Efficient synthesis of highly soluble and functionalized fulleropyrrolidines

Abstract

A series of new functionalized fulleropyrrolidines with varied alkyl chains were synthesized and characterized using various spectral techniques. The properties of the supramolecular architecture core C₆₀ were fine-tuned by the incorporation of mono- and di-alkyl chains attached via a phenyl ring attached with a pyrrolidine ring. Long alkyl chains attached [60] fullerene exhibited more solubility, high emission and absorption.

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1. Introduction

Over the past two decades, fullerene chemistry has been directed to create cage like carbon structures as building blocks in organic synthesis for several important applications.^1–3 Among the various fullerenes reported C₆₀ is widely studied and it shows a large range of interesting features, which include the spherical shape of fullerene bringing about delocalized π-electron systems. C₆₀ is also a good electron acceptor in both ground and excited states, used in the construction of photovoltaic devices⁴ because of their unique electrochemical properties, namely ability to accept up to six electrons.⁵ Apart from that, it is an excellent photosensitizer due to its long-lived triplet state, high absorption in the visible region, appearance of various stable oxidation states and the low degree of charge recombination. Chemical modification of fullerenes has been ardently explored because of their potential applications in biological activities such as, anti-HIV activity, neuroprotective, DNA cleavage, anti-oxidant and anti-microbial activities.⁶ Moreover it is used as organic semiconductors, photoconductors, photovoltaic devices, electronic and optical devices.^7,8

Modification of fullerene structures using synthetic methods is a new custom structure approach to control the morphology of fullerenes. Fulleropyrrolidine consists of a variety of organo-fullerene derivative in which, a pyrrolidine ring is fused to a 6,6-junction of the fullerene structure which is a widely studied fullerene derivative.⁹ These derivatives have been prepared by 1,3-dipolar cycloaddition of azomethine ylides to C₆₀ known as Prato reaction.¹⁰ A number of C₆₀ fulleropyrrolidines have been synthesized by reaction of C₆₀ with N-substituted glycine and suitable aldehydes.¹¹

One of the major problem with fullerene derivatives, especially C₆₀ is its poor solubility in organic solvents and insoluble nature in water. This attitude hinders their application in the solar cells¹² and solution based organic electronic devices.¹³ To overcome this obstacle, two different routes have been handled to increase the solubility.¹⁴ The first strategy is that the fullerene molecules are converted into soluble host molecules¹⁵ by non-covalently encapsulation method. Another method is the covalent functionalization of fullerene by the substitution of various groups through chemical modification.^16,17 Accordingly chemical modification in fullerene derivatives has not only alter the physical and chemical properties but also provides useful insight for molecular constructions. In this article an efficient synthesis, characterization and photo-physical aspects of functionalized new fulleropyrrolidines are reported.

2. Results and discussion

Prato reaction is an example of [3 + 2] cycloaddition reaction, in which the azomethine ylide is generated in situ after decarboxylation of iminium salts obtained by condensation of amino acid and aldehydes. These ylides react with the C₆₀ to form fulleropyrrolidines. The synthesis of fulleropyrrolidine 4 is described in Schemes 2–4. It is obtained by condensation of appropriate benzaldehyde 3 (Scheme 1) and N-methylglycine onto C₆₀. Table 1 describes the product yield 4a–4i and C₆₀ recovered after the column chromatography. Various alkyl chains were introduced onto the pyrrolidine ring through the lateral phenylic group. The introduction of alkyl chains will enhance the van der Waals interaction among the molecules and improve molecular connectivity. The power conversion efficiency of organic solar cells based on fullerene derivatives was reported to increase with the length of substituted aliphatic chains.^17,18 These fullerene derivatives are divided into two parts, sp² carbon rich C₆₀ moiety and sp³ aliphatic chain. C₆₀ contains carbon atoms with three sp² hybrid orbitals having one delocalized π-orbital such as benzene. As a result, it has a high affinity towards aromatic solvents such as toluene, xylene, and benzene but low affinity to aliphatic and polar solvents. However, aliphatic chains are usually soluble in sp³-carbon rich solvents like hexane, alcohols and ethers. Although both the parts are hydrophobic in nature, but shows different affinities to several solvents.

Scheme 1 Synthesis of aldehyde derivatives 3a–3g.

Scheme 2 Synthetic schedule of fulleropyrrolidines 4a–4g.

Scheme 3 Synthesis of 4h.

Scheme 4 Synthesis of 4i.

Table 1 Product yield, reaction time and recovered C₆₀ for the cycloaddition reactions of C₆₀ with N-methylglycine and aldehydes

Aldehyde	Reaction time (h)	Product	Yield (%)	Recovered C60 (%)
3a	9	4a	34	40
3b	13	4b	31	38
3c	15	4c	29	42
3d	15	4d	32	37
3e	18	4e	31	41
3f	17	4f	32	39
3g	13	4g	31	35
3h	16	4h	30	40
3i	17	4i	28	39

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The structure of the synthesized compounds was confirmed by FT-IR, ¹H and ¹³C NMR spectral data. The FT-IR spectra of the 4a–4i shows the occurrence of chemical reaction, which are characterized by the disappearance of characteristic aldehydic peaks at 1677, 1387, 1298 cm⁻¹ and appearance of C–N stretching peak at 1026 cm⁻¹. The aliphatic C–H stretching peaks appeared at 2921, 2849 cm⁻¹, C=C stretching peak observed at 1607 cm⁻¹, C–O–C stretching peak reported at 1265 cm⁻¹, aromatic C–H, wagging peaks appeared at 827, 718 cm⁻¹. In addition the characteristic peaks of C₆₀ observed at 1375, 1176 cm⁻¹ as well as a C₆₀ cage vibration peak was noted at 525 cm⁻¹. ¹H NMR spectra of 4a–4i shows (Table 2) the presence of pyrrolidine protons as singlet at 2.73–2.81 ppm and the two doublets in the region of 4.14–5.48 ppm (d, J ∼ 8.4–10.8 Hz). The ArCHN proton signal appeared at 4.84–5.48 ppm, aromatic ring proton signals are observed at 6.89–7.82 ppm (J = 6.89–8.40 Hz), alkyl chain proton peaks are observed in the region of 0.90–1.97 ppm as multiplets. Methyl protons peak appeared as a triplet at 0.80 ppm. The ¹³C NMR spectra of 4a–4i show the signals in the region between 129.0–156.0 ppm thus indicating C_2v symmetry of the fullerene moiety. A peak was seen near 83 ppm, corresponding to the sp³ hybridized carbon atom with a 6,6 ring junction on the fullerene core. N–CH₃ group signal appeared around at 40.0 ppm and signals for alkyloxy chains were observed between 14.0–70.0 ppm. Furthermore the other peaks between 14–27 ppm belongs to the alkyl groups present in the fullerene derivatives.

Table 2 ¹H NMR spectral data for fulleropyrrolidine derivatives 4a–4i

Compound	N–Me	H–2	H–5a (J Hz^−1)	H–5b (J Hz^−1)
4a	2.79	5.46	4.27 (9.2)	4.95 (9.2)
4b	2.81	4.85	4.23 (10.8)	4.97 (9.2)
4c	2.81	4.84	4.24 (9.6)	4.97 (9.2)
4d	2.78	5.38	4.25 (9.6)	4.96 (9.6)
4e	2.79	4.87	4.23 (9.2)	4.97 (9.2)
4f	2.79	4.87	4.23 (9.2)	4.95 (9.2)
4g	2.82	4.86	4.25 (9.2)	4.98 (9.6)
4h	2.73	5.48	4.14 (9.6)	5.00 (8.4)
4i	2.80	4.91	4.25 (9.2)	4.98 (9.6)

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2.1. Electrochemical studies

The electrochemical characterization of the compound 4c was performed at room temperature in 0.5 M tetrabutylammonium perchlorate (TBAP) in CHCl₃ solution, using three electrode cell unit, a glassy carbon as working electrode, a platinum wire as counter electrode and Ag/AgCl as reference electrode. The experiment was done under the scan rate of 0.5 mV s⁻¹ and the current sensitivity 100 mV s⁻¹. This method is very useful to get information about the electronic structure of fulleropyrrolidines. The electron acceptor properties of [60] fullerene,¹⁹ which include the ability to reversibly undergo up to six one-electron reduction.²⁰ The Fig. 1 shows the redox potential of compound 4c with three reversible reduction points which is potentially shifted to more negative values due to the saturation of one of the double bond on the fullerene C₆₀ cage. This should raise the LUMO energy of the ensuring fullerene derivative.²¹ Thus, the reduction peaks of 4c were observed at −0.923, −1.132, −1.951 V, more negative values than the [60] fullerene −0.60, −1.00, −1.52, −2.04 V reported by Nazario Martin et al.²² Reduction potential of fulleropyrrolidine is slightly higher than C₆₀, suggests that the parent C₆₀ has more tendency to accept electrons.

Fig. 1 Cyclic voltammogram of 4c in CHCl₃, 0.5 M TBAP, scan rate 100 mV s⁻¹.

2.2. Photophysical studies

In UV absorption spectra the sharp absorption bands appeared at 285 nm and 325 nm are (Fig. 2) characteristic signs of fullerene C₆₀ mono adduct.²³ The new shoulder observed at 432 nm corresponds to the fulleropyrrolidine. In addition a weak characteristic absorption band observed at 703 nm, indicates the S₁ → S₀ transition²⁴ of C₆₀ moiety. When compare the 4c spectrum with the parent C₆₀, the peaks observed at 284, 336, 405 nm of C₆₀ were totally dissimilar except the peak at 284 nm. The peak observed at 284 nm is the characteristic peak of [60] fullerene.

Fig. 2 UV-vis absorption spectra of 4c (a) and C₆₀ (b) in toluene at 1 × 10⁻⁶ mol L⁻¹.

The fluorescence spectra of 4c and the reference compound C₆₀ were measured with 365 nm excitation in toluene. The fulleropyrrolidine exhibits greater fluorescence intensity than parent C₆₀. On comparison, 4c has more than two fold increase in emission than that of C₆₀ under the same concentration. This shows the lowering of the symmetry of C₆₀ by derivatization which considerably increase the fluorescence intensity of the fullerene compound.^24,25 Further, the characteristic high emission in fluorescence spectra is due to the addition of the alkyl chain in the C₆₀ core (Fig. 3). A sharp peak appeared at 732 nm is due to interaction between alkyl group singlet excited state and fullerene ground state.²⁶ Corroborate this peak also assigned as a (6,6) – region addition on the fullerene. Compound 4c shows high emission in fluorescence spectra compare with other alkylated fulleropyrrolidines. Because the long alkyl chain substitution rationally may bring out more emission out off others. The chain length had an important impact on the fluorescence spectra of the compounds. Good efficiency and high electron rich properties make these compounds suitable for OLED devices. Due to light emitting and charge transporting capacity of this kind of alkylated π-compounds have encountered increasing attention in recent years for application in flexible organic electronics.^27,28

Fig. 3 Fluorescence spectra of 4c (a) and C₆₀ (b) in 1 × 10⁻⁶ mol L⁻¹ in toluene.

The resulted spectroscopic techniques, anticipate that the appropriate hybridization design of alkyl chains attached with fullerene moiety can direct the self-organized supramolecular structure as a fundamental subunit. In addition, generally the fullerene derivatives are known to show poor electron acceptor properties than the parent C₆₀, which occurs as a result of the saturation of double bond in the C₆₀ framework which in turn raised the LUMO energy level.²¹ Murakami et al. reported the different ratios of both C₆₀ and alkyl tail groups which results in a different aggregation naturally as considered by the self-assembly theory for typical amphiphilic surfactants in aqueous media.²⁹ Therefore, these design and development in alkyl group concepts are anticipated to produce novel alkylated compounds with coveted new applications in optoelectronic devices such as OLED.

3. Experimental

3.1. General

Buckminsterfullerene, C₆₀ (+99.95%), 2,4-dihydroxybenzaldehyde, 3,4-dihydroxybenzaldehyde, 1-bromohexadecane, 1-bromodecane, 1-bromooctadecane, K₂CO₃, sarcosine were purchased from Aldrich and used without further purification. N,N′-Dimethylformamide, toluene, chloroform, dichloromethane, hexane were used as reagent grade without further purification. Thin layer chromatography was carried on Merck 60F₂₅₄ silica gel plate and column chromatography was performed with Merck silica gel (100–200 mesh).

The ¹H and ¹³C spectra were recorded on BrukerAvance 400 MHz spectrometer in CDCl₃ with TMS as an internal standard. IR spectra were recorded on Nicolet Avatar 300 FT-IR spectrophotometer. MALDI-TOF MS were processed on an applied Biosystems voyager DE-PRO spectrometer. Which is equipped with a nitrogen laser (λ = 337 nm) operated in positive ion, linear mode. DCTB (3-methyl-4-(4-tert-butylphenyl)butadiene-1,1-dinitrile) matrix solution (10 mg mL⁻¹ DCTB in 1 mL of toluene) used as a matrix. A cyclic voltammetry experiment was performed by using three electrode cell units, consisting of polished 2 mm a glassy carbon as working electrode, a platinum wire as counter electrode and Ag/AgCl reference electrode. Tetrabutylammonium perchlorate (TBAP) was added as a supporting electrolyte. Cyclic Voltammetry were performed with CHI604C instrument, under the scan rate of 0.5 mV s⁻¹ and the current sensitivity given was 100 mV s⁻¹. The experiment was done at room temperature in a dry and inert atmosphere, accomplished by passing nitrogen gas for about 10 min before starting the experiment. The UV-visible spectral measurements were carried out with a Shimadzu Model UV-1650 UV-visible spectrophotometer. The fluorescence emission spectra were monitored by using a Shimadzu RF-5310 PC spectrofluorimeter with the excitation slit width of 5.0 nm. Both UV-visible and fluorescence measurements were carried out in 10 mm quartz cells.

3.2. Synthesis of functionalized benzaldehydes

Suitable benzaldehyde 1 (10 mmol) and alkyl halide 2 (20 mmol) were dissolved in 40 mL of N,N′-dimethylformamide and K₂CO₃ (5.52 g, 40 mmol) was added. The solution was stirred at room temperature for 24 h, and then the reaction mixture was poured into a mixture of water and dichloromethane. The organic layer was separated and dried with MgSO₄. The solvent was removed under reduced pressure and the resulting crude product was purified on a column chromatography using dichloromethane and hexane (8 : 2 v/v) as eluent.

3.2.1. 2,4-Bis(n-hexadecyloxy)benzaldehyde, 3a. FT-IR (KBr, cm⁻¹) ν 2920, 2850, 1677, 1606, 1467, 1387, 1298, 1267, 1192, 1113, 1019, 860, 719; ¹H NMR (CDCl₃, 400 MHz δ ppm) 0.65 (bs, 6H), 1.03–1.22 (m, 60H), 1.60 (m, 4H), 3.79 (bs, 4H), 6.19 (s, 1H), 6.28 (d, 1H, J = 8.2 Hz), 7.56 (d, 1H, J = 8.4 Hz), 10.10 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz δ ppm) 14.12, 22.70, 25.97, 26.06, 28.19, 28.78, 29.03, 29.10, 29.37, 29.45, 29.56, 29.60, 29.67, 29.70, 31.93, 32.86, 34.05, 68.42, 68.48, 98.95, 106.18, 118.92, 130.19, 163.38, 165.79, 188.44.

3.2.2. 3,4-Bis(n-decyloxy)benzaldehyde, 3b. FT-IR (KBr, cm⁻¹) ν 2921, 2851, 1685, 1591, 1465, 1439, 1394, 1275, 1236, 1134, 1066, 1019, 807, 724; ¹H NMR (CDCl₃, 400 MHz δ ppm) 0.88 (t, 6H), 1.27–1.47 (m, 36H), 1.84 (m, 4H), 4.06 (m, 4H), 6.95 (d, 1H, J = 8.0 Hz), 7.41 (d, 2H, J = 12.4 Hz), 9.82 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz δ ppm) 14.14, 22.71, 25.97, 26.01, 28.21, 28.80, 29.00, 29.09, 29.31, 29.38, 29.41, 29.47, 29.53, 29.60, 29.62, 29.64, 31.91, 31.94, 32.87, 34.07, 69.13, 110.91, 111.74, 126.64, 129.87, 149.44, 154.69, 191.04.

3.2.3. 3,4-Bis(n-octadecyloxy)benzaldehyde, 3c. FT-IR (KBr, cm⁻¹) ν 2919, 2850, 1685, 1591, 1457, 1439, 1278, 1237, 1134, 1017, 808, 722; ¹H NMR (CDCl₃, 400 MHz δ ppm) 0.87 (d, 6H), 1.25–1.47 (m, 56H), 1.84 (d, 4H, J = 6.4 Hz), 4.06 (d, 4H, J = 7.6 Hz), 6.95 (d, 1H, J = 8.0 Hz), 7.40 (d, 2H, J = 12.4 Hz), 9.82 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz δ ppm) 14.16, 22.73, 25.98, 26.02, 29.01, 29.09, 29.41, 29.66, 29.71, 29.76, 31.97, 69.14, 110.89, 111.72, 126.66, 129.86, 149.44, 154.69, 191.07.

3.2.4. 2,3,4-Tris(n-heptyloxy)benzaldehyde, 3d. FT-IR (KBr, cm⁻¹) ν 2927, 2858, 1681, 1587, 1456, 1375, 1292, 1182, 1089, 800; ¹H NMR (CDCl₃, 400 MHz δ ppm) 0.80 (s, 9H), 1.65–1.27 (m, 20H), 1.71 (m, 6H), 3.88 (t, 2H), 3.94 (t, 2H), 4.08 (t, 2H), 6.62 (d, 1H, J = 8.8 Hz), 7.47 (d, 1H, J = 8.8 Hz), 10.16 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz δ ppm) 14.04, 18.35, 22.45, 22.58, 22.63, 25.96, 26.00, 26.07, 27.82, 28.11, 28.42, 28.59, 29.01, 29.10, 29.17, 29.19, 29.35, 29.68, 30.15, 30.29, 30.93, 31.62, 31.76, 31.78, 31.87, 32.82, 33.74, 58.16, 68.85, 73.66, 75.23, 76.83, 108.01, 123.43, 123.67, 141.00, 156.63, 157.01, 159.10, 188.89.

3.2.5. 4-Decyloxybenzaldehyde, 3e. FT-IR (KBr, cm⁻¹) ν 2926, 2855, 1694, 1602, 1465, 1391, 1310, 1257, 1160, 1106, 1020, 832, 723; ¹H NMR (CDCl₃, 400 MHz δ ppm) 0.88 (t, 3H), 1.27–1.48 (m, 16H), 1.80 (m, 2H), 4.03 (t, 2H), 6.98 (d, 2H, J = 8.8 Hz), 7.81 (d, 2H, J = 8.8 Hz), 9.86 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz δ ppm) 14.13, 21.02, 22.70, 22.87, 25.83, 25.93, 25.98, 28.20, 28.52, 28.62, 28.80, 29.07, 29.21, 29.28, 29.34, 29.36, 29.57, 31.91, 32.86, 53.47, 64.13, 64.68, 68.44, 114.75, 128.34, 129.74, 131.99, 161.23, 164.29, 171.26, 190.82.

3.2.6. 4-Octadecyloxybenzaldehyde, 3f. FT-IR (KBr, cm⁻¹) ν 2922, 2852, 1686, 1602, 1468, 1309, 1254, 1157, 1015, 833, 722; ¹H NMR (CDCl₃, 400 MHz δ ppm) 0.87 (t, 3H), 1.25–1.48 (m, 32H), 1.80 (m, 2H), 4.03 (t, 2H), 6.98 (d, 2H, J = 8.4 Hz), 7.82 (d, 2H, J = 8.4 Hz), 9.87 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz δ ppm) 14.16, 22.74, 26.00, 29.09, 29.39, 29.41, 29.59, 29.63, 29.71, 29.75, 31.97, 68.44, 114.75, 129.74, 132.01, 164.30, 190.84.

3.2.7. 3-Methoxy-4-octadecyloxybenzaldehyde, 3g. FT-IR (KBr, cm⁻¹) ν 2912, 2848, 1676, 1636, 1589, 1464, 1402, 1264, 1136, 1026, 863, 724, 653; ¹H NMR (CDCl₃, 400 MHz δ ppm) 0.87 (t, 3H), 1.25–1.46 (m, 30H), 1.88 (m, 2H), 3.93 (s, 3H), 4.09 (t, 2H), 6.96 (d, 1H, J = 8.0 Hz), 7.42 (t, 2H), 9.84 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz δ ppm) 14.14, 22.72, 25.91, 28.93, 29.38, 29.55, 29.60, 29.72, 31.95, 56.06, 69.22, 109.24, 111.36, 126.86, 129.86, 149.86, 154.23, 190.96.

3.3. Synthesis of fulleropyrrolidine 4

A mixture of 3a (1 mmol), C₆₀ (0.50 mmol) and 66 mg of sarcosine (0.75 mmol) in 40 mL of toluene was refluxed till its colour turned from purple into reddish brown. The solvent was removed under reduced pressure and the reaction mixture was directly purified on a column chromatography using toluene/hexane 2 : 1 as an eluent.

3.3.1. N-Methyl-2-(2,4-bis(n-hexadecyloxy)phenyl)fulleropyrrolidine, 4a. FT-IR (KBr, cm⁻¹) ν 2921, 2849, 1607, 1460, 1375, 1265, 1176, 1111, 1026, 827, 718, 525; ¹H NMR (CDCl₃, 400 MHz δ ppm) 0.87 (t, 6H), 1.24–1.43 (bs, 56H), 1.75 (m, 4H), 2.79 (s, 3H), 3.95 (m, 4H), 4.27 (d, 1H, J = 9.2 Hz), 4.95 (d, 1H, J = 9.2 Hz), 5.46 (s, 1H), 6.46 (s, 1H), 6.58 (d, 1H, J = 8.4 Hz), 7.82 (d, 1H, J = 8.8 Hz); ¹³C NMR (CDCl₃, 100 MHz δ ppm) 14.16, 22.72, 29.39, 29.73, 31.96, 39.94, 68.44, 68.51, 74.28, 82.78, 130.28, 135.52, 135.52, 137.92, 141.17, 143.12, 144.02, 144.14, 144.18, 144.29, 144.34, 144.71, 144.99, 145.19, 145.30, 145.32, 145.40, 145.73, 145.78, 146.54, 146.78, 148.06, 148.32, 149.37, 149.90, 155.04.

3.3.2. N-Methyl-2-(3,4-bis(n-decyloxy)phenyl)fulleropyrrolidine, 4b. FT-IR (KBr, cm⁻¹) ν 2922, 2852, 1627, 1462, 1263, 1128, 1021, 871, 520; ¹H NMR (CDCl₃, 400 MHz δ ppm) 0.87 (bs, 6H), 1.25–1.42 (m, 34H), 1.77 (m, 4H), 2.81 (s, 3H), 3.98 (bs, 4H), 4.23 (d, 1H, J = 10.8 Hz), 4.85 (s, 1H), 4.97 (d, 1H, J = 9.2 Hz), 6.94 (d, 2H), 7.39 (s, 1H), 7.62 (d, 1H); ¹³C NMR (CDCl₃, 100 MHz δ ppm) 14.15, 22.72, 23.77, 26.08, 29.16, 29.34, 29.40, 29.50, 29.65, 29.73, 31.96, 40.07, 68.99, 69.06, 69.38, 69.99, 70.01, 83.46, 129.26, 135.79, 139.69, 141.59, 141.71, 142.06, 142.27, 142.59, 142.71, 143.17, 144.42, 144.69, 145.26, 145.57, 145.81, 145.97, 146.18, 146.34, 146.54, 146.97, 147.32.

3.3.3. N-Methyl-2-(3,4-bis(n-octadecyloxy)phenyl)fulleropyrrolidine, 4c. FT-IR (KBr, cm⁻¹) ν 2921, 2852, 1736, 1630, 1463, 1282, 1101, 1022, 876, 723, 613; ¹H NMR (CDCl₃, 400 MHz δ ppm)0.87 (t, 6H), 1.25 (bs, 64H), 1.79 (m, 4H), 2.81 (s, 3H), 3.97 (t, 2H), 4.06 (t, 2H), 4.24 (d, 1H, J = 9.6 Hz), 4.84 (s, 1H), 4.97 (d, 1H, J = 9.2 Hz), 6.88 (bd, 1H, J = 7.6 Hz), 6.94 (d, 1H, J = 8.0 Hz), 7.39 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz δ ppm) 14.17, 22.73, 26.05, 29.08, 29.14, 29.19, 29.33, 29.41, 29.42, 29.51, 29.68, 29.75, 29.78, 31.97, 40.07, 69.00, 69.06, 69.36, 70.01, 76.73, 83.46, 129.25, 135.78, 136.49, 140.13, 141.71, 141.85, 142.06, 142.09, 142.14, 142.16, 142.19, 142.27, 142.58, 142.60, 142.62, 142.71, 143.00, 143.17, 144.42, 144.68, 144.72, 145.18, 145.27, 145.30, 145.32, 145.37, 145.49, 145.53, 145.57, 145.81, 145.95, 145.98, 146.12, 146.16, 146.18, 146.24, 146.28, 146.34, 146.53, 146.96, 147.32, 153.68, 154.18, 156.31; MALDI-TOF mass calculated 1390.7 found 1389.2.

3.3.4. N-Methyl-2-(2,3,4-tris(n-heptyloxy)phenyl)fulleropyrrolidine, 4d. FT-IR (KBr, cm⁻¹) ν 2922, 2852, 1593, 1456, 1382, 1292, 1219, 1176, 1089, 1026, 840, 771, 669, 526; ¹H NMR (CDCl₃, 400 MHz δ ppm)0.84 (t, 9H), 1.22–1.32 (m, 30H), 1.77 (m, 6H), 2.78 (s, 3H), 3.87 (t, 2H), 3.96 (m, 2H), 4.10 (bs, 1H), 4.25 (d, 1H, J = 9.6 Hz), 4.96 (d, 1H, J = 9.6 Hz), 5.38 (s, 1H), 6.74 (d, 1H, J = 8.8 Hz), 7.59 (d, 1H, J = 8.8 Hz); ¹³C NMR (CDCl₃, 100 MHz δ ppm) 14.15, 14.25, 22.64, 22.68, 22.77, 26.05, 26.13, 26.31, 29.14, 29.23, 29.30, 29.45, 29.73, 30.02, 30.55, 31.83, 31.91, 32.05, 40.16, 68.62, 69.19, 69.97, 73.31, 73.83, 76.38, 76.72, 108.41, 122.44, 124.19, 136.62, 139.46, 140.09, 141.31, 141.65, 141.88, 142.22, 142.31, 142.65, 143.12, 144.47, 144.62, 145.26, 145.33, 145.57, 145.96, 146.09, 146.27, 147.31, 152.65, 153.10, 154.10; MALDI-TOF mass calculated 1196.4 found 1195.4.

3.3.5. N-Methyl-2-((4-decyloxy)phenyl)fulleropyrrolidine, 4e. FT-IR (KBr, cm⁻¹) ν 2918, 2850, 1607, 1460, 1370, 1242, 1169, 1112, 1029, 823, 722, 550; ¹H NMR (CDCl₃, 400 MHz δ ppm)0.87 (t, 3H), 1.25–1.44 (m, 18H), 1.76 (t, 2H), 2.79 (s, 3H), 3.95 (t, 2H), 4.23 (d, 1H, J = 9.2 Hz), 4.87 (s, 1H), 4.97 (d, 1H, J = 9.2 Hz), 6.94 (d, 2H, J = 8.0 Hz), 7.69 (bs, 2H); ¹³C NMR (CDCl₃, 100 MHz δ ppm) 14.17, 22.74, 26.11, 28.99, 29.20, 29.36, 29.41, 29.47, 29.61, 29.74, 31.93, 31.97, 33.87, 40.03, 68.01, 69.00, 70.01, 83.25, 114.11, 114.53, 128.68, 130.49, 135.80, 136.59, 136.80, 139.33, 139.61, 139.92, 140.14, 140.18, 141.56, 141.71, 141.86, 142.04, 142.14, 142.31, 142.57, 142.70, 143.00, 143.17, 144.42, 144.65, 144.73, 145.17, 145.26, 145.36, 145.58, 145.82, 145.97, 146.18, 146.34, 146.43, 146.56, 146.86, 147.33, 153.71, 154.17, 156.42, 159.22.

3.3.6. N-Methyl-2-((4-octadecyloxy)phenyl)fulleropyrrolidine, 4f. FT-IR (KBr, cm⁻¹) ν 2918, 2850, 1607, 1460, 1370, 1242, 1169, 1112, 1029, 832, 723, 544, 526; ¹H NMR (CDCl₃, 400 MHz δ ppm)0.87 (d, 3H), 1.25 (m, 32H), 2.02 (t, 2H), 2.79 (s, 3H), 3.94 (t, 2H), 4.23 (d, 1H, J = 9.2 Hz), 4.87 (s, 1H), 4.95 (d, 1H, J = 9.2 Hz), 6.94 (d, 2H, J = 7.6 Hz), 7.70 (bs, 2H); ¹³C NMR (CDCl₃, 100 MHz δ ppm) 14.15, 22.72, 26.10, 29.19, 29.39, 29.73, 31.95, 33.85, 40.04, 68.95, 68.98, 69.01, 83.25, 128.69, 128.88, 130.00, 134.46, 136.82, 136.97, 138.97, 139.32, 140.17, 140.18, 142.05, 142.12, 142.14, 143.00, 143.16, 143.44, 144.42, 145.26, 145.57, 146.16, 146.34, 146.55, 147.60, 147.62, 147.76, 148.28, 148.89, 150.13, 152.42, 153.02.

3.3.7. N-Methyl-2-((3-methoxy-4-octadecyloxy)phenyl)fulleropyrrolidine, 4g. FT-IR (KBr, cm⁻¹) ν 2922, 2852, 1629, 1589, 1458, 1382, 1265, 1128, 1028, 767, 669, 526; ¹H NMR (CDCl₃, 400 MHz δ ppm) 0.86 (d, 3H), 1.25–1.42 (m, 32H), 1.85 (m, 2H), 2.82 (s, 3H), 3.88 (s, 3H), 3.99 (t, 2H), 4.25 (d, 1H, J = 9.2 Hz), 4.86 (s, 1H), 4.98 (d, 1H, J = 9.6 Hz), 6.89 (d, 1H, J = 7.6 Hz), 7.35 (s, 1H), 7.53 (d, 1H, J = 8.8 Hz); ¹³C NMR (CDCl₃, 100 MHz δ ppm) 14.14, 22.71, 26.71, 26.00, 29.23, 29.38, 29.46, 29.58, 29.63, 29.72, 30.07, 30.22, 31.46, 31.95, 40.07, 56.27, 68.97, 70.03, 83.49, 135.77, 136.47, 139.83, 140.20, 141.85, 142.06, 142.27, 142.61, 143.01, 143.20, 144.42, 144.72, 145.30, 145.57, 145.81, 145.96, 146.17, 146.29, 146.51, 147.33, 153.65, 154.17; MALDI-TOF mass calculated 1152.3 found 1151.

3.3.8. N-Methyl-2-((2,4,6-trimethyl)phenyl)fulleropyrrolidine, 4h. FT-IR (KBr, cm⁻¹) ν 2921, 2852, 1742, 1611, 1457, 1374, 1172, 1123, 1023, 852, 729, 526; ¹H NMR (CDCl₃, 400 MHz δ ppm)2.25 (s, 3H), 2.59 (s, 3H), 2.73 (s, 3H), 3.06 (s, 3H), 4.14 (d, 1H, J = 9.6 Hz), 5.00 (d, 1H, J = 8.4 Hz), 5.48 (s, 1H), 6.89 (d, 2H, J = 6.8 Hz); ¹³C NMR (CDCl₃, 100 MHz δ ppm) 14.15, 20.87, 22.72, 22.79, 22.87, 28.99, 29.39, 29.73, 31.96, 33.85, 40.11, 69.69, 69.98, 70.22, 80.67, 130.14, 132.52, 138.36, 141.17, 141.53, 142.09, 142.18, 142.60, 143.13, 145.29, 145.32, 145.62, 145.82, 145.97, 146.06, 146.18, 146.27, 146.96, 147.29, 151.54, 153.57, 154.70, 157.29; MALDI-TOF mass calculated 895.9 found 895.9.

3.3.9. N-Methyl-2-((4-tert-butyl)phenyl)fulleropyrrolidine, 4i. FT-IR (KBr, cm⁻¹) ν 2918, 2848, 1737, 1674, 1591, 1462, 1373, 1288, 1176, 1093, 1024, 837, 721, 526; ¹H NMR (CDCl₃, 400 MHz δ ppm)1.25–1.30 (m, 9H), 2.80 (s, 3H), 4.25 (d, 1H, J = 9.2 Hz), 4.91 (s, 1H), 4.98 (d, 1H, J = 9.6 Hz), 7.42 (d, 2H, J = 7.6 Hz), 7.70 (s, 2H); ¹³C NMR (CDCl₃, 100 MHz δ ppm) 14.16, 22.73, 29.19, 29.40, 29.73, 31.37, 31.96, 34.66, 40.15, 69.11, 70.10, 83.42, 125.52, 128.76, 129.00, 133.76, 135.77, 136.76, 140.18, 141.53, 141.70, 141.86, 142.01, 142.07, 142.12, 142.29, 142.58, 142.69, 144.42, 144.62, 144.73, 145.26, 145.47, 145.56, 145.96, 146.14, 146.24, 146.33, 146.43, 146.54, 146.95, 147.32, 151.41, 153.69, 154.23.

Acknowledgements

One of the author (A. R. Parveen) is grateful to UGC New Delhi for the research fellowship.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra15057k

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