Yu-Jin Choi

Photoresponsive Carbohydrate-based Giant Surfactants: Automatic Vertical Alignment of Nematic Liquid Crystal for the Remote-Controllable Optical Device

Photoresponsive carbohydrate-based giant surfactants (abbreviated as CELAnD-OH) were specifically designed and synthesized for the automatic vertical alignment (VA) layer of nematic (N) liquid crystal (LC), which can be applied for the fabrication of remote-controllable optical devices. Without the conventional polymer-based LC alignment process, a perfect VA layer was automatically constructed by directly adding the 0.1 wt % CELA1D-OH in the N-LC media. The programmed CELA1D-OH giant surfactants in the N-LC media gradually diffused onto the substrates of LC cell and self-assembled to the expanded monolayer structure, which can provide enough empty spaces for N-LC molecules to crawl into the empty zones for the construction of VA layer. On the other hand, the CELA3D-OH giant surfactants forming the condensed monolayer structure on the substrates exhibited a planar alignment (PA) rather than a VA. Upon tuning the wavelength of light, the N-LC alignments were reversibly switched between VA and PA in the remote-controllable LC optical devices. Based on the experimental results, it was realized that understanding the interactions between N-LC molecules and amphiphilic giant surfactants is critical to design the suitable materials for the automatic LC alignment.

Self-Assembled Hierarchical Superstructures from the Benzene-1,3,5-Tricarboxamide Supramolecules for the Fabrication of Remote-Controllable Actuating and Rewritable Films

The well-defined hierarchical superstructures constructed by the self-assembly of programmed supramolecules can be organized for the fabrication of remote-controllable actuating and rewritable films. To realize this concept, we newly designed and synthesized a benzene-1,3,5-tricarboxamide (BTA) derivative (abbreviated as BTA-3AZO) containing photoresponsive azobenzene (AZO) mesogens on the periphery of the BTA core. BTA-3AZO was first self-assembled to nanocolumns mainly driven by the intermolecular hydrogen-bonds between BTA cores, and these self-assembled nanocolumns were further self-organized laterally to form the low-ordered hexagonal columnar liquid crystal (LC) phase below the isotropization temperature. Upon cooling, a lamello-columnar crystal phase emerged at room temperature via a highly ordered lamello-columnar LC phase. The three-dimensional (3D) organogel networks consisted of fibrous and lamellar superstructures were fabricated in the BTA-3AZO cyclohexane-methanol solutions. By tuning the wavelength of light, the shape and color of the 3D networked thin films were remote-controlled by the conformational changes of azobenzene moieties in the BTA-3AZO. The demonstrations of remote-controllable 3D actuating and rewritable films with the self-assembled hierarchical BTA-3AZO thin films can be stepping stones for the advanced flexible optoelectronic devices.

Flexible and Patterned Thin Film Polarizer: Photopolymerization of Perylene-based Lyotropic Chromonic Reactive Mesogens

A perylene-based reactive mesogen (DAPDI) forming a lyotropic chromonic liquid crystal (LCLC) phase was newly designed and synthesized for the fabrication of macroscopically oriented and patterned thin film polarizer (TFP) on the flexible polymer substrates. The anisotropic optical property and molecular self-assembly of DAPDI were investigated by the combination of microscopic, scattering and spectroscopic techniques. The main driving forces of molecular self-assembly were the face-to-face π-π intermolecular interaction among aromatic cores and the nanophase separation between hydrophilic ionic groups and hydrophobic aromatic cores. Degree of polarization for the macroscopically oriented and photopolymerized DAPDI TFP was estimated to be 99.81% at the λmax = 491 nm. After mechanically shearing the DAPDI LCLC aqueous solution on the flexible polymer substrates, we successfully fabricated the patterned DAPDI TFP by etching the unpolymerized regions selectively blocked by a photomask during the photopolymerization process. Chemical and mechanical stabilities were confirmed by the solvent and pencil hardness tests, and its surface morphology was further investigated by optical microscopy, atomic force microscopy, and three-dimensional surface nanoprofiler. The flexible and patterned DAPDI TFP with robust chemical and mechanical stabilities can be a stepping stone for the advanced flexible optoelectronic devices.

Butterfly-Shaped Diphenylpyrimidine Molecule: Tunable Photophysical Properties by Molecular Self-Assembly Pathways

To understand the relationships between chemical structures, molecular packing structures, and photophysical properties of organic materials, a butterfly shaped diphenylpyrimidine molecule (abbreviated as DPP-6C12) was newly synthesized [Park, M.; Choi, Y.-J.; Kim, D.-Y.; Hwang, S.-H.; Jeong, K.-U. Cryst. Growth Des. 2015, 15, 900-906]. By breaking the molecular symmetry and coplanarity of DPP-6C12, peculiar monotropic phase transitions were observed. Based on two-dimensional wide-angle X-ray diffraction and selected area electron diffraction, the molecular packing structures of ordered phases were identified, which were further confirmed by the computer simulations in the real and reciprocal spaces. Finally, we demonstrated that the photophysical properties of DPP-6C12 can be tuned by controlling the molecular packing structures with simple thermal treatments.

Multifunctional Optical Thin Films Fabricated by the Photopolymerization of Uniaxially Oriented Lyotropic Liquid Crystal Monomers for Electro-Optical Devices

A multifunctional optical thin film (MOTF) is fabricated by coating the newly synthesized perylene-based reactive mesogen (PBRM) and stabilized by the subsequent photopolymerization. Based on the spectroscopic results combined with morphological observations, it is found that nematic liquid crystal (NLC) is aligned parallel to the molecular long axis of PBRM not only due to the long-range physical anchoring effect but also due to the short-range molecular physical interactions between alignment layer and NLC molecules. From the electro-optical properties of LC test cells fabricated with the PBRM MOTF, it is clearly demonstrated that the PBRM MOTF can work as the planar LC alignment layer as well as the in-cell coatable polarizer. The coatable PBRM MOTF from lyotropic chromonic reactive mesogens can pave a new way for the flexible optoelectronic devices.

Stimuli-responsive liquid crystal physical gels based on the hierarchical superstructures of benzene-1,3,5-tricarboxamide macrogelators

For the fabrication of rewritable electro-optic devices, we prepared randomly dispersed liquid crystal physical gels (LCPG) by constructing a well-defined hierarchical superstructure of a benzene-1,3,5-tricarboxamide (BTA)-based macrogelator (abbreviated BTA3AZO) in a host nematic (N) LC medium. The programmed BTA3AZO macrogelator in the NLC medium built up three-dimensional networks not only by hydrogen-bonding between BTA cores but also by nanophase separations between BTA3AZOs and NLCs. On the basis of the anisotropic optical properties of LCPGs, it was realized that a light scattering state of BTA3AZO LCPGs can be switched to a transmittance state under a fairly low driving voltage (22 V) compared with those (around 100 V) of polymer-based systems. The lower driving voltage of BTA3AZO LCPGs should be mainly due to the formation of self-assembled BTA3AZO nanocolumn networks (only 1 wt%) which were finely dispersed in the NLC medium as well as the good affinity between NLCs and azobenzene mesogens (AZOs) which also responded to the applied electric fields. The BTA3AZO LCPGs can be remote-controlled by utilizing the trans–cis photoisomerization of the azobenzene moieties in the LCPGs. Additionally, the rewritable electro-optical properties of BTA3AZO LCPGs allowed us to demonstrate remote-controllable light shutters.

Construction of Polymer-Stabilized Automatic MultiDomain Vertical Molecular Alignment Layers with Pretilt Angles by Photopolymerizing Dendritic Monomers under Electric Fields

The synthesized itaconic acid-based dendritic amphiphile (Ita3C12) monomers and the methacryl polyhedral oligomeric silsesquioxane (MAPOSS) cross-linkers were directly introduced for the construction of automatic vertical alignment (auto-VA) layers in the host nematic liquid crystal (NLC) medium. The auto-VA layer can be stabilized by irradiating UV light. For the automatic fabrication of a polymer-stabilized multidomain VA (PS auto-MDVA) layer with a pretilt angle, Ita3C12 and MAPOSS were photopolymerized under the electric field by irradiating UV light on the multidomain electrode cell. Mainly because of the pretilted NLC at zero voltage, the electro-optic properties of the PS auto-MDVA cell were dramatically improved. From the morphological observations combined with surface chemical analyses, it was found that various sizes of protrusions on the solid substrates were automatically constructed by the two-step mechanisms. We demonstrated the PS auto-MDVA cell with the enhancement of electro-optic properties as a single-step process and investigated how the protrusions were automatically developed during the polymer stabilization.

Programmed Hierarchical Hybrid Nanostructures from Fullerene-Dendrons and Pyrene-Dendrons

The construction of fullerene (C60 ) hierarchical nanostructures with the help of amphiphilic molecules remains a challenging task in nanoscience and nanotechnology. Utilizing the host-guest complex concept, sub-10 nm layered superstructures are constructed from a monofunctionalized C60 dendron (C60 D, guest) and tweezer-like pyrene dendron (PD, host). Since C60 D and PD are asymmetric shape amphiphiles having liquid crystal (LC) dendrons, both C60 D and PD construct head-to-head bilayer superstructures by themselves. From fluorescence titration experiments, it is realized that the host-guest complex shows 1:1 stoichiometric binding with a binding constant (Ksv = 2.45 × 105 m-1 ). Based on the morphological observations and scattering analyses, it is found that buckle-like asymmetric building blocks (C60 D·PD) are self-assembled by the host-guest complex and construct multilayer hybrid nanostructures. The hierarchical hybrid nanostructures consist of the self-assembled C60 D·PD bilayer with a 2D C60 ·P nanoarray sandwiched between LC dendrons. This advanced strategy is expected to be a practicable and rational guideline for the fabrication of programmed hierarchical hybrid nanostructures.

Fabrication of Actuating and Rewritable Film by Self-AssembledHierarchical Superstructures

Extended Abstract The development of supramolecular chemistry provides a platform for the design and synthesis of multifunctional molecules which can evolve into the self-assembled superstructures with the targeted physical properties. 2 To fabricate the remote-controllable actuating and rewritable films, we newly designed and synthesized a benzene-1,3,5-tricarboxaimde (BTA) derivative containing photo-responsive azobenzene (AZ) mesogens (abbreviated as B3AZ). On the basis of the thermal, microscopic, and scattering results, B3AZ was first self-assembled to nanocolumns mainly driven by the intermolecular hydrogen-bond between BTA cores, and these self-assembled nanocolumns were further self-organized laterally to form the low-ordered hexagonal columnar liquid crystal (LC) phase (ΦLC) below the isotropic temperature. Upon cooling, lamello-columnar crystal phase (KCr) emerged at room temperature via a highly ordered lamella-columnar LC phase (KLC). Note that alkyl tails were disordered in the KLC phase, while the ordered alkyl tails did participate for the construction of well-ordered KCr phase. Because the B3AZ molecules exhibiting the LC behaviors can form the hierarchical three-dimensional (3D) superstructures, thin films and free-standing films were fabricated by the programmed self-assembly. Additionally, hydrogen bondable amide unit in the BTA core play a key role to form the macroscopic films. The 3D organogel networks consisted of fibrous and lamellar superstructures were fabricated in the B3AZ cyclohexane-methanol solutions. By tuning the wavelength of light, the shape and color of the 3D networked thin films were remote-controlled 3D actuating and rewritable films with the self-assembled hierarchical B3AZ thin films. It can be stepping stones for the advanced flexible optoelectronic devices. This work was supported by the BK21 plus, BRL2015042417, Mid-Career Researcher Program (2016R1A2B2011041), and Global Ph. D. Fellowship Program (NRF2016H1A2A1907561) of Republic of Korea.

Multilayer thin films for the construction of active repulsive hydrogen barriers

An active repulsive hydrogen (H2) barrier thin film (ARHB-tfilm) was fabricated by layer-by-layer (LbL), spin and plasma sequential coatings on a poly(ethylene terephthalate) (PET) film. Protons generated by the reduction of H2 on the palladium nanolayer can be trapped in the proton-rich Nafion region by the electrical repulsive force formed at the interface of the positively charged polymer nanolayer. The multilayer ARHB-tfilms showed a significant improvement in hydrogen gas barrier properties.