Radical Chemistry from Diazonium-Terminated Surfaces - Chemistry

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Radical Chemistry from Diazonium-Terminated Surfaces Hassan Hazimeh,† Sandie Piogé,‡ Nadège Pantoustier,‡ Catherine Combellas,† Fetah I. Podvorica,† and Frédéric Kanoufi*,† †

Physico-Chimie des Electrolytes, des Colloides et Sciences Analytiques, CNRS UMR 7195, and ‡Laboratoire de Physico-chimie des Polymères et Milieux Dispersés, CNRS UMR 7615, ESPCI ParisTech, 10 rue Vauquelin, 75231 Paris Cedex 05, France S Supporting Information *

ABSTRACT: Active diazonium groups are attached onto surfaces via oxidative grafting of the 4-phenylacetic diazonium salt. The fraction of the anchored aryldiazonium accessible to (electro)chemical transformation is determined by electrochemical interrogation. Because enough sites are available to react with radical traps, the ability of the aryldiazoniumanchored surfaces to induce radical surface chemistry is tested. Particularly, a new route to grow polymer brushes from surfaces under ATRP conditions is presented. Such an example of a controlled coupling reaction demonstrates how aryldiazoniums, which are irreversible sources of Ar• radicals, can sustain long-term radical cross-coupling reactions. Such a strategy of generating dormant radical sources from diazonium precursors is fruitful to the simple, versatile, and sustainable chemical decoration of materials. KEYWORDS: radical chemistry, surface chemistry, electrografting, diazonium, controlled radical polymerization from surfaces

I. INTRODUCTION The tailoring of the physical or chemical properties of surfaces or materials with molecular, biomolecular, or nanomaterial assemblies has received considerable attention in many applications and fundamental studies.1 One of the key issues in this field is to design the most appropriate synthetic strategy that will allow one to anchor, with a high selectivity, a functional group on a surface and make it responsive. If a wide range of synthetic approaches are available, the most popularly used coupling chemistries are carbodiimide peptidic and “click” reactions. Owing to their reactivity, approaches based on the generation of free radicals are also particularly appealing, provided their generation and reactivity are controlled. Electrochemistry is a straightforward way to generate radical species from various precursors by electron transfer coupled to bond dissociation processes. Among the different precursors, aryl diazonium salts are attractive and popular sources of reactive radical species in surface or material science.2 Indeed, their simple activation (by electrochemistry, temperature, or photochemistry) yields an aryl radical, which further grafts carbon, metal, semiconductor, and polymer surfaces. The simple introduction on surfaces of various functionalities by direct2 or further cross-coupling reactions ensues.2−5 Diazonium ions are also interesting synthons, recently revisited, to bring chemical diversity in organic synthetic approaches.6 Here again, the diazonium is used as a precursor of radicals for the introduction of a new chemical functionality, as in the Sandmeyer reaction, which converts a diazonium to a halogen,7 or in the Meerwein reaction, which allows the arylation of olefins (Scheme 1).8 Usually, generation of the aryl © 2013 American Chemical Society

radical and control of its reactivity require the use of a reducing organometallic complex [generally copper(I)], but photocatalytic9 or metal-free10 synthetic approaches under nucleophilic conditions are also reported. On the basis of their high reactivity, which can be externally triggered, the anchoring of diazonium terminal functions on surfaces is a promising alternative to direct the chemical derivatization of surfaces and materials. Here, we demonstrate the potential of surface-anchored diazonium moieties as controlled sources of radicals enabling surface derivatization processes based on radical C−C bond formation. On the basis of possible electrochemical radical generation from diazonium ions, our contribution is devoted, but not restricted, to electrode surfaces. It is, henceforth, liable (i) to the introduction of a diazonium function on a surface and (ii) to the control of radical chemistry from such a surface. The former aspect is a matter of current interest and has been obtained, from an electrografting procedure, using a symmetric bisfunctionnalized aryl precursor,11−13 imparted with an inherent selectivity issue. The latter concerns control of the generation and reactivity of radicals from these surfaces. The strategy, which is validated here, consists of making the highly reactive aryl radicals alive on surfaces so that they are available at will for any further chemical use. This strategy is actually involved in atom-transfer radical polymerization (ATRP).14 As a key ingredient of ATRP, a radical precursor Received: December 4, 2012 Revised: January 23, 2013 Published: January 29, 2013 605

dx.doi.org/10.1021/cm3039015 | Chem. Mater. 2013, 25, 605−612

Chemistry of Materials

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Scheme 1. Reactivity of Diazonium-Grafted Surfaces

inspected. The presence and reactivity of the surface-anchored diazonium moieties available for further chemical derivatization are evaluated by spectroscopic and electrochemical interrogation. Then, the possibility of using these diazonium anchors as potential sources of (“living”) radical-based crosscoupling reactions (Scheme 1) is explored through coupling of the aryl radical Ar• derived from the diazonium with an olefin under catalytic organometallic activation. The efficiency of this coupling is achieved by detecting the controlled growth of polymer chains under ATRP conditions. For that, we first validate in solution the use of aryldiazoniums as initiators of controlled radical polymerization, albeit scarcely used so far.22 Then, we transpose the polymerization reaction to diazoniumterminated surfaces in order to induce cross-coupling reactions and particularly to grow polymer brushes from surfaces in a controlled way.

(a dormant species), anchored on the surface, is activated by a copper(I) complex toward radical coupling with a monomer (chain propagation, kp in eq 1). Typical ATRP radical sources14

are bromoalkyl-terminated molecules, which readily generate alkyl radicals under dissociative15,16 or inner-sphere electron transfer.16a,17 Organometallic chemistry allows, as illustrated in eq 1, reversible conversion of the intermediate growing radical chain, R•, into a bromine-terminated dormant chain, RBr. This equilibrium controls the chain growth and maintains the radical source “alive” for further activation and cross-coupling chemistry within the material. Apart from this “living” character, ATRP is also an elegant way to design smart materials and objects with controllable size distribution, composition, and architecture. Moreover, electrochemistry was recently used to both trigger selectively18 and control19 the organometallic radical activation. However, unlike for RBr, the coppercatalyzed reductive cleavage of diazonium salts (eq 2) yields irreversible loss of the N2+ functionality:6,20 ka0

Cu I/Ln + ArN2+ → Cu II/Ln + Ar• + N2

II. MATERIALS AND METHODS Materials. (4-Aminophenyl)acetic acid (NH2C6H4CH2COOH, NH2BzCOOH, 98%) and 4-methylaniline (NH2C6H4CH3, 99%) were purchased from Aldrich and used without further purification. + N2C6H4CH2COOH (+N2BzCOOH), BF4−, +N2C6H5 (PhN2+), BF4− and +N2C6H4CH3 (BzN2+), BF4− were synthesized according to the literature.23 A total of 0.01 mol of (4-aminophenyl)acetic acid (respectively, aniline or 4-methylaniline) was dissolved in 0.03 mol of 50:50 HBF4/water. After the solution was cooled at ∼0 °C with ice, a concentrated solution of NaNO2 (0.015 mol) in water was added under stirring. After filtration, the precipitate was washed with a 5% cold solution of NaBF4 in water, cold methanol, and ether. The powder was dried and kept in a freezer at −5 °C. Methyl methacrylate (MMA, 99%) and glycidyl methacrylate (GMA, 97%) from Aldrich were purified by filtration through a basic alumina column and stored at −5 °C after purification. CuBr (99.999%), CuBr2 (98%), 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA, 97%), 2,2′-dipyridyl (99%), ferrocene (Fc, 98%), basic alumina, acetone, methanol, dichoromethane, acetonitrile (ACN), N,N-dimethylformamide (DMF), NBu4BF4, H2SO4, and diethyl ether were purchased from Aldrich and used without further purification. Typical ATRP of MMA in Solution. In a typical procedure, required amounts of the (4-methylphenyl)diazonium initiator (I = BzN2+, 72 mg, 3.46 × 10−4 mol), MMA (7.43 mL, 0.07 mol), and ACN (7.3 mL, 50% v/v) were introduced into a Schlenk tube (A) sealed with a rubber septum. The resulting solution was degassed by three freeze−pump−thaw cycles to remove oxygen. At the same time,

(2)

Therefore, making the aryl radical source dormant relies on its coupling with a potent dormant radical source. This is the strategy explored here through the use of a transition-metal complex to catalytically generate aryl and alkyl radicals and engage them in controlled and “living” cross-coupling reactions (respectively Meerwein and ATRP schemes). Different issues are then addressed here. First, an alternative way to anchor diazonium moieties onto a surface is depicted. High selectivity, and therefore high diazonium surface coverage, should be amenable by using an unsymmetrical precursor imparted with both a diazonium group and another electrocleavable group, for example, cleavable under anodic conditions. Different groups are known to yield surface electrografting under oxidation,2b for example, arylacetates, which allow grafting of phenyl moieties via the Kolbe reaction.21 Here, the grafting of a benzyldiazonium layer on electrode surfaces by oxidative electrografting of the phenylacetic diazonium salt is 606



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a second Schlenk tube (B) was charged with CuBr (48.3 mg, 3.46 × 10−4 mol) and CuBr2 (7.6 mg, 3.46 × 10−5 mol) and was degassed with three vacuum/nitrogen fill cycles. The resulting solution from the Schlenk tube (A) was transferred into the Schlenk tube (B) through a cannula. The solution was further degassed by freeze−pump−thaw cycles and then placed in an oil bath thermostatted at 40 °C. Once the reaction temperature was reached, the HMTETA ligand (93 μL, 3.46 × 10−4 mol) was added under nitrogen (t = 0). Samples were withdrawn at specified time intervals using a degassed syringe, and the conversion was determined by 1H NMR spectroscopy. Samples were dissolved in dichloromethane and filtered through a basic alumina column to remove the copper complex. The solutions were concentrated under vacuum and precipitated in a large excess of methanol. The resulting polymers recovered by filtration were dried under vacuum at room temperature before analysis. Size Exclusion Chromatography (SEC). The number-average molecular weights (Mn,SEC) and polydispersity indexes (PDIs) of the obtained poly(methyl methacrylate) (PMMA) were determined by SEC. The steric exclusion was carried out on a Stability GPC Linear Cluzeau column (5 μm, 300 × 8 mm2; separation limits, 100−1000 g mol−1) and two Stability GPC 100 Cluzeau columns (5 μm, 300 × 8 mm2; separation limits, 103−106 g mol−1), coupled with a refractive index (RI) detector (Viscotek, TDA 302) and a light scattering (LS) detector (Viscotek, TDA 302; laser λ = 670 nm; angles θ1 = 90° and θ2 = 7°), using tetrahydrofuran (THF) as a mobile phase at a flow rate of 1.0 mL min−1. The PDIs of the samples were derived from the RI signal by a calibration curve based on PS standards (Polymer Standards Service). The weight-average molar masses, Mw, of polymers were calculated from the LS signal with OmniSEC 4.5 software, using the average RI increment (dn/dc) of PMMA in THF, which is 0.089 mL g−1. NMR Spectroscopy. Polymers were characterized by 1H NMR spectroscopy, using a Bruker AC 400 MHz spectrometer and CDCl3 as the solvent. Calculated molecular weights (Mn,calc) of the resulting PMMA were determined from the conversion rate obtained from the signals of MMA and PMMA and eq 4 indicated in the text. Substrates. Gold (Au)-coated wafers (Aldrich, 1 × 1 cm2 Aucoated silicon wafer, 1000 Å coating) or a platinum (Pt) plate (0.5 mm thickness from Goodfellow) were cleaned with a piranha solution [1:3 (v/v) H2O2/H2SO4] for 10 min at room temperature and rinsed under sonication for 10 min in Milli-Q water. Glassy carbon (GC) plates (1 × 1 cm2 × 1 mm from Sigradur G, HTW) were cleaned by sonication in acetone. Before modification, the plates were dried under a stream of nitrogen. Caution! Piranha solutions are highly aggressive and explosive in contact with organic solvents; they should be handled with full protection, gloves, mask, etc. The electrodes for cyclic voltammetry (CV) were 1-mm-diameter Pt or 3-mm-diameter GC wires imbedded in epoxy resin or a small silicon shard. They were polished with different grades of polishing paper and finally with a 0.04 μm alumina slurry on a polishing cloth (DP-Nap, Struers, Denmark), using a Presi Mecatech 234 polishing machine. After polishing, the plates were rinsed with Milli-Q water and sonicated for 10 min in acetone to avoid organic contaminants. Electrode and Plate Functionalization by +N2BzCOOH. Electrografting was performed by chronoamperometry in ACN + 0.1 M NBu4BF4 solutions in a 10 mM solution of +N2BzCOOH with either Pt or GC electrodes and plates or Au plates. The reference electrode was either Ag/AgCl or saturated calomel electrode and the counter electrode a Pt foil. The grafting potential was 1.4 V vs Ag/ AgCl for Pt or GC electrodes and 1.1 V vs Ag/AgCl for Au plates. Surface-Initiated ATRP of GMA. First, a 100 mL Schlenk flask equipped with a magnetic stirring bar and sealed with a rubber septum was deoxygenated under vacuum followed by backfilling with nitrogen three times. The CuBr (52 mg, 0.37 mmol) and CuBr2 (22.4 mg, 0.10 mmol) powders and the initiator-grafted Au wafer were introduced into the flask under a nitrogen flow. A 14 mL of DMF + 7 mL of H2O mixture containing GMA (10.0 mL, 73.3 mmol) and 2,2′-dipyridyl (142.8 mg, 0.91 mmol), previously degassed, was added to the polymerization flask using a double-tipped needle under a nitrogen flow. The flask was placed at 50 °C for several hours. The

polymerization was stopped by cooling and opening of the flask in order to expose the catalyst to air. The Au wafer−poly(glycidyl methacrylate) (PGMA) hybrids were thoroughly rinsed in DMF and dichloromethane under sonication for two periods of 5 min. Infrared Reflection Absorption Spectroscopy (IRRAS). IRRAS spectra of the modified plates were recorded using a purged (low CO2, dry air) Jasco FT/IR-6100 Fourier transform infrared spectrometer equipped with a mercury−cadmium−telluride detector. For each spectrum, 1000 scans were accumulated with a spectral resolution of 4 cm−1. The background recorded before each spectrum was that of a clean substrate. Attenuated total reflectance (ATR) spectra were recorded with a diamond ATR accessory (Jasco ATR PR0470-H). Ellipsometry. The thicknesses of the films on Au and Pt were measured with a Sentech SE400 monowavelength ellipsometer. The following values were taken as refractive indexes, ns, and dispersion coefficients, ks, in agreement with reported values:24 for Au, ns = 0.195 and ks = 3.50; for Pt, ns = 2.32 and ks = 4.63; for GC, ns = 1.9 and ks = 0.75. These values were measured on clean surfaces before using the plates for grafting, and the film thicknesses were determined from the same plates after modification, taking ns = 1.46 and ks = 0 for the polymeric layer. Electrochemical Measurements. Electrochemical experiments were performed with an EG&G 263A potentiostat/galvanostat and Echem 4.30 software. All experiments were carried out in ACN solutions deoxygenated with nitrogen. All potentials are measured versus the Ag/AgCl electrode.

III. RESULTS AND DISCUSSION III.1. Electrografting of a Metal Surface by a Biscleavable Moiety. The introduction of a diazonium function on a surface has previously been described but mainly through the use of the symmetrical bisfunctionalized pphenylenediamine precursor. The latter yields the (4aminophenyl)diazonium salt, which can be electrografted to form 4-aminophenyl layers on GC or indium−tin oxide (ITO) surfaces.11 In situ conversion of the amine-terminated surface into a diazonium-terminated surface in both aqueous and nonaqueous media11,12 allows for the anchorage of Au nanoparticles or DNA and protein moieties. Alternatively, Marshall and Locklin isolated the benzene (p-didiazonium hexafluorophosphate) salt and controlled the selective grafting of only one diazonium group, owing to the careful selection of the reductive grafting potential.13 Higher selectivity should be amenable by using an unsymmetrical precursor imparted with two different electrocleavable groups. Here, we have derivatized electrode surfaces with a precursor, 4-phenylacetic diazonium (+N2C6H4CH2COOH designated as + N2BzCOOH), that possesses two different electrochemically activable sources of radicals: a diazonium and a carboxylic moiety, which can be activated independently. Indeed, diazoniums and acetates are expected to graft electrode surfaces upon respectively reductive and oxidative conditions (Figure 1A). The different electrografting experiments were performed on either macroelectrodes (plates of Pt, Au, or GC), allowing for further surface characterization (ellipsometry and FTIRRAS), while grafting on microelectrodes (1-mm-diameter Pt disks) allowed for quantitative characterization through electrochemical probing. Electrochemical reduction under potentiostatic conditions at −0.6 V vs Ag/AgCl for 300 s at a Pt microelectrode of a 10 mM solution of + N 2 BzCOOH in ACN allows grafting of −BzCOOH onto the electrode surface. This strategy was already proposed as a fast and efficient way to introduce biochemical functionalities on electrodes.4a,b The grafting is confirmed in Figure 1B because CV of the grafted micro607



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selected E = 1.1 V vs Ag/AgCl. A grafted Pt disk microelectrode is then characterized by CV (Figure 1C) and shows a reductive peak at −0.28 V vs Ag/AgCl, that is, the signature for the diazonium group. In second and third scans, the current decreases largely, in agreement with the diazonium reductive cleavage. Integration of the voltammogram indicates that a multilayer film containing 1.6 nmol cm−2 BzN2+ has been grafted onto the Pt microelectrode. Such derivatization by −BzN2+ is also observed on a GC microelectrode (not shown). Structural characterization of the grafted layer is obtained from ellipsometry on Pt and Au macroelectrodes. Ellipsometry shows that Pt or Au plates grafted under the latter conditions give respectively ∼8.3 ± 2.0 and 7.1 ± 0.6 nm thick films. The −BzN2+-derivatized electrodes are resistant to prolonged ultrasound activation and 24 h of exposure in air at room temperature because such treatments did not change the ellipsometric thickness of the grafted layer. The mechanical stability of the grafted layer is comparable to that of thin films obtained from diazonium electrografting.2 Moreover, a Pt microelectrode exposed for 24 h in air at room temperature still presents the electrochemical signature of the diazonium group, demonstrating the stability of the grafted layer and, above all, the stability of the diazonium moieties in the grafted layer. Electrografting processes usually form poorly compact layers, which present nanoporous defects (pinholes) or are permeable to various redox probes. Typically, electrografted layers slow down the electron transfer to redox probes without totally blocking it, as would be expected for >1 nm perfectly packed organic layers. Here, the BzN2+ multilayer grafted on the Pt microelectrode is also partially permeable to a ferrocyanide redox probe, as shown in Figure S1 in Supporting Information (SI). Analysis of Figure S1 in the SI shows that the grafted layer only attenuates by a factor of 3.2 the rate of charge transfer to the aqueous ferricyanide redox probe. The contributions of capacitive currents measured in the electrochemical characterization of the grafted microelectrodes (Figures 1B,C) are very similar (170 nA) for both grafted −BzCOOH and BzN2+ layers and much lower than that of a bare Pt microelectrode (650 nA), confirming the presence of a dielectric layer on the microelectrode upon grafting. The layer capacitance, C (Cl = ic/ 2Av ∼ 25 μF cm−2, with A the electrode surface area and v the scan rate), of the grafted microelectrodes is about 10 times larger than that observed for compact thiol monolayers selfassembled on Au.25 This demonstrates the poor compacity of the grafted layers on Pt, which is in favor of the possible transport and charge transfer within these layers. This also suggests that, because both films have the same thickness, they also possess similar dielectric structure and compacity. The transport and charge-transfer possibilities in the BzN2+ grafted layer are particularly interesting to use BzN2+ as a seeding layer for the introduction of further functionalities through chemically or electrochemically actuated processes. Indeed, many electrochemical sensors and, more recently, examples of the electrochemically induced initiation of “click” reactions26 or growth of polymer brushes18,27 use the permeability to charge transfer of organic layers assembled on electrodes. III.2. Further Derivatization by the Gomberg−Bachman Reaction. We have illustrated the potential reactivity of the immobilized BzN2+ layer as a seeding layer to introduce new functionalities in a stepwise approach by coupling reactions. Different chemical routes use aryldiazonium chemistry to couple aryl radicals with various molecules. The Gomberg−Bachmann reaction28 allows for aryl−aryl coupling

Figure 1. (A) Electrografting of unsymmetrical biscleavable moieties (10 mM +N2BzCOOH in ACN + 0.1 M NBu4BF4). Reductive activation of diazonium (−0.6 V vs Ag/AgCl for 300 s) yields immobilization of BzCO2H, as attested by (B) its oxidative signature. Oxidative activation (1.4 V vs Ag/AgCl for 600 s) of the acetic moiety yields the immobilization of BzN2+, as attested by (C) its reductive signature. (B and C) CV at 0.1 V s−1 of Pt electrodes (d = 1 mm) in ACN + 0.1 M NBu4BF4. (B) (a) Bare Pt and (b) Pt grafted with −BzCOOH. (C) Pt grafted with BzN2+: (a) first, (b) second, and (c) third scans.

electrode (Figure 1Bb), compared to a bare Pt microelectrode (Figure 1B,a), presents an oxidation peak at 1.06 V, which is the signature of arylacetate. This peak disappears upon a second scan, confirming the oxidative cleavage of COOH upon the Kolbe reaction.21 Its integration shows that 0.6 nmol cm−2 BzCOOH is immobilized onto the Pt electrode. Moreover, under oxidative conditions, radicals are generated from the same precursor and grafted onto different electrode surfaces as benzyldiazonium (BzN2+) moieties. This is achieved through the electrochemical oxidation of 10 mM +N2BzCOOH in ACN under potentiostatic conditions at 1.4, 1.4, or 1.1 V vs Ag/AgCl for 600 s on respectively Pt, GC, or Au. The electrografting potential used for Au was lower than that for Pt and GC in order to avoid competitive Au surface degradation at too anodic potentials (Au dissolution or gold oxide formation). However, as shown from its electrochemical signature in Figure 1B, arylacetate oxidative cleavage is fully operative at the 608



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reaction has only taken place efficiently in this outermost region in preferential contact with the reacting solution. III.3. Further Derivatization by the Meerwein Reaction. Next, we have inspected the reactivity of aryl radicals toward other trapping molecules such as olefins, M, through the Meerwein mechanism. Different prospects are sought in such reactions. On the one hand, as discussed in the Introduction, when the reaction is performed via the organometallic complex copper(I), it is expected to provide both Ar• generation (irreversible dediazotation, eq 2) and coupling to the olefin (eq 3):

and allowed coupling of an aryl radical to one of the cyclopentadienyl moieties of Fc.12,29 Here, the latter reaction was illustrated during the 4 min immersion of a BzN2+-grafted Pt microelectrode into a CH2Cl2 solution of 0.1 M Fc (Scheme 1, upper route). After ultrasonic rinsing in acetone, the cyclic voltammogram of the microelectrode shows the typical signature of Fc (Figure 2A). Again,

kc

Ar • + M → ArM•

(3)

The resulting aryl−alkyl, ArM•, radical may be converted to a dormant bromo-terminated aryl−alkyl, ArMBr, by the backward deactivating step of the reversible halogen-transfer reaction as depicted in eq 1. This dormant species can then act as a potent “living” source of radical species available for further cross-coupling reactions. Through such a procedure, the reactive diazonium moiety, even though it is an irreversible precursor of Ar•, can be transformed into a surface-immobilized reversible source of radical chemistry. Indeed, the ArM• radical may be engaged in further coupling with another olefin, ensuing, via the formation of an oligomeric ArMnBr, in a controlled radical polymerization. Such radical polymerization initiated from dediazotation of the BzN2+ layer is expected to yield the controlled growth of a polymer chain from the electrode surface. This is reminiscent of the growth of polymer brushes from surfaces by controlled living radical polymerization such as in ATRP initiated from bromo-terminated alkyl layers.14 Even though aryldiazoniums are well-known precursors of radical species, they have been rarely used as initiators of controlled radical polymerization reactions.22 On the other hand, engaging the Ar• radical in a polymerization reaction amplifies the characteristic signature of the olefin, which is polymerized, making the surface-bound reaction product easier to identify by standard surface analytical techniques. ATRP in Solution. To validate the hypothesis that aryldiazoniums may be used as initiators of ATRP reactions, we have first investigated in solution the kinetics of polymerization of MMA initiated by (4-methylphenyl)diazonium (initiator, I = BzN2+; Figure 3). The investigation in solution is also important for further transposition to diazoniumterminated surfaces. Indeed, the absence of any hydrolytic bond between the surface and macromolecules imparts good chemical stability but prevents ungrafting and, hence, direct analysis of the grafted polymer. Hence, several studies on ATRP from an inorganic surface showed that the average molecular weight and PDI of the grafted polymer are practically the same as those obtained for the soluble polymer formed in bulk solution from the added free initiator.32 The ATRP of MMA in solution was carried out at 40 °C in ACN with a molar ratio of [MMA]0/[BzN2+]0/[CuBr]0/ [CuBr2]0/[HMTETA]0 = 200/1/1/0.1/1. The kinetic experimental results are presented in Figure 3. The straight line of ln([M]0/[M]) versus time in Figure 3A indicates first-order kinetics and a constant number of active species throughout the polymerization process. Figure 3B illustrates the molecular weight and PDI dependence on monomer conversion. As can be seen, there is a good linear relationship between the polymer molecular mass determined by chromatography, Mn,SEC, and the

Figure 2. (A) CV of a Fc-derivatized Pt electrode (d = 1 mm) in ACN + 0.1 M NBu4BF4. CV at 0.1 V s−1 and variations of (B) the peak potential, Ep, and (C) the peak current, ip, with the scan rate, v.

the capacitive current measured on the cyclic voltammogram of the Fc-derivatized electrode is slightly higher (200 nA) than that of the grafted microelectrode before Fc coupling. This suggests that the grafted film thickness has not changed much (or has slightly decreased) during the coupling reaction and that the layer has mainly kept its integrity during the chemical reaction. The oxidation peak current is proportional to the electrode potential scan rate up to 0.1 V s−1 (Figure 2B), showing that the Fc moieties are immobilized. Meanwhile, at a higher scan rate, deviation from this linear law is observed, showing limitation by charge transport (hopping and/or counterion diffusion) within the layer. At low scan rates, integration of the current yields the amount of immobilized Fc moieties, 0.66 nmol cm−2, which are electrochemically addressed. This value is close to a monolayer surface coverage and is lower than the initial 1.6 nmol cm−2 surface coverage of the BzN2+ multilayer structure. This indicates that only a fraction of the grafted moieties is accessible either to Fc transport within the film during the reaction or to electron transport in the multilayer during electrochemical interrogation. Similar conclusions were proposed for electrografted layers bearing electroactive24,30 or derivatizable3c moieties. The variations of the anodic and cathodic peak potentials with the scan rate (Figure 2C) are compared to the theoretical prediction for a quasi-reversible system. An apparent electrontransfer rate to the layer-attached Fc of 1 s−1 ensues, a value 6 orders of magnitude lower than that found in monolayer systems.31 This suggests that (i) the electron-donating Fc group is not in close proximity to the electrode surface but rather is placed farther away at the outermost extremity of the organic layer and (ii) as was previously observed when etching electrografted layers in solution,3b−d the Gomberg−Bachmann 609



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that the amount of aryl radicals available for the ATRP is 2.4−3 times lower than that in classical ATRP in solution with 4bromoisobutyrate as the initiator. This is in line with the mismatch between the chemical structure of the initiator and of the growing chains. This is interpreted as a result of the high reactivity of aryl radicals and their possible deactivation, for example, by hydrogen-atom abstraction (eq 5) to any hydrogen-atom donor, SH, or the addition of trapping moieties identified as terminating routes in the Meerwein reaction. The addition of halogen [Sandmeyer (eq 6)] may be such a deactivating route because no polymerization is engaged with 4bromotoluene, BzBr, when the latter is used as a solution initiator (Table S1 in the SI). The 2.4−3 ratio would then indicate that these terminating reactions (eqs 5 and 6) have rates 2.4−3 times faster than the aryl−olefin coupling, kc, in eq 3.33 Assuming that the bromine ligand transfer is close to diffusion control, kBr ∼ 109 M−1 s−1,34 yielding kBr[CuII] ∼ 107 s−1, and from the estimated rate for hydrogen-atom abstraction from acetonitrile by phenyl radical kH[SH] = 6.7 × 106 s−1,35 it ensues that the initial coupling between the aryl radical and the MMA olefin is kc ∼ 106 M−1 s−1, a value 1 order of magnitude higher than that observed for a standard bromoalkyl ATRP initiator,36 in agreement with the high reactivity of aryl radicals. kH

Ar • + SH → ArH + S• kBr

Ar • + Cu IIBr2 → ArBr + Cu IBr

conversion. The PDIs of the polymers remain low (PDI ≤ 1.1) throughout the polymerization, indicating reasonable control of the polymerization. As in the Meerwein mechanism, the CuIBr complex is used as a potent reducing (and dediazotation) agent of aryldiazonium, generating the aryl radical, which further couples to MMA as in eq 3. This generated aryl radical is actually equivalent to the intermediate alkyl radical responsible for ATRP, allowing for the controlled growth of a polymeric chain. However, the observed molecular mass Mn,SEC is 2.4−3 times higher than that expected from the reactant proportions for an ATRP mechanism: [MMA]0 MMMA + MI [I]0

(6)

The following chain propagation operates at much slower rates, kapp = 1.2 × 10−5 s−1, in agreement with the reported values for the ATRP of MMA,37 suggesting that the propagation reaction rate is the same whatever the nature of the initiator. The ATRP scheme is then confirmed from extension of the experimental conditions for polymerization in solution with variation of the solvent (ACN, DMF, or THF), temperature (25, 40, or 60 °C), or monomer (GMA) (Table S1 in the SI). ATRP from the Surface. The polymerization reaction is then transposed to surfaces (lower route of Scheme 1) and tested on Pt, Au, and GC macroelectrode surfaces (plates) in order to grow polymer brushes from surface-anchored layers of +N2Bz obtained from the oxidative electrografting of +N2BzCOOH. The GMA monomer was chosen because of its potential further derivatization through catalytic epoxide ring opening. For all of the tested surfaces, deposition of the organic material is attested from an ellipsometry thickness increase. The chemical structure of the deposited material and the controlled growth of GMA polymer chains from different Au substrates submitted to the same ATRP conditions are evidenced by FT-IRRAS (Figure 4) and ellipsometry. The IR signature of PGMA is checked from characteristic bands: CO at 1735 cm−1, C−O at 1260 and 1150 cm−1, and the epoxy group at 910 cm−1. These signatures are only detected on Au surfaces coated with the BzN2+ layer. This shows that PGMA polymer brushes do not grow from (nor are physically adsorbed on) a bare Au surface or an Au surface coated with a Bz layer electrografted by oxidation of BzCOOH. Conversely, PGMA brushes grow from layers of BzN2+. As the polymerization time increases, both the height of the IRRAS characteristic bands of PGMA and the brush thickness measured by ellipsometry increase (Figures S2 and 4 in the SI). The brush thickness increases linearly with the polymerization time with a 2.5 nm h−1 growth rate. Finally, a PGMA-grafted surface whose brush growth had been halted (after 15 h of polymerization) was exposed to air

Figure 3. ATRP of MMA using (4-methylphenyl)diazonium as the initiator (I). [MMA]0/[I]0/[CuBr]0/[CuBr2]0/[HMTETA]0 = 200/ 1/1/0.1/1 at 40 °C. (A) Ln([M]0/[M]) (■, red) and monomer conversion (□) versus time. (B) Dependence of the molecular weight (Mn) and PDI (Mw/Mn) on MMA conversion.

M n,calc = conv (%)

(5)

(4)

where [X]0 and MX are respectively the initial concentration and molar mass of X, where X = monomer or initiator, I. Because the Mn,SEC/Mn,calc ratio is constant throughout the polymerization, eq 4 suggests that the amount of the growing chain is constant throughout the polymerization process. Equation 4 then relates the amount of radical available at the early stage of the polymerization initiation. The higher experimentally determined molecular mass, Mn,SEC, indicates 610



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for further chemical decoration of the electrode surface. The amount of immobilized seed moieties is assessed from electrochemical interrogation. This shows that, within the grafted multilayers, only a fraction of −N2+ can be detected electrochemically or is able to generate aryl radicals, which can be further trapped by the electroactive Fc label. The latter reaction demonstrates the ability of the −N2+ functionality to initiate radical chemistry from a surface. The manner in which aryldiazonium moieties can sustain controlled living radical polymerization is then particularly shown. The efficient crosscoupling between the generated aryl radical and an olefin by C−C bond formation is demonstrated through the formation of polymers in solution or from surfaces. Again, on the basis of the polymer molecular mass estimate, the Ar• radical reactivity alters its availability for complete C−C coupling. However, the controlled fashion of the radical polymerization process grants the use of easy-to-prepare diazonium species from cost-effective anilines as alternative synthons to alkyl halides for surface growth of polymers or more generally for cross-coupling strategies. It may be fruitful for the decoration of surfaces and materials with polymer brushes and, more generally, for the versatile and sustainable introduction of chemical diversity on surfaces. The aryl−olefin coupling proposed here is also a convenient way to transform the irreversible precursor of Ar• into a dormant “living” source of radical chemistry. This opens new ways of inspecting and controlling radical chemistry from surfaces by selective electrochemical activation.

■

ASSOCIATED CONTENT

S Supporting Information *

Attenuation of the electron-transfer rate by the −CH2C6H4N2+ (BzN2+) layer, ATRP of MMA and GMA in solution, and ATRP of GMA from a chemically derivatized Au surface and the influence of the polymerization time. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 4. FT-IRRAS of Au surfaces: (a) bare or grafted with (b) −CH2C6H5 or (c) −CH2C6H4N2+, submitted to ATRP of GMA: (A) spectra; (B) film thickness, δ, in (c) as a function of time.

■ ■

overnight and then submitted to a new PGMA reaction medium for 7 h. This resulted in the further growth of 15 nm of the PGMA layer (a 53 nm film thickness was obtained for the total process instead of 58 nm with no halt). This demonstrates the possible reinitiation of the polymerization process with a growth rate (2 nm h−1) similar to that found without interruption. This living character of the polymerization process suggests that the dormant species terminating the brush can be reactivated under ATRP conditions and are Br-terminated polymer chains. These results extend the conclusion drawn during homogeneous ATRP polymerization of MMA and demonstrate that polymer brushes can be grown by ATRP from diazonium-terminated surfaces. Moreover, the polymerization was also detected from ellipsometry on Pt and GC macroelectrode surfaces. This demonstrates that the different −N2+terminated surfaces can act as seeding layers for controlled growth of polymer and more generally for controlled phenyl radical chemistry. The concept proposed here can be generalized to a wide range of N2+-derivatized surfaces and materials.

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS H.H. and F.I.P. thank ESPCI ParisTech for financial support. M. Hanafi is thanked for his assistance in polymer characterization.

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REFERENCES

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Radical Chemistry from Diazonium-Terminated Surfaces - Chemistry

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