Electrochemical desorption of n-alkylthiol SAMs on polycrystalline gold: studies using a ferrocenylalkylthiol probe

The reductive voltammetric desorption of n-alkylthiol self-assembled monolayers (SAMs) was studied using ferrocenyldodecanethiol (FcC12SH) as a probe. A short (10 s) incubation in a 2 mM FcC12SH solution labels the pre-existing defect sites present in a tetradecanethiol (C14S-Au) SAM. Additional defects in a C14S-Au SAM are then created by voltammetric cycling to reductive potentials. The effects of changing the desorption potential and the hold time at the desorption potential were investigated by monitoring the change in the two characteristic FcC12S-Au peaks. These two peaks are associated with FcC12S-Au filling individual single site defects (peak I at 260 mV) and pinhole defects (peak II at 380 mV). The reductive desorption potential applied to a binary (FcC12S-/C14S-Au) SAM results in a partial desorption and a potential "remixing" of the phase-separated state.     

The redox chemistry of the covalently immobilized native and low-pH forms of yeast iso-1-cytochrome c

Cyclic voltammetry experiments were carried out on native Saccharomyces cerevisiae iso-1-cytochrome c and its C102T/N62C variant immobilized on bare polycrystalline gold electrode through the S-Au bond formed by a surface cysteine. Experiments were carried out at different temperatures (5-65 degrees C) and pH values (1.5-7). The E degrees ' value at pH 7 (+370 mV vs SHE) is approximately 100 mV higher than that for the protein in solution. This difference is enthalpic in origin and is proposed to be the result of the electrostatic repulsion among the densely packed molecules onto the electrode surface. Two additional electrochemical waves are observed upon lowering the pH below 5 (E degrees ' = +182 mV) and 3 (E degrees ' = +71 mV), which are attributed to two conformers (referred to as "intermediate" and "acidic", respectively) featuring an altered heme axial ligation. This is the first determination of the reduction potential for low-pH conformers of cytochrome c in the absence of denaturants. Since the native form of cytochrome c can be restored, bringing back the pH to neutrality, the possibility offered by this transition to reversibly modulate the redox potential of cytochrome c is appealing for bioelectronic applications. The immobilized C102T/N62C variant, which differs from the native protein in the orientation of the heme group with respect to the electrode, shows very similar reduction thermodynamics. For both species, the rate constant for electron transfer between the heme and the electrode increases for the acidic conformer, which is also found to act as a biocatalytic interface for dioxygen reduction.     

Relationship between electrochemical potentials and substitution reaction rates of ferrocene-containing β-diketonato rhodium(I) complexes; cytotoxicity of [Rh(FcCOCHCOPh)(cod)

A series of ferrocene-containing rhodium complexes of the type [Rh(FcCOCHCOR)(cod)] (cod = 1,5-cyclooctadiene) with R = CF(3), 1, (E(pa)(Rh) = 269; E(o)'(Fc) = 329 mV vs. Fc/Fc(+)), CCl(3), 2, (E(pa) = 256; E(o)' = 312 mV), CH(3), 3, (E(pa) = 177; E(o)' = 232 mV), Ph = C(6)H(5), 4, (E(pa) = 184; E(o)' = 237 mV), and Fc = ferrocenyl = (C(5)H(5))Fe(C(5)H(4)), 5, (E(pa) = 135; E(o)'(Fc1) = 203; E(o)'(Fc2) = 312 mV), have been studied electrochemically in CH(3)CN. Results indicated that the rhodium(I) centre is irreversibly oxidised to Rh(III) in a two-electron transfer process before the ferrocenyl fragment is reversibly oxidized in a one-electron transfer process. The peak anodic (oxidation) potential, E(pa), (in V vs. Fc/Fc(+)) of the rhodium core in 1-5 relates to k(2), the second-order rate constant for the substitution of (FcCOCHCOR)(-) with 1,10-phenanthroline in [Rh(FcCOCHCOR)(cod)] to form [Rh(phen)(cod)](+) in methanol at 25 °C with the equation lnk(2) = 39.5 E(pa)(Rh) - 3.69, while the formal oxidation potential of the ferrocenyl groups in 1-5 relates to k(2) by lnk(2) = 40.8 E(o)'(Fc)-6.34. Complex 4 (IC(50) = 28.2 μmol dm(-3)) was twice as cytotoxic as the free FcCOCH(2)COPh ligand having IC(50) = 54.2 μmol dm(-3), but approximately one order of magnitude less toxic to human HeLa neoplastic cells than cisplatin (IC(50) = 2.3 μmol dm(-3)).     

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of self-assembled monolayers of ruthenium complexes on gold

Self-assembled monolayers (SAMs) of three ruthenium complexes, [Ru(L)(2)](PF(6))(2), [Ru(L)(tpyPO(3))](PF(6))(2), and [Ru(L18)(tpyPO(3))](PF(6))(2), were prepared on evaporated gold films on glass or stainless steel plates; where L = 2, 6-bis(benzimidazoyl)pyridine, tpyPO(3) = 2,6-bis(2,2':6', 2"-terpyridyl)pyridine phosphanate, and L18 = 2, 6-bis(N-octadecylbenzimidazoyl)pyridine. Structures of these SAM complexes were studied by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The SAMs were either prepared by direct binding of Ru-complexes to Au films by alkanethiol or by the multilayer method. In the multilayer method 1,4-thiobutylphosphate was used to form a base layer on an Au film, and the base layer was then chemically bridged to the Ru-complexes by zirconium phosphate. MALDI-TOFMS of SAM1, that had been prepared by direct binding of [Ru(L)(2)](PF(6))(2) to the Au film by an octanethiol group, showed cleavage at the S-Au linkages and elimination of the counter anion to yield a molecular ion and its dimeric ion. On the other hand, SAM2 and SAM3, which had been prepared by bridging Ru-complexes [Ru(L)(tpyPO(3))](PF(6))(2) or [Ru(L18)(tpyPO(3))](PF(6))(2) to the base layers with zirconium phosphate, showed dissociation from the base layers and elimination of the counter anion to give ions of the Ru complex molecules and their fragmentation ions. No molecular ion containing the base layer resulting from the S-Au bond cleavage was observed. Copyright 2000 John Wiley & Sons, Ltd.     

An outer-sphere two-electron platinum reagent

A strategy for designing cooperative outer-sphere two-electron platinum reagents is demonstrated. The novel platinum(II) complex, [Pt(tpy)(pip2NCN)][BF4] (1(BF4-)) (tpy = 2,2':6',2' '-terpyridine, pip2NCN- = 2,6-(CH2N(CH2)5)2-C6H3-), in which the metal is bonded to two pincer type ligands, has been prepared. Treatment of 1 with protic acid results in protonation of the pendant piperdyl groups, allowing for the isolation of [Pt(tpy)(pip2NCNH2)][PF6]3 (2(PF6-)3). 1H NMR spectra of 1 and 2 establish that in each complex the terpyridyl ligand is tridentate, whereas the piperdyl ligand is monodentate, bonded to platinum through the phenyl ring. The structure of the protonated complex was confirmed by an X-ray crystallographic study of crystals of 2(Cl-)3.4H2O. The cyclic voltammagram of 1 exhibits two reversible one-electron reduction waves at E degrees ' = -0.98 V and E degrees ' = -1.50 V (E degrees ' = (Epc + Epa)/2), with a DeltaEp of 65 and 61 mV, respectively. In contrast to other Pt(II) complexes, including 2, this complex also undergoes a nearly reversible two-electron oxidation process at E degrees ' = 0.40 V (DeltaEp = 43 mV, 0.01 V/s). The accumulated data are consistent with the unusual ligand architecture of 1 being capable of stabilizing and allowing for facile interconversion between the Pt(II) and Pt(IV) oxidation states.     

Binding of nitrate to a CuII-cyclen complex bearing a ferrocenyl pendant: synthesis, solid-state X-ray structure, and solution-phase electrochemical and spectrophotometric studies

The reaction of Cu(NO3)2.3H2O with the ligand 1-(ferrocenemethyl)-1,4,7,10-tetraazacyclododecane (L) in acetonitrile leads to the formation of a blue complex, [Cu(L)(NO3)][NO3] (C1). The X-ray structure determination shows an unexpected binding of a nitrate anion in that the CuII center is surrounded by four N atoms of the 1,4,7,10-tetraazacyclododecane (cyclen) macrocycle and two O atoms from a chelating nitrate anion, both Cu-O distances being below the sums of the van de Waals radii. Hydrogen-bonding interactions in the crystal lattice and a weak interaction between a second nitrate O and the CuII center in C1 give rise to a highly distorted CuII geometry relative to that found in the known structure of [Cu(cyclen)(NO3)][NO3] (C5). Electrochemical studies in acetonitrile containing 0.1 M [Bu4N][NO3] as the supporting electrolyte showed that oxidation of C1 in this medium exhibits a single reversible one-electron step with a formal potential E degrees f of +85 mV vs Fc0/+ (Fc = ferrocene). This process is associated with oxidation of the ferrocenyl pendant group. Additionally, a reversible one-electron reduction reaction with an E degrees f value of -932 mV vs Fc0/+, attributed to the CuII/I redox couple, is detected. Gradual change of the supporting electrolyte from 0.1 M [Bu4N][NO3] to the poorly coordinating [Bu4N][PF6] electrolyte, at constant ionic strength, led to a positive potential shift in E degrees f values by +107 and +39 mV for the CuII/I(C1) and Fc0/+(C1) redox couples, respectively. Analysis of these electrochemical data and UV-vis spectra is consistent with the probable presence of the complexes C1, [Cu(L)(CH3CN)2]2+ (C2), [Cu(L)(CH3CN)(NO3)]+ (C3), and [Cu(L)(NO3)2] (C4) as the major species in nitrate-containing acetonitrile solutions. In weakly solvating nitromethane, the extent of nitrate complexation remains significant even at low nitrate concentrations, due to the lack of solvent competition.     

Electron transfer and electrocatalytic properties of the immobilized methionine80alanine cytochrome c variant

The M80A variant of yeast iso-1-cytochrome c (cytc), which features a noncoordinating Ala residue in place of the axial heme iron Met ligand, was chemisorbed on a gold electrode coated with 4-mercaptopyridine or carboxyalkanethiol self-assembled monolayers (SAM) and investigated by cyclic voltammetry at varying conditions of temperature, pH, and O2 concentration. The E degrees ' value (standard reduction potential for the heme Fe(III)/Fe(II) couple) of M80A cytc on both SAMs is of approximately -200 mV (vs the standard hydrogen electrode, SHE) at pH 7, which is more than 400 mV lower than that of native cytochrome c in the same conditions. The thermodynamics of Fe(III) to Fe(II) reduction and the kinetics of heterogeneous electron transfer (ET) are dominated by the presence of a hydroxide ion as the sixth axial heme iron ligand above pH 6. On both SAMs, protonation of the bound hydroxide ion is mainly responsible for the changes in these parameters at low pH, since the distances of ET between the heme and the electrode are found to be independent of pH in the range of 5-11. The invariance of the electrochemical features up to pH 11 indicates that no changes in heme iron coordination occur at high pH, at variance with native cytc. Most notably, immobilized M80A cytc is found to act as an efficient biocatalyst for O2 reduction from pH 5 to 11.0. This finding makes M80A cytc a suitable candidate as a constituent of a biocatalytic interface for O2 biosensing and opens the way for the exploitation of engineered cytochrome c in the bio-based detection of chemicals of environmental and clinical interest.     

A comparison of electron-transfer rates of ferrocenoyl-linked DNA

Electron transfer (ET) through two configurations of double-stranded (ds)-DNA was investigated by the attachment of a ferrocenoyl (Fc)-labeled and thiol-labeled DNA to Au electrodes. The first configuration positions the Fc moiety on the same strand as the thiolate, whereas the second configuration positions the Fc group on the complementary strand. The subtle difference in structure leads to a difference in E0' values (29 mV) and in ET rate constants (25 vs 115 s-1). The results have led to a further understanding of electron transfer in ds-DNA, and several models of ET are proposed.     

Control of cytochrome C redox potential: axial ligation and protein environment effects

Axial iron ligation and protein encapsulation of the heme cofactor have been investigated as effectors of the reduction potential (E degrees ') of cytochrome c through direct electrochemistry experiments. Our approach was that of partitioning the E degrees ' changes resulting from binding of imidazole, 2-methyl-imidazole, ammonia, and azide to both cytochrome c and microperoxidase-11 (MP11), into the enthalpic and entropic contributions. N-Acetylmethionine binding to MP11 was also investigated. These ligands replace Met80 and a water molecule axially coordinated to the heme iron in cytochrome c and MP11, respectively. This factorization was achieved through variable temperature E degrees ' measurements. In this way, we have found that (i) the decrease in E degrees ' of cytochrome c due to Met80 substitution by a nitrogen-donor ligand is almost totally enthalpic in origin, as a result of the stronger electron donor properties of the exogenous ligand which selectively stabilize the ferric state; (ii) on the contrary, the binding of the same ligands and N-acetylmethionine to MP11 results in an enthalpic stabilization of the reduced state, whereas the entropic effect invariably decreases E degrees ' (the former effect prevails for the methionine ligand and the latter for the nitrogenous ligands). A comparison of the reduction thermodynamics of cytochrome c and the MP11 adducts offers insight on the effect of changing axial heme ligation and heme insertion into the folded polypeptide chain. Principally, we have found that the overall E degrees ' increase of approximately 400 mV, comparing MP11 and native cytochrome c, consists of two opposite enthalpic and entropic terms of approximately +680 and -280 mV, respectively. The enthalpic term includes contributions from both axial methionine binding (+300 mV) and protein encapsulation of the heme (+380 mV), whereas the entropic term is almost entirely manifest at the stage of axial ligand binding. Both terms are dominated by the effects of water exclusion from the heme environment.     

Mediated oxidation of ascorbic acid on a homologous series of ferrocene-terminated self-assembled monolayers

The kinetics of electrocatalytic oxidation of ascorbate was studied on a series of redox self-assembled monolayers (SAMs) of the general formula Fc(CH2)4COO(CH2)nSH as electron-transfer mediators, where Fc is the ferrocenyl group and n = 3, 6, 9, and 11. We show that the rate of electron transfer from ascorbate to the surface-confined Fc+ decreases with increasing n. The rationale for the dependence of the rate of electrocatalytic activity and n, in the presence of ClO4, is obtained from Fourier-transform surface-enhanced Raman spectroscopy (FT-SERS), cyclic voltammetry, and electrochemical quartz crystal microbalance (EQCM) data. In particular, FT-SERS shows decreasing amounts of surface-bound ClO4- upon oxidation of the ferrocene with decreasing n, while EQCM data show the effective electrode mass increase was consistently higher on the shorter chain SAMs. This mass increase is likely due to increasing ferricinium cation hydration. As n decreases, the SAMs become less ordered (FT-SERS data), as is widely known from previous literature. Disorder favors water penetration into the SAM, which, in turn, increases the hydration of the Fc+ (EQCM data). Increased hydration of the Fc+ impedes the formation of Fc+-ClO4- ion pairs (EQCM and FT-SERS data), which, consequently, accelerates the electrocatalytic electron transfer from the solution-dissolved ascorbate.     

Metallocene-terminated allylium salts: the effect of end group on localization in polymethines

A range of 1,3-di(metallocenyl)allylium salts [Mc(CH)(3)Mc'](+)[X](-) [Mc, Mc' = ferrocenyl (Fc), 2,3,4,5,1',2',3',4'-octamethylferrocen-1-yl (Fc' '), ruthenocenyl (Rc); X = BF(4), PF(6)] was synthesized by reaction of (2-lithiovinyl)metallocenes with formylmetallocenes, followed by treatment of the resulting alcohols with HX. Two salts with X = BAr'(4) [Ar' = 3,5-(CF(3))(2)C(6)H(3)] were synthesized by anion metathesis from the corresponding PF(6) salts. The crystal structure of [Fc' '(CH)(3)Fc' '](+)[PF(6)](-) contains symmetrical termethine cations, while the same appears to be true in the disordered structure of [Fc(CH)(3)Fc](+)[PF(6)](-). The formally unsymmetrical cation in [Fc(CH)(3)Fc' '](+)[BF(4)](-) is only slightly unsymmetrical with little bond-length alternation in the allylium bridge. In contrast, the crystal structures of [Rc(CH)(3)Rc](+)[PF(6)](-) and [Rc(CH)(3)Rc](+)[BAr'(4)](-) both contain a bond-alternated "Peierls-distorted" cation, which can be considered as a ruthenocene bridged to a [(eta(6)-fulvene)(eta(5)-cyclopentadienyl)ruthenium] cation by a vinylene moiety. The strong similarity between solid-state and solution infrared and Raman spectra of [BF(4)](-), [PF(6)](-), and [BAr'(4)](-) salts of [Rc(CH)(3)Rc](+) indicates that the C-C stretching constant in the allylium chain and, therefore, the structure, of this ion are largely independent of the local environment, suggesting that the unsymmetrical structures observed in the crystal structures are not simply an artifact of packing. Differences in the solvatochromism of [Rc(CH)(3)Rc](+) and [Fc(CH)(3)Fc](+) also suggest a localized structure for the former cation in solution. Electrochemistry, UV-visible-NIR spectroscopy, and DF calculations give insight into the electronic structure of the metallocene-terminated allylium cations. Using an analogy between polymethines and mixed-valence compounds, the difference between the behaviors of [Fc(CH)(3)Fc](+) and [Rc(CH)(3)Rc](+) is attributed to larger reorganization energy associated with the geometry differences between metallocene and [(eta(6)-fulvene)(eta(5)-cyclopentadienyl)metal] structures in the ruthenium case.     

Synthesis, crystal structure, spectroscopic and electrochemical properties, and H2-evolving activity of a new [PtCl(terpyridine)]+ derivative with viologen-like redox properties

A new Pt(II)-based metalloviologen analog, [PtCl(Mepytpy)](2+) (abbreviated as PV(2+); Mepytpy(+) = 4'-(4-methyl-pyridinio)-2,2':6',2'-terpyridine), was synthesized and its spectroscopic and electrochemical properties were investigated. The structures of Mepytpy(+) and PV(2+) were solved by single-crystal X-ray diffraction analysis. Electrochemical studies in DMF (N,N-dimethylformamide) revealed that both the first and second reduction potentials of PV(2+) (E = -0.87 V vs. Fc/Fc(+) for PV(2+)/PV(+)˙; E = -1.31 V vs. Fc/Fc(+) for PV(+)˙/PV(0)) are nearly consistent with those of methylviologen (N,N'-dimetyl-4,4'-bipyridinium, MV(2+)) (E = -0.90 V vs. Fc/Fc(+) for MV(2+)/MV(+)˙; E = -1.28 V vs. Fc/Fc(+) for MV(+)˙/MV(0)), where the first reduction of PV(2+) is assigned as the reduction at the Mepytpy(+) ligand bound to the Pt(ii) ion. Upon the addition of a reducing agent Na(2)S(2)O(4) into an aqueous acetate buffer solution (pH = 5.0) of PV(2+), the generation of one-electron-reduced radical species (PV(+)˙) was spectrophotometrically observed and was assigned based on the time-dependent density functional theory (TD-DFT) calculations. The H(2)-evolving activity of PV(2+) was evaluated in the presence of a sacrificial electron donor (EDTA) in an aqueous acetate buffer solution (pH = 5.0). It was found that the stability of the metalloviologen PV(2+) during the photolysis is much higher than that of the parent compound [PtCl(tpy)](+).     

Monolayer structures of highly photoluminescent furan oligoaryls: an approach to improve packing crystallinity of dithiolated aromatics

We demonstrated that mono- and dithiolated furan-containing oligoaryls (II-IV, see Chart 2) can be successfully synthesized via a one-pot strategy starting from propargylic dithioacetals. IRAS (infrared reflection-absorption spectroscopy) and STM (scanning tunneling microscopy) experiments revealed that single-component monolayers of II, III, and IV are essentially disordered, an important property that prevents excited photoluminescent molecules from self-quenching in the organic layers of an OLED device. Surprisingly, localized lattice packing of crystalline dithiolated furan oligoaryls on Au(111) can be assembled by immersing preadsorbed n-dodecanethiol SAMs in the corresponding deposition solutions. The discrepancy in the formation of disordered or localized crystalline structures is discussed. For single-component monolayers, the facile formation of S-Au bonds generates chaotically distributed monolayers in which the arched molecules hinge each other and block the desorptive pathways. The absence of crystalline packing is mainly attributed to the difficulty for the dithiols to simultaneously break two S-Au bonds, to desorb, and then to readsorb, the key step to improve the intermolecular attractions for crystalline SAMs. By preassembling n-dodecanethiol SAMs, the space for dithiolated compounds III and IV to adsorb is limited to domain boundaries or packing defects where crystalline packing of III and IV can grow.     

Strong resistance of a thin crystalline layer of balanced charged groups to protein adsorption

Resistance of mixed self-assembled monolayers (SAMs) with various counter-charged terminal groups of different valence and protonation/deprotonation states to nonspecific protein adsorption is investigated. It is demonstrated that excellent nonfouling surfaces can be readily constructed from mixed positively and negatively charged components of equal valence in a wide range of thiol solution compositions. Furthermore, the lattice structure of one of the mixed SAM systems studied is revealed by atomic force microscopy (AFM) to be (5.2 +/- 0.2 A x 5.2 +/- 0.2 A)60 degrees . Results indicate that the packing structure of mixed charged SAMs is determined by strong charge-charge interactions of the terminal groups rather than S-Au and chain-chain interactions. This work provides direct evidence that conformational flexibility is not required for protein resistance of a surface and even a single compact layer of charged groups of balanced charge with a crystalline structure can resist nonspecific protein adsorption, suggesting that tightly bound water molecules on the topmost part of the mixed SAMs play a dominant role in surface resistance to nonspecific protein adsorption.     

Voltammetric determination of dihydroxybenzenes at a mechanically renewed electrode made from a graphite-epoxy composite

Differences of potentials of anodic and cathodic peaks (ΔE _p) are determined in cyclic voltammograms of dihydroxybenzene/p-benzoquinone redox systems at an electrode made of a graphite-epoxy composite in a wide pH range. The data obtained (ΔE _p = 29 ± 1 mV) are close to the thermodynamic values for two-electron reversible reactions. This indicates that the electrode mechanically renewed by cutting a 0.5-μm surface layer directly in a test solution exhibits a high activity in such electrochemical reactions. The potentials of anodic and cathodic peaks are proportional to the pH of the supporting electrolyte solution in the range from 1.0 to 9.0. A change of 58 ± 1 mV in E _p per unit pH for all isomers shows that the first stage of the oxidation of each dihydroxybenzene isomer involves one electron and is accompanied by the detachment of one hydrogen ion, that is, an intermediate oxidation product, semiquinone, is formed. Despite the closeness of the potentials of hydroquinone and pyrocatechol peaks (ΔE = 100 mV), a scheme is proposed for the selective voltammetric determination of dihydroxybenzene isomers in a 0.1 M HCl solution in hydroquinone-pyrocatechol, pyrocatechol-resorcinol, and hydroquinone-resorcinol binary mixtures. The concentrations of hydroquinone and pyrocatechol are found from cathodic peaks and that of resorcinol, from the anodic peak. The results are well reproducible and contain no systematic error.     

铁氰化钆修饰电极的制备及表征

应用电化学循环扫描法于玻碳电极表面沉积并形成铁氰化钆修饰电极(GdHCF/GC),扫描电镜(SEM)显示,有两种大小和外形明显不同的颗粒状GdHCF附着在电极表面.红外光谱表明,GdCHF的C≡N弯曲振动吸收峰出现在2062.5 cm-1处.循环伏安法测试表明,在0.2 mol/L NaC l溶液中,GdHCF/GC电极出现两对氧化还原峰,扫速为20 mV/s时,其氧化还原峰的式量电位分别为E0’(I)=192.5 mV和E0’(II)=338.5 mV.研究了不同支持电解质对GdHCF/GC电极电化学性能的影响,GdHCF对Na+离子有优先选择性.     

An electrochemical investigation of intermediates and processes involved in the catalytic reduction of dinitrogen by [HIPTN3N]Mo (HIPTN3N = (3,5-(2,4,6-i-Pr3C6H2)2C6H3NCH2CH2)3N)

The redox properties of [HIPTN(3)N]Mo complexes (where HIPTN(3)N = (3,5-(2,4,6-i-Pr(3)C(6)H(2))(2)C(6)H(3)NCH(2)CH(2))(3)N) involved in the catalytic dinitrogen reduction cycle were studied using cyclic voltammetry in fluorobenzene with Bu(4)NPF(6) as the electrolyte. MoN(2) (Mo = [HIPTN(3)N]Mo, E(1/2) = -1.96 V vs. Fc(+)/Fc at a Pt electrode), Mo≡N (E(1/2) = -2.68 V vs. Fc(+)/Fc (Pt)), and [Mo(NH(3))]BAr'(4) (Ar' = 3,5-(CF(3))(2)C(6)H(3), E(1/2) = -1.53 V vs. Fc(+)/Fc (Pt)) each undergo a chemically reversible one-electron reduction, while [Mo=NNH(2)]BAr'(4) (E(1/2) = -1.50 V vs. Fc(+)/Fc (Pt)) and [Mo=NH]BAr'(4) (E(1/2) = -1.26 V vs. Fc(+)/Fc (Pt)) each undergo a one-electron reduction with partial chemical reversibility. The acid employed in the catalytic reduction, [2,4,6-collidinium]BAr'(4), reduces irreversibly at -1.11 V vs. Fc(+)/Fc at Pt and at -2.10 V vs. Fc(+)/Fc at a glassy carbon electrode. The reduction peak potentials of the Mo complexes shift in the presence of acids. For example, the reduction peak for MoN(2) in the presence of [2,4,6-collidinium]BAr'(4) at a glassy carbon electrode shifts positively by 130 mV. The shift in reduction potential is explained in terms of reversible hydrogen bonding and/or protonation at a nitrogen site in Mo complexes. The significance of productive and unproductive proton-coupled electron transfer reactions in the catalytic dinitrogen reduction cycle is discussed.     

Preparation and electrochemistry of azobenzene self-assembled monolayers on gold--long range tunneling and end-group hydrogen bonding effect

A series of thiol-functionalied azobenzene derivatives (RAzoCnSH: R=H for n=3-6, abbreviated as AzoCnSH; R=CH(3)CONH for n=4, abbreviated as aaAzoC4SH) on gold electrodes were prepared and their self-assembly and electrochemical properties were studied by cyclic voltammetry. They all formed uniform and reproducible self-assembled monolayers (SAMs) on gold and showed well-behaved voltammetric responses in aqueous solution. Both the length of the alkyl chain spacer and the H-bonding of the end acetamino group had effects on the stability and the electrochemical kinetics of the SAMs, and the effect of the H-bonding was dominant. The surface coverage of the SAMs (AzoCnSH) is gradually increased with an increase of the alkyl chain spacer length, whereas the presence of the terminal acetamino group leads to a greater increase of the surface coverage. At a low scan rate, voltammetric responses corresponding to an irreversible two-electron, two-proton reduction/oxidation of the trans-azobenzene redox center were obtained in the range of +300 mV and -800 mV, which exhibited very large peak-to-peak splitting. At a high scan rate of 500 mV/s, two steps of reversible one-electron, one-proton reduction/oxidation corresponding to the cis-isomer in azobenzene-thiol SAMs (n is odd) was clearly observed between +300 and -200 mV. The apparent electron-transfer rate is decreased with increasing distance between the azobenzene redox center and the gold electrode. The existence of the end acetamino group which restricted the conformational change during the redox process also led to a decrease of the standard rate constant, and this restriction effect is more predominant than the distance effect.     

Intermixed terpyridine-functionalized monolayers on gold: nonlinear relationship between terpyridyl density and metal ion coordination properties

Aiming at the functionalization of surfaces with terpyridine anchors for the coordinative deposition of additional layers, mixed self-assembled monolayers (SAMs) were prepared from binary solutions of 12-(2,2':6',2″-terpyridine-4'-yl)dodecane-1-thiol (TDT) and 1-decanethiol (DT). The SAMs and the order of the constituting molecules were analyzed by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and time-of-flight-secondary ion mass spectrometry (ToF-SIMS). The composition of the (TDT/DT)-SAMs and with it the surface density of terpyridyl groups correlates linearly with the relative concentrations of the two compounds in the solution used for depositing them. In marked contrast, the amount of terpyridine-coordinated Pd(II) ions significantly deviates from this trend with an optimum at a 1:3 ratio of TDT/DT. This indicates a major fraction of the terpyridines in TDT-rich SAMs not to be accessible for Pd(II) ion coordination. In agreement, NEXAFS spectroscopy reveals the alkyl backbones in TDT-rich SAMs not to be ordered, while they are preferentially upright oriented in the optimal 1:3-(TDT/DT)-SAMs. We interpret this in terms of terpyridine backfolding in TDT-rich SAMs, while they are located in accessible positions on top of the SAM in the 1:3-(TDT/DT)-SAM. While the alkyl backbones in the 1:3-(TDT/DT)-SAM are ordered, NEXAFS spectroscopy shows the terpyridyl groups not to have a preferential orientation in this SAM and thus retain enough flexibility to adjust to molecules that are deposited on top of the mixed SAM. In conclusion, the novel SAM does not undergo phase separation and consists predominantly of intermixed phases with adjustable surface density of quite flexible terpyridine anchor groups. The terpyridine-Pd(II) anchors are not only available for a future deposition of the next layer, but the metal ions also represent a sensitive probe for the accessibility of the terpyridyl groups.     

Self-assembled monolayers of CH3COS- terminated surfactant-encapsulated polyoxometalate complexes

Self-assembled monolayers (SAMs) on gold surfaces based on three kinds of acetylthio-surfactant-encapsulated polyoxometalate clusters (thio-SECs) terminated with multiple CH(3)COS- groups, (NC(26)H(55)S(CO)CH(3))(6)H(2)[Co(H(2)O)CoW(11)O(39)], (NC(26)H(55)S(CO)CH(3))(13)H(3)[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)], and (NC(26)H(55)S(CO)CH(3))(13)[Fe(4)(H(2)O)(2)(P(2)W(15)O(56))(2)]Br, have been prepared, which is representative of a general methodology to fabricate polyoxometalate-based SAMs. Thio-SECs self-assembled into monolayers on gold surfaces through the hydrolysis of CH(3)COS- groups and the subsequent formation of S-Au bonds, which was confirmed by grazing angle infrared spectroscopy, X-ray photoelectron spectroscopy, and ellipsometric and scanning tunneling microscopy (STM) measurements. Furthermore, the SAMs of the thio-SECs possess closely packed structures, and the local short-range order is clearly observed in the magnified STM image. We have also investigated the electrochemical behavior of SAMs of thio-SECs by cyclic voltammetry in detail, and the redox potential of the original polyoxometalates has been well retained. The electrochemical signals of the SAMs are very weak because of the small moiety of thio-SECs that are electrochemically accessible in the cyclic voltammetry experiments. The polyoxometalate-modified electrodes that we prepared are not only highly ordered in the local short range but also stable in electrochemical cycling because of the multiple S-Au bonds of thio-SECs on the gold substrates that aid in the construction of functional materials such as electrochemical and electrocatalytic devices.