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Two experimental approaches that enable control of current flow through metal–molecules–metal junctions are described. A number of studies using two-electrode metal–molecules–metal junctions have shown that the current between the electrodes depends on the structures of the incorporated molecules. When a tunneling mechanism dominates electron transport through organic molecules, the molecules behave similar to resistors with resistivities that can be controlled by changing the structure. Incorporation of molecules with increasing conjugation into Hg-based junctions increases the current flow dramatically. Alternatively, by using four-electrode electrochemical junctions that allow the potential of the electrodes to be controlled with respect to the energy levels of the incorporated molecules, 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 molecular electronics. Electrochemical 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 “electrochemical” 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.

The fabrication of functional electronic devices based on ultrathin organic films, such as self-assembled monolayers (SAMs), requires the understanding and control of electron transport across organic molecules. Our group and others have shown that the electron-transfer rate, measured by the current flow through metal–molecule–metal junctions, sensitively depends on the structure of the molecules. Electron transfer through molecules is commonly described by a simple relation used for electron tunneling mechanisms

The synthesis, characterization, and electrochemical, photophysical, and photochemical properties of the binuclear compounds [(Ru(H-8-bpy)(2))(2)((Metr)(2)Pz)](PF6)(2) (1) and [(Ru(D-8-bpy)(2))(2)((Metr)(2)Pz)](PF6)(2) (2), where bpy is 2,2'-bipyridine and H-2(Metr)(2)Pz is the planar ligand 2,5-bis(5'-methyl-4'H-[1,2,4]triaz-3'-yl)-pyrazine, are reported. Electrochemical and spectro-electrochemical investigations indicate that the ground-state interaction between each metal center is predominantly electrostatic and in the mixed-valence form only a low level of ground-state delocalization is present. Resonance Raman, transient, and time-resolved spectroscopies enable a detailed assignment to be made of the excited-state photophysical properties of the complexes. Deuteriation is employed to both facilitate spectroscopic characterization and investigate the nature of the lowest excited states.

The synthesis and characterization of mixed ligand 2,2';6',2' '-terpyridine (tpy) ruthenium complexes with 2,6-bis([1,2,4]triazol-3-yl)pyridine, 2,6-bis(5-phenyl-[1,2,4]triazol-3-yl)pyridine, and 2,6-bis([1,2,3,4]tetrazol-5-yl)pyridine are reported. The species are characterized by HPLC, 1H NMR, UV/vis, and emission spectroscopy. The photophysical properties of the complexes are investigated as a function of temperature over the range 80-320 K. The emission lifetime observed for the fully deprotonated compounds at room temperature is about 80 ns. This increase by 2 orders of magnitude with respect to the parent "[Ru(tpy)2](2+)" complex is rationalized by an increase in the energy of the metal based dsigma orbital, rather than by manipulation of the pi* orbitals on the ligands. The acid-base and electrochemical properties of the compounds are reported also.

Hydrogen bond association between ruthenium bibenzimidazole and carboxylated polypyridylosmium complexes results in stable supramolecular aggregates. The determined stability constant of logK approximate to 6 +/- 0.3 allows efficient energy transfer from the ruthenium to the osmium moiety. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

Complexes of the type [Ru(H2bibzim)(R-bpy)2X2·nH2O (R = H, 2,2′-bipyridine, bpy, X = CF3SO3−1; R = Me, 4,4′-dimethyl-2,2′-bipyridine, dmbpy, X = PF6−2; R = tert-butyl, 4,4′-di-tert-butyl-2,2′-bipyridine, tbbpy, X = Cl−3; H2bibzim = 1,1′-bibenzimidazole) containing two NH functions acting as hydrogen bond donors formed different spatially highly organised supramolecular assemblies with water. X-Ray investigation revealed that the nature of the counter ion influences the hydrogen bonding pattern and extent of spatial organisation. In 2 one dimensional chains of hydrogen bonded water could be found. In the deprotonated complex [Ru(bibzim)(tbbpy)2] 4 water molecules serve as hydrogen bond donors. The diastereomeric forms 5a/5b of the homodinuclear ruthenium complex [{Ru(tbbpy)2}2(bibzim)][PF6]2 could be separated. No differences in their photophysical properties could be detected. The X-ray investigation of 5a and [{Ru(bpy)2}2(bibzim)][PF6]26 showed little influence of peripheral substitution on structural properties. Complexes 3–5 exhibit activity in electrochemical CO2 reduction which can be tuned by variation of the degree of protonation of the bibenzimidazole.

This paper compares the structural and electrical characteristics of self-assembled monolayers (SAMs) of n-alkanethiolates, SCn (n = 10, 12, 14), on two types of silver substrates: one used as-deposited (AS-DEP) by an electron-beam evaporator, and one prepared using the method of template-stripping. Atomic force microscopy showed that the template-stripped (TS) silver surfaces were smoother and had larger grains than the AS-DEP surfaces, and reflectance-absorbance infrared spectroscopy showed that SAMs formed on TS substrates were more crystalline than SAMs formed on AS-DEP substrates. The range of current densities, J (A/cm2), measured through mercury-drop junctions incorporating a given SAM on AS-DEP silver was, on average, several orders of magnitude larger than the range of J measured through the same SAM on TS silver, and the AS-DEP junctions failed, on average, 3.5 times more often within five current density-voltage (J-V) scans than did TS junctions (depending on the length of the alkyl chains of the molecules in the SAM). The apparent log-normal distribution of J through the TS junctions suggests that, in these cases, it is the variability in the effective thickness of the insulating layer (the distance the electron travels between electrodes) that results in the uncertainty in J. The parameter describing the decay of current density with the thickness of the insulating layer, beta, was either 0.57 A-1 at V = +0.5 V (calculated using the log-mean of the distribution of values of J) or 0.64 A-1 (calculated using the peak of the distribution of values of J) for the TS junctions; the latter is probably the more accurate. The mechanisms of failure of the junctions, and the degree and sources of uncertainty in current density, are discussed with respect to a variety of defects that occur within Hg-drop junctions incorporating SAMs on silver.

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 with 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 the Hg-SAM-R//R-SAM-Hg junction occurs through a self exchange mechanism between the redox sites organized at each electrode, while the current flowing through the Hg-SAM//R//SAM-Hg junction 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.

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.

Tetra-peri-(tert-butyl-benzo)-di-peri-(pyrimidino)-coronene 1, the parent compound of the nitrogen heterosuperbenzene family N-HSB, is employed as a novel monotopic ligand in the formation of [Pd(eta(3)-C3H5)(1)]PF6 2 and [Ru(bpy)(2)(1)](PF6)(2) (where bpy = 2,2'-bipyridine 3a and d(8)-2,2'-bipyridine 3b). These N-coordinated complexes are fully characterized by H-1 NMR and IR spectroscopy and ESI-MS. Metal coordination has a profound effect on both the absorption and the emission properties of 1. Pd(II) coordination causes a red-shift in the low-energy absorptions, a decrease in the intensity of the n-pi* absorptions, and a quenching of the emission. Ru(II) coordination causes absorption throughout the visible region and creates two new complexes that join an elite group of compounds known as "black" absorbers. 3a and 3b possess two discernible (MLCT)-M-1 bands. The one of exceptionally low energy (lambda(max) = 615 nm) has an associated (MLCT)-M-3 emission ( lambda(max) = 880 nm) due to the unprecedented electron delocalization and acceptor properties of the rigid aromatic N-HSB 1. Both Ru(II) complexes are near-IR emitters with unusually protracted emission lifetimes of 320 ns at 77 K. They are photochemically inert, and their electrochemical properties are consistent with the presence of a low-lying pi* orbital on 1. The first two reversible reductions (E-1/2 (CH3CN), -0.54 V, -1.01 V vs SCE) are due to the stepwise reduction of 1 and are anodically shifted as compared to [Ru(bpy)(3)](2+). Temperature- and concentration-dependent NMR studies on 2 and 3a suggest extensive aggregation is occurring in solution.

A photophysical study of the interaction between dihydrogen phosphate anions and a series of ruthenium polypyridyl complexes containing anion receptor 4,4'-bis[(2-methoxyethyl)carbamoyl]-2,2'-bipyridine (L1) is reported. The complexes investigated are of the type [Ru(L)[2](L1)]X[2] and [Ru(L1)[3]]X[2], where L is 2,2'-bipyridyl or 1,10-phenanthroline and X is chloride or PF[6]-. The emission properties of the compounds are studied as a function of the anion concentrations using emission lifetime and steady-state measurements. For the mixed ligand complexes the emission intensity and lifetime increases upon the addition of 2.5 molar equivalents of H[2]PO[4]-, further additions do not result in further increases. For [Ru(L1)[3]]X[2] the emission increases upon addition of the first 2.5 equivalents of dihydrogen phosphate, however, further increases lead to a decrease in both emission intensity and lifetime. The effect of the addition of trace amounts of water is also examined.

The luminescence of a ruthenium bibenzimidazole complex can be switched on by the presence of Zn(II), Mg(II), and Cu(I) compounds. Complexation of these metals to the ruthenium center is confirmed by electrospray mass spectrometry.

The mononuclear compound [Ru(terpy)L], where H2L is 2,6-bis(1,2,4-triazol-3-yl)pyridine, shows an emission lifetime of 65 ns, about 300 times longer than that observed for the parent [Ru(terpy)3]2+ complex.