4556
J. Phys. Chem. 1996, 100, 4556-4563
Exchange Dynamics of Redox-Active Self-Assembling Mixed Monolayers Jorge D. Tirado and He´ ctor D. Abrun˜ a* Department of Chemistry, Baker Laboratory, Cornell UniVersity, Ithaca, New York, 14853-1301 ReceiVed: August 30, 1995; In Final Form: December 14, 1995X
The exchange dynamics as well as the desorption and displacement reactions of redox-active self-assembling monolayers based on transition-metal complexes of osmium and ruthenium have been studied. On the basis of results from the above-mentioned processes, it appears that the exchange dynamics are controlled by the rate of desorption via a dissociative mechanism. We have also made use of wave-shape analysis in terms of shifts in the formal potentials and widths of the voltammetric waves with changes in the surface coverage and monolayer composition. The presence of mixed monolayers (Os/Ru) results in an increased broadening of the voltammetric wave associated with the ruthenium complex relative to the case of a monolayer of the ruthenium complex alone which we ascribe to an enhanced degree of electrostatic repulsion. Also based on an analysis of the voltammetric response, we believe that the mixed monolayers are randomly distributed rather than being composed of discrete and separated phases.
Introduction Self-assembled monolayers are currently the object of intense study because their well-defined structural character provides molecular surfaces of great potential for scientific studies and technological exploration. Mixed monolayers of self-assembling molecules have been the subject of several recent publications.1-14 Most of the work of self-assembling monolayers has involved alkane thiols on gold. Some of these systems involve molecules with long and short alkane chains which give rise to singlephase or multiphase monolayers, depending on the conditions under which the adsorption is carried out.1 Other systems combine alkane thiols with different end-group functionalities. For example, electroactive groups (such as ferrocene) mixed with alkane thiols have been studied.2 In addition, the effects of different organic functionalities on the properties of mixed monolayers have been the subject of various studies.3,4 Mixed self-assembling monolayers have been characterized by a several techniques including X-ray photoelectron spectroscopy (XPS),5 contact angle,6 ellipsometry,7 and, more recently, time-of-flight secondary ion mass spectrometry8 and scanning tunneling microscopy (STM),9 among others.10 As applications of the self-assembly method in electrochemistry are explored further, it will become increasingly desirable to prepare mixed layers for which the relative quantities of two or more different adsorbates on a surface can be systematically varied and controlled. This will enable complex molecular superstructures to be prepared with the ultimate goal of accomplishing specific functions such as chemical sensing or various molecular electronic functions.11 In this paper we discuss the exchange dynamics within a mixed-monolayer formed by two electroactive self-assembling molecules using cyclic voltammetry as the electroanalytical tool. Chidsey and co-workers2 previously studied coadsorption of ferrocene-terminated and unsubstituted alkanethiols, and similar experiments, without electroactive groups, have been described by Allara et al.12 and Whitesides and co-workers.13 Faulkner and Foster recently demonstrated the use of the difference in the kinetics of electron transfer of two species with identical formal potentials in a mixed-monolayer to determine their surface coverages along with their bulk concentrations.14 X
Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4556$12.00/0
Figure 1. General structure of the complexes under study and a model depicting a molecule adsorbed on a platinum electrode surface.
The molecules employed in this work have been previously characterized in terms of their thermodynamic parameters,15 dynamics of adsorption,16 and electron-transfer rates17 and consist of transition-metal complexes of the type [M(bpy)2LCl]+ where M is either Os or Ru, bpy ) 2,2′-bipyridine, and L ) 1,3-bis(4-pyridyl)propane which is referred to as dipy. The osmium and ruthenium complexes are referred to as Osdipy and Rudipy, respectively. The dipy ligand has a pendant pyridine group which serves as the anchor through which the complex binds to the surface of a platinum electrode. Figure 1 shows the structure of the molecule and a model of the adsorbed complex on a platinum surface. Because these two complexes are virtually identical in size whereas their redox responses are well separated (by about 400 mV), they provide an ideal system with which to probe the exchange dynamics of redox-active © 1996 American Chemical Society
Redox-Active Self-Assembling Mixed Monolayers self-assembling monolayers which constitutes the objective of the present work. In addition, we present data on the kinetics of desorption and displacement and employ data from all these experiments to propose a mechanism for the above-mentioned processes. Moreover, we show how the electrochemical response can be used to derive (albeit indirectly) information on the microenvironment within the mixed-monolayer system. Experimental Section Reagents. Water was purified with a Hydro purification system connected in series to a Millipore Milli-Q system. KClO4 (GFS Chemicals) was recrystallized twice from water and dried under vacuum at room temperature for 72 h. H2SO4 (Ultrex, J. T. Baker) and 1,3-bis(4-pyridyl)propane; dipy (Aldrich) and all other reagents were of at least reagent-grade quality and were used as received unless otherwise specified. The metal complexes (adsorbates) were synthesized following procedures described below. The synthesis of Osdipy was carried out by a procedure previously described17 and which follows procedures previously reported by Meyer et al.18a,b and Dwyer et al.18c About 200 mg of [Os(bpy)2Cl2] (0.35 mmol) was heated at reflux under a nitrogen atmosphere in deaerated ethylene glycol (ca. 25 mL) for 3 h with 2 equiv of the dipy ligand. After cooling, an equivalent volume of water was added, and the solution was subsequently filtered. The complex was precipitated with a saturated aqueous solution of NH4PF6. The product was collected, washed with water, and dried with ether. Purification was effected by chromatography on alumina with 15% methanol in toluene as eluent. The desired complex eluted as a brownish band. The synthesis and purification of Rudipy was similar to that described above with the following changes. About 200 mg of [Ru(bpy)2Cl2‚2H2O] (0.38 mmol) were refluxed under a nitrogen atmosphere in a deaerated 1:1 ethanol-water mixture (ca. 25 mL) for 3 h with 2 equiv of the ligand, until a brownish solution was obtained. After reaching room temperature, an equal volume of water was added and the solution was filtered. The product was precipitated with a saturated aqueous solution of NH4PF6. After filtering and washing with ether, a brownish precipitate was collected. Purification was by column chromatography on neutral alumina and 15% methanol/toluene as eluent. The Rudipy complex eluted as the first brown band. Instrumentation. Cyclic voltammograms were carried out with an EI-400 bipotentiostat from Ensman Instrumentation, and data were collected using a Gateway 2000 PC (486DX) system equipped with an interface card from National Instruments Model AT-MIO-16F-5 programmed with LabView. Potentials were measured against a sodium chloride saturated silver/silver chloride (Ag/AgCl) reference electrode without regard for the liquid junction potential. A 15 mL glass vial was used as the electrochemical cell, with a coiled platinum wire as the counter electrode. Procedures. The platinum disk working electrode was made by sealing, in soft glass, a 3-4 cm piece of annealed platinum wire (75 µm diameter, Engelhard Ind.). The electrode was sanded with 400 and 600 grit sand paper (Buehler) and polished with 1 µm diamond paste (Buehler). Electrical connection to the electrode was made with a drop of mercury and a piece of copper wire inserted in the glass tubing. Prior to use the electrode was polished with 1 µm diamond paste (Buehler) and washed with water and acetone. The electrode was pretreated by continuous cycling between the oxide formation and hydrogen adsorption potentials (about +1.40 and -0.20 V, respectively) in 1 M H2SO4 until the voltammetry of a clean
J. Phys. Chem., Vol. 100, No. 11, 1996 4557 polycrystalline platinum electrode was obtained. The electrochemically active area of the electrode was obtained by integration of the hydrogen adsorption waves using a conversion factor of 210 µC/cm2.19 Roughness factors, defined as the ratio of electrochemically determined area to geometric area were typically about 1.2-1.3. The electrode was rinsed with water and subsequently immersed in an aqueous 0.1 M solution of KClO4 and cycled between 0.0 and +1.0 V until a steady voltammogram was obtained. All the cyclic voltammograms shown in this paper were obtained at 100 V/s. This relatively fast scan rate was chosen so as to kinetically hinder the oxidation process of the platinum surface that can take place to a significant extent at potentials close to +1.0 V and slow scan rates (e.g. 100 mV/s). It was observed that at a scan rate of 100 V/s the contribution to the background from oxide formation was small. The complexes were injected as an acetone solution, typically 0.5 mM, and the solution was subsequently purged with prepurified nitrogen in order to homogenize the solution prior to immersing the electrode. The volume injected varied according to the desired final concentration and concentration ratios of the complexes but was typically in the microliter range. The potential of the electrode was held constant during deposition at 0.0 V. Cyclic voltammetry was used to determine the coverage of the complexes on the surface of the electrode by integration of the wave corresponding to the metal-based oxidation from M(II) to M(III) (where M ) Os, Ru) with formal potentials at about +0.35 V for Osdipy and +0.75 V for Rudipy, respectively. Since the solution concentration of the complexes was typically in the micromolar regime, their contribution to the measured current (charge) was negligible. Thus, surfacecoverage measurements could be carried out in the deposition solution with minimal error from the complex in solution. The coverage was monitored as a function of time for both complexes at different total concentrations and/or concentration ratios. Mixed monolayers of Osdipy and Rudipy on platinum electrodes were prepared by initially coating the surface with one complex and injecting into the solution the desired volume of an acetone solution of the other. Results and Discussion The adsorption process which governs the formation of selfassembling monolayers can be viewed as the competition of ions, solvent, and adsorbate molecules in solution for binding sites on the substrate surface. The following equilibrium expression can be used to describe such a situation:
Msol + nSads h Mads + nSsol
(1)
where Msol represents adsorbate molecules in solution, Sads is the solvent and/or ions adsorbed on the substrate, n of which are displaced as Ssol into solution by Msol to form Mads, the adsorbate bound to the surface. The amount of material adsorbed on the substrate will depend on the concentration of the adsorbate in solution. At low solution concentrations a fraction of a monolayer will be formed. As the solution concentration of the adsorbate increases so does the coverage, until a concentration value is reached at which the substrate’s surface is saturated. At concentrations above this value, a saturation coverage is always obtained. We previously reported15 that for the systems presented in this paper the minimum concentration to achieve a saturation coverage (which is about 1.0 × 10-10 mol/cm2) was about 0.5 µM. To maintain a full monolayer on the surface of the electrode at all times, a total concentration of 2 µM was used in the experiments presented in this paper.
4558 J. Phys. Chem., Vol. 100, No. 11, 1996
Tirado and Abrun˜a
Figure 2. Cyclic voltammograms at 100 V/s in aqueous 0.1 M KClO4 of a monolayer of (A) Osdipy (about 8.3 × 10-11 mol/cm2) and (B) Rudipy (about 8.1 × 10-11 mol/cm2) on a platinum electrode (area ) 1.0 × 10-4 cm2).
Let us assume that a monolayer of adsorbate M is formed on a substrate, as described by eq 1. If this substrate modified with M is exposed to a solution of a different adsorbate (N), an additional equilibrium is established which involves the displacement of adsorbate M by N. This equilibrium can be expressed as:
Mads + Nsol h Msol + Nads
(2)
On the other hand, if two different adsorbates (M and N) are initially present in solution, a competitive equilibrium is established between these two species for binding sites on the substrate. This equilibrium can be expressed as
Msol + Nsol + nSads h Mads + Nads + nSsol
Figure 3. Cyclic voltammograms at different times for the displacement experiments. (A) An initial monolayer of Rudipy displaced by Osdipy from a 2 µM solution. Times: (a) 0; (b) 2; (c) 6; (d) 10; (e) 57 min. (B) An initial monolayer of Osdipy displaced by Rudipy from a 2 µM solution. Times: (a) 0; (b) 1; (c) 3; (d) 6; (e) 110 min.
TABLE 1: Solution Concentration and Coverage Ratios for Osdipy and Rudipy Mixed Monolayers solution concn ratio [Ru]/[Os]
coverage ratio ΓRu/ΓOs
0.33 0.53 1.03 2.09 3.00
0.27 0.41 0.80 1.61 2.26
(3)
where M and N are different adsorbates competing for the surface. If there is no selectivity of the surface for a particular adsorbate, the monolayer composition on the substrate should reflect the bulk solution composition. The formation of a mixed monolayer of the Osdipy and Rudipy complexes was investigated to determine if the equilibrium situation described above was applicable. These studies may allow a better understanding of the mechanism of monolayer formation as well as the structure of these mixed monolayers which may provide the basis for further applications of the concept of mixed monolayers of redox-active selfassembling molecules. The cyclic voltammetric signatures of the adsorbed complexes studied are depicted in Figure 2 with the charge under the waves in the cyclic voltammograms corresponding to about one monolayer (1.0 × 10-10 mol/cm2) of Osdipy (A) and Rudipy (B), respectively. Since the characteristic formal potentials of the complexes are separated by about 400 mV, identification and quantification of each material in a mixed monolayer are easily achieved. A reversibly adsorbed monolayer in equilibrium with its complex in solution should be displaced (exchanged) when contacted with a different solution containing another adsorbate,
as described in eq 2. Cyclic voltammograms obtained at different times after exposure of a monolayer of Rudipy to an Osdipy solution (Figure 3A) show that this is indeed the case. Figure 3A shows an initially formed monolayer of Rudipy being displaced by Osdipy from a 2 µM solution of the latter containing no dissolved Rudipy. The areas under the voltammetric waves are a measure of the amount of each material adsorbed on the electrode surface. It can be seen that the area under the voltammetric wave for Rudipy (centered at +0.75 V) decreases to nearly zero while the area under the wave for Osdipy (centered at +0.35 V) exhibits a concomitant increase. The same results are obtained for the reverse case, that is a monolayer of Osdipy initially adsorbed is displaced when put in contact with a solution of Rudipy. Figure 3B shows the results obtained from such an experiment. The effects of different concentration ratios of the complexes in solution on the composition of the resulting monolayer were also investigated. If the system is under equilibrium at all times the composition of the monolayer should reflect the ratio of the complexes in solution. Table 1 shows the results obtained in five experiments where the concentration ratio of the complexes ([Ru]/[Os]) was varied from about 0.3 to 3.0 at a constant total concentration of 2 µM. The coverage ratio of the complexes, determined by cyclic voltammetry, showed the
Redox-Active Self-Assembling Mixed Monolayers
Figure 4. Plot of surface coverage ratio (ΓRu/ΓOs) vs solution concentration ([Ru]/[Os]) ratio.
Figure 5. (A) Cyclic voltammograms at different times (i.e., 0, 3, 10, 20 min) to monitor the exchange dynamics of Osdipy with Rudipy. (B) Charge variation with time for Osdipy (b), Rudipy (9), and total charge (2). Solid lines are fit to eqs 4 and 5 for Rudipy and Osdipy, respectively. The dotted line represents the sum of the two solid lines, and the dashed line is the average total charge.
same increasing trend although systematically lower (by about 25%) when compared to the solution composition. A plot of surface coverage ratio vs solution concentration ratio ([Ru]/[Os]) is presented in Figure 4. Although an excellent correlation (r ) 0.999) is obtained, the slope is less than unity (0.75) which would suggest that there is a slight preference of the surface for the osmium complex. Given that the free energies of adsorption of these materials are virtually identical, we are not, at this time, certain of the origin of this effect. This discrepancy may be attributed to the charge integration process which would affect the determination of the coverage by the Ru complex. Since the platinum oxide formation overlaps somewhat with the Rudipy signal, it makes it more difficult to subtract its background current when compared to the Osdipy complex. The results shown here are comparable to those reported by Faulkner et al. with a similar system.14 The exchange dynamics were studied by recording cyclic voltammograms at different times during the equilibration (from a homogeneous to a mixed monolayer) process. Figure 5A depicts the formation of mixed monolayer from a solution containing a 1:1 ratio of Rudipy/Osdipy. In this case the electrode was initially covered with a monolayer of Osdipy, and Rudipy was subsequently injected so as to achieve a 1:1 concentration ratio in solution. It is apparent from the figure that the voltammetric peak due to adsorbed Osdipy decreases
J. Phys. Chem., Vol. 100, No. 11, 1996 4559 while that for the Rudipy complex exhibits a concomitant increase. If the charges associated with these peaks are obtained at different times, the dynamics of exchange can be studied. Figure 5B shows the results obtained from the data in Figure 5A. The anticipated decrease in the Osdipy charge (coverage) as well as the increase in that due to Rudipy are observed until both reach an equilibrium value, determined by their respective solution concentrations. Also plotted in Figure 5B is the total charge which, as can be seen, remains virtually constant during the experiment. This demonstrates that the process being monitored is the surface exchange dynamics at constant surface coverage in which no stacking nor multiple layers are formed. The presence of isopotential points (at +0.45 V in Figure 5A) is also evident. Since isopotential points arise as a result of an equilibrium process between two surface species at constant total coverage,20 this provides compelling evidence that the exchange process is one that is at equilibrium at all times. This is an important observation since it implies that the time evolution of the changes in surface coverage can be employed to ascertain the kinetics of desorption, displacement, or exchange, depending on the specific experiment under consideration. These three processes are pictorially shown in Scheme 1. These processes are the focus of this paper and are described in detailed below. From the processes depicted in Scheme 1, adsorption and desorption rates can be obtained under different conditions which will help in understanding the mechanism of exchange between the two species under study. Simple exponential expressions were used to describe the increase (eq 4) or decrease
Qt,a ) Qe,a(1 - exp(-kat))
(4)
Qt,d ) (Qi,d - Qe,d) exp(-kdt) + Qe,d
(5)
(eq 5) of charge due to adsorption or desorption, respectively, of the complexes. In these equations Qt,a and Qe,a are the charge at time t and equilibrium charge for the adsorbing species, respectively; and Qt,d, Qe,d, and Qi,d are the charge at time t, equilibrium charge, and initial charge for the desorbing species, respectively. From fits of the experimental points to eqs 4 and 5 the rate constants of adsorption (ka) and desorption (kd) of the complexes in solution could be obtained. This system of equations was obtained from an analysis similar to that previously reported by this laboratory on studies of the adsorption dynamics of a family of compounds analogous to Osdipy.16 The solid lines in Figure 5B are fits of the data to eq 5 in the case of the desorption of Osdipy and to eq 4 for the adsorption of Rudipy. The dotted line represents the sum of the individual fits for Osdipy and Rudipy, and the dashed line represents the average of the total charge. The fits obtained are in excellent agreement with the data. The rate constants obtained from the fits to this and other experiments are shown in Table 2. Given that these two complexes are virtually identical in terms of their adsorptive behavior and that the total concentration of adsorbate used (2 µM) was in the saturation coverage regime, one would a priori anticipate that the rates of adsorption and desorption would be the same and that the total coverage would remain constant throughout. The data in Figures 5 and 6 and Table 2 are consistent with this although it should also be noted that (as can be seen in Table 2) there is significant error in some of the measurements. Nonetheless, and taking these considerations into account, the experimental observations are consistent with expectation. At a microscopic level, the exchange mechanism could involve a dissociative or an associative process with the slower of the two controlling the overall exchange dynamics of the system. We now look at each case in detail.
4560 J. Phys. Chem., Vol. 100, No. 11, 1996
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SCHEME 1: Pictorial Representation of the Experiments Presented in This Papera
a Desorption experiment: a monolayer is exposed to supporting electrolyte, and its desorption is monitored as a function of time. Displacement experiment: a monolayer is exposed to a solution of a different adsorbate, and the displacement is monitored as a function of time. Exchange experiment: an initial monolayer in equilibrium with its adsorbate in solution is exchanged with a different adsorbate injected into the original solution.
TABLE 2: Desorption and Adsorption Rate Constants for Osdipy and Rudipy from the Exchange Experiments
Figure 6. Plot of charge vs time for the desorption of a monolayer of (A) Osdipy and (B) Rudipy into 0.1 M KClO4 aqueous solution.
TABLE 3: Desorption and Adsorption Rate Constants for the Displacement of an Initial Monolayer by a Different Adsorbate from a 2 µM Solution in the Absence of the Initial Complex
initial monolayer
concn ratio [Ru]/[Os]
desorption rate (kd,i)/10-2 min-1
adsorption rate (ka,i)/10-2 min-1
initial monolayer
desorption rate (kd,i)/10-1 min-1
adsorption rate (ka,i)/10-1 min-1
Osdipy Rudipy Osdipy Osdipy Rudipy
0.33 0.53 1.03 3.00 3.00
kd,Os ) 4.9 ( 3.5 kd,Ru ) 5.7 ( 0.9 kd,Os ) 7.8 ( 1.4 kd,Os ) 5.0 ( 0.4 kd,Ru ) 3.5 ( 0.7
ka,Ru ) 6.4 ( 1.6 ka,Os ) 8.5 ( 1.2 ka,Ru ) 11.3 ( 1.5 ka,Ru ) 6.6 ( 0.9 ka,Os ) 3.4 ( 0.1
Osdipy Rudipy
kd,Os ) 2.6 ( 0.1 kd,Ru ) 1.9 ( 0.1
ka,Ru ) 6.6 ( 0.9 ka,Os ) 1.3 ( 0.1
In a dissociative mechanism where the desorption process is rate determining, the variation of the total charge (coverage) with time should be constant. For a molecule in solution to adsorb, a surface site must be made available. As soon as a site for adsorption is accessible, the adsorbing species occupies it, and as a result no change in the total charge is observed at different times. This is precisely what we observe in our experiments as shown in Figure 5B. Such behavior is also consistent with the fact that under the experimental conditions where the bulk concentration of adsorbate (2 µM) is at the saturation coverage plateau, the rate of adsorption would be expected to be rapid. An associative mechanism involving material in solution loosely associated with the monolayer, possibly intercalating within the film and helping adsorbed molecules to desorb could be a reasonable alternative if the intercalation or weak association occurs only to a small extent. The cyclic voltammetric technique would not detect this interaction if it represents 5-10% of a monolayer since this is the magnitude of the error in the coverage measurements. If the extent of interaction is higher than 10%, one would anticipate an increase in the total charge at the beginning of the experiment with a succeeding
decrease down to the monolayer value. This, however, is clearly not observed. If an associative mechanism were involved, desorption and displacement experiments (as shown in Scheme 1) should yield different results. The displacement experiments would have conditions that would favor the desorption if it takes place in an associative way when compared to the conditions present in simple desorption experiments, and this should be reflected in the rate constants for desorption in each case. Displacement experiments are shown in Figure 3 and Table 3 summarizes the rate constants obtained after fitting the data to eqs 4 and eq 5. Desorption experiments were done as follows: a monolayer of the complex, Osdipy or Rudipy, was adsorbed on the electrode, which was subsequently placed in a clean supporting electrolyte solution where the desorption was monitored with cyclic voltammetry. Figure 6A,B shows the results obtained for Osdipy and Rudipy, respectively. Solid lines are fits of the data to eq 5, and the rate constants obtained are presented in Table 4. All the desorption rate constants obtained are within the same order of magnitude; slightly larger values are observed for the displacement experiments which would suggest that some association might be taking place enhancing the rate of desorption. If this is taking place in the exchange experiments, it would have to be to a small extent because there is no increase in the total charge during the experiment, as mentioned earlier. On the basis of this analysis, we believe
Redox-Active Self-Assembling Mixed Monolayers
J. Phys. Chem., Vol. 100, No. 11, 1996 4561
TABLE 4: Desorption Rate Constants of the Individual Monolayers into Clean Supporting Electrolyte initial monolayer
desorption rate (kd,i)/10-1 min-1
Osdipy Rudipy
kd,Os ) 1.5 ( 0.1 kd,Ru ) 0.9 ( 0.2
that a dissociative mechanism and/or an associative mechanism, to a small extent, can be used to describe the exchange dynamics. Previous studies on the adsorption kinetics of this type of molecule provide additional evidence in support of the proposed mechanism in the exchange dynamics.16 We have previously shown that the adsorption process is under kinetic (rather than diffusion) control and that it obeys the equilibrium described in eq 1. Clearly, the conditions were such that the adsorption process dominated over the desorption, resulting in the formation of a monolayer on a platinum electrode in contact with a solution of the complex. From those experiments the measured rate constant for adsorption at a solution concentration similar to that employed in this work (e.g., 2 µM) was about 0.13 min-1, which is significantly faster than all of the desorption rates presented in Table 2. Desorption experiments shown in Figure 6A,B and their respective rate constants shown in Table 4 were used for comparison. The magnitude of these rate constants is comparable to the adsorption rate constant values quoted above. However, it is important to keep in mind that these desorption experiments represent a nonequilibrium situation since there is no complex in solution. Even though the experimental conditions for desorption are different from the adsorption experiments, the rate constants obtained for the former can be used as a reference point. Under experimental conditions where net adsorption is observed, the magnitude of the desorption rate has to be smaller than the rate of adsorption; so the exchange process is controlled by the desorption step, as is experimentally observed. The slope of the solid lines in Figure 5B is a measure of the rate of the exchange process. From the desorption equation (eq 5) an expression can be derived to obtain the value of the rate of the exchange dynamics. From eq 5, eq 6 can be obtained:
dQt /dt ) -kd(Qi,d - Qe,d)exp(-kdt)
(6)
If kd ) 5.4 × 10-2 min-1, the average value of kd from Table 2 is used, and time (t) is set to zero, the initial rate of exchange can be calculated. For the experiment shown in Figure 5B, an initial exchange velocity of 5% of a monolayer per minute is obtained, which indicates a very dynamic system. The rate of interchange slows down until equilibrium is reached and no further net exchange is observed. Wave-Shape Analysis and Near-Neighbor Interactions. The voltammetric response of electroactive molecules in a selfassembling monolayer can be used to probe the chemical environment within the film. In the ideal cyclic voltammogram for a surface-confined species, 20,21 the current is directly proportional to the scan rate (ν), the cathodic wave on scan reversal is the mirror image of the anodic wave reflected across the potential axis which means that Ep,a ) Ep,c, and the full width at half-maximum (∆Efwhm) is given by eq 7. Such ideal
∆Efwhm ) 90.6/n mV
(7)
behavior is frequently not observed experimentally. Cyclic voltammograms of adsorbed species are typically broader than predicted. Deviations from the ideal behavior can be caused by the nature of the chemical environment within the monolayer.
Figure 7. Cyclic voltammograms at different times (i.e., (a) 0, (b) 10, (c) 30 min) to monitor the exchange dynamics of an initially deposited monolayer of Rudipy in contact with an aqueous solution of Osdipy and Rudipy (1:1) at a total concentration of 2 µM in 0.1 M KClO4.
In general, the width of the peak depends on the types of interactions among the molecules making the monolayer. The voltammetric peak becomes narrower and sharper if attractive forces predominate and wider and more rounded if, on the contrary, repulsive forces prevail.21 In our experiments, the observed ∆Efwhm is larger than the ideal value indicating repulsive (destabilizing) interactions. This is expected since the molecules in the monolayer are positively charged (+1 in the reduced form, +2 in the oxidized form), and electrostatic repulsions between the headgroups would take place. Another interesting observation reported for similar systems is the dependence of the formal potential (E°′) on the total coverage.17 In a qualitative way one can say that the oxidation becomes more difficult (shift to more positive potentials) at higher coverage because the repulsions between molecules increase with coverage. The ideas described above will be considered within the context of the experiments performed on mixedmonolayer systems. Figure 7 shows the cyclic voltammograms for adsorbed Rudipy from an experiment in which a Rudipy monolayer was initially deposited (cyclic voltammogram a), and then Osdipy was injected to the solution to have an equivalent concentration of the former and to form the mixed monolayer. Cyclic voltammogram b was acquired after 10 min, and cyclic voltammogram c after 30 min. No significant changes were observed after this time period suggesting that equilibrium was reached. The expected decrease in the Rudipy signal is observed together with a concomitant increase in the Osdipy wave. Two other effects were observed. First, the apparent formal potential for the Rudipy complex was shifted toward more positive potentials (by about 40 mV); and second, the wave for the Rudipy complex became broader; after the presence of the Osdipy in the mixed monolayer. We believe that the shift in formal potential and the broadening of the Rudipy wave are due to the change in the chemical environment around the Rudipy complex when the Osdipy is present in the mixed monolayer. As mentioned before, the redox potential for Osdipy is about +0.35 V and for Rudipy it is about +0.75 V. For a monolayer containing only Rudipy at potentials prior to its oxidation; say +0.50 V, all the molecules have a +1 charge. The Rudipy molecules in the monolayer experience electrostatic repulsions which is reflected in the value of ∆Efwhm ) 200 mV, larger than the value of 90.6 mV when there are no interactions between particles in the monolayer. When Osdipy is introduced into the film at the same level (e.g., 1:1 ratio within the
4562 J. Phys. Chem., Vol. 100, No. 11, 1996
Figure 8. Sections of cyclic voltammograms in Figure 7 corresponding to the oxidation of (A) Rudipy in the monolayer by itself and (B) in the presence of Osdipy in a 1:1 ratio (c in Figure 7). Symbols are the experimental points, dashed lines are the background, dotted lines represent a Gaussian fit to the experimental points after subtraction of the background, and the solid lines combine the background with the fit.
monolayer) some of the Rudipy molecules will have Osdipy near neighbors which at a potential of +0.50 V are already oxidized to +2. This will give rise to larger electrostatic repulsions so that the Rudipy oxidation wave will be shifted to more positive potentials since a larger driving force will be needed to oxidize the Rudipy in this less favorable (for the oxidized state) environment. In addition, broader peaks are expected in the presence of Osdipy reflecting the higher electrostatic repulsion between particles. Figure 8A,B shows the regions of the cyclic voltammograms in Figure 7 corresponding to the oxidation of Rudipy in the case of a full Rudipy monolayer and for the 1:1 mixed monolayer (after 30 min), respectively. The symbols correspond to the data points, the dashed lines are the background, the dotted line represents a Gaussian fit to the peak, and the solid line through the symbols combines the Gaussian fit with the background. To compare the two fits with respect to their width, the current was normalized to the total charge in each case. Figure 9A shows the results obtained after normalization (relative to the total charge) with the solid line representing the Rudipy monolayer alone and the dashed line the mixed monolayer, where one can note the shift in potential in going from the Rudipy monolayer to the mixed-monolayer. To compare the width of the waves the Rudipy wave was shifted (by 28 mV) so as to overlap it with the mixed monolayer wave as shown in Figure 9B. It can be seen that in the mixed-monolayer case the Rudipy wave is broader. The ∆Efwhm value increased from 200 mV for the Rudipy monolayer to 230 mV for the mixed monolayer case. The changes described above were observed in all the experiments in which the Rudipy monolayer was initially present on the surface of the electrode. On the other hand, in experiments where the Osdipy was initially present on the surface of the electrode, after the introduction of Rudipy to the film, the Rudipy wave was always broad, as shown for example in Figure 5A.
Tirado and Abrun˜a
Figure 9. (A) Cyclic voltammograms in Figures 8A and 8B normalized by the total charge under the peak. (B) Overlapped normalized cyclic voltammograms from Figure 9A.
SCHEME 2: Pictorial Representation of the Two Possible Mixed States That Could Be Attained after Reaching Equilibriuma
a (A) Randomly mixed monolayer. (B) Phase-separated mixed monolayer.
The electrochemical response of the mixed monolayer can also provide some information as to the final arrangement of the monolayer after equilibrium is reached. Scheme 2 depicts two possible structures of the monolayer after the exchange experiment has reached equilibrium. One possibility (case A) is to have a random distribution of the Osdipy and Rudipy molecules. On the other hand the molecules could segregate
Redox-Active Self-Assembling Mixed Monolayers into two domains as shown in case B. We believe that case A, the random distribution of particles in the monolayer, better describes the system. The fact that the cyclic voltammetric wave of Rudipy shifts to positive potentials and broadens in the presence of Osdipy indicates the presence of close interactions between the molecules within the mixed monolayer. The formation of two phases would allow interactions of this kind only at the boundaries so that the effects would be less marked. For the mixed monolayer to segregate, there must be a preferential interaction between identical molecules. We do not believe that is the case, since except for the identity of the metal center, the complexes are virtually identical. A third possibility would be the result of a combination of the two cases shown. That is, one could have the surface randomly decorated with islands of each complex. To differentiate between this case and that discussed previously, additional studies will be required. These studies are currently being carried out, and their results will be reported elsewhere. Conclusions The exchange dynamics as well as the desorption and displacement reactions of redox-active self-assembling monolayers of transition-metal complexes of osmium and ruthenium of the type [M(bpy)2ClL]+ (M ) Os, Ru, bpy ) 2,2′-bipyridine and L is 1,3-bis(4-pyridyl)propane) have been studied. The above-mentioned processes appear to be controlled by the rate of desorption via a dissociative mechanism, although a small extent of association (less than 10%) is also possible. From studies of the exchange dynamics, the initial rate of exchange is estimated to be of the order of 5% of a monolayer per minute. We have also made use of wave-shape analysis in terms of shifts in the formal potentials and widths of the voltammetric waves with changes in the surface coverage and monolayer composition. The presence of mixed monolayers (Os/Ru) results in an increased broadening of the voltammetric wave associated with the ruthenium complex relative to the case of a monolayer of the ruthenium complex alone which we ascribe to an enhanced degree of electrostatic repulsion from the oxidized osmium centers. Also based on an analysis of the voltammetric response, we believe that the mixed monolayers are randomly distributed rather than being composed of discrete and separated phases. Acknowledgment. This work was supported by the National Science Foundation (DMR 91-07116), and the Office of Naval Research. J.T. gratefully acknowledges support by a Corning Foundation Fellowship.
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