Dae-Yoon Kim

Three-dimensional actuators transformed from the programmed two-dimensional structures via bending, twisting and folding mechanisms

Combining the physical principle of actuators with the basic concept of photonic crystals, colour-tunable three-dimensional (3D) photonic actuators were successfully fabricated. By controlling the d-spacings and the refractive index contrasts of the self-assembled 3D colloidal photonic crystals, colours of the photonic actuators were tuned. Various shapes of these 3D actuating objects were constructed by transforming the programmed 2D structures via bending, twisting and folding mechanisms. These 2D structures were first programmed by breaking the symmetry. The selective swellings were then applied as driving forces to control the shapes and colours of the photonic actuators. Scroll photonic actuators had been first demonstrated by bending the traditional 2D cantilever structure (K.-U. Jeong, et al., J.Mater.Chem., 2009, 19, 1956). By breaking the symmetry of a cantilever structure perpendicular to its long axis, polypeptide-/DNA-like 3D helical photonic actuators were obtained from the programmed 2D structure via twisting processes. Both left- and right-handed scrolls and helices with various colours can be achieved by changing the polarity of solvents. Different types of 3D actuators, such as cube, pyramid and phlat ball, were also demonstrated via the folding mechanism. The reversible 3D photonic actuators transformed from the programmed 2D structures via the bending, twisting and folding mechanisms may be applied in the field of mechanical actuators, and optoelectronic and bio-mimetic devices.

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.

Dual Photo‐functionalized Amphiphile for Photo‐reversible Liquid Crystal Alignments

Without the conventional polymer-based liquid crystal (LC) alignment process, a newly synthesized dual photo-functionalized amphiphile (abbreviated as ADMA1 ) was successfully applied as a robust photo-reversible LC alignment layer by self-assembly and photo-polymerization. The LC alignment layer constructed by directly adding dual photo-functionalized amphiphiles into LC media significantly cuts the manufacturing cost as well as opens new doors for the fabrication of novel electro-optical devices.

Reversible Actuating and Writing Behaviours of a Head-To-Side Connected Main-Chain Photochromic Liquid Crystalline Polymer

A photochromic liquid crystalline (LC) polymer {4-butoxy-2′-(4-hex-5-enyloxy)-4′-(4-hex-5-enyl-oxybenzyloxy)azobenzene, abbreviated as PLCP} was newly synthesized via the acrylic diene metathesis polymerization (H. J. Choi, et al., J. Mater. Chem., 2009, 19, 7124). Utilizing the combined techniques of differential scanning calorimetry, cross-polarized optical microscopy, and wide-angle X-ray diffraction (WAXD), it was realized that PLCP formed a nematic (N) phase below the isotropization temperature (TNI = 145 °C) and the N phase turned to the glassy N phase below the glass transition temperature (Tg = 24 °C). Especially, two-dimensional (2D) WAXD of the uniaxially oriented PLCP fibre indicated that the N phase contains the synclinically tilted smectic C (SmC)-type cybotactic clusters in which the molecular packing structure is related to the head-to-side connected chemical structure of PLCP. Without introducing any chemical crosslinks, PLCP fibres and films were reversibly deformed by exposing to ultraviolet and visible light. Additionally, we demonstrated the rewritable micro-patterned PLCP film. Because of the wireless remote-controlled actuating behaviours of PLCP combined with its excellent processability, PLCP can be applied in optoelectronic and bio-mimetic devices.

The biaxial lamello-columnar liquid crystalline structure of a tetrathiafulvalene sanidic molecule

A macroscopically oriented lamello-columnar mesophase was obtained from a newly designed and synthesized symmetric tetrathiafulvalene-based molecule (symTTF11) containing four flexible alkyl chains chemically attached to the periphery of TTF mesogens (L. Wang, et al., J. Mater. Chem., 2011, 21, 60), and its phase behaviors and physical properties were investigated by the combined experimental techniques. Based on the experimental results and structure analyses, it was revealed that there are two ordered columnar phases below the isotropic phase: a columnar crystalline phase (ColK) and a columnar smectic liquid crystal phase (ColL). From two-dimensional wide angle X-ray diffraction patterns of the macroscopically oriented samples, phase structures of ColK and ColL were identified and their biaxial molecular packing structures were also proposed. Since the assembled symTTF11 columns were well organized in the ordered layer structures over a macroscopic domain, symTTF11 exhibited a good charge carrier mobility in the ColL phase.

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.

Hierarchical superstructures of norbornene-based polymers depending on dendronized side-chains

For understanding the self-assembly behaviors of norbornene-based main-chain polymers depending on dendronized side-chains, a series of polynorbornenes containing the programmed dendrons (AT3P and BP3P) is newly designed and successfully synthesized via ring-opening metathesis polymerization (ROMP) with high molecular weights and narrow polydispersity indices. The phase transitions and molecular packing structures of AT3P and BP3P are investigated by the combination of thermal, scattering and microscopic analyses. AT3P with flexible and hydrophobic alkyl chain dendrons forms the columnar nematic (ColN) liquid crystal (LC) phase. By introducing a rigid biphenyl mesogen and a hydrophilic triethylene oxide linker between the hydrophobic alkyl chain and the polynorbornene main-chain, a smectic A (SmA) LC phase is formed, mainly due to the nanophase separations and the strong intermolecular interactions between the biphenyl mesogens. This study reveals that the hierarchical superstructures of norbornene-based polymers can be precisely controlled by tuning the chemical structures and the physical interactions of side-chain pendants.

Pseudo-rodlike molecules with hockey-stick-shaped mesogen

ABSTRACT Hockey-stick-shaped molecules were newly synthesised to obtain pseudo-rodlike molecules. The designed molecules consist of a polar terminal ring (i.e. 2,3,4-, 2,4,6- or 3,4,5-trifluorophenyl group), a rigid middle block (i.e. four rings with aligned ester linkages) and a flexible terminal chain (i.e. dodecyloxy group). We found that the compounds with 2,3,4- and 3,4,5-trifluorophenyl groups formed a smectic A mesophase with head-to-head bi-layer building blocks, whereas the compound with 2,4,6-trifluorophenyl group formed a nematic mesophase. This might be concerned with the behaviour of pseudo-rodlike molecules GRAPHICAL ABSTRACT

Macroscopically oriented hierarchical structure of the amphiphilic tetrathiafulvalene molecule

Since the physical properties of electronic and biological molecules strongly depend on the nature of molecular self-assembly and organization, it is essential to control their molecular packing structure and morphology on the different length scales. In this aspect, a programmed amphiphilic tetrathiafulvalene (TTF) molecule (abbreviated as amph-7TTF14) was newly designed and synthesized. Differential scanning calorimetry (DSC) combined with one-dimensional (1D) wide angle X-ray diffraction (WAXD) techniques revealed that a highly ordered crystalline phase emerged below the isotropic phase. From the 2D WAXD pattern of the macroscopically oriented amph-7TTF14 film, the crystal structure was identified to be a monoclinic unit cell. The face-to-face π–π interaction between TTF groups and the nanophase separation between rigid TTF groups and flexible hydrophobic alkyl and hydrophilic tri(ethylene oxide) tails were the main driving forces for the self-assembly of amph-7TTF14. The morphological observations using transmission electron microscopy (TEM), atomic force microscopy (AFM), and polarized optical microscopy (POM) indicated that the amph-7TTF14 formed not only flat ribbons but also scrolls and helixes, in which the ribbons further aggregated to create the fibrous hierarchical structures. Based on the experimental results and careful analyses, it was realized that the scrolls and helices were induced by the unbalanced surface stresses generated during the crystallization process. When the macroscopically oriented fibrous hierarchical structure is properly applied to the electrooptical and bio-mimetic devices, the targeted physical properties may be significantly improved and tuned for the specific practical applications.