Kent A. Nielsen

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.

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.

Ion-Regulated Allosteric Binding of Fullerenes (C60 and C70) by Tetrathiafulvalene-Calix[4]pyrroles

The effect of ionic species on the binding of fullerenes (C60 and C70) by tetrathiafulvalene-calix[4]pyrrole (TTF-C4P) receptors and the nature of the resulting supramolecular complexes (TTF-C4P + fullerene + halide anion + tetraalkylammonium cation) was studied in the solid state through single crystal X-ray diffraction methods and in dichloromethane solution by means of continuous variation plots and UV-vis spectroscopic titrations. These analyses revealed a 1:1 stoichiometry between the anion-bound TTF-C4Ps and the complexed fullerenes. The latter guests are bound within the bowl-like cup of the C4P in a ball-and-socket binding mode. The interactions between the TTF-C4P receptors and the fullerene guests are highly influenced by both the nature of halide anions and their counter tetraalkylammonium cations. Three halides (F(-), Cl(-), and Br(-)) were studied. All three potentiate the binding of the two test fullerenes by inducing a conformational change from the 1,3-alternate to the cone conformer of the TTF-C4Ps, thus acting as positive heterotropic allosteric effectors. For a particular halide anion, the choice of tetraalkylammonium salts serves to modulate the strength of the TTF-C4P-fullerene host-guest binding interactions and, in conjunction with variations in the halide anion, can be exploited to alter the inherent selectivity of the host for a given fullerene. Differences in binding are reflected in the excited state optical properties. Overall, the present four-component system provides an illustration of how host-guest binding events involving appropriately designed artificial receptors can be fine-tuned via the addition of simple ionic species as allosteric modulators.

Self-assembled monolayers of mono-tetrathiafulvalene calix[4]pyrroles and their electrochemical sensing of chloride.

Since the first reports more than a half-century ago, interest in self-assembled monolayers (SAMs) on metal surfaces has increased almost exponentially. At present, such supported SAMs are used for numerous applications, ranging from metallurgy—wherein the surface properties, such as conductivity, wettability, corrosion resistance, and etching resistance, are modified—to the preparation of sensors and materials for use in separations. In fact, the use of SAMs is now recognized as offering a convenient, flexible, and simple way to tailor the interfacial properties of metals. This is because the spontaneous organization and absorption of molecules on metal, metal oxide, or semiconducting surfaces often creates highly ordered systems with well-defined dimensions on the nanoscale. During the last decade, increasing attention has been dedicated to the design and elaboration of SAMs that integrate a molecular or ionic receptor unit with a transducer element so as to create sensing devices for environmental or biologic purposes. If a redox-active element is incorporated into the monolayer or the guest, the recognition event can be followed by cyclic voltammetry (CV). In other cases, the monitoring can be completed by using impedance spectroscopic measurements. Although, cation (generally metallic ions) recognition using SAMs is well documented, and several systems that exhibit both excellent selectivity and sensitivity have been described in the literature, much less work on anion recognition by using SAMs has been reported, perhaps reflecting the fact that anions have variable sizes and shapes and exhibit strong solvation in most solvents. The first anion recognition SAM systems were reported by Astruc et al. and involved the use of ferrocene derivatives for dihydrogenophosphate anion detection. More recently, Echegoyen et al. described SAMs for use in acetate anion recognition. Although these systems are very selective, their sensitivity is relatively low and the limits of detection do not reach the sub-millimolar scale. Calix[4]pyrrole and its derivatives have been extensively studied in the search for chemosensors capable of recognizing specific chemical species, with a number of optically active calix[4]pyrrole-based sensors having now been reported. Some of us have recently described the first fully successful examples of electrochemically active sensors based on calix[4]pyrroles; these were prepared by attaching one, two, or four redox-active tetrathiafulvalene (TTF) units directly to the calix[4]pyrrole platform so as to produce receptors with enhanced binding affinities toward anions as compared to the parent meso-octamethyl calix[4]pyrrole. In the case of the mono-TTF calix[4]pyrrole, we observed selectivity between the chloride anion (Ka= 2900m ) and the bromide anion (Ka=96m ) in CH2Cl2 solution. This success has led us to consider that TTF calix[4]pyrroles could be used to create anion-sensing SAMs. These kinds of receptors are attractive in this regard because they incorporate within one molecular framework both pyrrolebased anion recognition and TTF-derived transducer subACHTUNGTRENNUNGunits. We wish to report here that such SAMs may be prepared and that they are effective for chloride anion recognition at the sub-millimolar level, thus providing a sensitivity that differs dramatically from previously reported systems. [a] L. G. Jensen, Dr. K. A. Nielsen, Prof. J. O. Jeppesen Department of Physics and Chemistry University of Southern Denmark Campusvej 55, DK-5230, Odense M (Denmark) Fax: (+45)6615-8780 E-mail : joj@ifk.sdu.dk Homepage: http://www.jojgroup.sdu.dk [b] Dr. T. Breton, Prof. Dr. E. Levillain, Dr. L. Sanguinet Universit d’Angers—CNRS 2 boulevard Lavoisier 49045, Angers Cedex (France) E-mail : eric.levillain@iniv-angers.fr lionel.sanguinet@univ-angers.fr [c] Prof. J. L. Sessler Department of Chemistry and Biochemistry The University of Texas at Austin 1 University Station, A5300, Austin, TX 78712-0165 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200901394.

Synthesis and recognition properties of higher order tetrathiafulvalene (TTF) calix[n]pyrroles (n = 4–6)

Two new benzoTTF-annulated calix[n]pyrroles (n = 5 and 6) were synthesized via a one-step acid catalyzed condensation reaction and fully characterized via single crystallographic analyses. As compared to the known tetra-TTF annulated calix[4]pyrrole, which is also produced under the conditions of the condensation reaction, the expanded calix[n]pyrroles (n = 5 and 6) are characterized by a larger cavity size and a higher number of TTF units (albeit the same empirical formula). Analysis of the binding isotherms obtained from UV-Vis spectroscopic titrations carried out in CHCl3 in the presence of both anionic (Cl−, Br−, I−, CH3COO−, H2PO4−, and HSO4−) and neutral (1,3,5-trinitrobenzene (TNB) and 2,4,6-trinitrotoluene (TNT)) substrates revealed that as a general rule the calix[6]pyrrole derivative proved to be the most efficient molecular receptor for anions, while the calix[4]pyrrole congener proves most effective for the recognition of TNB and TNT. These findings are rationalized in terms of the number of electron rich TTF subunits and NH hydrogen bond donor groups within the series, as well as an ability to adopt conformations suitable for substrate recognition, and are supported by solid state structural analyses.

Acid/Base Controllable Molecular Recognition

The study of controllable molecular recognition in supramolecular receptors is important for elucidating design strategies that can lead to external control of molecular recognition applications. In this work, we present the design and synthesis of an asymmetric (TTF) tetrathiafulvalene-calix[4]pyrrole receptor and show that its recognition of 1,3,5-trinitrobenzene (TNB) can be controlled by an acid/base input. The new receptor is composed of three identical TTF units and a fourth TTF unit appended with a phenol moiety. Investigation of the host-guest complexation taking place between the TTF-calix[4]pyrrole receptor and the TNB guests was studied by means of absorption and (1)H NMR spectroscopy; this revealed that the conformation of the molecular receptor can be switched between locked and unlocked states by using base and acid as the input. In the unlocked state, the receptor is able to accommodate two TNB guest molecules, whereas the guests are not able to bind to the receptor in the locked state. This work serves to illustrate how external control (acid/base) of a receptor may be used to direct the molecular recognition of guests (TNBs). It has led to a new controllable molecular recognition system that functions as an acid/base switch.