Byoung-Ki Cho

Self-assembling behavior of amphiphilic dendron coils in the bulk crystalline and liquid crystalline states.

We prepared a series of amphiphilic dendron coils (1-3) containing aliphatic polyether dendrons with octadecyl peripheries and a poly(ethylene oxide) (PEO) coil (DP = 44). The molecular design in this study is focused on the variation of dendron generation (from first to third) with a fixed linear coil, upon which the thermal and self-assembling behavior of the dendron coils was investigated in the bulk. All the dendron coils exhibit two crystalline phases designated as k1 (both crystalline octadecyl chains and PEO) and k2 states (crystalline octadecyl chains and molten PEO). Crystallinities for both octadecyl peripheries and the PEO decrease as generation increases. In particular, the dendron coil (3) containing third generation shows a drastic reduction of the PEO crystallinity, which is attributed to the considerable chain folding and plasticization effects by the largest hydrophilic dendritic core segment. All the crystalline phases are bilayered lamellar morphologies. On going from k1 to k2, the periodic lamellar thickness decreases in the dendron coil (1) with first generation, but interestingly increases in 3. After melting of octadecyl peripheries, 1 shows no mesophase (i.e., liquid crystalline phase). Additionally, dendron coil 2 (3) displays a network cubic mesophase with Ia3d symmetry (micellar cubic with Pm3n) which is transformed into a lamellar (hexagonal columnar) mesophase upon heating. Remarkably, the temperature-dependent mesomorphic behavior in 2 and 3 is a completely reverse pattern in comparison with conventional linear-linear block copolymers. The unusual bulk morphological phenomena in the crystalline and liquid crystalline phases can be elucidated by the dendron coil architecture and the associated coil conformational energy.

Cubic and Columnar Supramolecular Architectures of Rod–Coil Molecules in the Melt State

The liquid crystalline behavior of compounds 1 (n = 7, 12, 15) differs significantly from that exhibited for conventional rodlike molecules. They organize into layered smectic, bicontinuous cubic or hexagonal columnar mesophases depending on the temperature or the volume fraction of coil segments.

Degree of chain branching-dependent assemblies and conducting behavior in ionic liquid crystalline Janus dendrimers

We prepared two Janus dendrimers 1 and 2 bearing a hydrophobic Percec-type minidendron (i.e., 3,4,5-tridodecyloxybenzyl group) and hydrophilic aliphatic ether dendrons with different degrees of branching via stepwise click reactions. The obtained Janus dendrimers have the same hydrophilic volume fraction (f = 0.34), but a different number of hydrophilic chains, i.e., tetrabranched and dibranched oligo(ethylene oxide) coils for 1 and 2, respectively. Upon doping with lithium salt, liquid crystalline (LC) ionic samples 1-Li+++ and 2-Li++ were obtained. As characterized by polarized optical microscopy (POM) and X-ray scattering techniques, 1-Li+++ showed an optically isotropic cubic LC phase, while 2-Li++ displayed a birefringent hexagonal columnar LC structure with a bilayered cross-section. Interestingly, the conductivity values of the cubic phase were considerably higher than those of the columnar phase, with which the cubic phase could be assigned as a bicontinuous cubic structure rather than a micellar one. On the basis of the experimental observations, the morphological contrast between two ionic Janus dendrimers is strongly attributed to the degree of chain branching, which is one of the major parameters governing the morphological features in the Janus dendrimer assemblies.

Self-Assembly of Molecular Dumbbells into Organized Bundles with Tunable Size

Dumbbell-shaped molecules consisting of three biphenyls connected through vinyl linkages as a conjugated rod segment and aliphatic polyether dendritic wedges with different cross-sections (i.e., dibranch (1), tetrabranch (2) and hexabranch (3)) were synthesized and characterized. The molecular dumbbells self-assemble into discrete bundles that organize into three-dimensional superlattices. Molecule 1, based on a dibranched dendritic wedge, organizes into primitive monoclinic-crystalline and body-centered, tetragonal liquid crystalline structures, while molecules 2 and 3, based on tetra- and hexabranched dendritic wedges, respectively, form only body-centered, tetragonal liquid crystalline structures. X-ray diffraction experiments and density measurements showed that the rod-bundle cross-sectional area decreases with increasing cross-section of the dendritic wedges. The influences of supramolecular structure on the bulk-state optical properties were investigated by measuring the UV/Vis absorption and steady state fluorescence spectroscopies. As the cross-section of the dendritic wedge of the molecule increases, the absorption and emission maxima shift to higher energy. This can be attributed to a quantum size effect of the three-dimensionally confined nanostructure.

Design, synthesis, and self-assembly behavior of C3-symmetry discotic molecules via click chemistry

We prepared a series of C3-symmetry discogens with three 1,2,3-triazoles and a central benzene ring by a copper-catalyzed “click reaction” between 1,3,5-triethynylbenzene and 3,4,5-trialkoxybenzyl azides. According to the differential scanning calorimetry (DSC) data, compounds with octyl and dodecyl peripheries showed stable liquid crystalline (LC) phases at room temperature, and upon heating underwent an isotropization at 105.3 °C and 92.2 °C, respectively. Compound with octadecyl peripheries melted into a LC phase at 63.5 °C, and changed into a disordered liquid at 84.4 °C. As characterized by optical polarized microscopy (POM) and X-ray scattering techniques, all LC phases revealed hexagonally packed columns, the axes of which are aligned vertically on the glass substrate. Interestingly, the experimentally accessible crystalline phase of the compound with octadecyl chains kept the 2-D hexagonal columnar structure, although the interdisc distance within the columns changed at the transition from the LC to crystalline phase.

Nanostructured organic electrolytes

This review introduces research work in the area of organic liquid crystalline (LC) electrolytes consisting of ionophilic and ionophobic groups. I discuss the LC phase behavior and morphology-dependent ion conduction of organic electrolyte systems based upon (i) coil–coil block copolymers (BCP), (ii) dendritic BCPs, (iii) mini-dendron-based liquid crystals, and (iv) rod-type liquid crystals. As a way to improve ionic conductivity and mechanical strength, the self-assembling approach of each LC system and other processes including macroscopic alignment techniques and post-polymerizations are described.

An Extraordinary Cylinder-to-Cylinder Transition in the Aqueous Assemblies of Fluorescently Labeled Rod-Coil Amphiphiles

We report an unprecedented cylinder-to-cylinder transformation in the self-assemblies of pyrene-labeled rod-coil molecules in water. The extraordinary morphological transformation can be monitored by a fluorescence variation from exciplex to excimer emissions, suggesting the rod-packing transition from antiparallel to interdigitated arrangements as a function of PEO coil length.

An Unusual Stacking Transformation in Liquid‐Crystalline Columnar Assemblies of Clicked Molecular Propellers with Tunable Light Emissions

The columnar liquid-crystalline (LC) and fluorescence properties of three-dimensional molecular propellers based on tetraphenylethylene is reported. X-ray scattering studies reveal an unusual transition from a rectangular (Colrec ) to a hexagonal columnar (Colhex ) phase. In contrast to second-order intercolumnar transitions based on a common tilt mechanism, the transition is first order and involves an unprecedented zigzag stacking of aromatic propellers in the Colrec phase. A sudden change in emission color from sky blue to green occurs rapidly and reversibly at this transition, which is due to the planarization of the propeller mesogen.

Self-Assembled Nanofibers and Related Nanostructures from Molecular Rods

This review introduces recent research in the area of self-assembled nanostructures from rod molecules in solutions; in particular one-dimensional nano-aggregates such as fibers, ribbons, tubules are highlighted. Self-assembled nanostructures are well-known to be strongly dependent upon the architecture of molecular building blocks; thus, in this review we discuss assembling behavior and the related functions associated with molecular shapes such as simple rod–coils, macrocycles, dendron–rod–coils, dumbbells, wedges and conjugated rods with lateral chains. In addition, biomimetic or bioconjugate amphiphilic rod systems are described.

Liquid crystalline and ion-conducting properties of mesogenic dendron–coil-dendron copolymers: characterization of LC phases using normalized conductivity

We synthesized three dendron–coil-dendron block copolymers consisting of ionophilic poly(ethylene oxide) (PEO) coils and mesogenic dendrons with four octadecyl peripheries via stepwise click reactions. The obtained polymers were doped with lithium triflate, whose concentration per ethylene oxide unit was 0.05. As characterized by optical polarized microscopy (POM) and X-ray scattering techniques, polymer 1 with the shortest PEO coil (Mn = 2000 g mol−1) did not show the liquid crystalline (LC) phase, while its ionic sample (1-Li+) exhibited a hexagonal columnar LC phase. On the other hand, 2 and 3 (with PEOs of Mn = 4000 and 8000 g mol−1, respectively) displayed identical LC morphologies to 2-Li+ and 3-Li+, respectively, although the phase transition temperatures increased upon salt doping. For 2 and 2-Li+, gyroid and lamellar LC phases were observed with increasing temperature, while 3 and 3-Li+ showed only a lamellar LC phase. The observed ion-transporting behavior was strongly dependent upon the connectivity of ion-conducting domain structures. The investigation of the morphology–conductivity correlation using a normalized conductivity (σ* = σ/f, where σ and f are the original conductivity and the PEO volume fraction) indicated that the 3-D gyroid LC phase showed the highest value, while the lowest conductivity was found in the 1-D columnar structure at identical temperatures. Additionally, the gyroid to lamellar phase transition temperature of 2-Li+ could be determined by the σ*, which was consistent with the X-ray data. Consequently, the results can be explained by the fact that the ionic pathway becomes complicated when going from higher to lower dimensional structures in polygrain samples.