Nam-Keun Oh

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.

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

Liquid-crystalline rod–coil polymers based on poly(ethylene oxide)s and the influence of the complexation of LiCF3SO3 on the liquid-crystalline assembly

The synthesis and characterization of rod–coil polymers of ethyl 4′-(4′-oxy-4-biphenylcarbonyloxy)-4-biphenylcarboxylate with polyethylene oxide of three (3–4), seven (7–4), twelve (12–4) and sixteen (16–4) ethylene oxide units, of 4′-(4′-oxy-4-phenylcarbonyloxy)-4-biphenylcarboxylate with poly(ethylene oxide) of sixteen ethylene oxide units (16–3), and of ethyl 4′-oxy-4-biphenylcarboxylate with poly (ethylene oxide) of sixteen ethylene oxide units (16–2) are described. All the rod-coil polymers except 16–2 display a layered smectic mesophase and, in particular, 16–4 shows a microphase-separated morphology. The complexes of 16–4 with up to 0.3 mol of LiCF3SO3 per mol of ethylene oxide units are also prepared. The rod-coil polymer 16–4 exhibits an enantiotropic smectic B (SB) mesophase. The complexes with 0.05 and 0.1 mol of LiCF3SO3, however, exhibit an enantiotropic smectic A (SA) mesophase in addition to an SB phase. In contrast with the complexes with 0.0–0.1 mol of LiCF3SO3, the complexes with 0.2 and 0.3 mol do not exhibit any smectic mesophases; however, they display a cylindrical micellar mesophase.

Hexagonal columnar liquid-crystalline phase from a rod–coil molecule

Liquid-crystalline behaviour of rod–coil molecules is normally associated with the formation of calamitic mesophases; by increasing the volume fraction of coil segments via substitution of poly(propylene oxide) with degree of polymerization of 12 to an elongated rod, the rod-like mesogen can be assembled into cylindrical micelles, showing a hexagonal columnar mesophase.

Self-Assembly of Rod-Coil Molecules into Cylindrical Supramolecular Architectures Through Ionic Interaction

Abstract The mesomorphic behaviors of two rod-coil molecules containing poly(ethylene oxide)s with degree of polymerization of 12 (12-4) and 16 (16-4) and their complexes with LiCF3SO3 are discussed. Both rod-coil molecules discussed in this paper exhibits a microphase separated lamellar crystalline phase as well as a layered smectic mesophase. The complexation with LiCF3SO3 induced a cylindrical micellar mesophase from a smectic phase of uncomplexed molecule. These results characterized by a combination of techniques by DSC, optical polarized microscopy and X-ray scattering experiments are discussed in terms of the change of the relative volume fraction of coil to rod segments through complexation with LiCF3SO3.

Tubular assembly of amphiphilic rigid macrocycle with flexible dendrons

An amphiphilic rigid macrocycle was shown to self-assemble into tubular aggregates that can solubilize SWNTs in aqueous solution through significant pi-pi interactions.

Thermotropic and lyotropic mesophase formation of poly(ethylene oxide) substituted rod-coil oligomer

SummaryThe synthesis and characterization of the rod-coil oligomer consisting of a molecular rod and a poly(ethylene oxide) with degree of polymerization of 12 (5) are presented. The rod-coil oligomer5 shows a crystalline phase with microphase segregation. However, the complexation of the rod-coil oligomer5 with LiCF3SO3 induces a thermotropic smectic liquid crystalline phase. In aqueous solution, the rodcoil oligomer5 shows various lyotropic mesophases such as lamellar, cubic and cylindrical micellar mesophases, depending on the oligomer concentration. These results characterized by differential scanning calorimetry and optical polarized microscopy are described.

Liquid Crystalline Assembly of Calamatic Mesogens and Rod-Coil Molecule by Pd and Ru Complexation

Abstract A series of cyanobiphenyl palladium complexed liquid crystals was synthesized. The Pd complexation of cyanobiphenyl mesogen results the enhancement of thermal stability and more ordered mesophase. π-Complexation of Cp*Ru(Cp*=C5Me5) moiety to the mesogenic group of rod-coil molecule shows the suppression of phase transition temperature without the change of the mesophase.

Organization of branched rod?coil molecules into a 3-D tetragonally perforated lamellar mesophaseElectronic supplementary information (ESI) available: synthetic details, polarized optical micrographs, and characterization of XRD. See http://www.rsc.org/suppdata/cc/b3/b317109d/

Tetramerization of coil-rod-coil ABC triblock copolymers to a tetrabranched molecule induces an unusual 3-D tetragonally perforated layered liquid crystalline phase as an intermediate structure between 1-D lamellar and 2-D hexagonal columnar phases.

Supramolecular assembly of fluorescent phasmidic diacetylene and its photopolymerization

The closed micellar triclinic structure of a fluorescent phasmidic diacetylene (PhDO) has been fixed by topochemical photopolymerization which induced fluorescence quenching to offer a novel method of image photopatterning.