Electron Self-exchange Rate Constants Research Articles

Electron-transfer kinetics of copper(II/I) tripodal ligand complexes.

The electron-transfer kinetics for each of three copper(II/I) tripodal ligand complexes reacting with multiple reducing and oxidizing counter reagents have been examined in aqueous solution at 25 degrees C, mu = 0.10 M. For all of the ligands studied, an amine nitrogen serves as the bridgehead atom. Two of the ligands (PMMEA and PEMEA) contain two thioether sulfurs and one pyridyl nitrogen as donor atoms on the appended legs while the third ligand (BPEMEA) has two pyridyl nitrogens and one thioether sulfur. Very limited kinetic studies were also conducted on two additional closely related tripodal ligand complexes. The results are compared to our previous kinetic study on a Cu(II/I) system involving a tripodal ligand (TMMEA) with thioether sulfur donor atoms on all three legs. In all systems, the Cu(II/I) electron self-exchange rate constants (k(11)) are surprisingly small, ranging approximately 0.03-50 M(-)(1) s(-)(1). The results are consistent with earlier studies reported by Yandell involving the reduction of Cu(II) complexes with four similar tripodal ligand systems, and it is concluded that the dominant reaction pathway involves a metastable Cu(II)L intermediate species (designated as pathway B). Since crystal structures suggest that the ligand reorganization accompanying electron transfer is relatively small compared to our earlier studies on macrocyclic ligand complexes of Cu(II/I), it is unclear why the k(11) values for the tripodal ligand systems are of such small magnitude.

Chelate ring sequence effects on thermodynamic, kinetic and electron-transfer properties of copper(ii/i) systems involving macrocyclic ligands with S4 and NS3 donor setsElectronic supplementary information (ESI) available: Tables S1–14: tabulations of experimental rate constants. See http://www.rsc.org/suppdata/dt/b3/b300602f/

The kinetic behavior of electron-transfer reactions involving several copper(II/I) complexes has previously been attributed to a dual-pathway “square scheme” mechanism in which changes in the coordination geometry occur sequentially, rather than concertedly, with the electron-transfer step. In the case of 14-membered macrocyclic quadridentate ligand complexes studied to date, the major geometric change appears to be the inversion of two coordinated donor atoms during the overall electron-transfer process. However, the relative importance of these two inversions has been a matter of speculation. In the current investigation, a comparison is made of Cu(II/I) systems involving two pairs of ligands with S4 and NS3 donor sets: 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4-a); 1,4,7,11-tetrathiacyclotetradecane ([14]aneS4-b); 1,4,8-trithia-11-azacyclotetradecane ([14]aneNS3-a); and 1,7,11-trithia-4-azacyclotetradecane ([14]aneNS3-b). In each pair of ligands, isomer a has the common chelate ring size sequence 5,6,5,6 while isomer b has the sequence 5,5,6,6. A crystal structure for [CuII([14]aneNS3-b)(H2O)](ClO4)2 demonstrates that, when coordinated to Cu(II), the b isomers stabilize the relatively rare ligand conformation designated as conformer II in which one donor atom is oriented opposite to the other three relative to the plane of the macrocycle. This eliminates one of the donor atom inversion steps which normally occurs during Cu(II/I) electron transfer. The copper complexes formed with these a and b isomers are examined in terms of (i) their CuIIL and CuIL stability constants, (ii) their CuIIL formation and dissociation rate constants, (iii) their CuII/IL redox potentials and (iv) their apparent electron self-exchange rate constants. Of the two donor atom inversions which occur in the case of the a-isomer complexes, the specific donor atom inversion which is common to the b-isomer complexes is judged to exhibit the larger energy barrier. Thus, it is presumed to represent the rate-limiting process responsible for the onset of “gated” electron transfer in previous studies on a-isomer complexes.

Active-site structure and electron-transfer reactivity of plastocyanins.

The active-site structures of Cu(II) plastocyanins (PCu's) from a higher plant (parsley), a seedless vascular plant (fern, Dryopteris crassirhizoma), a green alga (Ulva pertusa), and cyanobacteria (Anabaena variabilis and Synechococcus) have been investigated by paramagnetic (1)H NMR spectroscopy. In all cases the spectra are similar, indicating that the structures of the cupric sites, and the spin density distributions onto the ligands, do not differ greatly between the proteins. The active-site structure of PCu has remained unaltered during the evolutionary process. The electron transfer (et) reactivity of these PCu's is compared utilizing the electron self-exchange (ESE) reaction. At moderate ionic strength (0.10 M) the ESE rate constant is dictated by the distribution of charged amino acid residues on the surface of the PCu's. Most higher plant and the seedless vascular plant PCu's, which have a large number of acidic residues close to the hydrophobic patch surrounding the exposed His87 ligand (the proposed recognition patch for the self-exchange process), have ESE rate constants of approximately 10(3) M(-)(1) s(-)(1). Removal of some of these acidic residues, as in the parsley and green algal PCu's, results in more favorable protein-protein association and an ESE rate constant of approximately 10(4) M(-)(1) s(-)(1). Complete removal of the acidic patch, as in the cyanobacterial PCu's, leads to ESE rate constants of approximately 10(5)-10(6) M(-)(1) s(-)(1). The ESE rate constants of the PCu's with an acidic patch also tend toward approximately 10(5)-10(6) M(-)(1) s(-)(1) at higher ionic strength, thus indicating that once the influence of charged residues has been minimized the et capabilities of the PCu's are comparable. The cytochromes and Fe-S proteins, two other classes of redox metalloproteins, also possess ESE rate constants of approximately 10(5)-10(6) M(-)(1) s(-)(1) at high ionic strength. The effect of the protonation of the His87 ligand in PCu(I) on the ESE reactivity has been investigated. When the influence of the acidic patch is minimized, the ESE rate constant decreases at high [H(+)].

Ion atmosphere relaxation control of electron transfer dynamics in a plasticized carbon dioxide redox polyether melt.

The sorption of CO(2) into the highly viscous, semisolid hybrid redox polyether melt, [Co(phenanthroline)(3)](MePEG-SO(3))(2), where MePEG-SO(3) is a MW 350 polyether-tailed sulfonate anion, remarkably accelerates charge transport in this molten salt material. Electrochemical measurements show that as CO(2) pressure is increased from 0 to 800 psi (54 atm) at 23 degrees C, the physical diffusion coefficient D(PHYS) of the Co(II) species, the rate constant k(EX) for Co(II/I) electron self-exchange, and the physical diffusion coefficient of the counterion D(COUNTERION) all increase, from 4.3 x 10(-10) to 6.4 x 10(-9) cm(2)/s, 4.1 x 10(6) to 1.6 x 10(7) M(-1) s(-1), and 3.3 x 10(-9) to 1.6 x 10(-8) cm(2)/s, respectively. Plots of log(k(EX)) versus log(D(PHYS)) and of log(k(EX)) versus log(D(COUNTERION)) are linear, showing that electron self-exchange rate constants are closely associated with processes that also govern D(PHYS) and D(COUNTERION). Slopes of the plots are 0.68 and 0.98, respectively, indicating a better linear correlation between k(EX) and D(COUNTERION). The evidence indicates that k(EX) can be controlled by relaxation of the counterion atmosphere about the Co complexes in the semisolid redox polyether melts. Because the counterion relaxation is in turn controlled by polyether "solvent" fluctuations, this is a new form of solvent dynamics control of electron transfer.

Charge Transport Dynamics and Redox Induced Structural Changes within Solid Deposits of a Ruthenium Dimer

Solid particles of a ruthenium dimer, [(Ru(bpy)2)2 bpzt-]3+, have been abrasively attached to macroelectrodes and microelectrodes, where bpy is 2,2‘-bipyridine and bpzt- is 3,5-bis(pyrazin-2-yl)-1,2,4-triazole. The voltammetry of these solids deposits is unusually ideal in NH4PF6-containing aqueous solutions, and the response is characterized by semi-infinite linear diffusion for scan rates between approximately 50 and 2000 mV s-1. SEM imaging reveals that sparse films of solid particles (1−10 μm in diameter) are efficiently transformed into microcrystals by voltammetric cycling. The charge transport diffusion coefficient, DCT, has been determined by systematically varying the voltammetric scan rate. For reduction of the deposits, DCT increases from 2.4 to 3.6 × 10-10 cm2 s-1 as the NH4PF6 concentration is increased from 0.1 to 2.0 M, while, for oxidation of the deposit, DCT increases from 1.1 to 3.9 × 10-10 cm2 s-1 over the same concentration range. The maximum DCT observed would correspond to an electron self-exchange rate constant of 1.1 × 105 M-1 s-1. Despite the smaller electron-transfer distance expected within the solid, this apparent self-exchange rate constant is more than 2 orders of magnitude smaller than that typically found for ruthenium bis-bpy complexes in solution. This observation suggests that ion rather than electron transfer may limit homogeneous charge transport through these solid deposits.

Determination of electron self-exchange rate constants with MARY spectroscopy: Dependence on the fluorophore

The influence of electron self-exchange on the magnetic field effect of various exciplex–radical ion pair (RIP) systems (1,2- and 1,4-dicyanobenzene in combination with the fluorophores pyrene, N-methylcarbazole and N-ethylcarbazole in tetrahydrofurane) has been investigated using magnetic-field-effect-on-reaction-yield (MARY) spectroscopy. By varying the concentration of the diamagnetic counterpart of one of the radicals in the RIP, the dependence of the linewidth on the electron self-exchange rate has been studied. The functional behaviour of the linewidth-related parameter B1/2 with respect to the molecule concentration is governed by two opposing effects: lifetime uncertainty broadening of spin levels leads to a linear increase at low concentrations, whereas weakened hyperfine coupling of the corresponding radical ion at high concentrations causes B1/2 to decrease again until a saturation value is reached. From the linear relationship at low concentrations an estimate of the self-exchange rate constant is obtained.

Electron Transfer Dynamics in Molten Salts of Mono- and Dinuclear Ruthenium Complexes

We have synthesized highly viscous, room-temperature, molten salts by associating various Ru(NH3)5L cations (L = 4,4‘-bipyridine, pyrazine, pyridine, 3-chloropyridine, benzonitrile) with polyether-tailed sulfonate anions. Microelectrode voltammetry in the undiluted melts yields, on the basis of charge transport occurring by electron hopping, electron self-exchange rate constants (kEX) for the various Ru3+/2+ couples. The rate constant (and activation barrier) for the pyrazine bridged binuclear pentaamineruthenium melt (Creutz-Taube ion) is similar to those obtained for mononuclear pentaammine[ligand]ruthenium melts, meaning that charge transport in the former is dominated by the rate of intermolecular, not intramolecular, electron transfer. The data at 35 °C are (Creutz−Taube ion) kEX = 3.7 × 104 M-1 s-1, (pyridine) 1.1 × 104 M-1 s-1, (3-chloropyridine) 2 × 104 M-1 s-1, and (benzonitrile) 3 × 104 M-1 s-1. All kEX values are smaller than those for [Ru2+/3+(bpy)3] in semisolid melts having equivalent MePEG ...

High sensitivity of the fluorine NMR signals of difluorovinyl analogs of natural hemin: reconstituted heme proteins and self‐exchange electron transfer in model compounds

AbstractThe chemical shifts of fluorine in difluorovinyl deuteroporphyrin iron complexes were shown to be very sensitive to the spin state of the metal and the nature of the ligand(s). Reconstituted myoglobin was used as a model heme protein with an exchangeable heme. Large variations in the fluorine chemical shifts in both the ferric and ferrous states were observed. This strong sensitivity to the nature of the metal ligand and the structural resemblance to natural hemin make this fluorinated porphyrin a good probe for the study of heme proteins. The large variations of chemical shifts depending on the oxidation state also permitted the measurement of the electron self‐exchange rate constants of bis(1‐methylimidazole)iron complexes in various solvents by analysis of line broadening of the 19F NMR signals. The experimental rate constants were strongly affected by the nature of the solvent, varying from 3.9 × 107 to 24.1 × 108 mol l−1 s−1 for DMSO‐d6 and acetone‐d6, respectively. The solvent parameters were used to estimate the outer‐sphere reorganization energies. The experimental rate constants in chloroform and in DMSO are in good agreement with these calculated outer‐sphere reorganization energies. Copyright © 2001 John Wiley & Sons, Ltd.

Direct evidence for a geometrically constrained "entatic state" effect on copper(II/I) electron-transfer kinetics as manifested in metastable intermediates.

The absolute magnitude of an "entatic" (constrained) state effect has never been quantitatively demonstrated. In the current study, we have examined the electron-transfer kinetics for five closely related copper(II/I) complexes formed with all possible diastereomers of [14]aneS(4) (1,4,8,11-tetrathiacyclotetradecane) in which both ethylene bridges have been replaced by cis- or trans-1,2-cyclohexane. The crystal structures of all five Cu(II) complexes and a representative Cu(I) complex have been established by X-ray diffraction. For each complex, the cross-reaction rate constants have been determined with six different oxidants and reductants in aqueous solution at 25 degrees C, mu = 0.10 M. The value of the electron self-exchange rate constant (k(11)) has then been calculated from each cross reaction rate constant using the Marcus cross relation. All five Cu(II/I) systems show evidence of a dual-pathway square scheme mechanism for which the two individual k(11) values have been evaluated. In combination with similar values previously determined for the parent complex, Cu(II/I)([14]aneS(4)), and corresponding complexes with the two related monocyclohexanediyl derivatives, we now have evaluated a total of 16 self-exchange rate constants which span nearly 6 orders of magnitude for these 8 closely related Cu(II/I) systems. Application of the stability constants for the formation of the corresponding 16 metastable intermediates--as previously determined by rapid-scan cyclic voltammetry--makes it possible to calculate the specific electron self-exchange rate constants representing the reaction of each of the strained intermediate species exchanging electrons with their stable redox partners--the first time that calculations of this type have been possible. All but three of these 16 specific self-exchange rate constants fall within--or very close to--the range of 10(5)-10(6) M(-1) s(-1), values which are characteristic of the most labile Cu(II/I) systems previously reported, including the blue copper proteins. The results of the current investigation provide the first unequivocal demonstration of the efficacy of the entatic state concept as applied to Cu(II/I) systems.

Electron self-exchange in the solid-state: cocrystals of hydroquinone and bipyridyl triazole.

Solid-state voltammetry, spectroscopy, and microscopy studies have been used to probe the proton and electron conductivity within a self-assembled cocrystal, HQBpt. This crystallographically defined material contains 3,5-bis(pyridin-2-yl)-1,2,4-triazole, HBpt, dimers that are pi-stacked and hydrogen bonded to 1,4-hydroquinone, H(2)Q, in a herringbone arrangement. When deposited onto platinum microelectrodes, the cocrystal exhibits a well-defined voltammetric response corresponding to oxidation of H(2)Q to the quinone, Q, across a wide range of voltammetric time scales, electrolyte compositions, and pH values. Scanning electron microscopy reveals that redox cycling in aqueous perchlorate solutions in which the pH is systematically varied from 1 to 7 triggers electrocrystallization and the extensive formation of rodlike crystals. Fast scan rate voltammetry reveals that the homogeneous charge transport diffusion coefficient, D(app), is independent of the perchlorate concentration for 0.1 < [ClO(4)(-)] < 1.0 M (pH 6.6) at 3.14 +/- 0.11 x 10(-)(9) cm(2) s(-)(1). Moreover, D(app) is independent of the perchloric acid concentration for concentrations greater than approximately 2.0 M, maintaining a value of 4.81 +/- 0.07 x 10(-)(8) cm(2) s(-)(1). The observation that D(app) is independent of the supporting electrolyte suggests that the rate-determining step for homogeneous charge transport is not the availability of charge-compensating counterions or protons, but the dynamics of electron self-exchange between H(2)Q and Q. We have used the Dahms-Ruff formalism to determine electron self-exchange rate constants which are 2.84 +/- 0.22 x 10(9) and 9.69 +/- 0.73 x 10(10) M(-)(1) s(-)(1) for pH values greater than approximately 2.0 and less than -0.3, respectively. Significantly, these values are more than 2 orders of magnitude larger that those found for benzoquinone self-exchange reactions in aqueous solution. These results indicate that hydrogen bonds play an important role in supporting rapid electron transfer. The increase in D(app) between pH 1.0 and -0.3 is associated with protonation of the HBpt moieties, which triggers a reversible change in the material's structure.

Determination of the electron self-exchange rates of blue copper proteins by super-WEFT NMR spectroscopy.

An NMR approach for determining the electron self-exchange (ESE) rate constants in blue copper proteins is presented. The approach uses the paramagnetic relaxation enhancement of resonances in 1D 1H super-WEFT spectra of partly oxidized (paramagnetic) proteins. These spectra allow a more precise determination of the relevant paramagnetic linebroadenings than conventional 1D 1H spectra and, thus, permit a more detailed investigation of the applicability of the linebroadenings for determining the electron exchange rates. The approach was used to estimate the ESE rate constant of plastocyanin from Anabaena variabilis. It was found that, although the rate constant can be determined accurately from a series of resonances, precise but erroneous constants are obtained from the resonances of the copper-bound residues, unless a narrow splitting of these resonances caused by the presence of two conformations is taken into account. As demonstrated here, this complication can be overcome by a correct analysis of the paramagnetic broadening of the combined double signals. Because of the high resolution and specific sensitivity of the approach it should be generally applicable to estimate electron transfer rates, k, if the paramagnetic relaxation enhancement R2p of the resonances can be determined, and the conditions k << R2p or delta omega(p) >> k >> R2p are fulfilled, delta omega(p) being the frequency separation between corresponding diamagnetic and paramagnetic sites.

Effect of conformational constraints on gated electron-transfer kinetics. 3. Copper(II/I) complexes with cis- and trans-cyclopentanediyl-1,4,8,11-tetrathiacyclotetradecane.

Previous kinetic and electrochemical studies of copper complexes with macrocyclic tetrathiaethers-such as 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4)-have indicated that electron transfer and the accompanying conformational change occur sequentially to give rise to a dual-pathway mechanism. Under appropriate conditions, the conformational change itself may become rate-limiting, a condition known as "gated" electron transfer. We have recently hypothesized that the controlling conformational change involves inversion of two donor atoms, which suggests that "gated" behavior should be affected by appropriate steric constraints. In the current work, two derivatives of [14]aneS4 have been synthesized in which one of the ethylene bridges has been replaced by either cis- or trans-1,2-cyclopentane. The resulting copper systems have been characterized in terms of their Cu(II/I)L potentials, the stabilities of their oxidized and reduced complexes, and their crystal structures. The electron self-exchange rate constants have been determined both by NMR line-broadening and by kinetic measurements of their rates of reduction and oxidation with six or seven counter reagents. All studies have been carried out at 25 degrees C, mu = 0.10 M (NaClO4 and/or Cu(ClO4)2), in aqueous solution. Both Cu(II/I) systems show evidence of a dual-pathway mechanism, and the electron self-exchange rate constants representative of both mechanistic pathways have been determined. The first-order rate constant for gated behavior has also been resolved for the Cu(I)(trans-cyclopentane-[14]aneS4) complex, but only a limiting value can be established for the corresponding cis-cyclopentane system. The rate constants for both systems investigated in this work are compared to values previously determined for the Cu(II/I) systems with the parent [14]aneS4 macrocycle and its derivatives involving phenylene and cis- or trans-cyclohexane substituents. The results are discussed in terms of the influence of the fused rings on the probable conformational changes accompanying the electron-transfer process.

Loop-Directed Mutagenesis of the Blue Copper Protein Amicyanin fromParacoccus versutusand Its Effect on the Structure and the Activity of the Type-1 Copper Site

Four loop-mutants of the blue copper protein amicyanin from Paracoccus versutus have been constructed and characterized. The mutations replaced the loop containing three (Cys93, His96, Met99) of the four copper ligands in amicyanin by the “ligand loops” of P. aeruginosa azurin (AmiAzu), A. faecalis pseudoazurin (AmiPaz), P. nigra plastocyanin (AmiPcy), and P. aureofaciens nitrite reductase (AmiNiR). The copper centers of all variants appear to be perfect type-1 Cu sites although the AmiNiR variant exhibits diminished stability. The optical spectra of the AmiAzu and AmiPaz variants display a significant dependence on temperature. Excitation at 457 nm as well as 647 nm results in similar resonance Raman spectra. The reduction potentials of the three stable variants are all higher than that of wt amicyanin. The reduced forms of the loop-mutants protonate at the C-terminal histidine, with pKa values of 5.6 (AmiAzu), 5.4 (AmiPaz), and 5.7 (AmiPcy) (6.8 for wt amicyanin). AmiAzu is the first known cupredoxin with more than two amino acids between the cysteine and histidine ligands that undergoes this protonation. The electron self-exchange rate constants at 25 °C are 5.7−7.5 times lower for the loop mutants than for wt amicyanin (1.2 × 105 M-1 s-1). The results are interpreted by taking into consideration that three metal ligands are intimately connected with the stable β-sandwich structure of the cupredoxin. This leaves considerable freedom in positioning the fourth one, the C-terminal His, on the “ligand loop”. This explains the ease by which the C-terminal histidine ligand can be excised and replaced by external ligands without losing the metal binding property of the protein. The results also help understand the remarkable evolutionary success of the combination of cupredoxin fold and Cu site for mediating biological electron transfer.

An Interpretation of Gated Behavior: Kinetic Studies of the Oxidation and Reduction Reactions of Bis(2,9-dimethyl-1,10-phenanthroline)copper(I/II) in Acetonitrile.

Reduction reactions of Cu(dmp)(2)(2+) (dmp = 2,9-dimethyl-1,10-phenanthroline) by ferrocene (Fe(Cp)(2) = bis(cyclopentadienyl)iron(II)), decamethylferrocene (Fe(PMCp)(2) = bis(pentamethylcyclopentadienyl)iron(II)), and Co(bpy)(3)(2+) (bpy = 2,2'-bipyridine) and oxidation reactions of Cu(dmp)(2)(+) by Ni(tacn)(2)(3+) (tacn = 1,4,7-triazacyclononane) and Mn(bpyO(2))(3)(3+) (bpyO(2) = N,N'-dioxo-2,2'-bipyridine) were studied in acetonitrile for the purpose of interpreting the gated behavior involving copper(II) and -(I) species. It was shown that the electron self-exchange rate constants estimated for the Cu(dmp)(2)(2+/+) couple from the oxidation reaction of Cu(dmp)(2)(+) by Ni(tacn)(2)(3+) (5.9 x 10(2) kg mol(-)(1) s(-)(1)) and Mn(bpyO(2))(3)(3+) (2.9 x 10(4) kg mol(-)(1) s(-)(1)) were consistent with the directly measured value by NMR (5 x 10(3) kg mol(-)(1) s(-)(1)). In contrast, we obtained the electron self-exchange rate constant of Cu(dmp)(2)(2+/+) as 1.6 kg mol(-)(1) s(-)(1) from the reduction of Cu(dmp)(2)(2+) by Co(bpy)(3)(2+). The pseudo-first-order rate constant for the reduction reaction of Cu(dmp)(2)(2+) by Fe(Cp)(2) was not linear against the concentration of excess amounts of Fe(Cp)(2). A detailed analysis of the reaction revealed that the reduction of Cu(dmp)(2)(2+) involved the slow path related to the deformation of Cu(dmp)(2)(2+) (path B in Scheme 1). By using Fe(PMCp)(2) (the E degrees value is 500 mV more negative than that of Fe(Cp)(2)(+/0)) as the reductant, the mixing with another pathway involving deformation of Cu(dmp)(2)(+) (path A in Scheme 1) became more evident. The origin of the "Gated Behavior" is discussed by means of the energy differences between the "normal" and deformed Cu(II) and Cu(I) species: the difference in the crystal field activation energies corresponding to the formation of pseudo-tetrahedral Cu(II) from tetragonally distorted Cu(II) and the difference in the stabilization energies of the tetrahedral and tetragonal Cu(I) for the activation of Cu(I) species. The reduction reaction of Cu(dmp)(2)(2+) by Fe(PMCp)(2) confirmed that the mixing of the two pathways takes place by lowering the energy level corresponding to the less favorable conformational change of Cu(I) species.

Alkaline Transition of Pseudoazurin from Achromobacter cycloclastes Studied by Paramagnetic NMR and Its Effect on Electron Transfer

Paramagnetic NMR studies on the Cu(II) form of pseudoazurin have been used to demonstrate that the alkaline transition of this protein results in an active site in which the axial Cu−S(Met) interaction is considerably decreased. This observation confirms the conclusion made from various other spectroscopic methods. Furthermore, we show that the alkaline transition of pseudoazurin coincides with a dramatic increase in the electron self-exchange rate constant of the protein. The self-exchange rate constant (25 °C) at pH 8.2 is 3.5 × 103 M-1 s-1 (I = 0.10 M), consistent with a previously determined value (25 °C) of 2.9 × 103 M-1 s-1 (I = 0.10 M) at pH 7.5. Upon increasing the pH value to 10.9 the self-exchange rate constant (25 °C) increases to 1.7 × 104 M-1 s-1 (I = 0.10 M). The increased self-exchange reactivity at high pH is due to the deprotonation of a number of lysine residues that surround the hydrophobic patch of the protein, the most likely docking surface for the self-exchange process. The concomit...

Perylene Polyether Hybrids: Highly Soluble, Luminescent, Redox-Active Dyes

A new class of hybrid redox polyether melts has been prepared by covalent attachment of short polyethylene oxide “tails” to perylene, resulting in highly viscous materials with improved solubility in common solvents ranging from alcohols to toluene. Solid-state voltammetry of the neat, undiluted materials is made possible by dissolution of LiClO4 supporting electrolyte in them. The perylene0/1- electron self-exchange rate constants (kEX) are large. Temperature studies show that the energy barrier is also large but that the reactions are adiabatic. The ionic conductivity of the melts was measured to assess the effects of electron migration. The absorption and fluorescence properties of the perylene derivatives in dilute solution are independent of polyether tail length, while the fluorescence of thin undiluted films of the hybrids is red-shifted (relative to dilute solution) by ca. 125 nm and is dominated by excimer emission.

Structural study and electron exchange kinetics of the bis (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) copper(II)/(I) couple in acetonitrile

Kinetic studies of the electron exchange reaction between Cu(bcp) 2 2+ and Cu(bcp) 2 ' were carried out in acetonitrile (bcp = 2.9-dimethyl-4,7-diphenyl-1,10-phenanthroline). The electron self-exchange rate constant was k ex (25°C = 1.4×10 3 kg mol −1 s −1 with ΔH ∗ = 38.1±2.4 kJ mol −1 and ΔS ∗ = −57±8 J mol −1 K −1 (ionic strength 0.1 mol kg −1 (tetraethylammonium tetrafluoroborate)). A small dependence of the rate constant on the ionic strength was observed. Electron spin resonance study of the Cu(II) species revealed that the Cu-N distances and the dihedral angles between the two NCuN planes in Cu(bcp) 2 2+ and Cu(dmp) 2 2+ complexes (dmp = 2.9-dimethyl-1.10-phenanthroline) are identical in solution. Although the comparison of the crystal structures suggests that the dihedral angle in the Cu(dmp) 2 ' complex is closer to 90° than that of the Cu(bcp) 2 + complex, there was no significant difference between the structures of those complex ions in solution. The electron self-exchange rate constants for the Cu(bcp) 2 2+/+ and the Cu(dmp) 2 2+/+ couples were similar to each other, despite the difference in radii of these ions.

Charge Transport in Ferrocene-Containing Polyacrylamide-Based Redox Gels

The charge transport properties of the redox gels formed from the copolymerization of vinylferrocene (VF) with acrylamide and N,N‘-methylenebisacrylamide (PVAB gels) are described. The apparent electron diffusion coefficients (Dap) measured by cyclic voltammetry and chronocoulometry exhibit an upward bowlike curvature vs immobilized VF concentration ([VF]imm) within the range 0.1−1.60 mM in 0.10 M supporting electrolyte (NaH2PO4/Na2HPO4, NaClO4, or NaNO3). The range of measured Dap values [(0.6−6.0) × 10-7 cm2 s-1] can be interpreted in terms of the mean-field model developed by Blauch and Savéant (Blauch, D. N.; Savéant, J. M. J. Am. Chem. Soc. 1992, 114, 3323). The large Dap values are attributed to the large λ (range of molecular motion permitted to the ferrocene residues) and large kex (bimolecular electron self-exchange rate constant of the ferrocenes in the PVAB gels). It was further observed that Dap decreases on increasing the electrolyte (NaClO4 or NaNO3) concentration from 0.10 to 0.50 M, especially at low (0.1−0.6 mM) and high (1.1−1.6 mM) [VF]imm, resulting in almost flat Dap vs [VF]imm profiles at 0.50 M electrolyte. This observation is attributed to increased nonbonded interactions between the polymer matrix, the redox sites, and the electrolyte upon increasing the electrolyte concentration.

Redox Polyether Hybrid Copper Bipyridine Complex Molten Salts.

The covalent attachment of poly(ethylene oxide) chains to metal bipyridine complexes produces viscous molecular melts with sufficient ionic conductivity to permit microelectrode voltammetry. This paper describes the use of this chemistry to prepare a new copper complex with "tailed" bipyridine ligands, [Cu(bpy(CO(2)MePEG-350)(2))(2)](ClO(4))(2), where MePEG-350 is methyl poly(ethylene glycol) with molecular weight 350 g mol(-)(1). The coupling of physical diffusion with electron hopping during voltammetry allows measurement of the electron self-exchange rate constant, k(EX), for the Cu(II/I) couple. Activation studies indicate the reaction to be nearly adiabatic, with an activation barrier larger than expected for purely inner-sphere rearrangements. This study also examines the depression of both electron transport and ionic conductivity caused by the addition of 1.4 M LiClO(4) to the undiluted room-temperature melt.

EPR measurements on the diffusionless homogeneous electron-self exchange kinetics of the nitrobenzene–nitrobenzene radical anion couple in nitrobenzene as solvent †

Electron-self exchange rate constants of the nitrobenzene–nitrobenzene radical anion couple (NB/NB˙–) and their temperature dependence have been measured by EPR-linebroadening effects in the so-called ‘fast exchange limit’. Nitrobenzene itself serves as solvent. The first order rate constants of these diffusionless reactions vary between 2.8 × 108 and 9.7 × 108 s–1 within a temperature range of 296–353 K.The results are compared on the basis of Marcus theory of electron transfer reactions with corresponding results of the same redox couple in solvents like DMF and CH3CN.