Facile Covalent Modification of a Highly Ordered Pyrolytic Graphite

Aug 18, 2014 - *R.B.L.: fax, +1-514-398-3797; tel, +1-514-398-8304; e-mail, [email protected] ., *R.S.: fax, +1-514-340-7502; tel, +1-514-398-185...
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Facile Covalent Modification of a Highly Ordered Pyrolytic Graphite Surface via an Inverse Electron Demand Diels−Alder Reaction under Ambient Conditions Jun Zhu,†,‡ Jonathan Hiltz,† Mohamed Amine Mezour,† Vadim Bernard-Gauthier,‡ R. Bruce Lennox,*,† and Ralf Schirrmacher*,‡ †

Department of Chemistry and Centre for Self-Assembled Chemical Structures, McGill University, 801 Sherbrooke Street West, Montreal, Quebec H3A 2K6, Canada ‡ Montreal Neurological Institute & Hospital, McGill University, 3801 University Street, Montreal, Quebec H3A 2B4, Canada S Supporting Information *

ABSTRACT: Chemical derivatization of an sp2-hybridized carbon surface under ambient conditions is a challenge. Tetrazine-linked compounds are shown to react with an sp2hybridized 2D carbon surface (highly ordered pyrolytic graphite) under ambient conditions, via an inverse electron demand Diels−Alder reaction. Taking advantage of the facile derivatization of dichlorotetrazine with thiol- or hydroxylcontaining molecules, a variety of tetrazine-terminated functional molecules can be prepared and applied to the preparation of functional carbon materials. The methodology described here is the mildest approach reported to date for the derivatization of an sp2-hybridized carbon surface.

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INTRODUCTION The functionalization of sp2-hybridized carbon materials such as fullerenes, carbon nanotubes (CNT), graphene, and graphite has been the focus of considerable research effort over the past two decades. Noncovalent interactions, including van der Waals interactions, electrostatic interactions, and π−π stacking have been used for the modification of CNT and graphene.1,2 These relatively weak interactions do not, however, provide the eventual product stability that is needed in many applications. Covalent modification, on the other hand, is more challenging to achieve because of the low inherent reactivity of sp2hybridized carbons and the significant steric constraints imposed by reaction of a highly extended planar system. Whereas the curvature of fullerenes and CNTs lessens the steric constraints associated with covalent derivatization, the 2D nature of graphene introduces considerable steric demands on a derivatization reaction. Because chemical modification of graphene/graphite involves the conversion of sp2 carbon to sp3 carbon and a concomitant perturbation of the extended aromatic system, strain is introduced by the deformation of the planar system. Two factors may favor derivatization of graphene vs graphite. First, the strain associated with deformation is larger in graphite than in graphene, because the former’s interlayer cohesive energy must be locally overcome.3,4 Second, because graphene can undergo reactions on each side (e.g., top and bottom), the net strain of a derivatized product is lessened.5 Reactions that can be performed on graphite may thus be more facile when applied to graphene. The low reactivity of the sp2-hybridized carbon in graphite often leads to the use of residual functionalities (e.g., CHO, © XXXX American Chemical Society

CO2H, CHnOH) for chemical modification. These moieties arise on the edges or at internal defect sites of the carbon surface. For example, edge-localized carboxylic acid groups lead to ester formation when combined with hydroxy-terminated polymers.6 Alternatively, aryldiazonium chemistry has been extensively used to introduce new functionalities to graphene and highly ordered pyrolytic graphite (HOPG).7−10 Singlelayer graphene is more reactive than multilayer graphene, and its edges have been shown to be more reactive than the basal plane.11 Similarly, a diazonium salt covalently reacts at the edges and not the basal planes of defect-free exfoliated graphene.12 Multiple attachment of diazonium radicals is also observed, consistent with the tendency of the radical to react with surface-bound molecules rather than the carbon surface itself.13,14 Even though graphite or graphene can in principle serve as a dienophile or diene, few studies have been carried out with them using Diels−Alder chemistry. Haddon et al. have recently suggested that graphite/graphene can be treated as a diene when presented with a reactive dienophile (e.g., tetracyanoethylene, TCNE) at room temperature. Conversely, graphite/ graphene can serve as a dieneophile and reacts with the diene 2,3-dimethyl-1,3-butadiene (50 °C, 3 h).15,16 A follow-up study showed that cyclopentadienes exclusively react covalently with a graphene surface at edges and defect sites, under highpressure-facilitated Diels−Alder conditions.17 Even though the Received: June 23, 2014 Revised: August 15, 2014

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SiFA-S-Tz as a pink, crystalline solid (93% yield). 1H NMR (CDCl3, 400 MHz) δ (ppm): 7.73 (d, J = 8.1 Hz, 2H), 7.68 (d, J = 8.1 Hz, 2H), 1.08 (s, 18H). 13C NMR (CDCl3, 60 MHz) δ (ppm): 176.4, 166.1, 137.4 (d, J = 13.7 Hz), 135.3 (2C, d, J = 4.3 Hz), 134.2 (2C), 126.5, 27.3 (6C), 20.3 (2C). HR-MS (APCI): exact mass ([M + H]+, C16H25ClFN4SSi) calcd 385.1085, found 385.1089. Synthesis of 3-Chloro-6-((4-(di-tert-butylfluorosilyl)benzyl)oxy)-1,2,4,5-tetrazine (SiFA-O-Tz). SiFA-O-Tz was synthesized following the same procedure as described above. SiFA-O-Tz was generated as a red crystal with a 95% isolated yield. 1H NMR (CDCl3, 400 MHz) δ (ppm): 7.68 (d, J = 8 Hz, 2H), 7.56 (d, J = 8 Hz, 2H), 5.70 (s, 2H), 1.06 (s, 18H). 13C NMR (CDCl3, 60 MHz) δ (ppm): 166.5, 164.5, 135.2 (d, J = 13.7 Hz), 134.8, 134.4 (2C, d, J = 4.3 Hz), 127.8 (2C), 71.9, 27.3 (6C), 20.3 (2C). HR-MS (APCI): exact mass ([M + H]+, C17H25ClFN4OSi) calcd 383.1470, found 383.1480. Inverse Electron Demand Diels−Alder Reaction between Tetrazine Derivatives (SiFA-O-Tz or SiFA-S-Tz) with HOPG. One milligram of SiFA-O-Tz or SiFA-S-Tz was dissolved in anhydrous CH2Cl2. A fresh HOPG surface was generated by tape peeling immediately before use. Four portions of 5 μL of the tetrazine derivative solution were applied to the HOPG surface. Anhydrous CH2Cl2 was added dropwise to the HOPG surface in order to maintain the liquid/solid interfacial reaction for 5 min. The HOPG surface was then rinsed with copious amounts of CH2Cl2 to remove the unreacted tetrazine derivatives. The chemically modified HOPG was then characterized by Raman spectroscopy and XPS. Control experiments were carried out by applying the corresponding SiFA-SH or SiFA-OH under the same reaction and purification conditions. Raman and XPS measurements were conducted for comparison purpose. Characterization of the Tetrazine Derivative Modified HOPG Surface by Raman Spectroscopy and XPS. Raman scattering data were acquired from the HOPG samples using a JY LabRamHR confocal Raman microscope. A HeNe laser light at 633 nm was applied to the sample through a 100× microscope objective, with a nominal power at the source of approximately 17 mW. A neutral density filter was used to reduce the incident laser power at the sample to approximately 4 mW. Spectra were obtained using an 1800 line/cm grating with an integration time of 15 s. Ten spectra were obtained. The spectra were collected in the range from 400 to 3100 cm−1. Background corrections were performed using Labspec software. XPS spectra were referenced to the C 1s binding energy (BE) of 285 eV. The 2p3/2−2p1/2 peaks were constrained using a 2p3/2/2p1/2 peak ratio of 2.0 and a peak separation of 1.6 ± 0.1 eV. Peak-fitting procedures were performed using the Thermo Avantage (version 4.60) software. The Inverse Electron Demand Diels−Alder Reaction between Tetrazine−AuNP and HOPG. A 100 μL portion of the tetrazine−AuNP stock solution (1 mg of tetrazine−AuNP in 10 mL of anhydrous CH2Cl2) was added to 2 mL of anhydrous CH2Cl2. A double-sided, peeled HOPG disk (1 cm2), freshly prepared via tape peeling, was dipped into the solution for 5 min. The HOPG disk was purified by washing with copious amounts of CH2Cl2 and THF. STM Imaging Study of the Tetrazine−AuNP-Modified HOPG Surface. STM imaging was performed using a Digital Instruments Inc. (Veeco) NanoScope V. The STM tips were mechanically cut from Pt/ Ir wire (80/20, diameter 0.25 mm, Nanoscience). All STM-images were obtained in the constant current mode by applying a tunneling current ISET of 100−300 pA and a sample bias VSET of 500−1400 mV.

inverse electron demand Diels−Alder (IEDDA) reaction of tetrazines with strained alkenes or acetylenes has been extensively explored,18−26 to the best of our knowledge, this reaction has not been applied to the modification of 2D carbon surfaces. In this context, we recently reported a methodology that uses an IEDDA reaction between tetrazine-terminated ligands on gold nanoparticles (tetrazine−AuNP) and singlewall carbon nanotubes (SWCNT). Covalent modification of SWCNT under ambient conditions results.27 Tetrazine-linked Au nanoparticles react with the sidewalls of a carbon nanotube without pretreatment of the latter. Given this precedent, we have explored the chemical modification of an HOPG surface via the IEDDA reaction between tetrazine derivatives and the sp2-hybridized carbon surface of HOPG. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) measurements demonstrate that a reaction can occur within 5 min and that a longer reaction time leads to a higher extent of surface modification. Tetrazine−AuNP are shown to attach to the HOPG surface under the same mild reaction conditions. A relatively low loading density in the NP case results. A scanning tunneling microscopy (STM) study establishes that the tetrazine−AuNP are covalently bonded to the planar surface. In addition to the mild reaction conditions, an advantage of tetrazine Diels−Alder chemistry is the associated simple derivatization of the dichlorotetrazine with thiol- or hydroxylcontaining molecules, thus making this IEDDA reaction attractive for the preparation of a variety functionalized carbon surfaces.

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EXPERIMENTAL SECTION

Commercial Solvents and Reagents Used. HOPG (SPT-II) was purchased from SPI, and fresh HOPG surfaces were generated by tape peeling. The compounds guanidine hydrochloride, hydrazine monohydride, 2,4-pentanedione, sodium nitrite, trichloroisocyanuric, 2,4,6-collidine, hydrogen tetrachloroaurate(III) trihydrate, potassium tetrabromoaurate(III), tetraoctylammoniun bromide, tetrabutylammonium borohydride, dodecanethiol, and 11-mecaptoundecanol were all purchased from Aldrich and used as received. Potassium carbonate (Caledon) and chloroform-d6 (Cambridge Isotope Laboratories) were also used as received. Dichlorotetrazine was synthesized following a slightly modified procedure developed by Hiskey,28−30 as described in refs 27 and 31. SiFA-SH and SiFA-OH were synthesized following the referenced procedure.32,33 Tetrazine−AuNP were synthesized as described in ref 27. All solvents were purchased from Aldrich and were used as received. Instrumentation. 1H and 13C NMR spectra were obtained using a Varian Mercury 400 (400 MHz) in deuterated chloroform solution and are reported in parts per million (ppm), with the residual protonated solvent resonance used as a reference. HR-MS (ESI, APCI) analyses were recorded in the McGill University Department of Chemistry Mass Spectrometer Facility. X-ray photoelectron spectroscopy was collected using a ThermoFisher Scientific K-alpha instrument employing a monochromatic Al Kα X-ray source (1486.6 eV) and a hemispherical electrostatic analyzer. Raman scattering data were acquired from dried films using a JY LabRamHR confocal Raman microscope. Transmission electron microscopy images and EDX spectrum were collected from a Philips CM200 TEM. STM imaging was obtained using a Digital Instruments Inc. (Veeco) NanoScope V. Synthesis of 3-Chloro-6-((4-(di-tert-butylfluorosilyl)phenyl)thio)-1,2,4,5-tetrazine (SiFA-S-Tz). Dichlorotetrazine (21.0 mg, 0.14 mmol), 4-(di-tert-butylfluorosilyl)phenylthiol (SiFA-SH, 34.2 mg, 0.13 mmol), and 16.8 μL of anhydrous 2,4,6-collidine (0.13 mmol) were dissolved in 10 mL of anhydrous CH2Cl2 under Ar. The solution turned from orange to pink in 5 min. The reaction continued for 35 min and the solvent was then removed by rotary evaporation. SiFA-STz was then purified by flash chromatography and gave 45.4 mg of

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RESULTS AND DISCUSSION

We recently demonstrated that tetrazine-terminated gold nanoparticles can covalently react with a single-walled carbon nanotube surface.27 We thus reasoned that the same chemistry and relatively mild conditions might be expanded to other sp2hybridized carbon surfaces. In order to clearly demonstrate the feasibility of this chemistry, an HOPG surface (1 cm2) was selected as the substrate. This was reacted with tetrazinederivatized fluorosilanes (SiFA-S-Tz and SiFA-O-Ts) and B

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tetrazine-derivatized gold nanoparticles (tetrazine−AuNP) (Scheme 1). Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used in the characterization of this IEDDA reaction between the carbon surface and the derivatized tetrazine. Scheme 1. (a) Reaction of Dichlorotetrazine (1) with Thiol/ Hydroxy Nucleophiles (SiFA-SH, SiFA-OH, and 11undecanolthiolate-capped gold nanoparticles) under Mild Basic Conditions and (b) Schematic Representation of the Synthesis of the Covalently Functionalized HOPG (HOPG− SiFA-O-Tz) via IEDDA followed by the Retro-Diels−Alder Reaction (rDA) between HOPG and SiFA-O-Tz under Ambient Conditionsa

Figure 1. Raman spectra of SiFA-O-Tz on the HOPG surface: (a) HOPG substrate; (b) SiFA-O-Tz; (c) SiFA-O-Tz-HOPG, 5 min reaction time; (d) SiFA-O-Tz−HOPG, 60 min reaction time. The characteristic G band, 2D bands of HOPG, and the CH3 band originating from the tert-butyl groups (t-bu CH3) from the SiFA-O-TZ and HOPG adduct are highlighted.

characteristic G band and 2D band at 1579 and 2680 cm−1 (Figure 1a), respectively. A very small, broad peak located at 1340 cm−1 is attributed to the D band. The D band is due to the breathing vibration mode of the sp2 carbon ring and becomes Raman active when the nearby lattice symmetry changes, as in the conversion of an sp2 carbon to an sp3 carbon. A D′ band in the region of 1620 cm−1, which is associated with the defects in graphitic materials, can also serve as an indication of the conversion of an sp2 carbon to an sp3 carbon. As expected, the Diels−Alder reaction of SiFA-O-tetrazine with the HOPG surface introduces sp3 carbons, leading to an increase of the D band intensity, as shown in Figure 1. Introducing a Raman active group onto the tetrazine moiety also facilitates monitoring of the progress of the reaction. The multiplets (associated with the t-Bu groups) observed between 2860 and 2960 cm−1 are attributed to the symmetric and asymmetric C−H stretches, respectively. The two closely spaced peaks at 1446 and 1463 cm−1 are also due to CH3 deformation and bending modes associated with the t-Bu groups. Close inspection of the G band following reaction with SiFA-O-Tz reveals the presence of a very small shoulder in the region of 1620 cm−1 (Supporting Information, Figure S5, as indicated by arrow), which increases in intensity with reaction time. This shoulder is attributed to the evolution of the D′ band associated with defects in graphitic materials.34 As the reaction proceeds, the number of sp3 carbon sites in the surface increases, thus leading to the appearance of the D′ band in the Raman spectrum. The shoulder may also be attributed to a CH stretch associated with the para-substituted phenyl group contained within the SiFA-O-Tz moiety, which partially overlaps the G band itself. The intensity of this peak would also increase with increased surface loading of SiFA-O-Tz at longer reaction times. The formation of the HOPG adduct with SiFA-O-Tz is readily determined by comparing the Raman spectra of (a) HOPG, (b) SiFA-O-Tz, (c) SiFA-O-Tz−HOPG (5 min reaction time), and (d) SiFA-O-Tz−HOPG (60 min reaction time). A summary of the Raman assignment is shown in Table 1. SiFA-S-Tz undergoes an analogous reaction with HOPG under ambient conditions (Supporting Information, Figures S6 and S7). It is noteworthy that the efficient modification of an HOPG surface with tetrazine derivatives

a

Gray = carbon, blue = nitrogen, red = oxygen, green = chlorine, orange = silicon, and cyan = fluorine.

To simplify the characterization of the surface modification chemistry, tetrazine derivatives containing Raman-active groups and heteroatoms (Si, Cl, N, F, etc.) were synthesized (Scheme 1). Representative tetrazine derivatives (SiFA-S-Tz and SiFAO-Tz, shown in Scheme 1) were synthesized and characterized by 1H NMR, 13C NMR, and high-resolution mass spectrometry. In brief, 3,6-dichlorotetrazine (1) was prepared following a previously reported procedure.27−31 SiFA-S-Tz and SiFA-O-Tz were generated by reacting 1 with the corresponding thiol- or hydroxyl-functionalized SiFA compounds in the presence of a weak base (Scheme 1). 3,6-Dichlorotetrazine (1) is a very hygroscopic compound and is prepared immediately prior to use. The IEDDA reaction of these representative tetrazine derivatives with HOPG proceeded over 5 and 60 min at room temperature, via exposure of the freshly prepared HOPG surface to a solution of a tetrazine derivative in dichloromethane (DCM). In a typical reaction, 50 μL of a DCM/ tetrazine derivative solution (1 mg/mL) was applied to the HOPG surface at room temperature and ambient atmospheric pressure. DCM was periodically applied to the surface dropwise to compensate for the evaporation, maintaining a continuous liquid/solid interface reaction over 5 to 60 min. The progress of the functionalization reaction was monitored by Raman spectroscopy. Control experiments were carried out by treating the HOPG surface with SiFA analogs without the tetrazine moiety, under the same conditions. The IEDDA reaction between HOPG and the tetrazine derivative was assessed using Raman spectroscopy (Figure 1). Spectra obtained (633 nm line of a HeNe laser, 4 mW power using a 100× microscope objective) at different areas on the surface showed no statistically significant difference, consistent with a uniform modification of the HOPG surface. HOPG has a C

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eV are assigned to Cl 2p1/2/Cl 2p3/2 (202.3 eV/200.6 eV) and S 2p1/2/S 2p3/2 (166.1/164.6), respectively. The ratio of F:Cl:S is 1.0:1.0:1.1 (after correction for sensitivity factors). This is consistent with the elemental ratio (1:1:1) of the proposed Diels−Alder adduct. A control experiment was carried out by applying the corresponding SiFA compounds without the tetrazine moiety to an HOPG surface, under the same incubation and purification conditions. The resulting XPS spectra were similar to the pristine HOPG and no peaks associated with the SiFA compounds are observed (Supporting Information, Figure S10). This is consistent with the tetrazinetethered SiFA compounds being covalently bonded to the HOPG surface rather than associated by physical adsorption. Nanoparticle-modified carbon surfaces have promising applications in catalysis and microelectronics. Tetrazine− AuNP with a gold core of 2.2 nm were synthesized and characterized as described previously.27 STM is useful in assessing the outcome of the reaction between AuNP and an HOPG surface. The STM tip was repeatedly scanned over the same area for at least 10 cycles. Images (Figure 3) demonstrate

Table 1. Summary of the Raman Bands Assignment of SiFAO-Tz on HOPG Raman shift (cm−1)

vibration mode

1340 1579 1620 2680 637 810 1108 1205 1415 1446, 1463 1606 2860−2960

HOPG D band HOPG G band HOPG D′ band HOPG 2D band ring deformation (tetrazine adduct) C−Cl stretch Si−aryl stretch O−aryl stretch ring stretching (tetrazine adduct) CH3 deformation and bending (t-Bu) benzene ring stretch C−H stretch (t-Bu, sym and asym)

can be achieved within 5 min and that the extent of derivatization increases with a reaction time of 60 min. The SiFA-O-Tz- and SiFA-S-Tz-modified HOPG surfaces were further characterized by XPS. The C 1s peak originating from the sp2 carbon atoms of pristine HOPG arises at 285.0 eV. No other elements are detected in the high-resolution XPS spectra of the pristine HOPG (Supporting Information, Figure S8). The Diels−Alder adduct on the HOPG surface (SiFA-S-Tz with HOPG in Figure 2 and SiFA-O-Tz with HOPG in Figure S11, Supporting Information) leads to peaks at 686.7, 400.8, and 102.5 eV. These are assigned to F 1s, N 1s, and Si 2p, respectively. The two doublets centered around 202 and 165

Figure 3. (a) Schematic representation of the HOPG−Tz−AuNP functionalization via tetrazine reaction under ambient conditions (25 °C, CH2Cl2, 5 min). (b) STM images of the Tz−AuNP-modified HOPG; the tetrazine reaction occurs exclusively on defect-free HOPG regions. (c) High-resolution STM image of HOPG. (d) Highresolution STM images corresponding to repetitive scanning over the same area. STM images were obtained in constant current mode by applying a tunneling current ISET of 100−300 pA and a sample bias VSET of 500−1400 mV.

that the AuNP remain in the same position, strongly suggestive of covalent attachment to the HOPG surface. The high steric demand of the 2D HOPG surface and tetrazine moieties on the AuNP surface leads to relatively low levels of AuNP loading. As described above, chemical modification of HOPG is difficult because of its conjugated polyaromatic structure and interlayer van der Waals interactions. We have demonstrated that the IEDDA reaction can be applied to the modification of SWCNT with tetrazine-derivatized AuNP.27 However, the chemical modification of HOPG does not entirely parallel the IEDDA reaction of carbon nanotubes. In practice, an HOPG sample as prepared here invariably contains edges (including multilayer step edges), atom vacancies (defects), and other impurities. Any or all of these “defects” lead to an increased reactivity.35 After covalent modification, the HOPG surface was thoroughly purified by washing with a copious amount of solvent. Control experiments using similar SiFA compounds but which lack the tetrazine moiety were also performed under similar conditions. Both Raman and XPS measurements confirm that SiFA compounds are chemically attached to the

Figure 2. Survey spectrum and high-resolution XPS spectra of SiFA-STz-functionalized HOPG (SiFA-S-Tz−HOPG). The high-resolution spectra for each element of the pristine HOPG were collected for comparison. No Si, S, Cl, or F are detected in the pristine HOPG sample (Supporting Information, Figure S10). The appearance of the new elements are due to the Diels−Alder adducts. (a) Survey spectrum of SiFA-S-Tz, (b) Si 2p, (c) S 2p1/2 and S 2p3/2, (d) Cl 2p1/2 and Cl 2p3/2, and (e) F 1s. D

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(17) Bian, S.; Scott, A. M.; Cao, Y.; Liang, Y.; Osuna, S.; Houk, K. N.; Braunschweig, A. B. J. Am. Chem. Soc. 2013, 135, 9240. (18) Clavier, G.; Audebert, P. Chem. Rev. 2010, 110, 3299. (19) Devaraj, N. K.; Weissleder, R.; Hilderbrand, S. A. Bioconjugate Chem. 2008, 19, 2297. (20) Reiner, T.; Keliher, E. J.; Earley, S.; Marinelli, B.; Weissleder, R. Angew. Chem., Int. Ed. 2011, 50, 1922. (21) Budin, G.; Yang, K. S.; Reiner, T.; Weissleder, R. Angew. Chem., Int. Ed. 2011, 50, 9378. (22) Karver, M. R.; Weissleder, R.; Hilderbrand, S. A. Angew. Chem., Int. Ed. 2012, 51, 920. (23) Blackman, M. L.; Royzen, M.; Fox, J. M. J. Am. Chem. Soc. 2008, 130, 13518. (24) Liu, D. S.; Tangpeerachaikul, A.; Selvaraj, R.; Taylor, M. T.; Fox, J. M.; Ting, A. Y. J. Am. Chem. Soc. 2012, 134, 792. (25) Seitchik, J. L.; Peeler, J. C.; Taylor, M. T.; Blackman, M. L.; Rhoads, T. W.; Cooley, R. B.; Refakis, C. A.; Fox, J. M.; Mehl, R. A. J. Am. Chem. Soc. 2012, 134, 2898. (26) Hansell, C. F.; Espeel, P.; Stamenovic, M. M.; Baker, I. A.; Dove, A. P.; Du Prez, F. E.; O’Reilly, R. K. J. Am. Chem. Soc. 2011, 133, 13828. (27) Zhu, J.; Hiltz, J.; Lennox, R. B.; Schirrmacher, R. Chem. Commun. 2013, 49, 10275. (28) Chavez, D. E.; Hiskey, M. A. J. Heterocycl. Chem. 1998, 35, 1329. (29) Chavez, D. E.; Hiskey, M. A. J. Energy Mater. 1999, 17, 357. (30) Chavez, D. E.; Hiskey, M. A.; Gilardi, R. D. Angew. Chem., Int. Ed. 2000, 39, 1791. (31) Gong, Y.; Miomandre, F.; Meallet-Renault, R.; Badre, S.; Galmiche, L.; Tang, J.; Audebert, P.; Clavier, G. Eur. J. Org. Chem. 2009, 35, 6121. (32) Wangler, B.; Quandt, G.; Iovkova, L.; Schirrmacher, E.; Wangler, C.; Boening, G.; Hacker, M.; Schmoeckel, M.; Jurkschat, K.; Bartenstein, P.; Schirrmacher, R. Bioconjugate Chem. 2009, 20, 317. (33) Iovkova, L.; Wangler, B.; Schirrmacher, E.; Schirrmacher, R.; Quandt, G.; Boening, G.; Schurmann, M.; Jurkschat, K. Chem.Eur. J. 2009, 15, 2140. (34) Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S.; Cancado, L. G.; Jorio, A.; Saito, R. Phys. Chem. Chem. Phys. 2007, 9, 1276. (35) Yan, L.; Zheng, Y. B.; Zhao, F.; Li, S. J.; Gao, X. F.; Xu, B. Q.; Weiss, P. S.; Zhao, Y. L. Chem. Soc. Rev. 2012, 41, 97.

HOPG surface, and Raman and XPS spectra correspond to the products anticipated from an IEDDA reaction.

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CONCLUSIONS We have demonstrated that tetrazine-tethered compounds react with an sp2-hybridized 2D carbon surface (HOPG) under ambient conditions, via an inverse electron demand Diels− Alder reaction. Due to the efficient substitution reaction of dichlorotetrazine with thiol- and hydroxyl-containing molecules, a variety of tetrazine-containing functionalized molecules can be prepared and applied to the preparation of functional carbon materials. The methodology described here is the mildest chemical approach reported to date to modify a highly ordered and chemically inert 2D carbon surface.

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ASSOCIATED CONTENT

S Supporting Information *

NMR spectra, Raman spectra, and XPS spectra, as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*R.B.L.: fax, +1-514-398-3797; tel, +1-514-398-8304; e-mail, [email protected] . *R.S.: fax, +1-514-340-7502; tel, +1-514-398-1857; e-mail: ralf. [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from CHIR (R.S.), NSERC (R.S., R.B.L.), and FQRNT (R.B.L.). The authors thank Prof. Mark Andrews for his assistance with the Raman analysis.

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REFERENCES

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