Tushar K. Bera

Self-organization of supramolecular helical dendrimers into complex electronic materials

The discovery of electrically conducting organic crystals and polymers has widened the range of potential optoelectronic materials, provided these exhibit sufficiently high charge carrier mobilities and are easy to make and process. Organic single crystals have high charge carrier mobilities but are usually impractical, whereas polymers have good processability but low mobilities. Liquid crystals exhibit mobilities approaching those of single crystals and are suitable for applications, but demanding fabrication and processing methods limit their use. Here we show that the self-assembly of fluorinated tapered dendrons can drive the formation of supramolecular liquid crystals with promising optoelectronic properties from a wide range of organic materials. We find that attaching conducting organic donor or acceptor groups to the apex of the dendrons leads to supramolecular nanometre-scale columns that contain in their cores π-stacks of donors, acceptors or donor–acceptor complexes exhibiting high charge carrier mobilities. When we use functionalized dendrons and amorphous polymers carrying compatible side groups, these co-assemble so that the polymer is incorporated in the centre of the columns through donor–acceptor interactions and exhibits enhanced charge carrier mobilities. We anticipate that this simple and versatile strategy for producing conductive π-stacks of aromatic groups, surrounded by helical dendrons, will lead to a new class of supramolecular materials suitable for electronic and optoelectronic applications.

Designing functional aromatic multisulfonyl chloride initiators for complex organic synthesis by living radical polymerization

The similarities and differences between sulfonyl chloride and alkyl halide initiators for metal-catalyzed living radical polymerizations are discussed. The differences in the rates of formation, reactivities, and reactions of primary radicals derived from sulfonyl halides and alkyl halides demonstrated the design principles for monosulfonyl and multisulfonyl chlorides that provided quantitative initiation and higher rates of initiation than of propagation. Multifunctional initiators with two, three, four, six, and eight sulfonyl chloride groups that produced perfect star polymers in 95% conversions were designed and synthesized on the basis of these principles.

CuI and CuII salts of group VIA elements as catalysts for living radical polymerization initiated with sulfonyl chlorides

ABSTRACT: The living radical polymerization of methyl methacrylate initiated fromsulfonyl chlorides and catalyzed by the new catalytic systems Cu 2 Y/Bpy and CuY/Bpy,where Y is O, S, Se, or Te and Bpy is 2,29-bipyridine, is described. An induction time wasobserved in all polymerization experiments. The values of the experimental rate con-stants of polymerization ( k p exp ) increased whereas the corresponding induction timesdecreased in the order Y 5 O , S , Se , Te. For the entire series of catalysts, k p exp forCuY was less than k p exp for Cu 2 Y. A mechanistic interpretation that involves the in situ generation of the CuCl/CuCl 2 pair, starting from Cu 2 Y or CuY, is provided.

Cell membrane as a model for the design of semifluorinated ion-selective nanostructured supramolecular systems

Abstract The synthesis and polymerization of two AB3 tapered, self-assembling methacrylate monomers ( 12 and 13 ) based on a first generation semifluorinated alkyl-substituted monodendron (i.e. minidendron) are described. These monomers contain either a benzo-15-crown-5 derivative, which is incorporated via the esterification of 4′-hydroxymethyl-1,4,7,10,13-pentaoxabenzocyclopentadecane or a podant group, which is incorporated via the monoesterification of tetraethylene glycol at its focal point, and both bear polymerizable groups on their periphery. The corresponding polymers ( 14–16 ) self-assemble and subsequently self-organize into supramolecular networks, which form a 2-D hexagonal columnar lattice via the fluorophobic effect. These networks consist of a continuous phase, which is based on the semifluorinated, paraffinic barrier layer and which is perforated in a hexagonal array by ion-selective or ion-active channels constructed from the benzo crown-ether and tetraethylene glycol respectively. The replacement of alkyl groups from the periphery of the tapered monodendrons with semifluorinated ones induces the self-assembly and, subsequently, the self-organization into supramolecular networks. These networks form 2-D hexagonal columnar lattices with thermal stability in some cases of up to 110°C. The design of such functional supramolecular systems was inspired by the bilayer fluid mosaic structure of the cell membrane. The lipid bilayer of the cell membrane, which in its ordered state acts as a barrier to the passage of polar molecules, was replaced with the semifluorinated paraffinic barrier, while the protein-based ionic channels were replaced with benzo-15-crown-5 and tetraethylene glycol based channels. The resulting supramolecular material has the mechanical integrity required for both ion-active and ion-selective nanostructured supramolecular systems.

Erratum: Self-organization of supramolecular helical dendrimers into complex electronic materials (Nature (2002) 419 (384-387))

This corrects the article DOI: nature01072

Electronic transport in self-organizing columnar phases

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 cm2/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.