Jan O. Jeppesen

Nanoscale molecular-switch crossbar circuits

Molecular electronics offer an alternative pathway to construct nanoscale circuits in which the critical dimension is naturally associated with molecular sizes. We describe the fabrication and testing of nanoscale molecular-electronic circuits that comprise a molecular monolayer of [2]rotaxanes sandwiched between metal nanowires to form an 8 × 8 crossbar within a 1 µm 2 area. The resistance at each cross point of the crossbar can be switched reversibly. By using each cross point as an active memory cell, crossbar circuits were operated as rewritable, nonvolatile memory with a density of 6. 4G bits cm −2 .B ys etting the resistances at specific cross points, two 4 × 4s ubarrays of the crossbar were configured to be a nanoscale demultiplexer and multiplexer that were used to read memory bits in a third subarray.

Two-Dimensional Molecular Electronics Circuits

Addressing an array of bistable [2]rotaxanes through a two-dimensional crossbar arrangement provides the device element of a current-driven molecular electronic circuit. The development of the [2]rotaxane switches through an iterative, evolutionary process is described. The arrangement reported here allows both memory and logic functions to use the same elements.

Nanoscale molecular-switch devices fabricated by imprint lithography

Nanoscale molecular-electronic devices comprising a single molecular monolayer of bistable [2]rotaxanes sandwiched between two 40-nm metal electrodes were fabricated using imprint lithography. Bistable current–voltage characteristics with high on–off ratios and reversible switching properties were observed. Such devices may function as basic elements for future ultradense electronic circuitry.

Positive Homotropic Allosteric Receptors for Neutral Guests: Annulated Tetrathiafulvalene-Calix[4]pyrroles as Colorimetric Chemosensors for Nitroaromatic Explosives

The study of positive homotropic allosterism in supramolecular receptors is important for elucidating design strategies that can lead to increased sensitivity in various molecular recognition applications. In this work, the cooperative relationship between tetrathiafulvalene (TTF)-calix[4]pyrroles and several nitroaromatic guests is examined. The design and synthesis of new annulated TTF-calix[4]pyrrole receptors with the goal of rigidifying the system to accommodate better nitroaromatic guests is outlined. These new derivatives, which display significant improvement in terms of binding constants, also display a positive homotropic allosteric relationship, as borne out from the sigmoidal nature of the binding isotherms and analysis by using the Hill equation, Adair equation, and Scatchard plots. The host-guest complexes themselves have been characterized by single-crystal X-ray diffraction analyses and studied by means of UV-spectroscopic titrations. Investigations into the electronic nature of the receptors were made by using cyclic voltammetry; this revealed that the binding efficiency was not strictly related to the redox potential of the receptor. On the other hand, this work serves to illustrate how cooperative effects may be used to enhance the recognition ability of TTF-calix[4]pyrrole receptors. It has led to new allosteric systems that function as rudimentary colorimetric chemosensors for common nitroaromatic-based explosives, and which are effective even in the presence of potentially interfering anions.

A mono-TTF-annulated porphyrin as a fluorescence switchElectronic supplementary information (ESI) available: data for 4. See http://www.rsc.org/suppdata/cc/b2/b212456d/

Annulation of one TTF unit directly to the porphyrin chromophore produces an almost nonfluorescent species which can be transformed into a fluorescent species by oxidation of the TTF unit.

Tetrathiafulvalenes: from heterocyclic chemistry to molecular devices

Abstract This account highlight recent developments in the field of supramolecular tetrathiafulvalene (TTF) chemistry. Progress in synthetic TTF chemistry has enabled the preparation of elaborate molecular systems and architectures. System based on host–guest interactions may act as, for example, sensors or molecular switches. Among these systems mechanically interlocked architectures such as catenanes and rotaxanes are now prime candidates for the construction of artificial molecular machines and the fabrication of molecular electronic devices.

A chloride-anion insensitive colorimetric chemosensor for trinitrobenzene and picric acid

AbstractA new receptor, the bisTTF-calix[2]thiophene[2]pyrrole derivative 3, has been prepared from the Lewis acid-catalyzed condensation of 2,5-bis(1-hydroxymethylethyl)thiopheno-TTF and pyrrole. This new system is found to form complexes with the electron-deficient guests, trinitrobenzene (TNB) and picric acid (PA), which serve as models for nitroaromatic explosives. The binding phenomenon, which has been studied in organic solution using proton nuclear magnetic resonance and absorption spectroscopies, results in an easy-to-visualize color change in chloroform that is independent of the presence of chloride anion, a known interferant for an earlier tetrakisTTF-calix[4]pyrrole TNB chemosensor. Support for the proposed binding mode comes from a preliminary solid state structure of the complex formed from TNB, namely TNB⊂3. A color change is also observed when dichloromethane solutions of chemosensor 3 are added to solvent-free samples of TNB, PA, and 2,4,6-trinitrotoluene supported on silica gel. FigureA new bis-tetrathiafulvalene calix[2]thiophene[2]pyrrole derivative has been prepared that gives rise to an easy-to-visualize color change in the presence of the model nitroaromatic explosives trinitrobenzene and picric acid.

Turning on resonant SERRS using the chromophore-plasmon coupling created by host-guest complexation at a plasmonic nanoarray.

An active molecular plasmonics system is demonstrated where a supramolecular chromophore generated in a host-guest binding event couples with the localized surface plasmon resonance (LSPR) arising from gold nanodisc gratings. This coupling was achieved by wavelength-matching the chromophore and the LSPR with the laser excitation, thus giving rise to surface-enhanced resonance Raman scattering (SERRS). The chromophore is a broad charge-transfer (CT) band centered at 865 nm (epsilon = 3500 M(-1) cm(-1)) generated by the complexation of cyclobis(paraquat-p-phenylene) (CBPQT(4+)) and the guest molecule tetrathiafulvalene (TTF). The substrates consist of sub-1-microm gold nanodisc arrays which display dimension-tunable plasmon wavelengths (600-1000 nm). The vibrational spectra of the complex arising from SERRS (lambda(exc) = 785 nm) were generated by irradiating an array (lambda(LSPR) = 765 nm) through the solution to give a chromophore-specific signature with the intensities surface enhanced by approximately 10(5). Surface adsorption of the empty and complexed CBPQT(4+) is also implicated in bringing the chromophore into the electric field arising from the surface-localized plasmon. In a titration experiment, the SERRS effect was then used to verify the role of resonance in turning on the spectrum and to accurately quantify the binding between surface-adsorbed CBPQT(4+) and TTF. The use of a nonpatterned gold substrate as well as a color mismatched complex did not show the enhancement, thus validating that spectral overlap between the chromophore and plasmon resonance is key for resonance surface enhancement. Simulations of the electric fields of the arrays are consistent with interdisc plasmon coupling and the observed enhancement factors. The creation of a responsive plasmonic device upon the addition of the guest molecule and the subsequent coupling of the CT chromophore to the plasmon presents favorable opportunities for applications in molecular sensing and active molecular plasmonics.

Quantifying the working stroke of tetrathiafulvalene-based electrochemically-driven linear motor-molecules

A highly constrained [2]rotaxane, constructed in such a way that the tetracationic cyclobis(paraquat-p-phenylene) ring is restricted to reside on a monopyrrolotetrathiafulvalene unit, has been synthesised and characterised. This design allows the deslipping free energy barrier for the tetracationic ring in all three redox states of the rotaxane to be determined.

Molecular logic gates using surface-enhanced Raman-scattered light.

A voltage-activated molecular-plasmonics device was created to demonstrate molecular logic based on resonant surface-enhanced Raman scattering (SERS). SERS output was achieved by a combination of chromophore-plasmon coupling and surface adsorption at the interface between a solution and a gold nanodisc array. The chromophore was created by the self-assembly of a supramolecular complex with a redox-active guest molecule. The guest was reversibly oxidized at the gold surface to the +1 and +2 oxidation states, revealing spectra that were reproduced by calculations. State-specific SERS features enabled the demonstration of a multigate logic device with electronic input and optical output.