Two-dimensional molecular electronics circuits

Template-Directed Synthesis of Multiply Mechanically Interlocked Molecules Under Thermodynamic Control

The template-directed construction of crown-ether-like macrocycles around secondary dialkylammonium ions (R2NH2+) has been utilized for the expedient (one-pot) and high-yielding synthesis of a diverse range of mechanically interlocked molecules. The clipping together of appropriately designed dialdehyde and diamine compounds around R2NH2+-containing dumbbell-shaped components proceeds through the formation, under thermodynamic control, of imine bonds. The reversible nature of this particular reaction confers the benefits of "error-checking" and "proof-reading", which one usually associates with supramolecular chemistry and strict self-assembly processes, upon these wholly molecular systems. Furthermore, these dynamic covalent syntheses exploit the efficient templating effects that the R2NH2+ ions exert on the macrocyclization of the matched dialdehyde and diamine fragments, resulting not only in rapid rates of reaction, but also affording near-quantitative conversion of starting materials into the desired interlocked products. Once assembled, these "dynamic" interlocked compounds can be "fixed" upon reduction of the reversible imine bonds (by using BH3.THF) to give kinetically stable species, a procedure that can be performed in the same reaction vessel as the inital thermodynamically controlled assembly. Isolation and purification of the mechanically interlocked products formed by using this protocol is relatively facile, as no column chromatography is required. Herein, we present the synthesis and characterization of 1) a [2]rotaxane, 2) a [3]rotaxane, 3) a branched [4]rotaxane, 4) a bis [2]rotaxane, and 5) a novel cyclic [4]rotaxane, demonstrating, in incrementally more complex systems, the efficacy of this one-pot strategy for the construction of interlocked molecules.

A reversible molecular valve

In everyday life, a macroscopic valve is a device with a movable control element that regulates the flow of gases or liquids by blocking and opening passageways. Construction of such a device on the nanoscale level requires (i) suitably proportioned movable control elements, (ii) a method for operating them on demand, and (iii) appropriately sized passageways. These three conditions can be fulfilled by attaching organic, mechanically interlocked, linear motor molecules that can be operated under chemical, electrical, or optical stimuli to stable inorganic porous frameworks (i.e., by self-assembling organic machinery on top of an inorganic chassis). In this article, we demonstrate a reversibly operating nanovalve that can be turned on and off by redox chemistry. It traps and releases molecules from a maze of nanoscopic passageways in silica by controlling the operation of redox-activated bistable [2]rotaxane molecules tethered to the openings of nanopores leading out of a nanoscale reservoir.

Solvent-dependent assembly of terphenyl- and quaterphenyldithiol on gold and gallium arsenide

The assembly of terphenyldithiol (TPDT) and quaterphenyldithiol (QPDT) on gold and gallium arsenide from ethanol (EtOH), tetrahydrofuran (THF), and solutions consisting of both solvents has been characterized by near-edge X-ray absorption fine structure spectroscopy. The surface coverage and the average orientation of both TPDT and QPDT on gold are solvent-independent. These molecules readily form monolayers on gold with an ensemble-average backbone tilt of 30 degrees +/- 3 degrees from the substrate normal. In sharp contrast, the assembly of TPDT and QPDT on gallium arsenide is extremely solvent-sensitive. At high ethanol fractions, both molecules form monolayers with an ensemble-average orientation that is indistinguishable from those on gold substrates. At low ethanol fractions and in pure THF, however, these molecules are disordered on gallium arsenide and the surface coverage is poor.

Electrical addressing of confined quantum systems for quasiclassical computation and finite state logic machines

Conduction spectroscopy measures the current I through a nanosystem as a function of the voltage V between two electrodes. The differential conductance, dI/dV, has peaks that can be assigned to resonance conditions with different electronic levels of the system. Between these increments, the current has roughly constant plateaus. We discuss how measurements of the current vs. voltage can be used to perform Boolean operations and hence construct finite state logic machines and combinational circuits. The inputs to the device are the source-drain voltage, including its sign, and a gate voltage applied in a manner analogous to optical Stark spectroscopy. As simple examples, we describe a two-state set-reset machine (a machine whose output depends on the input and also on its present state) and a full adder circuit (a circuit that requires three inputs and provides two outputs).

The First Studies of a Tetrathiafulvalene-σ-Acceptor Molecular Rectifier

Langmuir-Blodgett monolayers of a donor-acceptor diad TTF-sigma-(trinitrofluorene) (8) with an extremely low HOMO-LUMO gap (0.3 eV) have been used to create molecular junction devices that show rectification behavior. By virtue of structural similarities and position of molecular orbitals, 8 is the closest well-studied analogue of the model Aviram-Ratner unimolecular rectifier (TTF-sigma-TCNQ). Compressing the monolayer results in aligning the molecules, and is followed by a drastic increase in the rectification ratio. The direction of rectification depends on the electrodes used and is different in n-Si/8/Ti and Au/8/C16H33S-Hg junctions. The molecular nature of such behavior was corroborated by control experiments with fatty acids and by reversing the rectification direction with changing the molecular orientation (Au/D-sigma-A versus Au/A-sigma-D).

Exploring Dynamics and Stereochemistry in Mechanically-Interlocked Compounds

The advent of self-assembly and template-directed synthetic protocols has led to a tremendous surge in the number of mechanically-interlocked compounds being prepared and studied. As these investigations are being carried out, it is becoming increasingly apparent that many of these compounds, known as catenanes and rotaxanes, possess unique dynamic and stereochemical properties. In addition, the drive to create molecular switches and machines for nanotechnological applications has generated a need to understand how to control those properties in condensed phases. Here, we present an overview of the field with regard to the solution dynamics and stereochemistry of mechanically-interlocked compounds - as well as to some related structural types - and review the recent results from our own research in some detail. 1H NMR spectroscopy has proven to be a powerful tool for probing both degenerate and nondegenerate dynamic processes in these compounds, as well as for identifying stereoisomers if they are present in solution. The results of several variable temperature NMR investigations on the effects of structural changes upon the dynamic processes and stimulated relative motions of components in catenanes and rotaxanes, as well as in some self-complexes and pretzelanes, are discussed. A review with 90 references.

Versatile Self-Complexing Compounds Based on Covalently Linked Donor-Acceptor Cyclophanes

A range of covalently linked donor-acceptor compounds which contain 1) a hydroquinone (HQ) unit, 2) a 1,5-dioxynaphthalene (DNP) ring system, or 3) a tetrathiafulvalene (TTF) unit as the pi-donor, and 4) cyclobis(paraquat-p-phenylene) (CBPQT(4+)) as the pi-accepting tetracationic cyclophane were prepared and shown to operate as simple molecular machines. The pi-donating arms can be included inside the cyclophane in an intramolecular fashion by virtue of stabilizing noncovalent bonding interactions. What amounts to self-complexing/decomplexing equilibria were shown to be highly temperature dependent when the pi-donating arm contains either an HQ or DNP moiety. The thermodynamic parameters associated with the equilibria have been unraveled by using variable-temperature (1)H NMR spectroscopy. The negative DeltaH degrees and DeltaS degrees values account for the fact that the "uncomplexed" conformation becomes the dominant species, since the entropy gain associated with the decomplexation process overcomes the enthalpy loss resulting from the breaking of the donor-acceptor interactions. The arm's in-and-out movements with respect to the linked cyclophanes can be arrested by installing a bulky substituent at the end of the arm. In the case of compounds carrying a DNP ring system in their side arm, two diastereoisomeric, self-complexing conformations are observed below 272 K in hexadeuterioacetone. By contrast, control over the TTF-containing arm's movement is more or less ineffective through the thermally sensitive equilibrium although it can be realized by chemical and electrochemical ways as a result of TTF's excellent redox properties. Such self-complexing compounds could find applications as thermo- and electroswitches. In addition, the thermochromism associated with the arm's movement could lead to some of the compounds finding uses as imaging and sensing materials.

Conformational diastereoisomerism in a chiral pretzelane

The introduction of a stereogenic center by a stereospecific synthesis into an optically active, donor-acceptor pretzelane, that exhibits helicity as well as fixed chirality, leads to a marked preference for one conformational diastereoisomer over the other in both acetone and dimethylsulfoxide that can be understood from computational models.

The Role of Physical Environment on Molecular Electromechanical Switching

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 (approximately 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.

Controllable Donor-Acceptor Neutral [2]Rotaxanes

In pursuit of a neutral bistable [2]rotaxane made up of two tetraarylmethane stoppers--both carrying one isopropyl and two tert-butyl groups located at the para positions on each of three of the four aryl rings--known to permit the slippage of the pi-electron-donating 1,5-dinaphtho[38]crown-10 (1/5DNP38C10) at the thermodynamic instigation of pi-electron-accepting recognition sites, in this case, pyromellitic diimide (PmI) and 1,4,5,8-naphthalenetetracarboxylate diimide (NpI) units separated from each other along the rod section of the rotaxane's dumbbell component, and from the para positions of the fourth aryl group of the two stoppers by pentamethylene chains, a modular approach was employed in the synthesis of the dumbbell-shaped compound NpPmD, as well as of its two degenerate counterparts, one (PmPmD) which contains two PmI units and the other (NpNpD) which contains two NpI units. The bistable [2]rotaxane NpPmR, as well as its two degenerate analogues PmPmR and NpNpR, were obtained from the corresponding dumbbell-shaped compounds NpPmD, PmPmD, and NpNpD and 1/5DNP38C10 by slippage. Dynamic 1H NMR spectroscopy in CD2Cl2 revealed that shuttling of the 1/5DNP38C10 ring occurs in NpNpR and PmPmR, with activation barriers of 277 K of 14.0 and 10.9 kcal mol(-1), respectively, reflecting a much more pronounced donor-acceptor stabilizing interaction involving the NpI units over the PmI ones. The photophysical and electrochemical properties of the three neutral [2]rotaxanes and their dumbbell-shaped precursors have also been investigated in CH2Cl2. Interactions between 1/5DNP38C10 and PmI and NpI units located within the rod section of the dumbbell components of the [2]rotaxane give rise to the appearance of charge-transfer bands, the energies of which correlate with the electron-accepting properties of the two diimide moieties. Comparison between the positions of the visible absorption bands in the three [2]rotaxanes shows that, in NpPmR, the major translational isomer is the one in which 1/5DNP38C10 encircles the NpI unit. Correlations of the reduction potentials for all the compounds studied confirm that, in this non-degenerate [2]rotaxane, one of the translational isomers predominates. Furthermore, after deactivation of the NpI unit by one-electron reduction, the 1/5DNP38C10 macrocycle moves to the PmI unit. Li+ ions have been found to strengthen the interaction between the electron-donating crown ether and the electron-accepting diimide units, particularly the PmI one. Titration experiments show that two Li+ ions are involved in the strengthening of the donor-acceptor interaction. Addition of Li+ ions to NpPmR induces the 1/5DNP38C10 macrocycle to move from the NpI to the PmI unit. The Li+-ion-promoted switching of NpPmR in a 4:1 mixture of CD2Cl2 and CD3COCD3 has also been shown by 1H NMR spectroscopy to involve the mechanical movement of the 1/5DNP38C10 macrocycle from the NpI to the PmI unit, a process that can be reversed by adding an excess of [12]crown-4 to sequester the Li+ ions.

The Influence of Constitutional Isomerism and Change on Molecular Recognition Processes

Three constitutionally isomeric bis(naphthylmethyl)ammonium ions, in which the two naphthyl groups are substituted 1) both at their 1-positions, 2) one at its 1-position and the other at its 2-position, and 3) both at their 2-positions, have been investigated separately in solution for their propensities to undergo spontaneous self-assembly with three different [24]crown-8 derivatives, namely, pyrido[24]crown-8 (P24C8), dipyrido[24]crown-8 (DP24C8) and dibenzo[24]crown-8 (DB24C8), in turn to form [2]pseudorotaxanes. The strengths of the 1:1 complexes depend on the composition of the secondary dialkylammonium ions and on the nature of the crown ether hosts; generally, as far as the guest cation is concerned, the 1/1- and 2/2-isomers form stronger complexes, as indicated by stability constant measurements, than the 1/2-isomer and, as far as the crown ethers are concerned, the more flexible P24C8 is a much more efficient host than either DP24C8 or DB24C8. The rates of formation of the [2]pseudorotaxanes are fast (i.e., taking no more than a few minutes) in solution with the exception of one case, that is, in which the crown ether host is DB24C8 and the guest cation is the 1/1-isomer, when it can take upwards of one month for the complexation-decomplexation equilibrium to be established at room temperature. In all cases, the equilibrium between complexed and uncomplexed species is slow on the NMR timescale, allowing the determination of stability constants to be made readily using the single-point method. X-ray crystallography and molecular modeling have been used to gain insight into ground and transition state interactions, respectively, in some of the [2]pseudorotaxanes. The relative stabilities of the three [2]pseudorotaxanes formed by each guest cation in the presence of the three crown ether hosts were also evaluated in solution by competition experiments that were monitored by (1)H NMR spectroscopy. By and large the results of the competition experiments could be predicted on the basis of the derived stability constants for the individual [2]pseudorotaxanes.

Electronic manipulation of DNA, proteins, and nanoparticles for potential circuit assembly

Using gold electrodes lithographically fabricated onto microscope cover slips, DNA and proteins are interrogated both optically (through fluorescence) and electronically (through conductance measurements). Dielectrophoresis is used to position the DNA and proteins at well-defined positions on a chip. Quadrupole electrode geometries are investigated with gaps ranging from 3 to 100 microm; field strengths are typically 10(6) V/m. Twenty nanometer latex beads are also manipulated. The electrical resistance of the electronically manipulated DNA and proteins is measured to be larger than 40 MOhms under the experimental conditions used. The technique of simultaneously measuring resistance while using dielectrophoresis to trap nanoscale objects should find broad applicability.

Mechanism of the Stoddart−Heath Bistable Rotaxane Molecular Switch

We use quantum mechanics to characterize the structure and current-voltage performance of the Stoddart-Heath rotaxane-based programmable electronic switch. We find that the current when the ring is on the DNP is 37-58 times the current when the ring is on the TTF, in agreement with experiment (ratio of 10-100). This establishes the basis for iterative experimental-theoretical efforts to optimize systems for molecule-based electronics which we illustrate by predicting the effect of adding a group such as CN to the rotaxane.

Differential Conductance Switching of Planar Tunnel Junctions Mediated by Oxidation/Reduction of Functionally Protected Ferrocene

Planar tunnel junctions were fabricated by self-assembling 1,1'- ferrocenedicarboxylic acid (FDCA) onto native oxides of thermally deposited aluminum films and subsequently depositing a second aluminum film. Junctions were characterized using Reflection-Absorption Fourier Transform Infrared Spectroscopy (RAIRS) and current-voltage (I-V) spectroscopy. Before deposition of the second aluminum film, RAIRS of FDCA and ferrocenecarboxylic acid (FCA) films revealed COO(-), C=O, and Fc ring stretching modes, indicating that both types of molecules can interact strongly with the oxide and remain intact. After deposition, systems exhibited prominent COO(-) modes and weakened C=O modes, indicating further reaction with aluminum/aluminum oxide. Fc ring modes persisted in FDCA systems but disappeared in FCA systems, suggesting that the second COOH group in the FDCA molecule can act as a protecting group for the ferrocene moiety. Cyclic I-V measurements of FDCA tunnel junction systems revealed very strong ( approximately 10-fold) hysteretic differential conductance switching that was both reversible and stable. Control measurements using as prepared junctions, as well as junctions containing 1,6-hexanedioic acid, 1,9-nonanedioic acid, 1,4-dibenzoic acid, or FCA revealed only very weak ( approximately 10%) differential conductance changes. We attribute FDCA junction switching to barrier profile modifications induced by oxidation/reduction of the functionally protected ferrocene moieties.

A molecular elevator

We report the incrementally staged design, synthesis, characterization, and operation of a molecular machine that behaves like a nanoscale elevator. The operation of this device, which is made of a platformlike component interlocked with a trifurcated riglike component and is only 3.5 nanometers by 2.5 nanometers in size, relies on the integration of several structural and functional molecular subunits. This molecular elevator is considerably more complex and better organized than previously reported artificial molecular machines. It exhibits a clear-cut on-off reversible behavior, and it could develop forces up to around 200 piconewtons.

Single-Walled Carbon Nanotube Based Molecular Switch Tunnel Junctions

This article describes two-terminal molecular switch tunnel junctions (MSTJs) which incorporate a semiconducting, single-walled carbon nanotube (SWNT) as the bottom electrode. The nanotube interacts noncovalently with a monolayer of bistable, nondegenerate [2]catenane tetracations, self-organized by their supporting amphiphilic dimyristoylphosphatidyl anions which shield the mechanically switchable tetracations from a two-micrometer wide metallic top electrode. The resulting 0.002 micron 2 area tunnel junction addresses a nanometer wide row of approximately 2000 molecules. Active and remnant current-voltage measurements demonstrated that these devices can be reconfigurably switched and repeatedly cycled between high and low current states under ambient conditions. Control compounds, including a degenerate [2]catenane, were explored in support of the mechanical origin of the switching signature. These SWNT-based MSTJs operate like previously reported silicon-based MSTJs, but differently from similar devices incorporating bottom metal electrodes. The relevance of these results with respect to the choice of electrode materials for molecular electronics devices is discussed.

Single-Molecule Designs for Electric Switches and Rectifiers

A design for molecular rectifiers is proposed. Current rectification is based on the spatial asymmetry of a molecule and requires only one resonant conducting molecular orbital. Rectification is caused by asymmetric coupling of the orbital to the electrodes, which results in asymmetric movement of the two Fermi levels with respect to the orbital under external bias. Results from numerical studies of the family of suggested molecular rectifiers, HS-(CH(2))(n)-C(6)H(4)(CH(2))(m)SH, are presented. Current rectification ratios in excess of 100 are achievable for n = 2 and m > 6. A class of bistable stator-rotor molecules is proposed. The stationary part connects the two electrodes and facilitates electron transport between them. The rotary part, which has a large dipole moment, is attached to an atom of the stator via a single sigma bond. Electrostatic bonds formed between the oxygen atom of the rotor and hydrogen atoms of the stator make the symmetric orientation of the dipole unstable. The rotor has two potential minima with equal energy for rotation about the sigma bond. The dipole can be flipped between the two states by an external electric field. Both rotor-orientation states have asymmetric current-voltage characteristics that are the reverse of each other, so they are distinguishable electrically. Theoretical results on conformation, energy barriers, retention times, switching voltages, and current-voltage characteristics are presented for a particular stator-rotor molecule. Such molecules could be the base for single-molecule switches, reversible diodes, and other molecular electronic devices.

Scalability Simulations for Nanomemory Systems Integrated on the Molecular Scale

Simulations were performed to assess the prospective performance of a 16 Kbit nanowire-based electronic nanomemory system. Commercial off-the-shelf microcomputer system modeling software was applied to evaluate the operation of an ultra-dense storage array. This array consists of demonstrated experimental non-volatile nanowire diode switches, plus encoder-decoder structures consisting of demonstrated experimental nanowire-based nanotransistors, with nanowire interconnects among all the switching devices. The results of these simulations suggest that a nanomemory of this type can be operated successfully at a density of 10(11) bits/cm(2). Furthermore, modest device alterations and system design alternatives are suggested that might improve the performance and the scalability of the nanomemory array. These simulations represent early steps toward the development of a simulation-based methodology to guide nanoelectronic system design in a manner analogous to the way such methodologies are used to guide microelectronic system design in the silicon industry.

Understanding Charge Transport in Molecular Electronics

For molecular electronics to become a viable technology the factors that control charge transport across a metal-molecule-metal junction need to be elucidated. We use an experimentally simple crossed-wire tunnel junction to interrogate how factors such as metal-molecule coupling, molecular structure, and the choice of metal electrode influence the current-voltage characteristics of a molecular junction.

Nanowire Crossbar Arrays as Address Decoders for Integrated Nanosystems

The development of strategies for addressing arrays of nanoscale devices is central to the implementation of integrated nanosystems such as biological sensor arrays and nanocomputers. We report a general approach for addressing based on molecular-level modification of crossed semiconductor nanowire field-effect transistor (cNW-FET) arrays, where selective chemical modification of cross points in the arrays enables NW inputs to turn specific FET array elements on and off. The chemically modified cNW-FET arrays function as decoder circuits, exhibit gain, and allow multiplexing and demultiplexing of information. These results provide a step toward the realization of addressable integrated nanosystems in which signals are restored at the nanoscale.

Molecular Recognition and Conductance in Crown Ethers

Crown ethers have the remarkable property of recognizing and binding specific metal cations in complex mixtures. We propose to combine molecular recognition with molecular electric conductance. The question we address is: can the event of binding a cation be sensed by a change in conductance? Specifically, we study a short molecular wire (MW) containing a crown-6 molecule connected via sulfur atoms to two gold atomic wires acting as metallic leads. Upon binding a cation, the density of states of the system is only slightly affected. This reflects the fact that the cation binding is largely electrostatic in nature and is accompanied by little electronic reorganization. Yet, the cationic binding does significantly lower conductance. We also identify strong interference affecting the conductance. A striking feature is the insensitivity of conductance to the type of ligand with the exception of the proton.

In the Twilight Zone between [2]Pseudorotaxanes and [2]Rotaxanes

A [2]pseudorotaxane, based on a semi-dumbbell-shaped component containing asymmetrically substituted monopyrrolotetrathiafulvalene and 1,5-dioxynaphthalene recognition sites for encirclement by cyclobis(paraquat-p-phenylene) and with a "speed bump" in the form of a thiomethyl group situated between the two recognition sites, has been self-assembled. This supramolecular entity is a mixture in solution of two slowly interconverting [2]pseudorotaxanes, one of which is on the verge of being a [2]rotaxane at room temperature, allowing it to be isolated by employing flash column chromatography. These two [2]pseudorotaxanes were both characterized in solution by UV/Vis and (1)H NMR spectroscopies (1D and 2D) and also by differential pulse voltammetry. The spectroscopic and electrochemical data reveal that one of the complexes behaves wholly as a [2]pseudorotaxane, while the other has some [2]rotaxane character to it. The kinetics of the shuttling of cyclobis(paraquat-p-phenylene) between the monopyrrolotetrathiafulvalene and the 1,5-dioxynaphthalene recognition sites have been investigated at different temperatures. The shuttling processes, which are accompanied by detectable color changes, can be monitored using UV/Vis and (1)H NMR spectroscopies; the spectroscopic data have been employed in the determination of the rate constants, free energies of activation, enthalpies of activation, and the entropies of activation for the translation of cyclobis(paraquat-p-phenylene) between the two recognition sites.

Linear assemblies of nanoparticles electrostatically organized on DNA scaffolds

A significant challenge faced in the use of nanoscale building blocks is developing parallel methods for interconnecting and patterning assemblies of the individual components. Molecular or polymeric scaffolds hold promise as a means of preparing closely spaced, specifically arranged nanoscale assemblies. Here we show how a biopolymer, DNA, can be used as a scaffold for the assembly of extended, close-packed, ligand-stabilized metal nanoparticle structures, including several desirable architectures (such as lines, ribbons, and branches). Electrostatic binding of ligand-stabilized nanoparticles to the DNA backbone results in extended linear chain-like structures, ribbon-like structures composed of parallel nanoparticle chains, and branched structures. High-resolution transmission electron microscopy shows that the particles are evenly spaced, separated only by the 15 A imposed by the intervening ligand shell. These studies demonstrate that biomolecular nanolithography (the arrangement of nanoscale building blocks on biomolecular scaffolds) is a viable approach to interconnecting individual devices into extended, closely spaced assemblies.

From metal complexes to fullerene arrays: exploring the exciting world of supramolecular photochemistry fifteen years after its birth

After over 15 years of extensive research work in many laboratories worldwide, supramolecular photochemistry is a well-established and highly recognized branch of science. A brief retrospective view on the birth and infancy of this research area is given and some of the latest developments are discussed. In supramolecular photochemistry Ru(II) and Cu(I) diimmine complexes and C60 fullerenes are some of the most widely investigated chromophores and over the years big efforts have been made to implement and tune their photophysical and excited state properties, which are briefly reviewed. Thanks to a huge amount of synthetic and analytical research work, it has been possible to insert or combine these organic and inorganic subunits in a variety of fascinating supramolecular architectures. Some results concerned with photoinduced processes occurring in dyads, triads, catenanes, rotaxanes, dendrimers, and protonated self-assembled architectures are briefly illustrated. The overall picture stemming form the current state of the art in supramolecular photochemistry is that of a discipline gaining an increasing degree of multidisciplinarity. Interconnections with biology, physics and information technology are being established at a very fast pace, suggesting a bright future for this still young research field.

Molecular logic circuits

Miniaturization has been an essential ingredient in the outstanding progress of information technology over the past fifty years. The next, perhaps ultimate, limit of miniaturization is that of molecules, which are the smallest entities with definite size, shape, and properties. Recently, great effort has been devoted to design and investigate molecular-level systems that are capable of transferring, processing, and storing information in binary form. Some of these nanoscale devices can, in fact, perform logic operations of remarkable complexity. This research--although far from being transferred into technology--is attracting interest, as the nanometer realm seems to be out of reach for the "top-down" techniques currently available to microelectronics industry. Moreover, such studies introduce new concepts in the "old" field of chemistry and stimulate the ingenuity of researchers engaged in the "bottom-up" approach to nanotechnology.

Redox-Induced Ring Shuttling and Evidence for Folded Structures in Long and Flexible Two-Station Rotaxanes

Two dumbbell-shaped components with tetraarylmethane-type stoppers - one hydrophobic and one hydrophilic - and a rod-like section containing a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) moiety as electron-donating units, and their [2]rotaxanes, incorporating the cyclobis(paraquat-p-phenylene) (CBPQT4+) cyclophane as their electron-accepting ring component, have been synthesized, the latter using template-directed protocols. The two amphiphilic [2]rotaxanes, which differ from each other only in the lengths of the polyether chains associated with their hydrophilic stoppers, were designed in order (i) to have them exhibit enhanced amphiphilicities and, by altering the lengths of polyether chains, (ii) to improve the qualities of their Langmuir-Blodgett films, and by removing the phenolic residues, (iii) to increase the oxidative stabilities of these switchable molecules, and so extend the lifetimes of electronic devices fabricated from amphiphilic hysteretic molecular switches of this type. UV-VIS absorption and 1H NMR spectra, as well as electrochemical measurements, show that both [2]rotaxanes exist to all intents and purposes in solution as the translational isomer in which the CBPQT4+ cyclophane surrounds the TTF unit. Evidence has also been obtained for the presence in solution of folded conformations of these [2]rotaxanes. While ox/red stimulation of the TTF unit causes shuttling of the CBPQT4+ cyclophane between the TTF and DNP stations, reduction of CBPQT4+ causes unfolding of the [2]rotaxane molecules.