Redox-Active Ruthenium-Organic Polyhedra with Tunable Surface Functionality and Porosities.

Dinuclear ruthenium paddlewheel complexes exhibit high structural stability in redox reactions. The use of these chemical motifs for the construction of Ru-based metal-organic polyhedra (RuMOPs) provides a route for redox-active porous materials. However, there are few studies on the synthesis and characterization of RuMOPs due to the difficulty in controlling the assembly process via the ligand-exchange reaction of equatorial acetates of the diruthenium tetraacetate precursors with dicarboxylic acid ligands. In this study, we synthesized three novel cuboctahedral RuMOPs based on the Ru2(II/III)-paddlewheel units with different alkyl functionalizations on the benzene-1,3-dicarboxylate moieties. We evaluated the effect of external functionalization on the molecular packing and the porous and redox properties. The electrochemical measurements revealed the multielectron transferred redox process where the electron-donating/-withdrawing nature of the functional groups allows the control of the redox behavior.

Photoexcited Anhydrous Proton Conductivity in Coordination Polymer Glass.

Optically switchable proton-conductive materials will enable the development of artificial ionic circuits. However, most switchable platforms rely on conformational changes in crystals to alter the connectivity of guest molecules. Guest dependency, low transmittance, and poor processability of polycrystalline materials hinder overall light responsiveness and contrast between on and off states. Here, we optically control anhydrous proton conductivity in a transparent coordination polymer (CP) glass. Photoexcitation of tris(bipyrazine)ruthenium(II) complex in CP glass causes reversible increases in proton conductivity by a factor of 181.9 and a decrease in activation energy barrier from 0.76 eV to 0.30 eV. Modulating light intensity and ambient temperature enables total control of anhydrous protonic conductivity. Spectroscopies and density functional theory studies reveal the relationship between the presence of proton deficiencies and the decreasing activation energy barrier for proton migrations.

Synthesis, X-ray structure, photophysical properties, and theoretical studies of six-membered cyclometalated iridium(iii) complexes: revisiting Ir(pnbi)2(acac).

We have determined the X-ray structure of Ir(pnbi)2(acac) (pnbi = 2-phenanthren-9-yl-1-phenyl-1H-benzimidazole; acac = acetylacetonate), which exhibits a six-membered metallocycle around the Ir center. This result stands in sharp contrast to previously postulated structures of Ir(pnbi)2(acac), which assumed a five-membered metallocycle. In this paper, we focus on the relative stability of five- and six-membered Ir(C^N) ring structures. DFT calculations of the total energies of Ir-(C^N) complexes indicated that six-membered structures are more stable when bulky substituents are present in the benzimidazole unit. When the phenanthrene group of pnbi was replaced with a naphthalene moiety, DFT calculations predicted that five-membered cycles are more stable than six-membered rings, which was confirmed experimentally by a single-crystal X-ray diffraction analysis. The steric bulk of the phenanthrene-containing polyaromatic ring ligand thus induces greater interligand repulsion between the two ligands, which plays an important role in determining the cyclometalation route. The Ir complexes examined in this study exhibit red emission (λem ≈ 660 nm) with relatively low quantum yields.

A Peanut-Shaped Polyaromatic Capsule: Solvent-Dependent Transformation and Electronic Properties of a Non-Contacted Fullerene Dimer.

Synthesis of molecular containers capable of incorporating multiple fullerenes remains challenging. Reported here is that room-temperature mixing of metal ions with W-shaped bispyridine ligands featuring polyaromatic panels results in the quantitative formation of a peanut-shaped M2 L4 capsule. The capsule reversibly converts into two molecules of an ML2 double tube in response to changes in the solvent. Notably, the capsule allows the incorporation of two fullerene molecules into the connected two spherical cavities at room temperature. The close proximity yet non-contact of the encapsulated C60 molecules, with a separation of 6.4 Å, was revealed by X-ray crystallographic analysis. The resultant, unusual fullerene dimer undergoes sequential reduction within the capsule to generate (C60 .- )2 , C60 .- ⋅C60 2- , and (C60 2- )2 species. Furthermore, temperature-controlled stepwise incorporation of two C60 molecules into the capsule is demonstrated.

Bio-inspired protonic memristor devices based on metal complexes with proton-coupled electron transfer.

A new type of memristor inspired by bio-membranes is presented, based on the proton movement resulting from proton-coupled electron transfer (PCET) processes in dinuclear Ru complexes, whereby a two-terminal device based on said Ru complexes and a proton-conducting polymer was constructed as a proof-of-concept. Two ITO electrodes were modified separately with dinuclear Ru complexes that bear tetraphosphonic acid linkers at both ends and a 2,6,2',6'-tetrakis(benzimidazol-2-yl)-4,4'-bipyridine (RuNH-OH) or 1,3,1',3'-tetrakis(benzimidazol-2-yl)-5,5'-biphenyl (RuCH-OH) bridging ligand, and both ITO electrodes exhibit PCET processes with different Ru(ii/iii) redox potentials and pKa values. Poly(4-vinylpyridine) (P4VP; pKa = 4-5), a proton-conducting polymer, was sandwiched between the two modified ITO electrodes to construct a two-terminal device of the type ITO|(RuNH-OH)3|P4VP|(RuCH-OH)3|ITO. Initially, the oxidation state of the metal centers in RuNH-OH and RuCH-OH is Ru(ii) and Ru(iii), respectively. Upon applying a bias voltage between the two ITO electrodes, the high and low current states switch at approximately ±1.10 V due to Ru(ii/iii) redox reactions. At the RuNH-OH|P4VP and RuCH-OH|P4VP interfaces, a proton is released from Ru(ii)NH-OH and subsequently captured by Ru(iii)CH-OH through the hydrogen-bonding interaction with the P4VP polymer, which is driven by the changes in the pKa values of the Ru complexes from 4.1-8.8 [Ru(ii)NH-OH] to <3.8 [Ru(iii)NH-OH] and from <8.4 [Ru(ii)CH-OH] to 5.2-9.8 [Ru(iii)CH-OH] under these conditions. The redox reactions on the modified Ru films create a large proton gradient between the two electrodes, enhancing the proton conductivity through the P4VP layer (pKa = 4-5). When the applied bias potential was inverted, the pKa gradient returned to the original state and the current decreased. Such a proton-conductivity enhancement is relevant to the transport of protons by proton gradients in bio-membranes. Therefore, the present protonic coordination-network films containing metal complexes that exhibit PCET should open new avenues for the design of a new type of memristor devices mimicking the function of synapses.

Proton-Rocking-Chair-Type Redox Capacitors Based on Indium Tin Oxide Electrodes with Multilayer Films Containing Ru Complexes.

A rechargeable proton-rocking-chair-type redox capacitor was fabricated using scalable layer-by-layer-(LbL)-assembled films composed of two dinuclear Ru complexes that exhibit proton-coupled electron-transfer (PCET) reactions with different Ru(II/III) redox potentials (RuNH-OH and RuCH-OH). RuNH-OH and RuCH-OH contain different coordination environments that involve two phosphonate linker ligands at both ends and bridging 2,6,2',6'-tetrakis(benzimidazol-2-yl)-4,4'-bipyridine or 1,3,1',3'-tetrakis(benzimidazol-2-yl)-5,5'-biphenyl ligands, respectively. The molecular units were assembled onto indium tin oxide (ITO) electrodes by complexation between the phosphonate groups and zirconium(IV) ions. The LbL growing process of these multilayer films was monitored by electrochemical or UV-vis spectroscopic measurements. The thus obtained LbL films on the ITO electrodes showed PCET reactions at different potentials, depending on the bridging ligands. The introduction of cyclometalated Ru-C bonds in the bridging ligand of RuCH-OH led to the stabilization of the ruthenium(III) oxidation state, and therefore, RuCH-OH exhibited lower p Ka values for the imino N-H protons in the bridging benzimidazole groups compared to those of the corresponding RuNH-OH complex. The proton movements that accompany the redox reaction in the Ru multilayer films on the ITO electrode were confirmed using a pH indicator probe. For the performance test of a proton-rocking-chair-type redox capacitor, a two-electrode system composed of RuNH-OH and RuCH-OH multilayer films on ITO electrodes was examined in an aqueous solution of NaClO4. Under galvanostatic conditions, stable, reversible, and repeatable charging/discharging processes occurred. The capacitance increased with an increasing number of LbL layers. For comparison, a similar redox capacitor composed of two RuNMe-OH and RuCMe-OH analogues, in which all four imino N-H protons on the benzimidazole moieties are protected by N-Me groups, was constructed and examined. In these complexes, the capacitance decreased by 77% compared to the PCET-type capacitor composed of a cell containing RuNH-OH and RuCH-OH; this result strongly suggests that the proton movement plays a more important role for the charge storage than the anion movement. In such LbL films composed of Ru complexes that exhibit PCET-type redox reactions, the capacitance is drastically improved with an increasing number of layers and using protons as charge carriers.

Hydrogen-bonded metallo-supramolecular polymers based on ruthenium or iron complexes for the selective extraction of single-walled carbon nanotubes.

The chemical functionalization of nano-carbon materials such as single-walled carbon nanotubes (SWNTs) and graphene by metal complexes has attracted much attention due to the multitude of potential applications in efficient energy-conversion and -storage devices. The solubilization and single-chirality separation of SWNTs by surface modifications is a useful approach to manipulate SWNTs in the liquid phase. In this study, several Ru and Fe complexes containing two terminal quadruple hydrogen-bonded (H-bonded) ureidopyrimidinedione (UPy) groups were synthesized (Ru-1, Ru-UPy, and Fe-UPy) to form H-bonded supramolecular polymers by self-association. In order to control the solubility of these complexes in nonpolar solvents, Ru-UPy and Fe-UPy were endowed with long alkyl side chain groups in the coordinated 2,6-bis(benzimidazol-2-yl)pyridine ligand, while Ru-1 and Ru-2 do not contain such long alkyl chain groups. AFM measurements revealed that Ru-1, Ru-UPy, and Fe-UPy form a fiber-like network morphology on HOPG surfaces, arising from the H-bonded aggregation. However, only Ru-UPy and Fe-UPy are able to solubilize SWNTs effectively upon simple sonication in chlorobenzene. After the solubilization of a CoMoCAT® SWNT in chlorobenzene using Ru-UPy or Fe-UPy, UV-Vis-NIR spectra showed sharp peaks at 996 and 1150 nm, which were attributed to (6, 5) and (7, 6)-SWNTs. The Raman spectra of the solubilized SWNTs revealed peaks that were attributed to the radial breathing mode (RBM), which suggests an enrichment of semiconducting SWNTs, i.e., Ru-UPy and Fe-UPy are able to selectively solubilize semiconducting SWNTs. Cyclic voltammograms of films of SWNTs covered with Ru-UPy or Fe-UPy on ITO electrodes showed a well-defined adsorbed Ru(ii/iii) or Fe(ii/iii) wave. Upon addition of acid, the redox response from the adsorbed H-bonded Ru-UPy and Fe-UPy disappeared and only SWNTs were left on the ITO electrode. Moreover, the Ru-UPy/SWNT and Fe-UPy/SWNT hybrids exhibited non-linear I-V characteristics.

Robust Nanowrapping of Reduced Graphene Oxide by Metal-Organic Network Films between Fe Ions and Tetra(Catechol-Substituted) Porphyrin.

We found the utilization of porphyrin-based metal-organic network films composed of tetra(catechol-substituted)porphyrin (cPor) and Fe ions for robust wrapping materials of graphene oxide (GO), which can keep the dispersion state under the chemical reduction of GO to reduced graphene oxide (rGO) in water. The tetra(catechol-substituted)porphyrin (cPor) was designed for soft-wrapping methods because the aromatic porphyrin moieties can be strongly adsorbed onto the surface of GO or rGO via both π-π interactions and the catechol-Fe coordination network formation. The GO sheets covered with the cPor-Fe films were reduced chemically in water under retention of the wrapped nanostructure of the cPor-Fe/GO sheets. The obtained rGO composites after chemical reduction are characterized by using UV-vis absorption, Raman, and X-ray photoelectron spectroscopy (XPS) spectra, as well as thermogravimetric analysis and energy-dispersive X-ray spectroscopy (EDX). XPS and EDX spectra showed the presence of Fe species, which originates from the coordinated Fe-catechol nodes in the wrapped cPor-Fe films. The wrapped rGO sheets could be easily handled in water because of their high solubility therein and exhibits electric conductivity. In dynamic light scattering analysis, the average diameter of rGO composites before and after reduction changed slightly from 419 ± 309 to 663 ± 697 nm, indicating that the chemical reduction is not significantly influenced by the size of the rGO composite or the solubility. It is expected that the soft wrapping cPor-Fe/rGO should employ the applications to prepare functional materials such as modified electrodes, catalysts, energy-storage materials, and electronic devices.

Wisely Designed Phthalocyanine Derivative for Convenient Molecular Fabrication on a Substrate.

An axial-substituted silicon phthalocyanine derivative, SiPc(OR)2 (R = C4H9), that is soluble in organic solvent is conveniently synthesized. This silicon phthalocyanine derivative reacts with a hydroxyl group on a substrate and then with another phthalocyanine derivative under mild conditions. The accumulation number of the phthalocyanine molecules on the substrates is easily controlled by the immersion time. On the basis of AFM (atomic force microscopy) images, the surface of the phthalocyanine-modified glass substrate has uneven structures on the nanometer scale. ITO electrodes modified with the composition of the phthalocyanine derivative and PCBM show stable cathodic photocurrent generation upon light irradiation.

Humidity-controlled rectification switching in ruthenium-complex molecular junctions.

Although molecular rectifiers were proposed over four decades ago 1,2 , until recently reported rectification ratios (RR) were rather moderate 2-11 (RR ~ 101). This ceiling was convincingly broken using a eutectic GaIn top contact 12 to probe molecular monolayers of coupled ferrocene groups (RR ~ 105), as well as using scanning tunnelling microscopy-break junctions 13-16 and mechanically controlled break junctions 17 to probe single molecules (RR ~ 102-103). Here, we demonstrate a device based on a molecular monolayer in which the RR can be switched by more than three orders of magnitude (between RR ~ 100 and RR ≥ 103) in response to humidity. As the relative humidity is toggled between 5% and 60%, the current-voltage (I-V) characteristics of a monolayer of di-nuclear Ru-complex molecules reversibly change from symmetric to strongly asymmetric (diode-like). Key to this behaviour is the presence of two localized molecular orbitals in series, which are nearly degenerate in dry circumstances but become misaligned under high humidity conditions, due to the displacement of counter ions (PF6-). This asymmetric gating of the two relevant localized molecular orbital levels results in humidity-controlled diode-like behaviour.

Controlling the Molecular Direction of Dinuclear Ruthenium Complexes on HOPG Surface through Noncovalent Bonding.

We synthesized three types of binuclear Ru complexes (1-3) that contain pyrene anchors for the adsorption of 1-3 onto nanocarbon materials via noncovalent π-π interactions, in order to investigate their adsorption onto and their desorption from highly ordered pyrolytic graphite (HOPG). The adsorption saturation for 1 (6.22 pmol/cm2), 2 (2.83 pmol/cm2), and 3 (3.53 pmol/cm2) on HOPG was obtained from Langmuir isotherms. The desorption rate from HOPG electrodes decreased in the order 3 (2.4 × 10-5 s-1) > 2 (1.4 × 10-5 s-1) ≫ 1 (1.8 × 10-6 s-1). These results indicate that the number of pyrene anchors and their position of substitution in such complexes strongly affect the desorption behavior. However, neither the free energy of adsorption (ΔGads) nor the heterogeneous electron-transfer rate (kET) showed any significant differences among 1-3, albeit that the surface morphologies of the modified HOPG substrates showed domain structures that were characteristic for each Ru complex. In the case of 3, the average height changed from ∼2 to ∼4 nm upon increasing the concentration of the solution of 3 that was used for the surface modification. In contrast, the height for 1 and 2 remained constant (1.5-2 nm) upon increasing the concentration of the complexes in the corresponding solutions. While the molecular orientation of the Ru-Ru axis of 3 relative to the HOPG surface normal changed from parallel to perpendicular, the Ru-Ru axis in 1 and 2 remained parallel, which leads to an increased stability of 1 and 2.

Synchronized Collective Proton-Assisted Electron Transfer in Solid State by Hydrogen-Bonding Ru(II)/Ru(III) Mixed-Valence Molecular Crystals.

A proton-coupled electron transfer (PCET) reaction was widely studied with isolated organic molecules and metal complexes in solution in view of the biological catalytic reaction, while studying this reaction in the crystalline or solid-state phase, which has a novel example, would give insight into the rather internal environment of proteins without solvation and a creation of new molecular materials. We tried to crystallize a hydrogen-bonded (H-bonded) coordination polymer with one-dimensional nanoporous channels, formed from redox-active RuIII complexes, [RuIII(Hbim)3] (Hbim- = 2,2'-biimidazolate monoanion). As a result, a synchronized collective PCET phenomenon was observed for the molecular nanoporous crystal by novel solid-state cyclic voltammetry (CV), which could be measured by only setting some crystals on the electrode surface. The nanoporous crystals, {[RuIII(Hbim)3]}n (1), are simultaneously induced to a synchronized collective RuIIRuIII mixed-valence state, {RuIIRuIII}n, with alternating arrays of RuII and RuIII complexes by PCET in a way of the reductive state of {RuIIRuII}n. Further, a new crystal with {RuIIRuIII}n, {[RuII(H2bim)(Hbim)2][RuIII(bim) (Hbim)2][K(MeOBz)6]}n (2), was also prepared, and the solid-state CV revealed the same electrochemical behavior of {RuIIRuIII}n with 1. The single crystal with {RuIIRuIII}n of 2 was unusually a semiconductor with 5.12 × 10-6 S/cm conductivity at 298 K by an impedance method (8.01 × 10-6 S/cm by a direct-current method at 277 K). Thus, an unprecedented electron-hopping conductor driven by a low-barrier proton transfer through a PCET mechanism (Ea = 0.30 eV) was realized in the H-bonding molecular crystal with {RuIIRuIII}n. Such studies on a PCET reaction in the crystalline state is not only worthwhile as a model of essential biological reactions without solvation, but also proposed to a new design of molecular materials to occur an electron transfer by using an intermolecular H-bond.

Stable anchoring chemistry for room temperature charge transport through graphite-molecule contacts.

An open challenge for single-molecule electronics is to find stable contacts at room temperature with a well-defined conductance. Common coinage metal electrodes pose fabrication and operational problems due to the high mobility of the surface atoms. We demonstrate how molecules covalently grafted onto mechanically robust graphite/graphene substrates overcome these limitations. To this aim, we explore the effect of the anchoring group chemistry on the charge transport properties of graphite-molecule contacts by means of the scanning tunneling microscopy break-junction technique and ab initio simulations. Molecules adsorbed on graphite only via van der Waals interactions have a conductance that decreases exponentially upon stretching the junctions, whereas the molecules bonded covalently to graphite have a single well-defined conductance and yield contacts of unprecedented stability at room temperature. Our results demonstrate a strong bias dependence of the single-molecule conductance, which varies over more than one order of magnitude even at low bias voltages, and show an opposite rectification behavior for covalent and noncovalent contacts. We demonstrate that this bias-dependent conductance and opposite rectification behavior is due to a novel effect caused by the nonconstant, highly dispersive density of states of graphite around the Fermi energy and that the direction of rectification is governed by the detailed nature of the molecule/graphite contact. Combined with the prospect of new functionalities due to a strongly bias-dependent conductance, these covalent contacts are ideal candidates for next-generation molecular electronic devices.

Energy-Storage Applications for a pH Gradient between Two Benzimidazole-Ligated Ruthenium Complexes That Engage in Proton-Coupled Electron-Transfer Reactions in Solution.

The judicious selection of pairs of benzimidazole-ligated ruthenium complexes allowed the construction of a rechargeable proton-coupled electron-transfer (PCET)-type redox battery. A series of ruthenium(II) and -(III) complexes were synthesized that contain substituted benzimidazoles that engage in PCET reactions. The formation of intramolecular Ru-C cyclometalation bonds stabilized the resulting ruthenium(III) complexes, in which pKa values of the imino N-H protons on the benzimidazoles are usually lower than those for the corresponding ruthenium(II) complexes. As a proof-of-concept study for a solution redox battery based on such PCET reactions, the charging/discharging cycles of several pairs of ruthenium complexes were examined by chronopotentiometry in an H-type device with half-cells separated by a Nafion membrane in unbuffered CH3CN/H2O (1/1, v/v) containing 0.1 M NaCl. During the charging/discharging cycles, the pH value of the solution gradually changed accompanied by a change of the open-circuit potential (OCP). The changes for the OCP and pH value of the solution in the anodic and cathodic half-cells were in good agreement with the predicted values from the Pourbaix diagrams for the pairs of ruthenium complexes used. Accordingly, the careful selection of pairs of ruthenium complexes with a sufficient potential gradient and a suitably large pKa difference is crucial: the charge generated between the two ruthenium complexes changes the OCP and the pH difference between the two cells in an unbuffered solution, given that the PCET reactions occur at both electrodes and that discharging leads to the original state. Because the electric energy is stored as a pH gradient between the half-cells, new possibilities for PCET-type rocking-chair redox batteries arise.

Simultaneous Formation and Spatial Patterning of ZnO on ITO Surfaces by Local Laser-Induced Generation of Microbubbles in Aqueous Solutions of [Zn(NH3)4]2.

We demonstrate the simultaneous formation and spatial patterning of ZnO nanocrystals on an indium-tin oxide (ITO) surface upon local heating using a laser (1064 nm) and subsequent formation of microbubbles. Laser irradiation of an ITO surface in aqueous [Zn(NH3)4]2+ solution (1.0 × 10-2 M at pH 12.0) under an optical microscope produced ZnO nanocrystals, the presence of which was confirmed by X-ray diffraction analysis and Raman microspectroscopy. Scanning the focused laser beam over the ITO surface generated a spatial ZnO pattern (height: ∼60 nm, width: ∼1 µm) in the absence of a template or mask. The Marangoni convection generated in the vicinity of the microbubbles resulted in a rapid concentration/accumulation of [Zn(NH3)4]2+ around the microbubbles, which led to the formation of ZnO at the solid-bubble-solution three-phase contact line around the bubbles and thus afforded ZnO nanocrystals on the ITO surface upon local heating with a laser.

Stepwise fabrication of donor/acceptor thin films with a charge-transfer molecular wire motif.

Novel thin films composed of a donor (D)/acceptor (A) charge-transfer chain compound were fabricated by a layer-by-layer technique using complexation of a paddlewheel-type diruthenium(ii, ii) complex with an N,N'-dicyanoquinonediimine derivative on an ITO substrate with a pyridine-substituted phosphonate anchor. The stepwise growth of an electron-transfer D+A--chain thin film was confirmed.

Synthesis and Single-Molecule Conductance Study of Redox-Active Ruthenium Complexes with Pyridyl and Dihydrobenzo[b]thiophene Anchoring Groups.

The ancillary ligands 4'-(4-pyridyl)-2,2':6',2''-terpyridine and 4'-(2,3-dihydrobenzo[b]thiophene)-2,2'-6',2"-terpyridine were used to synthesize two series of mono- and dinuclear ruthenium complexes differing in their lengths and anchoring groups. The electrochemical and single-molecular conductance properties of these two series of ruthenium complexes were studied experimentally by means of cyclic voltammetry and the scanning tunneling microscopy-break junction technique (STM-BJ) and theoretically by means of density functional theory (DFT). Cyclic voltammetry data showed clear redox peaks corresponding to both the metal- and ligand-related redox reactions. Single-molecular conductance demonstrated an exponential decay of the molecular conductance with the increase in molecular length for both the series of ruthenium complexes, with decay constants of ßPY =2.07±0.1 nm(-1) and ßBT =2.16±0.1 nm(-1) , respectively. The contact resistance of complexes with 2,3-dihydrobenzo[b]thiophene (BT) anchoring groups is found to be smaller than the contact resistance of ruthenium complexes with pyridine (PY) anchors. DFT calculations support the experimental results and provided additional information on the electronic structure and charge transport properties in those metal|ruthenium complex|metal junctions.

Controlling the Adsorption of Ruthenium Complexes on Carbon Surfaces through Noncovalent Bonding with Pyrene Anchors: An Electrochemical Study.

Surface modifications of carbon nanomaterials, such as graphene or carbon nanotubes, through noncovalent π-π interactions between π-conjugated carbon surfaces and pyrene anchors have received much attention on account of the applications of these materials in organic electronic and sensor devices. Despite the rapidly expanding use of pyrene anchors, little is known about the number of pyrene groups required in order to achieve a stable attachment of molecules on nanocarbon surfaces. So far, systematic studies on such surface modifications through adsorption isotherms and desorption behavior of molecules still remain scarce. In this study, we have investigated the effect of the number of pyrene anchors in redox-active Ru complexes on their adsorption on carbon nanomaterials through noncovalent π-π interactions. The Ru(II/III) couple was used as a redox marker in order to determine the surface coverage on nanocarbon surfaces such as highly oriented pyrolytic graphite (HOPG), single-walled carbon nanotubes (SWCNTs), and multiwalled carbon nanotubes (MWCNTs). The amount of surface coverage as well as the kinetic stability of the Ru complexes was thereby observed to be directly proportional to the number of pyrene groups present in the ligands. The desorption rate from HOPG electrode increased in the order Ru-1 with eight pyrene groups (k = 2.0 × 10(-5) s(-1)) < Ru-2 with four pyrenes (4.1 × 10(-5) s(-1)) < Ru-3 with two pyrenes (6.8 × 10(-5) s(-1)) ≪ Ru-4 with one pyrene (4.1 × 10(-3) s(-1)). Furthermore, the electrochemical polymerization of the Ru complex with four pyrene groups proceeded more efficiently compared to complexes with one or two pyrene groups. As a consequence, compounds having more than two and/or optimally four pyrene groups revealed a stable adsorption on the nanocarbon surfaces. The heterogeneous electron transfer rate between the Ru complex, Ru-2, and the carbon nanomaterials increased in the order SWCNTs (kET = 1.3 s(-1)) < MWCNTs (ϕ = 5-9 nm) (kET = 4.0 s(-1)) < MWCNTs (ϕ = 110-170 nm) (kET = 14.9 s(-1)) < HOPG (kET = 110 s(-1)).

Controlling the Direction of the Molecular Axis of Rod-Shaped Binuclear Ruthenium Complexes on Single-Walled Carbon Nanotubes.

We report the synthesis of a mixed-valence ruthenium complex, bearing pyrene moieties on one side of the ligands as anchor groups. Composites consisting of mixed-valence ruthenium complexes and SWNTs were prepared by noncovalent π-π interactions between the SWNT surface and the pyrene anchors of the Ru complex. In these composites, the long axis of the Ru complexes was aligned in parallel to the principal direction of the SWNT. The optimized conformation of these complexes on the SWNT surface was calculated by molecular mechanics. The composites were examined by UV/Vis absorption and FT-IR spectroscopy, XPS, and SEM analysis. Furthermore, their electrochemical properties were evaluated. Cyclic voltammograms of the composites showed reversible oxidation waves at peak oxidation potentials (Epox ) = 0.86 and 1.08 V versus Fc(+) /Fc, which were assigned to the Ru(II) -Ru(II) /Ru(II) -Ru(III) and the Ru(II) -Ru(III) /Ru(III) -Ru(III) oxidation events of the dinuclear ruthenium complex, respectively. Based on these observations, we concluded that the electrochemical properties and mixed-valence state of the dinuclear ruthenium complexes were preserved upon attachment to the SWNT surface.