Current Organic Chemistry, 2011, 15, 1121-1132 1121

1385-2728/11 $58.00+.00 © 2011 Bentham Science Publishers Ltd.

Microwave-assisted Functionalization of Carbon Nanostructured Materials

S. P. Economopoulos*, N. Karousis, G. Rotas, G. Pagona and N. Tagmatarchis*

Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, Athens 116 35, Greece

Abstract: Microwaves have proven to be a powerful ally in any synthetic effort, shortening reaction times considerably and in some cases facilitating the reaction by offering higher yields or fewer byproducts. Since the mid ‘90s microwave-assisted synthesis has found its way into carbon nanostructures, offering significant advantages towards their functionalization. In this review, an overview of the lat-est trends in microwave-assisted functionalization of carbon nanostructured materials is presented covering major breakthroughsachieved in the last years for this novel, intriguing and non-conventional synthetic approach.

Keywords: Fullerenes, carbon nanotubes, microwaves, functionalization, solubilization, hybrids, characterization.

INTRODUCTION

Ever since the first report on buckminsterfullerene by Kroto and coworkers [1], carbon nanostructured materials have attracted con-siderable scientific interest. The discovery of carbon nanotubes (CNTs) [2] and carbon nanohorns (CNHs) [3] by Iijima as well as the recent developments in exfoliation of nanocrystalline graphitic films known as graphene sheets [4,5] have only increased the po-tential of this novel class of materials. The remarkable thing is that this family of carbon-based nanostructures incorporates a wide variety of unique properties, ranging from electrical to mechanical, while at the same time, their nanoscale size allows for atomic con-trol and tuning that directly influences their macroscopic properties. It is only fair to say that, coupled with the term nanotechnology -that is beginning to take form as not only a buzz word and truly make an impact in everyday technology advancements- carbon nanostructures have evolved to a truly interdisciplinary research area. The current state-of-the-art is that carbon nanostructured ma-terials owing to their advantageous physical and chemical proper-ties have found their way into optoelectronic devices such as pho-tovoltaics [6-9] and photodiodes [10], light emitting devices, [11,12] as well as medicinal chemistry [13-15] as drug delivery carriers [16,17]. In addition, the excellent mechanical properties [5,18-20] exhibited by these materials only enrich further their po-tential for incorporation into wider variety of areas.

Despite their potential, the limiting factor into their wide-spread acceptance is the solubility problems that plague carbon nanostruc-ture materials in general. While fullerenes are fairly soluble in a number of organic solvents such as CS2 or o-dichlorobenzene (o-DCB), single walled CNTs (SWNTs), CNHs and graphene sheets are insoluble in every solvent. The answer to these problems and the way to take advantage of the unique properties of these materi-als for the aforementioned uses is to chemically attach functional units onto the graphitic skeleton, thus providing the solubility en-hancement necessary and in most cases, add the properties of the functional group, giving rise to a hybrid material. In order to en-hance solubility in these carbon based materials, two distinct ap-proaches of functionalization are present, either non-covalently [21-24], or covalently [25]. In this review we will focus on covalent functionalization of carbon nanostructures with the aid of micro

*Address correspondence to this author at the Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, Athens 116 35, Greece; Tel: + 30 210 7273835; Fax: + 30 210 7273794; E-mails: economop@eie.gr, tagmatar@eie.gr

waves. While covalent functionalization is an area of interest that is of the essence to almost all research groups who deal with carbon nanostructures, it is so wide that many different approaches to the desired objective are taken. Conventional approaches towards cova-lent attachment of functional groups have afforded many interesting products that either, simply, enhance solubility [26,27] or are de-signed to couple the properties of the added functional unit with the ones of the carbon nano-material [28-30].

MICROWAVE IRRADIATION

A relatively novel approach for functionalization of carbon nanostructures is microwave-assisted chemistry [31, 32]. Although relatively new, microwave-assisted chemistry has seen tremendous growth since the very first time its uses have been reported [33, 34]. Microwaves generate rapid intense heating of polar substances, which results in significantly lower reaction times [35]. Coupled by the fact that most of the times the reaction yields are higher and the product is easier to work up, these outline some of the advantages microwave assisted synthetic approaches offer. Another interesting advantage of microwave chemistry is the fact that the heating is created in the interior of the sample and in the process, transferred outwards, whereas in conventional synthetic approaches the heating is created outside and transferred towards the reaction mixture through a medium such as an oil bath and a solvent. In the case of microwaves, especially if the reactants facilitate this, the use of a solvent can be completely omitted. Avoiding solvent use in synthe-sis can reduce environmental contamination and even be more con-venient than using solvent-based syntheses. The field of solvent-free chemistry covers all aspects of organic synthesis and the ad-vantages of avoiding solvents and minimizing distillation residues should not be overlooked. Combination of microwave irradiation and solvent-free procedures constitutes an approach towards the green chemistry general concept.

Towards efficient functionalization of carbon-based materials Langa and Martin did pioneering work with fullerene functionaliza-tion using microwave irradiation [36-40]. Aside from the consider-able time efficiency that the microwave approach afforded, com-pared to conventional heating (minutes, instead of hours), Langa and coworkers noticed a universal increase in reaction yields and in some cases a relative control of mono and polyadducts of fullerene derivatives.

In the area of single walled carbon nanotubes, microwave irra-diation has been found useful in the purification process. Since their

1122 Current Organic Chemistry, 2011, Vol. 15, No. 8 Economopoulos et al.

discovery by Sumio Iijima in 1991, carbon nanotubes, despite their potential, have been plagued by issues of metal catalyst impurities as well as polydispersion of SWNTs bundles, all as a result of the synthetic procedures. As produced SWNTs consist of, apart from metallic and semiconducting, different diameter and size nanotubes. These issues greatly hamper the SWNTs uses in almost every nanotechnological application. Reports of removing metal catalyst by selectively interacting with the residual carbon coating of the catalyst nanoparticle have been presented [41]. This method proves advantageous compared to traditional acid treatment of SWNTs as the microwave assisted approach requires only mild hydrochloric acid treatment to achieve catalyst concentrations of only 0.2 wt%. Since the widely used nitric acid treatment is suitable for removing the catalyst from SWNT samples, but only at the expense of a sig-nificant destruction of the SWNTs [42], microwave irradiation proves a powerful ally for this process. Recently [43], another re-port of metallic nanotubes decomposing more rapidly than semi-conducting ones, under microwave irradiation was published, pro-posing a few minutes heating of SWNTs to greatly enrich the semi-conducting content of the sample, allowing better incorporation in electronic applications where semiconducting nanotubes only, are required. In this review we will mostly focus on the more recent achievements on microwave assisted functionalization on carbon nanostructures.

FULLERENE FUNCTIONALIZATION USING MICRO-WAVE IRRADIATION

Fullerenes and their derivatives have shown great promise in a large number of technological and medical applications. Function-alization of fullerenes, to afford soluble and hybrid materials has

been the main interest of all groups in the field of fullerene re-search. A fairly rich literature for fullerene functionalization using microwave irradiation can be found involving cycloadditions [36, 38, 44].

Wu et al. reported a series of successful additions of fluoroalkenylsufonylazides to C60 fullerene taking part under either mi-crowave or conventional heating conditions [45]. In Scheme 1, a series of reactions successfully carried out by microwave irradiation is presented. The reaction yields were either higher or comparable to conventional heating reaction conditions, but the time was con-siderably shortened from hours to minutes. Importantly, as it is shown in Scheme 1, bis- or tris-adducts 3c and 3d, respectively, were not detected, when microwave reaction conditions applied. Thus, compared with conventional heating conditions, the micro-wave-assisted formation of the fluoroalkenylsufonylazides 3 is a cleaner reaction.

Another example, deals with the widely applied pyrrolidine formation onto C60, first reported in 1993 [46] involving the [3+2] cycloaddition reaction of azomethine ylides with fullerenes. The 1,3-dipolar cycloaddition reaction of azomethine ylides to C60 is reversible and through a first highly efficient retrocycloaddition reaction, of differently substituted pyrrolidinofullerenes, quantita-tively affords the parent unsubstituted fullerene [47]. The addition of azomethine ylides can also be applied in higher fullerenes such as C70 [48]. As described earlier [37] the cycloaddition can be per-formed through microwave irradiation and this holds true for the C70 adducts as well. Due to the nature of higher fullerenes, like C70,there are implications towards their functionalization. Setting aside the higher cost and abundance issues, the lower symmetry of C70gives rise to the formation of more than one mono-adducts, as

N SO2R1

+ R1SO2N3

oDCB, N2

MW

1(a-c)

2(a-c)R1 = CF2CF2OCF2CF2I

CF2CHF2

C4F9

N

+ CF3CH2N3

oDCB, N2

MW

1d

3a

CF3

N CF3

3b

+

N CF32

N CF33

3c 3d

Scheme 1. Synthetic route of azafulleroids using microwave irradiation.

Microwave-assisted Functionalization of Carbon Nanostructured Materials Current Organic Chemistry, 2011, Vol. 15, No. 8 1123

structural isomers, as compared with the case of the highly symmet-rical icosahedral C60. Whereas C60 contains a single type of [6,6] bond, C70 contains four different [6,6] bonds. However, Langa et al.reported [49] that under microwave irradiation there is tunable re-gioselectivity on the fullerene adducts obtained. As depicted in Scheme 2 when conventional heating conditions are applied ad-ducts 1a through 1c are produced. When microwave irradiation is used with toluene and chlorobenzene as solvents, 1b is produced in 8% and 3% yields, respectively, while switching the solvent to o-DCB only yields products 1a and 1c. This is a major result, in terms of reaction selectivity, highlighting the importance of microwaves.

Recently, Prato et al. described that reversibility of the 1,3-dipolar cycloaddition of azomethine ylides reaction to C60, is also possible using microwaves [50], in the presence of ionic liquids as a solvent/dispersion agent. Ionic liquids have been successfully used in a variety of organic reactions as alternatives to molecular solvents, for many organic transformations [51]. Their advantages include tunable composition which effectively translates to “custom

made solvents”, polar character, nonvolatility, thermal resistance, complementarity with water or other green-media and liquid electrolyte behavior. Using a variety of ionic liquids (since the chemical structure, as expected, directly affects the outcome of the reaction) in a mixture with o-DCB, Prato et al. achieved retro-cycloaddition yields of >99% for different fulleropyrolidines (Scheme 3).

Martin et al. [47,52] proposed that the reaction mechanism for the retrocycloaddition, occurs through the formation of a reactive 1,3-dipolar intermediate which is expected to be stabilized by either electron-acceptors on a-carbon, or electron-donors on the nitrogen atom or both. Prato suggested that the ionic liquid medium does not affect this. Moreover the careful optimization and control of reac-tion temperature, through pressurized air, allowed the application of higher microwave irradiation intensities without overheating the sample and risking ionic liquid decomposition. Finally, the selec-tion of ionic liquid, as expected, plays a key role in the reaction outcome. Despite the satisfactory results obtained by the combined

Scheme 2. Structural isomers of C70 fullerene pyrrolidine adducts obtained.

R1CHO

R2NHCH2COOH

Tol, reflux

Ionic liquid

MW (50 W)

5-10 min

N

R2

R1

R1 = H, R2 = CH3

R1 = C6H13, R2 = CH3

R1 = p-CH3O-C6H4, R2 = CH3

R1 = H, R2 = p-HO-C6H4

R1 = R2 = CH3

N R2 = CH3R1 =

R1 = COOEt, R2 = CH3

Scheme 3. Synthetic route for direct and retro-cycloaddition of azomethine ylides to [60]fullerene.

1124 Current Organic Chemistry, 2011, Vol. 15, No. 8 Economopoulos et al.

use of ionic liquids and microwave, the retrocycloaddition is con-trolled in part by the substituent of the nitrogen atom. Due to the nature of the mechanism the electron-donating substituent on the nitrogen site turns out to be strongly influential to cycloreversion.

CARBON NANOTUBE FUNCTIONALIZATION USING MI-CROWAVE IRRADIATION

One of the first things that comes to mind is that intense micro-wave irradiation can damage the CNTs and affect their properties. During one of the first publications on microwave assisted func-tionalization of single-walled and multi-walled carbon nanotubes [53], Sano et al. addressed the issue in order to investigate whether thermal-only or microwave effects as well, come into play when exposing CNTs in microwaves. Microwave absorption depends heavily on the dielectric properties of the substance irradiated [35], and macroscopically this interaction can be regulated. Toluene is a low absorbing solvent of microwave energy and can, therefore, be used to keep the reaction mixture at reasonable temperatures, essen-tially eliminating hotspots in the reaction mixture, as well as violent thermal effects and allow the progress of the reaction to be carried out safely and focus on the non thermal microwave effects that might occur on CNTs. In this context, Sano sonicated CNTs along with HNO3 and H2SO4 for several hours and then treated them with H2O2/H2SO4 in order to produce cleaved nanotubes bearing carbox-ylic moieties. By filtration, a thin “bucky paper-like” nanotube mat was formed that was immersed in toluene. With a four-point probe, the mat was monitored with respect to its electric resistance, a value that will indicate sufficiently any changes in its electric properties as a direct result of any changes occurring due to microwave expo-sure. A microwave setting of 200W was applied for a period of 100h and the surface resistance remained unchanged, indicating that no notable change, with respect to their conductivity, occurs. How-ever, as expected, when the same amount of intensity is applied in air or when boiling of the liquid medium occurs, the mat is dam-aged. Sano proceeded in successfully functionalizing the carboxy-lated CNTs by addition of dicyclohexylcarbodiimide (DCC) and octadecylamine (ODA) preheated to 60 oC in a glass tube. The CNTs produced stable dispersions in CHCl3, that was attributed to covalent attachment of the octadecylamine unit through a formation of a peptide bond with the carboxylate unit. After various experi-mentations using a steady 100W microwave intensity over different periods of time, Sano concluded that an exposure of over 20min is adequate to produce stable chloroform CNT dispersions. Addition-ally keeping the exposure time at 30min and experimenting with microwave intensity revealed that an intensity of over 60W is re-quired for the desired stability of the functionalized nanonstructures in the solvent. With those sets of measurements, the optimum mi-crowave conditions for the efficient tip-functionalization and disso-lution of CNTs were obtained.

One of the most basic functionalization routes of CNTs is the introduction of carboxyl groups along the tube’s length or optimally at the end of the nanotube. The latter approach has the advantage that it does not disrupt the �-conjugation and can influence unfa-vorably the electronic properties of the tube when applied into e.g. optoelectronic devices [54, 55] as well as the structure of the nano-material [56]. In order to introduce –COOH groups on a CNT, a simple oxidation procedure is applied [57] that involved sonication in acid. These methods are successful in the solubilization of carbon nanotubes and affect their length depending on the sonication con-ditions (e.g. time). In an attempt to estimate the number of end

functional groups, Hammon and coworkers also employed mid-IR spectroscopy, [58] through the use of a “marker” that reacts with the carboxylate groups (e.g. alkylamine) offering another analytical tool apart from the more traditionally used thermogravimetric analysis measurements. Chen et al. introduced a novel technique for purification, primarily, of multi-walled carbon nanotubes (MWCNTs) from metal particles that opens up the ends of CNTs, introducing –COOH groups [59-61]. After treatment optimization, they proposed that gradually heating the sample up to 210 oC in 20min using 5M HNO3 and maintaining this temperature for 30min yielded open-enden MWCNTs (with low catalyst concentration) and more importantly with minimal damage to the nanotube back-bone. In a similar manner, Wang et al. reported the efficient water solubilization of CNTs via microwave irradiation in concentrations of up to 10 mg/ml [62]. The experimental procedure involved a 1:1 mixture of 70% nitric acid and 97% sulfuric acid and an exposure to microwave irradiation. FT-IR spectra showed that, additionally to the –COOH groups produced, a high concentration of acid sul-fonated groups was present. The group also observed an average length of 1�m for the HiPCO nanotubes used which would suggest a shortening effect due to the microwave effect coupled with the strong acidic treatment and some sidewall disordering also attrib-uted to the same factors. However through microscopy techniques, the nanotube samples were mostly individually separated and not in bundles. They exhibited excellent solubility, producing long term stable suspensions in deionized water and anhydrous alcohol with higher solubilities of 20 mg/ml observed in acidified water. It is also worth noting that due to the high functionalization degree (an estimated one out of every three carbons was carboxylated and one in every ten carbon atoms was sulfonated) the nanotubes were spontaneously aligned when drop casted, possibly due to the hydro-phobic and nanotube-nanotube interactions.

Regarding carbon nanohorns (CNHs), Yoshida et al. described a similar procedure [63] where CNHs’ dispersibility in tetrahydro-furan was increased 25-fold. CNHs were simply irradiated in a microwave oven, without any acid present. Decoration of the cone ends with Pt nanoparticles by a similar microwave treatment is also possible. On a side note, the authors speculated that microwave irradiation is directed at the carbon nanohorn tips, based at the fact that decoration with Pt nanoparticles was achieved around the dahlia-like structure, through microwave irradiation (~1-2 minutes at 250W) of the oxidized nanomaterial.

Similarly to CNT oxidation, the following functionalization technique is just as attractive due to the ease of implementation. In the context of CNT-based composites, researchers have reported high mechanical strength measured in nanotube-polymer nanocom-posites [64, 65]. As such, the aspect of CNT loading in a polymer matrix with the intent of increasing the polymer’s mechanical prop-erties is extremely appealing. As mentioned earlier CNTs are excel-lent microwave absorbers [66] and Tour and coworkers reported intense heat output in SWNTs when irradiated with microwaves [67] in the range of 2000 oC. When trying to take advantage of such a response, the microwave exposure on the material has to be regu-lated strictly, in order not to severely damage the polymer matrix, which in most cases cannot withstand temperatures comparable to most carbon nanostructures. There are reports of MWNT-polymer bonding using microwave irradiation by Baughman and coworkers [68] in which two plexiglass sheets can be “glued” together if a thin transparent MWNT sheet is sandwiched between them and irradi-ated by microwaves. Control of the temperature was provided with a water reference and the sample was heated, gradually up to the

Microwave-assisted Functionalization of Carbon Nanostructured Materials Current Organic Chemistry, 2011, Vol. 15, No. 8 1125

point that the water reached 100 oC and subsequently held that tem-perature for 1 min, before allowing the sample to cool down.

Keeping these results in mind, Chin et al. managed to achieve MWNT loading in a poly(ethylene terephthalate) PET matrix by way of a microwave irradiation method [69]. In this report, MWNTs were dispersed in ethanol and sprayed along a PET plate, irradiated in a microwave oven and rinsed with ethanol to remove unattached MWNTs. After only 1s of microwave exposure, the nanotubes start to penetrate the PET surface, with some PET drop-lets forming along the tubes’ axis, possibly due to excessive heat in those areas. Chin, speculated that this sudden increase in tempera-ture leads to melting of the adjacent polymer matrix, allowing sub-mersion of the MWNTs into the polymer. They also noticed that MWNTs are present away from the welding junction indicating that PET wets the nanotubes and that there is a wetting period of several seconds, during which the nanotube is allowed to penetrate the bulk polymer. In addition, there are no defects in the PET, such as bub-bles, that are common in other CNT-polymer nanocomposites de-riving from processes involving extruders [70] or solution mixing [71]. The latter indicates that a small region around the nanotube is involved in the welding process and that the bulk of the polymer remains intact. Higher Tg polymers than PET were also used, such as polycarbonate (PC) and polyimide (PI) showing again, that the local melting of these polymers caused by the heating of adjacent nanotubes is enough to intercalate the MWNTs into the matrix.

Having essentially the same effect as functionalization, the welded nanotubes in the polymer matrix can undergo processes such as alignment, by the application of force in the polymer composite.

Given the similarities between fullerenes and carbon nanotubes it is logical that the chemistry applied in C60 will, in part, be appli-cable in CNTs. Microwave-assisted synthesis of Diels-Alder ad-ducts of C60 with 2,3-pyrazinoquinodimethanes has been reported by Langa [40] previously. In 2004, Langa and coworkers reported the successful implementation of the reaction conditions on CNTs [72].

As depicted in Scheme 4 the acid purified HIPco CNTs contain-ing carboxylic groups at the ends, were converted to the corre-sponding acyl chlorides (through the use of thionyl chloride), fol-lowed by reaction with n-pentanol [73] to create soluble CNTs. The functionalized CNTs reacted with o-quinodimethane generated in situ from 4,5-benzo-1,2-oxathiin-2-oxide under microwave irradia-tion for 45 min. The resulting functionalized CNTs could be charac-terized by a variety of techniques, including 1H NMR in CD2Cl2.

Likewise, the Prato reaction has been successfully implemented on CNTs using microwave irradiation [74, 75]. Following, the im-plementation of azomethine ylide functionalization in DMF in clas-sical heating conditions [76], the strong microwave absorption of carbon nanotubes was used to facilitate the same reaction. As de-picted in Scheme 5 the reaction results in the formation of azome-thine ylides using aziridine precursors and microwave irradiation.

O

O

O

O

O

O

O

OO

O

O

O

MW (150 W)

50 min

O

O

O

O

O

O

O

OO

O

O

O

n

Scheme 4. Microwave-assisted functionalization of CNTs using Diels-Alder cycloaddition reaction.

1126 Current Organic Chemistry, 2011, Vol. 15, No. 8 Economopoulos et al.

The CNTs were left to react in solvent free conditions, taking ad-vantage of the liquid phase of the aziridine derivative and were irradiated for 1h. The resulting functionalized CNTs exhibited a degree of functionalization of about 27%, or 1 functional unit per 51 carbon atoms, as calculated by thermogravimetric analysis. The fused pyrrolidine ring formed onto the skeleton of CNTs can be removed, furnishing the pristine CNTs, by simple heating. Addi-tionally, with the addition of an excess amount of C60, the pyr-rolidine from the CNTs backbone is removed and transferred to the fullerene sphere.

The same reaction has been implemented in carbon nanohorns using the microwaves to expedite the functionalization process of this novel class of nanomaterials as Rubio et al. recently reported [77]. In Scheme 6 the functionalization procedure for the addition of azomethine ylides to CNHs, is shown. As is the case with CNTs, the microwave reaction offers good tolerance concerning the R1, R2substituents, allowing flexibility in the obtained functional CNHs. This reaction has been realized using conventional reaction condi-tions using DMF as solvent [78]. However, Prato et al., modified the protocol when using microwaves, avoiding the use of solvent and achieved a reaction time of 1h, offering a substantial improve-ment over the previously reported ~100h. The microwave function-alized CNHs were characterized towards their degree of function-

alization exhibiting from one functional unit per 192 carbon atoms, up to 1 unit per 99 carbon atoms.

In the same report [77] Prato’s group managed to perform func-tionalization of CNHs under microwaves using the Tour reaction conditions [79]. Concerning carbon nanotubes, this reaction will be discussed further down. The Tour reaction initially takes place in o-DCB/CH3CN 2/1, it has recently been implemented using more environmentally friendly solvents such as H2O [80]. Taking advan-tage of microwaves, the reaction was able to proceed to functional-ize CNHs (Scheme 7). Examining the functionalization degree, Prato’s group discovered that one functional group for every 54 carbon atoms was present, denoting that the Tour reaction pro-gresses better than the addition of azomethine ylides under micro-wave irradiation. Moreover, thermogravimetric analysis (TGA) showed that two plateaus are formed, suggesting that the BOC group decomposes before the rest of the organic matter. Coupling of these two functionalization methods gives rise to doubly func-tionalized CNHs bearing both azomethine ylides and BOC-protected functional groups.

Due to the fact that the cycloaddition reaction was more selec-tive than the arene addition, the process of functionalizing CNHs with two different functional units was performed in two steps. Firstly the 1,3-cycloaddition took place, forming the pyrrolidines

N

EtOOC

NEtOOC

NCOOEt

N COOEt

MW

oDCBPhCl

��

Scheme 5. Synthetic route for the functionalization of CNTs with azomethine ylides using aziridine precursors and their retrocycloaddition to C60.

Microwave-assisted Functionalization of Carbon Nanostructured Materials Current Organic Chemistry, 2011, Vol. 15, No. 8 1127

and after this procedure, the Tour reaction took place to afford the final functionalized CNHs as seen in Scheme 8. When the function-alized CNHs with either the phtalimido and/or the BOC group are cleaved, the resulting CNH material is left with NH3

+ groups, which makes the material soluble in polar solvents.

The same methodology was applied in single walled carbon nanotubes by Prato et al. [81]. This led to doubly functionalized CNTs as shown in Scheme 9. This approach essentially builds upon the method proposed by Tour in 2003 in which the functionaliza-tion of single-walled and multi-walled carbon nanotubes with the arylamine derivative takes place with mechanical stirring in the absence of a solvent [82]. With this previous knowledge in mind, Prato and coworkers proposed the synthetic route depicted in Scheme 9. At first, pristine SWNTs were functionalized using the 1,3-dipolar cycloaddition with various aldehydes and sarcosine, in the absence of any solvent using a microwave and various ramping and holding times ranging from 30W to 50W for intensity and from

20sec up to 1h reaction times. All the products exhibited a change in the D/G ratio in the Raman spectra and upon characterization with TGA, a weight loss of 11-21% denoting the organic uptake of the functionalized CNTs. Examination of the radial breathing mode (RBM) region below 400cm-1 in the Raman spectra did not show any selectivity of the microwave reaction towards metallic and/or semiconducting tubes. In contrast, reports of variations in intensity of the peaks and therefore ease of functionalization in metallic nanotubes rather than semiconducting have surfaced, which actu-ally resulted in efficient separation of metallic from semiconducting CNTs [83]. The degree of functionalization also seems independent of the aromatic or aliphatic nature of the substituent but the group reported that upon optimization of the process, smaller reaction times led to smaller degree of functionalization. These functional-ized CNTs were reacted with diazonium salts in order to produce the double-functionalized nanomaterials. In order to achieve that, the tubes were dispersed in water with p-toluidine and isoamyl

Scheme 6. Synthetic route for functionalized CNHs with azomethine ylides using microwave irradiation.

Scheme 7. Tour reaction conditions on CNHs under microwave irradiation.

1128 Current Organic Chemistry, 2011, Vol. 15, No. 8 Economopoulos et al.

Scheme 8. Microwave-assisted synthesis of doubly-functionalized CNHs.

Scheme 9. Synthesis of functionalized CNTs using microwave irradiation in Prato and Tour reaction conditions.

Microwave-assisted Functionalization of Carbon Nanostructured Materials Current Organic Chemistry, 2011, Vol. 15, No. 8 1129

Scheme 10. Bingel functionalized CNTs using microwave irradiation Bingel functionalized CNTs using microwave irradiation.

Scheme 11. Synthetic process for malonate-decorated CNHs.

1130 Current Organic Chemistry, 2011, Vol. 15, No. 8 Economopoulos et al.

nitrite was added before irradiation in a microwave oven for 90min, while keeping the temperature at 80 oC. Raman spectra showed a change in D/G ratio and thermogravimetric analysis showed an increased weight loss compared to the 1,3-cycloaddition. In short a total weight loss of 21% up to 36% is evidenced and a quantitative approximation of functional units from each functionalization proc-ess is estimated. About 1 unit owed to the 1,3-dipolar cycloaddition for every 87 to 216 carbon atoms is present (the 11% to 21% men-tioned above) and 1 unit owed to the arene radical addition for every 31 to 192 carbon atoms. Interestingly, the higher functional-ized CNTs were consistently (both in the “Prato” and the “Tour” reaction conditions) the ones with the di-methoxyphenyl functional group, while the CNTs exhibiting the lowest degree of functionali-zation were the ones with the tri-dodecyloxy phenyl groups denot-ing the possibility of steric hindrances having a key role in the final product.

Finally the Bingel reaction conditions have been successfully implemented, yielding functionalized carbon nanotubes and nano-horns with malonate derivatives. Imahori examined the feasibility of microwave-assisted functionalization in SWNTs using malonates as depicted in Scheme 10. Imahori and coworkers essentially pro-duced doubly-functionalized SWNTs, first using previously de-scribed methods to enhance solubility in organic solvents by adding alkyl substituents at the ends and at the defect sites of CNTs [75, 84, 85]. Afterwards the Bingel reaction was employed, either with conventional heating, or by microwave irradiation to yield the final functionalized nanostructure. As expected, Imahori reports [86] much shorter reaction times in microwave heating than conven-tional heating (30min vs 24h) and only 40W of microwave irradia-tion intensity to produce functionalized CNTs with the same degree of sidewall funtionalization as the conventionally synthesized one. Increasing the microwave output, resulted in increased organic uptake and produced higher side-functionalized CNTs. However exposure of the sample to an excess of 60W or for a prolonged period of time (over 30min) did not continue this trend.

Finally, Tagmatarchis et al. proposed that this approach is ap-plicable in the synthesis of CNH derivatives, bearing malonates (Scheme 11) [87]. Using microwaves and solvent-free conditions, malonate units were able to covalently attach themselves onto CNHs forming functionalized CNHs that produced stable disper-sions in a variety of solvents such as CH2Cl2 or DMF. This syn-thetic approach exhibited synthetic flexibility, allowing for different (aromatic and fairly bulky) malonates to be decorated around the CNHs dahlia-like three dimensional structure. Malonates bearing light-harvesting moieties like pyrene and anthracene were synthe-sized and successfully reacted using the Bingel conditions, in a microwave oven to produce CNH hybrid materials that were solu-ble in organic solvents. The microwave irradiation used did not exceed 60W and it was delivered in steps to ensure that the tem-perature did not exceed 120-140 oC and HRTEM measurements showed no damage to the CNH’s structure. The reaction times were under 2h for all the experiments, including the intermediate cool down times needed for the samples to return to lower temperatures. Owing to the absence of a solvent, the reaction mixture (comprised of the CNHs, the desired malonate derivative, the base, in our case 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and either I2 or CBr4took less than a minute to reach the threshold temperature of 120 oC, even at microwave intensities as low as 5W. The synthetic ap-proach provides another easy and extremely time-efficient way to produce a variety of soluble functionalized carbon nanohorns.

CONCLUSIONS

An overview of the latest trends in microwave assisted chemis-try towards functionalization of fullerenes, carbon nanotubes and carbon nanohorns is presented. The use of microwave irradiation in structures such as these, proves to be an important tool that facili-tates reactions and in some cases allows the realization of previ-ously unattainable products. Most commonly used functionalization synthetic routes for C60, CNTs and CNHs can be implemented suc-cessfully using mw with comparable results and with considerable time efficiency. A few other possibilities are opened through the use of microwaves exploiting their strong absorbance from carbon nanostructures. The added benefit of reduced or complete absence of solvents makes this approach even more appealing.

ACKNOWLEDGEMENTS

The authors would like to thank the EU FP7, Capacities Pro-gram, NANOHOST project (GA 201729) for financial support. S.P.E. would like to thank the State Scholarships Foundation of Greece (IKY) for financial support under the postdoctoral fellow-ship grant. Partial support through COST network MP0901 NanoTP is also acknowledged.

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Received: 28 December, 2009 Revised: 10 February, 2010 Accepted: 11 February, 2010