Search

We demonstrate that the potential dependence of the initial change (A/ΔTeq) in the open-circuit potential of Au(111) | (nonelectroactive) electrolyte solution interfaces (effected by the temperature perturbation (ΔTeq) induced by the indirect laser-induced temperature jump (ILIT) technique) is sensitive to the presence of the anion components of the electrolyte adsorbed onto the electrode surfaces for a wide variety of anions. Analysis of the potential dependence of A/ΔTeq for the ClO4− and SO42− anions, therefore, is used to ascertain (for ClO4−) or estimate (for SO42−) the temperature derivative of the dipole potential (i.e., (dVD)/(dT)) associated with the relevant Au(111) | electrolyte solution interface. The value and any potential dependence of (dVD)/(dT) are suitable probes of the structure of the pertinent electrode | electrolyte interface. The negative value determined here for the (dVD)/(dT) associated with a complete (saturated) layer of ClO4− ions adsorbed onto a Au(111) electrode surface (i.e...

For fast, interfacial ion-transfer (adsorption/desorption) reactions, I demonstrate the development of a relationship describing the time dependence of the open-circuit potential change (ΔVoc(t)) initiated by the coulostatic temperature perturbation associated with the indirect laser-induced temperature jump (ILIT) technique. The form of this relationship is exactly the same as the one developed for interfacial electron-transfer (ET) reactions of solution-dissolved redox couples. However, the definitions of the parameters associated with this relationship (e.g., the measured rate constant (km,a/d) and the mass-transfer parameter (P75,a/d)) are unique to and are functions of the chemical kinetic and (some of) the thermodynamic properties of the adsorption/desorption reactions. An additional analysis of the time dependence of the ILIT-induced open-circuit potential change (ΔVoc,et(t)) effected by the ET reaction of a reversibly adsorbed redox couple (where only the adsorbed redox species undergo the ET reaction) also results in a relationship that is the same as that developed for solution-dissolved redox couples, but the definitions of km,et and P75,et are, again, different for the reversibly adsorbed redox moieties. The analyses associated with these developments are relevant to possible studies (using ILIT or other coulostatic perturbation techniques) of the adsorption/desorption reactions pertinent to many technologically important electrochemical processes.

Two relatively simple approaches are developed and used to calculate (predict) the standard interfacial electron-transfer (ET) rate constants (k degrees) of the Ru(NH3)6(3+/2+) couple dissolved in aqueous electrolyte solutions in contact with Au electrodes coated with self-assembled monolayers (SAMs) composed of HS(CH2)nOH as functions of both n and temperature. These approaches are suggested by the conclusion reached by Smalley et al. (J. Electroanal. Chem. 2006, 589, 1-6) that the interfacial ET rate of a solution-dissolved redox couple in contact with a SAM is, within 1 order of magnitude, the same as the (normalized) interfacial ET rate of a similar attached (as a constituent of a similar SAM) couple. The calculations, therefore, employ the measured electronic coupling of the attached (to Au electrodes through alkanethiolate bridges) -PyRu(NH3)5(3+/2+) couple. The two approaches also both include dynamic solvent effects on the ET kinetics and the influence of electronic coupling on the activation barrier for the ET reaction. At T=298 K and n=3, 11, and 14, the predicted rate constants are in very good agreement with the existing measurements of k degrees. However, for n3 at 298 K, the predicted rate constants are extremely large (i.e.,4.5 cm s(-1)) and do not tend toward a limiting value. Additionally, even if the electronic coupling between a Au electrode and a Ru(NH3)6(3+/2+) moiety located at the surface of the SAM is0.1 eV, the calculated standard rate constant is not directly proportional to the inverse of the longitudinal dielectric time of the solvent. A primary reason for both the absence of a limiting value for the predicted k degrees's at 298 K and the attenuated influence of dynamic solvent effects is the activation energy barrier suppression caused by large values of the electronic coupling.

A relationship is developed that facilitates a comparison between the standard electron-transfer rate constant (k0) of a redox couple covalently attached (as a constituent of a self-assembled monolayer (SAM)) to an electrode and the k0 of a similar redox couple dissolved in an electrolyte solution in contact with a similar (but not electroactive) electrode/SAM assembly. Such comparisons performed for ferrocene and ruthenium redox couples demonstrate that the (normalized) rate of electron-transfer (and, also, the electronic coupling) for a solution-dissolved couple through an alkanethiol SAM/aqueous electrolyte solution interface is, within one order of magnitude, equal to that for a covalently attached redox species. A related comparison involving the Marcus theory limiting rate constants for solution-dissolved couples and the Arrhenius preexponential factors for attached couples supplies additional evidence for this surprising result.

The standard heterogeneous electron-transfer rate constants (k(n)0) between substrate gold electrodes and the ferrocene redox couple attached to the electrode surface by variable lengths of substituted or unsubstituted oligophenyleneethynylene (OPE) bridges as constituents of mixed self-assembled monolayers were measured as a function of temperature. The distance dependences of the unsubstituted OPE standard rate constants and of the preexponential factors (An) obtained from an Arrhenius analysis of the unsubstituted OPE k(n)0 versus temperature data are not monotonic. This surprising result, together with the distance dependence of the substituted OPE preexponential factors, may be assessed in terms of the likely conformational variability of the OPE bridges (as a result of the low intrinsic barrier to rotation of the phenylene rings in these bridges) and the associated sensitivity of the rate of electron transfer (and, hence, the single-molecule conductance which may be estimated using An) through these bridges to the conformation of the bridge. Additionally, the measured standard rate constants were independent of the identity of the diluent component of the mixed monolayer, and using an unsaturated OPE diluent has no effect on the rate of electron transfer through a long-chain alkanethiol bridge. These observations indicate that the diluent does not participate in the electron-transfer event.

The indirect laser-induced temperature-jump (ILIT) technique is used to measure the pKa's of self-assembled monolayers (SAMs on gold electrodes) composed of dibutanoic acid disulfide (DBAD) and 6-mercaptohexanoic acid (MHA) in contact with 1.00 M ionic strength NaClO4 electrolyte solutions. The ILIT technique may be used to determine the pKa of a surface-attached acid because the size of the ILIT response is related to the potential drop across the electrode/electrolyte interface that is, in turn, related to the extent of ionization of the acid. The pertinent data are the potentials of zero ILIT response (Epzr) versus pH. It is found that the measured Epzr versus pH data set for the DBAD monolayers is (within experimental error) the same as that measured for the MHA monolayers. Additionally, an analysis of the combined DBAD/MHA data set gives a pKa (4.5 ± 0.2) and a total concentration (ΓT) of either DBAD or MHA comprising the SAM (ΓT = (5.4 ± 0.3) × 10-10 mol/cm2) that (again within experimental error) a...

The indirect laser-induced temperature jump technique is used to study the heterogeneous electron transfer kinetics of Fe(CN)63−/4−, Ru(NH3)63+/2+ (both in aqueous 1 M KF) and dimethylferrocene/dimethylferrocenium (Me2Fc, in 1 M LiClO4 in CH3OH) on Au electrodes. Evidence is obtained demonstrating not only that all three of these redox couples adsorb on Au but also that the behavior of the measured electron transfer kinetics for these couples is significantly perturbed from that expected for simple heterogeneous electron transfer reactions. The interpretation of the results of these measurements is, therefore, made much more complex and uncertain. Such complexity and uncertainty is greatly reduced by irreversibly attaching the redox moiety to the electrode surface as a part of a stable, organized structure (e.g. a self-assembled monolayer).

Theoretical relationships are developed to describe the open-circuit responses associated with the indirect laser-induced temperature-jump (ILIT) method, a method for measuring fast electron-transfer rate constants of surface-attached redox species. The analysis is also applicable to data obtained using the coulostatic charge-injection method. The unique relationship between km, the relaxation rate constant for the ILIT (or coulostatic) response, and Ei, the potential at which the system is initially poised, exhibits a surprising sensitivity to the values of k0, Ei0′ (the standard rate constant and formal potential for the redox couple), α (the transfer coefficient in the Butler–Volmer equation) and γ (a dimensionless parameter which is directly proportional to the total surface concentration of the redox moiety). ILIT data for several examples of surface-attached ferrocene moieties confirm the theoretically predicted km vs Ei behavior. Values of Ei0′ and γ extracted from the ILIT data agree well with the values obtained from cyclic voltammetric data thereby confirming that the ILIT and cyclic voltammogram (CV) experiments are sampling the same ferrocene population. Keywords: Temperature-jump, Laser, ILIT, Coulostatic, Surface-attached redox species, Redox capacitance, Electron transfer resistance

The indirect laser-induced temperature jump (ILIT) method is used to determine the acidity (pKa) of monolayers composed of 11-mercaptoundecanoic acid (MUA) self-assembled on vapor-deposited gold film electrodes in contact with either 0.10 or 1.0 M ionic strength NaClO4 electrolyte solutions. The ILIT technique may be used to measure the pKa of a surface-attached acid because the magnitude of the ILIT response is related to the potential gradient at the electrode/electrolyte interface which is, in turn, related to the extent of ionization of the acid. The relevant data are the potentials of zero ILIT response versus pH. A simple two-layer model of the acid-modified electrode/electrolyte interface and Gouy−Chapman double layer theory (modified for the onset of dielectric saturation effected by the large concentration of charge present at this interface when a substantial fraction of the carboxylic acid moieties are ionized) were employed to analyze these data. This analysis gives pKa = 5.7 ± 0.2 at 0.10 M i...

The kinetics of electron transfer between a substrate gold electrode and a self-assembled monolayer formed from CH{sub 3}(CH{sub 2}){sub n-1}SH and ({eta}{sup 5} C{sub 5}H{sub 5})Fe ({eta}{sup 5}-C{sub 5}H{sub 4})CO{sub 2}(CH{sub 2}){sub n}SH were studied as a function of n, the number of methylenes in the alkyl chain tethering the ferrocene moiety to the electrode, using the indirect laser-induced temperature jump method (ILIT). For 5 {

The addition of K 3 Fe(CN) 6 and K 4 Fe(CN) 6 to a 1 M KF solution is shown to effect a significant change in the response of the open-circuit potential to a rapid change in the interfacial temperature. Using the indirect laser-induced temperature-jump (ILIT) method the interfacial temperature can be changed in a few nanoseconds—fast enough so that electron transfer between the electrode and Fe(CN) 6 3−/4− (adsorbed or in the bulk) does not have time to occur. The only explanation for a change in the ILIT response is that the Fe(CN) 6 3−/4− adsorbs on the gold surface and effects a change in the structure, and therefore the thermal response, of the double layer.

Understanding the physical and chemical factors that control the kinetics of interfacial electron-transfer (ET) reactions is important for a large number of technological applications. The present article describes electrochemical kinetic studies of these factors, in which standard interfacial ET rate constants (k(0)(l)) have been measured for ET between substrate Au electrodes and various redox couples attached to the electrode surfaces by variable lengths (l) of oligomethylene (OM), oligophenylenevinylene (OPV) and oligophenyleneethynylene (OPE) bridges, which were constituents of mixed self-assembled monolayers (SAMs). The k(0)(l) measurements employed the indirect laser-induced temperature jump (ILIT) technique, which permits the measurement of interfacial ET rates that are orders of magnitude faster than those measurable by conventional techniques using the macroelectrodes that are the most convenient substrates for the mixed SAMs. The robustness of the measured rate constants (k(0)(l)), together with the Arrhenius activation energies (E(a)(l)) and preexponential factors (A(l)), is demonstrated by their invariance with respect to several experimental system parameters (including the chemical nature and length of the diluent component of the mixed SAM). Analysis of the kinetic results demonstrates that all of the observed interfacial ET processes proceed through a common type of transition state (predominantly associated with solvent reorganization around the redox moiety) and that the actual ET step involves direct electronic tunnelling between the Au electrode and the redox moiety. However, for the full range of l investigated, a global exponential decay of A(l) is not found for any of the three types of bridges. Possible reasons for this behavior, including the role of rate determining steps associated with adiabatic mechanisms within or beyond the transition state theory framework, are discussed, and comparisons with related conductance measurements are presented.

The standard heterogeneous electron-transfer rate constants between substrate gold electrodes and either ferrocene or pentaaminepyridine ruthenium redox couples attached to the electrode surface by various lengths of an alkanethiol bridge as a constituent of a mixed self-assembled monolayer were measured as a function of temperature. The ferrocene was either directly attached to the alkanethiol bridge or attached through an ester (CO(2)) linkage. For long bridge lengths (containing more than 11 methylene groups) the rate constants were measured using either chronoamperometry or cyclic voltammetry; for the shorter bridges, the indirect laser induced temperature jump technique was employed to measure the rate constants. Analysis of the distance (bridge length) dependence of the preexponential factors obtained from an Arrhenius analysis of the rate constant versus temperature data demonstrates a clear limiting behavior at a surprisingly small value of this preexponential factor (much lower than would be expected on the basis of aqueous solvent dynamics). This limit is independent of both the identity of the redox couple and the nature of the linkage of the couple to the bridge, and it is definitely different (smaller) from the limit derived from an equivalent analysis of the rate constant (versus temperature) data for the interfacial electron-transfer reaction through oligophenylenevinylene bridges between gold electrodes and ferrocene. There are a number of possible explanations for this behavior including, for example, the possible effects of bridge conformational flexibility upon the electron-transfer kinetics. Nevertheless, conventional ideas regarding electronic coupling through alkane bridges and solvent dynamics are insufficient to explain the results reported here.

We measured rate constants of thermal, interfacial electron transfer through oligophenylenevinylene bridges between a gold electrode and a tethered redox species in contact with an aqueous electrolyte using the indirect laser-induced temperature jump technique. Analysis of the distance dependence indicates that, unlike other bridges studied to date, the rate constants are not limited by electronic coupling for bridges up to 28 angstroms long. The energy levels of the bridges relative to those of the redox species rule out hopping through the bridge. We conclude that, out to 28 angstroms, the transfer is limited by structural reorganization and that electron tunneling occurs in less than 20 picoseconds, suggesting that oligophenylenevinylene bridges could be useful for wiring molecular electronic elements.

A change in the interfacial temperature of an electrode causes a concomitant change in its open-circuit potential. A portion of this change in potential is due to the Soret potential, Δ V s , effected by Gibbs energy gradients of electrolyte ions associated with the extant temperature gradient. In our previously reported use of laser-induced interfacial temperature-jump (LIITJ) perturbations to investigate a variety of interfacial relaxation phenomena, it was necessary to characterize the temporal behavior of Δ V s , and related thermally induced concentration changes, Δ c j (x,t) , as a function of temperature, Δ T(x,t) . In the present work we present analytic and simulated analyses of the operative transport phenomena and show that Δ V s / Δ T (0, t ) and Δ c j (0, t )/ Δ T (0, t ) are constant for any electrolyte and independent of the temporal dependence of Δ T (0, t ) as long as Δ c j (0, t )/ c j o ⪡ 1 ( c j o = bulk concentration of the j th species) and ( t m c n > 10 −10 mol l −1 s ( t m = measurement time; c n = total ion concentration). This serendipitous result means that Soret phenomena will not introduce spurious relaxations into the LIITJ responses. The requisite conditions for this ideal behavior are established.

The branching ratio, ..cap alpha../sub 1/, for the H + C/sub 2/H/sub 3/O product channel of the O(/sup 3/P) + C/sub 2/H/sub 3/O reaction was determined from measured H- and O-atom profiles in this flash photolysis-resonance fluorescence study. The relative detection sensitivity of the system for H and O atoms was determined experimentally. A chemical model was used to describe the reaction mechanism together with the relative detection sensitivity, and a value of ..cap alpha../sub 1/ = 0.27 +/- 0.05 was derived at 300 K. At higher temperatures, the value of ..cap alpha../sub 1/ appears to increase slightly. Possible reasons for this increase are discussed. 30 references, 2 figures, 7 tables.

The formation of the complex between photogenerated magnesium octaethylporphyrin cation and hydroxide has been studied in mixed alcohol/water solvents. This complex has been postulated as a participant in the transport of hydroxide and/or hydrogen ion across bilayer lipid membranes. The absorption spectrum of the complex (lambda/sub max/=665nm) is red shifted from that of the cation (lambda/sub max/=680nm) in both methanol/water (M/W) and ethanol/water (E/W). The difference in the two constants is too great to be explained entirely by the Born equation (with correction for ion screening) in which the dielectric constant of the solvent system is the pertinent variable. Differential solvation effects appear to be involved. The rate constants, k/sub 2/, for the reaction of the cation with the p-dinitrobenzene anion, p-DNB/sup -/ (p-DNB is the electron acceptor for the photogeneration of the maagnesium octaethylporphyrin cation), and k/sub 4/, for the reaction of the complex with p-DNB/sup -/, are 1.1 X 10/sup 9/ and 2.1 X 10/sup 9/ M/sup -1/s/sup -1/ in M/W and 1.3 X 10/sup 9/ and 1.2 X 10/sup 9/ M/sup -1/s/sup -1/ in E/W. Although the rate constants are within an order of magnitude of being diffusion controlled, the fact that k/sub 4/less than or equalmore » tok/sub 2/ is indicative of some degree of activation control.« less

The production of ions from the first excited triplet state (T) of magnesium octaethylporphyrin (MgOEP or simply P) has been studied. Triplet-triplet annihilation is shown to be the major route for the production of the ions P/sup +/ and P/sup -/. Evidence is obtained indicating that the reaction of T with ground-state P is not a significant source of ions. Evidence is also presented indicating that the two triplets initially combine to form an excited charge-transfer complex. The relationship between the multiplicity of this charge-transfer complex and triplet quenching, delayed fluorescence, and ion formation is discussed. 13 figures, 3 tables.

A coulostatic perturbation of an electrode surface effected by a laser-induced temperature jump is described. A thin foil or film electrode exposed to dielectric (air, or glass backing) on one side and solution on the other is irradiated by a laser pulse from the dielectric side. Non-reflected photons are absorbed in a thin layer of the metal at the metal/dielectric interface and thermalize virtually instantaneously. The high thermal diffusivity in the metal causes rapid heating of the metal/solution interface and a concomitant change in the open-circuit potential of the electrode. This change in potential and its subsequent relaxation(s) are analyzed quantitatively in terms of: (1) A junction potential between the (hot) electrode and the (cold) contact wire; (2) a change in the potential across the electrode double layer which can be effected by a change in the capacitance and/or by a change in dipole orientation (equivalent to a change in the potential of zero charge) and/or by charge transfer (electron transfer between the electrode and a redox system located at the outer Helmholtz plane (OHP) or inner Helmholtz plane (IHP), or ion transfer between the IHP and the OHP); (3) a Soret potential. For small temperature changes and a constant charge-transfer resistance the theoretical analysis yields an analytic equation. Using a flashlamp-pumped dye laser (width at half-max. ~ 0.35 μs) and a 25 μs Pt-foil electrode, the thermal relaxation phenomena are verified and calibrated experimentally and (as a demonstration) the method is used to evaluate the heterogeneous rate constant for ferri/ferrocyanide. The possibility of probing electrode responses in the sub-microsecond and sub-nanosecond time domains is discussed.

Fluorescence quenching of trans-etiochlorin I (C) by magnesium octaethylporphyrin (P) has been studied in small (approx.250 A in diameter) phosphatidylcholine vesicles. It is shown that the quenching is due to the formation of a nonfluorescent statistical pair energy trap between P and C with a critical separation distance between 10 and 18 A. The formation of P/sup +/ as the result of the flash photolysis of C in vesicle suspensions containing both P and C in addition to an electron acceptor demonstrates that excitation of this trap results in an electron transfer from P to C. 31 references, 5 figures.

The rate constant for the reaction of ground-state atomic oxygen with ethylene was determined by using two techniques: flash photolysis-resonance fluorescence (FP-RF, 244-1052 K) and discharge flow-resonance fluorescence (DF-RF, 298-1017 K). Kinetic complications due to the presence of molecular oxygen in the FP-RF experiments at high temperatures (T > 800 K) were overcome by using NO as the photolytic source of the O atoms. The rate constant, k/sub 1/ (T), derived in this study exhibits extreme non-Arrhenius behavior, but it can be successfully fit to the sum of exponentials expression, 244-1052 K, k/sub 1/(T) = (1.02 +/- 0.06) x 10/sup -11/ exp(-753 +/- 17 K/T) + (2.75 +/- 0.26) x 10/sup -10/ exp(-4220 +/- 550 K/T), in units of cm/sup 3/ molecule/sup -1/ s/sup -1/. Additionally, a fit of the results of this work to a simple transition-state theory expression and the comparison of these results with those of other workers are discussed.

Spectra resulting from the pulse radiolysis of glasses CH/sub 3/OH and CD/sub 3/OH at 6 K were observed. The absorption maxima of these spectra (625 nm for CH/sub 3/OH and 720 nm for CD/sub 3/OD-observed 100 ns after the beginning of the electron pulse) are only slightly red shifted from the absorption maximum obtained when either of these alcohols is ..gamma.. irradiated at 4.2 K (i.e., 600 nm). Bleaching studies indicate that these ..gamma.. radiolysis spectra are composed of a visible part which is due to solvated electrons and an infrared part which is due to trapped electrons. The infrared portions of the pulse radiolysis spectra at 6 K are enhanced compared to the above ..gamma.. radiolysis spectra, and they decay to the latter spectra by a process which is approximately first order. Benzyl chloride also reduces the infrared absorptions which are produced by an electron pulse at 6 K more effectively than it reduces the visible absorptions. The species which is responsible for the transient infrared absorption at 6 K is tentatively identified as an electron localized in either a very shallow trap or a trap which is geminate to its parent cation. In either case, since these pulsemore » radiolysis spectra are fully developed immediately after the electron pulse, it is concluded that electrons are localized in preexisting potential wells which may be either deep or shallow in vitreous methanol. 50 references, 5 figures, 3 tables.« less