Dendron Rodcoils Research Articles

Twist Ferrocene Wires from Self-Assembly of Chiral Rod-Coil Organometallics

Inspired by research on organometallics, the incorporation of transition metals into organic derivatives through covalent attachment has become a mature field. Because of improvements in synthesis, the ferrocene-based moiety is now a well-known organometallic moiety. Tunable ferrocene residues provide an attractive method to expand the range of properties. By taking advantage of self-assembly, various self-assembled organometallic morphologies including spheres, cylinders, vesicles, and lamellae with nanoscale iron-rich microdomains can be used in applications such as switches, transistors, and sensors for integration into microelectronic technology. Also, ordered nanostructures transform into semiconducting materials after oxidation and yield iron-rich magnetic ceramics after pyrolysis treatment. Helical morphologies can be obtained from the self-assembly of chiral supramolecules such as chiral liquid crystals, folded oligomers, chiral counterions, chiral dendrimers, dendron rod–coils, chiral homopolymers, chiral block copolymers, and chiral discotic molecules. In our previous studies, the self-assembly of a series of chiral Schiff based rod–coil amphiphiles was systematically examined. The self-assembled helical superstructures could be induced by chiral sugar, and a morphological transformation from helical to platelet-like morphology was driven by simply increasing the hydrophobic chain length at the chain end of the chiral rod–coil amphiphiles. The helical morphology results from the steric-hindrance effect of the chiral sugar packing in the self-assembly. Herein we aim to create iron-rich spiral superstructures by taking advantage of the self-assembly of chiral Schiff based rod–coil molecules that are end-capped with ferrocene (namely, chiral rod–coil organometallics). Figure 1

Preface

Preface

The Molecular Basis of Self‐Assembly of Dendron–Rod–Coils into One‐Dimensional Nanostructures

We describe here a comprehensive study of solution and solid-state properties of self-assembling triblock molecules composed of a hydrophilic dendron covalently linked to an aromatic rigid rod segment, which is in turn connected to a hydrophobic flexible coil. These dendron-rod-coil (DRC) molecules form well-defined supramolecular structures that possess a ribbonlike morphology as revealed by transmission-electron and atomic-force microscopy. In a large variety of aprotic solvents, the DRC ribbons create stable networks that form gels at concentrations as low as 0.2% by weight DRC. The gels are thermally irreversible and do not melt at elevated temperatures, indicating high stability as a result of strong noncovalent interactions among DRC molecules. NMR experiments show that the strong interactions leading to aggregation involve mainly the dendron and rodlike blocks, whereas oligoisoprene coil segments remain solvated after gelation. Small-angle X-ray scattering (SAXS) profiles of different DRC molecules demonstrate an excellent correlation between the degree-of-order in the solid-state and the stability of gels. Studies on two series of analogous molecules suggest that self-assembly is very sensitive to subtle structural changes and requires the presence of at least four hydroxyl groups in the dendron, two biphenyl units in the rod, and a coil segment with a size comparable to that of the rodlike block. A detailed analysis of crystal structures of model compounds revealed the formation of stable one-dimensional structures that involve two types of noncovalent interactions, aromatic pi-pi stacking and hydrogen bonding. Most importantly, the crystal structure of the rod-dendron compound shows that hydrogen bonding not only drives the formation of head-to-head cyclic structures, but also generates multiple linkages between them along the stacking direction. The cyclic structures are tetrameric in nature and stack into ribbonlike objects. We believe that DRC molecules utilize the same arrangement of hydrogen bonds and stacking of aromatic blocks observed in the crystals, explaining the exceptional stability of the nanostructures in extremely dilute solutions as well the thermal stability of the gels they form. This study provides mechanistic insights on self-assembly of triblock molecules, and unveils general strategies to create well-defined one-dimensional supramolecular objects.

Cover Picture: The Molecular Basis of Self‐Assembly of Dendron–Rod–Coils into One‐Dimensional Nanostructures (Chem. Eur. J. 28/2006)

Supramolecular ribbons are formed by the self‐assembly of dendron–rod–coil (DRC) triblock molecules. In their Full Paper on page 7313 ff., S. I. Stupp et al. describe the properties and structural characterization of such molecules. The study provides mechanistic insights and unveils general strategies to create well‐defined one‐dimensional supramolecular objects.

Cover Picture: Structural Modifications to Polystyrene via Self-Assembling Molecules (Adv. Funct. Mater. 3/2005)

The cover shows tensile failure of a sample of pure polystyrene (left), and a polystyrene sample with greater impact strength containing 1% by weight of dispersed nanoribbons (right), as reported in work by Stupp and co-workers on p. 487. The nanoribbons are formed by self-assembly of molecules known as dendron rodcoils (DRCs) in styrene monomer, resulting in the formation of a gel. This gel can then be polymerized thermally. We have previously reported that small quantities of self-assembling molecules known as dendron rodcoils (DRCs) can be used as supramolecular additives to modify the properties of polystyrene (PS). These molecules spontaneously assemble into supramolecular nanoribbons that can be incorporated into bulk PS in such a way that the orientation of the polymer is significantly enhanced when mechanically drawn above the glass-transition temperature. In the current study, we more closely evaluate the structural role of the DRC nanoribbons in PS by investigating the mechanical properties and deformation microstructures of polymers modified by self-assembly. In comparision to PS homopolymer, PS containing small amounts (≤ 1.0 wt.-%) of self-assembling DRC molecules exhibit greater Charpy impact strengths in double-notch four-point bending and significantly greater elongations to failure in uniaxial tension at 250 % prestrain. Although the DRC-modified polymer shows significantly smaller elongations to failure at 1000 % prestrain, both low- and high-prestrain specimens maintain tensile strengths that are comparable to those of the homopolymer. The improved toughness and ductility of DRC-modified PS appears to be related to the increased stress whitening and craze density that was observed near fracture surfaces. However, the mechanism by which the self-assembling DRC molecules toughen PS is different from that of conventional additives. These molecules assemble into supramolecular nanoribbons that enhance polymer orientation, which in turn modifies crazing patterns and improves impact strength and ductility.

Structural Modifications to Polystyrene via Self-Assembling Molecules

We have previously reported that small quantities of self-assembling molecules known as dendron rodcoils (DRCs) can be used as supramolecular additives to modify the properties of polystyrene (PS). These molecules spontaneously assemble into supramolecular nanoribbons that can be incorporated into bulk PS in such a way that the orientation of the polymer is significantly enhanced when mechanically drawn above the glass-transition temperature. In the current study, we more closely evaluate the structural role of the DRC nanoribbons in PS by investigating the mechanical properties and deformation microstructures of polymers modified by self-assembly. In comparision to PS homopolymer, PS containing small amounts (≤ 1.0 wt.-%) of self-assembling DRC molecules exhibit greater Charpy impact strengths in double-notch four-point bending and significantly greater elongations to failure in uniaxial tension at 250 % prestrain. Although the DRC-modified polymer shows significantly smaller elongations to failure at 1000 % prestrain, both low- and high-prestrain specimens maintain tensile strengths that are comparable to those of the homopolymer. The improved toughness and ductility of DRC-modified PS appears to be related to the increased stress whitening and craze density that was observed near fracture surfaces. However, the mechanism by which the self-assembling DRC molecules toughen PS is different from that of conventional additives. These molecules assemble into supramolecular nanoribbons that enhance polymer orientation, which in turn modifies crazing patterns and improves impact strength and ductility.

Synthesis, Self-Assembly, and Characterization of Supramolecular Polymers from Electroactive Dendron Rodcoil Molecules

We report here the synthesis and self-assembly of a series of three molecules with dendron rodcoil architecture that contain conjugated segments of oligo(thiophene), oligo(phenylene-vinylene), and oligo(phenylene). Despite their structural differences, all three molecules yield similar self-assembled structures. Electron and atomic force microscopy reveals the self-assembly of the molecules into high aspect ratio ribbon-like nanostructures which at low concentrations induce gelation in nonpolar solvent. Self-assembly results in a blue-shifted absorption spectrum and a red-shifted, quenched fluorescence spectrum, indicating aggregation of the conjugated segments within the ribbon-like structures. The assembly of these molecules into one-dimensional nanostructures is a route to pi-pi stacked supramolecular polymers for organic electronic functions. In the oligo(thiophene) derivative, self-assembly leads to a 3 orders of magnitude increase in the conductivity of iodine-doped films due to self-assembly. We also found that electric field alignment of these supramolecular assemblies can be used to create arrays of self-assembled nanowires on a device substrate.

Dendron rodcoils: synthesis of novel organic hybrid structures.

Convergent and divergent syntheses of novel organic hybrid structures termed dendron rodcoils (DRC) containing dendritic, rodlike, and coillike segments are described. The aryl ester dendron masked with 32 trifluoromethyl groups is prepared via a convergent approach using 5-(tert-butyldimethylsiloxy)isophthalic acid as the monomer unit. The activation of the focal point of the dendron allows for successful coupling between the dendron and the diblock rodcoil molecules synthesized separately. In another example, the dendritic block is grown via divergent strategy from the terminus of rodcoil using 3,5-bis(tert-butyldimethylsiloxy)benzoic acid as an AB(2) monomer. A combination of catalyzed esterification reactions and silyl deprotection chemistry proved to be a very efficient method for construction of these nanosized structures with unusual molecular architecture. Both synthetic strategies allowed for the preparation of DRCs with nearly monodisperse dendritic blocks as demonstrated by NMR, MALDI-TOF, and GPC measurements.

Scaffolding of Polymers by Supramolecular Nanoribbons

Supramolecular nanoribbons formed by dendron rod–coil molecules show ordered birefringence! The authors describe here how self-assembling block molecules containing dendritic, rodlike, and flexible segments can cause certain monomers to gel into ordered birefringent media. As demonstrated in the Figure even an amorphous polymer such as polystyrene (left) can be transformed into a birefringent solid (right).

Self-assembly of dendron rodcoil molecules into nanoribbons.

One of the expectations in science is the discovery of materials with surprising properties or functionality based on designed molecules that self-order or fold. The focus on self-assembly throughout the 90s has generated very useful knowledge toward this expectation and many prospects are on the horizon.1-6 Learning how to control the dimensionality and shape of selfassembled structures through molecular design remains a challenge. We report here on the self-assembly of molecules 1 which we refer to as dendron rodcoils (DRC) because of their blocked covalent architecture consisting of coil-like, rodlike, and dendritic segments. These molecules are synthesized in 15 steps with an