Synthesis and characterisation of ruthenium complexes containing a pendent catechol ring

A series of [Ru(bipy)₂L]⁺ and [Ru(phen)₂L]⁺ complexes where L is 2-[5-(3,4-dimethoxyphenyl)-4H-1,2,4-triazol-3-yl]pyridine (HL1) and 4-(5-pyridin-2-yl-4H-1,2,4-triazol-3-yl)benzene-1,2-diol (HL2) are reported. The compounds obtained have been characterised using X-ray crystallography, NMR, UV/Vis and emission spectroscopies. Partial deuteriation is used to determine the nature of the emitting state and to simplify the NMR spectra. The acid-base properties of the compounds are also investigated. The electronic structures of [Ru(bipy)₂L1]⁺ and Ru(bipy)₂HL1]²⁺ are examined using ZINDO. Electro and spectroelectrochemical studies on [Ru(bipy)₂(L2)]⁺ suggest that proton transfer between the catechol and triazole moieties on L2 takes place upon oxidation of the L2 ligand

Gating Current flowing through Molecules in Metal-Molecules-Metal Junctions

We have assembled two junctions that incorporate redox sites between Hg electrodes by different interactions. In the first junction, Hg-SAM-R//R-SAM-Hg, the redox site (R) are covalently linked to each electrode in self assembled monolayers (SAM-R). In the second junction, Hg-SAM//R//SAM-Hg, the redox sites dissolved in solution are trapped by electrostatic interaction at the SAM formed at the electrodes. The current flowing through these junctions can be controlled by adjusting the potential applied at the electrodes respect to the redox potential of the species by using an electrochemical system. The current flowing in these two junctions is mediated by the redox sites through different mechanisms. In particular, the current flowing through junction Hg-SAM-R//R-SAM-Hg occurs through a self exchange mechanism between the redox sites organized at each electrode, while the current flowing through junction Hg-SAM//R//SAM-Hg is dominated by a redox- cycling mechanism. The systems described here are easy to assemble, well-characterized, yield reproducible data and make it easy to modify the electrical properties of the junctions by changing the nature of the redox centres. For these characteristics they are well suited for collecting fundamental information relevant to the fabrication of molecular switches

Two experimental approaches for controlling current flowing through metal-molecules-metal junctions

Two exptl. approaches that enable control of current flow through metal-mols.-metal junctions are described. A no. of studies using two-electrode metal-mols.-metal junctions have shown that the current between the electrodes depends on the structures of the incorporated mols. When a tunneling mechanism dominates electron transport through org. mols., the mols. behave similar to resistors with resistivities that can be controlled by changing the structure. Incorporation of mols. with increasing conjugation into Hg-based junctions increases the current flow dramatically. Alternatively, by using four-electrode electrochem. junctions that allow the potential of the electrodes to be controlled with respect to the energy levels of the incorporated mols., it is possible to change the mechanism of electron transfer and produce abrupt increases in the current flow. These signals, analogous to solid-state diodes, are particularly significant for mol. electronics. Electrochem. junctions also permit prediction of the value of the applied potential at which the current will start taking off and to identify the mechanism of charge transport. New and recently published results obtained using junctions based on Hg electrodes in an "electrochem." mode show that two junctions incorporating redox centers by different interactions behave as current switches, with the current flow dominated by different charge-transport mechanisms

Electron Transport through Hexa-peri-hexabenzocoronene Units in a Metal-SAMs-Metal Junction

Electron transport measurements along molecules incorporating hexa-peri-hexabenzocoronenes (HBC) and organized in self assembled monolayers (SAM) are performed by using a junction based on an Au and a Hg electrode, Au-SAM//SAM-Hg. It is well known that HBC generates well ordered supramolecular aggregates via π-π interactions between the hexabenzocoronene discs. While charge mobilty across the supramolecular aggregates have been measured, there are no information of electron transfer along the axial axis of the aromatic core. This work focuses on i) the synthesis of a the soluble compound, 2-(5-[1,2]dithiolan-3-yl-pentanoic acid dodecyl ester)-5,8,11,14,17-(3,7-dimethyloctanyl) hexa-peri-hexabenzocoronene (HBC), that allows for organization in SAM, ii) the characterization of the HBC SAMs by by XPS, NEXFAS spectroscopy and elipsometry and, iii) I-V measurements across four junctions incorporating the HBC SAMs (HBCS) and SAMs formed by alkyl chains of different length (Cn): Au-Cn//C12-Hg, Au-HBCS//Cn-Hg, Au-Cn//HBCS-Hg, Au-HBCS//HBCS-Hg. The results show that i-V curves measured in Au-C18//C18-Hg, Au-HBCS//C18-Hg, Au-HBCS//HBCS-Hg are overlapping, indicating that the HBC core behaves as transparent to electrons in respect with the aliphatic anchoring chains. The aliphatic chains are responsible for the total tunneling barrier to electron transport of the HBC monolayers. The 30 Å thick HBC SAM can be considered as effectively composed of two parts: a ”highly conductive” HBC layer with a thickness of 10 Å and an “low conductive” aliphatic part with a thickness of 20 Å. Both high mechanical stability and high electric “conductivity” of the HBC unit, qualify this material as promising building block for molecular electronics