J. Phys. Chem. B 2001, 105, 8829-8837
8829
Tetrazine Bridged Osmium Dimers: Electrochemical vs Photoinduced Electron Transfer† Robert J. Forster* and Tia E. Keyes‡ National Center for Sensor Research, School of Chemical Sciences, Dublin City UniVersity, Dublin 9, Ireland ReceiVed: March 13, 2001; In Final Form: June 27, 2001
Monolayers of the dimeric complex [p0p Os(bpy)2 4-tet Os(bpy)2 Cl]3+, where p0p is 4,4′-bipyridyl, bpy is 2,2′-bipyridyl, and 4-tet is 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine, have been formed by spontaneous adsorption onto platinum microelectrodes. These monolayers are stable, and both metal centers exhibit well-defined voltammetric responses for the Os2+/3+ redox reaction. Adsorption isotherms reveal that the areas of occupation for the dimer and a model monomer, [p0p Os(bpy)2 4-tet]2+, are identical at 158 ( 14 Å2. This result is consistent with the dimer adopting an extended configuration rather than lying coplanar with the electrode surface. High-speed chronoamperometry reveals that the standard heterogeneous electron-transfer rate constants, k, for the “inner” [p0p Os(bpy)2 4-tet]2+ and “outer” [4-tet Os(bpy)2Cl]+ moieties are (1.3 ( 0.2) × 106 and (1.1 ( 0.1) × 104 s-1, respectively. The reorganization energy is at least 0.6 ( 0.1 eV. The relatively small decrease in the heterogeneous electron-transfer rate constant across the 14 Å 4-tet bridge is interpreted in terms of electron superexchange. Solution phase transient emission measurements reveal that the rate of photoinduced electron transfer (PET) between the two metal centers is (1.6 ( 0.1) × 107 s-1. This rate constant is a factor of approximately 400 smaller than the ground-state electron-transfer rate constant for monomeric [4-tet Os(bpy)2 Cl]+ monolayers when the driving forces are identical. This significant difference is interpreted in terms of the energy separation between the ground or excited states and the bridge. These data also reveal that the strength of electronic coupling across the tetrazine bridge is significantly greater for two metal centers than for a metal electrode and a remote redox moiety.
Introduction The strength of electronic coupling, the distance of charge transfer, and the nature of the bridge linking the reactants are of central importance in the dynamics of nearly all molecule based electron-transfer reactions.1-4 The development of spontaneously adsorbed and self-assembled monolayers has greatly facilitated investigations into these effects for heterogeneous electron transfer.5,6 However, important issues such as the effect of switching the oxidation state of a bridge unit on the dynamics of electron transfer or comparisons of thermal vs photochemically driven reactions cannot be explored using traditional monolayers that contain a single redox center. In contrast, surface active dimers would allow these issues to be explored. Bridges that incorporate metal centers are expected to exhibit a different behavior compared to conventional organic linkers because their redox states can be switched, thus modulating the dynamics of electron transfer.7,8 Information about processes of this kind is especially important for molecular electronics applications ranging from information storage to the production of antennae complexes suitable for solar energy conversion.9,10 In this contribution, we report on monolayers assembled on platinum microelectrodes using the dimeric complex [p0p Os(bpy)2 4-tet Os(bpy)2 Cl]3+, where p0p is 4,4′-bipyridyl, bpy is 2,2′-bipyridyl, and 4-tet is 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine (Chart 1). The voltammetric response associated with the Os2+/3+ reaction is unusually ideal for both metal centers. The formal potential of the inner redox center, [p0p Os(bpy)2 4-tet]2+, is †
Part of the special issue “Royce W. Murray Festschrift”. * To whom correspondence should be addressed. ‡ Current address: School of Chemistry, Dublin Institute of Technology, Dublin 8, Ireland
CHART 1: Structure of the Dimer Used To Form Spontaneously Adsorbed Monolayers
more positive than that of the outer moiety. High scan rate cyclic voltammetry and potential step chronoamperometry conducted on a nanosecond to microsecond time scale have been used to measure the dynamics of electron transfer to the metal centers. In particular, we have probed the mechanism by which the outer redox center is reduced and the effect of switching the oxidation state of the inner complex on the extent of electronic coupling across the bridge. Moreover, by comparing the electron-transfer rates for the electrochemically and photochemically driven process, we reveal the profound impact that differences between the energies of redox and bridge states can have on the dynamics of electron transfer. Traditionally, it is assumed that the dynamics of electron transfer across electrode/monolayer interfaces is influenced by the structure of the bridge rather than the tunneling barrier between the electrode and the bridge, e.g., the gold-thiol link in alkanethiol monolayers.11-13 However, there have been
10.1021/jp010948v CCC: $20.00 © 2001 American Chemical Society Published on Web 08/21/2001
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Forster and Keyes
remarkably few experimental investigations of this issue, i.e., comparisons of electronic coupling across a bridge linking two metal centers compared with the same bridge linking remote redox centers to a metal surface. Here, we report on the extent of electronic coupling between the two metal centers and compare it with previously reported data for spontaneously adsorbed monolayers that contain the same tetrazine bridge. Experimental Section Synthesis. [p0p Os(bpy)2 4-tet](PF6)2. [Os(bpy)2 4-tet Cl]PF6 was synthesized and characterized as described previously.14 A 0.477 g (0.52 mmol) amount of the complex was dissolved in 20 cm3 of 50:50 ethanol:water and heated to reflux. Then 0.08 g (0.51 mmol) of 4,4′-dipyridyl (p0p) was added to the refluxing solution over 20 min. The reaction mixture was allowed to reflux for a further 12 h. After cooling, a solution of concentrated aqueous NH4PF6 was added. The resulting green solid (0.529 g, 86% yield) was collected by filtration and recrystallized from acetone:water (1:1 (v/v)). The purity of the complex was confirmed by CHN and HPLC. Anal. Calcd for OsC42H32N12P2F12: C, 42.6%; H, 2.7%; N, 14.2%. Found: C, 43.0%; H, 2.6%; N, 14.3%. [p0p Os(bpy)2 4-tet Os (bpy)2 Cl](PF6)3. [Os(bpy)2 p0p Cl]PF6 was synthesized and characterized as described previously.15 This complex was converted to [Os(bpy)2 p0p H2O]2+ by heating 300 mg in a minimum volume of water for 3 h. The complex was then precipitated by adding a 5-fold molar excess of LiClO4 followed by refrigeration for 2 h. The solid product was collected by vacuum filtration with a yield of approximately 75%. [p0p Os(bpy)2 4-tet Os (bpy)2 Cl](PF6)3 was synthesized by dissolving [Os(bpy)2 4-tet Cl]PF6 (0.477 g, 0.52 mmol) in 50 cm3 of ethanol and bringing it to reflux. A molar equivalent (0.463 g, 0.53 mmol) of [Os(bpy)2 p0p H2O](ClO4)2 dissolved in ethanol was then added over 20 min and the resulting solution refluxed for 36 h. After cooling, a concentrated solution of aqueous NH4PF6 was added to precipitate the dark brown complex. The product was recrystallized by dissolving the complex in 50:50 acetone:water followed by slow evaporation of the organic solvent. The purity of the recrystallized product was confirmed using cation exchange HPLC (single peak, retention time 5.2 min) and elemental analysis. Anal. Calcd for Os2C62H48N16P3F18Cl: C, 39.9%; H, 2.6%; N, 12.0%. Found: C, 39.7%; H, 2.8%; N, 12.4%. Instrumentation. In high-speed chronoamperometry,16 a custom-built function generator-potentiostat, with a rise time of less than 2 ns, was used to apply potential steps of variable pulse width and amplitude directly to a two-electrode cell. A Pt foil and an Ag/AgCl reference electrode were combined to form a counter electrode. The foil lowered the resistance and provided a high-frequency path. Cyclic voltammetry was performed using a CH Instruments Model 660 Electrochemical Workstation and a conventional three electrode cell. All solutions were degassed thoroughly using nitrogen, and a blanket of nitrogen was maintained over the solution during all experiments. Potentials are quoted with respect to a CH Instruments Ag/AgCl reference electrode filled with saturated KCl which had a potential of +0.190 V with respect to the normal hydrogen electrode. All experiments were performed at room temperature (22 ( 3 °C). Microelectrodes were prepared using platinum microwires of radii between 1 and 50 µm sealed in a glass shroud and were mechanically polished as described previously.15,17 Electrochemical cleaning of the electrodes was carried out by cycling in 0.1 M H2SO4 between potential limits chosen to initially
Figure 1. Voltammetric response for a 5 µm platinum electrode modified with a monolayer of [p0p Os(bpy)2 4-tet Os(bpy)2 Cl]3+. The scan rate is 10 V/s. The supporting electrolyte is 1.0 M LiClO4 in acetonitrile. The monolayer surface coverage is 9.8 × 10-11 mol cm-2. Cathodic currents are up, and anodic currents are down.
oxidize and then reduce the surface of the platinum electrode. Excessive cycling was avoided in order to minimize the extent of surface roughening. The real surface area of the electrodes was determined by calculating the charge under the platinum oxide reduction peak. Typically surface roughness values were between 1.6 and 2.0. Determining the real, as opposed to the geometric, area of the electrodes is important if the area of occupation of the adsorbate is to be accurately determined. Spontaneously adsorbed monolayers were formed by immersing the microelectrodes in micromolar solutions of the metal complex in acetone:water (50:50 (v/v)) for periods up to 12 h. The complex is stable toward aerial oxidation, and no precautions were taken to exclude atmospheric oxygen during monolayer formation. Before electrochemical measurements were made, the electrodes were rinsed with 50:50 acetone:water, Milli-Q water, and the electrolyte to remove any unbound material. Subsequent measurements were performed in blank electrolyte. Luminescent lifetimes were measured using the third harmonic (355 nm) of a Spectron Q-switched Nd:YAG laser for excitation. Emission was detected in a right angled configuration to the laser using an Oriel model IS520 gated intensified CCD coupled to an Oriel model MS125 spectrograph. With suitable signal averaging, this configuration allows a complete emission spectrum (spectral range, 250 nm) to be obtained within times as short as 10 ns. The emission spectra were typically recorded using the average of 20 laser shots. The gatewidth, i.e., the exposure time of the CCD, was never more than 5% of the excited-state lifetime. The step size, i.e., the time between the acquisition of discrete spectra, was typically 5% of the excitedstate half-life. Dilute solutions of the complex in acetonitrile (293 K) or 4:1ethanol:methanol (77 K; 10-4 to 10-5 M) were deaerated for 20 min under nitrogen prior to use. Low-temperature emission lifetime studies were carried out using an Oxford Instruments gas-exchange cryostat equipped with a Thor 3030 temperature controller. Standard iterative techniques were employed to determine the lifetimes of emission. Results and Discussion General Electrochemical Properties. Figure 1 shows representative background corrected cyclic voltammograms for a spontaneously adsorbed [p0p Os(bpy)2 4-tet Os (bpy)2 Cl]3+ monolayer in acetonitrile containing 1.0 M LiClO4 as supporting electrolyte. Osmium polypyridyl complexes of this type are extremely inert toward both photochemically and thermally driven ligand substitution reactions and the voltammetric peak
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heights change by less than 10% when the monolayer is repeatedly cycled for periods up to 5 h in blank electrolyte solutions. Dimers of similar structure, e.g., [Cl Os (bpy)2 4-tet Os(bpy)2 Cl]2+, that lack a pendant nitrogen do not exhibit strong adsorption onto the electrode surface and can be removed by washing with 50:50 acetone:water. This observation suggests that stable dimeric monolayers involve binding of the p0p ligand to the microelectrode surface. These voltammograms are consistent with those expected for an electrochemically reversible reaction involving a surface-confined species.18,19 The peak shape is independent of scan rate, υ, for 0.05 < υ < 50 V s-1, and the peak height increases linearly with increasing scan rate, rather than the υ1/2 dependence observed for the complex in solution. The full width at half-maximum (fwhm) values are between 90 and 110 mV, indicating that only very weak destabilizing interactions exist within the monolayers. Moreover, the ideality of the response indicates that under these relatively slow scan rate conditions, charge compensating counterions are freely available within the layer. In osmium polypyridyl complexes of this type, metal based oxidations are typically observed between 0.25 and 0.75 V, while ligand based reductions occur are potentials negative of -1.40 V.17,20 Solution phase studies of the corresponding monomeric fragments of the dimer, i.e., [p0p Os(bpy)2 4-tet]2+ and [4-tet Os(bpy)2 Cl]+, reveal that the formal potentials, E°′, for the Os2+/3+ redox reaction are 0.590 and 0.260 V, respectively.14 Therefore, the redox response centered at 0.620 V corresponds to the “inner” [p0p Os(bpy)2 4-tet]2+ fragment that is bound to the electrode surface through the 4,4′-dipyridyl bridging ligand. The redox response observed at 0.280 V corresponds to the “outer” [4-tet Os(bpy)2 Cl]+ moiety located at the monolayer/solution interface. In attempting to form spontaneously adsorbed monolayers using dimeric building blocks, it is essential to probe whether the adsorbates adopt a perpendicular or coplanar orientation with respect to the electrode surface. The area occupied per molecule, Amolec, will be significantly different in these two circumstances; i.e., if the adsorbates are oriented normal to the electrode surface, then Amolec will be dictated simply by the radius of the metal complex and will be similar to a monomeric species. In contrast, significantly larger areas of occupation will be observed for a coplanar orientation. Therefore, an insight into the monolayer structure can be obtained by comparing adsorption isotherms for monomeric and dimeric species. Adsorption Isotherms. To obtain the adsorption isotherm,21 the surface coverages at equilibrium of both [p0p Os(bpy)2 4-tet]2+ and [p0p Os(bpy)2 4-tet Os (bpy)2 Cl]3+ were determined by integrating the background corrected cyclic voltammograms as the bulk concentration in the deposition solution was systematically varied. The Langmuir adsorption isotherm21,22 describes adsorption where lateral interactions between the adsorbed molecules are absent, the limiting surface coverage is dictated simply by the size of the adsorbate, and there is an equilibrium between the bulk and surface concentration. The rate of monolayer formation is significantly larger than desorption in this system, and, as discussed by Anson and Campbell,23 this asymmetry in the adsorption/desorption kinetics means that the Langmuir adsorption isotherm is not strictly applicable. However, it does provide a convenient means of comparing the approximate free energies of adsorption, ∆Gads°, for the monomeric and dimeric species. The Langmuir adsorption isotherm is given in22
Γi/(Γs - Γi) ) exp(-∆Gads°/RT)CB
(1)
Figure 2. Dependence of the surface coverage on the bulk concentration of the [p0p Os(bpy)2 4-tet]2+, 9, and [p0p Os(bpy)2 4-tet Os(bpy)2 Cl]3+, 4. The supporting electrolyte is aqueous 1.0 M LiClO4. The solid lines represent the best fits to the Langmuir adsorption isotherm. The inset shows the fit of the experimental data to the linear form of the Langmuir isotherm.
where Γi is the surface excess of species i at equilibrium, Γs is the surface excess of species i at saturation, and CB is the bulk concentration of species i in solution. Equation 1 can be rearranged into a linear form given by
CB 1 1 ) C + Γi Γs B Γs exp(-∆Gads°/RT)
(2)
and the results are shown in the inset of Figure 2. The experimental data follow the Langmuir adsorption isotherm and yield ∆Gads° values of -34 ( 2 and -30 ( 2 kJ mol-1 for [p0p Os(bpy)2 4-tet]2+ and [p0p Os(bpy)2 4-tet Os (bpy)2 Cl]3+, respectively. Thus, while both complexes adsorb strongly onto platinum, the free energy of adsorption for the dimers is somewhat smaller than for the monomer. This reduced ∆Gads° may arise because the bulkier dimer inhibits stabilizing lateral interactions. The free energy of adsorption for a dimer adsorbing onto a clean platinum surface is likely to be significantly different from that associated with dimer adsorption onto a monolayer modified surface. Therefore, the simple Langmuir curve illustrated in Figure 2 suggests that, over this concentration range, multilayers are not formed. Significantly, the saturation coverages of the two complexes are indistinguishable with Γs values of (1.0 ( 0.1) × 10-10 and (1.1 ( 0.1) × 10-10 mol cm-2 being observed for the monomer and dimer, respectively. Consistent with crystallographic data24,25 which indicate that the radii of osmium and ruthenium polypyridyl complexes are of the order of 6.7 Å, this saturation coverage corresponds to an area of occupation of 158 ( 14 Å2. Energy minimized molecular modeling indicates that the area of occupation for the dimer would be of the order of 300 Å2 if it lay coplanar with the electrode surface. Therefore, these results strongly suggest that the dimer is oriented perpendicular to the electrode surface and that its area of occupation is dictated by the radius of the [Os(bpy)2]2+ moieties. Effect of Scan Rate on the Voltammetric Response. Voltammetry can provide a powerful insight into the rate of heterogeneous electron transfer across the electrode/monolayer interface. When the time constants for the voltammetric experiment and heterogeneous electron transfer become comparable, the peak-to-peak splitting, ∆Ep, between the anodic, Epa, and cathodic, Epc, peak potentials increases. Figure 3 illustrates how the voltammetric response associated with the Os2+/3+ redox
8832 J. Phys. Chem. B, Vol. 105, No. 37, 2001
Forster and Keyes couple and that the electrochemical double layer sets up at the electrode/film interface. Figure 3 indicates that heterogeneous electron transfer influences the response of the outer couple at high scan rates. The observation of slower heterogeneous electron transfer for the couple at 0.280 V, i.e., a larger ∆Ep for a given scan rate, is consistent with the significantly larger electron-transfer distance for the outer vs the inner redox center, i.e., approximately 23 vs 9 Å. As discussed elsewhere, k° depends on both a frequency factor and a Franck-Condon barrier and is described by27-29
k ) Aet exp(-∆Gq/RT) Figure 3. Effect of the scan rate on the voltammetry of an [p0p Os(bpy)2 4-tet Os(bpy)2 Cl]3+ monolayer. The surface coverage is 1.0 × 10-10 mol cm-2 and the platinum microdisk radius is 5 µm. The scan rates from top to bottom are 20 000, 10 000, 5000, and 2000 V/s. The open circles represent the theoretical fit obtained from the nonadiabatic tunneling model where k° is 1.1 × 104 and 1.3 × 106 s-1, for the couples centered at 0.280 and 0.620 V, respectively, and λ is 0.6 eV in both cases. The supporting electrolyte is aqueous 1.0 M LiClO4.
reaction changes as the scan rate, υ, is increased from 2000 to 20 000 V s-1. In sharp contrast, ∆Ep for the couple at 0.280 V increases from approximately 100 to 300 mV over the same range of scan rates. Uncompensated cell resistance, charge compensating ion transport, and slow heterogeneous electron transfer could contribute to the observed behavior. Where the working electrode is a 5 µm radius microelectrode, the uncompensated resistance as measured using potential step chronoamperometry is 7630 ( 332 Ω. Taken in conjunction with the maximum peak current of approximately 870 nA, this cell resistance leads to an iR drop of approximately 6 mV. This ohmic loss is negligible compared to the peak-to-peak separation of 280 mV observed for the [4-tet Os(bpy)2 Cl]+ moiety. Moreover, despite the similar peak current, ∆Ep for the [p0p Os(bpy)2 4-tet]2+ is less than 10 mV even at a scan rate of 20 000 V s-1. Both of these observations indicate that an ohmic drop does not significantly influence the voltammetry. As discussed by Smith and White,26 the presence of an adsorbed monolayer can significantly affect the interfacial potential distribution. For example, if ions are unable to permeate the monolayer, then the potential at the interface decreases linearly between the electrode surface and the plane of electron transfer followed by an exponential decay in solution. Given that the electron-transfer dynamics are potential dependent, it is important to have an insight into the ion permeability of the monolayers. Significantly, the peak shapes and ∆Ep values are independent of the supporting electrolyte concentration for 0.2 < [LiClO4] < 2.0 M. Moreover, the formal potentials for the dimer dissolved in acetonitrile are within 10 mV of those found for the monolayers. Also, the limiting interfacial capacitance of the monolayer found in aqueous media where the electrolyte concentration exceeds 1.2 M yields a monolayer dielectric constant of 52 ( 6. All of these observations suggest that the monolayer is well-solvated and is permeable to electrolyte ions. Finally, investigations of the heterogeneous electron-transfer dynamics using chronoamperometry, vide infra, reveal that the electrode kinetics of both metal centers are described by a single rate constant. If ion and electron transfer occurred on the same time scale, then the driving force for electron transfer would continuously vary during redox switching of the monolayer, causing complex kinetics to be observed. The combination of these results strongly suggests that ion permeation does not influence the voltammetry of either redox
(3)
where Aet is the preexponential factor and ∆Gq is the electrochemical free energy of activation.30 For an adiabatic reaction, the prefactor is the product of κel the electronic transmission coefficient and υn a frequency factor dictated either by nuclear or solvent motion. In contrast, for a nonadiabatic reaction where the reactants are weakly coupled, κel is significantly less than unity and the prefactor is dictated by the electron hopping frequency in the activated complex, υel. As discussed by Chidsey,6 Creager,31 and Murray,32 the voltammetric response can be modeled according to a nonadiabatic electron-transfer response to extract standard heterogeneous electron-transfer rate constants and reorganization energies. Following Chidsey,6 the potential dependent rate constants for monolayer reduction, kred.,η and oxidation, kox.,η, are given by
kred.,η ) κelFkBT
kox.,η ) κelFkBT
∫-∞+∞
exp{-(x - (λ + η)/kBT)2(kBT/4λ)}
+∞exp{-(x
∫-∞
1 + exp(x) - (λ - η)/kBT)2(kBT/4λ)} 1 + exp(x)
dx (4) dx (5)
where x is the electron energy relative to the Fermi level, κel is the distance dependent electronic coupling between the electrode and the redox sites, F is the density of electronic states in the metal electrode, kB is the Boltzmann constant, T is the absolute temperature, and λ is the reorganization energy. The voltammetric current for the reaction of an immobilized redox center following first-order kinetics is given by
iF ) nFA(kox.,η Γred.,η - kred.,ηΓox.,η)
(6)
where Γred.,η and Γox.,η are the instantaneous surface coverages of the oxidized and reduced species. Energy minimized molecular modeling indicates that the electron-transfer distances are approximately 9 and 23 Å for the inner and outer redox centers, respectively. Therefore, in using this approach to model the voltammetric response, there are only two freely adjustable parameters for each redox center, k° and ∆Gq ()λ/4). To fit the experimental voltammograms, we have used the Nelder and Mead Simplex33 algorithm to find the values of k° and ∆Gq that minimize the sum square residuals between the theoretical and experimental currents observed in anodic branches of the linear sweep voltammograms. Figure 3 shows that the theory satisfactorily fits the high scan rate voltammetry of [p0p Os(bpy)2 4-tet Os(bpy)2 Cl]3+, monolayers where the k° values are 1.1 × 104 and 1.3 × 106 s-1, for the couples centered at 0.280 and 0.620 V, respectively. Consistent with results obtained
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for similar redox centers,15,17,35 in both cases, λ is 0.6 eV, but a similar quality fit is obtained for 0.4 < λ < 1 eV. Significantly, the acceptable fit obtained suggests that the voltammetric response is satisfactorily described by a mechanism involving two independent heterogeneous electron-transfer reactions. This observation suggests that a sequential electron-transfer mechanism in which the inner redox center mediates the reduction of the outer site is not important. This behavior is expected since mediated reduction of the outer redox center by the inner center is thermodynamically uphill by more than 300 mV. Chronoamperometry. Long-range electron transfer through structurally heterogeneous bridges plays a key role in both biological and synthetic systems.34 It is known that the electronic structure of the bridge plays a critical role in dictating the rate of electron transfer across donor-bridge-acceptor systems.35 For example, when the bridge energy is significantly different from that of the donor or acceptor, the bridge population is close to zero at all times and electron transfer proceeds by a superexchange mechanism.36 In contrast, when the donor/ acceptor and bridge energies are similar, the bridge site is actually populated and a sequential transfer mechanism is observed. Dimeric monolayers represent attractive systems for probing these effects. For example, in the case where a fully oxidized monolayer is reduced, electron transfer to the inner redox center is likely to occur more rapidly because the electrochemical driving force will be larger and the electrontransfer distance smaller. Therefore, electron transfer to the outer redox center will occur through a bridge containing a reduced Os2+ center. The presence of an Os2+ will change the energy of the p0p-Os-tet bridge states which may modulate the strength of electronic coupling between the outer redox center, creating an electrochemical switch. We have probed these effects by measuring the potential dependence of the electron-transfer rate constants using chronoamperometry. For an ideal electrochemical reaction involving a surface bound species, the Faradaic current following a potential step that changes the redox composition of the monolayer exhibits a single-exponential decay in time according to37
iF(t) ) kQ exp(-kt)
(7)
where k is the apparent rate constant for the overall reaction and Q is the total charge passed in the redox transformation. Figure 4 illustrates typical examples of the current-time transients observed for the reduction of the [p0p Os(bpy)2 4-tet]3+ moieties, i.e., the inner redox centers, where the electrode radius is 1 µm. In these experiments the overpotential η (≡E - E°′) was systematically varied between -0.05 and -0.200 V. To use these data to accurately measure k, the response time of the electrode must be shorter than the time constant for heterogeneous electron transfer. The RC time constants, where R is the total cell resistance and C is the interfacial capacitance, were obtained by stepping the potential from -0.100 to -0.050 V. The monolayer is not redox active at these potentials, and the current decays follow single exponential kinetics due to double layer charging. The capacitance is approximately 0.2 pF, and the RC time constants are less than 5 ns. Thus, as illustrated in Figure 4, double layer charging will affect the current response only at time scales shorter than approximately 20 ns. Therefore, the heterogeneous electron-transfer rate constant can be measured by analyzing the current-time transients at relatively longer time scales. The linearity of the semilog current vs time plots shown in the inset of Figure 4 indicates that heterogeneous electron transfer is
Figure 4. Current-time responses for a 1 µm radius platinum microelectrode modified with an [p0p Os(bpy)2 4-tet Os(bpy)2 Cl]3+ monolayer following potential steps in which the potential is stepped (top to bottom on right-hand side) from 0.800 to 0.560, 0.512, 0.465, and 0.410 V. The corresponding overpotentials for the reduction of the [p0p Os(bpy)2 4-tet]3+ center are -50, -98, -145, and -200 mV. The inset shows the corresponding semilog plots of Faradaic current vs time.
Figure 5. Tafel plot for [p0p Os(bpy)2 4-tet Os(bpy)2 Cl]3+ monolayers adsorbed on platinum microelectrodes. The supporting electrolyte is 1.0 M aqueous LiClO4. The symbols O (data from Figure 4) and 0 (data from Figure 6) represent experimental data for the “inner” and “outer” couples centered at 0.620 and 0.280 V, respectively. The top solid curve represents the best fit to the data of the Butler-Volmer equation where k° and R are 1.3 × 106 s-1 and 0.44 ( 0.05, respectively. The lower solid curve represents the best fit derived from the nonadiabatic electron-transfer model where k° is 1.1 × 104 and the reorganization energy is 0.6 eV. Between monolayer error bars are approximately the same size as the symbols.
described by a single rate constant suggesting that the process is characterized by a single electron-transfer distance, reorganization energy, and microenvironment. Moreover, the observation of a single rate constant that is independent of the surface coverage for (0.3 ( 0.1) × 10-11 < Γ < (1.1 ( 0.1) × 10-11 mol cm-2 supports our assertion that multilayers are not formed in this system. Figure 5 illustrates Tafel plots of ln k vs η. The rapid nature of heterogeneous electron transfer for the inner redox couple limits the range of driving forces that can provide kinetically meaningful information to approximately |η| < 0.2 V. Over this modest range of overpotentials ln k increases linearly with increasing η, which is consistent with the predictions of the conventional Butler-Volmer formulation of electrode kinetics.38 The solid line illustrates the best fit equation to the ButlerVolmer relationship where k° and R are 1.3 × 106 s-1 and 0.44 ( 0.05, respectively. The similarity of the rate constants as measured using cyclic voltammetry and chronoamperometry indicates that the results are internally self-consistent.
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Figure 6. Semilog plots of current-time responses for a 50 µm radius platinum microelectrode modified with an [p0p Os(bpy)2 4-tet Os(bpy)2 Cl]3+ monolayer following potential steps in which the potential is stepped (top to bottom on right-hand side) from 0.800 to 0.218, 0.165, 0.120, and 0.065 V. The corresponding overpotentials for the reduction of the [4-tet Os(bpy)2 Cl]2+ center are -42, -95, -140, and -195 mV. The inset illustrates the corresponding semilog current-time plots. From top to bottom on the right-hand side the overpotentials are -42, -95, and -140 mV.
Dimeric monolayers of this kind represent an important opportunity to investigate how switching the oxidation state of a metal center affects electronic communication across the bridge. For example, when the potential is stepped to trigger the reduction of both Os3+ centers within an oxidized monolayer, then the outer center, [4-tet Os(bpy)2 Cl]2+, is reduced by electron transfer through a bridge containing an Os2+ center. Figure 6 illustrates current-time responses for a dimeric monolayer where the potential is stepped from 0.800 V to various potentials that are negative of the formal potential of the outer redox center. Only the current response at time scales longer than at least 5 RC are presented, i.e., double layer charging does not influence the data presented. At short times, the current is dominated by both double layer charging and the Faradaic current associated with reduction of the inner redox centers. The heterogeneous electron-transfer rate constant for the inner sites exceeds 107 s-1 since the electrochemical driving force is larger than 0.5 V. At longer times, the Faradaic current associated with reduction of the outer centers is observed. The inset of Figure 6 shows that semilog current-time plots are linear consistent with a first-order heterogeneous electrontransfer mechanism. Figure 5 illustrates Tafel plots for these data. The rate of electron transfer to the outer redox centers is relatively slower allowing the potential dependence to be probed over a much wider range of driving forces. Figure 5 shows that, consistent with the Marcus formulation of electrode kinetics, significant curvature is observed for η e 0.3 V. Fitting these data according to the nonadiabatic electron tunneling model provides best fit estimates of k° and λ of 1.1 × 104 s-1 and 0.6 eV, respectively. Importantly, the k° obtained agrees with that found by cyclic voltammetry to within experimental error. The potential independent limiting rate of heterogeneous electron transfer as predicted by the nonadiabatic tunneling model is approximately 2 × 107 s-1. While systems have been developed that exhibit very weak distance dependent photoinduced electron transfer,4 the difference in rate constants for the inner ((1.3 ( 0.2) × 106 s-1) vs the outer ((1.1 ( 0.1) × 104 s-1) redox centers is remarkably small given that the difference in electron transfer distance is approximately 14 Å. For example, taking a typical value of the
Forster and Keyes distance dependent tunneling factor, β, of 1.1 Å-1, one would expect the rate constant to be approximately 4 orders of magnitude smaller than what is experimentally observed. Similarly, if the electron-transfer mechanism switched from adiabatic for the inner couple to nonadiabatic for the outer, then a significantly larger difference in the two electron-transfer rate constants would be expected. Significantly, where electron transfer proceeds via a coherent superexchange mechanism,39 the rate depends algebraically on the difference between the energy levels of the bridge and donor/acceptor, ∆Ebridge-D/A. For the p0p-Os-4-tet bridge considered here, reducing the inner metal center creates a new bridge state centered at the E° for the Os2+/3+ couple (0.620 V). While the rapid nature of the electron transfer to the inner redox site prevents λ from being measured directly, the available evidence suggests that the reorganization energy is the same for both redox couples. Therefore, the larger k° observed for the inner redox couple arises because of a larger prefactor, i.e., coupling effects, in eq 3, suggesting that converting the metal center from the oxidized to reduced forms changes the strength of electronic coupling between the outer metal center and the electrode. This ability to electrochemically modulate electronic coupling across a bridge is highly desirable for developing molecular switches based on resonant tunneling approaches.17 Photoinduced Charge Transfer. Since the [p0p Os(bpy)2 4-tet]2+ moiety is luminescent, the dimers reported here offer an important opportunity to compare ground- and excited-state charge-transfer processes. For example, we recently,40 reported on superexchange across gold/[Os(bpy)2 4-tet Cl]+ monolayer interfaces. These investigations revealed that the system is charge localized and nonadiabatic with a preexponential factor of 1.2 × 105 s-1, i.e., approximately 5 orders of magnitude smaller than the prefactors associated with strongly coupled systems. By comparing the rates of photochemically and electrochemically triggered electron transfer across the tetrazine bridge, insight can be obtained into the effect of switching an electronically localized redox center for a delocalized metal electrode on the strength of electronic coupling. Photochemical measurements are possible for this dimer because the luminescent [p0p Os(bpy)2 4-tet]2+ center is bound to an [Os(bpy)2 Cl]+ moiety that may quench the excited state by either an energy- or electron-transfer mechanism. The chargetransfer rate constant, kCT, can be estimated using
1 1 kCT ) τ τ°
(8)
where τ and τ° are the luminescence lifetimes in the presence and absence of a linked quencher. Figure 7 illustrates emission transients for both the dimer and a structurally analogous monomeric complex, [Os(bpy)2 p0p 4-tet]2+, that does not include the quenching moiety. These data reveal that the excitedstate lifetime of the dimer is significantly shorter (42 ( 6 ns) than that of the monomer (142 ( 11 ns). This result indicates that the [Os(bpy)2 Cl]+ moiety quenches the excited [p0p Os(bpy)2 4-tet]2+* center. The rate constant for this chargetransfer process is (1.6 ( 0.1) × 107 s-1 at 298 K, suggesting that quenching takes place efficiently across the 14 Å tetrazine bridge. Temperature Dependence of Photoinduced Charge Transfer. In systems of this kind, charge transfer may involve energy or electron transfer between the two sites. Intramolecular energy transfer processes are typically temperature independent,41-43 because of the small reorganization energies involved. In contrast, electron transfer is a thermally activated process
Tetrazine Bridged Osmium Dimers
J. Phys. Chem. B, Vol. 105, No. 37, 2001 8835
Figure 7. Emission transients for [Os(bpy)2 p0p 4-tet]2+ (0) and [p0p Os(bpy)2 4-tet Os(bpy)2 Cl]3+ (solid line) dissolved in ethanol. The inset shows the corresponding semilog plots.
Figure 8. Temperature dependence of the photoinduced charge-transfer rate constant. The solvent is 50:50 ethanol:methanol.
involving a significant outer sphere reorganization energy, making it sensitive to the temperature of the surrounding medium. Therefore, temperature resolved measurements of kCT provide a convenient means of distinguishing between photoinduced energy and electron-transfer processes. Figure 8 illustrates a plot of ln kCT vs 1/T as the temperature of the ethanol: methanol solution is systematically varied from 298 to 118 K. This figure shows that the rate of photoinduced charge transfer decreases dramatically with decreasing temperature dropping from 1.6 × 107 to 3.7 × 103 s-1 on going from 298 to 118 K. This sensitivity to temperature indicates that photoinduced electron rather than energy transfer is the dominant quenching mechanism in this system. These measurements indicate that the free energy of activation, ∆Gq, is 12.3 ( 1.1 kJ mol-1. It is perhaps significant that if the entropy term is negligible, then this free energy of activation corresponds to a reorganization energy of 0.5 ( 0.05 eV, which is remarkably similar to that, 0.6 eV, found from analyzing the Tafel plot illustrated in Figure 5. Driving Force for PET. One of our objectives is to probe whether there are differences between the photo- and electrochemically induced electron-transfer processes. However, to quantify these effects, and indeed the driving force for the PET reaction, it is necessary to determine the excited-state redox potentials. The formal potential for oxidation, E°*ox., and reduction, E°*red., of the excited state can be calculated using eqs 9 and 10, respectively,
E°*ox. ) E°ox. + E°°
(9)
E°*red. ) E°red. - E°°
(10)
where E°ox. and E°red. are the formal potentials associated with
Figure 9. Emission spectra obtained at 298 (dashed line) and 77 K (solid line) for [p0p Os(bpy)2 4-tet Os(bpy)2 Cl]3+ dissolved in 50:50 ethanol:methanol.
the first oxidation (Os2+/3+) and reduction (bpy/bpy•-) of the luminescent moiety in the ground state and E°° is the energy difference between the lowest vibrational levels of the ground and excited states. Solution phase cyclic voltammetry indicates that E°ox. and E°red. are + 0.280 and -1.320 V, respectively. Figure 9 illustrates the emission spectrum for the dimer within an ethanol:methanol glass at 77 K. The value of E°°, as obtained from the wavelength of maximum emission at cryogenic temperatures, is 1.67 eV. Equations 9 and 10 yield values of -1.39 and +0.35 V for E°*ox. and E°*red., respectively. Quenching of the excited state by electron transfer may occur either by electron transfer from (reductive) or to (oxidative) the quenching moiety. One approach to elucidating whether one of these mechanisms predominates is to determine the driving force forces for two reactions. If the Coulombic stabilization energy of the products is negligible, then the free energies of activation can be determined using
∆G°ox. ) E°*ox. - E°red.
(11)
∆G°red. ) E°ox. - E°*red.
(12)
This analysis indicates that the driving forces for oxidative and reductive quenching are indistinguishable, with both driving forces being relatively modest at 70 mV. That ∆G°ox. ) ∆G°red. prevents us from determining whether quenching of the excited state occurs by an oxidative or reductive mechanism. Future investigations will use transient absorbance spectroscopy on the nanosecond time scale to probe the exact nature of the quenching mechanism. However, once ∆G° is known, the electrochemically and photochemically driven electron-transfer rate constants can be compared. The standard heterogeneous electron-transfer rate constant for a mononuclear [Os(bpy)2 4-tet Cl]2+ monolayer is (1.2 ( 0.1) × 104 s-1. At the driving force found for the photochemically triggered reaction, 70 mV, khetero is (4.0 ( 0.2) × 104 s-1, i.e., a factor of approximately 400 smaller than that found for the photoinduced reaction, (1.6 ( 0.1) × 107 s-1. It is possible that the [4-tet Os(bpy)2 Cl]+ moiety within the dimer creates a favorable vibronic deactivation pathway in the PET experiments causing the photoinduced electron-transfer rate constant to be overestimated. However, it is important to note that the proximity in energy of the donor/acceptor and those of the mediating bridge are distinctly different in the electrochemically and photochemically triggered processes. The voltammetric response illustrated in Figure 1 indicates that the tetrazine bridge is reduced only at potentials negative of -2.000 V. Therefore,
8836 J. Phys. Chem. B, Vol. 105, No. 37, 2001
Forster and Keyes
SCHEME 1: Schematic Diagram of Ground (Left-Hand Side) and Excited-State Energy Levels of Redox Centers and Bridge
as illustrated in Scheme 1, in the ground state the formal potentials of the metal based redox processes and the tetrazine bridge are separated by more than 2.3 V. In contrast, if quenching occurs by oxidizing the excited state, then the donor, acceptor, and tetrazine bridge are all within approximately 0.5 V of one another. We have previously demonstrated that achieving resonance of this kind can significantly enhance the rate of electron transfer by promoting enhanced electronic coupling across the bridge.17 Detailed kinetic analysis of the dynamics of electron transfer across gold/[Os(bpy)2 4-tet Cl]+ monolayer interfaces40 reveal that the system is charge localized and nonadiabatic with a preexponential factor of 1.2 × 105 s-1, i.e., approximately 5 orders of magnitude smaller than the prefactors associated with strongly coupled systems. Given the large differences found between khetero and kphoto, it is important to determine whether the D/A coupling across the 4-tet bridge is weaker for the delocalized metallic states of an electrode compared to the localized redox states of the dimer. As described by Marcus,27,28,30 in the case of the photoinduced electron-transfer reaction, the rate constant for electron crossexchange between the two osmium sites, kCE, depends on the difference in the redox potentials of the two reactants, ∆G°, and the reorganization energy, λ ()4∆Gq).
[
]
(λ + ∆G°)2 4λRT
kCE ) AET exp -
(13)
This analysis indicates that AET for the PET process is approximately, 1 × 1010 s-1. This value is dramatically larger (by almost 5 orders of magnitude) than that found for monolayers in which an [Os(bpy)2Cl]+ moiety is linked to a gold electrode surface using through the 3,6-bis(4-pyridyl)-1,2,4,5tetrazine bridge.40 This result suggests that there are significant differences in the electronic coupling of molecule-to-metal and molecule-to-molecule in systems of this kind. Conclusions Spontaneously adsorbed monolayers comprising dimeric metal complexes are attractive for investigating the mechanism of long-range electron transfer. For example, the effect of switching the oxidation state of the “inner” center on the dynamics of electron transfer to the “outer” center can be probed. The redox responses of the metal centers within the [p0p Os(bpy)2 4-tet Os(bpy)2 Cl]3+ monolayer described here are unusually ideal. Chronoamperometry conducted at nanosecond to microsecond time scales reveals that the redox state of the inner [p0p Os(bpy)2 4-tet]2+ and outer [4-tet Os(bpy)2 Cl]+
moieties are switched by heterogeneous electron transfer. In solution, these dimers undergo a photoinduced electron-transfer reaction, thus allowing the dynamics of photoinduced vs electrochemically driven reactions to be compared. The driving forces for oxidative and reductive electron-transfer quenching of the excited state as calculated using the Rehm-Weller equation are indistinguishable with a value of 70 mV. We find that the rate constant for the photoinduced electron-transfer process is (1.6 ( 0.1) × 107 s-1 compared to (4.0 ( 0.2) × 104 s-1 for the ground-state process with the same driving force. This result suggests that the proximity of donor/acceptor and bridge states, i.e., the achievement of resonance in the PET reaction, dramatically enhances the rate of electron transfer. These investigations into dimeric monolayers also reveal that the strength of electronic coupling across the tetrazine bridge is significantly higher when it links two metal complexes compared the situation where it acts as a bridge between a metal center and an electrode. This result suggests that the tunneling junction between the bulk metal and an adsorbed bridge can significantly influence heterogeneous electron-transfer rates. Acknowledgment. Financial support from Enterprise Ireland, the Irish Science and Technology Agency, under the Basic Research Program is gratefully acknowledged. The generous loan of potassium hexachloroosmate(IV) by Johnson Matthey under the loan scheme is deeply appreciated. We appreciate the valuable insight provided by the reviewers regarding the analysis of the electrochemical kinetics. References and Notes (1) Wasielewski, M. R. Chem. ReV. 1992, 92, 345. (2) Weaver, M. J. Chem. ReV. 1992, 92, 463. (3) Forster, R. J. J. Electrochem. Soc. 1997, 144, 1165. (4) Davis, W. B.; Svec, W. A.; Ratner, M. A.; Wasielewski, M. R. Nature 1998, 396, 60. (5) Finklea, H. O. Encyclopedia of Analytical Chemistry: Instrumentation and Applications; Wiley & Sons: New York, 2000. (6) Chidsey, C. E. D. Science 1991, 251, 919. (7) Keyes, T. E.; Forster, R. J.; Jayaweera, P. M.; Coates, C. G.; McGarvey, J. J.; Vos, J. G. Inorg. Chem. 1998, 22, 5925. (8) Williams, R. D.; Petrov, V. I.; Lu, H. P.; Hupp, J. T. J. Phys. Chem. A 1997, 101, 8070. (9) Collier, C. P.; Wong, E. W.; Belohradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391. (10) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (11) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (12) Sek, S.; Misicka, A.; Bilewicz, R. J. Phys. Chem. B 2000, 104, 5399. (13) Slowinski, K.; Slowinski, K. U.; Majda, M. J. Phys. Chem. B 1999, 103, 8544. (14) Forster, R. J.; Keyes, T. E.; Bond, A. M. J. Phys. Chem. B 2000, 104, 6389. (15) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5444. (16) Xu, C. Ph.D. Thesis, University of Illinois at Urbana-Champaign, 1992. (17) Forster, R. J. Inorg. Chem. 1996, 35, 3394. (18) . Laviron, E., J. Electroanal. Chem. 1974, 52, 395. (19) . Brown, A. P.; Anson, F. C. Anal. Chem. 1977, 49, 1589. (20) Forster, R. J.; Faulkner, L. R. Langmuir 1995, 11, 1014. (21) Trassati, S. J. Electroanal. Chem. 1974, 53, 335. (22) Laviron, E. J. Electroanal. Chem. 1982, 12, 53. (23) Campbell, J. L. E.; Anson, F. C. Langmuir 1996, 12, 4008. (24) Goodwin, H. A.; Kepert, D. L.; Patrick, J. M.; Skelton, B. W.; White, A. H. Aust. J. Chem. 1984, 37, 1817. (25) Ferguson, J. E.; Love, J. L.; Robinson, W. T. Inorg. Chem. 1972, 11, 1662. (26) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398. (27) Bagchi, G. Annu. ReV. Chem. 1989, 40, 115. (28) Sutin, N. Acc. Chem. Res. 1982, 15, 275. (29) Li, T. T.-T.; Guyer, K. L.; Barr, S. W.; Weaver, M. J. J. Electroanal. Chem. 1984, 164, 27.
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