Electrochemical Rectification of Redox Mediators Using Porphyrin

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Electrochemical Rectification of Redox Mediators Using Porphyrin-Based Molecular Multilayered Films on ITO Electrodes Marissa R. Civic, and Peter H. Dinolfo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05643 • Publication Date (Web): 13 Jul 2016 Downloaded from http://pubs.acs.org on July 19, 2016

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ACS Applied Materials & Interfaces

Electrochemical Rectification of Redox Mediators Using Porphyrin-Based Molecular Multilayered Films on ITO Electrodes Marissa R. Civic and Peter H. Dinolfo* Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, 125 Cogswell, 110 8th Street, Troy, NY 12180, USA *Corresponding author. E–mail: [email protected]

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Abstract

Electrochemical charge transfer through multilayer thin films of zinc and nickel 5,10,15,20tetra(4-ethynylphenyl) porphyrin constructed via copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) “click” chemistry has been examined. Current rectification towards various outersphere redox probes is revealed with increasing numbers of layers, as these films possess insulating properties over the neutral potential range of the porphyrin, then become conductive upon reaching its oxidation potential. Interfacial electron transfer rates of mediator-dye interactions towards [Co(bpy)3]2+, [Co(dmb)3]2+, [Co(NO2-phen)3]2+, [Fe(bpy)3]2+, and ferrocene (Fc), all outer-sphere redox species, have been measured by hydrodynamic methods. The ability to modify electroactive films’ interfacial electron transfer rates, as well as current rectification towards redox species, has broad applicability in a number of devices, particularly photovoltaics and photogalvanics.

Keywords: electrochemical rectification, layer-by-layer assembly, wall-jet electrochemistry, interfacial electron transfer rates, multilayer electrochemistry

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Introduction Tailoring electrode surfaces to control charge transfer reactions is a critical aspect in the design of many types of electronic devices. Macromolecular electronics1, biosensors2–4, catalytic devices5, and solar energy devices6, among others, all utilize current rectification in order to successfully function. Researching current rectification of electrodes is therefore of great interest due to its broad application.2,3,7–12 Electrochemical current rectification is the process by which charge transfer between a modified electrode surface and solution phase redox probe is promoted in one direction, while limited in the opposite. The direction of charge transfer is determined by the relative thermodynamic potentials of the surface bound species and redox probe. Figure 1 shows a schematic representation of anodic current rectification of the Faradaic response of redox probe M at a modified electrode surface. The direct charge transfer (both cathodic and anodic, right side of Figure 1) between M and the electrode is blocked by the insulating properties of the surface bound film. Anodic charge transfer is possible when the electrode reaches the oxidation potential of the redox active film, which then mediates oxidation of M (left side of Figure 1).1 The cathodic reaction, however, is blocked due to unfavorable thermodynamics of the film potential versus that required to reduce M.

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Figure 1. Schematic representation of anodic current rectification of a surface-confined redox active film towards a solution based redox probe, M. Electrochemical current rectification was first reported by Murray and co-workers between discrete layers of redox active polymers comprised of Fe, Ru, and Os polypyridyl complexes on electrode surfaces.13–15 Since then, numerous other examples have been described in the literature. Mallouk and co-workers described the rectifying properties between cationic and anionic redox probes at electrode surfaces modified with zeolites, pillared clay particles, and zirconium-phosphonate-based multilayers.16 Wrighton and co-workers showed rectification of electrodes coated with quinone- and viologen-based polymers could be controlled by the pH of the electrolyte.10,17,18 Kubiak and co-workers were able to show current rectification towards the methyl ester of cobaltocenium carboxylate and methyl viologen via nickel clusters assembled on Au electrodes.11 Several other examples are based on Au electrodes modified with nalkanethiols. Uosaki et. al. showed that ferrocene terminated undecanethiol on Au could mediate the reduction of FeIII in solution.19 Creager et. al. was able to show rectification with Au electrodes modified with ferrocenecarboximide terminated alkanethiols of various lengths towards ferrocyanide ions in solution.7 Crooks et. al. have shown ferrocyanide ions can be 4


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effectively rectified at a gold electrode via redox active dendrimers and n-alkanethiols.9 Recently, the Haga20–22 and van der Boom23,24 groups, among others,25 have examined the electrochemical properties of mixed multilayer films comprised of Ru, Os, and Fe polypyridyl complexes assembled on ITO electrodes. Through judicious selection of the linking groups and molecular precursors, they have been able to control the direction and rates of electron transport through the multilayer films. Our group has developed a methodology utilizing copper(I)-catalyzed azide–alkyne [3+2] cycloaddition chemistry (CuAAC)26,27 to construct molecular multilayer assemblies on electrode surfaces using a range of building blocks including porphyrins28–31 and perylene diimides.32 This method employs a layer-by-layer (LbL) approach that links alternating molecular components through self-limiting reactions at an interface. LbL methods, a bottom-up approach to nanoscale fabrication, are facile, broadly applicable, and easily implemented into surface functionalization techniques.33–36

Figure

2

shows

a

schematic

of

Zn(II)

5,10,15,20-tetra(4-

ethynylphenyl)porphyrin (ZnTPEP) and 1,3,5-tris(azidomethyl)benzene (N3-Mest) multilayers used in this study assembled on indium tin oxide (ITO) electrodes.

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Figure 2. Schematic representation of porphyrin-based molecular multilayers fabricated using CuAAC in a LbL approach on ITO electrodes. Our previous studies demonstrated an increase in performance of p-type dye-sensitized solar cells (DSC) incorporating ZnTPEP multilayer functionalized photocathodes.37 Our results indicated photovoltage was improved with increasing layers when the commonly used iodide/triiodide redox couple was replaced with cobalt-based outer-sphere redox shuttles [Co(bpy)3]3+ and [Co(sep)]3+ where “bpy” is 2,2’-bipyridine and “sep” is sepulchrate. We proposed based on results that thicker multilayer structures block electron recombination reactions between the reduced mediator and underlying electrode surface. One of the major contributing factors to DSC inefficiencies is photovoltage loss due to the relatively high potential of commonly used I−/I3−.38 Efforts to replace I−/I3− with cobalt complexes have increased significantly6 since an early report in 2001.39 One advantage of utilizing cobalt and other transition metal coordination complexes is the ability to tune the redox potential of the mediator to match that of the sensitizing chromophore.40,41 A drawback of these outer-sphere electron-transfer reagents is the relatively fast recombination kinetics between a photoinjected electron in the semiconductor and the oxidized mediator as compared to I−/I3−. In 6


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order to reduce the recombination rates, several approaches have been taken including deposition of ultra-thin layers of alumina on the semiconductor surface,42–44 as well as increasing the steric bulk of the mediators45 or dyes.46–48 In fact, the DSC with the current record efficiency utilizes a [Co(bpy)3]2+/3+ redox couple in conjunction with a bulky Zn porphyrin push-pull sensitizer.46 As the future of these devices lies with these types of redox mediators, the rectification property of our molecular multilayers is of current interest. In our earlier work on the development of this multilayer fabrication technique, we were able to observe electrochemical current rectification of the Faradaic response of [Co(bpy)3]2+/3+ with a ZnTPEP-multilayer modified ITO electrode such as the one shown in Figure 2. The multilayer film showed insulating properties in its neutral form which blocked the direct oxidation and reduction of [Co(bpy)3]2+/3+, and then became conductive upon reaching the porphyrin oxidation potential, mediating the oxidation of [Co(bpy)3]2+.31 This blocking layer experiment was initially performed to test the quality of the film and look for defects, but the aqueous electrolyte employed did not allow us to directly examine the electrochemical response of the porphyrin multilayer. Herein we report molecular multilayers of ZnTPEP assembled via CuAAC induce unidirectional current flow towards various outer-sphere redox species, effectively acting as a passivation layer on ITO working electrodes. We examine this rectification in films with one through four layers of ZnTPEP, as well as use wall-jet electrochemistry to determine interfacial electron transfer rates between four layers of ZnTPEP and outer-sphere redox species [Co(bpy)3]2+, [Co(dmb)3]2+, [Co(NO2-phen)3]2+, [Fe(bpy)3]2+, and Fc0/+.

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Results & Discussion Selection of Redox Probes Figure 3 contains each redox probe examined with its respective potential. All potentials listed herein are referenced to the ferrocene/ferrocinium redox (Fc0/+) couple. Polypyridyl cobalt complexes were chosen due their outer-sphere redox mechanism and their recent incorporation into DSCs.6,49 These species’ redox potentials are easily tunable and were chosen to approach that of ZnTPEP. An iron polypyridyl species [Fe(bpy)3](PF6)2 was selected with a midpoint potential past the first oxidation of ZnTPEP.

Figure 3. Midpoint potentials for redox probes examined. Current Rectification Studies The top panel of Figure 4 contains representative CVs for one through four layers of ZnTPEP assembled on an ITO electrode and are consistent with previous reports.30 All electrochemistry herein was collected using anhydrous acetonitrile with 0.1 M TBAPF6 electrolyte under a nitrogen atmosphere. The peak currents for the ZnTPEP0/+ at +0.50 V and ZnTPEP+/2+ at +0.81 V show a linear response with scan rate, consistent with a surface bound species (Figure S1).50

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The integrated charge for both waves also increases with additional layers of ZnTPEP added to the film, closely matching the increase in UV-visible absorbance (Figure S2).30 The lower panel of Figure 4 show CVs for [Co(bpy)3]2+ over varying layers of ZnTPEP on ITO. A clean working electrode produces a typical faradaic response for diffusion-controlled species centered at -0.27 V. Addition of one to four layers of ZnTPEP to an ITO working electrode via the CuAAC based LbL methodology changes the current response for [Co(bpy)3]2+/3+. The oxidation wave shifts anodically with increasing layers until it reaches the onset of ZnTPEP film oxidation at approximately +0.30 V and remains consistent after two layers. The oxidation wave sharpens and maintains approximately the same current density at about +0.25 V from two to four layers. The ZnTPEP film has a driving force of approximately 0.77 V towards [Co(bpy)3]2+ oxidation, thus allowing an alternate pathway for mediator oxidation via the multilayer film. The lack of any observed current at the standard potential of the [Co(bpy)3]2+ implies the film is densely packed with minimal pinhole defects. On the other hand, when examining the reductive return wave for [Co(bpy)3]3+/2+, adding successive layers of ZnTPEP results in a cathodic shift and a diminished current density. This effect is due to the insulating properties of the neutral ZnTPEP film, which would require a tunneling mechanism for charge transfer from the underlying ITO electrode to [Co(bpy)3]3+. The combined effects of mediated oxidation of [Co(bpy)3]2+ by ZnTPEP+ and diminished cathodic response due insulating nature of the neutral film result in a large current rectification response for [Co(bpy)3]2+/3+ that increases with thickness of the ZnTPEP multilayer film.

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Figure 4. Top: CVs of one through four layers of ZnTPEP on ITO. Bottom: CVs of a 1 mM solution [Co(bpy)3]2+ at a clean ITO electrode (solid red line), SAM-functionalized ITO (long dashed orange), and one (dashed yellow), two (short dashed green), three (dash-dot cyan), and four (purple dash-dot-dot) layers of ZnTPEP on ITO. The vertical dashed black lines are located at the ZnTPEP0/+ and ZnTPEP+/2+ potentials. 10


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Figure 5 shows CVs of all mediators at an ITO electrode modified with four layers of ZnTPEP. The rectification effect towards the mediators is greatest for the thickest films examined. CVs of each mediator at one through four layers of ZnTPEP are available in the Supporting Information. A significant peak splitting is seen for [Co(dmb)3]2/3+ (Figure S3), [Co(bpy)3]2/3+ (Figure 4), and [Co(NO2-phen)3]2/3+ (Figure S4) after addition of only one layer of ZnTPEP due to the insulating nature of the multilayer film. CVs of these mediators at two or more layers shifts the oxidation wave anodically to the onset of the film oxidation, consistent with mediated charge transfer via ZnTPEP0/+ as outlined in Figure 1. The reductive wave continues to decrease in peak current and shift cathodically due to the increasing thickness of the neutral film, consistent with a tunneling process. This results in an increasing rectification effect increases with additional ZnTPEP layers. The hypothesis for ZnTPEP0/+ mediated charge transfer from the solution mediators is supported by CVs taken over a range of mediator concentrations. Figure 6 shows CVs of 0.1, 0.3, and 1.0 mM [Co(dmb)3]2/3+ at an ITO electrode modified with four layers of ZnTPEP. As concentration of the redox probe is increased, the anodic current increases as expected with the onset potential remaining the same.

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Figure 5. CVs for [Co(bpy)3]2+ (purple line) , [Co(dmb)3]2+ (dash-dotted cyan), [Co(NO2phen)3]2+ (dashed blue), Fc (medium dashed green), and [Fe(bpy)3]2+ (short dashed red) over four layers of ZnTPEP on ITO. The vertical dashed black lines are located at the ZnTPEP0/+ and ZnTPEP+/2+ oxidation potentials, while the smaller solid lines are located at each mediator’s thermodynamic potential.

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Figure 6. CVs of [Co(dmb)3]2+ over a film of four layers of ZnTPEP on ITO. Black solid line, blue long dash, and red medium dash are 1000, 300, and 100 µM respectively. The dashed black line at 0.5 V corresponds to the midpoint potential of ZnTPEP0/+.

In order to confirm the observed oxidative shift of redox probes’ oxidation is due to electrochemical mediation via ZnTPEP+, an analogous study was done over a film of four layers of NiTPEP. NiTPEP has an oxidation potential of 0.74 V, which is 240 mV more positive than that of ZnTPEP (0.50 V). Due NiTPEP’s higher potential, the anodic current response for the redox probes should shift a corresponding amount if the mediated process dominates the anodic current response for thicker films. Figure 7 compares CVs of [Co(dmb)3]2+ over four layers of ZnTPEP and NiTPEP. The oxidative peak for [Co(dmb)3]2+ at a NiTPEP4 modified electrode is shifted +680 mV vs. its standard potential, as compared to a +490 mV shift for ZnTPEP4. The additional +190 mV shift is similar to the difference in oxidation potential of the two porpohyrins and is consistent with the mediated oxidation of [Co(dmb)3]2+ by the films. The CVs of [Co(bpy)3]2+ and [Co(NO2-phen)3]2+ at a NiTPEP4 modified electrode show similar anodic shifts (Figure S5). 13



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Figure 7. CVs of 1 mM [Co(dmb)3]2+ over four layers of ZnTPEP (red line) and NiTPEP (black line) on ITO. The oxidation of the NiTPEP4 films appears as a reversible wave at +0.7V.

The electrochemistry of Fc at ZnTPEP and NiTPEP modified ITO electrodes do not result in an anodic shift of the oxidation wave as is seen for the larger charged cobalt complexes. The smaller, neutral Fc may be able to penetrate the multilayer films, resulting in oxidation at the thermodynamic potential. There is a slight decrease in the cathodic current at higher layers of ZnTPEP modified electrodes. This apparent rectification effect is likely the result of the ferrocinium ions being expelled from the non-polar neutral film. However, Fc oxidation is clearly not mediated by the ZnTPEP as the peak current does not shift towards the ZnTPEP0/+ wave; rather, it remains centered at the thermodynamic potential (Figure S6).51–54 A similar response is seen for NiTPEP films (Figure S5). The electrochemical response of Fe(bpy)32+ over a ZnTPEP film presents a different situation. The midpoint potential of Fe(bpy)32/3+ (+0.60 V) is +0.10 V more positive than ZnTPEP0/+ and +0.21 V less than ZnTPEP+/2+. As a result, electrochemically generated ZnTPEP+ cannot oxidize the [Fe(bpy)3]2+. However, ZnTPEP2+, does have the driving force required to oxidize 14


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[Fe(bpy)3]2+. Thus, there is a slight anodic shift of the oxidation of Fe(bpy)32+ to the onset of the ZnTPEP2/+ wave with increasing layers (Figure S7). A rectifying effect is expected here, similar to the Co mediators, although Fe(bpy)33+ has the thermodynamic driving force required to oxidize ZnTPEP. Thus, the reduction peak of Fe(bpy)33+ is shifted cathodically to the ZnTPEP0/+ wave. However, the maximum peak splitting of Fe(bpy)33+ and rectifying effect is limited by the close spacing at the ZnTPEP0/+ and ZnTPEP2/+ potentials. The response of [Fe(bpy)3]2+ at a NiTPEP4 film further supports this (Figure S5-D). The oxidation potential of NiTPEP is almost identical to that of [Fe(bpy)3]2+. At four layers of NiTPEP there is no observable peak splitting as compared to a bare electrode, suggesting the NiTPEP film can also mediate both the oxidation and reduction process.

Interfacial Electron Transfer Rates In order for these multilayer films to be applicable in devices utilizing electrochemical rectification, the interfacial electron transfer rates need to be determined and optimized. We employed a forced-convection hydrodynamic flow method to provide a quantitative analysis of the interaction between mediators and the oxidized films. Rotating disk electrochemistry (RDE) is the typical hydrodynamic method used to evaluate interfacial electron transfer rates; however, it is difficult to design RDE electrodes containing conductive ITO glass.55 Wall-jet electrochemistry offers an alternative forced-convection hydrodynamic method that allows for the use of a wider range of planar electrode materials, including ITO.56–60 In order to conduct wall-jet electrochemistry experiments, we constructed our own apparatus based upon various groups’ designs (Figure S11).55,57 This apparatus, in brief, is composed of a planar electrode with a defined working area and a jet that delivers a centered stream of electrolyte to continuously replenish the redox species under investigation. The stream is normal 15



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to the electrode and applies a hydrodynamic flow profile, creating a limiting current characteristic of a diffusion-controlled electrochemical system. Provided that the electrode diameter is sufficiently larger than that of the jet, the current density across the electrode interface can be treated similarly to RDE.57,59 Equation 1 is the representative Levich equation for a wall-jet hydrodynamic system, where ilim is the limiting current (A), n is the number of electrons, F is Faraday’s constant (C/mol e-), C is the concentration of the redox species (M), DS is the diffusion coefficient of the species, ν is the kinematic viscosity (cm2/s), a is the diameter of the jet determined by the gauge of the needle (cm), A is the working electrode area (cm2), and υ is the flow rate (cm3/s).57,59

= 0.898

(1)

Taking advantage of this flow profile allows determination of interfacial rate constants through the Koutecky-Levich relation (Equation 2)50,55,57 where ik is the kinetic limiting current.

* * !."#"$%&'( ) + , -

= +

(2)

The following rate law (Equation 3) can then be applied where Γfilm is the film coverage (mol/cm2) and kET is the interfacial electron transfer rate constant. Film coverage was determined by integrating surface CVs for four layers of ZnTPEP, and dividing that obtained coverage value by four to approximate the coverage at the surface of the film in contact with the mediator.

. = Γ0 123 (3) 16


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Equation 4 introduces keff, which represents the rate constant without factoring in the coverage of the surface-bound electroactive species.

1400 = 50 123

(4)

Figure 8 contains two examples of linear sweep voltammograms (LSVs) of redox mediators (Fc0 and [Co(dmb)3]2+) over four layers of ZnTPEP on ITO taken using our wall-jet forced convection instrument. Corresponding LSVs for remaining mediators are included in the Supporting Information (Figure S12-Figure S14).

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Figure 8. Linear sweep voltammograms of 1 mM Fc (top) and 1 mM [Co(dmb)3]2+ (bottom) at varying flow rates. Vertical line is at the midpoint potential of the ZnTPEP0/+ film. A Levich analysis (Figure S15) performed on each redox mediator over four layers of ZnTPEP on ITO reveals a linear limiting current with respect to ν0.75 (flow rate), confirming an electrochemical system that is mass-transfer limited.50,57 A Koutecky-Levich analysis for each mediator is shown in Figure 9. If the inverse of the limiting current is linear with respect to ν0.75, the intercept reveals the kinetic limiting current, as shown previously in Equation 2.

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Figure 9. Koutecky-Levich analysis of each mediator: Fc (orange down triangle); [Co(bpy)3]2+ (red circles); [Co(dmb)3]2+ (yellow squares); [Co(NO2-phen)3]2+ (green diamonds); and [Fe(bpy)3]2+ (teal upward triangle). Limiting current was taken at 0.50 V (ZnTPEP0/+) for all mediators except [Fe(bpy)3]2+, which was taken 0.99 V. Application of the rate law (Equations 3 and 4) reveals the interfacial electron transfer rate constants between the redox mediators and the ZnTPEP film on the working electrode. These values are included in Table 1. Limiting currents were taken at the potential of ZnTPEP, except for in the case of [Fe(bpy)3]2+. A completely limiting current of [Fe(bpy)3]2+ was difficult to achieve, as scanning beyond than 1V irreversibly damages the films. In this case, the ilim was taken at 0.99 V, the section of the LSV that exhibited the most ilim character. The Levich analysis for [Fe(bpy)3]2+, therefore, is less linear than the other mediators.

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Table 1. Interfacial electron transfer rates and driving force for electron transfer for oxidation of various redox mediators (1 mM) at a working electrode with four layers of ZnTPEP.

Mediator

kET [(cm3)(mol-1)(s-1)]

keff [(cm)(s-1)]

∆GET (V)

Fc

9.4× 108

1.5 × 10-1

-0.50a

[Co(bpy)3]2+

5.4 × 108

4.4 × 10-2

-0.70a

[Co(dmb)3]2+

6.5 × 108

5.3 × 10-2

-0.77a

[Co(NO2-phen)3]2+

3.4 × 108

2.8 × 10-2

-0.12a

[Fe(bpy)3]2+

3.3 × 108

2.7 × 10-2

-0.22b

a

Calculated from the ZnTPEP0/+ potential at 0.50V. b Calculated from the ZnTPEP+/2+ potential at 0.81V. Examining the rates for each redox probe, values vary; Fc0 has the highest while [Fe(bpy)3]2+ has the lowest, as evidenced by its incomplete rectification seen in Figure S6. As Fc0 is the smallest of the five, it is likely that it penetrates the films, increasing the current and hence the kET. Outer-sphere cobalt redox mediators are considerably larger species and their kET values are an order of magnitude smaller than that of Fc0. Cobalt species also undergo a change in spin upon oxidation/reduction, hence lengthening the time of electron transfer processes. Comparing ∆GET for the oxidation of cobalt redox probes by ZnTPEP+ (ZnTPEP2+ for [Fe(bpy)3]2+) to the trend in kET for this process, an increase in rate is seen for species with the greatest driving force for electron transfer ([Co(dmb)3]2+ and [Co(bpy)3]2+). Driving force for [Co(NO2-phen)3]2+ is 0.65 V less than the bipyridyl-based species, and hence a slower kET is observed. With a driving force of -0.22 V for oxidation of [Fe(bpy)3]2+ by ZnTPEP2+, this redox probe results in an electron transfer rate comparable to cobalt-based species. However, the 20


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similarity in kET for these species (less than a factor of 2) for a wide range of ∆GET (0.65 V) suggest that the interfacial electron transfer process is not the limiting factor. Rather, electron transfer, or hopping, through the porphyrin multilayer film may be limiting the overall interfacial rate as the calculated values in Table 1 will be a product of these rates. We are currently exploring the thickness and molecular linker dependence on the through-film charge transfer rates of these porphyrin multilayers systems.

Conclusions We have examined the surface passivation properties of films assembled via CuAAC click chemistry. We have shown our films possess a rectifying property that we have demonstrated towards five different outer-sphere redox mediators (Fc0/+, [Co(bpy)3]2+, [Co(dmb)3]2+, [Co(NO2phen)3]2+, [Fe(bpy)3]2+). In these systems, current is mediated by the electroactive film when in contact with a redox probe. This process was examined by cyclic voltammetry. Using hydrodynamic forced convection voltammetry, we calculated interfacial electron transfer rates between the mediators and films of ZnTPEP. The inherent ability for films assembled via this LbL method to encourage unidirectional current flow towards outer-sphere redox species has broad application in many types of devices.1,2,13

Experimental Materials: All solvents, ACS grade or higher, were purchased from Sigma-Aldrich or Fisher Scientific and used as received. Anhydrous acetonitrile used for all electrochemical experiments was purged with nitrogen and recirculated for 48 hours through a solid-state column purification system (Vacuum Atmospheres Company, Hawthorne, CA) prior to use.61 Toluene was purged with nitrogen and stored under nitrogen over 4A molecular sieves. Sodium ascorbate (Aldrich) 21



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Page 22 of 38

was used as received. Zinc(II) 5,10,15,20-tetra(4-ethynylphenyl) porphyrin (ZnTPEP),62 nickel (II) 5,10,15,20-tetra(4-ethynylphenyl) porphyrin (NiTPEP),29 tris-(benzyltriazolylmethyl)amine (TBTA),63

1,3,5-tris(azidomethyl)benzene64,

and

(3-azidopropyl)trimethoxysilane65

were

synthesized according to literature methods. Indium tin oxide (ITO) coated glass was purchased from Delta Technologies (polished float glass slides, one-side coated, Rs of 4-8 Ω). Tetrabutylammonium hexafluorophosphate (Aldrich) was recrystallized twice from hot ethanol and dried under vacuum overnight. Ferrocene (Aldrich) was purified via sublimation. Cobalt and Iron complexes. All polypyridyl cobalt and iron complexes, [Co(bpy)3](PF6)2, [Co(dmb)3](PF6)2, and [Co(NO2-phen)3](PF6)2 were synthesized following literature procedure47 with slight modifications. In minimal methanol, 1 equivalent of CoCl2•6H2O and 3.3 equivalents of the corresponding ligand were stirred until dissolved. The resulting solution was yellow to brown. The solution was stirred under reflux for approximately four hours. In order to form the complex with the hexafluorophosphate counterion, a saturated, aqueous solution of potassium hexafluorophosphate was added drop-wise until no further precipitation occurred. The resulting solid was filtered, washed well with methanol then diethyl ether, and dried under high vacuum overnight. The product was further purified via anion exchange from the PF6- salt to the Cl- salt and back to PF6- in order to ensure high purity. Exchange to the Cl- salt was accomplished by dissolving the PF6- salt in acetone and adding saturated TBACl until no further precipitate formed. The resulting solid was filtered, washed with acetone, methanol, and diethyl ether. After each step involving solvation, the dissolved complex was filtered through a fine frit. [Fe(bpy)3](PF6)2 was synthesized following literature procedure with slight modifications.66 Briefly, FeCl2•6H2O was dissolved in a minimal amount of water. Three equivalents of 2,2’dipyridyl was added slowly with stirring to form a dark red solution. A saturated aqueous 22


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solution of sodium chloride was added to assist with recrystallization of dark red solid, [Fe(bpy)3]Cl2. The chloride salt was anion exchanged to the hexafluorophosphate salt as described vide supra for cobalt complexes. Purity of all complexes was confirmed with both NMR and cyclic voltammetry. Cyclic voltammograms for each redox probe are included in Figure 4 and Figure S7.

Azido Self-Assembled Monolayer Formation. Prior to SAM formation, ITO-coated glass slides were sonicated in dilute Alconox for 10 minutes then rinsed well with DI water. Slides were rinsed successively with acetone, dichloromethane, methanol, and water and dried with a stream of nitrogen, and submersed in 1:1:5 NH3:H2O2:H2O for 30 minutes. Upon removal, the slides were rinsed well with DI water, dried under a stream of nitrogen, and placed to further dry in a Schlenk flask at 0.1 mTorr for 1 hour. The appropriate volume of neat (3azidopropyl)trimethoxysilane to make a 1 mM solution was added to the Schlenk tube, which was then evacuated and purged with nitrogen three times. Dry toluene was transferred via cannula to the Schlenk tube, ensuring slides were submerged. The reaction vessel was then heated to 65-70 °C overnight. Upon cooling to room temperature, the ITO was removed and sonicated in toluene for 5 minutes then rinsed with acetone, dichloromethane, methanol, and water and dried under a stream of nitrogen. Slides were annealed at 75 °C for 3 hours under vacuum. Multilayer Formation. Multilayers were assembled using our previously reported method.31 In brief, for ethynyl porphyrin layers, the SAM functionalized ITO surface is placed for 10 minutes in contact with a DMSO solution containing less than 2% water, 1.3 mM ZnTPEP, 0.33 mM CuSO4, 0.36 mM TBTA, and 0.48 mM sodium ascorbate. After the 10 minute treatment, the 23



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Page 24 of 38

slide was rinsed with acetone, dichloromethane, methanol, 5 mM EDTA in 50/50 water/ethanol, and water then dried under a stream of nitrogen. The azido-linker reaction consists of a DMSO solution containing less than 8% water, 2.2 mM 1,3,5-tris(azidomethyl)benzene, 4.4 mM CuSO4, 4.8 mM TBTA, and 8.9 mM sodium ascorbate. Treatment for the linker layer was also 10 minutes and the same rinsing scheme was used. Multilayer growth was confirmed using UV-Vis spectroscopy. Electronic absorbance spectra were collected using a Perkin-Elmer Lambda 950 spectrophotometer with a solid sample holder that orients the sample normal to incident light. Background spectra of the SAM-functionalized ITO surface were subtracted from each spectrum. Electrochemistry. All cyclic voltammetry experiments were performed using a CHI440A (CH Instruments, Austin, TX) potentiostat. Electrolyte solution for all experiments was 0.1 M tetrabutylammonium hexafluorophosphate in anhydrous acetonitrile and was both prepared and deoxygenated with nitrogen immediately before use. All CVs were referenced against a ferrocene internal standard. Solution CVs were collected using a 1 mM solution of the corresponding mediator in 0.1 M TBAPF6 in anhydrous acetonitrile. The three electrode system used consisted of a Pt counter electrode, a Pt disk working electrode, and a Ag/AgCl wire pseudoreference electrode. The headspace of the cell was purged with a very low stream of nitrogen during scans. Electrochemical rectification CVs were collected using a three electrode setup containing a Pt wire counter electrode and a Ag/AgCl wire pseudoreference electrode. The working electrode was a functionalized ITO electrode defined by the cylindrically bored area of a Teflon cone pressed against the slide and filled with either 0.1 M TBAPF6 electrolyte or 1 mM solution of the

24


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corresponding mediator in 0.1 M TBAPF6.67 Scan rates for all surface scans were 1000 mV/s and 100 mV/s for solutions. All scans were directed anodically. Wall-jet experiments were conducted using an instrument consisting of a Harvard Apparatus Model 11 Plus single syringe pump (70-2208) containing a 10 mL Gas-Tight syringe with a needle that was cut and filed flat. The needle was lowered into a cell so that it was 4 mm above the surface of the working electrode. The cell design is described elsewhere.31,67 In brief, a hole bored in the bottom of a cylindrical Teflon cone defined the working area of the functionalized ITO electrodes (0.15 cm2 area). The Teflon cone was pressed firmly against the ITO electrode and filled with electrolyte and mediator (0.1 M TBAPF6 and 1 mM mediator in SPS acetonitrile). A Teflon plug was placed in the top of the cone with holes bored to fit snugly a nitrogen line, the wall-jet syringe, a Pt wire counter electrode, and a Ag/AgCl wire pseudo-reference electrode. A Levich analysis of the redox mediators at a bare ITO electrode are included in the supporting information (Figure S16).

ASSOCIATED CONTENT Supporting Information Additional electrochemical scans of the multilayer films with redox mediators. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS M.R.C. acknowledge a Slezak Memorial Fellowship. This material is based upon work supported by the National Science Foundation under CHE-1255100.

25



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TOC Graphic

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Electrochemical Rectification of Redox Mediators Using Porphyrin

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