Convenient syntheses of fullerynes for ‘clicking’ into fullerene polymers

Abstract Alkyne-functionalized fullerenes (fullerynes) were designed and conveniently synthesized via Bingel reaction in one step with high yields. They were used to react with azido-functionalized polystyrene (PS) via Huisgen [3 + 2] cycloaddition ‘click’ chemistry to form two fullerene polymers: one with C60 tethered to the end of a PS chain (C60-1PS) and the other with C60 tethered at the junction point of two PS chains of identical molecular weight (C60-2PS). The fullerene polymers were characterized by 1H NMR, 13C NMR, FT-IR, MALDI-TOF mass spectrometry and SEC. The results showed that the fullerene polymers are well-defined with narrow polydispersity and high fullerene functionality. Besides, aggregation of C60 in THF was observed in the SEC traces. The optical properties of the fullerene polymers were studied by UV–Vis absorption spectroscopy, and the results suggested that the PS chain(s) on the fullerene core has no remarkable effect on the optic property of C60. The thermal properties of the fullerene polymers were studied by TGA and DSC, and the results indicated that the two fullerene polymers with different C60 content and distinct molecular topology may have different self-assemble architectures in the solid state. The well-defined fullerene polymers can be used as model compounds to study the self-assemble architecture of shape amphiphiles based on polymer-tethered molecular nanoparticles.


Materials and methods
Fourier transform infrared (FT-IR) spectra were taken using a Nicolet Magna-IR 550 FT-IR spectrometer in the range of 400-4000 cm −1 by making a KBr pellet from the mixture with the sample. The resolution was 4 cm −1 and 32 scans were averaged.
Matrix-assisted laser desorption/ionization time-offlight (MALDI-TOF) mass spectra were recorded on a Kratos Axima CFR plus spectrometer (Shimadzu Biotech, Manchester, UK) with a 337 nm nitrogen laser and the acceleration voltage of 20 kv. The matrix was 2,5-dihydroxybenzoic acid (DHB). Mass spectra in positive ion mode were measured in both linear and reflection modes. Data analyses were conducted with Kompact software.
Size exclusion chromatography (SEC) analysis were performed using a Waters 515 Plus instrument equipped with three waters Styragel columns (103, 104, 105 Å) and three detectors (DAWN HELEOS, ViscoStar and Optilab rEX). THF was used as the eluent at a flow rate of 1.0 mL/min at 35 °C. The system was calibrated by a set of monodispersed standard polystyrenes.
UV-Vis spectra were measured on a CARY 100 UV-Vis spectrophotometer. Samples were prepared in THF at a concentration of 1 × 10 −5 M, and the spectra were recorded between 200 and 600 nm.
Thermogravimetric analyses (TGA) were carried out with a NETZSCH STA 409 PC/PG thermogravimetric analyzer at a heating rate of 20 °C/min under a nitrogen atmosphere. Differential scanning calorimetry (DSC) was performed on DSC Q2000 V24.10 with a heating rate of 5 K min −1 under N 2 flow.

Mono-Fulleryne (3)
A 500 mL three-necked flask equipped with dropping funnel was charged with C 60 (360 mg, 0.50 mmol), CBr 4 (199 mg, 0.86 mmol), and dry degassed toluene (200 mL). After C 60 was dissolved, compound 2 (138 mg, 0.75 mmol) was added, and a solution of DBU (152 mg, 1.0 mmol) in toluene (40 mL) was then added dropwise during a period of 1 h. After stirring for another 6 h at room temperature under nitrogen in the absence of light, the mixture was concentrated and eluted with cyclohexane/toluene (3/2,v/v) on silica gel column. After the first fraction containing unreacted C 60 , mono-Fulleryne (3)

Di(3-butyn-1-yl) malonate (4)
A 250 mL three-necked flask equipped with the deanstark strap was charged with malonic acid (10.4 g, 0.10 mol), 3-butyn-1-ol (28 g, 0.40 mol), toluene (60 mL) and four drops of concentrated sulfuric acid as catalyst. The mixture was refluxed under nitrogen for 5 h before cooled to room temperature. Water and diethyl ether was added, and the organic layer was separated. The water layer was extracted with diethyl ether for three times, and then the combined organic layers were washed with brine. After dried over anhydrous Na 2 SO 4 , the solvent of the organic layer was removed under reduced pressure. The crude product was purified by column chromatography by using petroleum ether/EtOAc (5/1, v/v) as eluent to yield a colorless oil. (

Design and synthesis of mono-fulleryne and difulleryne
Fullerynes were synthesized in one step from C 60 with high yield by reacting terminal-alkyne-functionalized malonates with fullerene via Bingel reaction (Scheme 1). The synthesis of mono-alkyne-functionalized malonate could be achieved by reacting alcohol with Meldrum's acid as reported in literature [32,48] or the selective hydrolysis of diethylmalonate. [47] Acid 1 was synthesized as described in literature [47] and then reacted with 3-butyn-1-ol using Steglich esterification to afford ethyl-3-butyn-1-yl malonate (2) in a yield of 27.7%. Then, malonate 2 was directly reacted with fullerene in presence of CBr 4 and DBU to give mono-Fulleryne (3) as a brown powder. The yield of 41.5% is good for fullerene functionalization. The synthesis of both malonate 2 and mono-Fulleryne were conveniently performed under mild reaction conditions. In literature, bifunctional fulleryne has been reported by Nierengarten and was synthesized via Bingel reaction. It was used as building blocks for copper-catalizedazide-alkyne cycloaddition (CuAAC) reaction. [37] Here we report a similar structure, di-Fulleryne. Bifunctional malonate (4) was synthesized by Fisher esterification of malonic acid with 3-butyn-1-ol in a yield of 57.7%. Its reaction with C 60 under Bingel-Hirsch condition gives a bifunctional fulleryne, the di-Fulleryne. The yields for mono-Fulleryne and di-Fulleryne are very close. Both fullerynes were fully characterized by 1 H-NMR, 13 C NMR, FT-IR and MALDI-TOF mass spectrometry. In the 1 H-NMR spectra (Figure 1(a) and (b)), resonance signals attributed to the α-protons of the malonate (~3.5 ppm) disappeared completely, evidencing the success of the Bingel reaction. Signals attributed to the sp 2 and sp 3 carbons of C 60 can be clearly observed in the 13 C NMR spectra (Figure 2(a) and (b)). The characteristic absorbance for C-C vibration on C 60 can be clearly observed at 525 cm −1 in the FT-IR spectra. The MALDI-TOF mass spectra (Figure 3) further confirm the structures, where the observed molecular ion peaks have m/z values matching that of the calculated monoisotopic masses. All of the evidence clearly proves the chemical structure and purity of the new fullerynes.

'Clicking' for fullerene polymers
Azido-functionalized polystyrene can be facilely synthesized by atom transfer radical polymerization (ATRP) and subsequent nucleophilic substitution as reported. [

C 60 -2PS (7)
The synthesis was performed in a procedure similar to that for 6 except using 5 instead of 3. Yield: 78.7%. 1  power that is readily soluble in common organic solvents such as THF, CH 2 Cl 2 etc. The number and position of the tethered polymer tails are pre-determined by the location and number of alkyne groups on fullerynes, leading to shape amphiphiles of distinct architectures: C 60 -1PS and C 60 -2PS.
The fullerene polymers were fully characterized by 1 H NMR, 13 C NMR, FT-IR and MALDI-TOF mass spectrometry. The product exhibited the characteristic resonances of protons both near the 1,2,3-triazole (4.9-5.2, 3.0-3.3 ppm) and C 60 unit (4.3-4.8 ppm) in the 1 H-NMR spectra ( Figure  1(c) and (d)). However, the signal attributed to the proton on the triazole ring cannot be observed, which may be overlapped with the peaks of the aromatic protons of amphiphiles using hydrophilic fullerene as the polar head and polystyrene as hydrophobic tail. [12] Versatile selfassembled micellar morphologies were observed and can be tuned by changing various parameters, such as molecular topology, polymer tail length and the molecular concentration. The C 60 was used mainly as a structural scaffold in this work and the electronic properties of C 60 were largely lost in these shape amphiphiles. It is of great interest to study the self-assemble behavior of shape amphiphiles containing pristine C 60 s. Thus, we try to synthesize welldefined fullerene polymers with distinct topologies as model shape amphiphiles.
The fullerene polymers synthesized using mono-Fulleryne and di-Fulleryne were also found to be a brown  confirming the stability and purity of the resulting fullerene polymers. The Gaussian distribution of the molecular weights was due to the different chain lengths of the polymers obtained during the polymerization. The difference in molecular weight between the two neighboring peaks was 104.1, which equals exactly to that of a single styrene repeating unit. The PDI calculated from MALDI-TOF is 1.01 for both C 60 -1PS and C 60 -2PS, which is very narrow. The results indicate that the fullerene polymers are pure with high C 60 functionalization and well-defined as designed.
In the SEC traces (Figure 6), the retention time of C 60 -1PS was slightly larger than that of PS-Br. Zhang et al. explained that the polystyrene chain tends to wrap around the fullerene ball due to the insoluble of C 60 in THF, thus decreasing the hydrodynamic volume of the fullerene polymer. [5] The C 60 -2PS showed a smaller retention volume, due to its larger molecular weight. A high molecular weight shoulder peak was observed in both fullerene polymers. The shoulder may be caused by the aggregation of C 60 in THF, [11,13,50] as the fullerene polymers were well defined without multi-addition product evidenced form the high sensitive MALDI-TOF. The shoulder peak of C 60 -2PS is smaller than that of C 60 -1PS indicating the tendency of aggregation in C 60 -2PS is weaker. Interestingly, previous study by Zhang et al. did not show the aggregation phenomena of the PS-C 60 polymer. [5] So we predicted that the aggregation of our C 60 -PS polymers may caused by the high concentration of the fullerene polymer solution or the low dissolve time in the SEC test, which need a systemic study in the further work. Ignoring the shoulder peak, the PDI calculated from SEC is 1.01 for both C 60 -1PS and C 60 -2PS, evidencing the well defined of the fullerene polymers.
The steady state UV-Vis absorption spectra were measured in THF at room temperature. Figure 7 displays the absorption spectra of the alkyne-functionalized fullerenes, and the fullerene polymers. In the UV-Vis spectra of fullerynes, the absorption peaks at ~330 and 430 nm were typical fingerprints of C 60 monoadducts (methanofullerene). [51][52][53] The UV-Vis absorption spectra of both C 60 -1PS and C 60 -2PS are very similar with that of fullerynes, indicated that the PS chain has no remarkable effect on the optic property of C 60 .
The thermal properties of the alkyne-functionalized fullerenes and the fullerene polymers were studied by TGA and DSC (Figures 8 and 9). C 60 and polystyrene were used as controls. The pristine fullerene exhibits outstanding thermal stability when heated to 550 °C. In comparison, the residue of mono-fulleryne and di-fulleryen at this temperature is 80.9 and 79.7% respectively, which is in good agreement with the theoretical estimation if assuming that the 'non fullerene' moiety had been completely decomposed and removed (79.7% for mono-fulleryen and 77.7% for di-fulleryne). The thermal decomposition of the PS. [36,49] Nevertheless, the success of the 'click' reaction and the precisely defined structures of both C 60 -1PS and C 60 -2PS can be validated from the combination of other techniques. In the 13 C NMR spectra (Figure 2(c) and (d)), the signals attributed to the alkyne carbons (79.8 and 70.8 ppm) on fullerynes disappeared completely, and the sp 3 carbons as well as sp 2 carbons of the C 60 moiety can be observed. FT-IR spectra (Figure 4) showed the complete disappearance of the azide group (2095.2 cm −1 ) and the appearance of a sharp peak at 525 cm −1 , which is the characteristic for C 60 . [1] The MALDI-TOF mass spectra ( Figure 5) showed a single Gaussian distribution with molecular weight in accordance to the proposed structure,  region 150-300 °C in the DSC profile, which may be attributed to the cross-linking reaction of the alkyne group. Di-fulleryne showed a similar DSC profile (thermal behavior) as that of mono-fulleryne, except that the exothermal peaks appeared at a lower temperature, indicating the lower thermal stability of di-fulleryene. Polystyrene undergoes a thermal decomposition between 386 and 450 °C. The onset decomposition temperature of the fullerene polymers (374 °C for C 60 -1PS and 382 °C for C 60 -2PS) are very similar with that of PS, indicated that the existence of C 60 has no remarkable effect on the thermal stability of the PS chain. The residue of the fullerene polymer at 550 °C (18.9% for C 60 -1PS and 10.7% for C 60 -2PS) can be considered as the fullerene part, which is in good agreement with the theoretical content of fullerene in the polymer (19.1% for C 60 -1PS and 10.5% for C 60 -2PS). In the DSC profile, PS showed a melting point at 84 °C. In contrast, only one glass transition process was detected for fullerene polymers, alkyne-functionalized fullerene occurred at 330 and 328 °C respectively. Before this temperature, mono-fulleryne showed two distinctive exotherms process between the    indicated that the incorporation of C 60 restricts the crystal of the PS chain. The T g of fullerene polymers was reported to be varied with C 60 content. [24,[54][55][56][57] In our case, the T g of C 60 -1PS (99 °C) is lower than the T g of C 60 -2PS (105 °C), whereas the C 60 content in C 60 -1PS is higher. This indicated that the two fullerene polymers with different C 60 content and distinct molecular topology may have different selfassemble architectures in the solid state.

Conclusions
In summary, mono-Fulleryne and di-Fulleryne were designed and conveniently synthesized in one step via Bingel reaction. These fullerynes were employed for 'click' into two fullerene polymers of distinct topology: one being C 60 tethered with one polymer tail (C 60 -1PS) and the other being C 60 tethered at the junction point between two polymer chains (C 60 -2PS). The fullerene polymers were well defined with narrow polydispersity and high degree of C 60 functionalization. The PS chain(s) on the fullerene core has no remarkable effect on the optic property of C 60 . Aggregation of C 60 in THF was observed in the SEC traces. The thermal behavior studies indicated that the two fullerene polymers with different C 60 content and distinct molecular topology may have different self-assemble architectures in the solid state. The well-defined fullerene polymers can be used as model compounds to study the self-assemble architecture of shape amphiphiles based on polymer-tethered molecular nanoparticles.