Whence Molecular Electronics? Flood, Amar H.; Stoddart, J. Fraser; Steuerman, David W. ...
Science (American Association for the Advancement of Science),
12/2004, Letnik:
306, Številka:
5704
Journal Article
Recenzirano
The drive toward further miniaturization of silicon-based electronics has led to a revival of efforts to build devices with molecular-scale components. Here, Flood et al present the reports of ...passive molecular electronic devices, such as tunnel junctions and rectifiers, which have withstood scientific scrutiny. Such devices could show electron tunneling or one-way flow of current through the molecule.
Starched Carbon Nanotubes Star, Alexander; Steuerman, David W.; Heath, James R. ...
Angewandte Chemie (International ed.),
July 15, 2002, Letnik:
41, Številka:
14
Journal Article
Recenzirano
Common‐or‐garden starch can render single‐walled carbon nanotubes (SWNTs) readily soluble in water. The secret is to preorganize the linear amylose component in the starch into a helix with iodine ...prior to bringing the SWNTs on the scene. The SWNTs displace the iodine molecules in a “pea‐shooting” type of mechanism (see scheme). After some physical cajoling of the aqueous solution containing the starch–SWNT complex, a fine “bucky paper” is formed. Spitting in the aqueous solution, followed by sitting around for a few hours, also enables equally fine “bucky paper” to be harvested.
We report on the kinetics and ground‐state thermodynamics associated with electrochemically driven molecular mechanical switching of three bistable 2rotaxanes in acetonitrile solution, polymer ...electrolyte gels, and molecular‐switch tunnel junctions (MSTJs). For all rotaxanes a π‐electron‐deficient cyclobis(paraquat‐p‐phenylene) (CBPQT4+) ring component encircles one of two recognition sites within a dumbbell component. Two rotaxanes (RATTF4+ and RTTF4+) contain tetrathiafulvalene (TTF) and 1,5‐dioxynaphthalene (DNP) recognition units, but different hydrophilic stoppers. For these rotaxanes, the CBPQT4+ ring encircles predominantly (>90 %) the TTF unit at equilibrium, and this equilibrium is relatively temperature independent. In the third rotaxane (RBPTTF4+), the TTF unit is replaced by a π‐extended analogue (a bispyrrolotetrathiafulvalene (BPTTF) unit), and the CBPQT4+ ring encircles almost equally both recognition sites at equilibrium. This equilibrium exhibits strong temperature dependence. These thermodynamic differences were rationalized by reference to binding constants obtained by isothermal titration calorimetry for the complexation of model guests by the CBPQT4+ host in acetonitrile. For all bistable rotaxanes, oxidation of the TTF (BPTTF) unit is accompanied by movement of the CBPQT4+ ring to the DNP site. Reduction back to TTF0 (BPTTF0) is followed by relaxation to the equilibrium distribution of translational isomers. The relaxation kinetics are strongly environmentally dependent, yet consistent with a single electromechanical‐switching mechanism in acetonitrile, polymer electrolyte gels, and MSTJs. The ground‐state equilibrium properties of all three bistable 2rotaxanes were reflective of molecular structure in all environments. These results provide direct evidence for the control by molecular structure of the electronic properties exhibited by the MSTJs.
Thermodynamics to the rescue: the nuances of a molecular switch, perched in its lower‐conducting ground state on the brink between being ON and OFF, and after WRITING in its fully ON higher‐conducting metastable state, lend credence to the postulated mechanism of action of such switches. No surprises: the amplitude of switching in a molecular‐switch tunnel junction is controlled by a finely balanced bistable rotaxane (B), fulfilling the role of such an indecisive molecular switch that is extremely sensitive (A) to changes in temperature on READING.
You only need eyes to appreciate the color change that occurs in a polymer matrix when the bistable rotaxane shown is switched between its ground‐state (green) and metastable‐state (red) ...co‐conformers. Not only is an electrochromic device within reach, but a universal switching mechanism seems to be on the cards.
The influences of different physical environments on the thermodynamics associated with one key step in the switching mechanism for a pair of bistable catenanes and a pair of bistable rotaxanes have ...been investigated systematically. The two bistable catenanes are comprised of a cyclobis(paraquat‐p‐phenylene) (CBPQT4+) ring, or its diazapyrenium‐containing analogue, that are interlocked with a macrocyclic polyether component that incorporates the strong tetrathiafulvalene (TTF) donor unit and the weaker 1,5‐dioxynaphthalene (DNP) donor unit. The two bistable rotaxanes are comprised of a CBPQT4+ ring, interlocked with a dumbbell component in which one incorporates TTF and DNP units, whereas the other incorporates a monopyrrolotetrathiafulvalene (MPTTF) donor and a DNP unit. Two consecutive cycles of a variable scan rate cyclic voltammogram (10–1500 mV s−1) performed on all of the bistable switches (∼1 mM) in MeCN electrolyte solutions (0.1 M tetrabutylammonium hexafluorophosphate) across a range of temperatures (258–303 K) were recorded in a temperature‐controlled electrochemical cell. The second cycle showed different intensities of the two features that were observed in the first cycle when the cyclic voltammetry was recorded at fast scan rates and low temperatures. The first oxidation peak increases in intensity, concomitant with a decrease in the intensity of the second oxidation peak. This variation changed systematically with scan rate and temperature and has been assigned to the molecular mechanical movements within the catenanes and rotaxanes of the CBPQT4+ ring from the DNP to the TTF unit. The intensities of each peak were assigned to the populations of each co‐conformation, and the scan‐rate variation of each population was analyzed to obtain kinetic and thermodynamic data for the movement of the CBPQT4+ ring. The Gibbs free energy of activation at 298 K for the thermally activated movement was calculated to be 16.2 kcal mol−1 for the rotaxane, and 16.7 and 19.2 kcal mol−1 for the bipyridinium‐ and diazapyrenium‐based bistable catenanes, respectively. These values differ from those obtained for the shuttling and circumrotational motions of degenerate rotaxanes and catenanes, respectively, indicating that the detailed chemical structure influences the rates of movement. In all cases, when the same bistable compounds were characterized in an electrolyte gel, the molecular mechanical motion slowed down significantly, concomitant with an increase in the activation barriers by more than 2 kcal mol−1. Irrespective of the environment—solution, self‐assembled monolayer or solid‐state polymer gel—and of the molecular structure—rotaxane or catenane—a single and generic switching mechanism is observed for all bistable molecules.
No matter what the surroundings, be it solution, a self‐assembled monolayer on gold, or a solid‐state polymer matrix, the same molecular electromechanical switching mechanism is obeyed for bistable rotaxanes (see illustration) and bistable catenanes in appropriate settings. However, the kinetics and thermodynamic parameters change quite dramatically from one environment to another.
The chemical interactions between single walled carbon nanotubes (SWNTs) and two structurally similar polymers, poly{(m-phenylenevinylene)-co-(2,5-dioctyloxy-p-phenylene)vinylene}, or PmPV, and ...poly{(2,6-pyridinylenevinylene)-co-(2,5-dioctyloxy-p-phenylene)vinylene}, or PPyPV, are investigated. The fundamental difference between these two polymers is that PPyPV is a base and is readily protonated via the addition of HCl. Both polymers promote chloroform solubilization of SWNTs. We find that the SWNT/PPyPV interaction lowers the pK a of PPyPV. Optoelectronic devices, fabricated from single polymer-wrapped SWNT structures, reveal a photogating effect on charge transport which can rectify or amplify current flow through the tubes. For PmPV wrapped tubes, the wavelength dependence of this effect correlates to the absorption spectrum of PmPV. For PPyPV, the wavelength dependence correlates with the absorption spectrum of protonated PPyPV, indicating that SWNTs assist in charge stabilization.
HA‐coated CNTs are very stable in aqueous solution and are able to transport through porous media to a significant extent, indicating high environmental mobility. Use of magnetic metaloxide ...nanoparticles as affinity probes provides a permanent and efficient removal strategy for HA‐coated CNTs from aqueous media (see image).
A family of poly(m-phenylenevinylene)-co-(p-phenylenevinylene)s, functionalized in the synthetically accessible C-5 position of the meta-disubstituted phenylene rings have been designed and ...synthesized: they are essentially poly{(5-alkoxy-m-phenylenevinylene)-co-(2,5-dioctyloxy-p-phenylene)vinylene} (PAmPV) derivatives. A range of these PAmPV polymers have been prepared both (1) by the polymerization of O-substituted 5-hydroxyisophthaldehydes and (2) by chemical modifications carried out on polymers bearing reactive groups at the C-5 positions. PAmPV polymers solubilize SWNT bundles in organic solvents by wrapping themselves around the nanotube bundles. PAmPV derivatives which bear tethers or rings form pseudorotaxanes with rings and threads, respectively. The formation of the polypseudorotaxanes has been investigated in solution by NMR and UV/vis spectroscopies, as well as on silicon oxide wafers in the presence of SWNTs by AFM and surface potential microscopy. Wrapping of these functionalized PAmPV polymers around SWNTs results in the grafting of pseudorotaxanes along the walls of the nanotubes in a periodic fashion. The results hold out the prospect of being able to construct arrays of molecular switches and actuators.
Bistable 2rotaxanes display controllable switching properties in solution, on surfaces, and in devices. These phenomena are based on the electrochemically and electrically driven mechanical shuttling ...motion of the ring-shaped component, cyclobis(paraquat-p-phenylene) (CBPQT4+) (denoted as the ring), between a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) ring system located along a dumbbell component. When the ring is encircling the TTF unit, this co-conformation of the rotaxane is the most stable and thus designated the ground-state co-conformer (GSCC), whereas the other co-conformation with the ring surrounding the DNP ring system is less favored and so designated the metastable-state co-conformer (MSCC). We report here the structure and properties of self-assembled monolayers (SAMs) of a bistable 2rotaxane on Au (111) surfaces as a function of surface coverage based on atomistic molecular dynamics (MD) studies with a force field optimized from DFT calculations and we report several experiments that validate the predictions. On the basis of both the total energy per rotaxane and the calculated stress that is parallel to the surface, we find that the optimal packing density of the SAM corresponds to a surface coverage of 115 Å2/molecule (one molecule per 4 × 4 grid of surface Au atoms) for both the GSCC and MSCC, and that the former is more stable than the latter by 14 kcal/mol at the optimum packing density. We find that the SAM retains hexagonal packing, except for the case at twice the optimum packing density (65 Å2/molecule, the 3 × 3 grid). For the GSCC and MSCC, investigated at the optimum coverage, the tilt of the ring with respect to the normal is θ = 39° and 61°, respectively, while the tilt angle of the entire rotaxane is ψ = 41° and 46°, respectively. Although the tilt angle of the ring decreases with decreasing surface coverage, the tilt angle of the rotaxane has a maximum at 144 Å2/molecule (the 4 × 5 grid/molecule) of 50° and 51° for the GSCC and MSCC, respectively. The hexafluorophosphate counterions (PF6 -) stay localized around the ring during the 2 ns MD simulation. On the basis of the calculated density profile, we find that the thickness of the SAM is 40.5 Å at the optimum coverage for the GSCC and 40.0 Å for MSCC, and that the thicknesses become less with decreasing surface coverage. The calculated surface tension at the optimal packing density is 45 and 65 dyn/cm for the GSCC and MSCC, respectively. This difference suggests that the water contact angle for the GSCC is larger than for the MSCC, a prediction that is verified by experiments on Langmuir−Blodgett monolayers of amphiphilic 2rotaxanes.
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