Enhanced Electron‐Transfer Properties of Cofacial Porphyrin Dimers through π–π Interactions

π–π assisted : Photoinduced electron transfer from cofacial porphyrin dimers to electron acceptors is prominently accelerated, whereas the back electron transfer is decelerated, relative to the corresponding porphyrin monomer (see figure).

     

Contrasting Effects of Axial Ligands on Electron‐Transfer Versus Proton‐Coupled Electron‐Transfer Reactions of Nonheme Oxoiron(IV) Complexes

The effects of axial ligands on electron‐transfer and proton‐coupled electron‐transfer reactions of mononuclear nonheme oxoiron(IV) complexes were investigated by using [Fe^IV(O)(tmc)(X)]ⁿ⁺ ( 1 ‐X) with various axial ligands, in which tmc is 1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane and X is CH₃CN ( 1 ‐NCCH₃), CF₃COO^? ( 1 ‐OOCCF₃), or N₃^? ( 1 ‐N₃), and ferrocene derivatives as electron donors. As the binding strength of the axial ligands increases, the one‐electron reduction potentials of 1 ‐X (E_red, V vs. saturated calomel electrode (SCE)) are more negatively shifted by the binding of the more electron‐donating axial ligands in the order of 1 ‐NCCH₃ (0.39) > 1 ‐OOCCF₃ (0.13) > 1 ‐N₃ (?0.05 V). Rate constants of electron transfer from ferrocene derivatives to 1 ‐X were analyzed in light of the Marcus theory of electron transfer to determine reorganization energies (λ) of electron transfer. The λ values decrease in the order of 1 ‐NCCH₃ (2.37) > 1 ‐OOCCF₃ (2.12) > 1 ‐N₃ (1.97 eV). Thus, the electron‐transfer reduction becomes less favorable thermodynamically but more favorable kinetically with increasing donor ability of the axial ligands. The net effect of the axial ligands is the deceleration of the electron‐transfer rate in the order of 1 ‐NCCH₃ > 1 ‐OOCCF₃ > 1 ‐N₃. In sharp contrast to this, the rates of the proton‐coupled electron‐transfer reactions of 1 ‐X are markedly accelerated in the presence of an acid in the opposite order: 1 ‐NCCH₃ < 1 ‐OOCCF₃ < 1 ‐N₃. Such contrasting effects of the axial ligands on the electron‐transfer and proton‐coupled electron‐transfer reactions of nonheme oxoiron(IV) complexes are discussed in light of the counterintuitive reactivity patterns observed in the oxo transfer and hydrogen‐atom abstraction reactions by nonheme oxoiron(IV) complexes (Sastri et al. Proc. Natl. Acad. Sci. U.S.A. 2007 , 104, 19 181–19 186).     

Energy‐Level Alignment for Single‐Molecule Conductance of Extended Metal‐Atom Chains

The use of single‐molecule junctions for various functions constitutes a central goal of molecular electronics. The functional features and the efficiency of electron transport are dictated by the degree of energy‐level alignment (ELA), that is, the offset potential between the electrode Fermi level and the frontier molecular orbitals. Examples manifesting ELA are rare owing to experimental challenges and the large energy barriers of typical model compounds. In this work, single‐molecule junctions of organometallic compounds with five metal centers joined in a collinear fashion were analyzed. The single‐molecule i–V scans could be conducted in a reliable manner, and the E_FMO levels were electrochemically accessible. When the electrode Fermi level (E_F) is close to the frontier orbitals (E_FMO) of the bridging molecule, larger conductance was observed. The smaller |E_F?E_FMO| gap was also derived quantitatively, unambiguously confirming the ELA. The mechanism is described in terms of a two‐level model involving co‐tunneling and sequential tunneling processes.     

Effect of Gold Nanoparticles on the Structure and Electron‐Transfer Characteristics of Glucose Oxidase Redox Polyelectrolyte‐Surfactant Complexes

Efficient electrical communication between redox proteins and electrodes is a critical issue in the operation and development of amperometric biosensors. The present study explores the advantages of a nanostructured redox‐active polyelectrolyte–surfactant complex containing [Os(bpy)₂Clpy]²⁺ (bpy=2,2′‐bipyridine, py= pyridine) as the redox centers and gold nanoparticles (AuNPs) as nanodomains for boosting the electron‐transfer propagation throughout the assembled film in the presence of glucose oxidase (GOx). Film structure was characterized by grazing‐incidence small‐angle X‐ray scattering (GISAXS) and atomic force microscopy (AFM), GOx incorporation was followed by surface plasmon resonance (SPR) and quartz‐crystal microbalance with dissipation (QCM‐D), whereas Raman spectroelectrochemistry and electrochemical studies confirmed the ability of the entrapped gold nanoparticles to enhance the electron‐transfer processes between the enzyme and the electrode surface. Our results show that nanocomposite films exhibit five‐fold increase in current response to glucose compared with analogous supramolecular AuNP‐free films. The introduction of colloidal gold promotes drastic mesostructural changes in the film, which in turn leads to a rigid, amorphous interfacial architecture where nanoparticles, redox centers, and GOx remain in close proximity, thus improving the electron‐transfer process.     

Topological and Electron‐Transfer Properties of Yeast Cytochrome c Adsorbed on Bare Gold Electrodes

The redox metalloprotein yeast cytochrome c was directly self‐chemisorbed on “bare” gold electrodes through the free sulfur‐containing group Cys102. Topological, spectroscopic, and electron transfer properties of the immobilised molecules were investigated by in situ scanning probe microscopy and cyclic voltammetry. Atomic force and scanning tunnelling microscopy revealed individual protein molecules adsorbed on the gold substrate, with no evidence of aggregates. The adsorbed proteins appear to be firmly bound to gold and display dimensions in good agreement with crystallographic data. Cyclic voltammetric analysis showed that up to 84 % of the electrode surface is functionalised with electroactive proteins whose measured redox midpoint potential is in good agreement with the formal potential. Our results clearly indicate that this variant of cytochrome c is adsorbed on bare gold electrodes with preservation of morphological properties and redox functionality.     

Covalent Assembly and Characterization of Nonsymmetrical Single‐Molecule Nodes

The covalent linking of molecular building blocks on surfaces enables the construction of specific molecular nanostructures of well‐defined shape. Molecular nodes linked to various entities play a key role in such networks, but represent a particular challenge because they require a well‐defined arrangement of different building blocks. Herein, we describe the construction of a chemically and geometrically well defined covalent architecture made of one central node and three molecular wires arranged in a nonsymmetrical way and thus encoding different conjugation pathways. Very different architectures of either very limited or rather extended size were obtained depending on the building blocks used for the covalent linking process on the Au(111) surface. Electrical measurements were carried out by pulling individual molecular nodes with the tip of a scanning tunneling microscope. The results of this challenging procedure indicate subtle differences if the nodes are contacted at inequivalent termini.     

Formation and Isolation of a Four‐Electron‐Reduced Porphyrin Derivative by Reduction of a Stable 20π Isophlorin

The two‐electron reduction of a diprotonated dodecaphenylporphyrin derivative by Na₂S₂O₄ gave a corresponding isophlorin ( Iph ) selectively. Formation of Iph was confirmed by spectroscopic measurements and the isolation of tetramethylated Iph . Further reduction of Iph proceeded to form an unprecedented four‐electron‐reduced porphyrin ( IphH₂ ), which was fully characterized by spectroscopic and X‐ray crystallographic analysis. IphH₂ , with a unique conformation, could be oxidized to reproduce the starting porphyrin, resulting in a proton‐coupled four‐electron reversible redox system.     

A Single‐Molecule‐Level Mechanistic Study of Pd‐Catalyzed and Cu‐Catalyzed Homocoupling of Aryl Bromide on an Au(111) Surface

On‐surface Pd‐ and Cu‐catalyzed C?C coupling reactions between phenyl bromide functionalized porphyrin derivatives on an Au(111) surface have been investigated under ultra‐high vacuum conditions by using scanning tunneling microscopy and kinetic Monte Carlo simulations. We monitored the isothermal reaction kinetics by allowing the reaction to proceed at different temperatures. We discovered that the reactions catalyzed by Pd or Cu can be described as a two‐phase process that involves an initial activation followed by C?C bond formation. However, the distinctive reaction kinetics and the C?C bond‐formation yield associated with the two catalysts account for the different reaction mechanisms: the initial activation phase is the rate‐limiting step for the Cu‐catalyzed reaction at all temperatures tested, whereas the later phase of C?C formation is the rate‐limiting step for the Pd‐catalyzed reaction at high temperature. Analysis of rate constants of the Pd‐catalyzed reactions allowed us to determine its activation energy as (0.41±0.03) eV.     

Photoinduced Formation and Characterization of Electron–Hole Pairs in Azaxanthylium‐Derivatized Short Single‐Walled Carbon Nanotubes

2‐Azaxanthone, a nitrogenated derivative of the well‐studied organic chromophore xanthone, has been covalently bound through 2‐(ethylthio)ethylamido linkers to the carboxylic acid groups of short, soluble single‐walled carbon nanotubes (CNTs) of 450 nm average length, and the resulting azaxanthylium‐functionalized CNTs (AZX‐CNT, 8.5 wt % AZX content) characterized by solution ¹H NMR, Raman and IR spectroscopy and thermogravimetric analysis. Comparison of the quenching of the triplet excited state of AZX (steady‐state and time‐resolved) and of the transient optical spectra of CNTs and AZX‐CNT shows that the covalent linkage boosts the interaction between the azaxanthylium moiety and the short CNT units. The triplet excited state of the azaxanthylium derivative is quenched by CNT with and without covalent bonding, but when it is covalently bonded, the singular transient spectrum is compatible with the photogeneration of electron holes through electron transfer from CNT to excited azaxanthylium units.     

Barrierless Single‐Electron‐Induced cis–trans Isomerization

Lowering the activation energy of a chemical reaction is an essential part in controlling chemical reactions. By attaching a single electron, a barrierless path for the cis–trans isomerization of maleonitrile on the anionic surface is formed. The anionic activation can be applied in both reaction directions, yielding the desired isomer. We identify the microscopic mechanism that leads to the formation of the barrierless route for the electron‐induced isomerization. The generalization to other chemical reactions is discussed.