Self-exchange Constants Research Articles

Rapid Electron Transfer Self-Exchange in Conformationally Dynamic Copper Coordination Complexes.

We report the electron transfer (ET) self-exchange rate constants (k11) for a pair of CuII/I complexes utilizing dpaR (dpa = dipicolylaniline, R = OMe, SMe) ligands assessed by NMR line broadening experiments. These ligands afford copper complexes that are conformationally dynamic in one oxidation state. With R = OMe, the CuI complex is dynamic, while with R = SMe, the CuII complex is dynamic. Both complexes exhibit unexpectedly large k11 values of 2.48(6) × 105 and 2.21(9) × 106 M-1 s-1 for [CuCl(dpaOMe)]+/0 and [CuCl(dpaSMe)]+/0, respectively. Among the fastest reported molecular copper coordination complexes to date, that of [CuCl(dpaSMe)]+/0 exceeds all others by an order of magnitude and compares only with those observed in type 1 blue copper proteins. The dynamicity of these complexes establishes pre-steady-state conformational equilibria that minimize the inner-sphere reorganization energies to 0.71 and 0.62 eV for R = OMe and SMe, respectively. In contrast to the emphasis on rigidity in the formulation of entatic states applied to blue copper proteins, the success of these two systems highlights the relevance of conformational dynamicity in mediating rapid ET.

Molecular Switch Cobalt Redox Shuttle with a Tunable Hexadentate Ligand.

Strong-field hexadentate ligands were synthesized and coordinated to cobalt metal centers to result in three new low-spin to low-spin Co(III/II) redox couples. The ligand backbone has been modified with dimethyl amine groups to result in redox potential tuning of the Co(III/II) redox couples from -200 to -430 mV versus Fc+/0. The redox couples surprisingly undergo a reversible molecular switch rearrangement from five-coordinate Co(II) to six-coordinate Co(III) despite the ligands being hexadentate. The complexes exhibit modestly faster electron self-exchange rate constants of 2.2-4.2 M-1 s-1 compared to the high-spin to low-spin redox couple [Co(bpy)3]3+/2+ at 0.27 M-1 s-1, which is attributed to the change in spin state being somewhat offset by this coordination switching behavior. The complexes were utilized as redox shuttles in dye-sensitized solar cells with the near-IR AP25 + D35 dye system and exhibited improved photocurrents over the [Co(bpy)3]3+/2+ redox shuttle (19.8 vs 18.0 mA/cm2). Future directions point toward pairing the low-spin to low-spin Co(II/III) tunable series to dyes with significantly more negative highest occupied molecular orbital potentials that absorb into the near-IR where outer sphere redox shuttles have failed to produce efficient dye regeneration.

Thermal and photoinduced electron transfer reactions of phthalocyanine complexes of Zn(II) and Cu(II) in acetonitrile.

Phthalocyanine that has four peripheral 2-methoxyphenyl substituents at the α-position and its Zn(II) and Cu(II) complexes were synthesized. Chemical oxidation by the Cu(II) ion and electrochemical oxidation of these metal complexes were investigated spectrophotometrically in acetonitrile. The UV-visible absorption spectra of these metal complexes and their one-electron oxidized π-cation radicals showed no concentration dependence, indicating that these species exist as monomers in solution. Kinetics of the thermal electron transfer reaction from each phthalocyanine complex to Cu2+ and the photoinduced electron transfer reaction of the Zn(II) phthalocyanine complex with V(V) and V(IV) Schiff base complexes were studied using conventional spectrophotometric and transient absorption techniques, and the electron transfer rate constants were analysed using the Marcus cross relationship. The obtained rate constants of the electron self-exchange reaction between the parent phthalocyanine complexes and their π-cation radicals were in the order of 109 to 1011 M-1 s-1 at T = 298.2 K. These large electron self-exchange rate constants are consistent with the phthalocyanine-centred redox reactions where small reorganization energies are required with little structural change during the electron transfer process.

Reactions of Plasmodium falciparum Ferredoxin:NADP+ Oxidoreductase with Redox Cycling Xenobiotics: A Mechanistic Study.

Ferredoxin:NADP+ oxidoreductase from Plasmodium falciparum (PfFNR) catalyzes the NADPH-dependent reduction of ferredoxin (PfFd), which provides redox equivalents for the biosynthesis of isoprenoids and fatty acids in the apicoplast. Like other flavin-dependent electrontransferases, PfFNR is a potential source of free radicals of quinones and other redox cycling compounds. We report here a kinetic study of the reduction of quinones, nitroaromatic compounds and aromatic N-oxides by PfFNR. We show that all these groups of compounds are reduced in a single-electron pathway, their reactivity increasing with the increase in their single-electron reduction midpoint potential (E17). The reactivity of nitroaromatics is lower than that of quinones and aromatic N-oxides, which is in line with the differences in their electron self-exchange rate constants. Quinone reduction proceeds via a ping-pong mechanism. During the reoxidation of reduced FAD by quinones, the oxidation of FADH. to FAD is the possible rate-limiting step. The calculated electron transfer distances in the reaction of PfFNR with various electron acceptors are similar to those of Anabaena FNR, thus demonstrating their similar “intrinsic” reactivity. Ferredoxin stimulated quinone- and nitro-reductase reactions of PfFNR, evidently providing an additional reduction pathway via reduced PfFd. Based on the available data, PfFNR and possibly PfFd may play a central role in the reductive activation of quinones, nitroaromatics and aromatic N-oxides in P. falciparum, contributing to their antiplasmodial action.

Charge Transport and Transfer Mechanisms for Solid State Metal Complex Systems

Recently there have been excited reports that copper-based molecular redox couples can be effective hole collection and transport materials when dissolved in liquid as well as in the solid state. The mechanism of charge transport in the solid state system is not understood, however, which inhibits further improvements or the development of design rules for intentionally made solid state DSSCs utilizing molecular redox complexes. In this talk, a change of diffusion mechanism for copper redox complexes will be presented during the solidification by an in-situ electrochemical analysis. The modified Dahms-Luff equation, which was partially substituted by the viscosity term, was used to evaluate the overall mechanism, and it was confirmed that the calculated self-exchange rate constant varies with phase and the viscosity-dependent value. Eventually, as the solidification progressed, the transport mechanism dominating was changed from ionic to electrical diffusion, which led to an increase in the apparent diffusion coefficient, indicating a high conductivity and a high self-exchange rate constant. However, in the case of cobalt complexes, which are often used in the liquid state, the transport mechanism continued to be dominated by ionic diffusion during solidification with a decrease in apparent diffusion and device performance. In addition to charge transport, the solidification affects rates of heterogeneous charge transfer and ultimately the performance in DSSCs, which will also be described. Finally, the results will be interpreted in design rules for metal complexes as solid state hole transport materials.

Real-Time Observation of the Diffusion Mechanism Progression from Liquid to Solid State of Transition Metal Complexes

Copper complexes have recently shown remarkable performance upon conversion from liquid-based to solid-state hole transport materials (HTMs) in mesoscopic solar cells; however, the diffusion mechanism is not clear. In this work, we apply an in situ solidification analysis of the charge diffusion and find that the dominant mechanism of [Cu(dmbpy)2]2+/+ (dmbpy = 6,6′-dimethyl-2,2′-bipyridine) changes from ionic to electronic diffusion. Through use of the modified Dahms–Luff equation, a fast self-exchange rate constant of hole-hopping in the HTM of 8.3 × 108 (±5 × 107) M–1 s–1 is calculated, which indicates a small reorganization energy of 0.47 eV. These results introduce a new methodology to analyze the transport mechanism of solids, reveal the mechanism of charge transport in molecular-based HTMs, and offer insight into ways to control the flow of charge in optoelectronic systems.

An Insulating Al2O3 Overlayer Prevents Lateral Hole Hopping Across Dye-Sensitized TiO2 Surfaces.

Three chromophores of the general form [Ru(bpy')2(4,4'-(PO3H2)2-2,2'-bipyridine)]2+, where bpy' is 4,4'-(C(CH3)3)2-2,2'-bipyridine (Ru(dtb)2P); 4,4'-(CH3O)2-2,2'-bipyridine (Ru(OMe)2P), and 2,2'-bipyridine (RuP) were anchored to mesoporous thin films of TiO2 nanocrystallites at saturation surface coverages to investigate lateral self-exchange RuIII/II intermolecular hole hopping in 0.1 M LiClO4/CH3CN electrolytes. Hole hopping was initiated by a potential step 500 mV positive of the E1/2 (RuIII/II) potential or by pulsed laser (532 nm, 8 ns fwhm) excitation and monitored by visible absorption chronoabsorptometry and time-resolved absorption anisotropy measurements, respectively. The hole hopping rate constant kR extracted from the potential step data revealed self-exchange rate constants that followed the trend: TiO2|Ru(OMe)2P (ket = 1.4 × 106 s-1) > TiO2|RuP (7.1 × 105 s-1) > TiO2|Ru(dtb)2P (6.5 × 104 s-1). Analysis of the anisotropy data with Monte Carlo simulations provided hole hopping rate constants for TiO2|RuP and TiO2|Ru(dtb)2P that were within experimental error the same as that measured with the potential step. The hole hopping rate constants were found to trend with the TiO2(e-)|RuIII → TiO2|RuII charge recombination rate constants. The atomic layer deposition of an ∼10 Å layer of Al2O3 on top of the dye-sensitized films was found to prevent hole hopping by both initiation methods even though the chromophore surface coverage exceeded the percolation threshold and excited-state injection was efficient. The dramatic hole hopping turnoff was attributed to a larger outer-sphere reorganization energy for self-exchange due to the restricted access of electrolyte to the redox active chromophores. The implications of these findings for solar energy conversion applications are discussed.

Spin-Doctoring Cobalt Redox Shuttles for Dye-Sensitized Solar Cells.

A new low-spin (LS) cobalt(II) outer-sphere redox shuttle (OSRS) [Co(PY5Me2)(CN)]+, where PY5Me2 represents the pentadentate ligand 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine, has been synthesized and characterized for its potential application in dye-sensitized solar cells (DSSCs). Introduction of the strong field CN- ligand into the open axial coordination site forced the cobalt(II) complex, [Co(PY5Me2)(CN)]+, to become LS based upon the complex's magnetic susceptibility (1.91 ± 0.02 μB), determined by the Evans method. Interestingly, dimerization and subsequent cobalt hexacyanide cluster formation of the [Co(PY5Me2)(CN)]+ monomer was observed upon long-term solvent exposure or addition of a supporting electrolyte for electrochemical characterization. Although long-term stability of the [Co(PY5Me2)(CN)]+ complex made it difficult to fabricate liquid electrolytes for DSSC applications, short-term stability in neat solvent afforded the opportunity to isolate the self-exchange kinetics of [Co(PY5Me2)(CN)]2+/+ via stopped-flow spectroscopy. Use of Marcus theory provided a smaller than expected self-exchange rate constant of 20 ± 5.5 M-1 s-1 for [Co(PY5Me2)(CN)]2+/+, which we attribute to a Jahn-Teller effect observed from the collected monomer crystallographic data. When compared side-by-side to cobalt tris(2,2'-bipyridine), [Co(bpy)3]3+, DSSCs employing [Co(PY5Me2)(CN)]2+ are expected to achieve superior charge collection, which result from a smaller rate constant, ket, for recombination based upon simple dark J- E measurements of the two redox shuttles. Given the negative redox potential (0.254 V vs NHE) of [Co(PY5Me2)(CN)]2+/+ and the slow recombination kinetics, [Co(PY5Me2)(CN)]2+/+ becomes an attractive OSRS to regenerate near IR absorbing sensitizers in solid-state DSSC devices.

Influence of 4 and 4′ Substituents on RuIII/IIBipyridyl Self-Exchange Electron Transfer Across Nanocrystalline TiO2Surfaces

Lateral self-exchange electron transfer across oxide surfaces is important to many solar energy capture and conversion schemes. Substituent effects on lateral RuIII/II self-exchange electron transfer were studied using a series of RuII polypyridyl compounds of the type [Ru(R2bpy)2(P)]2+, where P is 2,2′-bipyridyl-4,4′-diphosphonic acid and R2bpy was a 4,4′-substituted-2,2′-bipyridine with six different R groups: −OCH3, −C(CH3)3, −CH3, −H, −Br, and −CF3. These functional groups were chosen mainly for their electron-withdrawing or -donating ability. Chronoabsorptometry was used to probe the apparent diffusion coefficient, DCA, that was proportional to the self-exchange rate constants, and these values were found to be between 2.8 × 10–11 and 7.9 × 10–9 cm2/s. The measured DCA values showed no correlation with the electron-withdrawing or -donating ability of the functional groups, but were instead correlated with the steric size of the substituents that also influenced the saturation surface coverage and thu...

Behavior of Ionic Liquids Around Charged Metal Complexes: Investigation of Homogeneous Electron Transfer Reactions Between Metal Complexes in Ionic Liquids

The second-order electron transfer reaction between the photo-excited triplet state of [Zn(TPP)]* (TPP = 5,10,15,20-tetraphenylporphyrin) and [Co(sep)]3+ (sep = sepulchrate = 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane) was investigated in three ionic liquids (ILs, 1-R-3-methylimidazolium bis(trifluoromethylsulfonyl)imide with R = butyl, pentyl, and hexyl) and in acetonitrile. Results of electrochemical and kinetic measurements indicated that ILs dissociate in the vicinity of charged metal complexes and at electrodes, although the dissociated anionic and cationic components of the ILs seem to exist as pairs around the metal complexes. Second-order rate constants for the electron transfer reaction are 1.88 × 109, 3.65 × 107, 2.63 × 107, and 2.01 × 107 kg·mol−1·s−1 in acetonitrile and in the butyl, pentyl and hexyl ILs, respectively, at 298 K, after correction of the contribution of diffusion. The average slope of the plot of the logarithmic second-order rate constants observed in acetonitrile and ILs against the logarithmic viscosity of each solvent was − 0.84. However, the slope of the same plot was much steeper (− 4.1) when data for only the three ILs were used. Detailed analyses of the experimental results on the basis of the Latner–Levin cross relation and the Marcus theory lead to the conclusion that the solvent properties such as the dielectric constant and refractive index around the polarized/charged transition states are different from those for the bulk ILs: observed self-exchange rate constants did not exhibit the Pekar factor dependence when dielectric constants and refractive indices for bulk ILs are used.

Selective Proton/Deuteron Transport through Nafion|Graphene|Nafion Sandwich Structures at High Current Density.

Ion current densities near 1 A cm-2 at modest bias voltages (<200 mV) are reported for proton and deuteron transmission across single-layer graphene in polyelectrolyte-membrane (PEM)-style hydrogen pump cells. The graphene is sandwiched between two Nafion membranes and covers the entire area between two platinum-carbon electrodes, such that proton transfer is forced to occur through the graphene layer. Raman spectroscopy confirms that buried graphene layers are single-layer and relatively free of defects following the hot-press procedure used to make the sandwich structures. Area-normalized ion conductance values of approximately 29 and 2.1 S cm-2 are obtained for proton and deuteron transport, respectively, through single-layer graphene, following correction for contributions to series resistance from Nafion resistance, contact resistance, etc. These ion conductance values are several hundred to several thousand times larger than in previous reports on similar phenomena. A ratio of proton to deuteron conductance of 14 to 1 is obtained, in good agreement with but slightly larger than those in prior reports on related cells. Potassium ion transfer rates were also measured and are attenuated by a factor of many thousands by graphene, whereas proton transfer is attenuated by graphene by only a small amount. Rates for hydrogen and deuterium ion exchange across graphene were analyzed using a model whereby each hexagonal graphene hollow site is assumed to transmit ions with a specific per-site ion-transfer self-exchange rate constant. Rate constant values of approximately 2500 s-1 for proton transfer and 180 s-1 for deuteron transfer per site through graphene are reported.

Bifurcation of Regeneration and Recombination in Dye-Sensitized Solar Cells via Electronic Manipulation of Tandem Cobalt Redox Shuttles.

A cobalt(IV/III) redox shuttle, cobalt tris(2-(p-tolyl)pyridine), [Co(ptpy)3]+/0, was synthesized and investigated for use in dye-sensitized solar cells, DSSCs. An incredibly fast self-exchange rate constant of (9.2 ± 3.9) × 108 M-1 s-1 was determined for [Co(ptpy)3]+/0, making it an ideal candidate for dye regeneration. To avoid fast recombination and solubility limitations, we utilized a tandem electrolyte containing [Co(ptpy)3]+/0 and cobalt tris(2,2'-bipyridine), [Co(bpy)3]3+/2+. An improved short circuit current density is achieved for DSSCs employing the tandem electrolyte, compared to electrolytes containing only [Co(bpy)3]3+/2+, consistent with superior dye regeneration expected based on predictions using Marcus theory, which is also discussed.

Kinetics of the outer-sphere oxidation of thiourea by heteropoly-α2-17-tungsto-1-vanadodiphosphate anion

Kinetics of the oxidation of thiourea (tu) by heteropoly-α2-17-tungsto-1-vanadodiphosphate anion, α2-[P2VVW17O62]7−, have been studied spectrophotometrically in aqueous acidic medium at 25 °C. At low pH (2.4–3.0), the neutral form of tu is the only reactive species. At higher pH (4.2–4.9), both neutral and deprotonated forms of tu participate in the reaction. The observed mixed-order kinetics suggest two parallel reactions: one in which the order in [tu] is unity, and a second in which it is two. In both cases, the order in [α2-[P2VVW17O62]7−] is unity. Based on the kinetic studies, a mechanism is proposed, in which a second-order proton-coupled electron transfer involving NH2CSNH2 and α2-[P2VVW17O62]7− proceeds through a sequential electron transfer, followed by proton transfer such that the reaction is an “activation-controlled” outer-sphere electron transfer process. By applying the Marcus equation, the self-exchange rate constants for the couples $${\text{NH}}_{2} {\text{CSNH}}_{2}^{ \cdot + }$$ /NH2CSNH2 and α2-[P2VVW17O62]7−/α2-[P2VIVW17O62]8− were evaluated.

Tuning Electron Delocalization and Transfer Rates in Mixed-Valent Ru3O Complexes through "Push-Pull" Effects.

Electron transfer rates in a series of oxo-centered triruthenium clusters featuring an extended aromatic ancillary ligand of the type [Ru3(OAc)6(μ3-O)(CO)(L)(pep)], where L = 4-cyanopyridine (cpy), pyridine (py), or 4-(dimethylamino)pyridine (dmap) and pep = 4-(phenylethynyl)pyridine were investigated. The electron self-exchange rate constants for the 0/- couple were determined by (1)H NMR line broadening experiments and found to range from 4.3 to 9.2 (× 10(7) M(-1) s(-1)) in deuterated acetonitrile (ACN-d3). Relative rates of self-exchange can be rationalized on the basis of increased contact area between self-exchanging pairs, and a push-pull modulation of electron density between the pep vs ancillary pyridine ligands. Faster self-exchange was observed with increasing electron-donating character of the ancillary pyridine ligand substituent (dmap > py > cpy), and this was attributed to increased orbital overlap between self-exchanging pairs. These results are supported by trends observed in (1)H NMR contact shifts of the pep ligand that were found to depend on the electron-donating or -withdrawing nature of the ancillary pyridine ligand.

Kinetics and mechanism of the oxidation of a cobaloxime by sodium hypochlorite in aqueous solution: Is it an outer-sphere mechanism?

The kinetics and mechanism of the oxidation of [Co(dmgBF2)2(OH2)2] (where dmgBF2=difluoroboryldimethylglyoximato) by sodium hypochlorite (NaOCl) were investigated by stopped-flow spectrophotometry at 450nm over the temperature range of 10°C⩽θ⩽25°C, pH range of 5.0⩽pH⩽7.8, and at an ionic strength of 0.60M (NaCl). The pKa1 value for [Co(dmgBF2)2(H2O)2] was calculated as 5.27±0.14 at I=0.60 (NaCl). The redox process was dependent on pH and oxidant concentration in a complex manner, that is, kobs=((k2[H+]+k1Ka)/([H+]+Ka))[OCl−]T, where at 25.3°C, k1 was calculated as 3.54×104M−1s−1, and k2 as 2.51×104M−1cm−1. At a constant pH value, while varying the concentration of sodium hypochlorite two rate constants were calculated, viz., k′1=7.56s−1 (which corresponded to a reaction pathway independent of the NaOCl concentration) and k′2=2.26×104M−1s−1, which was dependent on the concentration of NaOCl. From the variation in pH, ΔH1≠, ΔH2≠, ΔS1≠, and ΔS2≠ were calculated as 58±16kJmol−1, 46±1kJmol−1, 34±55Jmol−1K−1, and −6±4Jmol−1K−1, respectively. The self-exchange rate constant, k11, for sodium hypochlorite (as ClO−) was calculated to be 1.2×103M−1s−1, where an outer-sphere electron transfer mechanism was assumed. A green product, [Co(dmgBF2)2(OH2)(OH)]·1.75NaOCl, which can react with DMSO, was isolated from the reaction at pH 8.04 with a yield of 13%.

A Distance Dependence to Lateral Self-Exchange across Nanocrystalline TiO2. A Comparative Study of Three Homologous RuIII/II Polypyridyl Compounds

Self-exchange intermolecular RuIII/II electron transfer, a process commonly referred to as “hole-hopping”, is of great interest as it provides a means of charge transport across the surface of nanocrystalline (anatase) TiO2 mesoporous thin films without the loss of free energy. This process was characterized by cyclic voltammetry and chronoabsorptometry for three homologous Ru diimine compounds of the general form [Ru(LL)2(dcbH2)](PF6)2, where LL is 2,2′-bipyridine (bpy), 4,4′-dimethyl-2,2′-bipyridine (dmb), or 4,4′-di-tert-butyl-2,2′-bipyridine (dtb) and dcbH2 is 2,2′-bipyridyl-4,4′-dicarboxylic acid. Apparent electron diffusion coefficients, D, abstracted from this data increased with dtb < bpy < dmb. Both techniques were consistent with this trend, despite differences in the magnitude of D between the two methods. Temperature dependent measurements revealed an activation barrier for electron self-exchange of 250 ± 50 meV that was within this error the same for all three diimine compounds, suggesting the total reorganization energy, λ, was also the same. Application of Marcus theory, with the assumption that the 900 ± 100 meV total reorganization energy for self-exchange electron transfer was independent of the Ru compound, revealed that the electronic coupling matrix element, HAB, followed the trend dtb (0.02 meV) < bpy (0.07 meV) < dmb (0.10 meV). The results indicate that insulating side groups placed on redox active molecules can be utilized to tune the electronic coupling and hence self-exchange rate constants without significantly altering the reorganization energy for electron transfer on TiO2 surfaces.

Kinetics of Regeneration and Recombination Reactions in Dye-Sensitized Solar Cells Employing Cobalt Redox Shuttles

The key to achieving high-efficiency dye-sensitized solar cells (DSSCs) is the realization of a redox shuttle which exhibits quantitative dye regeneration with a minimal driving force. Since the electron diffusion length, Ln, is controlled by recombination to the redox shuttle, an optimal redox shuttle must balance the kinetics of these two key electron-transfer reactions. In this work the dye regeneration efficiency, ηreg, and the electron diffusion length were determined for DSSCs employing cobalt tris(bipyridine), [Co(bpy)3]3+/2+, and cobalt bis(trithiacyclononane), [Co(ttcn)2]3+/2+, redox shuttles from optical and incident photon to current efficiency (IPCE) measurements of the cells under front side and back side illumination directions. The regeneration of the D35cpdt dye was found to be quantitative with [Co(ttcn)2]3+/2+; however, dye regeneration with the current champion redox shuttle [Co(bpy)3]3+/2+ is suboptimal despite a larger driving force of the reaction. The electron diffusion length was found to be shorter for DSSCs with the [Co(ttcn)2]3+/2+ redox shuttle compared to [Co(bpy)3]3+/2+, however, due to faster recombination. The self-exchange rate constants of the two redox shuttles were determined from cross-exchange measurements and were found to differ by over 4 orders of magnitude. Application of Marcus theory allowed the difference in self-exchange rate constants to quantitatively account for the differences in regeneration efficiency and electron diffusion length of the two redox shuttles. Atomic layer deposition (ALD) was used to add a single layer of alumina on the TiO2 film prior to immersing it in the sensitizer solution. This treatment resulted in improved performance for DSSCs employing both redox shuttles; however, the improvement was shown to arise from different causes. The alumina layer reduces recombination to the redox shuttle and thereby increases Ln for [Co(ttcn)2]3+/2+. The alumina layer was also shown to improve the dye regeneration efficiency for the [Co(bpy)3]3+/2+ redox shuttle through reduction of recombination to the oxidized dye. These findings clearly demonstrate the fine balance between the regeneration and recombination reactions when outer-sphere redox shuttles are employed in DSSCs. Isolation of the efficiency-limiting reactions, however, allows for strategies to overcome these barriers to be identified and implemented.

A kinetic study of the oxidations of 2-mercaptoethanol and 2-mercaptoethylamine by heteropoly 11-tungsto-1- vanadophosphate in aqueous acidic medium

The kinetics of oxidation of 2-mercaptoethanol and 2-mercaptoethylamine by the heteropoly 11-tungsto-1- vanadophosphate anion, [PVVW11O40]4−, have been studied spectrophotometrically in aqueous perchloric acid at 25 °C. EPR and optical studies show that [PVVW11O40]4− is reduced to the one-electron reduced heteropoly blue, [PVIVW11O40]5−, whilst the thiols are oxidized to the corresponding disulphides, RSSR. Spectrophotometric titrations show that the stoichiometry of both reactions is 1:1. At constant pH, the reactions show simple second-order kinetics with first-order dependence of rate on both [oxidant] and [thiol]. At constant [thiol], the rate of the reaction increases with increasing pH. Plots of kobs/[thiol]t versus 1/[H+] are linear with finite intercepts, showing that both the undissociated thiol (RSH) and thiolate ion (RS−) are reactive species. Generation of RS· from RSH proceeds via a separated-concerted proton–electron transfer mechanism. The reaction of thiolate ion is a simple outer-sphere electron transfer reaction. By applying the Marcus theory, the self-exchange rate constants for the couples HOCH2CH2S·/HOCH2CH2S− and H3N+CH2CH2S·/H3N+CH2CH2S− were evaluated as 3 × 109 and 2.2 × 108 M−1 s−1, respectively, at 25 °C.

Lateral Intermolecular Self-Exchange Reactions for Hole and Energy Transport on Mesoporous Metal Oxide Thin Films.

Intermolecular self-exchange energy and electron-transfer reactions occur without a loss of free energy. This behavior can be exploited for energy-transport applications when molecules that undergo self-exchange transfer reactions are immobilized on a solid support. This Article focuses upon lateral self-exchange reactions and the relevant interfacial chemistry that occurs on the mesoporous nanocrystalline (anatase) TiO2 thin films that are commonly used in dye-sensitized solar cells. It has been known for some time that all of the dye molecules (termed sensitizers) within such thin films can be reversibly oxidized and reduced by lateral self-exchange electron transfer provided that the sensitizer surface coverage exceeds a percolation threshold. Under conditions where excited-state electron injection into TiO2 is unfavored, lateral intermolecular energy-transfer reactions are also known to occur. The self-exchange rate constants have been quantified by electrochemical, absorption, and/or time-resolved anisotropy techniques and understood within the framework of Marcus theory. Such analysis reveals that the reorganization energy and the electronic coupling are sensitive to the identity of the molecular compound. Time-resolved anisotropy measurements have shown that lateral charge and energy-transfer reactions across the TiO2 surface occur in kinetic competition with charge recombination and excited-state relaxation, respectively. The extent to which lateral self-exchange reactions might be exploited for solar energy conversion applications is discussed, as are critical fundamental issues that remain unresolved.

Electron self-exchange in hemoglobins revealed by deutero-hemin substitution

Hemoglobins (phytoglobins) from rice plants (nsHb1) and from the cyanobacterium Synechocystis (PCC 6803) (SynHb) can reduce hydroxylamine with two electrons to form ammonium. The reaction requires intermolecular electron transfer between protein molecules, and rapid electron self-exchange might play a role in distinguishing these hemoglobins from others with slower reaction rates, such as myoglobin. A relatively rapid electron self-exchange rate constant has been measured for SynHb by NMR, but the rate constant for myoglobin is equivocal and a value for nsHb1 has not yet been measured. Here we report electron self-exchange rate constants for nsHb1 and Mb as a test of their role in hydroxylamine reduction. These proteins are not suitable for analysis by NMR ZZ exchange, so a method was developed that uses cross-reactions between each hemoglobin and its deutero-hemin substituted counterpart. The resulting electron transfer is between identical proteins with low driving forces and thus closely approximates true electron self-exchange. The reactions can be monitored spectrally due to the distinct spectra of the prosthetic groups, and from this electron self-exchange rate constants of 880 (SynHb), 2900 (nsHb1), and 0.05M−1s−1 (Mb) have been measured for each hemoglobin. Calculations of cross-reactions using these values accurately predict hydroxylamine reduction rates for each protein, suggesting that electron self-exchange plays an important role in the reaction.