a liquid crystal display (LCD) whose viewing angle can be switched from wide view to narrow view using only one panel. In the device, each pixel is divided into two parts: a major pixel and a sub-pixel. The main pixels display the images while the sub-pixels control the viewing angle. In the main pixels, birefringence is induced by horizontal electric fields through inter-digital electrodes leading to a wide viewing angle, while in the sub-pixels, birefringence is induced by the vertical electric field so that phase retardation occurs only at oblique angles. As a result, the dark state (or contrast ratio) of the entire pixel can be controlled by the voltage of the sub-pixels. Such a switchable viewing angle LCD is attractive for protecting personal privacy.

Self-assembling discotic liquid crystal porphyrin into more controllable ordered nanostructure mediated by fluorophobic effect

The novel nanoscale discotic liquid crystal porphyrin with partial chain perfluorination, which has the same basic structure as the best photoreceptor in nature (chlorophyll), shows an exceptionally enhanced tendency to self-assemble into ordered nanostructure. Defect-free homeotropically aligned fluorinated porphyrin thin films were, for the first time, fabricated and characterised. The ability to self-assemble large π-conjugated discotic molecules into highly ordered nanostructure via partial chain perfluorination provides new insight for the bottom-up nanofabrication of molecular devices. The controllable ordered porphyrin nanostructure with directed molecular arrangement holds great promise for use in high-performance electronic devices.

Self-assembly, condensation, and order in aqueous lyotropic chromonic liquid crystals crowded with additives†

Dense multicomponent systems with macromolecules and small solutes attract a broad research interest as they mimic the molecularly crowded cellular interiors. The additives can condense and align the macromolecules, but they do not change the degree of covalent polymerization. We chose a lyotropic chromonic liquid crystal with reversibly and non-covalently assembled aggregates as a much softer system, reminiscent of “living polymers”, to demonstrate that small neutral and charged additives cause condensation of aggregates with ensuing orientational and positional ordering and nontrivial morphologies of phase separation, such as tactoids and toroids of the nematic and hexagonal columnar phase coexisting with the isotropic melt. Scanning transmission X-ray microscopy (STXM) with near edge X-ray absorption fine structure (NEXAFS) analysis as well as fluorescent microscopy demonstrates segregation of the components. The observations suggest that self-assembly of chromonic aggregates in the presence of additives is controlled by both entropy effects and by specific molecular interactions and provide a new route to the regulated reversible assembly of soft materials formed by low-molecular weight components.

Advanced bistable cholesteric light shutter with dual frequency nematic liquid crystal

In this paper, a direct switching between a transparent (or reflecting) planar (P) state to an opaque (or transparent) focal conic (FC) state and vice-versa of a polymer free bistable cholesteric light shutter without any homogeneous polyimide (PI) layer, is demonstrated based on the sign inversion of dielectric anisotropy of dual frequency liquid crystal (DFLC). The direct switching was achieved by applying square wave field at low (1 kHz) and high (50 kHz) frequency. As a result, the DFLC light shutter sustains bistable bright and dark states in electric field off state and exhibits excellent electro-optic performance. The direct switching from the FC to P states not only supports more uniform P state but also significantly reduces switching voltage by eliminating the high field homeotropic (H) state required for the switching in the conventional polymer stabilized cholesteric texture (PSCT) light shutter. The driving voltage applied to make a transition from the P to FC one is relatively low (3Vp-p/µm). Further, switching time from FC to P state was reduced drastically with homeotropic PI layer. Results show that dual frequency cholesteric liquid crystal (DFCLC) light shutter holds a great promise for use in energy efficient display devices and switchable windows.

Designing bent-core nematogens towards biaxial nematic liquid crystals

The nematic phase occurring in bent-core (or banana-shaped) molecule systems is conducive to the occurrence of biaxial (Nb) phase due to the inherent biaxial molecular shape. The tendency of bent-core molecules to stabilise a layered (smectic) structure poses one of the difficulties in obtaining the thermotropic Nb phase. To investigate the factors favouring the formation of the nematic phase, a number of bent-core nematic liquid crystals derived from 1, 3-phenylene central bent motifs were synthesised. Results of an investigation into the influence of the nature and position of substitutions on the central core and two rigid arms as well as the type of linkage units on the occurrence and stability of the nematic phase reveal important structure–property relationships. Textural and X-ray diffraction measurements have helped us identify molecules with the potential of exhibiting the Nb phase.

Tailoring Monodomain in Blue Phase Liquid Crystal by Surface Pinning Effect

Surface pinning effect of the optically isotropic, blue phase liquid crystal (BPLC) has been studied. Polycrystalline, platelet, multi-domain topological defects without surface treatment have been transformed to uniform monodomain in rubbed surfaces of the cell. The operating voltage was reduced by 27%, owing to a dramatic increase in the Kerr constant. Hysteresis was reduced by 63%, whereas the transmission efficiency gets doubled. The surface anchoring effect on the blue phase plays a significant role in the elastic free energy, coherence length and topological defects. BPLC displays with a low operating voltage, and reduced hysteresis can be realized with surface treatment.

In situ homeotropic alignment of nematic liquid crystals based on photoisomerization of azo-dye, physical adsorption of aggregates, and consequent topographical modification.

In situ homeotropic alignment is achieved by photochromic trans- to cis-isomerization of an azo-dye doped in a nematic host. The augmented dipole moment of the cis-isomer formed under UV-irradiation expedites molecular assembly into crystalline aggregates. Subsequent deposition of the aggregates creates a roughened surface and induces an anchoring transition from the initial planar to a homeotropic alignment of the LCs.