Large protonation-gated photochromism of an OPE-embedded difurylperfluorocyclopentene

A recently reported protolytic gating effect on the ring closing reaction of an oligo(phenylene ethynylene) (OPE) embedded difurylperfluorocyclopentene (S) with a dimethylaminophenyl chain link in each of the side arms, was quantitatively analyzed in detail. The reaction system (So, SoH+, SoH22+, Sc' ScH+, ScH22+) comprising three protolytic forms in both open and closed configuration, is characterized by four protolytic equilibrium constants and six photochemical quantum yields of ring closing and ring opening. The absorption spectra, conductivity, and effective photochemical quantum yields were measured in acetonitrile as functions of solvent acidity varied by addition of trifluoroacetic acid and triethylamine and characterized by an effective pHnon-aq. Based on the derivation of a rigorous method for assessing the individual quantum yields of ring closure and ring opening of the six species, it was shown that it is specifically the second protonation step that is responsible for a more than 10-fold increase in the quantum yield of ring closure

Statistical investigation of current-voltage characterization in single molecule-metal junctions

We show statistical measurements of single molecule-metal contacts using the mechanically controllable break junction technique. The measurements are carried out in a solvent, in order to allow in situ binding of the molecules to the metallic contacts during the measurements. Statistics is gathered by opening and closing the junctions repeatedly and recording current-voltage characteristics at various stages of the opening and closing curves. By modeling the data with a single level model we can extract parameters such as the position of the molecular energy level, which carries the current, and the coupling between the metal and the molecule. In first experiments we use this method to characterize different anchoring groups, which mediate the mechanical and electrical coupling between the metallic electrodes and the molecules. We use tolane molecules, which are structurally simple, as model systems for this purpose

Synthesis and Photoswitching Studies of OPE-Embedded Difurylperfluorocyclopentenes

We report the synthesis and photochemical behavior of five photochromic molecular switches <b>7a</b>–<b>e</b> with attached molecular wires based on differently substituted oligo­(phenylene ethynylene) (OPE) building blocks. The switchable molecular wires <b>7a</b>–<b>e</b> were built in a convergent approach from substituted iodotolans <b>6a</b>–<b>e</b> and 1,2-bis­(2-methyl-5-ethynylfuran-3-yl)­perfluorocyclopentene <b>5</b> by 2-fold Sonogashira coupling. Compound <b>5</b> was prepared from the corresponding bis-aldehyde <b>2</b> by Wittig-type olefination with [PPh₃CHBr₂]­Br·CH₃CN, followed by elimination to the bromoalkyne under mild phase-transfer conditions at 0 °C. Halogen–metal exchange with <i>i</i>-PrMgCl·LiCl and hydrolysis furnished <b>5</b> in good overall yield. Substituents R¹ and R² in the OPE portion were either electron-withdrawing or electron-donating, and their influence on the photostability and photoswitching characteristics of <b>7a</b>–<b>e</b> was studied. All resulting molecules show reversible photochromism between the colorless off and the deeply colored on states when irradiated with light of 313 and 576 nm wavelengths, respectively. The quantum yields of these photoreactions increased when electron-withdrawing groups were used. This was further corroborated by reversible protonation/deprotonation of <b>7e</b> (R¹ = NMe₂, R² = H) for which the ring-closing quantum yield increased 10-fold upon switching off the donor by protonation

Wide Band-Gap 3,4-Difluorothiophene-Based Polymer with 7% Solar Cell Efficiency: An Alternative to P3HT

Wide Band-Gap 3,4-Difluorothiophene-Based Polymer with 7% Solar Cell Efficiency: An Alternative to P3H

Large Magnetoresistance in Single-Radical Molecular Junctions

Organic radicals are promising building blocks for molecular spintronics. Little is known about the role of unpaired electrons for electron transport at the single-molecule level. Here, we examine the impact of magnetic fields on electron transport in single oligo(p-phenyleneethynylene) (OPE)-based radical molecular junctions, which are formed with a mechanically controllable break-junction technique at a low temperature of 4.2 K. Surprisingly huge positive magnetoresistances (MRs) of 16 to 287% are visible for a magnetic field of 4 T, and the values are at least 1 order of magnitude larger than those of the analogous pristine OPE (2-4%). Rigorous analysis of the MR and of current-voltage and inelastic electron-tunneling spectroscopy measurements reveal an effective reduction of the electronic coupling between the current-carrying molecular orbital and the electrodes with increasing magnetic field. We suggest that the large MR for the single-radical molecular junctions might be ascribed to a loss of phase coherence of the charge carriers induced by the magnetic field. Although further investigations are required to reveal the mechanism underlying the strong MR, our findings provide a potential approach for tuning charge transport in metal-molecule junctions with organic radicals.publishe

Flexible Molecular-Scale Electronic Devices Composed of Diarylethene Photoswitching Molecules

The electrical properties of diarylethene photoswitching molecular devices on flexible substrates are studied. When exposed to UV or visible light, diarylethene molecular devices show two electrical states (a high and a low conductance state) with a discrepancy of an order of magnitude in the level of current between the two states. The diarylethene flexible molecular devices exhibit excellent long-time stability and reliable electrical characteristics in both conductance states when subjected to various mechanical stresses

A combined experimental and theoretical study of 1,4-bis(phenylethynyl)-2,5-bis(ethoxy)benzene adsorption on Au(111)

The electronic and geometrical structure of 1,4-bis(phenylethynyl)-2,5-bis(ethoxy)benzene (PEEB) molecules adsorbed on a Au(111) surface is investigated by low temperature scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) in conjunction with density-functional-based tight-binding (DFTB) simulations of the density of states and the interaction with the substrate. Our density functional theory calculations indicate that the PEEB molecule is physisorbed on the Au(111) substrate, with negligible distortion of the molecular geometry and charge transfer between molecule and substrate.publishe

Light-Induced Switching of Tunable Single-Molecule Junctions

A major goal of molecular electronics is the development and implementation of devices such as single-molecular switches. Here, measurements are presented that show the controlled in situ switching of diarylethene molecules from their nonconductive to conductive state in contact to gold nanoelectrodes via controlled light irradiation. Both the conductance and the quantum yield for switching of these molecules are within a range making the molecules suitable for actual devices. The conductance of the molecular junctions in the opened and closed states is characterized and the molecular level E 0, which dominates the current transport in the closed state, and its level broadening Γ are identified. The obtained results show a clear light-induced ring forming isomerization of the single-molecule junctions. Electron withdrawing side-groups lead to a reduction of conductance, but do not influence the efficiency of the switching mechanism. Quantum chemical calculations of the light-induced switching processes correlate these observations with the fundamentally different lowlying electronic states of the opened and closed forms and their comparably small modification by electron-withdrawing substituents. This full characterization of a molecular switch operated in a molecular junction is an important step toward the development of real molecular electronics devices