10004
J. Phys. Chem. B 1998, 102, 10004-10012
Redox Properties of Ground and Electronically Excited States: [Ru(bpy)2Qbpy]2+ Monolayers Robert J. Forster* and Tia E. Keyes† School of Chemical Sciences, Inorganic Chemistry Research Centre, Dublin City UniVersity, Dublin 9, Ireland ReceiVed: July 7, 1998; In Final Form: September 30, 1998
Dense monolayers of [Ru(bpy)2Qbpy]2+, where bpy is 2,2′-bipyridyl and Qbpy is 2,2′:4,4′′:4′4′′-quarterpyridyl, have been formed by spontaneous adsorption onto clean platinum microelectrodes. Cyclic voltammetry of these monolayers is nearly ideal, and five redox states are accessible over the potential range from +1.3 to -2.0 V. Chronoamperometry conducted on a microsecond time scale has been used to measure the heterogeneous electron-transfer rate constant, k, for both metal- and ligand-based redox reactions. Heterogeneous electron transfer is characterized by a single unimolecular rate constant (k/s-1). Standard heterogeneous electron-transfer rate constants, k°, have been evaluated by extrapolating Tafel plots of ln k vs overpotential, η, to zero driving force to yield values of (5.1 ( 0.3) × 105 s-1, (3.0 ( 0.1) × 106 s-1, and (3.4 ( 0.2) × 106 s-1 for k°3+/2+, k°2+/1+, and k°1+/0, respectively. Temperature-resolved measurements of k reveal that the electrochemical activation enthalpy, ∆Hq, decreases from 12.1 ( 1.7 kJ mol-1 for the 3+/2+ reaction to 7.5 ( 0.8 kJ mol-1 for the 2+/1+ process. Probing the temperature dependence of the formal potential gives the reaction entropy, ∆Src°. Significantly, the free energy of activation is constant at 6.9 ( 0.6 kJ mol-1 for all three redox couples investigated. The electronic transmission coefficient, κel, describing the probability of electron transfer once the transition state has been reached, is considerably less than unity for all three redox processes. Following photoexitation using a laser pulse at 355 nm, emission is observed from the monolayers with an excited-state lifetime (6.2 µs) that exceeds that of the complex in solution (1.4 µs). It appears that weak electronic coupling between the adsorbates and the electrode means that the excited states are not completely deactivated by radiationless energy transfer to the metal. For the first time, we have used voltammetry conducted at megavolt per second scan rates to directly probe the redox potentials and electron-transfer characteristics of electronically excited species.
Introduction Electronically excited states play pivotal roles in areas as diverse as dye sensitization of semiconductors for solar energy conversion1 to photosynthesis.2,3 Photoexcited reactants have been used extensively to study electron transfer because excited states, created upon absorption of a photon, are simultaneously better electron donors and acceptors than their ground-state precursors.4 However, while our understanding of heterogeneous electron transfer in the ground state has evolved to a high degree on both theoretical and experimental fronts, there have been few reports on direct measurements of exited-state redox potentials.5 Moreover, the direct measurement of oxidation and reduction kinetics of electronically excited states remains completely unexplored. This situation has arisen largely because electronically excited states are transient species and typically have submicrosecond lifetimes. Conventional electrochemical methods cannot provide a meaningful insight into the redox properties of these fleeting species since traditional voltammetry is restricted to millisecond, or longer, time scales.6 However, with the advent of ultrafast electrochemical techniques and microelectrodes, it is now possible to directly probe the properties of species having submicrosecond lifetimes.7 While Fox and co-workers have demonstrated that it is possible to directly measure the excited-state redox potentials for solution-phase species with lifetimes on the order of hun† Present address: School of Chemistry, Dublin Institute of Technology, Kevin Street, Dublin 2, Ireland.
dreds of nanoseconds,5 the slow time scale of diffusion prevents the measurement of dynamic parameters. This diffusion limitation can be eliminated by forming a spontaneously adsorbed monolayer. Electrochemical measurements on excited states within monolayers are not often performed8,9 because, beyond the reductive or oxidative quenching of the excited state that we wish to investigate, other deactivation pathways exist. First, if the monolayers are formed on mirror smooth surfaces, then energy transfer from the excited state to the electrode is expected to be efficient if the electron transfer distance is sufficiently short to allow heterogeneous electron transfer to occur within the lifetime of the electronically excited state.10,11 These apparently contradictory demands, i.e., a short electron-transfer distance for fast heterogeneous electron transfer, vs a large separation of the excited state from the surface to prevent energy transfer, coupled with the extreme demands on experimental time scale, represent significant barriers to progress in this area. However, it is important to note that efficient interaction between the excited molecules and metallic states of the electrode is only expected for near perfect metallic mirrors. Indeed, as has been reported many times, depending on the length of the bridging ligand, and the quantum efficiency for fluorescence, excitation of the electronic plasma resonance within the metal can cause fluorescence enhancement at rough electrodes.12,13 Second, because the surface concentration of the adsorbates can be high, lateral energy or electron transfer may lead to quenching of the excited states. Therefore, the ability to control the surface coverage is important.
10.1021/jp9828890 CCC: $15.00 © 1998 American Chemical Society Published on Web 11/11/1998
[Ru(bpy)2Qbpy]2+ Monolayers CHART 1
In this contribution, we describe ultrafast electrochemical and transient photochemical measurements on [Ru(bpy)2Qbpy]2+ (Chart 1) monolayers; bpy is 2,2′-bipyridyl and Qbpy is 2,2′: 4,4′′:4′4′′-quarterpyridyl. We have used high-speed chronoamperometry to measure the rate of heterogeneous electron transfer across the metal/ monolayer interface in the ground state. Despite the rather short electron-transfer distance, there is weak electronic coupling between the metal center, or the bipyridyl ligands, and the metal surface leading to a nonadiabatic reaction in the ground state. Such weak coupling, together with nonmirror finish microelectrodes, appears to impede radiationless deactivation, making the excited state lifetime on the surface similar to that found in a frozen glass. We have used fast-scan voltammetry to allow us probe redox processes within the lifetime of the electronically excited state and report the first cyclic voltammograms of an excited-state species. Experimental Section cis-Ru(bpy)2Cl2 was prepared by standard synthetic methods.14 All chemicals were purchased from Aldrich and were used without further purification. 2,2′:4,4′′:4′4′′-Quaterpyridyl. This ligand was prepared using a modification to the method described by Morgan et al.15 4,4′-Bipyridyl (5 g, 0.02 M) was placed in a sealed Teflon bomb over dried Pd/C (1 g, 10% Pd). The bomb was heated in an oven at 200 °C for 1 week. The product was dissolved in chloroform, filtered to remove the catalyst, and dried. Then it was extracted and recrystallized from acetone. Yield, 1.2 g, 20%, m.p. 235 °C (literature value 235 °C)15 1H NMR data [(CD3)2SO]: H3, H3′′(d) 8.06, (d) (2H), H5, H6, H5′,H6′, 7.97 ppm (d) (4H), H2′′′, H3′′′, H2′′, H3′′, 8.81 ppm, (d) (4H)), H5′′′, H6′′′, H5′′, H6′′, 8.92 ppm, (d) (4H)). Anal. Calcd. for C20H16N4O: C, 73.3; H, 4.8; N, 17.07. Found: C, 74.2; H, 4.64; N, 17.18. [Ru(bpy)2(Qbpy)]2+. 2,2′:4,4′′:4′4′′-Quarterpyridyl (0.12 g, 0.38 mM) was dissolved in ethanol/water (20 cm3 6/4 v/v) and heated to reflux. cis-Ru(bpy)2Cl2 (0.276 g, 0.38 mM) dissolved in ethanol was added to the refluxing solution over 20 min. Then, the reaction mixture was left refluxing for a further 4 h. The solvent was removed to a volume of 10 cm3, and a solution of concentrated aqueous NH4PF6 was added. The resulting orange solid was collected by filtration and recrystallized from acetonitrile/water (1/1 v/v). Apparatus. Electrochemical cells were of conventional design and were thermostated within (0.2 °C using a Julabo F10-HC refrigerated circulating bath. All potentials are quoted with respect to a BAS Ag/AgCl gel-filled reference electrode. The potential of the ferrocene/ferricenium couple dissolved in acetonitrile containing 0.1 M TEAP as supporting electrolyte was +0.322 V. Cyclic voltammetry was performed using a CH Instruments Model 660 Electrochemical Workstation and a conventional three-electrode cell. All solutions were degassed
J. Phys. Chem. B, Vol. 102, No. 49, 1998 10005 using nitrogen, and a blanket of nitrogen was maintained over the solution during all experiments. In high-speed chronoamperometry,16,17 a custom-built function generator-potentiostat, with a rise time of less than 10 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 highfrequency path. In studies of the voltammetry of electronically excited species, the application of the voltage sweep was triggered by a custom-built silicon diode such that the potential sweep commenced simultaneously with the arrival of the laser pulse at the electrode surface. Alternatively, a Stanford Research Instruments Model DG 535 Four Channel Digital Pulse Generator/Delay Generator was used as controller to simultaneously trigger the laser pulse and the application of the potential sweep. In this case, the optical path (1.2 m) was sufficiently short so that it did not introduce any significant difference between the triggering and arrival of the laser pulse. Microelectrodes were fabricated from platinum microwires (Goodfellow Metals Ltd.) of radii between 1 and 25 µm by sealing them in soft glass using a procedure described previously.16 Microdisk electrodes were exposed by removing excess glass using 600 grit emery paper followed by successive polishing with 12.5, 5, 1, 0.3 and 0.05 µm alumina. The polishing material was removed between changes of particle size by sonicating the electrodes in deionized water for at least 5 min. The polished electrodes were electrochemically cleaned by cycling in 0.1 M HClO4 between potential limits chosen to first oxidize and then reduce the surface of the platinum electrode. Excessive cycling was avoided in order to minimize the extent of surface roughening. Finally, the electrode was cycled between -0.300 and 0.900 V in aqueous 0.1 M NaClO4 until hydrogen desorption was complete. Macroscopic platinum electrodes were polished in an identical manner. The real or microscopic surface area of the electrodes was found by calculating the charge under the oxide or hydrogen adsorption-desorption peaks.18 Typically, the surface roughness factor was between 1.3 and 1.6. Obtaining the real, as opposed to the projected or geometric, surface area of the electrodes is important if the area occupied per molecule is to be accurately measured. For electrochemical measurements in the ground state, spontaneously adsorbed monolayers were formed in situ using a 50 µM solution of the metal complex in the electrolyte solution. A low concentration of the surface active complex in solution improved the stability of the monolayers when exposed to organic solvents yet minimized the diffusional contribution to the overall current in chronoamperometry or cyclic voltammetry. In the case of excited-state measurements, monolayers were formed by placing clean electrodes in a degassed solution of 50:50 methanol:water containing 1 mM of the complex for 24 h. Subsequent measurements were performed in solutions that did not contain any of the dissolved complex. A nonisothermal cell, where the reference electrode was isolated from the main compartment by a salt bridge and held at room temperature, was used for the temperature-resolved experiments. The high electrolyte concentration and the design of the bridge minimize any systematic error in the reported temperature effects on E°′ due to changes in the liquid junction potential with temperature.19 Fluorescence. The surfaces of platinum macroelectrodes modified with spontaneously adsorbed monolayers of [Ru(bpy)2Qbpy]2+ were positioned against an Epiflan Neofluar
10006 J. Phys. Chem. B, Vol. 102, No. 49, 1998
Figure 1. Cyclic voltammogram of a spontaneously adsorbed [Ru(bpy)2Qbpy]2+ monolayer. The scan rate is 1 V/s, and the surface coverage is 1.04 × 10-10 mol cm-2. The supporting electrolyte is 0.1 M TBABF4 in acetonitrile. The radius of the platinum microelectrode is 25 µm. The cathodic currents are up, and the anodic currents are down. The complex is in the 2+ form between approximately (1.0 V.
water-immersion objective (40× magnification) and observed through a Zeiss Axioskop microscope. The fluorescence was monitored using a Peltier-cooled TH 7895B CCD that was operated at -25 °C through a Photometrics NU-200 controller interfaced to a Macintosh personal computer. The microscope was equipped with a 200 W mercury-arc lamp for illumination (epifluorescence configuration). The intensity of the incident light was controlled using neutral density filters between 50 and 1% transmittance. Exposure of the modified surface to the mercury-arc lamp was minimized to reduce the extent of photochemical decomposition of the monolayer. 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 2 ns. The monolayer emission intensity was weak, and transients were typically recorded using the average of 20 laser shots. The gate width, i.e., the exposure time of the CCD, was never more than 2% of the excited-state lifetime. The step size, i.e., the time between the acquisition of discrete spectra, was typically 5% of the excited-state half-life. Where necessary, monolayer emission spectra were smoothed using an eight-point Savitsky-Golay algorithm. Dilute solutions of complex in acetonitrile (RT) or EtOH/ MeOH 4:1 (77 K) (10-4-10-5 M) were deaerated for 20 min under nitrogen prior to use. Low-temperature emission lifetime studies were carried out using an Oxford Instruments gasexchange cryostat equipped with a Thor 3030 temperature controller. Standard iterative techniques were employed to determine the lifetimes of emission. Absolute error on emission lifetimes are (6%. Results and Discussion General Electrochemical Properties. Figure 1 shows a representative cyclic voltammogram for a [Ru(bpy)2Qbpy]2+ monolayer, where the supporting electrolyte is 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) in acetonitrile that contains 50 µM of the complex. The formal potentials, E°′, are 1.15, -1.09, -1.38, and -1.55 V. Strekas, Baker, and coworkers previously reported the solution-phase voltammetry of
Forster and Keyes this complex,20 and their redox potentials are in excellent agreement with those observed here. By comparison with the analogous [Ru(bpy)3]2+ complex, the peak observed at 1.15 V is attributed to the metal-based Ru2+/3+ redox reaction. The processes observed at negative potentials correspond to successive ligand-based reductions, respectively.21-23 In complexes of this type, if the Ru2+/1+ redox process occurs, then it does so only at potentials more negative than -2.3 V.24 Furthermore, reduction of the metal center would lead to an unstable d7 electronic configuration causing irreversible voltammetric responses to be observed. In heteroleptic systems of this kind, it is possible that the first ligand based reduction occurs either on the bipyridyl or on the quarterpyridyl ligand. The noncomplexed quarterpyridyl ligand15 has a less negative reduction potential than that of free 2,2′-bipyridyl. Therefore, the reduction occurring at -1.09 V in the monolayer is assigned to the quarterpyridyl. It is perhaps important to note that in solutionphase experiments of both the ruthenium complex and the free quarterpyridyl, we find that rapid adsorption occurs causing the first reduction peak to shift by approximately 250 mV in a positive potential direction. This sensitivity of the bridge redox potential to surface immobilization was not observed previously for complexes adsorbed through linear 4,4′-dipyridyl type linkers.25-28 This experimental voltammetric response observed for the monolayer has all of the characteristics of a redox system confined to an electrode surface.6 For example, the peak shapes are symmetrical and independent of scan rate, υ, at least over the range 1 to 50 V/s, and the peak height scales linearly with the scan rate unlike the υ1/2 dependence expected for a freely diffusing species. The difference between the anodic and cathodic peak potentials, ∆EP, was ca. 30 mV, and the full width at half-maximum was approximately 110 mV, i.e., about 20 mV larger that the ideal value.29,30 These observations suggest that the Qbpy complex adsorbs onto the surface of a platinum microelectrode to give an electroactive film. Experiments conducted using a complex, [Ru(bpy)3]2+, that did not contain any unbound nitrogens did not form stable spontaneously adsorbed monolayers. The peak height and area of the wave centered at 1.15 V do not change by more than 10% when cycled repeatedly over a 5 h period at temperatures up to 40 °C. This behavior indicates that this redox couple is electrochemically reversible and that the monolayers are stable for long periods even at relatively elevated temperatures. Reversible responses for the ligandbased reductions were obtained provided that the electrolytic solvent was distilled and dried. The Faradaic charge associated with converting the monolayer from the 2+ to the 3+ oxidation states has been estimated from the area under the wave centered at 1.15 V after correcting for double-layer charging. This charge, together with the real surface area of the electrode, has been used to calculate the surface coverage, or the number of moles of [Ru(bpy)2Qbpy]2+ per cm2. The limiting surface coverage, Γ, was (1.05 ( 0.1) × 10-10 mol cm-2, corresponding to an area occupied per molecule of 158 ( 17 Å2. Molecular modeling suggests that the radius of the complex as dictated by the outermost edge of the quarterpyridyl bridge is approximately 6.5 Å. This radius would lead to a theoretical area of occupation that is approximately 20% smaller than that observed experimentally. Allowing for charge-compensating counterions and a solvation sheath, the experimental voltammetric data confirm that closepacked monolayers are formed in which all of the adsorbates are electrochemically active.
[Ru(bpy)2Qbpy]2+ Monolayers
Figure 2. Room-temperature emission spectra for [Ru(bpy)2Qbpy]2+ dissolved in 4:1 ethanol:methanol (s) and as a spontaneously adsorbed monolayer (---). In the case of the monolayer, the contacting solvent is 4:1 ethanol:methanol. The y-axes are normalized relative to each peak’s maximum emission intensity.
Monolayer Fluorescence. Figure 2 illustrates fluorescence spectra obtained at 298 K for a platinum macroelectrode modified with a [Ru(bpy)2Qbpy]2+ monolayer where the solvent is blank degassed 4:1 v/v ethanol:methanol and for a solution of the same complex. In both cases the excitation wavelength was 355 nm. The emission signal observed for the monolayer is significantly lower in intensity than that observed for the solution-phase species. Therefore, to facilitate comparison of the spectral features of the solution phase vs bound species, the y-axis has been normalized relative to their respective peak intensities at λmax. The monolayer surface coverage, as measured by voltammetry, is 1.05 × 10-10 mol cm-2. It is important to confirm that the monolayer fluorescence arises from ruthenium centers that are intimately bound to the electrode surface. For example, luminescence from solution-phase, or physisorbed, species must be excluded. As discussed above, in the absence of dissolved complex, the half-life of the monolayer is approximately 12 h. However, even when electrochemical measurements indicate that the monolayer has fully desorbed, significant fluorescence cannot be detected from the solution. This result suggests that the signal shown in Figure 2 does not come from diffusive species. A second possibility is that the luminescence arises from physically adsorbed material. We find that if the electrode is not carefully rinsed with the electrochemical solvent after removal from the deposition solution then a different voltammetric response to that illustrated in Figure 1 is observed. Specifically, a diffusionlike tail appears, and the charge associated with the Ru2+/3+ redox reaction is between 5 and 10 times that expected for monolayer coverage. This behavior suggests that, beyond a chemisorbed monolayer, material remains physically adsorbed to the surface. That physisorbed material can be detected electrochemically makes it unlikely that the monolayers comprise an intimately bound electrochemically active, but efficiently quenched inner layer, and an electrochemically inactive, but fluorescent, outer layer. Although it is difficult to estimate the quantum yield for emission for the adsorbate, that luminescence is observed for the monolayers indicates that the excited states are not completely quenched by energy transfer to the electrode surface.31,32 The two spectra are qualitatively similar, although the emission spectrum for the monolayer shows a shoulder on the low-energy side, and the wavelength of maximum emission is shifted to
J. Phys. Chem. B, Vol. 102, No. 49, 1998 10007
Figure 3. Emission transients for an ethanolic solution of [Ru(bpy)2Qbpy]2+(fine upper line of transient) and for a spontaneously adsorbed monolayer (thick lower line of transient) in contact with 0.1 M LiClO4 dissolved in ethanol. The inset shows semilog plots of the responses.
lower energy by approximately 25 nm. The peak broadening observed suggests slow dipolar relaxation around the excited state. The shift of the emission to lower energy is somewhat unexpected as the lower dielectric constant and less efficient solvation of the polar excited state within the monolayer would be anticipated to shift the emission spectrum to higher energy. However, reaction entropies (vide infra) suggest that the more highly charged states are stabilized on the electrode surface. This shift is similar to that described previously by Bard and Zhang for Langmuir-Blodgett monolayers incorporating functionalized [Ru(bpy)3]2+ centers.33 These authors attributed the shift to interactions between the adsorbates and between the adsorbates and the electrode surface. Figure 3 shows the emission transients obtained for the complex dissolved in degassed ethanol and for an adsorbed monolayer. Repeated application of the laser pulse onto the modified surface causes the cyclic voltammetry response corresponding to the Ru2+/3+ redox reaction to change, causing the peak to become less well-defined and the peak current to decrease. Thus, while 5 laser pulses typically cause the peak current to decrease by less than 5% and the peak shape to remain essentially unchanged, 50 pulses increase the fwhm to 150 ( 20 mV and decrease the peak current by 30%. Both thermal desorption and photolysis of the adsorbed monolayer contribute to these changes. To minimize the effect of laser-induced changes on the measurements, data were collected only from films exposed to less than 20 pulses. That photochemical decomposition is observed for these monolayers suggests that rapid quenching by electron transfer to the metal surface does not occur efficiently in these systems. Where rapid charge injection does occur, e.g., in the Graetzel cell,34 compounds that are normally photosensitive become highly photostable upon immobilization because the 3MC state that leads to decomposition is not efficiently populated. Figure 3 shows that the complex exhibits a relatively longlived excited-state both in solution and as an adsorbed species. The inset of Figure 3 shows semilog intensity vs time plots. The emission observed for the solution-phase reactant exhibits the expected single-exponential decay behavior with a lifetime
10008 J. Phys. Chem. B, Vol. 102, No. 49, 1998
Forster and Keyes
of 1420 ( 60 ns. This excited-state lifetime is in excellent agreement with the value of 1418 ns reported by Strekas and Baker.20 In contrast, the response observed for the monolayer is more complex, and simple first-order kinetics are not followed. The lifetimes were fit to a biexponential decay eq 1, using standard nonlinear optimization procedures
I(t) ) Ae-k1t + Be-k2t
(1)
where A and B are preexponential factors and k1 and k2 are the rates of each first-order decay. Such nonidealities are not unusual for luminescent species immobilized within polymer films or sol-gels and may reflect different local microenvironments around discrete sites within the monolayer.35 Excitedstate lifetimes of approximately 1.2 (63%) and 6.2 µs (37%) are obtained at short and long times, respectively. Therefore, beyond the nonidealities observed for the monolayers, Figure 3 indicates that at least some of the immobilized complexes exhibit increased excited-state lifetimes compared to their lifetime in solution. It is likely that this behavior arises because of decreased freedom of rotation when the complex is adsorbed.36 This interpretation is supported by excited-state lifetimes measured for the complex in frozen ethanol:methanol glass at cryogenic temperatures where the lifetime is approximately 6 µs. Moreover, that the excited-state lifetimes of solution-phase and monolayer reactants are different discounts an emission mechanism in which the impinging laser pulse causes rapid thermal desorption of the adsorbates, which then undergo emission in solution before readsorbing onto the electrode surface. Heterogeneous Electron-Transfer Dynamics. As discussed in the Introduction, in trying to probe the redox properties of electronically excited states, weak electronic coupling between the adsorbate and the electrode is desirable in order to prevent quenching of the excited state by the metal surface. However, as revealed by the emission lifetimes, the redox properties of the excited states within the monolayer must be probed on a microsecond time scale. This short time scale demands the use of microelectrodes and ultrafast electrochemical techniques. An even more important consideration is whether the rate of heterogeneous electron transfer is sufficient to switch the oxidation state of the monolayer within a few microseconds. Therefore, we have probed the dynamics of heterogeneous electron transfer for the Ru2+/3+-and, more importantly, the ligand-based couples. 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 to26,28,37
iF(t) ) kQ exp(-kt)
(2)
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 chronoamperometric response observed for the Ru2+ - e- f Ru3+ redox reaction of an [Ru(bpy)2Qbpy]2+ monolayer formed on a 5 µm radius platinum microelectrode, where the electrolyte is 0.1 M TBABF4 in acetonitrile. In these experiments, the potential was stepped from 0.600 V to potentials, E, that were 50, 75, and 100 mV more positive than the formal potential E°′. These transients are structured, and the response observed for η ) 50 mV (η ≡ E - E°′) shows that on a low-microsecond time
Figure 4. Current response for the Ru2+/3+ redox reaction occurring within an [Ru(bpy)2Qbpy]2+ monolayer formed on a 5 µm platinum microelectrode. From top to bottom on the left-hand side, the overpotentials η are 100, 75, and 50 mV, respectively. The supporting electrolyte is 0.1 M TBABF4 in acetonitrile. The inset shows ln iF(t) vs t plots for the Faradaic reactions. From top to bottom on the right-hand side, the overpotentials η are 50, 75, and 100 mV, respectively.
domain two current decays can be separated. These responses are time-resolved owing to the shorter time constant of doublelayer charging compared to that of the Faradaic reaction. In these investigations, the electron-transfer rate constant has been determined only when the time constant of double-layer charging is at least five times shorter than the time constant of the Faradaic reaction. This condition has been satisfied by carefully selecting the radius of the microelectrode employed. Beyond the issue of fast double-layer charging, it is important to consider the effect of ohmic drop on the observed currenttime transients.6 When Faradaic and charging currents flow through a solution, they generate a potential that acts to weaken the applied potential by an amount iR, where i is the total current flow through the solution and R is the total cell resistance. This ohmic drop can lead to severe distortions of experimental responses resulting in inaccurate measurements of the heterogeneous electron-transfer rate. As illustrated in Figure 4, the Faradaic currents that flow in these high-speed chronoamperometric experiments are typically in the low microampere range even for 5 µm radius electrodes. Given that the cell resistance in this experiment is approximately 15 000 Ω, the average iR drop is less than 70 mV for this system. We have used three strategies to minimize the effects of uncompensated resistance in the large potential step experiments. First, we use a relatively high supporting electrolyte concentration (g0.1 M). Second, we use smaller electrodes to measure k at large overpotential. This approach is useful since the resistance increases with decreasing electrode radius, but the current decreases as the square of the radius leading to reduced ohmic effects for smaller electrodes, e.g., the maximum ohmic loss observed at the highest overpotential employed is reduced from approximately 3000 mV at a 12.5 µm microelectrode to 240 mV at a 1 µm microelectrode. Third, and perhaps most importantly, we extract rate constants only from data obtained relatively late in the lifetime of the current decay transient, i.e., when the anticipated iR drop is less than 15 mV. The linearity of the semilog plots shown in the inset of Figure 4 indicates that heterogeneous electron-transfer associated with oxidizing an [Ru(bpy)2Qbpy]2+ monolayer is a first-order process. Since uncompensated resistance causes the applied potential, and hence the apparent rate, to evolve with time, nonlinear responses would be expected if substantial ohmic drop
[Ru(bpy)2Qbpy]2+ Monolayers
Figure 5. Tafel plots where 9 and 2 denote experimental data for the 3+/2+ and 2+/1+ redox couples, respectively. The supporting electrolyte is 0.2 M TBABF4 in acetonitrile. The errors on the rate constants are approximately equal to the size of the symbols.
effects were present. Beyond the issue of iR drop, the linear responses illustrated in Figure 4 indicate that the local microenvironments and electron-transfer distances are similar for individual adsorbates within the monolayer. Given that the absolute value of the slope of the semilog plot represents the rate constant, it is apparent that heterogeneous electron transfer occurs very rapidly in this system. For example, even for a modest overpotential of 50 mV, the heterogeneous electrontransfer rate constant, k, for oxidation of the ruthenium centers is (1.3 ( 0.2) × 106 s-1. As given by eq 2, the intercept of these semilog plots equals ln(kQ). The total charge passed, as calculated using these intercepts, agrees with the slow scan voltammetric data to within 10% confirming that all of the adsorbates are electrochemically active on a microsecond time scale. Potential Dependence of k. One of the great advantages of using redox-active monolayers as model systems for understanding those factors that control electron-transfer rates is the ability to tune the driving force for the reaction.6 We have probed how k depends on driving force for both metal- and ligand-based redox reactions. Figure 5 illustrates Tafel plots of ln k vs overpotential, η, for the 3+/2+ and 2+/1+ redox reactions, where the supporting electrolyte is 0.2 M TBABF4 in acetonitrile. This figure shows that ln k depends approximately linearly on η for the range of overpotentials investigated. This behavior is consistent with the ButlerVolmer formulation6 of electrode kinetics with the slopes being equal to -RcnF/RT and (1 - Ra)nF/RT, for the reduction and oxidation processes, where Rc and Ra are the cathodic and anodic transfer coefficients, respectively. The Tafel slopes yield Rc and Ra values of 0.49 ( 0.05 and 0.51 ( 0.05, respectively. This behavior would not be expected if the determination of the heterogeneous electron-transfer rate constant were adversely affected by ohmic losses. That both transfer coefficients equal the value of 0.5 expected for a reversible reaction indicates that the energy barrier to electron transfer is highly symmetrical. The standard heterogeneous electron-transfer rate constant, k°, has been determined by linearly extrapolating ln k to zero overpotential. The standard heterogeneous rate constant depends on the redox process being probed, and while a k° of (5.1 ( 0.8) × 105 s-1 is observed for the metal-based 3+/2+ reaction, a significantly higher standard rate constant of (3.0 ( 0.5) × 106 s-1 is observed for the ligand-based reduction.
J. Phys. Chem. B, Vol. 102, No. 49, 1998 10009 Our interest in the dynamics of the oxidation of the reduced ligand arises because there is a considerable body of work available that indicates that the same orbital is populated by ligand-based reduction and photoexcitation.24,38,39 Therefore, the rate at which the reduced bpy ligand, bpy•-, can be reoxidized is pivotal to probing the dynamics of electronically excited states since photochemical excitation initially promotes an electron into the same singlet orbital. Thus, while recognizing that the electronic charge on the two reactants differ by a whole unit and that emission occurs from a triplet state, one might expect the oxidation kinetics for bpy•- and the electronically excited state to be similar. Strekas and Baker20 report excited-state lifetimes of 1418 and 63 ns for the [Ru(bpy)2Qbpy]2+ and [Ru(bpy)2Qbpyme2]4+ complexes, respectively, where Qbpyme2 is the methylated derivative of the quarterpyridyl ligand. Photoinduced electron transfer from the bipyridyl-based excited state to the viologen electron acceptor site is a likely mechanism for this significant reduction in excited-state lifetime. The rate of this excited-state electron transfer can be calculated according to eq 3
kET )
1 1 τ[Ru(bpy)2 (Qbpyme2)]4+ τ[Ru(bpy)2 (Qbpy)]2+
(3)
where τ[Ru(bpy)2(Qbpyme2)]4+ and τ[Ru(bpy)2Qbpy]2+ are the lifetimes for the quaternized and free pyridine complexes. This analysis yields an electron-transfer rate of 1.5 × 107 s-1 where the driving force is approximately 0.32 V. When the data in Figure 5 are extrapolated to this driving force, a rate constant of 6.6 × 108 s-1 is obtained for the ground-state oxidation of the bpy•- species. Given the limitations of these calculations and the complexity of the reaction in question, this analysis suggests that the oxidation of the ground-state bpy anion radical is a reasonable model for oxidative excited-state electrochemistry. However, a more reliable conclusion is that the rate of heterogeneous electron transfer from the excited-state to the electrode ought to be sufficiently fast to allow an excited state voltammetric response be observed; i.e., the time constant for electron transfer from the excited state is approximately 300 ns under conditions of zero driving force compared to an emission lifetime of approximately 6 µs. This monolayer is somewhat unusual in that the electronic coupling between the emitting state and the electrode surface is sufficiently weak so that complete quenching of the electronically excited states by energy transfer to the metal does not occur, yet the coupling is sufficiently strong to support heterogeneous electron transfer on a submicrosecond time scale. An insight into this balance can be achieved by determining the electronic transmission coefficient, κel, describing the probability of electron transfer from the transition state. One cannot assume that differences in the electronic transmission coefficients alone cause the observed differences in the standard rate constants since both preexponential and free energies of activation influence k°. As discussed previously by Chidsey,37 Creager,40 and Murray,41 the reorganization energy λ and hence κel can be determined by modeling Tafel plots. However, the upper limit on the heterogeneous electron-transfer rate that is measurable with our current system is of the order of 107 s-1. Therefore, as illustrated in Figure 5, it is possible to determine k only over a limited range of overpotentials. The accessible range of η is approximately (75 mV for the bpy-based reactions, which is insufficient for an accurate determination of λ. An alternate approach that we have employed previously26,28 is to explicitly measure the enthalpy and entropy of
10010 J. Phys. Chem. B, Vol. 102, No. 49, 1998
Forster and Keyes
TABLE 1: Standard Rate Constants, Activation Parameters, and Preexponential Factors for Metal- and Ligand-Based Electron-Transfer Reaction within [Ru(bpy)2Qbpy]n+ Monolayersa charge
10-5k°, s-1
∆Hq, kJ mol-1
∆Gcq,b kJ mol-1
10-6Aet, c -1 s
3+/2+ 2+/1+ 1+/0
5.1(0.3) 30.2(1.3) 34.2(2.2)
12.1(1.7) 7.5(0.8) 8.9(1.1)
6.8(0.4) 6.3(0.5) 7.6(0.9)
7.9(1.1) 38.2(1.9) 73.6(3.6)
a Numbers in parentheses represent the standard deviations for at least three individual monolayers. Supporting electrolyte is 0.2 M TBABF4 in acetonitrile. b Free energy of activation determined from the cathodic ideal electrochemical enthalpies as measured from the temperature dependence of the heterogeneous rate constant and the reaction entropy, ∆Src°. c Preexponential factor extracted from the standard heterogeneous electron-transfer rate constant using ∆Gcq.
activation by investigating the temperature dependence of k and E°′, respectively. Enthalpy and Entropy of Activation. The electrochemical enthalpy of activation has been determined from an Arrhenius plot of ln k vs T-1. The rate constant was measured using an overpotential of 50 mV as determined at 298 K. Also, the entropy of activation was measured from the temperature dependence of the formal potential. In both cases, the experiments and analysis was performed as described previously,25,26,28 and the results are presented in Table 1. This table clearly show that the enthalpic and entropic barriers to electron transfer depend on the identity of the redox couple. However, a significant observation is that there is mutual compensation between these two contributions such that experimentally indistinguishable free energies of activation are observed for the ruthenium- and ligand-based redox processes. Preexponential Factor. That indistinguishable values of ∆Gq are observed for both metal- and ligand-based processes is significant given the differences in their charge states. Table 1 contains values of Aet (≡κelυn) that have been determined using the experimental values of ∆Gq and k°. Previous studies on related systems26,28 indicate that the frequency factor for heterogeneous electron transfer is of the order of 1012 s-1. Therefore, for both metal- and ligand-based processes, κel is considerably less than unity (of the order of 10-5-10-6) indicating weak electronic coupling between the redox centers and the electrode. Under these circumstances, the electron transfer is considered to be nonadiabatic. This weak coupling is likely to be the reason energy transfer to the surface from the electronically excited state does not completely quench the monolayer emission. Excited-State Electrochemistry. Since the time constant for heterogeneous electron transfer from the reduced ligand is considerably shorter than the fluorescence lifetime, it ought to be possible to probe the dynamics of electron transfer to, and from, the excited-state species. However, having established the time scale demands, it is important to consider where on the potential axis a voltammetric response corresponding to the electronically excited state would occur. The most commonly used method to estimate the excited-state oxidation potential is the Rehm-Weller equation,42 which is given by eq 4 for oxidative quenching of an excited state.
E(A+/A*) ) E(A+/A) - E0-0
(4)
where E(A+/A-) is the potential for the first reduction and E0-0 is the energy difference between the lowest vibrational levels of the ground and excited states. E(A+/A-) can be obtained from ground-state solution-phase voltammetry and E0-0 from
Figure 6. Cyclic voltammetry of a 5 µm radius platinum microelectrode modified with an [Ru(bpy)2Qbpy]2+ monolayer following laser excitation at 355 nm. The scan rate is 4300 V/s, the surface coverage is 1.1 × 10-10 mol cm-2, and the supporting electrolyte is 0.1 M TBABF4 in acetonitrile. The initial potential is -1.2 V.
emission spectra obtained at cryogenic temperatures. The excited-state oxidation potential obtained in this way using the data presented in Figures 1 and 2 is approximately -0.67 V. However, this value is only an estimate of the true excitedstate redox potential and typically contains uncertainties of 100 mV or more. For example, in the case of ruthenium and osmium polypyridyl complexes, the vibrational fine structure is never well-resolved, making it difficult to accurately determine E0-0. The inherent inaccuracies in using ground-state parameters to approximate excited-state characteristics makes a direct measurement of the excited-state redox properties invaluable. Moreover, direct measurement of E* offers a means of unequivocally confirming that the redox and spectroscopic orbitals are the same, thereby confirming the validity of the Rehm-Weller equation for determining excited-state potentials. Measurements of this type were first reported by Bock et al.,43 where the excited-state oxidation potential of [Ru(bpy)3]2+ was indirectly estimated from kinetic studies of quenching by a series of nitrobenzene complexes. However, Jones and Fox5 have described how phase-modulated voltammetry can be used to directly measure the excited-state redox potential. This technique is essentially a steady-state method and is applicable only when the excited-state lifetime exceeds 500 ns. Figure 6 shows the 4300 V/s voltammetric response of an [Ru(bpy)2Qbpy]2+ monolayer formed on a 5 µm radius platinum microelectrode. The voltammetric scan was optically triggered so that it commenced simultaneously with the arrival of the incident laser pulse on the electrode surface. The potential limits are chosen so that the monolayer exists in the Ru2+ state prior to photoexcitation and allows the excited-state response expected at approximately -0.67 V to be captured. Given that this initial potential is negative of the excited-state oxidation potential, quenching of the excited state by electron transfer to the electrode surface is not expected. Beyond a small current oscillation at approximately -0.62 V, the voltammetry is entirely consistent with that observed in the absence of laser excitation indicating that pulsing the laser does not cause artifacts in the cyclic voltammetry, at least not at this scan rate. This behavior is entirely expected since the voltammetric scan rate is too slow to capture the excited state response prior to its return to the ground state by emission or quenching. However,
[Ru(bpy)2Qbpy]2+ Monolayers
Figure 7. Conditions as in Figure 6 except that the scan rate is 3 × 105 V/s. The dashed line illustrates the voltammetric response obtained for a bare electrode under identical conditions.
this response confirms that impinging a laser pulse onto the monolayer surface does not significantly alter the ideality of the voltammetry. Figure 7 shows the response observed at a scan rate of 3 × 105 V/s immediately after photoexcitation. Now the voltammetric time scale is shorter than the excited-state lifetime, and voltammetry characteristics of the electronically excited state can be obtained. This figure shows that on the first scan an oxidative current response is observed at approximately -0.42 V. The peak potential of this process is in approximate agreement with the calculated excited-state redox potential of -0.67 V. That this current response is not observed for the second or subsequent scans confirms the transient nature of the phenomenon. This figure also illustrates the voltammetric response obtained for a bare microelectrode. That a flat featureless voltammogram is obtained confirms that currents induced by thermal, photoelectric, or photoacoustic effects, e.g., through double-layer restructuring, are absent at this time scale. However, chronoamperometry experiments in which the current is measured on a submicrosecond time scale indicate that for both bare and monolayer-modified microelectrodes, a significant photoinduced current is observed. This current decays approximately exponentially over time but with a significant damped oscillation superimposed on the decay. The time scale of this decay depends on the applied potential and is more rapid for the monolayer-coated electrodes; e.g., at a potential of -1.0 V, the decay is complete within 100 ns for the monolayer-coated electrode but takes 300 ns for the unmodified surface. An important test of internal self-consistency is to correlate the amount of excited-state reactant with the value obtained from ground-state cyclic voltammetry such as that illustrated in Figure 1. The oxidative charge under the peak centered at -0.42 V in Figure 7 corresponds to approximately 5.8 × 10-17 mol of reactant on the electrode surface. Voltammetric measurements conducted both before and after the excited-state voltammetry indicate that there are 8.7 × 10-17mol of reactant on the surface. This value is consistent with that expected for a 5 µm radius microelectrode after taking a surface roughness of 1.5 and a surface coverage of 1.1 × 10-10 mol cm-2 into account. While recognizing the difficulties caused by other deactivation pathways and the influence of heterogeneous kinetics, that approximately 66% of the total surface coverage appears to undergo an excited-state redox reaction is an important observa-
J. Phys. Chem. B, Vol. 102, No. 49, 1998 10011
Figure 8. Conditions as in Figure 6 except that the scan rate is 22 000 V/s.
tion. Taking into account the time taken to scan the potential from its initial value to the excited-state peak potential, this result suggests that close to 100% of the adsorbed complexes were excited during the laser pulse. It is significant that the area under the anodic peak at -0.42 V is larger than the cathodic peak observed at approximately 0.7 V. This observation is consistent with the heterogeneous electron-transfer rate constants discussed previously. These data indicate that k° for the bpy•- oxidation was approximately a factor of 6 larger than that found for the Ru2+/3+ reaction. Therefore, we anticipate more facile kinetics for the oxidation of the excited state and a consequently larger charge than for the Ru2+/3+ reaction. Indeed, the large peak-to-peak separation observed for the Ru2+/3+ redox reaction confirms that the voltammetric response is significantly influenced by the dynamics of heterogeneous electron transfer. The area under the cathodic peak located at approximately 0.7 V increases by less than 10% between the first and successive scans indicating that the number of Ru3+ centers on the electrode surface does not change significantly between scans. A complication in the voltammetry shown in Figure 7 is that the product obtained by oxidizing the electronically excited state is the ground-state oxidized product [Ru(bpy)2Qbpy]3+. The complication arises because this ruthenium 3+ species is created at a potential that is approximately 1.5 V negatiVe of the groundstate formal potential. Under these circumstances, one would expect that the ground-state product would be rapidly reduced. Thus, as shown in the following scheme, the overall reaction would consist of removal of an electron from the electronically excited species followed by electron injection into the groundstate oxidized product, i.e., two opposing electron transfers.
[Ru(bpy)2Qbpy]2+* - e- f [Ru(bpy)2Qbpy]3+ (irreversible excited-state oxidation Epeak ≈ -0.42 V) [Ru(bpy)2Qbpy]3+ + e- f [Ru(bpy)2Qbpy]2+ (irreversible reduction of ground-state oxidized product) An insight into the reason this mechanism is not followed at megavolt per second scan rates can be obtained by systematically varying the scan rate. Figure 8 shows the voltammetric response observed at the slower scan rate of 22 000 V/s. In this figure, a current oscillation of the type expected for opposing oxidation-reduction reactions that are proceeding on compa-
10012 J. Phys. Chem. B, Vol. 102, No. 49, 1998 rable time scales is observed. Although the exact rate constants have not yet been determined, this response is consistent with the rate constant for oxidation of the electronically excited state being only about an order of magnitude smaller than reduction of the ground-state product. This result is not expected on the basis of the kinetic analysis presented earlier. The standard heterogeneous rate constant for the Ru2+/3+ reaction is 5.1 × 105 s-1. Although too fast for us to determine experimentally, given that the reorganization energy is approximately 0.3 eV (Table 1, λ ) 4∆Gcq), one would expect a limiting rate constant of approximately 108 s-1 to be observed for very large overpotentials. Given this large rate constant, we do not understand why a significant Ru3+ population remains on the surface to generate the unusual current oscillation observed at approximately -0.5 V. Conclusions Ruthenium polypyridyl complexes that include a quarterpyridyl ligand within their coordination shell are surface active and yield highly stable monolayers. Five distinct oxidation states are electrochemically accessible, and the electrochemical responses are nearly ideal as the potential, temperature, and experimental time scale are varied over a wide range. Chronoamperometry has been used to probe the rate of heterogeneous electron transfer across the monolayer/microelectrode interface. This process can be characterized by a single rate constant at high electrolyte concentrations suggesting that heterogeneous electron transfer across these metal/monolayer interfaces is mechanistically uncomplicated. Significantly, upon laser excitation at 355 nm the monolayers fluoresce; i.e., despite having the excited state located close to a metal surface, energy transfer does not completely quench the excited state. Moreover, the excited-state lifetime of the adsorbate is approximately three times longer than that found for the complex in solution and is indistinguishable from that observed for the complex in an ethanol:methanol glass at cryogenic temperatures. In the first report of its kind, we have attempted to probe the energetics and dynamics of excited-state oxidation using voltammetry conducted at scan rates on the megavolts per second range. The voltammetric data reported are qualitatively consistent with the response anticipated for an electronically excited state in terms of the behavior of bare electrodes, as well as the peak potential and voltammetric time scale over which the transient response is observed for monolayer-modified electrodes. Specifically, although influenced by the dynamics of heterogeneous electron transfer, the measured excited-state redox potential agrees with that predicted by the Rehm-Weller equation to within 180 mV. Fast-scan voltammetry reveals that the heterogeneous electron-transfer rate constant is larger for the excited-state than for the ground-state bpy anion radical, bpy•-, suggesting that the redox and spectroscopic orbitals may not be identical. We expect that further studies including electrochemical Stern-Volmer plots and the effects of separating the excited states using diluents will provide further insight into this challenging and exciting area. Beyond the useful insight into the energetics and dynamics of excited-state quenching, we are exploring the use of luminescent monolayers as fast-responding active displays and in trace analysis. Acknowledgment. R.J.F. gratefully acknowledges Professor Larry R. Faulkner of the University of Texas at Austin for the generous loan of the high-speed potentiostat that made part of this work possible. Financial support from Enterprise Ireland,
Forster and Keyes the Irish Science and Technology Agency, under Basic Research Grant SC/96/424, and the European Union Joule Program is gratefully acknowledged. The generous loan of ruthenium trichloride and platinum microwires by Johnson Matthey under the loan scheme is deeply appreciated. We appreciate the many insightful comments provided by the referee. This paper is dedicated to Professor Allen J. Bard whose outstanding experimental and theoretical contributions underpin this work. References and Notes (1) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (2) Knox, R. S. Photosynth. Res. 1996, 48, 35. (3) Ward, M. D. Chem. Soc. ReV. 1997, 26, 365. (4) Juris, A.; Balzani, V.; Barigelletti, Campagna, S.; Belser, P.; Von Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85. (5) Jones, W. E.; Foxe, M. A. J. Phys. Chem. 1994, 98, 5095. (6) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 1980. (7) Forster, R. J. Ultrafast Electrochemical Techniques; Encyclopedia of Analytical Chemistry; Wiley and Sons: New York, 1998. (8) Pflug, J. S.; Faulkner, L. R. J. Am. Chem. Soc. 1980, 102, 6144. (9) Pflug, J. S.; Faulkner, L. R.; Seitz, W. R. J. Am. Chem. Soc. 1983, 105, 4890. (10) Kuhn, H. J. Chem. Phys. 1970, 53, 101. (11) Chance, R. R.; Prock, A. Sibley, R. AdV. Chem. Phys. 1978, 37, 1. (12) Weitz, D. A.; Garoff, S.; Gersten, J. I.; Nitzan, A. J. Chem. Phys. 1983, 78, 5324. (13) Wokuan, A.; Lutz, H.-P.; King, A. P.; Wild, U. P.; Ernst, R. R. J. Chem. Phys. 1983, 79, 509. (14) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem., 1978, 17, 3334. (15) Morgan R. J.; Baker, A. D. J. Org. Chem, 1990, 55, 1986. (16) Xu, C. Ph.D. Thesis, University of Illinois at Urbana-Champaign, 1992. (17) Faulkner, L. R.; Walsh, M. R.; Xu, C. Contemporary Electroanalytical Chemistry; Plenum Press: New York, 1990. (18) Trasatti, S.; Petrii, O. A. J. Electroanal. Chem. 1992, 327, 354. (19) Yee, E. L.; Cave, R. J.; Guyer, K. L.; Tyma, P. D.; Weaver, M. J. J. Am. Chem. Soc. 1979, 101, 1131. (20) Bierig, K.; Morgan, R. J.; Tysoe, S.; Gafney, H. D.; Strekas, T. C.; Baker, A. D. Inorg. Chem. 1991, 30, 4898. (21) Griffith, W. P. In ComprehensiVe Coordination Chemistry; Wilkinson, G., Ed.; Pergamon: Oxford, England, 1987; Vol. 4, Chapter 46. (22) Sullivan, B. P.; Conrad, D.; Meyer, T. J. Inorg. Chem. 1985, 24, 3640. (23) Tokel-Takvoryan, N. E.; Hemmingway, R. W.; Bard, A. J. J. Am. Chem. Soc. 1973, 95, 6582. (24) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser P.; Von Zelewsky, A., Coord. Chem. ReV., 1988, 84, 85. (25) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5444, (26) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5453. (27) Forster, R. J.; Faulkner, L. R. Langmuir 1995, 11, 1014. (28) Forster, R. J.; O′Kelly, J. P. J. Phys. Chem. 1996, 100, 3695. (29) Laviron, E. J. Electroanal. Chem. 1974, 52, 395. (30) Brown, A. P.; Anson, F. C. Anal. Chem. 1977, 49, 1589. (31) Kalyanasundaram, K. Coord. Chem. ReV. 1982, 46, 218. (32) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 5007. (33) Zhang, X, Bard, A. J. J. Phys. Chem. 1988, 92, 5566. (34) O’Regan, B.; Graetzel, M. Nature 1991, 353, 737. (35) Kalyanasundaram, K. In Photochemistry in Organized and Constrained Media; Ramamurthy, V., Ed.; VCH Publishers: New York, 1991. (36) Reisfeld, R. J. Non-Cryst. Solids 1990, 121, 254. (37) Chidsey, C. E. D. Science 1991, 251, 919. (38) Dodsworth, E. S.; Lever, A. B. P. Chem. Phys. Lett. 1986, 124, 152. (39) Vlcek, A. A.; Dodsworth, E. S.; Pietro, W. J.; Lever, A. B. P. Inorg. Chem. 1995, 34, 1906. (40) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307. (41) Tender, L.; Carter, M. T.; Murray, R. W. Anal. Chem. 1994, 66, 3173. (42) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259. (43) Bock, C. R.; Meyer, T. J.; Whitten, D. J. J. Am. Chem. Soc., 1975, 97, 2909.
