Functionalization of graphene by tetraphenylethylene using nitrene chemistry

Abstract

We have demonstrated that via nitrene addition, graphene was covalently functionalized by tetraphenylethylene, which is a typical aggregation-induced emission (AIE) molecule with a twisted conformation. The obtained new nanocomposite TPE–C₄N₃–G–S was characterized by transmission electron microscopy (TEM), fourier transform infrared (IR) spectroscopy, fluorescence spectroscopy, UV-vis, thermo-gravimetric analysis (TGA), and ¹³C NMR. The twisted conformation of TPE–C₄N₃ was effective in improving the solubility and dispersion stability of graphene, and the resulting composite was totally soluble in organic solvents. The resulting product was determined to have one functional group per 48 carbons. In addition, this new type of molecule may open up a new avenue for the synthesis of graphene-based materials and nanodevice fabrications.

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Introduction

Graphene is a single layer of carbon atoms in a closely packed honeycomb two-dimensional lattice. Since its discovery by K. S. Novoselov and A. K. Geim in 2004,¹ it has been a rising star due to its unique properties, such as high surface area, excellent thermal and electrical conductivity, and strong mechanical strength.² This new nanomaterial has attracted increasing attention from various research fields for their potential applications in many technological areas, such as supercapacitors,³ solar cells,⁴ field effect transistors,⁵ biosensors,⁶ and so on. Despite its great promises, one of the major obstacles to these applications is the insolubility of graphene, since the strong π–π interaction allows it to form agglomerates easily.

In order to improve its solubility to meet the requirements of various applications, great efforts have been made by scientists. Till now, the surface modification of graphene has been achieved by electrostatic stabilization, covalent modification and non-covalent functionalization,⁷ and the covalent modification plays an important role in improving the solubility of graphene. Because of the inert surface of graphene, many reports of covalent functionalization were based on graphene oxide (GO), e.g., the isocyanate-modified GO,⁸ or the covalent modification by amidation of the carboxylic groups with octadecylamine,⁹ amine-terminated ionic liquid¹⁰ or porphyrin,¹¹ and so on.¹² However, the drawback was that GO contains some oxygen functional groups, which disrupt the π-conjugation, and thus lose the conductivity. Recently, a series of methods have been utilized for the direct functionalization of graphene, such as aryne cycloaddition,¹³ diazonium coupling,¹⁴ 1,3-dipolar cycloaddition,¹⁵ and nitrene addition.¹⁶ Fortunately, the solubility of graphene has been improved to a certain extent.

In our previous work, graphene could be functionalized via nitrene addition, which was generated in situ by thermal decomposition of azido starting materials, to realize the functionalization of graphene through polyacetylenes.¹⁷ Owing to the good solubility of polyacetylenes, the polymer wrapped graphene became soluble in organic solvents with enhanced emission. Although polymers have good solubility and wrapping ability towards graphene, the preparation of polymers often uses catalysts based on various transition-metal species, which are environmentally unfriendly, need careful preservation, and are difficult to obtain. On the other hand, the polymerization conditions are often strict. So we were wondering if small molecules can be used instead of polyacetylenes to efficiently improve the solubility of graphene, because the synthesis is easy, in comparison with polymers.

As reported in the literatures, molecules bearing π-electron-rich frameworks could efficiently improve the dispersion stability of graphene through nondestructive π–π interactions, for example, pyrene units were widely used in non-covalent functionalizations.^7a,7b Perhaps pyrene derivates having azido groups could be used for the covalent functionalization of graphene through nitrene chemistry. Therefore, we synthesized compound PB–N₃ (Fig. S1, ESI†), but unfortunately, the reaction system of PB–N₃ and graphene was stable for only one week (Fig. S2, ESI†). And when the mixture was filtrated, the filtration showed a yellow color, just as PB–N₃ itself, demonstrating that no soluble PB–G was obtained. Considering this result, we supposed that besides π-electron-rich frameworks, there should be another important factor to successfully improve the solubility of graphene. As we know, the π–π stacking interactions between layers makes graphene sheets easily aggregate to form multi-layers, and therefore losing the unique properties of graphene. So forces should be made to detach two layers from each other, in other words, steric hindrance should be introduced to overcome the large interlayer cohesive energies. Combining these two factors, large steric hindrance and rich π-electron frameworks, in this article we present the design and synthesis of a tetraphenylethylene derivative for the functionalization of graphene (Scheme 1).

Scheme 1 The synthetic route of TPE–C₄N₃–G, inset: photographs of TPE–C₄N₃ (A) and TPE–C₄N₃–G–S (B) in o-dichlorobenzene.

Experimental

Materials and instrumentations

Tetrahydrofuran (THF) was dried over and distilled from a K–Na alloy under an atmosphere of dry argon. Dichloromethane (CH₂Cl₂) was dried over CaH₂ and distilled under normal pressure before use. All the other reagents were used as received.

¹H and ¹³C NMR spectra were measured on a Varian Mercury 300 using tetramethylsilane (TMS; δ = 0 ppm) as the internal standard. Elemental analyses were performed by a CARLOERBA-1106 micro-elemental analyzer. Raman spectroscopy was performed on the LabRAM HR 800UV (Horiba Jobin Yvon Company) using 632.8 nm laser excitation. Scans were taken on an extended range (150–3400 cm⁻¹) and the exposure time was 50 s. The samples were viewed under a maximum magnification of ×10. Transmission electron microscopy (TEM) was performed on the JEM-2010-FEF at an accelerating voltage of 200 kV. The TEM samples were prepared by drying a droplet of the suspension on a lacy carbon grid. Atomic force microscopy (AFM) images were obtained from an Agilent 5500 using the tapping mode. IR spectra (400–4000 cm⁻¹) were measured using a Nicolet 380 FT-IR with pure KBr as the background. The X-ray powder diffraction (XRD) analyses were performed on a Bruker D8 Advanced X-ray diffractometer with Cu-K_α radiation (λ = 1.5418 Å). The diffraction data was recorded for 2θ angles from 5 to 60°. UV-vis spectra between 200 and 850 nm were measured on a Shimadzu UV-3600 Spectrophotometer. Photoluminescence (PL) spectra were taken on a Hitachi F-4500 Fluorescence Spectrophotometer. Thermal analysis was performed on NETZSCH STA449C thermal analyzer at a heating rate of 10 °C min⁻¹ in nitrogen at a flow rate of 50 cm³ min⁻¹ (TGA).

Synthesis of compound 3

Into a 100 mL Schlenk tube were added 2 (1.81 g, 5 mM),¹⁸ and CH₂Cl₂ (40 mL). The tube was evacuated under vacuum and then flushed with argon three times through the sidearm. The tube was placed into a reactor at −78 °C, after stirring for 20 min, boron tribromide (5 mM) was carefully added to the mixture. The resultant mixture was stirred for 4 h, and then warmed to room temperature and stirred overnight. The reaction was hydrolyzed by shaking with H₂O (20 mL), the organic phase was separated and concentrated by a rotary evaporator. The crude product was purified by recystallization using THF and methanol (1.65 g, 97% yield). ¹H NMR (CDCl₃, 300 MHz), δ (TMS, ppm): 7.10–6.98 (m, 15H, Ar–H), 6.89 (m, 2H, Ar–H), 6.56 (m, 2H, Ar–H), 4.73 (s, 1H, –OH).

Chart 1 The synthetic routes of TPE–C₄N₃.

Synthesis of compound 4

Into a 100 mL flask were added 3 (1.5 g, 4.3 mM), potassium carbonate (771 mg, 1.7 mM), 1,4-dibromobutane (0.6 mL, 5 mM) and acetone (50 mL); the mixture was refluxed overnight. After the reaction, the mixture was cooled to room temperature and concentrated by a rotary evaporator, and then the crude product was purified by a silica gel column using chloroform/petroleum ether (1/5, v/v) as eluent to afford a white solid (1.66 g, 80% yield). ¹H NMR (CDCl₃, 300 MHz), δ (TMS, ppm): 7.15–6.99 (m, 15H, Ar–H), 6.93 (m, 2H, Ar–H), 6.62 (m, 2H, Ar–H), 3.91 (t, J = 9.3 Hz, 2H, –O–CH₂–), 3.47 (t, J = 9.9 Hz, 2H, –CH₂–Br), 2.04 (m, 2H, –CH₂–), 1.89 (m, 2H, –CH₂–).

Synthesis of TPE–C₄N₃

Into a 50 mL Schlenk tube were added 4 (1.45 g, 3 mM), sodium azide (390 mg, 6 mM), and DMF (3 mL); the mixture was stirred at 80 °C overnight. After the reaction, the mixture was cooled down to room temperature and poured into water, the solid was filtrated and the crude product was purified by a silica gel column using chloroform/petroleum ether (1/5, v/v) as eluent to afford a yellow solid (1.2 g, 90% yield). ¹H NMR (CDCl₃, 300 MHz), δ (TMS, ppm): 7.07–7.02 (m, 15H, Ar–H), 6.93 (m, 2H, Ar–H), 6.62 (m, 2H, Ar–H), 3.91 (t, 2H, –O–CH₂–), 3.34 (t, 2H, –CH₂–N₃), 1.85 (m, 2H, –CH₂–), 1.79 (m, 2H, –CH₂–). ¹³C NMR (CDCl₃, 75 MHz), δ (TMS, ppm): 157.9, 144.57, 144.48, 141.06, 140.62, 136.64, 133.09, 131.86, 128.28, 128.17, 126.91, 126.80, 114.12, 67.35, 51.56, 26.97, 26.23. Anal. Calcd for C₃₀H₂₇N₃O: C, 80.87; H, 6.11; N, 9.43. Found: C, 80.90; H, 6.10; N, 9.05.

Functionalization of graphene by TPE–C₄N₃

Into a baked 100 mL Schlenk tube with a stopcock in the sidearm were added graphene (10 mg), and o-dichlorobenzene (10 mL). The tube was evacuated under vacuum and then flushed with argon three times through the sidearm. After being sonicated for 20 min at room temperature, TPE–C₄N₃ (250 mg) was added, and the black suspension was heated to 110 °C under argon protection and stirred for 3 days. When the reaction was finished, it was cooled to room temperature, the insoluble dark solid was filtered out, which was further washed with THF for several times, and named as TPE–C₄N₃–G–In; the soluble part was purified by a silica gel column using chloroform/petroleum ether (1/5, v/v) to remove excess TPE–C₄N₃. The one left was named as TPE–C₄N₃–G–S. Finally, TPE–C₄N₃–G–In (6 mg) and TPE–C₄N₃–G–S (103 mg) were yielded after drying under vacuum at 40 °C.

Results and discussion

Graphene was synthesized via the chemical reduction of graphene oxide at a low-temperature (exfoliation temperature as low as 200 °C) under a negative pressure environment.¹⁹ TPE–C₄N₃ was synthesized as shown in Chart 1. The preparation of graphene functionalized by TPE–C₄N₃ was depicted in Scheme 1. After the reaction was completed, the mixture was filtrated and the solution exhibited a dark color (TPE–C₄N₃–G–S, B in Scheme 1), indicating the presence of graphene, and it was stable for at least two months. TPE–C₄N₃–G–S was soluble in common organic solvents, such as THF, DMF, CHCl₃ and acetone, but insoluble in ethanol, methanol and water, similar to TPE–C₄N₃. Besides, TPE–C₄N₃–G–In also showed good dispersion stability. As shown in Fig. S3, ESI† after sonication, the dispersion of graphene sheets in THF could stay for only 5 min, and 30 min later, graphene was nearly totally precipitated at the bottom of bottle, but TPE–C₄N₃–G–In was stable with no sediment. The dispersibility of TPE–C₄N₃–G–In in other solvents, including polar and nonpolar solvents (toluene, DMF, CHCl₃, ethanol and acetone) were also tested in detail (Fig. S4–S5, ESI†). With the aid of covalent functionalization by TPE–C₄N₃, the solubility was much improved. Stable dispersions of TPE–C₄N₃–G–In were formed just by shaking without sonication. After standing for 15 min, except for acetone and toluene, no precipitate was found. For comparison, the dispersion stability of graphene was also studied, though some reports demonstrated that graphene itself had certain solubility in organic solvents,²⁰ and our results displayed limited solubility, which might be related to its dimensions. As shown in Fig. S4, ESI† the graphene dispersion in DMF was stable, but in all the other solvents formed clear solutions 15 min later. In addition, we also studied the dispersibility with a mild sonication for 5 min (Fig. S5, ESI†). In contrast with graphene, TPE–C₄N₃–G–In displayed good dispersion stability in both polar and nonpolar solvents. Taking the low boiling point solvent CHCl₃ for example, the solubility of TPE–C₄N₃–G–In was about 0.3 mg mL⁻¹, while the control experiment showed that graphene could not form a stable dispersion. It is known that graphene is unstable in nonpolar solvents (e.g., toluene). The TPE–C₄N₃–G–In dispersion in toluene also displayed improved dispersion stability even after standing for 1 h when compared to graphene. These results demonstrate that our methods are effective in improving the solubility or dispersibility of graphene.

The successful grafting of TPE–C₄N₃ onto graphene could be partially proved by their IR spectra (Fig. 1). Comparing with TPE–C₄N₃, the apparent peak centered at ~2100 cm⁻¹, assigned to the stretching vibration of the azido groups, disappeared in the IR spectrum of TPE–C₄N₃–G–S, which is attributed to its reaction with graphene. Besides, almost all the typical bands of TPE–C₄N₃ could be found in TPE–C₄N₃–G–S: the bands at 1600, 1572 and 1514 cm⁻¹ should be ascribed to the stretching vibration of the skeleton of the aromatic rings, 1450 and 1370 cm⁻¹ are the scissor vibrations of the methylene groups, with the asymmetrical and symmetrical C–H stretching vibration of the methylene groups at 2929 and 2875 cm⁻¹, and in the fingerprint region, the bands at 761 and 699 cm⁻¹ demonstrated the monosubstituted phenyl. In addition, a new peak at 1700 cm⁻¹ appeared, which could be ascribed to the C=O stretching vibration of graphene, due to the incomplete reduction process. These results indicated that TPE–C₄N₃ was covalently grafted to graphene.

Fig. 1 IR spectra of TPE–C₄N₃, graphene and TPE–C₄N₃–G–S.

Raman spectra of the samples were taken using 633 nm laser excitation. Unfortunately, only those of graphene and TPE–C₄N₃–G–In were obtained (Fig. S6, ESI†), and we failed to get the Raman spectrum of TPE–C₄N₃–G–S. The possible reason is the presence of TPE–C₄N₃, whose fluorescence signal is higher than the Raman signal under the same conditions. The intensity ratio of the D and G bands (I_D/I_G) is often used as an indicator for the level of chemical modification. The Raman spectrum of TPE–C₄N₃–G–In showed a disorder mode with a ratio of 1.1, a little higher than that of graphene (1.0), possibly due to the distribution of the functional groups on the surface or an effect of the size of the sheets. The G band of TPE–C₄N₃–G–In shifted from 1590 to 1593 cm⁻¹, this blue shift was ascribed to the influence of defects or isolated double bonds.²¹ XRD patterns of TPE–C₄N₃, TPE–C₄N₃–G–S and graphene were also studied (Fig. S7, ESI†). Graphene displayed a typical peak at 2θ = 25.32° with an inter-planar spacing of 3.5 Å, while, after modification, TPE–C₄N₃–G–S showed a broaden band at 20.68°, and the d-spacing increased to 4.3 Å, which should be ascribed to the covalent modification by TPE–C₄N₃.

The presence of graphene could be detected by HRTEM (Fig. 2). The morphology of TPE–C₄N₃–G–S exhibited a large graphene sheet with many wrinkles, with a dimension ranging from 500 nm to micrometers. We also used TPE–C₄N₃ for comparison (Fig. S8, ESI†), because the small molecule could not endure the irradiation of the powerful electron beam, coffee-ring-like photographs were taken. Combining these two images, the grafting of TPE–C₄N₃ to graphene could be further confirmed. For the measurement of AFM, we failed to get the result of graphene itself, perhaps because of its bad processablity. Fortunately, we got the result of TPE–C₄N₃–G–In easily (Fig. 3). The average thickness of TPE–C₄N₃–G–In was about 4–5 nm, the height of single layer graphene sheet was about 1 nm, thus, this increase suggested that the molecules were covalently attached to both sides of the graphene, which was in good accordance with the reported literature.^15b,16d In principle, there was another possibility that bi-layer or multi-layer structures of graphene were obtained.

Fig. 2 TEM image of TPE–C₄N₃–G–S.

Fig. 3 AFM image of TPE–C₄N₃–G–In, scan size:2 μm×2 μm.

Because TPE–C₄N₃–G–S was totally soluble in organic solvents, we also tested its photochemical properties. TPE–C₄N₃ is a well-known type of aggregation-induced emission (AIE) molecule with a special character: it is nonluminescent in solutions, but highly emissive in the aggregation state or in the solid state.²² Examination of the AIE properties was carried out in a mixture of water-tetrahydrofuran under different water fractions (Fig. S9–S10, ESI†). TPE–C₄N₃ was insoluble in water, so increasing the water fraction could change the existing form from solution to the aggregated state, and the non-emissive TPE–C₄N₃ became highly emissive as a result of the restriction of its phenyl rotors in the aggregation state. The fluorescence (FL) intensity of TPE–C₄N₃ was significantly enhanced when the water fraction exceeded 90%, and at the fraction of 99%, the FL intensity increased 20 fold, but for TPE–C–₄N₃–G–S, no obvious change occured. The FL intensity of TPE–C₄N₃–G–S was quenched (about 80%) by the presence of graphene (Fig. 4), indicating the strong electronic interaction between TPE–C₄N₃ and graphene. UV-vis absorption spectra of TPE–C₄N₃ and TPE–C₄N₃–G–S were measured (Fig. S11, ESI†) and the spectrum of TPE–C₄N₃–G–S showed its absorption peak at 315 nm, which is similar to that of TPE–C₄N₃ and gave no changes after modification.

Fig. 4 Fluorescence spectra of TPE–C₄N₃ and TPE–C₄N₃–G–S in a H₂O/THF mixture with 99% of water (v/v).

We also used ¹³C and ¹H NMR spectra to characterize TPE–C₄N₃ and TPE–C₄N₃–G–S (Fig. S12–S15, ESI†), the peak at 51.56 ppm disappeared, which was the carbon atoms linked to the azido group, demonstrating that azido was thermally generated and reacted with graphene. In the preparation process of graphene, the oxygen functional groups of GO could not be totally reduced; generally, some oxygen-containing groups were left even under strict reaction conditions, such as C=O-type oxygen and epoxy (1, 2-ether) groups. In the ¹³C NMR spectrum of TPE–C₄N₃–G–S, resonances at 178.34, 68.87 and 108.14 ppm appeared, and these could be possibly assigned to the ketones, epoxy (1,2-ether) groups and graphitic carbons in graphene, respectively.²³

For the sake of determining the extent of TPE–C₄N₃ grafting, thermo-gravimetric analysis (TGA) of TPE–C₄N₃–G–S was performed over the temperature range of 20–600 °C in an argon atmosphere. Fig. 5 displays the thermograms of graphene, TPE–C₄N₃ and TPE–C₄N₃–G–S. The overall weight loss of graphene was 15%, showing its good thermal stability, while TPE–C₄N₃ showed poor stability with respect to graphene, with a total weight loss of about 95%. In the temperature range of 320–495 °C, the weight loss of TPE–C₄N₃–G–S could be attributed to the decomposition of TPE–C₄N₃ with a weight loss of 44%, indicating the degree of functionalization was one functional group per 48 carbons, and a moderate grafting efficiency was achieved according to the reported literature.¹⁵ Additionally, in order to explain the different solubility of TPE–C₄N₃–G–S and TPE–C₄N₃–G–In, the TGA curve for TPE–C₄N₃–G–In was also collected (Fig. S16, ESI†). It displayed a total weight loss of 22%, which is similar to that of graphene. In the temperature range of 320 to 495 °C, the weight loss of graphene, TPE–C₄N₃ and TPE–C₄N₃–G–In was 4.49%, 68.93% and 7.9%, respectively, and the content of the TPE–C₄N₃ moieties in TPE–C₄N₃–G–In was calculated to be only 5.3%, which was much lower than TPE–C₄N₃–G–S. Therefore, the different functional efficiency accounted for their different solubilities.

Fig. 5 Thermo-gravimetric analysis of graphene, TPE–C₄N₃ and TPE–C₄N₃–G–S.

In order to confirm our concept, we used PB–N₃ and TPP–C₄N₃ for comparison, and a weak dispersion stability was observed for both of them (Fig. S17, ESI†). The reaction solutions of PB–N₃ and TPP–C₄N₃ could stay for no more than one week, while TPE–C₄N₃ lasted for at least two months. Although the pyrene moiety has been reported to have a strong affinity with graphene via π-stacking, the small stereospecific blockade could not overcome the large interlayer cohesive energies: for TPP–C₄N₃ the stereospecific blockade is larger than PB–N₃, but the π-interactions are weaker. TPE–C₄N₃ has the merits of both PB–N₃ and TPP–C₄N₃, and thus has the best activity towards graphene. At the same time, the experiment in our article actually demonstrated that TPE–C₄N₃ effectively improved the solubility or dispersibility of graphene. In order to confirm that large steric hindrance was important to improve the solubility of graphene, we also performed molecular calculations on their energy levels based on DFT/B3LYP/6-31G(d): the optimized configurations of TPE–C₄N₃, PB–N₃ and TPP–C₄N₃ are shown in Fig. 6. PB–N₃ was totally flat, but TPE–C₄N₃ and TPP–C₄N₃ had twisted conformations and the dihedral angles between C=C and each of the phenyl rings of TPE–C₄N₃ were in the range from 48 to 68°, larger than TPP–C₄N₃, which was in the range of 38 to 48°. When graphene is grafted by TPE–C₄N₃, the non-planar conformation of TPE–C₄N₃ helps to reduce the π–π stacking of the graphene sheets and enhance its solubility or dispersion stability. This result further confirmed that our concept about the functionalization of graphene was correct.

Fig. 6 Optimized configurations of TPE–C₄N₃, PB–N₃ and TPP–C₄N₃.

Conclusions

In summary, we have reported that molecules having the characteristics of stereospecific blockades and rich π-electron frameworks could efficiently improve the dispersion stability of graphene. In this article, TPE–C₄N₃ was covalently attached to graphene using nitrene chemistry and soluble graphene was prepared successfully. By the way, this method presented here could be expanded to other molecules, which may open up a new avenue for the synthesis of graphene-based materials and nanodevice fabrications.

Acknowledgements

We are grateful to the National Fundamental Key Research Program (2011CB932702) and the National Natural Science Foundation of China (No. 21161160556) for financial support, and the Centre for Electron Microscopy of Wuhan University for the aid of TEM tests.

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Footnote

† Electronic supplementary information (ESI) available: details of the experimental procedures for PB-N₃ and TPP-C₄N₃, Raman, UV-vis, FL spectra, ¹³C NMR, ¹H NMR, and TEM images. See DOI: 10.1039/c2ra20460f

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