3-Aryl-3-(trifluoromethyl)diazirines as Versatile Photoactivated “Linker

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3-Aryl-3-(trifluoromethyl)diazirines as Versatile Photoactivated “Linker” Molecules for the Improved Covalent Modification of Graphitic and Carbon Nanotube Surfaces Elliot J. Lawrence,† Gregory G. Wildgoose,*,† Leigh Aldous,‡ Yimin A. Wu,§ Jamie H. Warner,§ Richard G. Compton,‡ and Paul D. McNaughter† †

Energy & Materials Laboratory, School of Chemistry, University of East Anglia, Norwich, NR4 7TJ, United Kingdom Physical & Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom § Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, United Kingdom ‡

bS Supporting Information ABSTRACT: 3-Aryl-3-(trifluoromethyl)diazirines are shown to be synthetically useful photoactivated carbene precursors that can be used as molecular “tethers” to facilitate the improved covalent surface modification of graphitic carbon and carbon nanotubes with a potentially large variety of chemical species. Proof-of-concept is demonstrated by the synthesis, as well as spectroscopic and electrochemical characterization, followed by photoactivated attachment of the organometallic diazirine derivative, 3-[3-(trifluoromethyl)diazirin-3-yl]phenyl ferrocene monocarboxylate, to the surface of vitreous carbon, and also to two different morphologies of multiwalled carbon nanotubes (“bamboo-like” and “hollow-tube”, denoted as b-MWCNTs and h-MWCNTs, respectively). The latter differ only in the relative amounts of “edge-plane-like” defect sites (at the termini of the nanotubes) and “basal-plane-like” pristine sidewall regions. The facile covalent coupling of the ferrocenyl “probe” moiety to the diazirine “linker” was confirmed by UV
vis, 1H and 19F NMR spectroscopy, and cyclic voltammetry (CV). Upon exposure to UV irradiation in the presence of graphitic materials, the resulting covalent surface attachment of the ferrocenyl groups via the diazirine “linker” was characterized by Raman and X-ray photoelectron spectroscopy (XPS) and by CV experiments performed in nonaqueous electrolyte. The surface coverage of 3-[3-(trifluoromethyl)diazirin-3-yl]phenyl ferrocene monocarboxylate, analyzed from both CV and XPS experiments was found to be 7%
11% of that estimated for a complete monolayer, and was 20-fold greater than that achieved in control experiments that employed conventional covalent modification strategies to form esters between ferrocene methanol and surface carboxylate groups on the graphitic materials. The surface loading of ferrocene groups on the b-MWCNTs was found to be only ca. 60%
70% that achieved on h-MWCNTs, reflecting the ability of the functionalized carbene intermediate formed upon photolysis of the parent diazirine to insert into CdC bonds in the otherwise relatively inert sidewalls of the nanotubes. This was further confirmed by Raman spectroscopic characterization, which revealed that the h-MWCNTs experienced significantly more sidewall functionalization than the b-MWCNTs, yet still retained good electronic conduction in electrochemical experiments. The relative chemical stability of 3-aryl-3-(trifluoromethyl)diazirines, the ease with which they can be potentially be coupled to a large range of different organic, inorganic, and biological species, and the enhanced surface loading that can be achieved as a result of the reactive carbene intermediate formed during their photolysis, render diazirines highly versatile and potent “linker” molecules for the development of chemically modified materials. KEYWORDS: diazirine, carbene, chemical modification, surface modification, carbon nanotubes, ferrocene-functionalized carbene, photolysis, sidewall functionalization, voltammetry

’ INTRODUCTION Research into the development and application of chemically modified carbon-based materials such as graphite and carbon nanotubes (CNTs) is rapidly maturing. The many varied applications of these materials range from sensor components, such as in (electro)analytical applications, to heterogeneous support materials for catalysts in “green” chemical processes and energy storage/ generation through to therapeutic materials such as drug delivery agents in medicinal chemistry.1
5 There are many approaches now available for chemists to chemically modify graphitic surfaces with a plethora of modifiers from simple organic molecules, inorganic r 2011 American Chemical Society

complexes, nanoparticles, polymers, and biological components (such as enzymes, proteins, and DNA).6 Broadly speaking, current modification methods can be divided into those involving either physisorption of a modifier onto the surface or covalent bond formation between the modifier and the graphitic material. For our purposes, we will limit the discussion to only those methods involving the chemisorption Received: May 24, 2011 Revised: June 30, 2011 Published: July 26, 2011 3740

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Chemistry of Materials approach. Among the most common methods of covalently modifying graphite or CNTs are reactions involving amide or ester formation, or C
C bond formation through radical attack, for example, by the reduction of aryldiazonium species.4 However, these predominantly modify only the reactive “edge-plane sites”—the termini of the nanotubes or hole defects in the tube walls—which are the sites of electron transfer7 for radical attack8,9 or are the locations where surface oxo-groups, such as carboxyl and hydroxyl groups, reside10
13 for amide/ester formation.5,6,14,15 This leaves the majority of the CNT surface— the relatively inert sidewall or “basal plane-like” regions— unmodified. The terms “basal-plane” or “edge-plane” describe the regions on the CNT surface by analogy to the structure of graphite.7,10 We are interested in supporting catalyst molecules on CNTs for electrocatalysed reactions in nonaqueous electrolytes. As such, we must develop an approach that covalently attaches small molecules to the CNTs surface and maximizes the loading of modifier by attacking both edge-plane-like tube ends and the more inert basal-plane-like sidewall regions. Covalent sidewall functionalization of graphitic materials such as CNTs is most commonly achieved using pericyclic reactions. Modification of CNTs by [2 + 1] cycloadditions with nitrene or carbene intermediates is common (see, for example, the work of Hu et al.16). Alternatively 1,3-dipolar cycloadditions are possible as demonstrated by Prato and co-workers, who modified CNTS via the in situ generation of azomethine ylides from the condensation of aldehydes and amino acids to form 5-membered pyrrolidine tethers.17,18 This method was used by Callegari et al. to immobilize ferrocenyl moieties onto the surface of SWCNTs in the construction of an amperometric biosensor.19 Other 1,3dipolar cycloadditions using ozone as the reactive species have also been used to modify the surface of CNTs with ozonide moieties.20,21 While carbenes themselves are sufficiently reactive to attack the CdC bonds in the CNT sidewall regions, they are often generated from precursors that are often very reactive and unstable themselves or require special handling, such as the PhHgCCl2Br precursor used by Hu et al.16 This drawback limits the usefulness of most carbene precursors where further synthetic steps are required to couple the desired modifier to the carbene-precursor “linker”. One class of molecules that overcomes this problem is the family of diazirines. Diazirines are strained three-membered heterocyclic ring systems that contain an sp3-hybridized carbon atom bonded to an azo group. Diazirines readily form carbene species upon liberating molecular nitrogen, on exposure to stimuli such as light22 or heat.23 Isomerization of the diazirine may occur on decomposition resulting in the formation of a linear diazo species, which can also eliminate nitrogen to afford the carbene species. The carbene that forms is capable of undergoing [2 + 1] cycloadditions with CdC bonds and will insert into C
O, N
H, and C
H bonds.22,24 While the carbene intermediate is very reactive and lacks chemoselectivity, certain types of diazirine precursor can be prepared, which are chemically stable under a broad range of reaction conditions, rendering them synthetically useful.22,25,26 The syntheses and characterization of diazirine,27 dialkyl diazirines,28
30 alkyl diazirines,31 and 3-halodiazirines32 were first reported in the 1960s. The potential of early diazirine compounds as synthetic reagents is rather limited. 3-Halodiazirines have an explosive nature (3-chloro-3-phenyldiazirine is more shock sensitive than nitroglycerine), while carbenes formed from dialkyl diazirines and alkyl 3H-diazirines have a tendency to selfreact in both intramolecular and intermolecular reactions.33,34

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Intramolecular carbene reactions such as 1,2-hydride and 1,3hydride migrations have been reported to afford alkenes and cyclopropanes.34 The formation of an azine (vide infra) from the corresponding diazirine via intermolecular carbene attack has also been reported, although several alternative mechanistic pathways are possible.33 The 3-aryl-3-(trifluoromethyl)diazirine family, reported by Brunner et al. in 1980, is currently the most synthetically useful class of diazirine, because they do not undergo intramolecular rearrangement, unlike previously explored diazirines.25,35 Brunner’s diazirines may be synthesized with relative ease and have good thermal and chemical stability; 3-(trifluoromethyl)-3-phenyldiazirine (TPD) has been reported to be stable in 1 M HCl or NaOH solution for at least 2 h, and was found to be stable at 75 °C for at least 30 min if kept in darkness.25 Despite the relative chemical stability of the diazirine precursor, photolysis of TPD afforded a reactive carbene that was capable of inserting into the C
H bond of cyclohexane.25 The desirable properties of 3-aryl-3-(trifluoromethyl)diazirine as carbene precursors has led to their successful application for photoaffinity labeling, used to study ligand-protein complexes, and other biological applications. This was reviewed extensively by Blencowe and Hayes.36 The Hayes group,22,37
39 and others40 have employed diazirine technology for many applications in materials science, including the preparation of polymers and surface modification. For example, Blencowe et al. covalently modified nylon-6,6 powder by photolysis of a fluorenone-derived 3-(trifluoromethyl)diazirine compound.22 The generated carbene is believed to have inserted into the N
H and C
H bonds, and evidence that suggests the successful covalent modification of nylon-6,6 with the fluorenone moiety was obtained using ultraviolet
visible light (UV
vis) spectroscopy. Despite the chemical stability and versatility of diazirines, coupled with the reactive and nonchemoselective nature of carbenes, there are few reports of the use of diazirine technology for the functionalization of graphitic materials. Brooks et al. functionalized glassy carbon with biotin via photolysis of the biotin-derived 3-(trifluoromethyl)diazirine; they also performed the functionalization using an analogous nitrene approach.41 The covalently attached biotin was fluorescently labeled, and fluorescence imaging was employed to visualize the extent of surface coverage. The carbene (diazirine) approach demonstrated increased loading of biotin moieties onto the substrate surface compared to the analogous nitrene, which was unable to insert into C
H bonds. More recently, while our group was independently developing the use of diazirines for the improved modification of CNTs with small organic and inorganic molecules, Ismaili et al. successfully modified the surface of MWCNTs with gold nanoparticles (AuNP) through photolysis of a diazirinebased precursor “linker” molecule.42 The diazirine moiety, a 3-aryl-3-(trifluoromethyl)diazirine variant, was bound to the AuNP through a thiol group, as part of a monolayer that was predominantly composed of dodecanethiol.40 Transmission electron microscopy (TEM) images clearly showed the decoration of MWCNT sidewalls with AuNPs; this was supported by X-ray diffraction (XRD) patterns that were indicative of the presence of AuNPs on the nanotube surface. In this report, we demonstrate that the use of 3-(trifluoromethyl)3-(3-hydroxylphenyl)diazirine as carbene precursor “linker” molecules offer a much improved route to the surface modification of graphitic carbon and CNT surfaces with small molecules with enhanced surface coverage of the modifier, compared to 3741

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Scheme 1. Preparation of the m-Diazirine Linker (6): a

a Conditions were as follows: (a) n-BuLi, Et2NCOCF3, THF,
78 °C, 3 h; (b) NH2OH 3 HCl, EtOH, 78 °C, 3.5 h; (c) TsCl, NEt3, DMAP, DCM, room temperature (RT), 42 h; (d) NH3, DCM,
78 °C, 19 h; (e) Ag2O, Et2O, RT, 20 h; and (f) BBr3, DCM, RT, 21 h.

traditional covalent modification methods. To this end, we have coupled ferrocene monocarboxylic acid to 3-(trifluoromethyl)3-(3-hydroxylphenyl)diazirine to form what is, to the best of our knowledge, the first organometallic-modified diazirine of its kind, 3-[3-(trifluoromethyl)diazirin-3-yl]phenyl ferrocene monocarboxylate. The ferrocenyl moiety can then be used as a model “probe” molecule with which to characterize the effectiveness of the modification as it has both chemically and electrochemically stable with a well-characterized redox behavior for electrochemical characterization in nonaqueous electrolytes. It also has an iron heteroatom that is a useful label for spectroscopic surface characterization by X-ray photoelectron spectroscopy (XPS).

2. EXPERIMENTAL SECTION 2.1. Reagents and Equipment. All reagents were purchased from either Sigma
Aldrich (Gillingham, U.K.) or Fisher Scientific (Loughborough, U.K.) and were of the highest grade available and used without further purification, unless stated otherwise. All synthetic reactions and manipulations were performed under a dry nitrogen atmosphere (N2 BOC Gases, Guildford, U.K.), using standard Schlenk-line techniques and apparatus, unless stated otherwise. “Bamboo-like” and “hollowtube” multiwalled carbon nanotubes (b-MWCNT and h-MWCNT, respectively) were purchased from Nanolab (Brighton, MA, USA; purity >95%, diameter = 30 ( 15 nm, length = 2
20 μm). All solvents were degassed to remove dissolved oxygen and dried prior to use by distillation over either sodium/benzophenone (tetrahydrofuran (THF) and diethyl ether (DEE)) or calcium hydride (dichloromethane (DCM)) under a nitrogen atmosphere. Acetonitrile was dried for 24 h over 3 Å molecular sieves prior to use. Nonaqueous electrochemical measurements were performed under an inert atmosphere using a computer-controlled potentiostat (Model PGSTAT 30, Autolab, Utrecht, The Netherlands) in a standard three-electrode configuration. A glassy carbon electrode (GCE) (BasiTechnicol, diameter of 3 mm) served as the working-electrode substrate with a platinum wire counter electrode (99.99%, GoodFellow, Cambridge, U.K.) and a silverwire quasi-reference electrode (99.99% GoodFellow, Cambridge) completing the cell assembly. The working electrode surface was renewed by successive polishing with diamond paste slurries of decreasing particle size from 3.0 μm to 0.1 μm (Kemmet, U.K.). The electrode was sonicated in deionized water and rinsed with ethanol between each polishing step. All

nonaqueous electrolyte solutions contained 0.1 M tetrabutylammonium tetrafluoroborate (TBAP) as the electrolyte salt. Infrared spectra were recorded using a Perkin
Elmer Model Spectrum 100 FT-IR spectrometer fitted with a Perkin
Elmer universal attenuated total internal reflectance (ATR) sampling accessory. Raman spectra were recorded using a JY Horiba Labram Aramis imaging confocal Raman microscope equipped with a 532-nm wavelength laser. The samples were mounted as solid powders on a glass slide. UV
vis spectra were recorded using a Perkin
Elmer Lambda 25 spectrometer. Nuclear magnetic resonance (NMR) spectra were recorded using either a Bruker Advance DPX-300 MHz or a Bruker Advance DPX-400 MHz spectrometer. X-ray photoelectron spectroscopy (XPS) was performed with a VG clam 4 MCD analyzer system using X-ray radiation from the Al KR band (1486.6 eV). All experiments were recorded with an analyzer energy of 100 eV and a takeoff angle of 90°. The base pressure in the analyzer chamber was maintained at no more than 2  10
9 mbar. Each sample was mounted on a stub using double-sided adhesive tape and then placed in the ultrahigh vacuum analysis chamber of the spectrometer. To prevent the sample from becoming positively charged when irradiated due to emission of photoelectrons, the sample surface was bombarded with an electron beam (10 eV) from a “flood gun” within the analysis chamber. Analysis and deconvolution of the resulting spectra was performed using the XPSPeak v4.1 software.43 Assignment of the spectral peaks was made using the UKSAF43 and NIST44 databases. All spectra were corrected relative to the graphitic C 1s peak position (284.6 eV)44 to account for the effect of the flood gun. Photolysis experiments were peformed using a Hanovia medium-pressure mercury lamp. 2.2. Synthetic Procedures. The synthesis of 3-(trifluoromethyl)3-(3-hydroxyphenyl)diazirine, detailed in the Supporting Information, was adapted from the report of Hayes et al.22 (Scheme 1) and performed to avoid exposing the samples to direct sources of light. Synthesis of Ferrocene Monocarboxylic Acid (7). The synthesis of ferrocene monocarboxylic acid was carried out according to the method of Little and Eisenthal.45 A blue solution of diphenyl carbamoyl chloride (12.51 g, 54 mmol) in anhydrous 1,2-dichloroethane (60 mL) was added to a yellow suspension of AlCl3 (10.80 g, 81 mmol) in 1,2-dichloroethane (30 mL). An orange solution of ferrocene (10.00 g, 54 mmol) in 1,2-dichloroethane (140 mL) was added dropwise over 10 min, and the dark reaction mixture was brought to reflux under a nitrogen atmosphere for 16 h; the mixture then was quenched via the addition to water (150 mL). The organic layer was extracted and the 3742

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Chemistry of Materials aqueous layer was washed with 1,2-dichloroethane (2  50 mL). The combined organic layers were concentrated in vacuo to give a brown residue, which was suspended in 20% w/v ethanolic KOH solution (100 mL) and refluxed under a nitrogen atmosphere for an additional 21 h before being concentrated in vacuo, resuspended in water (100 mL), cooled, and acidified with concentrated HCl to yield a dark orange precipitate. The precipitate was filtered and washed with water to give the crude product as a brown solid. Next, the crude product was purified by dissolution in 5% w/v NaOH solution to give a brown suspension, which was stirred with activated carbon before filtration. The brown/orange filtrate was concentrated in vacuo to half its initial volume before being cooled and reprecipitated with concentrated HCl. Filtration gave (7) (0.98 g, 4.3 mmol) as golden brown crystals in 7.9% yield. UV
vis (MeOH): λ 448.5 nm. νmax (solid)/cm
1: 3386 (broad), 2940, 2870, 2628, 2554, 1650, 1473, 1281, 1158, 825. 1H NMR (400 MHz, CDCl3) δ 4.85 (m, 2H), 4.47 (m, 2H), 4.26 (s, 5H). Synthesis of 3-[3-(Trifluoromethyl)diazirin-3-yl]phenyl Ferrocene Monocarboxylate (8). Ferrocene monocarboxylic acid (7) (0.23 g, 1.0 mmol), DCC (0.20 g, 1.0 mmol), DMAP (60 mg, 0.49 mmol), and (6) (0.21 g, 1.0 mmol) were dissolved in degassed, anhydrous DCM (20 mL) to form a red/brown solution. The reaction mixture was stirred under a nitrogen atmosphere, in darkness, for 48 h to form a white precipitate suspended in a red/brown solution. The mixture was filtered and concentrated in vacuo to give the crude product (0.52 g) as a brown viscous oil containing some solid material. Purification via column chromatography on silica (1:2 hexane:DCM) and concentration in vacuo gave the crude product (0.22 g) as an orange oil. The crude product (0.10 g) was taken up in DCM (20 mL) and extracted with 0.1 M NaOH(aq) solution (3  10 mL). The aqueous phase was extracted with DCM (10 mL) and combined organic phases were concentrated in vacuo to give (8) (0.09 g, 0.22 mmol) as an orange oil in 48% yield. [Mass and yield are reported for the diazirine component of the product mixture.] UV
vis (MeOH) λ 349, 453 nm. νmax (oil)/cm
1 3100, 2930, 1730, 1610, 1586, 1493, 1455, 1270, 1239, 1193, 1153, 1094. 1H NMR (300 MHz, CDCl3): δ 7.47* (m, 2H), 7.39 (m, 1H), 7.30* (m, 2H), 7.26 (m, 1H), 7.11* (m, 2H), 7.08 (m, 1H), 7.03* (m, 2H), 6.99 (m, 1H), 5.07 (t, J = 1.9 Hz, 2H), 4.98* (t, J = 1.9 Hz, 4H), 4.65 (t, J = 1.9 Hz, 2H), 4.55* (t, J = 1.9 Hz, 4H), 4.33* (s, 6H), 4.34 (s, 3H). 19F NMR (282 MHz, CDCl3): δ
65.1* (s, 6F),
65.2 (s, 3F). 19F NMR (282 MHz, CDCl3): δ
65.1* (s, 6F),
65.2 (s, 3F). [Asterisks (*) denote the corresponding azine side-product assignments; see section 3.2.] 2.3. Surface Modification of MWCNTs Using 3-[3-(Trifluoromethyl)diazirin-3-yl]phenyl Ferrocene Monocarboxylate 2.3.1. Photolysis. A sonicated suspension of either unmodified b-MWCNTs or unmodified h-MWCNTs (2.5 mg in 4 mL of DCM) was drop-cast as a thin film onto a microscope slide and the solvent allowed to evaporate. An aliquot of 3[3-(trifluoromethyl)diazirin-3-yl]phenyl ferrocene monocarboxylate (8) (1.8 mg, 0.004 mmol) in DCM (1 mL) was then dropcast over the MWCNT film ensuring complete coverage, and the solvent allowed to evaporate. [Mass and molar quantity is reported for the diazirine component of the mixture.] The microscope slide was then irradiated under UV light for 24 h before being washed off the microscope slide with EtOH and filtered on a B€uchner funnel. The black residue was successively washed with copious amounts of DCM, acetone, water, Et2O and acetone until the filtrate ran colorless, and then the material was allowed to

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Figure 1. Twenty (20) overlaid cyclic voltammograms recorded for 1.0 mM m-diazirine (6) in MeCN at a scan rate of 100 mV s
1.

air-dry to yield the 3-[3-(trifluoromethyl)diazirin-3-yl]phenyl ferrocene monocarboxylate modified b-MWCNTs or h-MWCNTs (Fc-b-MWCNTs Fc-h-MWCNTs) as a fine black powder. 2.3.2. Pyrolysis. Unmodified b-MWCNTs (23 mg) were suspended in an orange solution of 3-[3-(trifluoromethyl)diazirin-3yl]phenyl ferrocene monocarboxylate (8) (7.8 mg, 0.019 mmol) in Et2O (2.5 mL) to give a black suspension. [Mass and molar quantity is reported for the diazirine component of the mixture.] The solvent was allowed to evaporate and the sample was heated at 120 °C, in darkness, for 70 h. The MWCNTs were washed onto a B€uchner funnel with EtOH, and the residue was washed with copious amounts of DCM, acetone, water, Et2O, and acetone and allowed to air-dry. 2.4. Surface Modification of MWCNTs with Ferrocenemethanol at Edge-Plane Defect Sites. The covalent coupling of ferrocenemethanol to surface carboxylate groups that decorate the edge-plane-defect sites of b-MWCNTs was performed as follows. A suspension of b-MWCNTs (200 mg) was first oxidized by stirring in a mixture of 3 M HNO3 (25 mL) and 3 M H2SO4 (25 mL) for 4 h at 85 °C in order to increase the number of surface carboxyl groups for covalent attachment.11 The suspension was then filtered, washed with water until the filtrate ran to neutral pH, and then allowed to dry in an oven at 90 °C for 2 h to give oxidized b-MWCNTs as a fine black powder. Next, thionyl chloride (5 mL, 68.8 mmol) was added to the oxidized b-MWCNTs (5 mg) under a dry nitrogen atmosphere and the suspension was subjected to sonication before being left to stir at room temperature for 3 h. The excess thionyl chloride was removed in vacuo to afford dry acyl-chloride-modified b-MWCNTs, to which a yellow solution of ferrocenemethanol (10 mg, 0.05 mmol) in anhydrous DCM (20 mL) was added together with 1 mL of anhydrous triethylamine. The reaction mixture was sonicated to resuspend the MWCNTs, which were then allowed to stir for an additional 17.5 h, under nitrogen at room temperature. The suspension was filtered, washed with copious amounts of EtOH and acetone until the filtrate ran colorless, and then allowed to air-dry to give ferrocene-functionalized edge-plane-defect-modified b-MWCNTs (epd-Fc-MWCNTs) as a fine black powder.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of 3-(Trifluoromethyl)3-(3-hydroxylphenyl)diazirine. 3-(Trifluoromethyl)-3-(3-hydro-

xylphenyl)diazirine (6) was prepared according to the method reported by Hayes et al.22 and shown in Scheme 1. 3743

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Scheme 2. Preparation of 3-[3-(trifluoromethyl)diazirin-3yl]phenyl Ferrocene Monocarboxylate (8) (Also Shown is the Structure of the Ferrocene-Functionalized Azine Side Product)a

Figure 2. UV
vis spectrum of 8. Inset shows the UV
vis spectrum of 6.

The overall yield was 57%; the products from each step of the synthesis were characterized by Fourier transform infrared spectroscopy (FT-IR), 1H NMR, 19F NMR, and UV
vis spectroscopy and were in excellent agreement with the literature. In particular, the m-diazirine (6) exhibits a singlet present at
65.1 ppm in the 19F NMR spectrum, in agreement with the chemical shift reported for the deprotected diazirine product,22 and an adsorption band is observed in the UV
visible spectrum at 345 nm, corresponding to an electronic π-to-π* transition in the azo (NdN) moiety of the diazirine 6 (see Figure 2).22 Electrochemical characterization of 6 (1.0 mM), in acetonitrile containing 0.1 M TBAP, was performed using cyclic voltammetry (see Figure 1). The m-diazirine was observed to undergo an apparently electrochemically irreversible reduction at
1.86 V vs ferrocene (labeled as system I in Figure 1), at a scan rate of 100 mV s
1. At scan rates above 900 mV s
1, a very small corresponding oxidative peak was observed on the reverse scan (see Figure 1 in the Supporting Information). This behavior is suggestive of an EC mechanism, using Testa-Reinmuth notation, where the initial heterogeneous electron transfer step is followed by a homogeneous chemical step. The appearance of an oxidation wave for system I indicates that, at scan rates of >900 mV s
1, the kinetics of the follow-up chemical step have begun to be outrun on the voltammetric time scale. This phenomenon has been observed previously for 3-n-butylphenyldiazirine,46 where the radical anion intermediates formed upon the initial reduction undergo a follow-up homogeneous chemical decomposition. Further kinetic analysis was not attempted. Upon scanning to more oxidizing potentials, a separate irreversible oxidative peak (system II) was observed at +1.42 V vs Fc. This can be assigned to the oxidation of the phenol moiety to form a reactive radical cation intermediate, which undergoes rapid polymerization and renders the process electrochemically irreversible. After recording 20 repeat cycles at 100 mV s
1 in the presence of 1.0 mM diazirine, the glassy carbon electrode (GCE) was removed from the electrolyte, carefully rinsed with acetonitrile, and placed in a fresh solution of acetonitrile containing only 0.1 M TBAP. Upon subsequent examination using cyclic voltammetry, no evidence for the electrochemically induced surface modification of the GCE by 6 was observed. 3.2. Synthesis and Characterization of 3-[3-(Trifluoromethyl) diazirin-3-yl]phenyl Ferrocene Monocarboxylate. Having successfully synthesized the m-diazirine (6) as our “linker” molecule, we next coupled this via an ester linkage to ferrocene monocarboxylic acid (7) to form 3-[3-(trifluoromethyl)diazirin3-yl]phenyl ferrocene monocarboxylate (8) (see Scheme 2).

a

(a) denotes the following conditions: FcCO2H (7), DCC, DMAP, DCM, RT, 48 h. 1

H NMR characterization of the crude product indicated the presence of DMAP and unreacted DCC coupling agent methyl proton signals at δ 3.09 ppm and pyridine proton signals at δ 6.57 and 8.19 ppm; unreacted DCC signals observed in the δ 1
2 ppm range) and ATR FT-IR indicated the presence of some unreacted ferrocene monocarboxylate (a broad O
H stretch at 3385 cm
1 and a carboxylic acid CdO stretch at 1702 cm
1). Subsequently, column chromatography, followed by basic extraction, was performed in an attempt to refine the crude product. 1H NMR and ATR FT-IR spectroscopy indicated that the residual DMAP, DCC, and FcCOOH impurities had been removed. A CdO stretching mode vibration observed in the ATR FT-IR spectrum of the refined product at 1730 cm
1 gives evidence for the successful preparation of the ester (8). UV
vis spectroscopy of the final product, 8, showed the presence of two absorption bands. The MLCT band resulting from electronic transitions within the ferrocene moiety was observed at 453 nm, while the presence of a shoulder at 349 nm was indicative of the presence of diazirine, corresponding to π-to-π* transitions in the azo moiety of the molecule (see Figure 2). The 1H and 19F NMR spectra of the refined product, 8, are indicative of a mixture of products that we identified (see the Supporting Information) as the ferrocene-functionalized metadiazirine product 8 and the corresponding inert azine formed from the intermolecular reaction of two diazirine molecules (see Scheme 2). The ferrocene-functionalized azine and diazirine 8 have similar physicochemical properties and, as such, resisted further, extensive attempts at separation and purification. Fortunately, the azine impurity is not reactive under the photolysis conditions used to modify the MWCNTs in the next section;22 it is simply washed off the MWCNTs after modification with the active diazirine (8). Cyclic voltammetric characterization of 8 (1.0 mM) in acetonitrile was performed and compared to the observed voltammetry of 6 (see Figure 3). The diazirine (system I) was again observed to undergo electrochemical reduction at a peak potential of
1.83 V vs ferrocene at a scan rate of 100 mV s
1, in excellent agreement with the observed behavior of 6. An irreversible oxidation peak observed at more positive potentials in the case of 6, corresponding to the oxidation of the phenolic 3744

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Figure 3. Overlaid cyclic voltammograms comparing the m-diazirine (6) and the ferrocene-functionalized diazirine (8) recorded for 1.0 mM solutions in MeCN at a scan rate of 100 mV s
1.

moiety (system II), is not observed in the voltammetry of 8, confirming that (i) the coupling to the ferrocene derivative 7 was successful and (ii) system II does indeed likely correspond to the deprotected phenol moiety. Furthermore, a reversible oneelectron redox process (system III) corresponding to the ferrocene/ferrocenium redox couple was observed at a midpeak potential (E1/2) of +0.29 V vs Fc. The shift to a more positive potential for the ferrocenyl moiety in 8, compared to ferrocene itself, reflects the increased electron-withdrawing character of the ester group. A smaller, quasi-reversible redox couple is also observed at a midpeak potential (E1/2) of +0.57 V vs Fc, and it is likely attributable to the ferrocene-coupled azine impurity that was observed in the 1H and 19F NMR spectra of 8. 3.3. Surface Modification of Carbon Nanotubes with Ferrocene Moieties Using Diazirine Molecular “Tethers”. We are interested in the relative ability of our proposed diazirine “linker” molecules to covalently modify the sidewalls of CNTs versus modification of the surface oxo-group-rich edge-plane defects at the termini of the tubes. To this end, we compared the surface modification of MWCNTs of two common morphologies, namely, “bamboo-like” (b-MWCNTs) and “hollow-tube” (h-MWCNTs) multiwalled carbon nanotubes, the morphologies of which are shown in Scheme 1 and Figure 10 of ref 7. The concentric carbon tubes from which the MWCNTs are formed are aligned parallel to the CNT axis in the case of h-MWCNTs, and the h-MWCNTs remain open along their entire length. In the case of b-MWCNTs, the concentric graphene tubes are “rolled-up” at an angle to the principal CNT axis and terminate regularly along the outer wall of the b-MWCNT. The b-MWCNTs are also compartmentalized along the nanotube length, much like the structure of bamboo from which their name is derived. The result of this is that the b-MWCNTs have a much greater degree of edge-plane sites along the sides of the b-MWCNTs where the individual tubes terminate compared to the h-MWCNTs, which have a more pristine sidewall, and only have edge-plane defects located at the ends of the h-MWCNTs and around hole defects in the nanotube structure. Carbenes have been generated from the parent diazirines using either photolysis or pyrolysis. We attempted to modify samples of b-MWCNTs or h-MWCNTs separately, using each method in order to compare their efficacy, as described in section 2.3. Cyclic voltammetric characterization of the MWCNTs resulting from the pyrolysis experiments indicated that this approach was not successful in modifying the MWCNTs. No

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redox features consistent with the presence of surface ferrocene groups were observed; indeed, the voltammetry of the MWCNTs after pyrolysis and subsequent washing (to remove any physisorbed material) was identical to the unmodified MWCNTs. Upon prolonged irradiation of 8 with UV light in the presence of either the b- or h-MWCNTs, the functionalized diazirine eliminated dinitrogen to afford the highly reactive carbene intermediate, which could then undergo [2 + 1] cycloaddition reactions with the MWCNTs. Note that solvent was not employed in the photolysis reactions, to avoid quenching of the carbene intermediates via solvolysis. The resulting ferrocenemodified MWCNTs (Fc-b-MWCNTs or Fc-h-MWCNTs) were characterized using nonaqueous cyclic voltammetry in conjunction with X-ray photoelectron (XPS) and Raman spectroscopic surface analysis techniques. 3.3.1. Spectroscopic Surface Characterization of Fc-bMWCNTs and FC-h-MWCNTs. XPS characterization of the Fcb-MWCNTs and Fc-h-MWCNTs was performed using the Fe and F heteroatoms as spectroscopic labels, because these are exclusive to the ferrocenyl and trifluoromethyl moieties within the structure of the modifier. A survey scan was first performed on each of the modified MWCNT materials from 0 eV to 1400 eV (see Figure 4). In both cases, peaks corresponding to emission from the Fe 3s, C 1s, O 1s, and F 1s levels were observed. A broad, somewhat indistinct signal corresponding to electrons emitted from the Fe 2p level is evident upon closer inspection of the survey scans at ∼715 eV.44 The observed F and Fe signals give evidence for the presence of modifier on the MWCNT surfaces. The magnitude of the F 1s peak, in comparison to that of the C 1s peak, which arises predominantly from the underlying MWCNTs, suggests that the photolysis of the diazirine 8 has resulted in a significant degree of surface functionalization, even taking into account the relative atomic sensitivities of the C 1s and F 1s peaks (0.25 and 1, respectively).43 No peaks corresponding to emission from the N 1s region at 400 eV were observed, suggesting that any unreacted diazirine and/or azine side products were washed off the MWCNT surface during the workup. Next, several cumulative scans were recorded over the F 1s and Fe 2p regions of the Fc-b-MWCNT and Fc-h-MWCNT samples, shown in Figure 5. Deconvolution of the Fe 2p1/2, Fe 2p3/2, and F 1s peaks was achieved by fitting optimized Gaussian
Lorentzian functions within the XPSPeak software43 to the experimental data using a Shirley baseline interpolation. The Fe 2p peaks were deconvoluted into spin
orbit-coupled contributions with binding energies of ca. 711 and 719 eV, corresponding to electrons from the Fe 2p3/2 and Fe 2p1/2 electronic levels, respectively.44 The elemental surface compositions of each sample given in Table 1 were determined using the peak areas, which were then normalized to correct for the relative atomic sensitivity factors of each element.43 Note that the atomic sensitivity factor of Fe is variable, depending on its chemical environment; therefore, a value of 2.00, suggested in the UKSAF database,43 was used throughout. The resulting F:Fe ratios were determined to be 2.2:1 and 2.8:1 for the Fc-b-MWCNTs and Fc-h-MWCNTs, respectively. This implies that 70%
90% of the diazirine “linker” molecules attached to the surface are coupled to a ferrocene moiety, in good agreement with the 1H NMR data discussed in section 3.2, which suggested that 60%
80% of the diazirine 6 had been successfully coupled to ferrocene groups in the synthesis of 8. 3745

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Figure 4. XPS survey spectra recorded for (a) Fc-b-MWCNTs and (b) Fc-h-MWCNTs. Peaks marked with an asterisk (*) indicate signals resulting from the underlying copper XPS stub.

Figure 5. Cumulative XPS spectra recorded over the F 1s and Fe 2p region with the deconvoluted simulated spectra overlaid for (a) Fc-b-MWCNTs and (b) Fc-h-MWCNTs.

Table 1. Elemental Surface Composition of b-MWCNTs and h-MWCNTs Modified with 8, As Determined from XPS Analysis Elemental Surface Composition (%) sample

C

O

F

Fe

Fc-b-MWCNTs

63.6

18.6

12.2

5.6

Fc-h-MWCNTs

57.4

18.9

17.5

6.2

factor of iron), which is exclusive to the covalently bound modifier molecule, were employed for a semiquantitative analysis. If we define the relative surface coverage of the ferrocenefunctionalized modifier as θ* = kθ (where θ is the true surface coverage and k is a constant of proportionality), then θ* is simply one-third of the ratio of the F 1s peak area to the sum of the C 1s and O 1s peak areas (since there are three F atoms per modifier molecule): θ ¼

It is not possible to reliably distinguish between the C 1s and O 1s signal contributions that result from the MWCNTs and those that result from the ferrocene-functionalized modifier molecules. However, in the case of the h-MWCNTS, there is clearly an anomalously large contribution from the O 1s peak. One possible explanation for this is that prolonged photolysis under open atmosphere can lead to a degree of aerial oxidation of the MWCNTs,47 which occurs to a greater extent on the h-MWCNTs than for the b-MWCNTs as the latter intrinsically possess a larger number of surface oxo-groups at the edge defects. While this prevents a quantitative analysis from being performed, the peak areas arising from the F 1s peak (rather than the Fe peaks, again due to uncertainty in the relative atomic sensitivity

AF 1s 3ðAC 1s þ AO 1s Þ

Doing this yields the relative surface coverage of diazirine molecules (θ*) of 5.0% for the Fc-b-MWCNTs and 7.6% for the Fc-h-MWCNTs. As the modifier contribution toward the C 1s and O 1s peaks was not corrected for, the relative surface coverages are underestimates of the true values. However, if the relative surface coverages are expressed as a ratio comparing b-MWCNT to h-MWCNT (θb/θh), thus removing the proportionality constant, k, and this ratio is compared to the same ratio of surface coverages obtained independently from the cyclic voltammetric characterization of the two different modified MWCNTs (see section 3.3.2 below), we obtain values of 0.64 from the XPS analysis and 0.63 from the voltammetric analysis. 3746

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Figure 6. (a,b) Raman spectra (offset for clarity) obtained using 532-nm excitation recorded before and after modification of the MWCNTs with 8 ((a) b-MWCNT samples and (b) h-MWCNT samples). (c) TEM images of the h-MWCNT sample after modification with the ferrocenefunctionalized diazirine.

Reassuringly, the results from these two markedly different techniques are in excellent agreement. Next, the Fc-b-MWCNTs and Fc-h-MWCNTs were characterized using Raman spectroscopy to determine the extent of the sidewall modification that had occurred during photolysis of 8. The Raman spectra of MWCNTs gives rise to two principal spectral bands of interest, namely, the D and G bands. The D band corresponds to the CNT radial breathing mode and is typically found at a Raman shift of ∼1330 cm
1, while the G band corresponds to the CNT tangential stretching mode, and is typically found at a Raman shift of ∼1580 cm
1.10 The intensity ratio of the D and G bands (ID/IG) and the corresponding full width at half maximum (fwhm) values can be used as an indicator for the degree of structural disorder present in the CNTs. This is directly related to the extent of sidewall functionalization and originally stems from a Raman spectroscopic investigation on single graphite crystals, performed by Tuinstra and Koenig.48 A relative ID/IG ratio that is much greater than 1 implies a large degree of disorder and suggests sidewall functionalization. Conversely, a relative ID/IG ratio close to 0 is indicative of a highly ordered, crystalline sample and is typical of pristine (unmodified) SWCNTs. Commercially available MWCNTs typically have ID/IG ratios of ∼0.5
1.5.10 An increase in the fwhm value, for either of the bands, suggests an increase in disorder and is indicative of sidewall functionalization. The Raman spectra recorded for the Fc-b-MWCNTs and the Fc-h-MWCNTs, using a 532-nm laser, are shown in Figure 6,

along with the Raman spectra recorded for samples of the unmodified MWCNTs, for comparison. The D and G bands are clearly observed in the unmodified band h-MWCNT samples at 1333
1340 cm
1 and 1565
1575 cm
1, respectively, in agreement with previous studies.10 After photolysis and functionalization with the ferrocene-modified diazirine species, the Raman spectra of the Fc-b-MWCNTs is observed to hardly change. However, the Raman spectrum of the Fc-h-MWCNTs is altered dramatically. Disruption of the nanotube sidewall structure by the attack of the ferrocenefunctionalized carbene intermediate has occurred to such an extent that the symmetry of the h-MWCNTs has been lost, and no Raman D or G bands are observable. Only a very small band was observed for the Fc-h-MWCNT sample at a Raman shift of 1389 cm
1. The Raman spectra for both samples were recorded a second time in case the Fc-h-MWCNTs was a spurious experimental artifact. However, identical spectra were recorded for both the Fc-b-MWCNTs and crucially for the Fc-h-MWCNTs. Reassuringly, TEM images of the h-MWCNTs, after modification with the ferrocene-functionalized diazirine (8) (also shown in Figure 6), reveal that the basic structure of the h-MWCNTs remains intact, and no evidence of significant polymerization of the diazirine can be observed. A broad feature at ca. 1100 cm
1 was observed in both the Fc-b-MWCNT and Fc-h-MWCNT Raman spectra. By comparison with the Raman spectrum of a sample of ferrocene, this can be assigned to the ferrocene groups covalently bound to the MWCNT surfaces in both cases. 3747

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Table 2. Raman Spectral Data Obtained for the b-MWCNTs and h-MWCNTs before and after Modification with 8 Position (cm
1) sample

Full Width at Half Maximum, fwhm (cm
1)

Intensity (counts) G-band

D-band

G-band

intensity ratio, ID/IG

208

195

57

55

1.07

3256

3340

65

60

0.97

233

56

53

0.84

D-band

G-band

D-band

b-MWCNT

1334

1574

h-MWCNT

1334

1565

Fc-b-MWCNT

1340

1573

196

Fc-h-MWCNT

Figure 7. Ten (10) overlaid cyclic voltammograms recorded in MeCN containing 0.1 M TBAP (scan rate = 100 mV s
1) for (a) Fc-b-MWCNTs and (b) Fc-h-MWCNTs supported on a glassy carbon electrode (GCE).

A comparison between the observed D and G bands of unmodified and ferrocene-functionalized MWCNTs is given in Table 2. The ID/IG ratio and the fwhm values were not observed to change considerably upon functionalization of the pristine b-MWCNTs with the ferrocene-functionalized diazirine linker 8. This suggests that the carbene intermediate did not significantly change the structure of the b-MWCNT basal-plane-like regions; there is almost no loss of pristine structure. Since the results from cyclic voltammetry (see below) and XPS characterization provide strong evidence for the surface modification of the b-MWCNT with ferrocene-modified diazirine groups, it may be concluded that the carbene species has mainly inserted into the C
O and O
H bonds of the surface oxo-groups decorating the numerous edge-plane defects within the b-MWCNT structure. The majority of the available b-MWCNT surface that is exposed to the carbene species is predominantly edge-plane-like in nature, and therefore is rich in surface oxo-groups such as hydroxyl, carboxyl, and quinonyl.10
13 Any basal-plane-like regions within the b-MWCNTs likely are effectively shielded from attack by either the steric bulk of the oxygen-containing functional groups or by overlap with neighboring tube walls arranged like a stack of paper cups (e.g., “,,”). Ismaili et al. observed a similar result when they functionalized MWCNTs using a gold nanoparticle-functionalized diazirine linker, although they did not state the morphology of the MWCNTs that they used.42 3.3.2. Cyclic Voltammetric Characterization of Fc-bMWCNTs and FC-h-MWCNTs. Having spectroscopically characterized the Fc-b/h-MWCNTs cyclic voltammetry was then performed in MeCN containing 0.1 M TBAP as supporting electrolyte, to confirm that the diazirine “linker” had covalently

tethered the Fc groups to the MWCNT surfaces and that these materials had not simply been physisorbed onto the surface of the nanotubes. The Fc-b-MWCNTs or Fc-h-MWCNTs were suspended in DCM (1 mg/mL) with sonication and a 20 μL aliquot was dropped onto the surface of a GCE and the solvent allowed to evaporate, leaving the modified MWCNTs as an immobilized layer on the surface of the GCE. Twenty (20) repeat cycles between 0.0 V and +1.75 V vs Ag were then performed at a scan rate of 100 mV s
1 (see Figure 7). An electrochemically quasireversible one-electron redox process was observed for either morphology of MWCNTs corresponding to the ferrocene/ ferrocenium ion redox couple, at a midpeak potential (E1/2) of +0.31 V vs ferrocene itself. The almost-symmetric voltammetric peak shape is consistent with a surface-bound species rather than that of a species in solution where the voltammetry is under diffusion control.49 The observed midpeak potential is in excellent agreement with the voltammetry of 8 recorded in solution. Unlike the voltammetry of 8, however, no redox peaks were observed corresponding to the ferrocene-modified azine impurity. Over an increasing number of scans, the Faradaic oxidative and reductive peak currents were observed to very slowly decrease. For a surface-bound redox active species, the peak currents should remain constant, and this might lead one to naively conclude that the ferrocene was actually physisorbed, not covalently bound to the surface, and was slowly leaching from the electrode over time. However, upon closer examination, the background capacitive charging current (which is directly proportional to the electroactive surface area of the electrode) was also found to be slowly decreasing over time. A plot of the peak 3748

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Figure 8. Plots showing the relative change (expressed as a percentage relative to the first scan values) in both the peak currents (ip) and capacitive background charging currents (Cdl) (recorded at 0.500 V) with successive scans for the voltammograms shown in Figure 6 for (a) Fc-b-MWCNTs and (b) Fc-h-MWCNTs.

currents (ip) measured for each successive cycle versus the background capacitive charging current (ΔCdl) measured for each cycle at 0.50 V vs Ag (where no Faradaic current flows) is given in Figure 8. Both quantities were expressed as a percentage of their respective first scan (n = 1) values, (ip)n/(ip)1 and (ΔCdl)n/(ΔCdl)1. Figure 8 clearly demonstrates that the decrease in peak current is directly correlated to the decrease in the electroactive surface area. In other words, the modified MWCNTs are simply gradually dropping off the electrode surface in the nonaqueous electrolyte, taking many covalently attached redox active species with them. The instability of immobilized CNT films on electrode surfaces in contact with a nonaqueous electrolyte is an often-encountered problem, exacerbated by the common practice of placing electrodes into solution in an inverted (facedown) orientation. We have developed a universal solution to this problem that we will publish in a separate report currently in preparation. Next the voltage scan rate (ν) was varied from 50 mV s
1 to 750 mV s
1 (see Figure 2 in the Supporting Information). For an ideal reversible redox system, the oxidative and reductive peakto-peak separation (ΔEp) remains constant with increasing scan rate. Tsierkezos previously demonstrated that ferrocene exhibits reversible oxidation when in acetonitrile-based solution, because no variation in peak-to-peak separation (ΔEp) from the ideal value of 57 mV (at 293 K) was observed with increasing scan rate.50 For a reversible surface-bound species, ΔEp should be 0 mV.49 The peak-to-peak separation for the Fc-b-MWCNTs and Fc-h-MWCNTs were found to be larger than zero, increasing from 42 and 31 mV, respectively, at scan rates of 50 mV s
1 to 228 and 191 mV at 750 mV s
1. This is indicative of a system with quasi-reversible electron transfer kinetics. MWCNTs typically exhibit fast electron transfer kinetics, with electron transfer predominantly occurring at the edge-plane defects.7,10 Therefore, this quasi-reversible behavior is related to the nature of the ferrocene-functionalized MWCNT surface structure. The carbene that forms from the ferrocene-functionalized diazirine 8 passivates the edge-plane defects by insertion into C
O, O
H, or terminal C
H bonds, which may reduce the number of available electron transfer sites.51 It is also likely that the conductivity of the MWCNTs is reduced due to disruption of the graphitic basal-plane-

like sidewall resulting from carbene [2 + 1] cycloaddition reactions reducing the degree of unsaturation of the nanotubes and forming localized sp3 hybridized carbon atoms. Finally, plots of measured peak currents versus the variation in voltage scan rate were constructed. These can be used as a diagnostic criterion to distinguish between redox active species in solution, where ip is proportional to ν1/2 for a fully reversible system, and surface-bound species where ip is usually linearly proportional to ν.49 For both the Fc-b-MWCNTs and Fc-hMWCNTs, the peak currents were found to be linearly proportional to ν1/2, suggesting that the voltammetry is under diffusion control and is not that of a surface-bound species (see Figure 3 in the Supporting Information). However, both the voltammetric wave shape and the peak-to-peak separation data are consistent with a surface-bound quasi-reversible system. This apparent contradiction has been seen in previous studies of chemically modified CNTs in aqueous solutions, and an explanation has been put forward by Thompson et al.52 Estimates of the diffusion coefficient obtained from the data in Figure 3 in the Supporting Information are inconsistent with a ferrocene derivative diffusing in solution (differing by 4 orders of magnitude). Amatore et al. were first to report this apparently contradictory phenomenon for a system of hemispherical redox active dendrimers that were attached to an electrode surface.53 Thompson et al., who had observed this effect previously for covalently surface-functionalized MWCNTs, rationalized the phenomenon by considering charge movement over the surface of a sphere.52 They found that their proposed model was in excellent agreement with experimental results. From this, they concluded that the observed square-root scan-rate dependence of the Faradaic currents resulting from molecules that were covalently attached to micrometer- or nanometer-sized spherical or cylindrical supports on a planar electrode surface is a result of the diffusion of charge via an “hopping” mechanism between redox centers around the curvature of the conducting support material.52 Therefore, the unusual square-root scan-rate dependence of ip is not contradictory with a surface-bound species, as indicated by the other data analysis and characterization. The problem may also be further exacerbated by the removal of redox active material as the CNTs gradually fall off the electrode during the experiments. Finally, we note that, when a thin film of 8 was photolyzed 3749

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directly onto the surface of a GCE working electrode, the identical voltammetric behavior was observed, except that the peak currents were reassuringly found to vary linearly with the voltage scan rate, as expected for a surface-bound species. This indicates that the use of diazirines is a versatile approach to modifying the different commonly encountered forms of graphitic materials. The surface concentrations (Γ) of the ferrocene-functionalized modifier present on the MWCNT surfaces were estimated using Faraday’s First Law. The peak area (charge passed) measured using the first scan recorded for each modified MWCNT material is proportional to the surface coverage fo redox active material at a given scan rate, given by eq 1: Γ¼

Q nFA

ð1Þ

where Q is the charge passed ((peak area)/C), n the number of electrons transferred (n = 1 in this case), and F the Faraday constant (F = 96 485 C mol
1). The surface area (A) in eq 1 was estimated assuming a geometric surface area of MWCNTs based on knowledge of their average dimensions and density, and the amount of material placed on each electrode. The modifier surface concentration (Γ) on the ferrocene-functionalized band h-MWCNTs was estimated to be (1.4 ( 0.3)  10
11 and (2.3 ( 0.3)  10
11 mol cm
2, respectively. Using the same approximations for the surface area of each CNT, and considering that the geometric volume is occupied by one ferrocene-functionalized diazirine molecule, allows us to estimate the theoretical minimum possible surface concentration for the formation of a monolayer (i.e., complete surface saturation). In turn, this then allows us to estimate the surface coverage (θ) obtained for the Fc-b-MWCNTs and Fc-hMWCNTs as being 0.07 and 0.11, respectively. This result implies that the surface loading achieved using the ferrocene-functionalized diazirine “linker” was as much as 7% and 11% of the theoretical capacity. Note that these values are based on several geometrical assumptions and, therefore, are subject to a relatively large error and should be used only as a qualitative guide. The surface coverage that was achieved using the b-MWCNTs was only ca. 63% of that achieved with the h-MWCNTs, which is in excellent agreement with the results obtained from XPS analysis in the previous section. This again likely reflects the relative availability of CdC bonds in the exposed basal-plane-like sidewall versus edge-plane functionalization at surface oxogroups between the two different MWCNT morphologies. 3.4. Surface Modification of Carbon Nanotubes via an Edge-Plane Defect Functionalization Method. Finally, as a control experiment, and to examine if the use of diazirines as molecular “tethers” allows for the improved surface modification of CNTs, a sample of b-MWCNTs—which should have the highest number of edge-plane-like sites for modification of the h-MWCNTs—was covalently modified using a traditional approach that only modifies the edge-plane-like defect sites on the surface of the nanotubes. Ferrocene methanol was covalently bound to the b-MWCNTs via esterification with the surface carboxylate groups decorating the edge-plane sites, to form “edge-plane defectmodified” ferrocene-functionalized b-MWCNTs (epd-Fc-MWCNTs; see section 2.4). Two key steps were taken to ensure that the comparison between the control method (forming epd-FcMWCNTs) and our diazirine-based approach (forming Fcb-MWCNTs) was unbiased toward the latter. First, to ensure

that the maximum edge-plane modification was achieved, the b-MWCNTs underwent a further oxidizing pretreatment step before the coupling reaction with ferrocene methanol to increase the number of surface carboxylate groups.2,11,13,15 Second, the sample of oxidized b-MWCNTs was exposed to ∼6 times as many molar equivalents of ferrocene methanol than the amount of 8 used in the diazirine-based approach to form the Fc-bMWCNTs. Again, identical voltammetric characterization of the epd-FcMWCNTs was performed as described above. This revealed that the modification of the b-MWCNTs with ferrocene groups was successful. An electrochemically reversible one-electron process, corresponding to the ferrocene/ferrocenium ion redox couple, was observed at a midpeak potential (E1/2) of +0.10 V vs ferrocene at 100 mV s
1. Using the same approximations as above, and the experimentally determined peak areas, the surface coverage (θ) of ferrocenyl moieties achieved using the edgeplane functionalization control method was determined to be 0.3% of the theoretical maximum for a complete geometric monolayer. The result is very significant. It implies that the use of diazirine “linker” molecules method allows more than 20 times the surface loading that can be achieved using a conventional, commonly used functionalization method.

4. CONCLUSIONS The surface modification of two different morphologies of carbon nanotubes (CNTs)—“bamboo-like” multiwalled carbon nanotubes (b-MWCNTs) and “hollow-tube” multiwalled carbon nanotubes (h-MWCNTs), as well as the surface of glassy carbon, was achieved using a ferrocene-functionalized meta-diazirine “linker” molecule that undergoes facile photolytic activation. No CNT pretreatment was required before the functionalization step. The carbene intermediate that forms upon photolysis of the diazirine was shown to react with the basal-plane-like sidewalls of the MWCNTs and at the edge plane defects. Voltammetric and XPS characterization of the resulting materials indicate that the MWCNT were modified with ferrocene (as a redox active “probe” molecule) moieties to as much as 7% and 11% of the maximum theoretical capacity for b-MWCNTs and h-MWCNTs, respectively. When compared to a conventional method for modifying carbon surfaces with molecules such as ferrocene, namely, ester-bond formation, the diazirine-based approach was found to improve the surface loading of modifier by as much as 20 times. The extent of sidewall functionalization using the diazirine as a molecular tether was assessed by Raman spectroscopy. The results of which suggest that the structure of the Fc-b-MWCNTs remained relatively unchanged, while there was strong evidence for disruption of the sidewall structure after functionalization of the h-MWCNT sample. Although some might argue that the six-step synthesis required to make the diazirine “tether” and then to couple it to the desired modifier reduces the utility of this approach, we would argue the converse. The synthetic steps are all facile, using readily available, standard laboratory glassware and standard synthetic techniques, which is quite within the technical ability of even physical electrochemists such as this author (G.G.W.)! The diazirine can be prepared on the multigram scale, and, if stored cold and in darkness, is stable and available for use for several months. The photolytic coupling step is facile and more convenient to perform than many conventional modification approaches, such as ester/amide-forming reactions or reduction of 3750

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Chemistry of Materials diazonium salts, for example. Furthermore, our method requires no pretreatment of the MWCNTs. Finally, derivatives of the 3-aryl-3-(trifluoromethyl)diazirines are sufficiently stabilized, to be compatible with a wide range of different chemistries. This, in turn, allows for the functional group meta to the diazirine group on the aryl ring to be readily substituted for many other useful groups or introduced in the para-position for different chemical coupling reactions toward a potentially vast range of organic, inorganic, and biological modifiers. While the example given here is targeted toward electrochemical applications, the scope of potential modifiers that are compatible with this diazirine-based approach lends itself to the possibility that this method of modifying carbonaceous materials may have applications in many other active areas of chemistry.

’ ASSOCIATED CONTENT

bS

Supporting Information. Synthetic procedures including FTIR, UV
vis, 1H NMR, and 19F NMR characterization data for the preparation of 3-(trifluoromethyl)-3-(3-hydroxylphenyl)diazirine (6); discussion of the 1H NMR, and 19F NMR characterization data for the preparation of 8 and assignment of the azine impurity; voltammetric characterization of 6 at variable scan rates; voltammetric characterization of the Fc-b-MWCNTs and Fc-h-MWCNTs materials at variable scan rates. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT G.G.W. and J.H.W. thank the Royal Society for support via University Research Fellowships. ’ REFERENCES (1) Guiseppi-Elie, A.; Shukla, N. K.; Brahim, S. Nanotechnol. Biol. Med. 2007, 27/1–27/14. (2) Karousis, N.; Tagmatarchis, N.; Tasis, D. Chem. Rev. 2010, 110, 5366–5397. (3) Martinez-Hernandez, A. L.; Velasco-Santos, C.; Castano, V. M. Chem. Carbon Nanotubes 2008, 2, 161–190. (4) Wildgoose, G. G.; Banks, C. E.; Leventis, H. C.; Compton, R. G. Microchim. Acta 2006, 152, 187–214. (5) Ravindran, S.; Chaudhary, S.; Colburn, B.; Ozkan, M.; Ozkan, C. S. Nano Lett. 2003, 3, 447–453. (6) Wong, S. S.; Joselevich, E.; Woolley, A. T.; Cheung, C. L.; Lieber, C. M. Nature 1998, 394, 52–55. (7) Banks, C. E.; Davies, T. J.; Wildgoose, G. G.; Compton, R. G. Chem. Commun. 2005, 829–841. (8) Abiman, P.; Wildgoose, G. G.; Compton, R. G. J. Phys. Org. Chem. 2008, 21, 433–439. (9) Abiman, P.; Wildgoose, G. G.; Compton, R. G. Int. J. Electrochem. Sci. 2008, 3, 104–117. (10) Holloway, A. F.; Wildgoose, G. G.; Compton, R. G.; Shao, L.; Green, M. L. H. J. Solid State Electrochem. 2008, 12, 1337–1348. (11) Masheter, A. T.; Xiao, L.; Wildgoose, G. G.; Crossley, A.; Jones, J. H.; Compton, R. G. J. Mater. Chem. 2007, 17, 3515–3524. (12) Thorogood, C. A.; Wildgoose, G. G.; Crossley, A.; Jacobs, R. M. J.; Jones, J. H.; Compton, R. G. Chem. Mater. 2007, 19, 4964–4974. (13) Wildgoose, G. G.; Abiman, P.; Compton, R. G. J. Mater. Chem. 2009, 19, 4875–4886.

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(14) Nguyen, C. V.; Delzeit, L.; Cassell, A. M.; Li, J.; Han, J.; Meyyappan, M. Nano Lett. 2002, 2, 1079–1081. (15) Majumder, M.; Chopra, N.; Hinds, B. J. J. Am. Chem. Soc. 2005, 127, 9062–9070. (16) Hu, H.; Zhao, B.; Hamon, M. A.; Kamaras, K.; Itkis, M. E.; Haddon, R. C. J. Am. Chem. Soc. 2003, 125, 14893–14900. (17) Georgakilas, V.; Kordatos, K.; Prato, M.; Guldi, D. M.; Holzinger, M.; Hirsch, A. J. Am. Chem. Soc. 2002, 124, 760–761. (18) Tagmatarchis, N.; Prato, M. J. Mater. Chem. 2004, 14, 437–439. (19) Callegari, A.; Cosnier, S.; Marcaccio, M.; Paolucci, D.; Paolucci, F.; Georgakilas, V.; Tagmatarchis, N.; Vazquez, E.; Prato, M. J. Mater. Chem. 2004, 14, 807–810. (20) Banerjee, S.; Wong, S. S. J. Phys. Chem. B 2002, 106, 12144–12151. (21) Mawhinney, D. B.; Naumenko, V.; Kuznetsova, A.; Yates, J. T., Jr.; Liu, J.; Smalley, R. E. J. Am. Chem. Soc. 2000, 122, 2383–2384. (22) Blencowe, A.; Cosstick, K.; Hayes, W. New J. Chem. 2006, 30, 53–58. (23) Liu, M. T. H.; Choe, Y.-K.; Kimura, M.; Kobayashi, K.; Nagase, S.; Wakahara, T.; Niino, Y.; Ishitsuka, M. O.; Maeda, Y.; Akasaka, T. J. Org. Chem. 2003, 68, 7471–7478. (24) Admasu, A.; Gudmundsdottir, A. D.; Platz, M. S.; Watt, D. S.; Kwiatkowski, S.; Crocker, P. J. J. Chem. Soc., Perkin Trans. 2 1998, 1093–1100. (25) Brunner, J.; Senn, H.; Richards, F. M. J. Biol. Chem. 1980, 255, 3313–3318. (26) Church, R. F. R.; Weiss, M. J. J. Org. Chem. 1970, 35, 2465–2471. (27) Graham, W. H. J. Am. Chem. Soc. 1962, 84, 1063–1064. (28) Paulsen, S. R. Angew. Chem. 1960, 72, 781–782. (29) Schmitz, E.; Ohme, R. Chem. Ber. 1961, 94, 2166–2173. (30) Schmitz, E.; Ohme, R. Angew. Chem. 1961, 73, 220–221. (31) Schmitz, E.; Ohme, R. Tetrahedron Lett. 1961, 612–614. (32) Graham, W. H. J. Am. Chem. Soc. 1965, 87, 4396–4397. (33) Rosenberg, M. G.; Brinker, U. H. J. Org. Chem. 2003, 68, 4819–4832. (34) Frey, H. M.; Scaplehorn, A. W. J. Chem. Soc. A 1966, 968–970. (35) Brunner, J.; Semenza, G. Biochemistry 1981, 20, 7174–7182. (36) Blencowe, A.; Hayes, W. Soft Matter 2005, 1, 178–205. (37) Blencowe, A.; Fagour, W.; Blencowe, C.; Cosstick, K.; Hayes, W. Org. Biomol. Chem. 2008, 6, 2327–2333. (38) Blencowe, A.; Caiulo, N.; Cosstick, K.; Fagour, W.; Heath, P.; Hayes, W. Macromolecules 2007, 40, 939–949. (39) Blencowe, A.; Blencowe, C.; Cosstick, K.; Hayes, W. React. Funct. Polym. 2008, 68, 868–875. (40) Ismaili, H.; Lee, S.; Workentin, M. S. Langmuir 2010, 26, 14958–14964. (41) Brooks, S. A.; Dontha, N.; Davis, C. B.; Stuart, J. K.; O’Neill, G.; Kuhr, W. G. Anal. Chem. 2000, 72, 3253–3259. (42) Ismaili, H.; Lagugne-Labarthet, F.; Workentin, M. S. Chem. Mater. 2011, 23, 1519–1525. (43) http://www.uksaf.org/. (44) http://srdata.nist.gov/xps/. (45) Little, W. F.; Eisenthal, R. J. Am. Chem. Soc. 1960, 82, 1577–1580. (46) Elson, C. M.; Liu, M. T. H. J. Chem. Soc., Chem. Commun. 1982, 415–416. (47) Savage, T.; Bhattacharya, S.; Sadanadan, B.; Gaillard, J.; Tritt, T. M.; Sun, Y. P.; Wu, Y.; Nayak, S.; Car, R.; Marzari, N.; Ajayan, P. M.; Rao, A. M. J. Phys.: Condens. Matter 2003, 15, 5915–5921. (48) Tuinstra, F.; Koenig, J. L. J. Chem. Phys. 1970, 53, 1126–1130. (49) Compton, R. G.; Banks, C. E. Understanding Voltammetry; World Scientific: Singapore, 2007. (50) Tsierkezos, N. G. J. Solution Chem. 2007, 36, 289–302. (51) Nichols, J. A.; Saito, H.; Hoefer, M.; Bandaru, P. R. Electrochem. Solid-State Lett. 2008, 11, K35–K39. (52) Thompson, M.; Wildgoose, G. G.; Compton, R. G. ChemPhysChem 2006, 7, 1328–1336. (53) Amatore, C.; Bouret, Y.; Maisonhaute, E.; Goldsmith, J. I.; Abruna, H. D. Chem.—Eur. J. 2001, 7, 2206–2226. 3751

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