Jeong-Kyu Lee

Silicon nanoparticles–graphene paper composites for Li ion battery anodes

Composites of Si nanoparticles highly dispersed between graphene sheets, and supported by a 3-D network of graphite formed by reconstituting regions of graphene stacks exhibit high Li ion storage capacities and cycling stability. An electrode was prepared with a storage capacity >2200 mA h g(-1) after 50 cycles and >1500 mA h g(-1) after 200 cycles that decreased by <0.5% per cycle.

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.

Solid‐State Scrolls from Hierarchical Self‐Assembly of T‐Shaped Rod–Coil Molecules

On a roll: Attachment of flexible coils to the middle of a rigid rod generates T-shaped rod-coil molecules that self-assemble into layers that roll up to form filled cylindrical and hollow tubular scrolls, depending on the coil length, in the solid state (see picture); the rods are arranged parallel to the layer plane.

Self‐Dissociating Tubules from Helical Stacking of Noncovalent Macrocycles

The construction of tubular structures by molecular selfassembly is a topic of great current interest because of the potential applications of such assemblies in the fields of biotechnology and materials science. Inspired by natural tubules created in biological systems, diverse synthetic tubular structures have been developed through self-assembly of designed molecular modules including lipid molecules, aromatic amphiphiles, and helical polymers. The organization of shape-persistent macrocycles into supramolecular structures is an alternative way to construct tubular structures. The macrocyclic segments with conformational rigidity stack on top of each other through p–p stacking interactions to create a hollow tubular interior that is separated from the exterior. The shape-persistent macrocyclic structures can also be constructed by non-covalent interactions such as hydrogen-bonding interactions of nucleotide mimic base pairs and metal-coordination bonding of bentshaped ligands. Although this strategy is well established, the construction of the shape-persistent macrocycles through non-specific interactions has been rarely reported. Noncovalent macrocyclic structures may be constructed by self-assembly of laterally grafted bent-shaped rigid segments with an internal angle of 1208 through a combination of shape complementarity and phase separation of dissimilar blocks. The resulting noncovalent macrocycles are expected to stack on top of each other to form tubular structures. In addition to noncovalent synthesis of 1D structures, another attractive aspect regarding these 1D structures is their possibility to dynamically respond to external stimuli, including stimuli-responsive sol–gel interconversion, thermoresponsive supramolecular chirality, and fluorescence switching. Accordingly, we synthesized the laterally grafted bent-rod amphiphile 1, which consists of a meta-linked aromatic segment and an oligoether dendron side-group. Herein we present the formation of hexameric macrocycles from the self-assembly of small block molecules based on an m-linked aromatic segment. The macrocycles stack on top of each other to form an elongated tubular structure (Figure 1). Notably, the resulting tubules dissociate into discrete toroidal stacks in response to addition of a silver salt. The rigid-flexible block molecules described here were prepared in a stepwise fashion according to previously reported similar procedures.

Self-Organization of Bent Rod Molecules into Hexagonally Ordered Vesicular Columns

Bent-shaped rigid-core molecules with flexible chiral dendrons grafted to the outer side of the bend were synthesized and characterized by circular dichroism, differential scanning calorimetry, X-ray scatterings, and transmission electron microscopy in solution and the solid state. The bent aromatic rods based on hepta- and nonaphenylene with nitrile groups at both ends self-assemble into well-ordered hollow tubular structures in aqueous solution, while the bent rod based on heptaphenylene without nitrile groups showed no apparent aggregations in aqueous solution. In the solid state, the rigid-flexible molecules based on heptaphenylene rod without the nitrile group self-assemble into a 2D oblique columnar structure with the columnar cross-section containing two interlocked molecules. Remarkably, the rigid flexible molecules based on hepta-, nona-, and undecaphenylene with nitrile groups self-assemble into a hexagonal columnar structure with weak 3D order. A model of vesicular channel structure is proposed based on small- and wide-angle X-ray diffraction on oriented fibers, density measurement, reconstruction and simulation of electron density maps, and molecular dynamics simulation. In contrast to the hollow tubular structure found in solution, in the solid both the outside and the interior of the columns are filled by the pendant aliphatic coils. Filling of the interior of these vesicular channels is made possible by some bent rod molecules turning their obtuse apex inward. One in 7, 2 in 8, and 4 in 10 molecules are thus inverted in a column slice in compounds with hepta-, nona-, and undecaphenylene cores, respectively. These are new examples of vesicular double-segregated columnar structures recently discovered in some dendrons.

Striking confinement effect: AuCl4- binding to amines in a nanocage cavity

Binding of AuCl(4)(-) to amine groups tethered to the interior of a 2 nm siloxane nanocage was determined in solutions containing various concentrations of acid. The mode of binding was inferred from EXAFS and UV-vis spectra to be by ligand exchange of amine for chloride, which implies that the amines remain unprotonated. Cyclic voltammetry confirmed that the Au complexes bind to the nanocage interior and established a 1:1 relationship between bound Au complex and amine groups. The results suggested a 5-7 pH unit shift in the protonation constant of the interior amines relative to free amines in solution.

Stepped Strips from Self-Organization of Oligo(p-phenylene) Rods with Lateral Dendritic Chains

We have demonstrated that the rod segments with lateral dendritic chains self-assemble into unique stepped strips in which the rods are aligned parallel to the strip long axis. This unique organization of the rod segments arises from a balance between the energetic gain of a parallel arrangement of the rods and the resulting entropic penalty associated with stretching of the lateral flexible chains.

Thickness-dependent morphological behavior of dendritic (PS)2-b-PLA copolymer thin films on a SiO2 substrate

In contrast to the known perpendicular ordering of conventional PS-b-PLA thin films, branched (PS)(2)-b-PLA thin films showed two different parallel orientations whose locations are strongly dependent upon film thickness.

Sequential transformation of the crystalline rod domain from a flat ribbon to segmented columnar to micellar structure

Abstract