New Insights into the Redox Properties of Pyridinium Appended 1,2-Dithienylcyclopentenes.

The optical and redox properties of a methyl pyridinium appended 1,2-dithienylethene photochromic derivative have been thoroughly investigated. A complex multi-step photo/redox mechanism is proposed for the closed isomer on the ground of spectro-electrochemical and theoretical data. The generated compounds are not stable over the time because of chemical reactions associated to the redox processes and a new dithienylethene derivative incorporating a seven-membered ring has been isolated and characterized.

Unprecedented ON/OFF Ratios in Photoactive Diarylethene-Bisthienylbenzene Molecular Junctions.

Photoactive molecular junctions, based on 4 nm thick diarylethene (DAE) and 5 nm thick bisthienylbenzene (BTB) layers, were fabricated by electrochemical deposition. Total thickness was around 9 nm, that is, above the direct tunneling limit and in the hopping regime. The DAE units were switched between their open and closed forms. The DAE/BTB bilayer structure exhibits new electronic functions combining photoswitching and photorectification. The open form of DAE/BTB shows low conductance and asymmetric I-V curves while the closed form shows symmetric I-V curves and high conductance. More importantly, unprecedented ON/OFF current ratios of over 10 000 at 1 V were reproducibly measured.

Long-Range Charge Transport in Diazonium-Based Single-Molecule Junctions.

Thin layers of cobalt and ruthenium polypyridyl-oligomers with thicknesses between 2 and 8 nm were deposited on gold by electrochemical reduction of diazonium salts. A scanning tunneling microscope was used to create single-molecule junctions (SMJs). The charge transport properties of the Au-[Co(tpy)2]n-Au (n = 1-4) SMJs do not depend markedly on the oligomer length, have an extremely low attenuation factor (ß âˆ¼ 0.19 nm-1), and do not show a thickness-dependent transition between two mechanisms. Resonant charge transport is proposed as the main transport mechanism. The SMJ conductance decreases by 1 order of magnitude upon changing the metal from Co to Ru. In Au-[Ru(tpy)2]n-Au and Au-[Ru(bpy)3]n-Au SMJs, a charge transport transition from direct tunneling to hopping is evidenced by a break in the length-dependent ß-plot. The three different mechanisms observed are a clear molecular signature on transport in SMJs. Most importantly, these results are in good agreement with those obtained on large-area molecular junctions.

Highly Efficient Photoswitch in Diarylethene-Based Molecular Junctions.

Thin layers of diarylethene oligomers (oligo(DAE)) were deposited by electrochemical reduction of a diazonium salt on glassy carbon and gold electrodes. The layers were fully characterized using electrochemistry, XPS, and AFM, and switching between open and closed forms using light was evidenced. Solid-state molecular junctions (MJs), in which a C-AFM tip is used as the top contact, were fabricated with total layer thicknesses fixed at 2-3 nm and 8-9 nm, i.e. below and above the direct tunneling limit. DAE was then photoswitched between its open and closed forms. Oligo(DAE) MJs using the open form of DAE are highly resistive while those with DAE in the closed form are more conductive. ON/OFF ratios of 2-3 and 200-400 were obtained for 3-nm- and 9-nm-thick DAE MJs, respectively.

Catalytic Light-Triggered Reduction Promoted by a Dithienylethene Derivative.

A pyridinium substituted dithienylethene derivative was used for the first time as an efficient photoreducing agent of two different substrates. This reaction exhibits high catalytic yields due to the continuous regeneration of the initial state of the photochromic molecule.

Highly Efficient Long-Range Electron Transport in a Viologen-Based Molecular Junction.

Thin layers of viologen-based oligomers with thicknesses between 3 and 14 nm were deposited on gold electrodes by electrochemical reduction of a diazonium salt, and then a Ti/Au top contact was applied to complete a solid-state molecular junction (MJ). MJs show symmetric J- V curves and highly efficient long-range transport, with an attenuation factor as small as 0.25 nm-1. This is attributed both to the fact that the viologen LUMO energy lies between the energies of the Fermi levels of the two contacts and to strong electronic coupling between molecules and contacts. As a consequence, resonant tunneling is likely to be the dominant transport mechanism within these MJs, but the temperature dependence of the transport properties suggests that activated redox hopping plays a role at high temperature.

Robust Bipolar Light Emission and Charge Transport in Symmetric Molecular Junctions.

Molecular junctions consisting of a Ru(bpy)3 oligomer between conducting carbon contacts exhibit an exponential dependence of junction current on molecular layer thickness (d) similar to that observed for other aromatic devices when d < 4 nm. However, when d > 4 nm, a change in transport mechanism occurs which coincides with light emission in the range of 600-900 nm. Unlike light emission from electrochemical cells or solid-state films containing Ru(bpy)3, emission is bipolar, occurs in vacuum, has rapid rise time (<5 ms), and persists for >10 h. Light emission directly indicates simultaneous hole and electron injection and transport, possibly resonant due to the high electric field present (>3 MV/cm). Transport differs fundamentally from previous tunneling and hopping mechanisms and is a clear "molecular signature" relating molecular structure to electronic behavior.

Control of Rectification in Molecular Junctions: Contact Effects and Molecular Signature.

Thin layers of oligomers with thickness between 7 and 9 nm were deposited on flat gold electrode surfaces by electrochemical reduction of diazonium reagents, then a Ti(2 nm)/Au top contact was applied to complete a solid-state molecular junction. The molecular layers investigated included donor molecules with relatively high energy HOMO, molecules with high HOMO-LUMO gaps and acceptor molecules with low energy LUMO and terminal alkyl chain. Using an oligo(bisthienylbenzene) based layer, a molecule whose HOMO energy level in a vacuum is close to the Fermi level of the gold bottom electrode, the devices exhibit robust and highly reproducible rectification ratios above 1000 at low voltage (2.7 V). Higher current is observed when the bottom gold electrode is biased positively. When the molecular layer is based on a molecule with a high HOMO-LUMO gap, i.e., tetrafluorobenzene, no rectification is observed, while the direction of rectification is reversed if the molecular layer consists of naphtalene diimides having low LUMO energy level. Rectification persisted at low temperature (7 K), and was activationless between 7 and 100 K. The results show that rectification is induced by the asymmetric contact but is also directly affected by orbital energies of the molecular layer. A "molecular signature" on transport through layers with thicknesses above those used when direct tunneling dominates is thus clearly observed, and the rectification mechanism is discussed in terms of Fermi level pinning and electronic coupling between molecules and contacts.

Efficient Photoswitch System Combining a Dimethyldihydropyrene Pyridinium Core and Ruthenium(II) Bis-Terpyridine Entities.

Terpyridine ruthenium complexes linked to the dimethyldihydropyrene (DHP) photochromic unit have been synthesized and fully characterized by cyclic voltammetry and absorption and emission spectroscopy. The study of the photoisomerization reaction undergone by the DHP motif under visible light irradiation is reported. In comparison to previous work, the introduction of an electron-withdrawing pyridinium spacer between the chelating terpyridine unit and the DHP skeleton has considerably tuned the photochromic properties of the free ligands and their corresponding complexes in term of time response and photoreversibility. A rapid, reversible, and complete conversion between the closed and the open forms has been clearly evidenced under visible light irradiation. Only slight perturbations have been induced by the presence of ruthenium centers. Experimental findings and their interpretation have been supported by theoretical calculations.

Unprecedented Self-Organized Monolayer of a Ru(II) Complex by Diazonium Electroreduction.

A new heteroleptic polypyridyle Ru(II) complex was synthesized and deposited on surface by the diazonium electroreduction process. It yields to the covalent grafting of a monolayer. The functionalized surface was characterized by XPS, electrochemistry, AFM, and STM. A precise organization of the molecules within the monolayer is observed with parallel linear stripes separated by a distance of 3.8 nm corresponding to the lateral size of the molecule. Such organization suggests a strong cooperative process in the deposition process. This strategy is an original way to obtain well-controlled and stable functionalized surfaces for potential applications related to the photophysical properties of the grafted chromophore. As an exciting result, it is the first example of a self-organized monolayer (SOM) obtained using diazonium electroreduction.