Recognition and One-Pot Extraction of Right- and Left-Handed Semiconducting Single-Walled Carbon Nanotube Enantiomers Using Fluorene-Binaphthol Chiral Copolymers

Synthesized single-walled carbon nanotubes (SWNTs) are mixtures of right- and left-handed helicity and their separation is an essential topic in nanocarbon science. In this paper, we describe the separation of right- and left-handed semiconducting SWNTs from as-produced SWNTs. Our strategy for this goal is simple: we designed copolymers composed of polyfluorene and chiral bulky moieties because polyfluorenes with long alkyl-chains are known to dissolve only semiconducting SWNTs and chiral binaphthol is a so-called BINAP family that possesses a powerful enantiomer sorting capability. In this study, we synthesized 12 copolymers, (9,9-dioctylfluorene-2,7-diyl)<i>x</i>((<i>R</i>)- or (<i>S</i>)-2,2′-dimethoxy-1,1′-binaphthalen-6,6-diyl)<i>y</i>, where <i>x</i> and <i>y</i> are copolymer composition ratios. It was found that, by a simple one-pot sonication method, the copolymers are able to extract either right- or left-handed semiconducting SWNT enantiomers with (6,5)- and (7,5)-enriched chirality. The separated materials were confirmed by circular dichroism, vis-near IR and photoluminescence spectroscopies. Interestingly, the copolymer showed inversion of SWNT enantiomer recognition at higher contents of the chiral binaphthol moiety. Molecular mechanics simulations reveal a cooperative effect between the degree of chirality and copolymer conformation to be responsible for these distinct characteristics of the extractions. This is the first example describing the rational design and synthesis of novel compounds for the recognition and simple sorting of right- and left-handed semiconducting SWNTs with a specific chirality

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 p<i>K</i>_a 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 p<i>K</i>_a 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

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 (<b>RuNH–OH</b> and <b>RuCH–OH</b>). <b>RuNH–OH</b> and <b>RuCH–OH</b> 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 <b>RuCH–OH</b> led to the stabilization of the ruthenium­(III) oxidation state, and therefore, <b>RuCH–OH</b> exhibited lower p<i>K</i>_a values for the imino N–H protons in the bridging benzimidazole groups compared to those of the corresponding <b>RuNH–OH</b> 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 <b>RuNH–OH</b> and <b>RuCH–OH</b> 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 <b>RuNMe–OH</b> and <b>RuCMe–OH</b> 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 <b>RuNH–OH</b> and <b>RuCH–OH</b>; 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

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 <b>Ru-1</b> with eight pyrene groups (<i>k</i> = 2.0 × 10^–5 s^–1) < <b>Ru-2</b> with four pyrenes (4.1 × 10^–5 s^–1) < <b>Ru-3</b> with two pyrenes (6.8 × 10^–5 s^–1) ≪ <b>Ru-4</b> 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, <b>Ru-2</b>, and the carbon nanomaterials increased in the order SWCNTs (<i>k</i>_ET = 1.3 s^–1) < MWCNTs (ϕ = 5–9 nm) (<i>k</i>_ET = 4.0 s^–1) < MWCNTs (ϕ = 110–170 nm) (<i>k</i>_ET = 14.9 s^–1) < HOPG (<i>k</i>_ET = 110 s^–1)

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

Tuning of Metal–Metal Interactions in Mixed-Valence States of Cyclometalated Dinuclear Ruthenium and Osmium Complexes Bearing Tetrapyridylpyrazine or -benzene

New dinuclear ruthenium or osmium complexes with cyclometalated bonds in either tridentate bridging (BL) or ancillary ligands (L), [(L)­M­(BL)­M­(L)] (where M = Ru, Os; L = bis­(<i>N</i>-methylbenzimidazolyl)­pyridine, -benzene; BL= tetrapyridylpyrazine (tppz), -benzene (tpb)), were synthesized, and their mixed-valence-state characteristics were investigated. All of the complexes showed successive one-electron redox processes, each of which correspond to M­(II/III) (M = Ru, Os) or ligand reduction waves. In addition, an M­(III/IV) couple was observed in cyclometalated [M₂(bis­(benzimidazolyl)­benzene)₂(BL)] complexes (M = Ru, Os). Effects of the cyclometalated bonds on the redox behaviors and the accessibility to the mixed-valence M­(II)–M­(III) dinuclear complexes are discussed. Introduction of a cyclometalated bond induced a large negative potential shift in the redox potentials of dinuclear ruthenium and osmium complexes, depending on either bridging or ancillary sites of the cyclometalated bonds: the change falls within the range of −1.0 to −1.2 V for the bridging sites and −0.65 to −0.7 V for the ancillary ones. This large negative potential shift arises from the strong electron-donating property of the phenyl anion in a metal–C bond. Replacing the ruthenium by osmium in the dinuclear complexes with the same bridging ligand results in an increase of the potential separation (Δ<i>E</i>(1)) and the comproportionation constant (<i>K</i>_com) of the mixed-valence complexes having the tppz bridging ligand (Δ<i>E</i>(1) and <i>K</i>_com values: Os > Ru); however, complexes having the tpb bridging ligand showed the opposite trend (Δ<i>E</i>(1) and <i>K</i>_com: Os < Ru). In addition to the results of EPR and DFT calculation, it was found that the orbital energy levels of the central metal ion (namely, either Ru or Os) in the mixed-valence complex determines the degree of orbital mixing between metal dπ orbitals and bridging-ligand π or π<b>*</b> orbitals, which leads to either hole- or electron-transfer exchange mechanisms