Wang-Cheol Zin

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.

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.

Structure and phase behaviour of polyazomethines having flexible (n-alkyloxy)methyl side chains

Abstract Structure and phase behaviour of a series of thermotropic polyazomethines having ( n -alkyloxy)methyl side chains (-CH 2 O- n -C m H 2m+1 , m = 4, 6, 8, 12) have been studied by wide-angle X-ray scattering, differential scanning calorimetry (d.s.c.) and polarizing optical microscopy. In d.s.c. thermograms the samples showed basically three transitions. The first transitions were ascribed to solid-solid transitions resulting from a large increase in disorder of the side chains. The second ones were assigned to crystal-mesophase transitions arising from melting of the side chain crystals. The third ones were attributed to mesophase-isotropic melt transitions. All the samples showed layered structures in the mesophase as well as in the crystalline phase, in which the side chains were fully interdigitated. In the crystalline phase the repeat units of rigid main chain were found not to align parallel with those of neighbouring main chains, but slipped by 49°. In the mesophase the side chain crystals were molten and the layer spacings increased proportionally with increasing side chain length. The structural regularity in the slipped repeat unit disappeared in the polymers bearing long side chains, whereas it was retained in those having short side chains. A unidirectional shearing made the layer plane align parallel on the surface.

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.

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.

Nanoscale Organization of Conjugated Rods in Rod–Coil Molecules

An important challenge in the preparation of self-assembling materials is the control of supramolecular structure with well-defined shape and size, which has potential implications both fundamentally and practically in areas such as materials science, molecular electronics, and biomimetic chemistry. A typical example of a self-assembling system is provided by rod±coil molecules consisting of stiff rod and flexible coil segments. The repulsion between the covalently connected rod and coil segments leads to self-organization into a variety of supramolecular structures whose shape and size are determined by the relative volume fraction of the rod block. Recent observations from our laboratory have shown that both rod±coil diblock molecules and rod±coil multiblock copolymers containing poly(propylene oxide) as a coil segment self-assemble into layered smectic, bicontinuous cubic, and hexagonal columnar liquid-crystalline superlattices as the coil segment of the molecule increases in length relative to the rod segment. In a preliminary communication, we have demonstrated that introduction of a hydrophobic docosyl chain into the rod±coil diblock molecule based on the hydrophilic poly(ethylene oxide) coil gives rise to the formation of a discrete micellar phase with a lack of three-dimensional (3D) symmetry. In a more recent publication, rod±coil ABA triblock molecules based on poly(propylene oxide) as the coil segment were proved to self-assemble into discrete rod bundles that organize into a 3D tetragonal structure. Our approach to controlling supramolecular architecture can be extended to rod±coil systems based on a conjugated rod, which create a novel class of self-assembling materials with unique optical and electronic properties. As a result of great interest in the optically and electronically active properties of highly conjugated and stiff rod-like molecules, a variety of oligomers and polymers have been synthesized to establish the molecular structure±property relationship. Recently, however, supramolecular structure as well as chain structure has been reported to have a dramatic effect on the physical properties of conjugated molecules. Thus, manipulation of supramolecular structure in conjugated molecules is of paramount importance in achieving efficient physical properties in solid-state molecular materials. A strategy to manipulate the supramolecular structure may be accessible by incorporation of the conjugated rod into a rod±coil architecture, which would allow formation of well-defined electronically and optically active 2D sheet-like, strip-like, and hockey puck cylinder-like domains in nanoscale dimensions. It is in this context that we have synthesized coil±rod±coil triblock molecules consisting of three biphenyls connected through vinylene linkages as a conjugated rod segment and poly(propylene oxide) (PPO) as the coil segment. The synthesis of coil±rod±coil molecules with a variety of coil lengths was performed as outlined in Scheme 1. Monophenylbenzyl alcohol-terminated PPOs 1±5 were prepared from the reaction of

Micropatterning of a single layer of nanoparticles by lithographical methods with diblock copolymer micelles

We have demonstrated micropatterning of a single layer of nanoparticles by combining a self-assembly of diblock copolymer micelles with conventional and soft lithographical methods. On a photoresist micropattern fabricated by conventional photolithography, a single layer of diblock copolymer micelles containing precursors of nanoparticles was spin-coated. By plasma and lift-off processes, nanoparticles with the preservation of a pseudo-hexagonal order of micelles were synthesized in the micropattern without their aggregation. In addition, soft lithography of the microcontact printing technique was combined with the process of diblock copolymer micelles to produce micropatterns of nanoparticles. As an ink of the microcontact printing process, a single-layered film of diblock copolymer micelles was spin-coated directly onto the stamp and then transferred on the substrate by stamping. Formation of a single layer of nanoparticles in the micropattern was carried out by plasma treatment.

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.