Side‐Group‐Mediated Mechanical Conductance Switching in Molecular Junctions

A key target in molecular electronics has been molecules having switchable electrical properties. Switching between two electrical states has been demonstrated using such stimuli as light, electrochemical voltage, complexation and mechanical modulation. A classic example of the latter is the switching of 4,4′‐bipyridine, leading to conductance modulation of around 1 order of magnitude. Here, we describe the use of side‐group chemistry to control the properties of a single‐molecule electromechanical switch, which can be cycled between two conductance states by repeated compression and elongation. While bulky alkyl substituents inhibit the switching behavior, π‐conjugated side‐groups reinstate it. DFT calculations show that weak interactions between aryl moieties and the metallic electrodes are responsible for the observed phenomenon. This represents a significant expansion of the single‐molecule electronics “tool‐box” for the design of junctions with electromechanical properties.     

Long‐Lived 1H Nuclear Spin Singlet in Dimethyl Maleate Revealed by Addition of Thiols

Nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) have become important techniques in many research areas. One major limitation is the relatively low sensitivity of these methods, which recently has been addressed by hyperpolarization. However, once hyperpolarization is imparted on a molecule, the magnetization typically decays within relatively short times. Singlet states are well isolated from the environment, such that they acquire long lifetimes. We describe herein a model reaction for read‐out of a hyperpolarized long‐lived state in dimethyl maleate using thiol conjugate addition. This type of reaction could lend itself to monitoring oxidative stress or hypoxia by sensitive detection of thiols. Similar reactions could be used in biosensors or assays that exploit molecular switching. Singlet lifetimes of about 4.7 min for ¹H spins in [D₄]MeOH are seen in this system.     

Voltage-induced swelling and deswelling of weak polybase brushes

We have investigated a novel method of remotely switching the conformation of a weak polybase brush using an applied voltage. Surface-grafted polyelectrolyte brushes exhibit rich responsive behavior and show great promise as "smart surfaces", but existing switching methods involve physically or chemically changing the solution in contact with the brush. In this study, high grafting density poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes were grown from silicon surfaces using atom transfer radical polymerization. Optical ellipsometry and neutron reflectivity were used to measure changes in the profiles of the brushes in response to DC voltages applied between the brush substrate and a parallel electrode some distance away in the surrounding liquid (water or D(2)O). Positive voltages were shown to cause swelling, while negative voltages in some cases caused deswelling. Neutron reflectometry experiments were carried out on the INTER reflectometer (ISIS, Rutherford Appleton Laboratory, UK) allowing time-resolved measurements of polymer brush structure. The PDMAEMA brushes were shown to have a polymer volume fraction profile described by a Gaussian-terminated parabola both in the equilibrium and in the partially swollen states. At very high positive voltages (in this study, positive bias means positive voltage to the brush-bearing substrate), the brush chains were shown to be stretched to an extent comparable to their contour length, before being physically removed from the interface. Voltage-induced swelling was shown to exhibit a wider range of brush swelling states in comparison to pH switching, with the additional advantages that the stimulus is remotely controlled and may be fully automated.     

Redox-responsive gel-sol/sol-gel transition in poly(acrylic acid) aqueous solution containing Fe(III) ions switched by light

Redox-responsive gel-sol/sol-gel transition in aqueous PAA system containing Fe(III)-citrate complex was realized by switching the redox states of Fe(III)/F(II) ions conjugated with photoreduction and oxidation. This reversible transition can be indicated chromatically by the Fe(III) ions and repeated many times as long as there is sufficient citric acid.     

Reversible light switch for macrocycle mobility in a DNA rotaxane

A recent trend in DNA nanotechnology consists of the assembly of architectures with dynamic properties that can be regulated by employing external stimuli. Reversible processes are important for implementing molecular motion into DNA architectures as they allow for the regeneration of the original state. Here we describe two different approaches for the reversible switching of a double-stranded DNA rotaxane architecture from a stationary pseudorotaxane mode into a state with movable components. Both states only marginally differ in their respective topologies but their mechanical properties are fundamentally different. In the two approaches, the switching operation is based on strand-displacement reactions. One of them employs toehold-extended oligodeoxynucleotides whereas in the other one the switching is achieved by light-irradiation. In both cases, multiple back and forth switching between the stationary and the mobile states was achieved in nearly quantitative fashion. The ability to reversibly operate mechanical motion in an interlocked DNA nanostructure opens exciting new avenues in DNA nanotechnology.     

Bistable stochastic biochemical networks: large chemical networks and systems with many molecules

In this paper we continue the program started in Hwang and Velázquez (J Math Chem, to appear). We describe some chemical systems exhibiting bistable behavior with reaction constants of order one, but where bistability is due to the presence of a large number of chemical species or a large number of molecules of some of the species. We derive generalizations of the classical Kramers’ formula that gives the switching times for some particular systems exhibiting a large number of species.     

Redox-driven conductance switching via filament formation and dissolution in carbon/molecule/TiO2/Ag molecular electronic junctions

Carbon/molecule/TiO2/Au molecular electronic junctions show robust conductance switching, in which a metastable high conductance state may be induced by a voltage pulse which results in redox reactions in the molecular and TiO2 layers. When Ag is substituted for Au as the "top contact", dramatically different current/voltage curves and switching behavior result. When the carbon substrate is biased negative, an apparent breakdown occurs, leading to a high conductance state which is stable for at least several hours. Upon scanning to positive bias, the conductance returns to a low state, and the cycle may be repeated hundreds of times. Similar effects are observed when Cu is substituted for Au and for three different molecular layers as well as "control" junctions of the type carbon/TiO2/Ag/Au. The polarity of the "switching" is reversed when the Ag layer is between the carbon and molecular layers, and the conductance change is suppressed at low temperature. Pulse experiments show very erratic transitions between high and low conductivity states, particularly near the switching threshold. The results are consistent with a switching mechanism based on Ag or Cu oxidation, transport of their ions through the TiO2, and reduction at the carbon to form a metal filament.     

Why do Physicists Love Charge-Transfer Salts?

I describe some of the phenomena encountered in charge-transfer salts that make them very attractive for condensed-matter physicists. These materials exhibit many interesting electronic properties, including reduced dimensionality, strong electron-electron and electron-phonon interactions and the proximity of antiferromagnetism, insulator states and superconductivity. A wide variety of low-temperature groundstates have been observed in the salts; frequently, one is able to move between these states by applying magnetic field, temperature, pressure or “chemical pressure”. In spite of this complex behavior, the charge-transfer salts possess very simple electronic bandstructure which it is often possible to measure in great detail. Hence, one can use the salts as “model systems” in which tractable theoretical calculations for phenomena such as superconductivity are compared directly with experiment.     

Intrinsic optical bistability of thin films of linear molecular aggregates: the one-exciton approximation

We perform a theoretical study of the nonlinear optical response of an ultrathin film consisting of oriented linear aggregates. A single aggregate is described by a Frenkel exciton Hamiltonian with uncorrelated on-site disorder. The exciton wave functions and energies are found exactly by numerically diagonalizing the Hamiltonian. The principal restriction we impose is that only the optical transitions between the ground state and optically dominant states of the one-exciton manifold are considered, whereas transitions to other states, including those of higher exciton manifolds, are neglected. The optical dynamics of the system is treated within the framework of truncated optical Maxwell-Bloch equations, in which the electric polarization is calculated by using a joint distribution of the transition frequency and the transition dipole moment of the optically dominant states. This function contains all the statistical information about these two quantities that govern the optical response and is obtained numerically by sampling many disorder realizations. We derive a steady-state equation that establishes a relationship between the output and input intensities of the electric field and show that within a certain range of the parameter space this equation exhibits a three-valued solution for the output field. A time-domain analysis is employed to investigate the stability of different branches of the three-valued solutions and to get insight into switching times. We discuss the possibility to experimentally verify the bistable behavior.     

Nonlocal behavior of an electron in the ring-opening of cyclobutene

The nonlocal behavior of an electron in the ring-opening of cyclobutene is analyzed using the sharing amplitude and tools based thereupon. The sharing amplitude is a generalization of the absolute value squared of the wave function (a probability density according to the Born interpretation) to a measure that gives, for a single particle in a many particle system, both the relative phase of a wavelike quantity between any two space/spin points and a measure of the distribution of that quantity between those points. The sharing indices are related to the absolute value squared of the amplitude, thereby ensuring that the amplitude and the indices are consistent. To provide prototypical behavior of the sharing quantities in single and double bonds and to identify nonbonding and antibonding behavior, these tools are used to describe single electron behavior in ethane and ethylene, in the latter including separate σ and π contributions and two excited states. Similar analyses of the single and double carbon-carbon bonds in the reactant cyclobutene and in the product s-cis-butadiene (a planar transition state allowing for σ and π separation) are carried out. Comparisons are made to the prototypical molecules. The sharing quantities in the locally stable forms of s-gauche-butadiene are then considered. A remnant of the bond that is broken in cyclobutene is found in a form that is consistent with a conrotatory ring-opening, a behavior suggested by Woodward and Hoffmann on the basis of the symmetry of the highest occupied molecular orbital in the product, but here without appeal to only that molecular orbital. Finally, the nonlocal behavior of an electron during the breaking and making of bonds in the reaction is discussed for several geometries along the reaction path.     

Electrical switching between vesicles and micelles via redox-responsive self-assembly of amphiphilic rod-coils

An aqueous vesicular system that is switchable by electric potential without addition of any chemical redox agents into the solution is demonstrated using redox-responsive self-assembly of an amphiphilic rod-coil molecule consisting of a tetraaniline and a poly(ethylene glycol) block. The vesicle membrane is split by an oxidizing voltage into smaller pucklike micelles that can reassemble to form vesicles upon exposure to a reducing voltage. The switching mechanism is explained by the packing behavior of the tetraaniline units constituting the membrane core, which depends on their oxidation states.     

Block copolymer micelles as switchable templates for nanofabrication

Block copolymer inverse micelles from polystyrene-block-poly-2-vinylpyridine (PS-b-P2VP) deposited as monolayer films onto surfaces show responsive behavior and are reversibly switchable between two states of different topography and surface chemistry. The as-coated films are in the form of arrays of nanoscale bumps, which can be transformed into arrays of nanoscale holes by switching through exposure to methanol. The use of these micellar films to act as switchable etch masks for the structuring of the underlying material to form either pillars or holes depending on the switching state is demonstrated.     

Complex dynamics in a periodically perturbed electro-chemical system

Dynamical response of a passivation model subjected to parametric periodic and stochastic perturbations is studied numerically. In response to weak periodic modulation, the system exhibits a rich variety of resonance behavior and induced dynamics, including periodically induced oscillation, birhythmicity, switching between two bistable states, selection of one of the bistable states, mixed-mode and chaotic oscillations. These phenomena are discussed in terms of the stability of saddle focus and an incomplete homoclinic connection. Our numerical results are relevant for a wide class of electro-chemical oscillatory systems, where the re-injection of unstable trajectory on the neighborhood of a saddle focus is a typical feature in the phase space.     

Correlated electron dynamics: how aromaticity can be controlled

In this Article, we show that the aromaticity of a molecule can be turned off by controlling the electron dynamics. We present a controlled switching from the aromatic ground state of benzene to two different nonaromatic states, using a laser pulse. The propagation of the molecular wave function is carried out with the time-dependent configuration interaction method. The laser pulse for switching between the ground and excited states is optimized using optimal control theory. Bond orders and Mulliken charges serve as an aromaticity criterion. The nonaromatic target states exhibit localized bonds and partial charges on the carbon atoms; these localized electrons circulate on an attosecond time scale in the ring system.     

Phase-stabilized two-dimensional electronic spectroscopy

Two-dimensional (2D) spectroscopy is a powerful technique to study nuclear and electronic correlations between different transitions or initial and final states. Here we describe in detail our development of inherently phase-stabilized 2D Fourier-transform spectroscopy for electronic transitions. A diffractive-optic setup is used to realize heterodyne-detected femtosecond four-wave mixing in a phase-matched box geometry. Wavelength tunability in the visible range is accomplished by means of a 3 kHz repetition-rate laser system and optical parametric amplification. Nonlinear signals are fully characterized by spectral interferometry. Starting from fundamental principles, we discuss the origin of phase stability and the precise calibration of excitation-pulse time delays using movable glass wedges. Automated subtraction of undesired scattering terms removes experimental artifacts. On the theoretical side, the response-function formalism is extended to describe molecules with three electronic levels, and the shape of 2D spectral features is discussed. As an example for this technique, experimental 2D spectra are shown for the dye molecule Nile Blue in acetonitrile at 595 nm, recorded for a series of population times. Simulations explore the influence of different model parameters and qualitatively reproduce the experimental results. We show that correlations between different electronically excited states can be determined from the spectra. The technique described here can be used to measure the third-order response function of complex systems covering several electronic transitions.     

Determination of the density and energetic distribution of electron traps in dye-sensitized nanocrystalline solar cells

Electron transport and recombination in dye-sensitized nanocrystalline solar cells (DSCs) are strongly influenced by the presence of trapping states in the titanium dioxide particles, and collection of photoinjected electrons at the contact can require times ranging from milliseconds to seconds, depending on the illumination intensity. A direct method of determining the density and energetic distribution of the trapping states responsible for slowing electron transport has been developed. It involves extraction of trapped electrons by switching the cell from an open circuit to a short circuit after a period of illumination. An advantage of this charge extraction method is that it is less sensitive than other methods to shunting of the DSC by electron transfer at the conducting glass substrate. Results derived from charge extraction measurements on DSCs (with and without compact TiO(2) blocking layers) are compared with those obtained by analysis of the open circuit photovoltage decay.     

The effective concentration of unbound ink anchors at the molecular printboard

Self-assembled monolayers terminating in beta-cyclodextrin cavities can be used to bind ink molecules and so provide a molecular printboard for nanopatterning applications. Multivalent or multisite binding strengthens the attachment of large inks and provides more robust patterns. In the present work we use computer simulations to probe the behavior of functionalized dendrimer inks at the printboard. We performed a series of long 10 ns fully atomistic molecular dynamics (MD) simulations to measure the effective local concentration of unbound ink anchor groups at the printboard for a variety of binding modes and also for the partial unbinding prerequisite for ink diffusion on the printboard. These simulations allow us to describe the conformational space occupied by partially bound inks and estimate the likelihood of an additional binding interaction. Furthermore, by simulating the shift from a divalent to monovalent binding mode we show that the released anchor quickly moves to the periphery of the dendrimer binding hemisphere but then reapproaches the printboard and remains in the vicinity of alternative binding sites. Secondary electrostatic interactions between the protonated dendrimer core and hydroxyl groups at the entrance to the beta-cyclodextrin cavities give "flattened" dendrimer binding orientations and may aid dendrimer diffusion on the printboard, allowing the dendrimer to "walk" along the printboard by switching between different partially bound states and minimizing complete unbinding to bulk solution, crucial for the application of the printboard in, for example, medical diagnostics.     

Global behavior and asymptotic reduction of a chemical kinetics system with continua of equilibria

We consider a model chemical kinetics system describing the dynamics of species concentrations taking part is consecutive-competitive reaction in a continuopusly stirred tank reactor. Corresponding dynamical system has a continua of equilibria. Particular equilibrium to which the solution of the system tends depends on the initial conditions. The global behavior of the system and its reductions via the invariant manifold and the boundary function methods are studied.AMS Subject Classification: 34D15, 34D35, 37C10     

Generalized CC-TDSCF and LCSA: The system-energy representation

Typical (sub)system-bath quantum dynamical problems are often investigated by means of (approximate) reduced equations of motion. Wavepacket approaches to the dynamics of the whole system have gained momentum in recent years and there is hope that properly designed approximations to the wavefunction will allow one to correctly describe the subsystem evolution. The continuous-configuration time-dependent self-consistent field (CC-TDSCF) and local coherent-state approximation (LCSA) methods, for instance, use a simple Hartree product of bath single-particle-functions for each discrete variable representation (DVR) state introduced in the Hilbert space of the subsystem. Here we focus on the above two methods and replace the DVR states with the eigenstates of the subsystem Hamiltonian, i.e., we adopt an energy-local representation for the subsystem. We find that stable and semiquantitative results are obtained for a number of dissipative problems, at the same (small) computational cost of the original methods. Furthermore, we find that both methods give very similar results, thus suggesting that coherent-states are well suited to describe (local) bath states. As a whole, present results highlight the importance of the system basis-set in the selected-multiconfiguration expansion of the wavefunction. They suggest that accurate and yet computationally cheap methods may be simply obtained from CC-TDSCF/LCSA by letting the subsystem states be variationally optimized.