The C70 δ-adducts with closed [5,6]-ring fusion are an important type of compound in classifying bond delocalization in the equatorial belt of C70. However, the formation of such compounds is severely restricted due to the low reactivity of the carbon atoms in the flat equatorial region. Such a restriction is lifted when reduced anionic C70 species are used, where the inert equatorial carbon atoms are activated.

C60 o-quinodimethane bisadducts [C60(QM)2] are promising electron acceptors for bulk heterojuction (BHJ) organic solar cells (OSCs). However, previous production of C60(QM)2 often resulted in excessive regioisomers, which were difficult to purify and might consequently obscure the structure–performance study of the organofullerene acceptors. Herein, the electrosynthesis of C60(QM)2 is reported. The reaction exhibits a strong regiocontrol with generation of fewer regioisomers. Pure regioisomers of cis-2, trans-3, and e C60(QM)2 are obtained, and the structures are solved with single-crystal X-ray diffraction. Interestingly, the cis-2 and trans-3 regioisomers exhibit better photovoltaic performance than the e regioisomer in the OSCs based on poly(3-hexylthiophene) (P3HT), which can be correlated with their structural difference.

C70 multiadducts with a novel 1,2,3,4,41,56,57,58-configuration were prepared via oxazolination and benzylation reactions. The structures of the adducts were determined by single crystal X-ray diffraction and spectral characterization. The compounds exhibit a high LUMO level, and result in a high open-circuit voltage in devices with P3HT.

Negative ion photoelectron spectroscopy shows interesting regioisomer-specific electron affinities (EAs) of 2,5- and 7,23-para-adducts of C70 [(ArCH2)2C70] (Ar = Ph, o-, m-, and p-BrC6H4). Their EA values are larger than that of C70 by 5–150 meV with the 2,5-polar adducts' EAs being higher than their corresponding 7,23-equatorial counterparts, exhibiting appreciable EA tunable ranges and regioisomeric specificity. Density functional theory (DFT) calculations reproduce both the experimental EA values and EA trends very well.

The base-promoted oxidative cycloaddition reaction of [60]fullerene with ethyl acetoacetate was investigated. The reaction resulted in C60 bis-2′,3′-dihydrofuran derivatives, but only with the trans-1, trans-2, trans-3, and e configurations rather than any cis or trans-4 structures, demonstrating a new regioselectivity resulting from steric influence on C60 bisaddition. In addition, distribution of the bisadducts was complicated by the unsymmetrical nature of the addends, where each individual configuration may consist of several regioisomers.

Methoxylation of the singly bonded 1,4–1′,4′-BnC60–C60Bn dimer afforded 1,4-OMe(Bn)C60, a 1,4-C60 adduct with sterically less demanding addends, as the major adduct. The situation was different from that of direct functionalization of C60, where 1,2-OMe(Bn)C60 was obtained as the major product. The reaction was studied with in situ vis-NIR spectroscopy and computational calculations to obtain a better understanding of this unusual regioselectivity. The stability difference between 1,2- and 1,4-OMe(Bn)C60 was studied.

[60]Fullerene derivatives with novel 1,4,9,25- and 1,4,9,12-configurations were obtained by reactions of C60 with aliphatic ketones and benzyl bromide under basic conditions. The structures of the products were determined by X-ray single-crystal diffraction and spectroscopic characterization. The reactions were rationalized by a monoenolate addition experiment and in situ vis–NIR spectroscopy.

C70 bis-heterocyclic derivative (1) bearing one oxazoline ring and one imidazoline ring with the 2 o’clock configuration is obtained with high chemio- and regioselectivity via the reaction of C70 with hydroxide and benzonitrile quenched with I2. Further study with benzylation experiment and theoretical calculations indicate that the oxazoline ring is the one first formed on the C70 cage, while the imidazoline ring is the one formed after the addition of I2 via a radical coupling reaction mechanism.

Upon reduction, singly bonded 1,2,4,15-C60 dimers with an oxazoline or imidazoline heterocycle dissociate into monoanionic 1,2,4-C60 intermediates, which surprisingly leads to the formation of 1,2,3,16-C60 rather than 1,2,4,15-C60 adducts of the original configuration by further benzylation, even though the analogue of dibenzylated C60 oxazoline with a 1,2,4,15-configuration is stable and has been obtained. These results are corroborated by computational calculations, which rationalize the reaction and clarify the structure of the 1,2,3,16-C60 adducts, providing new insights into the intrinsic reactivity of singly bonded C60 dimers.

The stability of the anionic species of the ortho and para regioisomers of (MeO)BnC2n (Me = methyl, Bn = benzyl, n = 30 or 35) has been examined. The results show that the ortho adducts (electronically favored regioisomers) are stable upon receiving one or two electrons, while the para ones (sterically favored adducts) decompose by removing the methoxy group under similar conditions. Computational calculations indicate that the stability of the anionic species is significantly affected by the electronic structure, where the [5,6]-double bond is responsible for the instability of the reduced species of the para adducts. Further study with 1,15-(MeO)2-2,4-Bn2C60, an adduct with both the ortho and para positioned methoxy, shows that the reduced species is stable, indicating that the 1,2,4,15-configuration is an electronically preferential structure even though it has a [5,6]-double bond.

Reduced fullerenes and fullerene derivatives exhibit intense absorptions in the vis-near-IR (vis-NIR) region. The absorptions are sensitive toward the addition pattern, number of addends, and oxidation state of the fullerene species and are used as an important benchmark for identifying anionic fullerene species. Similar absorptions are also shown for the reduced singly bonded C60 species, which are electronically different from reduced fullerene derivatives. However, much less work has been carried out on the vis-NIR spectroscopic study of the anionic singly bonded C60 species, likely due to the difficulty in controlled production of these species. Herein, we report the vis-NIR spectroscopic study of the mono- and dianions of the singly bonded C60 species (RC60–, R–C60–, and RC602–•), which are selectively generated by controlled-potential bulk electrolysis (CPE) of the singly bonded C60 dimer and C60 oxazolino heterocycles. Time-dependent density functional theory (TD-DFT) calculations were performed to rationalize the experimental results.

Reductive benzylation of C60 imidazoline with a bulky addend affords two 1,2,3,16-adducts (2 and 4) and one 1,2,3,4-adduct (3). Experimental and computational results indicate that the sterically favored 2 is more stable than the electronically favored 3. However, an opposite stability order is shown for the dianions of 2 and 3.

Reductive benzylation of C70 imidazolines bearing a bulky addend has been carried out under conditions similar to that reported for C60 analogues. However, different from the reaction of C60 analogues, the reaction of C70 imidazolines not only results in adducts with 1,2,3,16-configuration due to the steric effect, but also a considerable amount of dibenzylated and monobenzylated products with 1,2,3,4-configuration, demonstrating a reactivity difference between C60 and C70. Interestingly, the anions of the 1,2,3,16-C70 adduct are rather stable as shown by the electrochemical study, which is in contrast to the anions of 1,2,3,16-C60 counterparts, and can be rationalized by the electronic structure difference between C70 and C60 derivatives.

Oxazoline and imidazoline functionalization of a singly bonded C60 dimer is achieved via a one-pot reaction of C60HBn with OH– and aromatic nitriles, where the OH– not only functions as a Brønsted base that deprotonates C60HBn but also as a nucleophile that initiates the nucleophilic addition to the fullerene cage. The structures of the obtained oxazolinated and imidazolinated C60 dimeric compounds have been established by HRMS, UV–vis, and 1H, 13C, and HMBC NMR characterizations and computational calculations. The reaction mechanism is studied with the in situ vis–near-IR spectra, which shows that the use of I2 is crucial for the functionalization of the C60 dimer, indicating that it is likely the dimeric molecule rather than the fragment of the dimer that is involved in the reaction.

Transformation of aromatic nitriles to imidazolines has been achieved under basic conditions with the electron-deficient C60 and C70 fullerenes, but not with the electron-deficient olefin of tetracyanoethylene (TCNE). In situ UV–vis–NIR indicates that the ability of RC60– to undergo single-electron transfer (SET) to C60 is crucial for the reaction.

Negative ion photoelectron spectroscopy shows interesting regioisomer-specific electron affinities (EAs) of 2,5- and 7,23-para-adducts of C70 [(ArCH2)2C70] (Ar = Ph, o-, m-, and p-BrC6H4). Their EA values are larger than that of C70 by 5–150 meV with the 2,5-polar adducts' EAs being higher than their corresponding 7,23-equatorial counterparts, exhibiting appreciable EA tunable ranges and regioisomeric specificity. Density functional theory (DFT) calculations reproduce both the experimental EA values and EA trends very well.

C70 multiadducts with a novel 1,2,3,4,41,56,57,58-configuration were prepared via oxazolination and benzylation reactions. The structures of the adducts were determined by single crystal X-ray diffraction and spectral characterization. The compounds exhibit a high LUMO level, and result in a high open-circuit voltage in devices with P3HT.