ARTICLE pubs.acs.org/Langmuir
Reductive Electrografting of Benzene (p-Bisdiazonium Hexafluorophosphate): A Simple and Effective Protocol for Creating Diazonium-Functionalized Thin Films Nicholas Marshall and Jason Locklin* Department of Chemistry, Faculty of Engineering, and Center for Nanoscale Science and Engineering, University of Georgia, Athens, Georgia 30602, United States ABSTRACT: In this Article, we describe a protocol for surface functionalization of benzenediazonium hexafluorophosphate monolayers by in situ electrochemical reduction of bis(benzenediazonium) hexafluorophosphate. Due to the considerable difference in potential between the first and second reduction of this species, it is possible to form a high density of surface-bound diazonium groups by use of a mild potential which selectively reduces only one diazonium group per ring. The resulting diazonium-containing monolayer reacts readily with solutions of electron-rich aromatic compounds. The reaction with ferrocene produces a dense (2.7 1010 mol/cm2) ferrocene-containing monolayer through a GombergBachmann type arylation. The resulting ferrocene group exhibits relatively rapid electron transfer to the electrode due to the conjugated linker layer as measured by alternating current voltammetry (ACV) and cyclic voltammetry. Aromatic systems with π-donor substitutents (N,N-dimethylaniline, N,N,N0 ,N0 tetramethyldiaminobenzophenone, and hydroquinone) react through an azo-coupling to form monolayers linked to the surface through an azobenzene moiety. The redox properties of these electron-rich species tethered to the surface were observed and quantified using cyclic voltammetry. This simple and versatile functionalization procedure has a wide variety of potential applications in surface science and materials research.
’ INTRODUCTION The use of diazonium salts for surface functionalization has gained tremendously in popularity over the past decade, since the initial report by Saveant et al.1 of a simple procedure for electrografting functionalized benzenes onto a glassy carbon electrode. Since then, the procedure has been expanded to include a wide variety of increasingly complex systems attached to the benzene ring,24 heteroaryl rings,5 and perhaps most importantly a considerable range of substrate materials including gold,2,6 reduced silicon,7,8 and indium tin oxide (ITO)4,7 surfaces. The layers formed on these materials are generally very stable relative to those formed by self-assembly chemistries, being particularly resistant to heat, chemical degradation, and ultrasonication.6 Further advantages over common monolayer forming techniques such as self-assembly of silane monolayers on oxide or thiols on noble metals include the speed of electrografting diazonium salts (deposition times on the order of 10 s, compared to ca. 10 h for self-assembly) and the easy availability of fully conjugated monolayers. Conjugated monolayers are desirable for electronic applications due to their enhanced electron-transfer rates relative to alkyl-based monolayers. In light of the versatile, clean chemistry which diazonium salts display in reacting with various surfaces, it is perhaps surprising r 2011 American Chemical Society
that the complementary reaction where a surface-bound diazonium group reacts with an electron donor in solution has remained relatively unexplored. The Tour group has used the in situ diazotization of an oligo-phenylethynylene species terminated at one end with an aniline group and at the other with a diazonium tetrafluoroborate moiety9 to functionalize singlewalled carbon nanotubes10 and fullerenes.11 Schmelmer et al. have utilized the reduction of nitroaromatic monolayers with an electron beam, followed by diazotization and reaction with an enolate to form a surface bound azo radical initiators for polymer brush formation.12 Similarly, polyarylamine films have been formed on gold by reduction with metallic iron of monodiazotized p-phenylenediamine in the presence of bare gold.13 The resulting arylamine-functionalized film can be diazotized, and the authors report the immobilization of pyridine compounds, poly(amido amine) dendrimers, copper nanoparticles, and graphene flakes, as well as the initiation of radical polymerization by electrochemical reduction of the diazotized film. These approaches have yielded promising initial results, but require considerable synthetic Received: June 29, 2011 Revised: September 7, 2011 Published: September 20, 2011 13367
dx.doi.org/10.1021/la2024617 | Langmuir 2011, 27, 13367–13373
Langmuir work or complex experimental apparatus to form diazoniumterminated monolayers. A simple procedure based on electrografting of benzidine or p-phenylenediamine diazonium tetrafluoroborates was reported by Saveant et al.14 and degraded to prepare phenol-terminated monolayers on glassy carbon, although neither the electrochemistry of the diazonium salt nor further reaction of the monolayer was described. The potential advantage of diazonium-functionalized surfaces is considerable. While surface functionalization through electrografting of diazonium salts is a common strategy, it is not without limitations. Most isolable diazonium salts capable of clean electrografting are electron-deficient benzene derivatives with a substituent at the para position. Electron-rich diazo salts are known to be less stable, with extreme cases such as ferrocenediazonium salt apparently being available only by in situ preparation and in poor yield. More broadly, it is obvious that many systems will be incompatible with the extremely harsh conditions for diazotization, which involve either strong acids or oxidizing agents or both. In these cases, it is desirable to avoid the need to prepare a diazonium derivative of the species which is being grafted onto the surface. In fact, due to the tremendous reactivity of the diazonium group, immobilization of diazonium-functionalized monolayers should allow grafting of certain types of chemical species without any chemical modification, an approach that has obvious advantages in terms of convenience and simplicity. In this work, we demonstrate the facile surface immobilization of several prototypical electron-rich aromatic species using the electrografting of bis(benzenediazonium) hexafluorophosphate.
’ EXPERIMENTAL SECTION Materials. Acetonitrile, tetrahydrofuran (THF), concentrated sulfuric acid, ferrocene, p-phenylenediamine, sodium nitrite, 4,40 -bis(N,Ndimethylamino)benzophenone, sodium hexafluorophosphate, and tetrabutylammonium hexafluorophosphate were obtained from VWR. 3,4Ethylenedioxythiophene (EDOT) was obtained from Sigma-Aldrich. Ferrocene was purified by sublimation in an Erlenmeyer flask on a hot plate in air. N,N-Dimethylaniline was obtained from Eastman Kodak and passed over basic alumina before use. THF was distilled from sodium ketyl. Except where a purification procedure is indicated, materials were used as received. Benzene(bisdiazonium)hexafluorophosphate. A modified procedure based on that given by Vogel et al.15 was used for the diazotization of p-phenylenediamine. A suspension of 1.4 g of sodium nitrite (20.3 mmol) in 15 mL (0.27 mol) of conc. sulfuric acid was warmed to 70 °C to form nitrous acid and then cooled to 0 °C. Then, 1.0 g of p-phenylenediamine (9.3 mmol) was dissolved in 10 mL of glacial acetic acid and added dropwise to the diazotizing reagent over 10 min. CAUTION: Benzene(bisdiazonium) bisulfate may be explosive if isolated. After the addition was complete, the reaction was stirred for 30 min; during this time, the reaction mixture becomes a thick slush. A cool, saturated solution of sodium hexafluorophosphate (3.6 g, 21.4 mmol) was added dropwise with mechanical stirring followed by 25 mL of icecold deionized water. The beige precipitate of crude insoluble benzene(bisdiazonium)hexafluorophosphate was collected by filtration in a Buchner funnel and washed with ice-cold water, cold methanol, and diethyl ether. The salt was purified by dissolution in HPLC-grade acetonitrile, filtration, and precipitation by slow addition of an equal volume of diethyl ether. The microcrystalline white product was stored at 20 °C in the dark. To reuse the stored product, precipitation from acetonitrile was repeated before use. 2.6 g, 67% yield. 1H NMR (300 MHz, d3-CH3CN, rel. to TMS): 8.98 (s). 13C NMR (75 MHz, d3CH3CN, rel. to TMS): 135.35, 117.03.
ARTICLE
Electrochemical Measurements. Electrochemical measurements were made using a CH Instruments CHI832B bipotentiostat. Tetrabutylammonium hexafluorophosphate (0.1 M) in HPLC-grade acetonitrile was used as the supporting electrolyte. Potentials reported as vs Ag/Ag+ were taken vs a silver wire pseudoreference electrode for which the Fc/Fc+ redox couple in acetonitrile is found at 0.375 V. Potentials reported as vs AgNO3 are vs a nonaqueous Ag/Ag+ reference electrode containing 10 mM AgNO3 in acetonitrile for which the Fc/Fc+ couple is at 0.275 V. Alternating current voltammetry (ACV) measurements were taken using the control software of the CHI832B bipotentiostat. Applied frequencies of 100, 200, 500, 1000, and 2000 Hz were used. A silver wire pseudoreference electrode (Ag/Ag+) was used in all ACV experiments. Before collecting ACV data on a film, cyclic voltammetry was used to confirm the presence of the ferrocene group. In other respects, the same conditions were used in ACV as in cyclic voltammetry described above. Polarization-Modulation Infrared ReflectionAbsorption Spectroscopy (PM-IRRAS) Measurements. All PM-IRRAS measurements were made on a Nicolet model 6700 spectrometer equipped with a photoelastic modulation (PEM) accessory (ThermoNicolet, Madison, WI) using OMNIC 8.0 software. All measurements were made with the grazing angle set at 80°. Ellipsometry. Ellipsometric measurements were performed on a Multiskop (Optrel GbR) apparatus with a 632.8 nm HeNe laser light source. Thickness data for films were obtained by fitting ellipsometric data using integrated specialized software. Measurements reported are the average of at least three data points across the substrate.
Electrografting of Benzene(bisdiazonium)hexafluorophosphate. A 20 mg/mL solution of the bisdiazonium salt was prepared using 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile as the solvent. Gold slides (EMF Corporation, Ithaca, NY) were cut with a glass cutter, rinsed with isopropyl alcohol, dried with a stream of nitrogen, and cleaned in a plasma cleaner (Harrick Plasma) using argon plasma (18 W for 3 min). Gold and glassy carbon working electrodes were cleaned by polishing with 20 nm alumina abrasive. After cleaning, working electrodes were immediately transferred to the solution of bisdiazonium salt in supporting electrolyte, connected to the bipotentiostat, and cycled once from 0.2 V to 0.1 V vs the nonaqueous reference electrode. The freshly prepared diazonium-functionalized surface was immediately used in the next step.
Reaction of Aromatic Species with Diazonium-Containing Monolayers. A 0.1 M solution of the aromatic species was prepared before deposition of the monolayer. Dichloromethane was used as the solvent in the case of ferrocene, 4,40 -bis(N,N-dimethylamino)benzophenone, and N,N-dimethylaniline (DMA). Acetonitrile was used for hydroquinone. Immediately after deposition, the working electrode substrate with the diazonium containing monolayer was transferred to the solution and allowed to react for 30 s at room temperature. Afterward, the substrate was removed and rinsed with acetone, dichloromethane, and HPLC-grade acetonitrile.
Protonation and Deprotonation of Acid/Base Bearing Monolayers. Monolayers functionalized with hydroquinone and DMA were immersed in 1 M NaOH, 1 M HCl, or 0.01 M phosphate-buffered saline solutions for 30 s and rinsed with 18 MΩ water briefly and then blown dry in a stream of nitrogen gas. The thickness and static contact angle of the substrates were measured immediately after the protonation/ deprotonation reaction.
’ RESULTS AND DISCUSSION A schematic showing the process of electrografting benzene(bisdiazonium) hexafluorophosphate to generate benzenediazonium monolayers at a gold or glassy carbon electrode, and subsequent functionalization with an electron-rich aromatic species 13368
dx.doi.org/10.1021/la2024617 |Langmuir 2011, 27, 13367–13373
Langmuir
ARTICLE
Figure 1. Deposition of a benzenediazonium hexafluorophosphate monolayer at a gold or glassy carbon electrode, and subsequent functionalization with an electron-rich aromatic species.
Figure 2. Cyclic voltammetry vs Ag/Ag+ of benzenediazonium hexafluorophosphate in solution. The first cycle (1) displays both reduction waves; subsequent cycles (2 and 3) show only charging current and no further reduction. Supporting electrolyte: 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile.
is shown in Figure 1. Benzene(bisdiazonium) hexafluorophosphate has two distinct reduction waves observable by cyclic voltammetry, corresponding to the reductive cleavage of the first and second aryl CN bonds (Figure 2). While the second wave occurs at potential consistent with the reduction of other diazonium salts, the first occurs at a remarkably low potential, clearly distinct from and sharper than the second. By reducing the bisdiazonium salt at a potential above the first reduction wave but well below the second wave, it is possible to directly form monolayers on the electrode surface containing an active diazonium group. Figure 3 shows the cyclic voltammogram of a representative deposition, where the film was cycled from 0.3 to 0.02 V vs Ag/Ag+ pseudoreference. Gold substrates electrochemically treated in this manner showed the expected reduction wave on the first cycle, followed by dramatically diminished current upon subsequent cycles due to the formation of a blocking monolayer, which retards further reaction at the electrode interface. This surface electrochemistry qualitatively
Figure 3. Reductive deposition of the benzenediazonium layer from benzene(bisdiazonium) hexafluorophosphate solution in acetonitrile. Potentials are given vs Ag/Ag+ pseudoreference electrode. Supporting electrolyte: 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile.
resembles that of p-nitrobenzenediazonium salts,16 displaying a sharp reduction peak at a high potential corresponding to reduction of diazonium groups in solution followed by a broad wave at a lower potential caused by reduction of the surfacebound electron-withdrawing group. Subsequent cycles display only charging current due to the formation of a blocking layer on the electrode surface. Electrode substrates functionalized by a single voltammetric cycle through the first reduction wave of the bis-diazo salt displayed the reactivity expected of a diazonium-grafted surface. To perform an initial screening, we reacted diazonium substrates with 0.1 M solutions of hydroquinone and DMA in acetonitrile and dichloromethane. DMA-functionalized monolayers immersed in a buffer solution (pH 7.2) had a static contact angle of 71°. After immersion in a solution of dilute HCl, the static contact angle decreased to 23° due to protonation of the nitrogen moiety. Similarly, hydroquinone-functionalized monolayers in pH 7.2 buffer had a contact angle of 65°, and after treatment with aqueous NaOH solution the static contact angle became hydrophilic (19°). This simple experiment demonstrates that dimethylaniline and hydroquinone are immobilized at the diazonium surface and that active 13369
dx.doi.org/10.1021/la2024617 |Langmuir 2011, 27, 13367–13373
Langmuir
ARTICLE
Figure 4. Mechanism of ferrocene arylation with a diazonium salt.
Figure 5. (a) Cyclic voltammetry (100 mV/s) of ferrocene immobilized through the diazonium monolayer. (b) Laviron plot (E Ef) vs ln(v) of immobilized ferrocene. (c) Peak current vs scan rate for the film shown in (a).
Figure 6. (a) AC voltammogram with an applied frequency of 100 Hz and (b) fit of Ipeak/Ibackground vs AC frequency. Rate constant from fit: 298 s1. Inset: rms error of the fit versus rate constant.
dimethylamine and phenolic hydroxyl groups remain after surface functionalization. We also attempted to immobilize 3,4-ethylenedioxythiophene (EDOT), an electron rich monomer used in the formation of conjugated polymers; however, we did not observe formation of a consistent EDOT monolayer either spectroscopically or electrochemically. We speculate that this failure may be due to competing side reactions such as a
double azo-coupling in which both α-positions react with surface-bound diazonium units. In order to probe the surface density of diazonium groups quantitatively, the integration of charge upon oxidation of a stable electrochemical probe immobilized at a surface provides a reliable method using cyclic voltammetry. Ferrocene has been reported to react readily with diazonium salts in a direct arylation reaction17 13370
dx.doi.org/10.1021/la2024617 |Langmuir 2011, 27, 13367–13373
Langmuir
ARTICLE
similar to the GombergBachmannHay coupling, accompanied by loss of N2. The proposed mechanism for this reaction, involving a single-electron transfer process, is shown in Figure 4. When the freshly prepared diazonium containing substrate is immersed in a 0.1 M solution of ferrocene for 30 s, the arylaryl coupling occurs, yielding a conjugated, surface-bound ferrocene species. The surface coverage of ferrocene can then be calculated from the cyclic voltammogram, and serve as a lower bound on the surface coverage of diazonium groups in the monolayer. Figure 5a shows the CV of a ferrocene-coupled monolayer. By careful choice of the reducing potential for diazonium immobilization, a surface coverage of 2.7 1010 mol/cm2 was achieved, which corresponds to a moderately packed monolayer of ferrocene with coverage about half of the theoretical maximum.17,18 The linear relationship (R2 = 0.997) between current and scan rate demonstrates that the ferrocene is surface bound (Figure 5c). The electrochemical surface coverage of ferrocene, as well as the rate of electron transfer in this system, is very sensitive to the structure of the monolayer and therefore to the deposition conditions. By varying the conditions and monitoring the resulting ferrocene redox wave in the voltammogram, we observed that the monolayers yielding the highest coverage of ferrocene are formed in a single CV cycle rather than multiple cycles, with the voltage swept from approximately 0.3 to 0 V. These values are determined from the solution CV of the bisdiazonium salt and are selected such that the high potential is in a plateau region of the CV with no processes other than Table 1. Ellipsometric Thicknesses and Electrochemical Surface Coverage for Diazonium Films Functionalized with Various Aromatic Species species
thickness (nm)
Γ (mol/cm2)
ferrocene DMA
3.3 3.8
2.66 1010 3.89 1010
MK
5.0
charging occurring and the low potential is within the first reduction peak (Figure 2). This rate constant of electron transfer can provide information about the quality of the surface-bound redox probe immobilized to the electrode surface in terms of homogeneity and structural integrity. Using the Laviron formalism,19 the rate constant of electron transfer between the conjugated ferrocene redox center and the gold electrode is estimated to be 25 s1, which is approximately 1 order of magnitude greater than values reported for an alkyl monolayer. On glassy carbon, the calculated rate constant is 129 s1 (Figure 5b), which approaches the lower end of reported values for conjugated thiol monolayers on gold and is comparable to the rate of transfer through an oligo(phenyleneethynylene) bridge to ferrocene grafted from the diazonium salt.20 To render the Laviron formalism applicable, the achieved overpotential (E Ef) for the measurements used to estimate the critical scan rate should be greater than 200 mV. This rate is not achievable with this system due to the fast electron transfer. Accordingly, electron transfer rates determined in this work using the Laviron approximation should be regarded only as lower bounds. Furthermore, the Laviron method only provides an order-of-magnitude estimate of the ET rate constant even when the overpotential condition is met. ACV measurements on systems which had been previously characterized by cyclic voltammetry have revealed actual electron transfer rate constants several times higher than the number observed in CV measurements.21 Using this technique allows for a more precise measurement of the electron transfer rate through these conjugated monolayers. Fitting of the normalized current vs the logarithm of alternating current frequency according to the procedure of Creager and Wooster22 yielded a first-order electron-transfer rate constant between 295 and 300 s1 on gold and a similar value on glassy carbon (Figure 6). Table 1 shows the ellipsometric measurements of the ferrocene-functionalized films (3.3 nm) along with other diazocoupled species. In each case, the layer thicknesses were higher than the expected value for a true monolayer, indicating some degree of oligomerization of the underlying diazonium
Figure 7. Cyclic voltammograms showing oxidation of electron-rich aromatic species immobilized at the diazonium monolayer. (a) Irreversible oxidation of N,N-dimethylaniline layer in acetonitrile/TBAPF6. (b) Pseudoreversible redox of hydroquinone layer in water/0.1 M NaH2PO3. All scans were performed at 100 mV/s. 13371
dx.doi.org/10.1021/la2024617 |Langmuir 2011, 27, 13367–13373
Langmuir
ARTICLE
stretching mode is visible in all monolayers, appearing at 1504 cm1 in MK, 1479 cm1 in ferrocene, and 1500 cm1 in DMA. The CdC stretch in ferrocene is consistent with literature values for the free compound (1464 cm1)25 and is due to the electron-rich nature of the cyclopentadienide ring in ferrocene. The peak observed at 1278 cm1 in MK is likely the CN stretch, intensified relative to the same mode in DMA due to the contribution of a charge-separated resonance form, which leads to a greater dipole moment change along with a blue shift (relative to DMA) due to the effect of the electron-withdrawing ketone group and the steric proximity of the NdN group. The corresponding peak in DMA occurs at 1225 cm1 with a shoulder at 1240 cm1. The peaks appearing around 1170 cm1 in MK and DMA are attributed to CN and CO stretches.
Figure 8. PM-IRRAS of aromatic species immobilized at diazonium monolayer on gold substrates. (a) Michler’s ketone (4,40 -bis(dimethylamino)benzophenone), (b) ferrocene, and (c) N,N-dimethylaniline.
containing layer. A drawback of the system is that the bisdiazonium salt is slightly unstable even when stored in a freezer and must be freshly recrystallized before use. This step is important to ensure maximized surface coverage and high electron transfer rates through the film. A glassy carbon electrode functionalized with ferrocene using bisdiazonium salt aged for 1 week at 20 °C had 10 lower surface coverage and an ET rate constant of only 50 s1. Figure 7 shows the electrochemistry of the diazo monolayers formed by reacting N,N-dimethylaniline and hydroquinone with the diazonium monolayers. N,N-Dimethylaniline has an irreversible oxidation wave near 0.8 V corresponding to the formation of a radical cation.23,24 The hydroquinone monolayer in aqueous phosphate buffer displays the characteristic pseudoreversible two-electron proton-coupled redox reaction of hydroquinone. The formal potential of this reaction is approximately 0.2 V vs Ag/Ag+, consistent with other reports of hydroquinone bound to a conjugated thin film.21 Infrared spectral analysis of the diazonium-functionalized films using PM-IRRAS after reaction with aromatic species yielded confirmation of the predicted nature of the couplings, with a strong peak corresponding to the NdN stretching mode visible near 1600 cm1 in all immobilized species other than the Fc film8. In the film functionalized with 4,40 -bis(N,N-dimethylamino)benzophenone (Michler’s ketone, MK), the intense CdO stretching frequency appears at 1589 cm1. This is a reasonable match for the free compound, which has a CdO stretch at 1599 cm1. The moderate red shift of the primary mode is likely due to the slightly π-donating nature of the azo linkage. A Fermi resonance is also possible between the stretching mode of the CdO bond and the NdN stretch, due to the strong red shift of the CdO stretch produced by the electrondonating dimethylamine substituents. The CdO stretch occurs at a notably low frequency compared to, for example, benzophenone, resulting in an overlap with the NdN stretch. This possibility is consistent with the appearance of a peak at 1662 cm1, which is assigned as the NdN peak in resonance with the CdO, accounting for the blue shift. The CdC ring
’ CONCLUSIONS The electrochemical grafting of the salt benzene(bisdiazonium)hexafluorophosphate has proven to be an effective, simple, and versatile means of creating fully conjugated monolayers terminated with an aromatic system on a conducting substrate. The use of a small aromatic system with strong electronic coupling between two diazonium groups makes it possible to selectively reduce one diazonium moiety, leaving the other to be incorporated into the monolayer, as the reduction potential of the single diazonium group on the surface is much lower (ca. 0.5 V) than that of the bisdiazonium species in solution. Subsequent functionalization of the surface with electron-rich aromatics proceeds through a monolayer with a benzenediazonium hexafluorophosphate end group, which reacts with aromatic species in solution by an azo coupling to form an azobenzene derivative or a direct Gomberg arylation to form a new biaryl compound. The route that the reaction takes depends on the substitution and nature of the aromatic ring. Ferrocene undergoes a Gomberg arylation through the rapid reduction of the diazonium group to a reactive radical species by a single-electron transfer process. On the other hand, aryl rings with π-donor substituents undergo the expected diazo coupling reaction. The monolayers resulting from these surface-confined reactions can participate in a rich variety of further chemistry, including redox cycling by application of a further potential, in situ reduction of the immobilized diazonium group, acidbase reactions, and presumably many more. The monolayers are fully conjugated and display rapid electron-transfer rates. The ease and simplicity of this protocol combined with the good electrontransfer properties of the resulting monolayers renders it likely that this platform will be useful in the construction of thin-film organic electronic devices, particularly in light of the robust nature of electrografted diazonium monolayers compared to those formed by thiol or silane self-assembly. The successful preparation of monolayers terminated with common photosensitizers such as 4,40 -bis(N,N)-dimethylaminobenzophenone demonstrates the robustness of the method even in the presence of the electron-withdrawing ketone moiety, and suggests the use of the technique for the immobilization of chromophores for photovoltaic or photochemical layers. Furthermore, the ease of creating azobenzene-containing thin films with this technique can easily lead to the creation of photoswitchable thin-film based systems. In general, due to its synthetic and electrochemical simplicity, this technique makes diazonium monolayers easily accessible. Future work in our laboratory based on this method 13372
dx.doi.org/10.1021/la2024617 |Langmuir 2011, 27, 13367–13373
Langmuir will include its use in surface-initiated polymerization and the preparation of functional photoactive thin films.
ARTICLE
(25) Spectral Database for Organic Compounds (SDBS). http:// riodb01.ibase.aist.go.jp/sdbs/ (accessed December 3, 2010).
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the Donors of the American Chemical Society Petroleum Research Fund (Grant 48917-DNI7) and the National Science Foundation (CHE 1058631). We would like to thank Stephen Creager for sharing details and an Excel spreadsheet to aid in AC voltammetry analysis. ’ REFERENCES (1) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1992, 114, 5883. (2) Hansen, M. N.; Farjami, E.; Kristiansen, M.; Clima, L.; Pedersen, S. U.; Daasbjerg, K.; Ferapontova, E. E.; Gothelf, K. V. J. Org. Chem. 2010, 75, 2474. (3) Harper, J. C.; Polsky, R.; Wheeler, D. R.; Brozik, S. M. Langmuir 2008, 24, 2206. (4) Gam-Derouich, S.; Carbonnier, B.; Turmine, M.; Lang, P.; Jouini, M.; Ben Hassen-Chehimi, D.; M. Chehimi, M. Langmuir 2010, 26, 11830. (5) Stockhausen, V.; Ghilane, J.; Martin, P.; Trippe-Allard, G.; Randriamahazaka, H.; Lacroix, J.-C. J. Am. Chem. Soc. 2009, 131, 14920. (6) Gui, A. L.; Liu, G.; Chockalingam, M.; Le Saux, G.; Harper, J. B.; Gooding, J. J. Electroanalysis 2010, 22, 1283. (7) Allongue, P.; de Villeneuve, C. H.; Cherouvrier, G.; Cortes, R.; Bernard, M. C. J. Electroanal. Chem. 2003, 550551, 161. (8) Ciampi, S.; Harper, J. B.; Gooding, J. J. Chem. Soc. Rev. 2010, 39, 2158. (9) Kosynkin, D. V.; Tour, J. M. Org. Lett. 2001, 3, 993. (10) Flatt, A. K.; Chen, B.; Tour, J. M. J. Am. Chem. Soc. 2005, 127, 8918. (11) Chen, B.; Lu, M.; Flatt, A. K.; Maya, F.; Tour, J. M. Chem. Mater. 2008, 20, 61. (12) Schmelmer, U.; Paul, A.; K€uller, A.; Steenackers, M.; Ulman, A.; Grunze, M.; G€olzh€auser, A.; Jordan, R. Small 2007, 3, 459. (13) Viel, P.; Le, X. T.; Huc, V.; Bar, J.; Benedetto, A.; Le Goff, A.; Filoramo, A.; Alamarguy, D.; Noel, S.; Baraton, L.; Palacin, S. J. Mater. Chem. 2008, 18, 5913. (14) Delamar, M.; Desarmot, G.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. M. Carbon 1997, 35, 801. (15) Vogel, A. I.; Furnis, B. S.; Hannaford, A. J.; Smith, P. W. G. Vogel’s Textbook of Practical Organic Chemistry, 5th ed.; Prentice Hall: Upper Saddle River, NJ, 1996. (16) Corgier, B. P.; Belanger, D. Langmuir 2010, 26, 5991. (17) Rosenblum, M.; Howells, W. G.; Banerjee, A. K.; Bennett, C. J. Am. Chem. Soc. 1962, 84, 2726. (18) Ju, H.; Leech, D. Langmuir 1998, 14, 300. (19) March, G.; Reisberg, S.; Piro, B.; Pham, M. C.; Delamar, M.; Noel, V.; Odenthal, K.; Hibbert, D. B.; Gooding, J. J. J. Electroanal. Chem. 2008, 622, 37. (20) Lu, M.; He, T.; Tour, J. M. Chem. Mater. 2008, 20, 7352. (21) Trammell, S. A.; Seferos, D. S.; Moore, M.; Lowy, D. A.; Bazan, G. C.; Kushmerick, J. G.; Lebedev, N. Langmuir 2007, 23, 942. (22) Creager, S. E.; Wooster, T. T. Anal. Chem. 1998, 70, 4257. (23) Jones, P. R.; Drews, M. J.; Johnson, J. K.; Wong, P. S. J. Am. Chem. Soc. 1972, 94, 4595. (24) Larumbe, D.; Gallardo, I.; Andrieux, C. P. J. Electroanal. Chem. Interfacial Electrochem. 1991, 304, 241. 13373
dx.doi.org/10.1021/la2024617 |Langmuir 2011, 27, 13367–13373
