Introduction

Esters are ubiquitous and an important class of organic compounds, which have been widely applied in the chemical and pharmaceutical industries, perfumery and cosmetics, and serve as precursors for many other types of organic molecules. Esters can undergo various functional group interconversions. It is also well known that the most common α-C−H functionalizations of esters are realized through enolate formation with a base followed by alkylation, acylation, or addition1. In addition, the α-arylation of esters can be achieved by the reactions of enolate intermediates with aryl halides under the catalysis of transition metals2,3,4. However, the α-C−H functionalizations of esters via a radical process are rarely investigated. The in situ formed enolates transfer a single electron to oxidants such as ferrocenium and copper salts to generate persistent radical intermediates, which can undergo further reactions5,6.

On the other hand, various functionalized derivatives of buckyballs have attracted extensive attention due to their excellent performances in materials and biological science7,8,9,10,11. [6,6]-Phenyl-C61-butyric acid methyl ester (PCBM, 1), a star molecule in fullerene chemistry and materials science, has been widely used in solar cell devices12,13,14,15,16 due to its unique advantages in excellent solubility and electronic transmission capability. Derivatives of PCBM have also been synthesized and utilized in organic photovoltaics (OPVs)12,17,18 and perovskite solar cells (PSCs)13,14,15,16,18. For example, bis-PCBM and PCBM(CH2) were applied in organic solar cells with P3HT as the electron donor. As desired, both P3HT:bis-PCBM and P3HT:PCBM(CH2) cells showed higher open circuit voltage (Voc) and power conversion efficiency (PCE) than the P3HT:PCBM cell (0.76 V and 0.73 V vs 0.61 V; 4.4% and 4.6% vs 4.0%, respectively)19. The methano CH2 moiety in PCBM(CH2) was introduced first through the reaction of [60]fullerene (C60) with iPrO-Me2SiCH2MgCl followed by Cu(II)-promoted cyclization to provide C60(CH2). Then, the reaction of C60(CH2) with the diazo compound generated from p-tosylhydrazide and NaOMe afforded PCBM(CH2) as a mixture of regioisomers in an overall yield of 15%20. Isomer effects on the device performance of OPVs and PSCs have been demonstrated18. Taking the previous complex synthetic steps for an isomeric mixture of PCBM(CH2)20 into consideration and in continuation of our interest in the electrosynthesis of fullerene derivatives21,22,23,24,25,26, we anticipated that the straightforward cyclopropanation of the electrochemically generated dianion of PCBM with CH2I2 via dual nucleophilic reactions would regioselectively afford a single isomer of PCBM(CH2). Surprisingly, CH2I2 was not involved, and two classes of buckyballs with fused bi- and tricyclic carbocycles via intramolecular radical α-C−H functionalization of the ester moiety were isolated for both PCBM and its analogue, i.e., [6,6]-phenyl-C61-propionic acid methyl ester (PCPM, 2)27. The formed buckyballs fused with a [3-6]/[3-5] bicycle or a [3-6-3]/[3-5-3] tricycle (vide infra) have different fused-ring systems compared to the previously reported buckyballs with a [5-6]/[5-5] bicycle28,29 or a [3-7-3]/[5-5-5] tricycle30,31. Furthermore, an unknown type of bisfulleroid with two tethered [6,6]-open orifices could be obtained for PCPM. Herein, we describe the details of these intramolecular annulations of the electrochemically generated dianions of PCBM and PCPM in the presence of a trace amount of oxygen, and preliminarily investigate the application of a representative derivative in inverted planar PSC devices.

Results

The reaction of 12–, which was generated by controlled potential electrolysis (CPE) of 1 at −1.38 V vs. saturated calomel electrode (SCE) for ca. 1 h, with CH2I2 for 5 h failed to afford the desired PCBM(CH2) and instead provided a buckyball with fused bicyclic carbocycle 3 as the major product and its regioisomer 4 as the minor product under an argon atmosphere. Products 3 and 4 were intramolecularly annulated isomers of PCBM. It turned out that the trace amount of O2 (15.3 ppm) in Ar was crucial for the formation of 3 and 4. Products 3 and 4 were formed in 25% and 6% yields, respectively, along with another buckyball with fused tricyclic carbocycle 5 as a minor product in 5% yield after stirring for 5 h at a flow rate of 2.6 mL s–1 for Ar/O2 gas. Notably, the electrochemically intramolecular radical α-C−H annulation of the ester moiety has been achieved. A fullerenyl proton existed in product 3 and was believed to come from protonation of an annulated anionic intermediate. Therefore, trifluoroacetic acid (TFA) was added to achieve higher protonation efficiency. Satisfactorily, the yields of 3 and 4 were improved to 39% and 12%, respectively, after the generated 12– was stirred for another 5 h under an Ar/O2 atmosphere, followed by treatment with 1 equiv. of TFA for 10 min (Fig. 1a). Simultaneously, tricyclic carbocycle 5 could be obtained as a minor product in 6% yield. Further study showed that similar results were obtained without acid treatment, probably because the protonation process occurred during column chromatography on silica gel32. Products 3, 4, and 5 could be obtained in lower yields of 31%, 6%, and 4%, respectively, when 1 equiv. of TFA was immediately added after the theoretical number of coulombs required for full conversion of 1 into 12– was reached, indicating that the intermediates for 3, 4, and 5 were formed during electrolysis and continued to be generated upon stirring for another 5 h.

Fig. 1: Electrosynthesis of buckyballs.
figure 1

a Electrosynthesis of buckyballs with fused bi- and tricycles 3, 4, and 5 from PCBM. b Electrosynthesis of buckyballs with fused bi- and tricycles 6 and 7 from PCPM. c Electrosynthesis of buckyball with fused bicycle 6 and bisfulleroid 8 from PCPM. d Electrochemical conversion of 7 to 8. All the redox potentials mentioned are relative to saturated calomel electrode. PCBM: 1, [6,6]-phenyl-C61-butyric acid methyl ester. PCPM: 2, [6,6]-phenyl-C61-propionic acid methyl ester. TFA: trifluoroacetic acid. The red color indicates the characteristic addition site for the fused ring system or the 14-membered-ring opening.

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Subsequently, the corresponding electrochemical reaction of PCPM (2), an analogue of PCBM, was also explored. Detailed investigations demonstrated that buckyballs with fused bi- and tricycles 6 and 7 could be formed in 11% and 14% yields, respectively, after the generated 22– was stirred for another 3 h without acid treatment (Fig. 1b). Compared to those of 3, 4, and 5, the fused bi- and tricycles of 6 and 7 shrank from the [3-6] and [3-6-3] patterns to the [3-5] and [3-5-3] patterns, respectively. Interestingly, product 6 could be selectively isolated in a higher yield of 32% when the generated 22– was immediately treated with 1 equiv. of TFA for 10 min (Fig. 1b). These results indicated that the electrochemical reaction of 22– under the Ar/O2 atmosphere initially afforded an intermediate for 6, which was then further transformed into a species for 7 upon prolonging the reaction time.

Remarkably, bisfulleroid 8 (vide infra) could be produced in 14% yield together with 6 in 21% yield when the generated 22– was stirred for another 3 h under an Ar/O2 atmosphere and subsequently treated with 1 equiv. of TFA for 10 min (Fig. 1c). Intriguingly, bisfulleroid 8 has two tethered orifices at [6,6]-bonds with a 14-membered-ring opening instead of the previous common bisfulleroids at [5,6]-bonds30,33,34,35, exhibiting a unique addition pattern for open-cage fullerene derivatives. Further study showed that bisfulleroid 8 could be more selectively and efficiently formed in 43% yield if anionic species 22.5– was generated by CPE with the acceptance of 2.5 electrons per molecule and stirred for another 5 h under an Ar/O2 atmosphere, followed by treatment with 5 equiv. of TFA for 10 min (Fig. 1c). The above results hinted that prolonging the reaction time gradually led to the formation of tricycle 7 and then bisfulleroid 8. Further experiments demonstrated that 7 could undergo highly efficient electrochemical conversion to 8 in 72% yield, suggesting that 7 was the precursor of 8 (Fig. 1d).

It was found that the amount of O2 in the system, which was measured by a trace oxygen analyser, played a crucial role in the present electrochemical reactions. Taking the electrosynthesis of bisfulleroid 8 from PCPM as an example, the product yield was lowered from 43% to 39% and 30% when the O2 content in the argon gas was changed from 15.3 ppm to 3.8 ppm and 27.0 ppm, respectively, while maintaining a flow rate at 2.6 mL s–1. The product yield dropped to 32% when the flow rate was reduced from 2.6 mL s–1 to 1.3 mL s–1, and the O2 content of 15.3 ppm in the Ar gas remained unchanged. Thus, a smaller amount of O2 in Ar (3.8 ppm) and a lower gas flow rate (1.3 mL s–1) resulted in lower product yields, while a larger amount of O2 in Ar (27.0 ppm) also led to reduced product yield with more unidentified residues. These results suggested that an appropriate amount of O2 was very important to control the electrochemical reactions and achieve high yields of the desired products. It should be noted that all efforts failed for the electrosynthesis of the analogous bisfulleroid from PCBM. The exact reason for this phenomenon is currently unclear.

Products 3–8 were unambiguously characterized by MALDI-TOF MS, 1H NMR, 13C NMR, FT-IR and UV/Vis spectra. The structural assignments were further confirmed by the single-crystal X-ray structures of 3, 4, 7, and 8. Single crystals of 3 were obtained through slow evaporation of a saturated solution of 3 in carbon disulfide (CS2) at ambient temperature. As shown in Fig. 2a, the structure of 3 unambiguously reveals that a [3,6]-fused bicycle is joined at the C1, C2, and C3 atoms and that a hydrogen is appended to the C4 atom. The α-carbon of the side-chain ester is bonded to the C3 atom. The bond lengths of C1–C2, C2–C3, and C3–C4 are 1.590, 1.554, and 1.587 Å, respectively, and are within the range of typical functionalized C–C bond lengths of the C60 cage. Interestingly, the two methylenes in the fused 6-membered ring are twisted. Luckily, as the stereoisomer of 3, single crystals of 4 showing an opposite orientation of the ester moiety (Fig. 2b) were obtained through slow evaporation of its solution in a mixture of toluene and decapyrrylcorannulene (DPC)36 at room temperature. Single crystals of 7 were obtained by slow evaporation of a mixture of a saturated solution containing product 7 in CS2 and a saturated solution of DPC in toluene at about 0 °C. As seen in Fig. 2c, a [3,5,3]-fused tricycle is joined at the C1, C2, C3, and C4 atoms. In addition, the α-carbon of the ester moiety is connected to both C3 and C4 atoms. The bond lengths for C1–C2, C2–C3, and C3–C4 are 1.602, 1.502, and 1.573 Å, respectively. Black block crystals of 8 were obtained via slow diffusion of hexane into a CS2 solution of 8. Figure 2d displays the single-crystal structure of 8. This clearly shows that the α-carbon of the ester group is anchored to the C3–C4 bond, which is broken after addition. The nonbonded C3···C4 distance is 2.452 Å. The bond length of C2–C3 is 1.355 Å, hinting that it is a C=C double bond. The C1–C2 bond is also ruptured, and the C1···C2 separation is 2.452 Å. These two [6,6]-open fulleroid motifs of 8 make its largest orifice a 14-membered ring (Supplementary Fig. 60). Both C1 and C4 bearing hydrogen are notably uplifted from the spherical surface because of their sp3 characteristics.

Fig. 2: Oak Ridge thermal ellipsoid plot (ORTEP) diagrams and proposed reaction mechanisms.
figure 2

a–d ORTEP diagrams of 3, 4, 7, and 8 with 20%, 20%, 10%, and 20% thermal ellipsoids, respectively. The solvent and decapyrrylcorannulene molecules are omitted for clarity. e–g Proposed reaction mechanisms for the three types of fullerene products.

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On the basis of the above experimental results, plausible reaction mechanisms are proposed and shown in Fig. 2e–g. 1/2 is electroreduced by CPE to form dianion I (12–/22–), which undergoes single electron transfer (SET) to O2 to produce radical anion II and O2•−. Hydrogen abstraction of the sp3 α-C−H adjacent to the ester group by O2•− generates radical species III and HO2−. Subsequent intramolecular radical coupling gives fullerenyl anion IV with a fused bicyclic carbocycle. Final protonation affords the buckyball with fused bicycle 3(4)/6 (Fig. 2e). Upon prolonging the reaction time, another hydrogen abstraction of the sp3 α-C−H adjacent to the ester group in IV by O2•− generates intermediate V, which undergoes intramolecular cyclization to result in radical anion VI. Finally, oxidation of VI by O2 provides a buckyball with fused tricyclic carbocycle 5/7 (Fig. 2f). Alternatively, hydrogen abstraction of the sp3 α-C−H and subsequent oxidation of the fullerenyl carboanion would generate a diradical species, which undergoes intramolecular radical coupling to give 5/7. As shown in Fig. 1a, 3, 4, and 5 were obtained 31%, 6%, and 4% yields, respectively, when the reaction mixture was immediately treated with TFA after the theoretical number of coulombs required for full conversion of 1 into its dianion was reached, suggesting that intermediates IV, V, and VI should be produced from 12– triggered by a trace amount of oxygen during electrolysis. This deduction can logically explain the failure for the attempted synthesis of PCBM(CH2) from 12– due to its preferential conversion into the abovementioned intermediates during electrolysis. For the formation of 8, intermediate 7•− (VI when n = 1) can be reduced to dianion VII, most likely by 22−. The fact that the second redox potential of 2 is much more negative than the first redox potential of 7 (Supplementary Figs. 2 and 11) supports the proposed SET process. Dual protonations of VII via monoanion VIII provide bisfulleroid 8 (Fig. 2g).

To better understand the observed stereochemistries and regioselectivities, computational studies with PCPM and PCBM as the starting materials by density functional theory (DFT) calculations at the B3LYP/6-31 G(d) level were performed. The partial natural bond orbital (NBO) charge distributions of fullerenyl anion IV showed that C4 (–0.128 for n = 1; –0.127 for n = 2) was the most negatively charged carbon atom among the nonfunctionalized C60 carbon atoms and was thus more prone to protonation, followed by C16 (–0.086 for n = 1; –0.087 for n = 2) as the second most negative site (Supplementary Fig. 61). Product 3 was slightly more stable than 4 by 1.03 kcal mol–1. As a result, both 3 and 4 were formed, and 3 was preferentially generated, consistent with the experimentally observed 3:4 ratio of 3.3–5.2. Nevertheless, the calculated energy of 6 was lower than that of 6′ by 2.21 kcal mol–1 (Fig. 3), agreeing with the isolation of only 6 (see Supplementary Section 11). In addition, 6 was calculated to be more stable than isomeric 6″ by 4.30 kcal mol–1. Notably, the structural optimization of dianion 72− automatically resulted in cleavage of both the C1–C2 bond and C3–C4 bond and generated open-cage intermediate VII. The NBO charge distributions of dianion VII exhibited the four most negative sites at C1 (–0.082), C4 (–0.075), C10 (–0.076), and C16 (–0.073). The corresponding protonated monoanion VIII was more stable than other isomeric monoanions by at least 3.46 kcal mol–1. As a result, VIII was favorably formed. Similarly, the NBO charge distributions of VIII showed the three most negative sites at C1 (–0.073), C10 (–0.076), and C12 (–0.049). Theoretical calculations indicated that the protonated product 8 was more stable than isomers 8′ and 8″ by at least 7.26 kcal mol–1. These computational results provide rationales for the experimentally obtained stereochemistries and regioselectivities.

Fig. 3: Relative energies for optimized 6, 6′ and 6″.
figure 3

Computations were performed at the B3LYP/6-31 G(d) level.

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The redoxes and energy levels of products 1–8 are summarized in Supplementary Table 1. In particular, the calculated lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) energy levels of 3 were –3.70 and –5.40 eV, which were comparable to those (–3.72 and –5.43 eV) of PCBM. Compared to PCBM, adducts 3–8 have limited solubility and are not suitable to serve as electron acceptors in organic solar cells or as electron transport layer (ETL) materials in PSCs. Given the wide application of fullerene derivatives in PSCs13,14,15,16,17,18,24,25,37, the presentative product 3, an intramolecularly annulated isomer of PCBM, was successfully applied as an additive to inverted planar PSCs with the configuration of FTO/NiOX/MAPbI3/PCBM:3/BCP/Ag (Fig. 4a). A reference device without 3 was also fabricated for comparison. After incorporation of 3 into the PCBM layer, the PCE was enhanced from 15.83% to 17.67% (increased by 11.6%, Fig. 4b) owing to a dramatically increased Voc from 1.00 V to 1.07 V, an improved short-circuit current (Jsc) from 19.85 mA/cm2 to 20.42 mA/cm2 and a higher fill factor (FF) from 79.46% to 80.80%. Electron transport materials play an important role in the performance of PSCs. We, therefore, investigated the effect of the ETL film without and with incorporation of 3 on the electron mobility (μe) for the electron-only device structures of ITO/MAPbI3/PCBM/Ag and ITO/MAPbI3/PCBM:3/Ag by using the space charge limited current (SCLC) method38. The electron mobility for the device with incorporation of 3 was 1.40 × 10−3 cm2 V−1 S−1. Noticeably, this value was nearly twice as high as that (7.16 × 10−4 cm2 V−1 S−1) of the reference device, resulting in an increase in Voc, Jsc, and FF and thus PCE39,40. These results demonstrated that isomer 3 had enormous potential in solar cell devices.

Fig. 4: Application to perovskite solar cells.
figure 4

a Illustration of the inverted planar perovskite solar cell structure used in this work. b J–V curves (reverse scan) of the device with PCBM:3 (red line) and the reference device with PCBM (black line). BCP: bathocuproine, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline. PCBM: [6,6]-phenyl-C61-butyric acid methyl ester. MA: methylammonium. FTO: fluorine doped tin oxide.

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Discussion

In summary, the electrosynthesis of buckyballs with fused-ring systems has been achieved via intramolecular radical α-C−H functionalization of the side-chain ester for both PCBM and its analogue PCPM in the presence of a trace amount of oxygen. Two classes of buckyballs with fused bicycles or tricycles can be electrochemically synthesized. Furthermore, an unknown type of bisfulleroid with two tethered [6,6]-open orifices can be efficiently obtained from PCPM. The chemical structures for all three types of products have been established by single-crystal X-ray crystallography. An intramolecularly annulated isomer of PCBM has been applied as an additive to inverted planar perovskite solar cells, and the power conversion efficiency increases significantly from 15.83% to 17.67%, demonstrating its enormous potential in solar cell devices.

Methods

Generation of 12– and subsequent treatment with CH2I2

Dianionic 12– was electrochemically obtained from 1 (9.2 mg, 0.010 mmol) by CPE at −1.38 V vs. SCE for ca. 1 h, followed by treatment with CH2I2 (0.80 μL, 0.010 mmol) and stirring at 25 °C for 5 h. All operations were performed under an argon atmosphere (15.3 ppm O2) and the gas flow rate was 2.6 mL s–1. The resulting mixture was purified on a silica gel column (200–300 mesh) with CS2/CH2Cl2 (1:1 v/v) to afford a mixture (3.3 mg) containing products 3 (2.27 mg, 25%), 4 (0.55 mg, 6%), and 5 (0.48 mg, 5%) based on the integrals of the methoxy group in the 1H NMR spectrum (Supplementary Fig. 52). The mixture was further separated by high-performance liquid chromatography (HPLC) on a Cosmosil Buckyprep column (10 × 250 mm) using toluene/isopropanol (7:3 v/v) as the eluent with a flow rate of 3 mL min–1 to obtain pure products 3, 4, and 5.

Generation of 12– and subsequent treatment with TFA

Dianionic 12– was electrochemically obtained from 1 (9.2 mg, 0.010 mmol) by CPE at −1.38 V vs. SCE for ca. 1 h, and then stirred at 25 °C for 5 h, followed by treatment with trifluoroacetic acid (TFA, 0.76 μL, 0.010 mmol) for 10 min. All operations were performed under an argon atmosphere, in which the O2 content was determined to be 15.3 ppm by a trace oxygen analyser, and the gas flow rate was 2.6 mL s–1. The resulting mixture was purified on a silica gel column (200–300 mesh) with CS2/CH2Cl2 (1:1 v/v) to afford a mixture (5.3 mg) containing products 3 (3.63 mg, 39%), 4 (1.11 mg, 12%), and 5 (0.56 mg, 6%) based on the integrals of the methoxy group in the 1H NMR spectrum (Supplementary Fig. 53). The mixture was further separated by HPLC on a Cosmosil Buckyprep column (10 × 250 mm) using toluene/isopropanol (7:3 v/v) as the eluent at a flow rate of 3 mL min–1 to afford pure products 3, 4, and 5.

Dianionic 12– was obtained by electroreduction from 1 (8.9 mg, 0.010 mmol) at −1.38 V vs. SCE by CPE, and the electrolysis time lasted for about 1 h. The reaction mixture was immediately treated with TFA (0.76 μL, 0.010 mmol) for 10 min. All operations were performed under an argon atmosphere (15.3 ppm O2), and the gas flow rate was 2.6 mL s–1. The resulting mixture was purified on a silica gel column (200–300 mesh) with CS2/CH2Cl2 (1:1 v/v) to afford a mixture (7.0 mg) containing products 3 (2.79 mg, 31%), 4 (0.55 mg, 6%), and 5 (0.32 mg, 4%) along with recovered 1 (3.34 mg, 38%) based on the integrals in the 1H NMR spectrum (Supplementary Fig. 54). The mixture was further separated by HPLC on a Cosmosil Buckyprep column (10 mm × 250 mm) using toluene/isopropanol (7:3 v/v) as the eluent with a flow rate of 3 mL min–1 to obtain pure products 3, 4, and 5.

Generation of 22– and subsequent treatment without or with TFA

Dianionic 22– was electrochemically obtained from 2 (9.2 mg, 0.010 mmol) by CPE at −1.34 V vs. SCE for ca. 1 h and then stirred at 25 °C for 3 h. All operations were performed under an argon atmosphere (15.3 ppm O2), and the gas flow rate was 2.6 mL s–1. The resulting mixture was filtered and then purified on a silica gel column (300–400 mesh) with CS2 as the eluent to give products 6 (1.0 mg, 11%) and 7 (1.3 mg, 14%).

Dianionic 22– was electrochemically obtained from 2 (8.8 mg, 0.010 mmol) by CPE at −1.34 V vs. SCE for ca. 1 h, followed by immediate treatment with TFA (0.76 μL, 0.010 mmol) for 10 min. All operations were performed under an argon atmosphere (15.3 ppm O2), and the gas flow rate was 2.6 mL s–1. The resulting mixture was filtered and then purified on a silica gel column (300–400 mesh) with CS2 as the eluent to give product 6 (2.8 mg, 32%).

Dianionic 22– was electrochemically obtained from 2 (8.9 mg, 0.010 mmol) by CPE at −1.34 V vs. SCE for ca. 1 h and then stirred at 25 °C for 3 h, followed by treatment with TFA (0.76 μL, 0.010 mmol) for 10 min. All operations were performed under an argon atmosphere (15.3 ppm O2), and the gas flow rate was 2.6 mL s–1. The resulting mixture was purified on a silica gel column (200–300 mesh) with CS2/CH2Cl2 (1:1 v/v) to afford a mixture containing products 6 and 8, which was further separated by HPLC on a Cosmosil Buckyprep column (10 × 250 mm) using toluene as the eluent with a flow rate of 3 mL min–1. From the 1H NMR spectrum of the mixture (3.1 mg), 6 (1.87 mg, 21%) and 8 (1.23 mg, 14%) were afforded as amorphous brown solids (Supplementary Fig. 55).

Synthesis of product 8

Dianionic 22.5– was electrochemically obtained from 2 (8.8 mg, 0.010 mmol) by CPE at −1.34 V vs. SCE for ca. 1.5 h and then stirred at 25 °C for 5 h, followed by treatment with TFA (3.8 μL, 0.050 mmol) for 10 min. All operations were performed under an argon atmosphere (15.3 ppm O2), and the gas flow rate was 2.6 mL s–1. The resulting mixture was filtered and then purified on a silica gel column (300–400 mesh) with CS2 as the eluent to give product 8 (3.8 mg, 43%).