Kwang-Un Jeong

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.

Devices and materials for high-performance mobile liquid crystal displays

Mobile liquid crystal displays (LCDs), often playing a role in hand-held computers, have become indispensable electronic tools to human beings in modern society. Innovative technological developments in the devices and materials have paved a successful road toward the development of mobile LCDs. Herein, after reviewing the trends and performance requirements of mobile LCDs, organic–inorganic materials as the key components of high performance mobile LCDs are addressed. Additionally, the developing trends in mobile LCDs are discussed with respect to these materials.

From crystals to columnar liquid crystal phases: molecular design, synthesis and phase structure characterization of a series of novel phenazines potentially useful in photovoltaic applications

It is known that in photovoltaic applications, columnar discotic liquid crystal (LC) phases of conjugated compounds are useful to align the molecules for improving their charge mobilities. However, conjugated compounds are usually either crystalline or amorphous. For compounds to form columnar discotic LC phases, specific molecular design is required for their ordered structural packing. In our recent report, a series of conjugated compounds, 6,7,15,16-tetrakis(alkylthio)quinoxalino-[2′,3′:9,10]-phenanthro[4,5-abc]phenazine (TQPP-[SCn]4) (n = 6, 8, 10 and 12), which display p-channel characteristics, were synthesized and characterized. This series of compounds was crystalline and did not exhibit LC behavior (S. Leng, B. Wex, L. H. Chan, M. J. Graham, S. Jin, A. J. Jing, K.-U. Jeong, R. M. Van Horn, B. Sun, M. Zhu, B. R. Kaafarani and S. Z. D. Cheng, J. Phys. Chem. B, 2009, 113, 5403–5411). In order to create a columnar LC phase with the lowest free energy within a broad applicable temperature region, we specifically designed and synthesized several series of electron-deficient phenazine derivatives to disrupt the molecular crystal packing and force the compounds to enter the columnar LC phase. These phenazine derivatives were designed to control the fused rigid ring size and shape as well as the location, lengths, and chemical structures of their flexible tails. These series include a series of 2,11-bis(1-methylethyl)-6,7,15,16-tetrakis(alkoxy)quinoxalino[2′,3′:9,10]phenanthro-[4,5-abc]-phenazines (TQPP-[t-Bu]2-[OR(B)]4), a series of 2,13-bis(1-methylethyl)-7,8,18,19-tetrakis(alkoxy)pyrazino[2,3-i]pyrazino[2″,3″:6′,7′]quinoxalino[2′,3′:9,10]phenanthro[4,5-abc]-phenazines (TPPQPP-[t-Bu]2-[OR(B)]4), and a series of 3,4,11,12,19,20-hexaalkoxy-2,5,7,8,10,13,15,16,18,21,23,24-dodecaazatri-anthracenes (HDATAN-[OR]6), where R is the alkyl chain in the substituents and B represents that they are branched structures. The different phase structures and transition behaviors of these series of compounds were studied, and based on the experimental results, we can conclude that tailoring the alkyl tail size, the core size, and the core shape leads to a promising way to design molecules that exhibit the columnar LC phase. In particular, changes in alkyl tail architecture affect the phase behaviors more significantly than changes in its length.

Phase behaviors and supra-molecular structures of a series of symmetrically tapered bisamides

A series of symmetrically tapered 1,4-bis[3,4,5-tris(alkan-1-yloxy)benzamido] benzene bisamides (CPhBA, where is the number of carbon atoms in the alkyl chains, = 10, 12 and 16), was synthesized in order to investigate the effect of alkyl chain length on supra-molecular ordered structures induced by hydrogen (H)-bonding and micro-phase separation. These bisamides consist of a rigid aromatic bisamide core with three flexible alkyl chains at each end of the core. Major phase transitions and their origins in CPhBA bisamides were studied with differential scanning calorimetry, one-dimensional (1D) wide angle X-ray diffraction (WAXD), infrared spectroscopy, and solid-state carbon-13 nuclear magnetic resonance experiments. The structures of these compounds in different phases were identified using 2D WAXD from oriented samples and were also confirmed by selected area electron diffractions in transmission electron microscopy from stacked single crystals and by computer simulations. All of the CPhBA bisamides in this series formed a highly ordered oblique columnar () phase and a low-ordered oblique columnar () phase, similar to a recent report on C14PhBA. The two main driving forces in the formation of these two supra-molecular columnar structures were identified: One was the H-bond formation between N-H and C[double bond, length as m-dash]O groups, and the other was the micro-phase separation between the bisamide cores and the alkyl chains. With increasing the length of alkyl tails, the isotropization temperature decreased, while the disordering temperature of the alkyl tails increased. The 2D lattice structures perpendicular to the columnar axis also increasingly deviated from the pseudo-hexagonal packing with increasing the alkyl tail length. However, the alkyl tail length did not have a significant influence on the packing along the columnar axis direction. Utilizing polarized optical microscopy, the phase identifications were also supported by the observation of texture changes and molecular arrangements inside of the micro-sized domains.

Surface-Modification on Vertical Alignment Layer Using UV-Curable Reactive Mesogens

The patterned vertical alignment (PVA) liquid crystal (LC) mode shows a wide viewing angle and a perfect dark state at a normal direction. However, it is inevitable to avoid the formation of disclinations and the movement of defect points during stabilization of LCs reorientation. It is due to fact that the LC directors tilt downward in different directions with collisions between them by the fringe-electric field. Consequently, the transmittance decreases and the response time gets slower. In order to overcome this barrier, the pretilt angles in four different directions are introduced on the substrates utilizing UV-curable reactive mesogen (RM) monomers. According to our studies, concentration of RM, UV curing condition, and applied voltage to the cell are critical to achieve an optimized surface-modified PVA mode which provides the well-defined reorientation of the LCs with respect to an electric field. Moreover, morphological behaviors on surface of substrate depending on curing conditions were investigated in order to confirm the existence of the stabilized polymer.

Photochromic 3-Dimensional Actuator Based on an Uncrosslinked Liquid Crystal Elastomer

A chemically programmed photochromic liquid crystal (LC) polymer was newly synthesized and used to fabricate an actuator exhibiting reversible photo-activated bending behavior at ambient temperature. Due to the lack of chemical crosslinking in this photochromic LC elastomer, it was possible to fabricate an actuator in the form of fiber or film by melt or solution processing, respectively. The three-dimensional (3D) bending of the film actuator can be precisely controlled in various directions by polarized ultraviolet (UV) light. The novel photochromic LC elastomer, exhibiting reversible photomechanical bending with controlled directions at room temperature, is beneficial for the realization of wireless remote-controlled 3D actuators.

Colour-tunable spiral photonic actuators

Combining the multi-faceted environmental responsiveness of polymers with photonically active structures, we developed spiral photonic actuators which can reversibly change both shape and colour in response to the chemical environment.

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.

From Random Coil Polymers to Helical Structures Induced by Carbon Nanotubes and Supramolecular Interactions

A simple method is reported for the preparation of double-helical structures through a series of achiral random and block copolymers poly(styrene-co-4-vinylbenzyl triazolylmethyl methylthymine) (PS-co-PVBT) with various T units on the side chains through click reactions of poly(styrene-co-4-vinylbenzyl azide) (PS-co-PVBN(3)) with propargyl thymine (PT) and also the synthesis of the A-appended pyrene derivative (A-Py) through click chemistry. This double-helical structure is observed from achiral random-coil polystyrene (PS) main chains, stabilized through the combination of multiple A-T hydrogen bonds, and π-π stacking between pyrene units and single-walled carbon nanotubes (SWCNTs).