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Myongsoo Lee

Publications and source records attributed to Myongsoo Lee.

27 records · Page 2Linked to original sources
PubMed2004

Stimuli-responsive supramolecular nanocapsules from amphiphilic calixarene assembly.

We synthesized tetrameric amphiphilic molecules based on a calixarene building block that self-assembles into a tunable and stable aggregation structure in aqueous solution. The amphiphilic calixarene molecules with a small hydrophilic part were observed to assemble into a vesicular structure that decreases significantly in diameter with only small increases in the hydrophilic chain length. Further increasing the chain length induced the collapse of the vesicles into spherical micelles. Remarkably, the vesicles were also observed to transform into small globular micelles at lower pH, which can be used to trigger the release of the encapsulated hydrophilic guest molecules.

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PubMed2004

Shape-persistent macromolecular disks from reactive supramolecular rod bundles.

We have synthesized a coil-rod-coil molecule based on a reactive rod block that self-organizes into a 3-D hexagonal close-packed (hcp) bundle structure in the melt state. Photo-cross-linking within an aromatic bundle yielded macromolecular disklike objects (molecular weight of 330 kDa) with a narrow size distribution that was essentially the same as that of the supramolecular bundles. In addition, cross-linking proceeded with the retention of the 3-D hcp symmetry of the supramolecular bundles. The 3-D symmetry of the macromolecular objects was observed to recover after their isolation from the solutions and transform into an isotropic liquid in the bulk state in a reversible way, suggesting that the macromolecular objects are shape-persistent in solutions as well as an isotropic liquid phase of the bulk.

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PubMed2004

Supramolecular reactor from self-assembly of rod-coil molecule in aqueous environment.

We synthesized an amphiphilic coil-rod-coil triblock molecule consisting of hexa-p-phenylene as a rod block and poly(ethylene oxide) with the number of repeating units of 17 as coil blocks and investigated aggregation behavior in aqueous environment. The rod-coil molecule was observed to aggregate into discrete micelles consisting of hydrophobic disklike rod bundles encapsulated by hydrophilic poly(ethylene oxide) coils. The aromatic bundles of the micelles were demonstrated to be used as an efficient supramolecular reactor for the room temperature Suzuki cross-coupling reaction of a wide range of aryl halides, including even aryl chlorides with phenylboronic acids in aqueous environment. These results demonstrate that self-assembly of amphiphilic rod-coil molecules can provide a useful strategy to construct an efficient supramolecular reactor for aromatic coupling reaction.

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PubMed2004

Anion-directed self-assembly of coordination polymer into tunable secondary structure.

A bent-shaped bipyridine ligand containing a dendritic aliphatic side chain has been synthesized as a ligand and complexed with silver ion through a self-assembling process. The resulting complexes were observed to self-assemble into supramolecular structures that differ significantly as a function of the counteranion size in the solid state, as confirmed by 1-D and 2-D X-ray diffraction experiments. The secondary structure of a cationic coordination chain appears to be dependent on the size of the counteranion. As the size of anion increases, the secondary structure of the coordination chain changes, from a helical chain, via a dimeric cycle, to a zigzag chain in the solid state. Interestingly, dilute solutions of the complexes exhibiting a columnar structure in polar solvents undergo spontaneous gelation and the resulting gels display a significant Cotton effect in the chromophore of the aromatic unit. These results represent a significant example that small variation in the anion size can provide a useful strategy to manipulate the secondary structure of linear chain and thereby solid-state supramolecular structure.

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PubMed2004

Self-assembling molecular trees containing octa-p-phenylene: from nanocrystals to nanocapsules.

Tree-shaped molecules consisting of octa-p-phenylene as a stem segment and oligoether dendrons as a flexible head were synthesized and characterized. The molecular tree based on a small flexible head self-assembles into a lamellar structure, whereas the molecule based on a larger headgroup self-assembles into a discrete heptameric bundle that organizes into a 3-D primitive orthorhombic supercrystals, as confirmed by X-ray scatterings and transmission electron microscopic (TEM) observations. Optical studies revealed that the absorption and emission maxima and absorption edge of the 3-D structure shift to higher energy compared to those of the lamellar structure. The molecules in dilute solution (THF/water = 1:10 v/v) were observed to self-assemble into capsule-like hollow aggregates, as confirmed by dynamic and static light scatterings, scanning electron microscopy (SEM), and TEM investigations. These results demonstrate that tree-shaped molecules are capable of packing into organized discrete nanocrystals with parallel arrangement as well as hollow nanocapsules with radial arrangement, depending on the presence of selective solvents for flexible headgroup.

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PubMed2004

Self-assembly of rod-coil molecules into molecular length-dependent organization.

A series of rod-coil molecules (n-x, where n represents the number of repeating units in a PPO coil and x the number of phenyl groups in a rod segment) with variation in the molecular length, but an identical rod to coil volume ratio was synthesized, and their self-assembling behavior was investigated by using DSC and X-ray scatterings. The molecule with a short rod-coil molecule (16-4) shows a 3-D tetragonal structure based on a body-centered symmetry of the discrete bundles in addition to a lamellar structure. This 3-D lattice, on heating, collapses to generate a disordered micellar structure. Remarkably, the molecules based on longer molecular length (21-5 and 24-6) were observed to self-organize into, on heating, lamellar, tetagonally perforated lamellar, 2-D hexagonal columnar and finally disordered micellar structures. Further increase in the molecular length as in the case of 29-7 and 32-8 induces a 3-D hexagonally perforated lamellar structure as an intermediate structure between the lamellar and tetragonally perforated lamellar structures. Consequently, these systems demonstrate the ability to regulate the domain nanostructure, from 2-dimensionally continuous layers, long strips to discrete bundles via periodic perforated layers by small changes in the molecular length, at an identical rod-to-coil volume fraction.

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PubMed2002

Amphiphilic hairy disks with branched hydrophilic tails and a hexa-peri-hexabenzocoronene core.

Amphiphilic discotic molecules with hydrophilic side branches consisting of hexaphenyl hexa-peri-hexabenzocoronene and hexabiphenyl hexa-peri-hexabiphenylcoronene as the aromatic core and hexa-substituted oligoethers as the branched peripheral chains have been synthesized, and their microstructure has been characterized. The discotic molecules based on dibranched oligoether side chains have been observed to self-organize into a well-ordered hexagonal columnar structure within liquid crystalline phases, which possessed an exceptionally high thermal stability and an unusually wide temperature range over >300 degrees C. We suggest that a combination of the large lateral dimensions of the rigid cores and disordered structure of the oxygen-containing branches tails is a driving force to the formation of a highly ordered columnar structure in the bulk state with enhanced molecular segregation. In contrast to the thermotropic phase behavior that favors the formation of highly ordered columnar aggregates through a strong stacking interaction, the hexabenzocoronene cores are packed in a face-on arrangement at the air/water interface and on solid surfaces with surface domains composed of an array of 7 x 7 molecules. We suggest a crablike molecular conformation and cluster-segregated monolayers with 6-fold symmetry and unusual face-on packing on a solid surface. Preliminary spectroscopic studies in the bulk state have shown that the molecules based on a hexaaromatic-substituted core may serve as functional supramolecular materials with high energy transfer characteristic within the columns due to near-perfect columnar ordering, which is unchanged over a wide temperature range. We believe that an absence of the crystallization phenomenon of side-branched oligoether chains is critical for the formation of long-range columnar ordering with strong intracolumnar correlation of conjugated disks important for high carrier mobility.

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PubMed2002

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.

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