Single-molecule Switching Research Articles

Reversible Photo-Switching of Single Azobenzene Molecules in Controlled Nanoscale Environments

We drive reversible photoinduced switching of single azobenzene-functionalized molecules isolated in tailored alkanethiolate monolayer matrices on Au{111}. We designed molecular tethers to suppress excited-state quenching from the metal substrate and formed rigid assemblies of single tethered azobenezene molecules in the domains of monolayer to limit steric constraints and tip-induced and stochastic switching effects. Single molecules were reversibly photoisomerized between trans and cis conformations by cycling exposure to visible and UV light. Trans and cis conformations were imaged as high (2.1 +/- 0.3 A) and low (0.7 +/- 0.2 A) protrusions in STM images and were assigned to the on and off states of the molecule, respectively.

New Cruciform Structures: Toward Coordination Induced Single Molecule Switches

New cruciform structures 1-4 were synthesized to investigate a new single molecule switching mechanism arising from the interplay between the molecule and the electrode surface. These molecular cruxes consist of two rod-type substructures, namely an oligophenylenevinylene and an oligophenyleneethynyl. While the oligophenylenevinylene rods are functionalized with acetyl protected sulfur anchor groups, the oligophenyleneethynyl rods provide terminal pyridine units. The hypothesized switching mechanism should arise from the electrochemical potential dependent coordination of the pyridine unit to the electrode surface. The assembly of the oligophenylenevinylene substructure was based on a Wittig reaction whereas its perpendicular oligophenyleneethynyl rod was assembled by Sonogashira-Hagihara coupling reactions. Preliminary transport investigations with molecular cruciforms 2 and 4 in a mechanical controllable break junction in a liquid environment displayed the trapping of single molecules between two gold electrodes via the terminally sulfur functionalized oligophenylenevinylene rod.

Dynamics of the central phenylene ring torsional motion in halogenated phenylene ethynylene oligomers: A quantum-theoretical contribution to the study of conformational basis for single-molecule switching phenomena

The dynamics of intramolecular torsional motion of central phenylene ring in a series of phenylene ethynylene oligomer derivatives was investigated. On the basis of calculated hindered rotational potentials corresponding to this motion, the torsional energy levels were obtained by solving the torsional Schrödinger equation. Subsequently, the torsional correlation time and transition probability was computed within the Bloembergen–Purcell–Pound (BPP) formalism, considering both the classical and quantum mechanical tunneling contributions to the intramolecular rotation. The results were interpreted in the context of molecular conductivity switching behavior of the considered series of compounds. Also some other parameters relevant to molecular admittance were calculated, such as the HOMO–LUMO energy difference and the spatial extent of the frontier molecular orbitals. Classical electrostatic arguments were applied to understand the physical basis of the conformational stability differences in the studied compounds. It was found that halogenation of the central phenylene ring may be used for fine-tuning of molecular conduction behavior, in the sense of modulating the HOMO–LUMO energy difference, the spatial extent of frontier MOs, as well as the barrier height to torsional motion of the central phenylene ring. The time scale of the temperature induced stochastic conformational switching between the “on” and “off” states, along with the corresponding transition probability could be varied by an order of magnitude upon halogenation of the central phenylene ring. The tunneling contributions to the torsional correlation time were found to be of minor importance in this context, and this quantity may be quite correctly estimated with the classical BPP approach.

Three-Terminal Devices to Examine Single-Molecule Conductance Switching

We report electronic transport measurements of single-molecule transistor devices incorporating bipyridyl-dinitro oligophenylene-ethynylene dithiol (BPDN-DT), a molecule known to exhibit conductance switching in other measurement configurations. We observe hysteretic conductance switching in 8% of devices with measurable currents and find that dependence of the switching properties on gate voltage is rare when compared to other single-molecule transistor devices. This suggests that polaron formation is unlikely to be responsible for switching in these devices. We discuss this and alternative switching mechanisms.

Stochastic and photochromic switching of diarylethenes studied by scanning tunnellingmicroscopy

We investigate switching of photochromic, thiophene-substituted diarylethene switches ongold. For this, the conjugated isomer is inserted in a dodecanethiol self-assembledmonolayer. This layer is subsequently scanned by tunnelling microscopy. First, we studythe statistics of stochastic switching events, using two different counting methods. Onemethod monitors reversible switching, whereas the other counts irreversible switchingevents. For each STM bias voltage, we observe a saturation to a dynamic equilibrium of‘on’ and ‘off’ state molecules. This equilibrium shifts to the ‘off’-side when the voltage biasis increased. Second, we separate light-induced switching from stochastic switching. In thisway, we demonstrate photochromic switching of single molecules, from the conjugated tothe non-conjugated form. The switching times we measure are comparable tothose found for conjugated switches self-assembled on gold colloids in solution.

Two and multilevel spectral switching of single molecules in polystyrene at room temperature

Various switching processes with jumps between two or more spectral states were observed for single molecules of the fluorescent dyes PI and DAPI in polystyrene at room temperature. Switching processes were found in the temporal trajectories of almost each spectral parameter. Statistical analyses on the distribution of jumpwidths and on correlations between them were performed. By this means a discrimination between extrinsic and intrinsic mechanisms, which trigger the observed switching dynamic, has been possible in some cases even without anticipated assumption on the nature of the mechanism itself. It has been found that PI and DAPI behave considerably different in single molecule spectroscopy, opposed to their almost identical appearance on bulk level.

Molecular Electronics and Resistance of Molecule-Metal Interfaces

Molecular electronics is an emerging field that aims at controlling physical systems on the level of individual molecules. While initial applications are thought to be in digital memory and logic, potential benefits include manipulation of light on the sub-wavelength level. In this presentation, I will describe the basic ideas and current status of the field. The recent demonstrations of the single-molecule conduction, rectification, amplification, and switching, will be briefly reviewed. I will specifically address the problems of conformational switching and the resistance of a metal – self-assembled molecular film interface. A class of recently designed bistable stator-rotor molecules, whose shape can be changed by external electric field, will be described. Also, the resistance of the metal – molecular monolayer contact will be shown to be strongly dependent on the orientation of the molecules' main axis relative to the metal surface. The sensitivity originates from the angular anisotropy of the conductive molecular orbitals that are usually built from very directional atomic wave functions. This effect should be taken into account in designing low resistance interfaces.

Supramolecular staircase via self-assembly of disklike molecules at the solid-liquid interface.

A series of soluble hexabenzocoronene (HBC) derivatives with pendant optically active (S)-3,7-dimethyloctanyl and (R,S)-3,7-dimethyloctanyl (mixture of stereoisomers) hydrocarbon side chains with and without a phenylene spacer were assembled into differently ordered arrays at the interface between a solution and the basal plane of highly oriented pyrolytic graphite (HOPG). Molecularly resolved scanning tunneling microscopy (STM) images revealed that all derivatives self-assemble into oriented crystals in quasi-two dimensions. However, while for the alkyl-substituted HBCs (1,4) all of the single aromatic cores within a monolayer exhibit the same contrast in the STM, the single aromatic cores with a phenylene group between the alkyl side chains and the aromatic core (2a,2b,3) exhibit different contrasts within a monolayer. For the disks carrying racemic branched or n-alkyl side chains (2b,3) a random distribution of the two different contrasts within the 2D-crystal is observed, while the optically active phenylene-alkyl-substituted HBC (2a) exhibits a periodical distribution of three contrasts within the monolayer. We attribute the different contrasts of the aromatic cores in the presence of the phenylene groups to a loss of the planarity of the whole molecule and different conformations, which allow the conjugated disks to attain different equilibrium positions above the surface of HOPG. In the case of the optically active side chains a regular superstructure with three distinctly different positions such as in a staircase is attained. The self-assembly processes are governed by the interplay of intramolecular as well as intermolecular and interfacial interactions. In the present case, the interactions may induce both the molecules to acquire well distinct positions along the z axis and to adopt different conformations. The reported results open new avenues of exploration. For instance, the different couplings of conjugated molecules with the substrate at different separations can be investigated by means of scanning tunneling spectroscopy (STS). Furthermore, experiments on the STM tip-induced switching of single molecules embedded in a monolayer appear feasible.

Elucidation of optical switching of single guest molecules in terrylene/ p-terphenyl mixed crystals

An empirical molecular model is used to explore the ground-state potential energy surface of a mixed crystal in which reversible, reproducible, photo-induced frequency jumps of single molecules were recently reported. In the model, which accounts for many observed features of terrylene in p-terphenyl, optical switching of the guest transition frequency corresponds to relaxation of host molecules in the second solvation shell.

Single-molecule optical switching of terrylene in p-terphenyl

The controlled manipulation and switching of single atoms and molecules raise the prospect of ultra-high-density data storage. Switching by motion of a single atom has been reported1, and techniques of single-molecule optical detection and spectroscopy2 in the condensed phase have been refined to a degree that allows the modification of the absorption properties of a single chromophore3. Light-induced jumps in single-molecule excitation frequencies have been reported3,4,5, but in none of these cases could the process be controlled: the jumps varied from molecule to molecule, they were interrupted by spontaneous jumps, and the new excitation frequencies could not be identified unambiguously. Here we report light-induced reversible frequency jumps ofsingle molecules of the aromatic hydrocarbon terrylene embedded in a particular site of a p-terphenyl host crystal6 at temperatures of around 2 K. The changes in absorption frequency for different terrylene molecules were identical (within 0.5%) for all samples studied. Thus we were able to switch single-molecule absorption lines in a controlled way between well-defined frequency positions.