Showing papers by "Virgil Percec published in 2003"

Giant Supramolecular Liquid Crystal Lattice

Abstract: Self-organized supramolecular organic nanostructures have potential applications that include molecular electronics, photonics, and precursors for nanoporous catalysts. Accordingly, understanding how self-assembly is controlled by molecular architecture will enable the design of increasingly complex structures. We report a liquid crystal (LC) phase with a tetragonal three-dimensional unit cell containing 30 globular supramolecular dendrimers, each of which is self-assembled from 12 dendron (tree-like) molecules, for the compounds described here. The present structure is one of the most complex LC phases yet discovered. A model explaining how spatial arrangement of self-assembled dendritic aggregates depends on molecular architecture and temperature is proposed.

Living radical polymerization of vinyl chloride initiated with iodoform and catalyzed by nascent Cu0/tris(2‐aminoethyl)amine or polyethyleneimine in water at 25 °C proceeds by a new competing pathways mechanism

Abstract: The first example of living radical polymerization of vinyl chloride carried out in water at 25 °C is reported. This polymerization was initiated by iodoform and catalyzed by nascent Cu0 produced by the disproportionation of CuI in the presence of strongly CuII binding ligands such as tris(2-aminoethyl)amine or polyethyleneimine. The resulting poly(vinyl chloride) was free of structural defects, had controlled molecular weight and narrow molecular weight distribution, contained two ∼CHClI active chain ends, and had a higher syndiotacticity (62%) than the one obtained by conventional free-radical polymerization at the same temperature (56%). This novel polymerization proceeds, most probably, by a combination of competitive pathways that involves activation by single electron transfer mediated by nascent Cu0 and degenerative chain transfer. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3283–3299, 2003

Universal Iterative Strategy for the Divergent Synthesis of Dendritic Macromolecules from Conventional Monomers by a Combination of Living Radical Polymerization and Irreversible TERminator Multifunctional INItiator (TERMINI)

Abstract: A new synthetic concept named TERMINI that stands for irreversible TERminator Multifunctional INItiator is reported. Suitable combinations of TERMINI and living polymerizations provide access to strategies for the design and synthesis of unprecedented complex molecular and macromolecular architectures from a diversity of commercial monomers. TERMINI represents a masked multifunctional initiator designed to quantitatively and irreversibly interrupt a chain organic reaction or a living polymerization. After demasking, the TERMINI repeat unit enables the quantitative reinitiation, in the presence or absence of a catalyst, of the same or a different living polymerization or a chain organic reaction in more than one direction, thus becoming a branching point. The demonstration of this concept was made by using a combination of metal-catalyzed living radical polymerization (LRP) and (1,1-dimethylethyl)[[1-[3,5-bis(S-phenyl 4-N,N‘-diethylthiocarbamate)phenyl]ethenyl]oxy]dimethylsilane as TERMINI, to elaborate a ...

Hierarchical self-assembly, coassembly, and self-organization of novel liquid crystalline lattices and superlattices from a twin-tapered dendritic benzamide and its four-cylinder-bundle supramolecular polymer.

Abstract: The synthesis and structural analysis of the twin-dendritic benzamide 10, based on the first-generation, self-assembling, tapered dendrons 3,4,5-tris(4′-dodecyloxybenzyloxy)benzoic acid and 3,4,5-tris(4′-dodecyloxybenzyloxy)-1-aminobenzene, and the polymethacrylate, 20, which contains 10 as side groups, are presented. Benzamide 10 self-assembles into a supramolecular cylindrical dendrimer that self-organizes into a columnar hexagonal (Φh) liquid crystalline (LC) phase. Polymer 20 self-assembles into an imperfect four-cylinder-bundle supramolecular dendrimer, and creates a giant vesicular supercylinder that self-organizes into a columnar nematic (Nc) LC phase which displays short-range hexagonal order. In mixtures of 20 and 10, 10 acts as a guest and 20 as a host to create a perfect four-cylinder-bundle host–guest supramolecular dendrimer that coorganizes with 10. A diversity of Φh, simple rectangular columnar (Φr-s) and centered rectangular columnar (Φr-c), superlattices are produced at different ratios between 20 and 10. This diversity of LC lattices and superlattices is facilitated by the architecture of the twin-dendritic building block, polymethacrylate, the host–guest supramolecular assembly, and by hydrogen bonding along the center of the supramolecular cylinders generated from 10 and 20.

Supramolecular assembly of dendritic polymers elucidated by 1H and 13C solid-state MAS NMR spectroscopy.

Abstract: Advanced solid-state NMR methods under fast magic-angle spinning (MAS) are used to study the structure and dynamics of large supramolecular systems, which consist of a polymer backbone with dendritic side groups and self-assemble into a columnar structure. The NMR experiments are performed on as-synthesized samples, i.e., no isotopic enrichment is required. The analysis of (1)H NMR chemical-shift effects as well as dipolar (1)H-(1)H or (1)H-(13)C couplings provide site-specific insight into the local structure and the segmental dynamics, in particular, of phenyl rings and -CH(2)O- linking units within the dendrons. Relative changes of (1)H chemical shifts (of up to -3 ppm) serve as distance constraints and allow protons to be positioned relative to aromatic rings. Together with dipolar spinning sideband patterns, pi-pi packing phenomena and local order parameters (showing variations between 30% and 100%) are selectively and precisely determined, enabling the identification of the dendron cores as the structure-directing moieties within the supramolecular architecture. The study is carried out over a representative selection of systems which reflect characteristic differences, such as different polymer backbones, sizes of dendritic side groups, or length and flexibility of linking units. While the polymer backbone is found to have virtually no effect on the overall structure and properties, the systems are sensitively affected by changing the generation or the linkage of the dendrons. The results help to understand the self-assembly process of dendritic moieties and aid the chemical design of self-organizing molecular structures.

Application of isomorphous replacement in the structure determination of a cubic liquid crystal phase and location of counterions.

Abstract: A second generation monodendron with dodecyl end-groups based on the AB3 monomer 3,4,5-trihydroxy benzoate has previously been shown to form a thermotropic cubic phase with Pm3n symmetry (Balagurusamy et al., J. Am. Chem. Soc. 1997, 119, 1539). A structure consisting of spherical “micelles” was proposed originally, but an alternative choice of structure factor phases, giving a structure of interlocked squashed columns, could not be ruled out by diffraction data on the original material alone. We have therefore synthesized two selectively fluorinated equivalent compounds, the carboxylic acid and its Rb salt, to be able to apply a variant of the isomorphous replacement crystallographic technique. On the basis of the electron density maps of the new labeled compounds, reconstructed using small-angle X-ray diffraction intensities, the interlocking columns model is unequivocally rejected and the spheres model is upheld. Furthermore, the location of the metal cation in the center of the “micelles” is directly ...

Charge transport in hexagonal columnar liquid crystals self-organized from supramolecular cylinders based on acene-functionalized dendrons

Abstract: We report measurements of time-of-flight charge carrier mobility and thermal optical polarized microscopy on a new class of hexagonal columnar (Φ h ) liquid crystals (LC) self-organized from supramolecular cylinders self-assembled from acene functionalized dendrons. We discuss the temperature and electric field-independent high hole mobility of 1-3.5×10 - 3 cm 2 /V s of fluorinated tapered first generation dendrons that contain pyrene and naphthalene groups in their core and self-assemble into supramolecular nanocylinders. The anomalous temperature behavior of the mobility is consistent with polaron transport mechanisms. Studies of time-of-flight transients, which reveal evidence for dynamic defects with lifetimes in the range of 10 - 6 s, are presented as aspects of charge generation and recombination kinetics.

Apparent tricritical behavior at a nearly second-order nematic-isotropic phase transition of a cyclic liquid crystalline trimer.

Abstract: The cyclic liquid crystalline trimer TPB-(c)9(3) was investigated by optical retardation and Freedericksz techniques within a few tens of millikelvins of the superheating limit of the nearly second-order nematic-isotropic phase transition. Both the optical retardation and the Freedericksz bend threshold voltage are in good agreement with tricritical behavior for the transition.

Fluorinated dendrons and self-organizing ultrahigh density nanocylinder compositions

Abstract: This invention relates to novel fluorinated dendrons and to a universal strategy for producing functional fluorinated dendrons programmed to self-assemble into supramolecular nanocylinder compositions containing pstacks of high electron or hole mobility donors (D), acceptors (A), or D-A complexes in the core. Such nanocylinder compositions are uniquely applicable to devices spanning from single supramolecule to nanoscopic and to macroscopic scales including transistors, photovoltaics, photoconductors, photorefractives, light emissives, and optoelectronics.

Grazing-incidence x-ray diffraction study of Langmuir films of amphiphilic monodendrons.

Abstract: We have used pressure-area isotherms and grazing-incidence x-ray diffraction to study structures of Langmuir films of first-generation monodendrons with two or three peripheral alkyl chains. Unlike the structures observed in their bulk liquid crystalline mesophases, these multichain monodendrons form either a centered rectangular lattice with molecular axes tilted toward nearest neighbors or an oblique lattice with molecular axes tilted in low-symmetry directions.

Chemistry: catalyst hunt accelerates.

Abstract: High-throughput methods of synthesis and analysis are streamlining the search for new catalysts, reducing the timescale from years to days. A new class of polymerization catalysts is the latest discovery.

Fast Photoconductivity in Self-Assembled Columnar Liquid Crystalline Photorefractive Materials

Abstract: : We have elaborated a novel strategy for the self-assembly and co-assembly of functional supramolecular columns that self-organize in a hexagonal columnar liquid crystalline phase forming high carrier mobility organic semiconductor materials for applications in photorefractive and photovoltaic devices. Among our accomplishments are, (1) synthesis and elucidation of the structure of self-organizing dendron materials into a helical superstructure supporting an axial transport channel suggesting that structure and function can be separately tailored, (2) demonstration of high mobility carrier transport of nearly 10-2 cm2/Vs, (3) synthesis of robust glass forming self-organizing dendron materials, (4) control of ionization potential and electron affinity by synthetic methods using various donor and acceptor moeities, (5) demonstration of the photorefractive effect in appropriate composites, (6) demonstration of the photovoltaic effect in a nanostructured inorganic/organic composite cell, (7) elucidation of carrier generation and transport mechanisms in columnar and smectic liquid crystalline phases, and (8) high mobility carrier transport in polymer networks. Such arrays are applicable to devices spanning from single supramolecule to nanoscopic and to macroscopic scales including transistors, photovoltaics, photoconductors, photorefractives, light emissives, and optoelectronics.

Electronic transport in self-organizing columnar phases

Abstract: This paper presents measurements of time-of-flight charge carrier mobility and thermal optical polarized microscopy on a new class of columnar liquid crystals (LC) self-organized into hexagonal columnar mesophase self-assembled form functionalized dendrons. We discuss the temperature and electric field-independent high hole mobility of 1-3.5×10 -3 cm 2 /Vs. The anomalous temperature behavior of the mobility is consistent with polaron transport mechanisms. Studies of time-of-flight transients, which reveal evidence for dynamic defects with lifetimes in the range of 10 -6 s, are presented as aspects of charge gneeration and recombination kinetics. We demonstrate the enhancement of electron transport in acene-based dendron LCs by dopign wtih TNF-based dendron molecules.