Effects of Counterion Mobility, Surface Morphology, and Charge

Mar 25, 2008 - The standard electron-transfer rate constants (k0) for the oxidation of porphyrin monolayers are reported for a number of solvent/elect...
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J. Phys. Chem. C 2008, 112, 6173-6180

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Effects of Counterion Mobility, Surface Morphology, and Charge Screening on the Electron-Transfer Rates of Porphyrin Monolayers Jieying Jiao,† Eric Nordlund,† Jonathan S. Lindsey,*,‡ and David F. Bocian*,† Department of Chemistry, UniVersity of California, RiVerside, California 92521-0403 and Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204 ReceiVed: January 7, 2008; In Final Form: February 5, 2008

The standard electron-transfer rate constants (k0) for the oxidation of porphyrin monolayers are reported for a number of solvent/electrolyte systems and electroactive surfaces. The goal is to explain the inverse correlation between the electron-transfer rates and the porphyrin surface concentration (Roth et al., J. Phys. Chem. B 2002, 106, 8639-8648). Each porphyrin is a zinc chelate and contains three meso-mesityl groups and a benzyl alcohol or benzyl thiol for surface attachment. The solvent/electrolyte systems include (i) the organic solvent propylene carbonate containing electrolytes with a common cation and anions of different size/mobility (PF6-, ClO4-, and Cl-) and (ii) neat ionic liquids with a common cation and anions of different size/mobility [(CF3SO2)2N- and (NC)2N-]. The substrates include Si(100), Au(111), and TiN. The k0 values observed using electrolytes with PF6-, ClO4-, and (CF3SO2)2N- counterions are similar to one another, whereas those observed using electrolytes Cl- and (NC)2N- counterions are 2-5 times faster. The faster rates for the latter anions are attributed to their smaller size/higher mobility. The k0 values observed for monolayers on Si(100) and Au(111) are similar to one another; the k0 values for monolayers on TiN are ∼5-fold faster. The faster rates for the TiN substrate are attributed to a rougher surface morphology (as determined via atomic force microscopy measurements) which results in an actual surface concentration that is lower than the concentration based on the geometrical area of a planar substrate. The k0 values determined for mixed monolayers where the electroactive porphyrin is co-deposited with an electroinactive porphyrin are dependent only on the concentration of the redox-active species, not on the total porphyrin concentration. This behavior is consistent with space-charge effects being the principal determinant of the inverse correlation between the electrontransfer rates and the porphyrin surface concentration. The space charge effects can be mitigated, but not eliminated, by using smaller, more mobile counterions and rougher surfaces.

I. Introduction The prospect that devices relying on the bulk properties of semiconductors will fail to retain their characteristic properties as sizes reach nanoscale dimensions has motivated studies of molecular materials as potential active media in electronic devices.1 Toward this goal, we have been engaged in a program aimed at constructing devices that use the properties of molecules to store information.2-5 In this approach, a collection of redox-active molecules (monolayer or polymer film) attached to an electroactive surface functions as the storage medium; information is stored in the discrete redox states of the molecules. Our studies have primarily utilized porphyrinic molecules as the charge-storage medium. The molecules have been covalently attached to both metal (Au via S anchor atoms)2,3 and semiconductor (Si via O, S, Se, and C anchor atoms)4,5 surfaces. Semiconductor substrates are of particular interest because the first molecular-based electronic devices are likely to be hybrid designs wherein molecules are integrated onto semiconductor platforms.6 One important characteristic of a memory device that relies on charge storage is the capacity of the storage medium. This capacity ultimately limits the ability to accurately read out * To whom correspondence should be addressed. E-mail: David. [email protected]; [email protected]. † University of California. ‡ North Carolina State University.

charge and hence, stored information. A key advantage of employing molecules for charge storage is that suitably designed molecular materials can afford significantly higher charge density than the materials commonly in use in memory devices. For example, the charge densities achievable with a monolayer of a porphyrin-based storage medium typically fall in the range of 10-40 µC cm-2, depending on the tether and surfaceattachment group,2-5 compared with 1-2 µC cm-2 for the Si/ SiO2 capacitors currently used in dynamic random access memories.6,7 The higher charge densities afforded by the porphyrin-based storage medium would permit more robust information storage, particularly as feature sizes are reduced beyond current dimensions. A second important characteristic of a memory device is its read/write speed, faster speeds being generally more desirable. In a molecular-based memory device based on charge storage, the read/write speed is directly related to the electron-transfer rate of the surface-attached molecules. In this regard, our previous studies of porphyrin monolayers on both metal and semiconductor surfaces have shown that the electron-transfer rates depend on factors such as the surface concentration (packing density) of the molecules.2d,f,3,5a-d,g In particular, we have observed that the electron-transfer rates monotonically decrease as the surface concentration of the molecules increases. This observation does not necessarily imply a change in the intrinsic electron-transfer rate between the porphyrin and

10.1021/jp800123u CCC: $40.75 © 2008 American Chemical Society Published on Web 03/25/2008

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Jiao et al. charge screening. The solvent/electrolyte systems include an organic solvent (propylene carbonate) containing electrolytes with a common cation and anions of different size/mobility (PF6-, ClO4-, and Cl-), and neat ionic liquids with a common cation and anions of different size/mobility [(CF3SO2)2N- and (NC)2N-]. The electroactive surfaces investigated included both Au(111) and Si(100), as well as TiN. TiN was investigated because this type of semiconductor is a common material for device construction. The morphology of all three surfaces was further characterized via atomic force microscopy (AFM). In a second series of studies, the electron-transfer rates of ZnPBzSwere measured in mixed monolayers of zinc porphyrins and free base porphyrins bearing thiol tethers (ZnPBzS-/FbPBzS-) on Au(111). In these experiments, the free base porphyrin FbPBzS- serves as a non-redox-active diluent. These experiments probe the difference in electron-transfer rates on sparsely covered surfaces versus more densely packed surfaces that contain varying amounts of redox-active species. Collectively, the studies reported herein elucidate the origin of the inverse correlation between the electron-transfer rates and surface concentration of the porphyrin monolayers. II. Experimental Section

electroactive surface, but rather reflects a change in the effective electron-transfer rate for the entire monolayer/counterion/solvent system. Regardless, the decrease in effective electron-transfer rate is undesirable inasmuch as the most efficacious molecularbased, charge-storage medium would exhibit both high capacity and fast rates. The origin of the inverse correlation between the surface concentration and electron-transfer rates of the porphyrin monolayers is uncertain. However, we have speculated that this effect might be due to restricted access of counterions at higher surface concentrations and/or other charge-screening or spacecharge effects.2d In this regard, space-charge effects are known to be important for controlling ion flow across lipid bilayers.8 In addition, previous studies of alkylferrocene monolayers have shown that when the redox center is buried within an insulating matrix of longer alkane chains, the redox kinetics are dramatically slower.9,10 Related studies in which large polymeric counterions are employed yield similar results.9,10 Indeed, the combination of these two factors can render a redox-active monolayer completely electro-inactive.11 Solvent/counterion exclusion also strongly attenuates the rate-distance behavior of the redox kinetics of self-assembled monolayers.12 In this work, we probe the origin of the inverse correlation between the surface concentration and electron-transfer rates of porphyrin monolayers. The porphyrins used in the studies are shown in Chart 1. These molecules have a common linker motif, benzyl-X (X ) O or S) and were chosen because we have previously investigated the electron-transfer characteristics of the two zinc porphyrins, ZnPBzS- and ZnPBzO-, on Au(111) (S anchor)2d and Si(100) (S and O anchors)4a,5a surfaces. These earlier studies used a single type of solvent/electrolyte system. Herein, we examine the electron-transfer rates for the first oxidation process of the ZnPBzS- and ZnPBzO- monolayers versus the surface concentration of the molecules as a function of (1) the size/mobility of the counterion, (2) the type/ morphology of the electroactive surface, and (3) the degree of

A. Chemicals and Materials. Porphyrins ZnPBzOH, ZnPBzSAc, and FbPBzSAc were synthesized as previously described.4a,13 The solvents used in the preparation of the monolayers were anhydrous CH2Cl2 (Aldrich, 99%) and anhydrous benzonitrile (Aldrich, 99%); the solvent used in the electrochemical measurements was propylene carbonate (PC, Aldrich, 99%). The supporting electrolytes n-Bu4NPF6 (TBAH, Aldrich) and n-Bu4NClO4 (TBAP, Aldrich) were recrystallized three times from methanol and dried in a vacuum at 100 °C. The supporting electrolyte n-Bu4NCl (TBACl, Fluka) was used as received. The ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-IM, Covalent Associates) and 1-ethyl-3-methylimidazolium dicyanamide (EMI-DCA, Fluka) were provided by A. Hawkins (ZettaCore, Inc.) and used as received. Silicon(100) wafers (Silicon Valley Microelectronics) were purchased as thermally oxidized B-doped (p-type) Si(100) (F ) 0.01-0.03 Ω cm). The Au(111) substrates were prepared by e-beam vapor deposition of 20 nm Cr (99.999%) followed by 200 nm of Au (99.995%) onto the surface of a precleaned B-doped Si(100) wafer. The chemicals used in microelectrode fabrication were AZ 5214 positive photoresist (Baker), AZ 400 K photoresist developer (Baker), Baker Aleg 355 resist stripper (Baker), nanostrip (Cyantek), HF dip (5:1), buffered oxide etch (10:1) and J. T. Baker clean (Baker). Ar and N2 (99.995%) were passed through Drierite (Fisher) and Oxyclear (Supelco) gas purifiers prior to use. Deionized water (from a Milli-Q system) had a F g 16 MΩ cm. B. Electrode Preparation. The electrochemical measurements on the monolayers were performed using Si(100) (100 × 100 µm), Au(111) (100 × 100 µm), and TiN (30 × 30 µm) microelectrodes. Microelectrodes were used to ensure that the RC time constant of the electrochemical cell was sufficiently short so that the electron-transfer kinetics of the porphyrin monolayers could be measured accurately. The Si microelectrodes were prepared via photolithographic methods as described previously.4a The Au microelectrodes were prepared in a similar manner as used for preparing the Si microelectrodes. The Au microelectrode was defined by a silicon oxide boundary layer (thickness ≈ 300 nm) that was deposited by plasma-enhanced chemical vapor deposition. The TiN microelectrodes were

Electron-Transfer Rates of Porphyrin Monolayers provided by T. DeBolske (ZettaCore, Inc.). Before use, the TiN microelectrodes were cleaned by sonicating in acetone, followed by water, and then isopropanol (1 min in each solvent) and etched with 0.1% HF for 4 min. A bare Ag wire was used as the counter/reference electrode. The electrode was prepared by sonicating a section of 500 µm diameter Ag wire (Alfa Aesar) in 7.0 M NH4OH, rinsing the wire in deionized water and ethanol, and then sonicating the wire in CH2Cl2 containing 1.0 M TBAH. The Ag wire prepared in this manner was placed inside a 10 µL polypropylene pipet tip containing ∼5 µL of supporting electrolyte solution. The supporting electrolytes solutions were 1.0 M TBAH, TBAP or TBACl in propylene carbonate, or the neat ionic liquids EMIIM or EMI-DCA. C. Monolayer Preparation. The monolayers composed of ZnPBzS- on Au(111) were prepared by dispensing 5 µL of ZnPBzSAc porphyrin solution in CH2Cl2 onto the surface of the microelectrode contained in a sparged vial sealed under Ar for ∼30 min. The S-acetyl protecting group is cleaved during the surface-attachment process.2,3 Additional details of the monolayer preparation on Au(111) can be found in ref 3, parts a and d. The mixed monolayers of ZnPBzS-/ FbPBzS- were prepared using solutions containing mixtures of the parent S-acetyl-protected porphyrins in the desired ratio. The ZnPBzO- monolayers on Si(100) and TiN were prepared using a high-temperature (400 °C) and short-time (2 min) “baking” attachment procedure previously described.14 Briefly, the monolayers were prepared by dispensing 2 µL of ZnPBzOH porphyrin solution in benzonitrile onto the microelectrode substrate in a vial. After deposition, the vial containing substrate was heated on a hot plate at 400 °C for 2 min, then removed, purged with Ar until it cooled to room temperature. Finally, the substrate was rinsed and sonicated five times with CH2Cl2 and purged dry under Ar. The surface concentration and conditions for achieving saturation coverage were determined electrochemically in a series of experiments wherein the concentration of the porphyrin in the deposition solvent (CH2Cl2 or benzonitrile) was varied systematically. These experiments revealed that the surface concentration could be varied in a controlled fashion from the low 10-12 to the mid 10-11 mol cm-2 range by varying the porphyrin concentration from 2 µM to 2 mM. D. Electrochemical Measurements. The electrochemical measurements on the monolayers were performed in a twoelectrode configuration using the fabricated microelectrode and the Ag counter/reference electrode described above. All electrochemical measurements with electrolytes TBAH, TBAP, or TBACl were made in the solvent propylene carbonate. The RC time constant for the microelectrode/electrochemical cell, measured to be ∼4 µs, is sufficiently short to preclude any significant interference with the measurement of the electrontransfer rates. The cyclic voltammograms were recorded using a Gamry Instruments PC4-FAS1 femtostat running PHE 200 Framework and Echem Analyst software. The surface concentration of the porphyrins in the monolayer was determined by integration of the total charge in the first anodic wave and by using the geometric dimensions of the microelectrode. The standard electron-transfer rate constant, k0, of the porphyrin monolayers was obtained using the same SWAV method used previously to obtain k0 values for porphyrin monolayers on both Au and Si surfaces.2d E. AFM Measurements. The AFM measurements were made in air using a Dimension 5000 instrument and a Nanoscope IV controller housed in a class 1000 facility. To minimize

J. Phys. Chem. C, Vol. 112, No. 15, 2008 6175 environmental noise, the experiments were run under a metal hood and the samples were locked into place by magnetic suction. The AFM tips were rotated tapping-mode etched silicon probes (RTESP) with resonant frequencies of ∼250 kHz (Veeco Probes). The measurements were made in tapping mode. All images were acquired with a scan rate of 1.0 Hz and were flattened with a first-order polynomial before analysis. III. Results A. Electrochemical Studies of ZnPBzX- Monolayers on Si(100), Au(100), and TiN. The first series of electrochemical studies examined neat ZnPBzX- monolayers on the three different surfaces using the different electrolytes. We first describe the general characteristics of the redox behavior of the monolayers, followed by an examination of the electron-transfer rates. The trends observed as a function of porphyrin surface concentration were the specific focus of the studies. 1. Voltammetric Characteristics. Representative fast scan (100 V s-1) cyclic voltammograms of the ZnPBzX- monolayers at similar surface concentrations on the three substrates in the different electrolytes are presented in Figure 1. The voltammetric characteristics of all the monolayers are qualitatively similar to one another and similar to those we have previously reported for ZnPBzO- and ZnPBzS- monolayers on Si(100) and Au(111) (TPAH electrolyte), respectively.2d,4a,5a In particular, all of the monolayers exhibit robust and reversible electrochemical behavior over multiple cycles under ambient conditions. At oxidizing potentials, the monolayers in the TBAH, TBAP, and EMI-IM, exhibit two resolved voltammetric waves, indicative of the mono- and dication porphyrin radicals. The potentials of the first and second oxidation in TBAH, TBAP, and EMI-IM are similar to one another. In the case of the TPACl and EMIDCA, the potentials are shifted to more positive values. The positive shift pushes the second oxidation wave of the porphyrin out of the observation window. This potential shift is attributed to a change in the reference potential due to complexation of the Cl- and (NC)2N- anions with the Ag reference electrode. In the case of the monolayers on Au(111), no voltammetric signature was observed with the ionic liquid EMI-DCA. This observation suggests that the (NC)2N- anion may also complex with the Au(111) surface. Finally, we note that the potentials of the porphyrin redox waves also vary somewhat as a function of the surface (Si(100) < Au(111) < TiN). The electrochemical characteristics of all of the porphyrin monolayers on the three surfaces in the different electrolytes were also investigated as a function of porphyrin surface concentration. These studies were prompted by the results of our previous studies of porphyrin monolayers, which showed that both the value of the redox potential and the line width of the redox wave increase as the surface concentration increases.2d,3a These trends indicate that both the thermodynamic driving force and the degree of heterogeneity in the redox process increase with increasing surface concentration.15,16 These general trends are also observed for all the ZnPBzX- monolayers studied herein as revealed in the data for the ZnPBzO- monolayers on Si(100) shown in Figure 2. This figure plots the potential for the first oxidation (E0/+1) and the full-width at half-maximum for the anodic peak of this oxidation (∆Ep,1/2) in the top and bottom panels, respectively. Inspection of the data in Figure 2 reveals other noteworthy trends. (1) For all of the electrolytes, the shifts of E0/+1 to more positive values as the surface concentration is increased from the lowest (Γ ≈ 2 × 10-12 mol cm-2) to highest (Γ≈ 7 × 10-11 mol cm-2) value are ∼100 mV. (2) For all the electrolytes, the

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Figure 2. Redox potentials, E0/+1 (top panel), and full-width at halfmaxima, ∆Ep,1/ 2 (anodic peak) (bottom panel), versus surface concentration of ZnPBzO- monolayers on Si(100) in various supporting electrolytes.

Figure 1. Representative fast-scan (100 V s-1) voltammograms of ZnPBzO- monolayers on Si(100) (top), ZnPBzS- monolayers on Au(111) (middle), and ZnPBzO- monolayers on TiN (bottom) in various supporting electrolytes.

increase in ∆Ep,1/2 as the surface concentration is increased from the lowest to highest value are ∼70-80 mV. (3) At low surface concentrations in TBAH, TPAP, and EMI-IM, the ∆Ep,1/2 values for all the monolayers are similar to one another and fall in the 90-100 mV range, near the thermodynamic minimum for a redox-homogeneous monolayer.15,17 At low surface concentrations, in TBACl and EMI-DCA, the ∆Ep,1/2 values are larger (∼110 and ∼130 mV, respectively), which is in the range where a distribution of formal potentials exists.16 2. Electron-Transfer Characteristics. The standard electrontransfer rate constants, k0, were measured for the first oxidation processes of all of the porphyrin monolayers on the three surfaces in the different electrolytes. As noted in the Introduction, our previous studies on porphyrin monolayers have shown that the k0 depends on surface concentration; higher surface coverage generally results in slower rates of electron transfer.2d,f,3,5a-d,g This general trend is also observed for all the

ZnPBzX- monolayers studied herein as revealed in the plots of k0 as a function of surface concentration shown in Figure 3. [Note that the k0 values on Si(100) cannot be determined accurately at high surface concentrations due to experimental limitations.] 5a-d These data reveal the following trends: (1) The k0 values in TBAP, TBAH, and EMI-IM are generally similar to one another; the rates in TBACl and EMI-DCA are faster. (2) The k0 values for the monolayers on Si(100) and Au(111) in a given electrolyte are generally similar to one another, consistent with our earlier studies that utilized TBAH.2d,4a,5a The k0 values for the monolayers on TiN in a given electrolyte are faster than those on Si(100) or Au(111). B. Electrochemical Studies of ZnPBzS-/FbPBzS- Mixed Monolayers on Au(111). The second series of electrochemical studies examined mixed monolayers of ZnPBzS- and FbPBzSporphyrins on Au(111). The free-base porphyrin undergoes oxidation at a potential ∼300 mV more positive than that of the zinc porphyrin; accordingly, the free-base porphyrin serves as a redox-inactive diluent having the same size and molecular shape as that of the redox-active zinc porphyrin.18 The Au(111) surface was chosen owing to the relative ease of preparing monolayers on Au versus Si or TiN. As noted in the Introduction, the aim of these studies was to probe the difference in electron-transfer rates on sparsely covered surfaces versus more densely packed surfaces that contain varying amounts of redoxactive species. We first describe the general redox characteristics of the mixed monolayers, followed by an examination of the electron-transfer rates. 1. Voltammetric Characteristics. Representative fast scan (100 V s-1) cyclic voltammograms of the ZnPBzS- and FbPBzSmonolayers at similar surface concentrations on Au(111) are

Electron-Transfer Rates of Porphyrin Monolayers

Figure 3. Standard electron-transfer rate constants, k0, versus surface concentration, Γ, for the first oxidation process (E0/+1) of ZnPBzOmonolayers on Si(100) (top), ZnPBzS- monolayers on Au(111) (middle), and ZnPBzO- monolayers on TiN (bottom) in various supporting electrolytes.

shown in Figure 4, top and middle panels, respectively. The voltammogram of a 50/50 mixed ZnPBzS-/FbPBzS- monolayer is shown in Figure 4, bottom panel. As can be seen, the voltammetry of the FbPBzS- monolayer is characterized by a single oxidation wave versus the two waves observed for the ZnPBzS- monolayer. The single oxidation wave observed for the FbPBzS- monolayer is ∼300 mV more positive than the first wave of the ZnPBzS- monolayer and corresponds to the analogous process, namely oxidation to the monocation radical. The second oxidation wave for the FbPBzS- monolayers is shifted out of the observation window. The difference in the oxidation potentials for the FbPBzS- and ZnPBzS- monolayers is comparable to that observed for the two types of porphyrins in solution (not shown) and as expected for a free base versus zinc chelate.18 In the mixed ZnPBzS-/ FbPBzS- monolayers, the first oxidation wave of the FbPBzS- component of the monolayer overlaps with the second wave of the ZnPBzS-

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Figure 4. Representative fast-scan (100 V s-1) voltammograms of ZnPBzS- monolayers, FbBzS- monolayers (middle), and a mixture of ZnPBzS- and FbPBzS- monolayers (bottom) on Au(111) (TPAH electrolyte).

component. The relative concentrations of the two species in the mixed monolayers can be evaluated via integration of the first wave (due to ZnPBzS- and the second wave (due to ZnPBzS- and FbPBzS-). An important observation for the mixed monolayers is that E0/+1 and ∆Ep,1/2 depend only on the concentration of redox-active porphyrin, not on the total porphyrin concentration. As is observed for the neat porphyrin monolayers, both E0/+1 and ∆Ep,1/2 increase as the concentration of redox-active species in the mixed monolayers increases (not shown). 2. Electron-Transfer Characteristics. The large difference between potentials for the first oxidation process of ZnPBzSand FbPBzS- allows the latter molecule to be used as a nonredox active diluent in studies of the electron-transfer rates for the first-oxidation of the former molecule versus both the surface concentration of the redox-active species, ΓZnP, and the total surface concentration, Γtotal (ZnPBzS- + FbPBzS-). The results

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Figure 6. AFM images of the (a) Si(100), (b) Au(111), and (c) TiN microelectrode surfaces before porphyrin attachment. Figure 5. (Top panel) Standard electron-transfer rate constants, k0, versus surface concentration for the first oxidation process (E0/+1) of ZnPBzS- in mixed ZnPBzS-/FbPBzS- monolayers on Au(111) (TBAH electrolyte). The total surface concentration, Γtotal (ZnPBzS- + FbPBzS-) is indicated on the abscissa. The points marked by the symbols are for the indicated surface concentration of ZnPBzS-, ΓZnP. The solid lines are least-squares fits to the data. (Bottom panel) Standard electrontransfer rate constants, k0, versus surface concentration of ZnPBzSmonolayers on Au(111) (TBAH supporting electrolyte). The solid squares are for neat ZnPBzS- monolayers. The open circles are for mixed ZnPBzS-/FbPBzS- monolayers with varying amounts of ZnPBzS- at the same total surface concentration of ZnPBzS- + FbPBzS-, Γtotal ≈ 4.8 × 10-11 mol cm-2. This value of Γtotal is marked by the vertical dashed line in the top panel.

of such studies are presented in Figure 5. The top panel of the figure plots the k0 values versus Γtotal. The points marked by the symbols are for the indicated surface concentration of ZnPBzS-. The solid lines are least-squares fits to the data. As can be seen, the electron-transfer rate depends only on the surface concentration of the redox-active species, ZnPBzS-, and not on the total surface concentration, ZnPBzS- + FbPBzS-. The bottom panel of Figure 5 plots the k0 values for mixed ZnPBzS-/FbPBzS- monolayers with varying amounts of ZnPBzS- (open circles) at the same total surface concentration of ZnPBzS- + FbPBzS-, Γtotal ≈ 4.8 × 10-11 mol cm-2. This value of Γtotal is marked by the vertical dashed line in the top panel. For comparison, the k0 values for a neat ZnPBzS- monolayer versus surface concentration (solid squares) are also plotted in the figure. The key observation is that the electron-transfer rates for similar values of the surface concentration of ZnPBzS- in the neat versus mixed monolayers are the same. Plots at other

values of Γtotal (not shown) yield similar results. Accordingly, more densely packed mixed ZnPBzS-/FbPBzS- monolayers that contain relatively small amounts of redox-active porphyrin exhibit relatively fast electron-transfer rates, similar to those of sparse monolayers of neat ZnPBzS-. C. AFM Studies of the Si(100), Au(100), and TiN Surfaces. The observation that the electron-transfer rates of the porphyrins in monolayers on TiN are uniformly faster than those in monolayers on Si(100) and Au(111) prompted us to investigate the morphology of the surfaces using AFM. AFM images of the three types of microelectrodes are shown in Figure 6. The images shown are for clean microelectrodes prior to deposition of the porphyrin. Because the Si(100) and TiN microelectrodes are subjected to the high-temperature baking procedure for porphyrin attachment, AFM images of these electrodes were also obtained after solvent deposition (without porphyrin) and baking. The AFM profiles of the three types of microelectrodes indicate that the TiN surface is significantly rougher (14 ( 1 Å) than either the Si(100) or Au(111) surface (4 ( 1 Å and 7 ( 1 Å, respectively). In the case of the Si(100) and TiN surfaces, the high-temperature baking treatment had no measurable effect on the surface roughness. The rougher surface of the TiN microelectrode indicates that the true surface area versus the geometrical (planar) area is larger than that of the true versus geometrical areas of the smoother Si(100) and Au(111) microelectrodes. Owing to heterogeneity across the surface profile (Figure 6), it is difficult to estimate just how much larger is the true versus geometrical area. Regardless, the rougher surface of the TiN microelectrode dictates that the porphyrin surface

Electron-Transfer Rates of Porphyrin Monolayers concentration on this microelectrode is lower than that calculated based on the geometrical area versus the porphyrin surface concentrations on the Si(100) and Au(111) microelectrodes. IV. Discussion The studies reported herein on the ZnPBzX- monolayers permit a systematic exploration of the factors that influence the electron-transfer rates and how these factors interplay with the surface concentration of the redox-active molecules. The factors explored include the effects of varying the counterions, the type/ morphology of the electroactive surface, and the degree of charge screening of the molecules. One very general observation is that the electron-transfer rates, while sensitive to these factors, are confined to a relatively narrow range. At a given surface concentration of redox-active species, variations in the counterion and surface alter the rates by no more than a factor of 5. Screening effects result in a somewhat larger variation in the rates, ∼20-fold over the surface concentration range examined. In the sections below, we discuss the electron-transfer characteristics of the porphyrin monolayers in more detail. The discussion is confined to the electron-transfer behavior and does not deal with other factors such as the thermodynamics of the redox process or redox heterogeneity. We have previously discussed these issues in detail for porphyrin monolayers and the reader is referred to this earlier work.2d A. Effects of Counterions. The observation that the electrontransfer rates of the porphyrin monolayers are influenced by the nature of the size/mobility of the counterions is generally consistent with earlier studies of other types of monolayers that have shown that bulky counterions generally lead to slower rates.9,10 In the case of the porphyrin monolayers, the similar electron-transfer rates observed in TBAH and TBAP electrolytes are consistent with the fact that the PF6- and ClO4- anions are approximately spherical and comparable in size and likely exhibit similar mobilities in the solvent (propylene carbonate). Perhaps more surprising is the observation that the electrontransfer rates in the EMI-IM ionic liquid are similar to those in TBAH and TBAP electrolytes. The (CF3SO2)2N- anion of the ionic liquid is significantly larger than either PF6- or ClO4-, and there is no reason a priori to expect a similar mobility to these smaller anions. Regardless of the relative mobilities of the anions, it might be expected that at higher porphyrin surface concentrations, the larger (CF3SO2)2N- anion would be less able to interpenetrate the monolayer and provide charge compensation for the oxidized molecules. One possible explanation for the similarity in the electron-transfer behavior of TBAH, TBAP, and EMI-IM could be that the PF6- or ClO4- are effectively larger and less mobile than might be expected due to the formation of a solvation shell. The observation that the electron-transfer rates in TBACl are generally faster than those in the TBAH or TBAP (or EMI-IM) is qualitatively consistent with the fact the Cl- is much smaller and should be more mobile than the PF6- or ClO4- [or (CF3SO2)2N-] anions. The smaller Cl- anion should also be able to better interpenetrate the monolayers at higher surface coverage. This view is supported by the observation that the electron-transfer rates do not fall off as rapidly as a function of surface concentration in TBACl. The faster electron-transfer rates in the EMI-IM ionic liquid are also qualitatively consistent with the fact that the (NC)2N- anion is relatively small and mobile. The increased mobility of (NC)2N- relative to (CF3SO2)2N- is reflected in the significantly larger conductivity of EMI-DCA (∼27 mS cm-1) versus EMI-IM (∼8 mS cm-1) (A. Hawkins, private communication). The (NC)2N- anion is

J. Phys. Chem. C, Vol. 112, No. 15, 2008 6179 also planar and bent and might more readily penetrate between the planes of the porphyrin macrocycles, particularly at higher surface concentrations. B. Effects of Surface Type/Morphology. The observation that the electron-transfer rates of the porphyrin monolayers on TiN are faster than those on either Si(100) or Au(111) might suggest that either the electrical characteristics of the former surface are distinctly different from those of the latter two or that other factors are at play. However, if electrical properties of the surfaces were governing the rates, it might be expected that a systematic trend in the rates would be observed from the less to more conductive substrate. Contrary to this observation, the electron-transfer rates on the conductor Au(111) are similar to those on the p-type semiconductor Si(100), whereas the rates on the semiconductor TiN, which is more metal-like than Si(100), are faster than on either Au(111) or Si(100). A more plausible explanation for the faster electron-transfer rates on TiN versus Si(100) or Au(111) lies in the surface morphologies of the former versus latter substrates. In particular, the AFM studies indicate that the TiN surface is much rougher than either the Au(111) or Si(100) surfaces. The larger surface roughness for TiN requires that the actual surface area is larger than the geometrical area of the surface. Thus, the actual surface concentration of the porphyrins on the TiN surface is lower than that calculated based on a planar surface. This in turn indicates that the abscissa of the rate plot shown for TiN in Figure 3 should be rescaled to reflect the lower actual surface concentration of the porphyrins. The amount of rescaling is not certain; however, any rescaling to lower surface concentrations would bring the rates on TiN more in line with those observed for the porphyrins on the smoother Si(100) and Au(111) surfaces. C. Effects of Charge Screening. The observation that the electron-transfer rates of the porphyrin monolayer are most strongly affected by the concentration of the redox-active species versus the total porphyrin surface concentration strongly suggests that space-charge effects among the oxidized molecules are predominant in determining the redox kinetics. This view is further supported by the observations. (1) The increases in E0/+1 and ∆Ep,1/2 with increases in surface concentration are similar for all the electrolytes and surfaces. (2) The values of E0/+1 and ∆Ep,1/2 in the mixed monolayers only depend on the concentration of redox-active species, not the total porphyrin surface concentration. The space-charge effects are somewhat mitigated by smaller more mobile counterions; however, they cannot be eliminated. This picture is generally consistent with the results of studies on other types of monolayers.9,10,19-21 The apparent inability of the redox-inactive porphyrins to electrically insulate/screen the redox-active porphyrins in the mixed monolayers is puzzling. Previous studies of other types of monolayers have shown that burying a redox center in an insulating molecular matrix has a significant effect on the electron-transfer kinetics.9,10 One possible explanation for the absence of such effects for the porphyrin monolayers is that all of the surface concentrations attainable (