Autonomic molecular transport by polymer films containing programmed chemical potential gradients.

Materials which induce molecular motion without external input offer unique opportunities for spatial manipulation of molecules. Here, we present the use of polyacrylamide hydrogel films containing built-in chemical gradients (enthalpic gradients) to direct molecular transport. Using a cationic tertiary amine gradient, anionic molecules were directionally transported up to several millimeters. A 40-fold concentration of anionic molecules dosed in aerosol form on a substrate to a small region at the center of a radially symmetric cationic gradient was observed. The separation of mixtures of charged dye molecules was demonstrated using a boronic acid-to-cationic gradient where one molecule was attracted to the boronic acid end of the gradient, and the other to the cationic end of the gradient. Theoretical and computational analysis provides a quantitative description of such anisotropic molecular transport, and reveals that the gradient-imposed drift velocity is in the range of hundreds of nanometers per second, comparable to the transport velocities of biomolecular motors. This general concept of enthalpy gradient-directed molecular transport should enable the autonomous processing of a diversity of chemical species.

Chemical gradients on graphene to drive droplet motion.

This work demonstrates the production of a well-controlled, chemical gradient on the surface of graphene. By inducing a gradient of oxygen functional groups, drops of water and dimethyl-methylphosphonate (a nerve agent simulant) are "pulled" in the direction of increasing oxygen content, while fluorine gradients "push" the droplet motion in the direction of decreasing fluorine content. The direction of motion is broadly attributed to increasing/decreasing hydrophilicity, which is correlated to high/low adhesion and binding energy. Such tunability in surface chemistry provides additional capabilities in device design for applications ranging from microfluidics to chemical sensing.

Effect of film thickness on the columnar packing structures of discotic supramolecules in thin films.

The effects of film thickness on the columnar packing structure of discotic supramolecules in a thin supported film have been investigated by grazing-incidence small-angle X-ray scattering technique using magnetically aligned cobalt octa(n-decylthio)porphyrazine (CoS10) films on octadecyltrichlorosilane (OTS)-functionalized substrates as model systems. Magnetically aligned CoS10 films with a range of film thicknesses (49-845 nm) form uniaxially oriented 'edge-on' columnar superstructures with their columnar directors perpendicular to the applied magnetic field. However, the orientational ordering of the columnar packing in the plane perpendicular to the applied magnetic field is strongly dependent on the film thickness. While being damped by the elasticity of the side chains of CoS10, the strong interfacial interaction at the film-substrate interface propagates up to 50-100 nm from the substrate, maintaining the orientation of columnar packing in the plane perpendicular to the applied magnetic field. When the distance from the film-substrate interface becomes larger than about 100 nm, symmetric tilting of columnar layer orientation, which saturates at 11.5 degrees , occurs due to longitudinal edge dislocations induced by accumulated elastic deformation.

perpendicular organization of macromolecules: synthesis and alignment studies of a soluble poly(iptycene).

We describe herein a polymeric material that prefers to align perpendicular to a stretch-aligned polymer host in the solid state. Poly(iptycene) poly-1 was synthesized from monomer 1 under hyperbaric techniques via a Diels-Alder polymerization. Polarized excitation spectra of the anthracene end groups in this material in a stretch-aligned, solution-cast poly(vinyl chloride) (PVC) film showed that the poly(iptycene) prefers to align normal (counter aspect ratio) to the stretching direction of the PVC. This is explained by a "threading" mechanism, whereby the PVC intercalates through the internal free volume presented by poly-1, similar to effects observed in small molecule iptycenes under similar conditions.

Cations M2(O2CtBu)4+, where M = Mo and W, and MoW(O2CtBu)4+. Theoretical, spectroscopic, and structural investigations.

With the aid of density function theory, the molecular and electronic structures of the molecules Mo2(O2CMe)4, MoW(O2CMe)4, and W2(O2CMe)4 and their single-electron oxidized radical cations have been determined; this includes calculated observables such as v(MM) and the delta --> delta* electronic transition energies. The calculated properties are compared with those for the corresponding pivalates, M2(O2CtBu)4 (M = Mo or W) and MoW(O2CtBu)4 and their radical cations prepared in situ by oxidation with Cp2FePF6. The EPR spectra of the radical cations are also reported. The EPR spectrum of the MoW(O2CtBu)4+ cation reveals that the unpaired electron is in a polarized MM delta orbital having 70% Mo and 30% W character. The MM stretching frequencies show good correlation with the MM bond lengths obtained from single-crystal X-ray diffraction studies of MoW(O2CtBu)4, W2(O2CtBu)4, and W2(O2CtBu)4+PF6- compounds, along with previously reported structures. These data provide benchmark parameters for valence trapped dicarboxylate bridged radical cations of the type [(tBuCO2)3M2]2(micro-O2C-X-CO2)+ (X = conjugated spacer).

9,10-Anthracene dicarboxylate bridged complexes with M2 quadruply bonded dimetal units: [[M2(O2CtBu)3]2(mu-9,10-An(CO2)2)], where M = Mo or W.

From the reactions between [M2(O2CtBu)4] and 9,10-anthracenedicarboxylic acid in toluene, the dicarboxylate bridged complexes [[M2(O2CtBu)3]2(mu-9,10An(CO2)2)], have been obtained as microcrystalline yellow (M = Mo) and red (M = W) powders. The powders are soluble in THF forming intense red (M = Mo) and green (M = W) solutions. The electronic absorption spectra in 2-MeTHF have been recorded as a function of temperature (2-298 K) and show a small bathochromic shift on cooling. The electronic structures have been investigated by molecular orbital calculations employing density functional theory on the model compounds [(HCO2)3M2]2(mu-9,10-An(CO2)2) where the M4 unit is constrained to lie in a plane. These reveal a minimum energy, gas-phase structure wherein the plane of the anthracene is twisted by ca. 54 degrees with respect to its 9,10-carboxylate units for both Mo and W. The results of these calculations are correlated with the electronic absorption spectral data and the electrochemical measurements (CV and DPV) of the first and second oxidation waves. The EPR spectra of the radical cations formed by single-electron oxidation with [Cp2Fe](+)[PF6]- in a THF-CH2Cl2 solvent mixture show that the complexes are valence trapped at ambient temperature on the EPR timescale. These results are discussed in the light of recent studies of dicarboxylate-linked MM quadruple bonds.

On the electron delocalization in the radical cations formed by oxidation of MM quadruple bonds linked by oxalate and perfluoroterephthalate bridges.

Electron paramagnetic resonance, electronic absorption, and resonance Raman spectroscopy reveal that in the oxalate-bridged compounds, [[(tBuCO2)3M2]2(mu-O2CCO2)]+[PF6]-, the unpaired electron is delocalized over four metal centers (M = Mo or W) as a result of M2 delta to bridge pi conjugation, but in the related cationic perfluoroterephthalate-bridged species, the tungsten complex is delocalized and the molybdenum analogue valence trapped.

Third-order nonlinear optical properties of complexes with MM triple and quadruple bonds (M = Mo, W) at 1064 nm by degenerate four-wave mixing.

Measurements of the third-order nonlinear optical responses of solutions of the metal-metal multiply bonded complexes Mo(2)(OPr(i))(6), W(2)(OBu(t))(6), M(2)(NMe(2))(6), M(2)(O(2)CBu(t))(4), and M(2)Cl(4)(PMe(3))(4) (M = Mo, W), using picosecond degenerate four-wave mixing at 1064 nm, are reported. These complexes display only very small instantaneous electronic polarizations when excited with cross-polarized beams. When the excitation beams are similarly polarized, a significant third-order optical response is detected, which is attributable to the formation of bulk thermal excitation gratings. Time-dependent measurements support this view.