Jean M. J. Fréchet

Molecular-weight-dependent mobilities in regioregular poly(3-hexyl-thiophene) diodes

We have investigated the transport properties in the direction perpendicular to the substrate of regioregular poly(3-hexyl-thiophene) of different molecular weights (MW) in a diode geometry. In these devices, which exhibit space-charge-limited behavior, we find that the mobility values at room temperature increase from 1.33×10−5cm2∕Vsto3.30×10−4cm2∕Vs as the MW is increased from 2.9to31.1kg∕mol. The mobility is found to be field independent for high MW films, but field dependent for the low MW films. The current–voltage characteristics of the diodes are also studied as a function of temperature from 160Kto300K. The activation energy for carrier transport, extracted from the Arrhenius plot, is found to decrease gradually from 143meVto126meV as the MW is increased.

Dendrimers and Hyperbranched Polymers: Two Families of Three-Dimensional Macromolecules with Similar but Clearly Distinct Properties

Abstract Dendrimers and hyperbranched polymers are globular macromolecules that are characterized both by a highly branched structure, in which all bonds converge to a focal point or core, and a multiplicity of reactive chain-ends. Because of the obvious similarity of their building blocks, many assume that the properties of these two families of dendritic macromolecules are almost identical and that the terms “dendrimer” and “hyperbranched polymer” can be used interchangeably. This assumption is incorrect because only regular dendrimers have a precise end-group multiplicity and functionality and exhibit properties that are totally unlike those of all other families of macromolecules. For example, regular dendrimers display a maximum in the relationship between their intrinsic the applications that are contemplated for the dendritic polymers. Using living systems as a model, one must anticipate that the regular placement of reactive groups at a precise location, as opposed to throughout a structure, has g...

Printable polythiophene gas sensor array for low-cost electronic noses

A route for generating arrays of printable polythiophene-based gas sensor materials suitable for low-cost manufacturing is demonstrated. Materials with complementary sensor responses are synthesized by incorporating functional groups into the molecule, either along the polymer backbone or as end-capping groups. Using these materials as printable sensor inks, a functional, integrated gas sensor array chip is fabricated using additive deposition techniques. The sensor array shows sensitivity to a range of volatile organic compounds down to concentrations of 10ppm. A three-terminal thin film transistor structure is used, allowing the extraction of multiple parameters that help to elucidate the mechanisms responsible for sensor response and the role of the functional groups in this response.

Two-photon degradable supramolecular assemblies of linear-dendritic copolymers

Micelles of dendritic-linear copolymers have been developed to release a payload after infrared stimulus.

Dendritic fullerenes; a new approach to polymer modification of C60

A new route for the preparation of polymer modified fullerences by the cycloaddition reaction of azides is demonstrated and the physical properties of the dendritic fullerence obtained show influences from both the parent dendrimer and C60

Porous polymer monoliths: an alternative to classical beads.

Porous polymer monoliths are a new category of materials developed during the last decade. These materials are prepared using a simple molding process carried out within the confines of a closed mold. Polymerization of a mixture that typically contains monomers, free-radical initiator, and porogenic solvent affords macroporous materials with large through-pores that enable flow-through applications. The versatility of the preparation technique is demonstrated by its use with hydrophobic, hydrophilic, ionizable, and zwitterionic monomers. The porous properties of the monolith can be controlled over a broad range. These, in turn, determine the hydrodynamic properties of the devices that contain the molded media. Since all the mobile phase must flow through the monolith, the mass transport within the molded material is dominated very much by convection, and the monolithic devices perform well even at very high flow rates. The applications of monolithic materials are demonstrated on the chromatographic separation of biological compounds and synthetic polymers, electrochromatography, gas chromatography, enzyme immobilization, molecular recognition, and in advanced detection systems. Grafting of the pore walls with selected polymers leads to materials with completely changed surface chemistries.

Polarity‐Directed One‐Pot Asymmetric Cascade Reactions Mediated by Two Catalysts in an Aqueous Buffer

Modern organic synthesis predominately relies on functional group reactivity differences to achieve the chemoselective formation of desired products. Herein we report a one-pot multistep asymmetric catalytic reaction in which substrates with similar chemical reactivities are differentiated on the basis of polarity. The one-pot reaction involves two catalysts and three substrates in the presence of water. The reaction mixture consists of two phases: a polar aqueous phase and a hydrophobic organic phase. The biphasic nature of the reaction medium and the polarity properties of the substrates and catalysts enable the selective formation of a major product instead of a statistical mixture of four possible products. We chose a two-step reaction involving condensation and subsequent conjugate addition as a model to develop a polarity-directed cascade reaction. Both reaction steps can involve linear aliphatic aldehydes as substrates (Scheme 1). Our aim was to combine the two reaction

Micromachined porous polymer for bubble free electro-osmotic pump

A novel porous polymer was microfabricated to serve as a porous plug for a new device, the porous plug electro-osmotic pump (pp-EOP). The plug eliminates any back pressure effects while enhances electro-osmotic flow in a channel. The pp-EOP was batch fabricated by surface micromachining on top of a silicon wafer. The pp-EOP device is driven by a periodic, zero-average injected current signal at low frequencies producing bubble-free electro-osmotic flow with reversible net movement. Testing of the device produced an average water-air interface velocity of 1.8 /spl mu/m/s at 0.8 Hz. The velocity was increased to 4.8 and to 13.9 /spl mu/m/s by necking the channel size.

Surface anchoring and dynamics of thiolated azobenzene molecules on Au(111)

We have investigated the temperature-dependent behavior of thiolated azobenzene molecules on Au(111) using scanning tunneling microscopy. The addition of a thiol functional group to azobenzene molecules leads to increased surface anchoring of single azobenzene molecules to gold. Thiolated azobenzene shows diverse surface morphology and does not form well-ordered structures at low coverage. At elevated temperatures, anchored molecules are observed to spin in place via hindered rotation. By measuring the number of rotating molecules as a function of temperature and using a simple model, we are able to estimate the energy barrier and attempt frequency for thermally induced hindered rotation to be 102+/-3 meV and 110+/-2 GHz, respectively.

Single-monolayer inkjetted oligothiophene organic TFTs exhibiting high performance and low leakage

Through the use of a novel oligothiophene precursor, we have demonstrated organic TFTs exhibiting relatively high mobility while simultaneously retaining ultra-low leakage and excellent on-off ratios. The unique tendency of this material to self-assemble into a crystalline morphology allows non-uniform printed droplets to reorganize into high-quality monolayers. The resulting structure provides excellent electrostatic characteristics, ideal for low power analog applications.