Oxygenated hollow cage fullerenes have been intensively studied due to their applications in biomedicine in recent years. Clusterfullerenes have become a focus of endohedral fullerene researches for their exceptionally high yield and thermal stabilities. However, oxide derivatives of clusterfullerene remain unexplored to date. Herein, we present the photochemical synthesis of an oxide derivative of clusterfullerene, Lu3N@C80O, for the first time. The compound was characterized by matrix-assisted laser desorption ionization time-of-flight mass spectrometry, UV–vis-NIR, cyclic voltammetry, and FTIR spectroscopy. The results suggest that one oxygen atom bridges with the fullerene cage after the oxidation of Lu3N@C80. Moreover, the oxidation has a major impact on the electrochemical behavior of Lu3N@C80.

The field of endohedral metallofullerenes has developed extraordinarily since the synthesis and characterization of Sc3N@Ih-C80 in 1999, the third most abundant fullerene after C60 and C70. During these almost two decades other clusterfullerenes have been trapped inside different IPR and non-IPR fullerenes. Sc2O has demonstrated to be a good template for middle size fullerenes, between C70 and C82, permitting to characterize many structures and determining different physical properties. This mini-review will allow the reader to gain insight into the field of endohedral metallofullerenes and in particular into the richness of the fullerenes containing scandium oxide clusters as well as into experimental and theoretical techniques used to characterize them.Download high-res image (70KB)Download full-size image

Indene-C60 bisadduct (IC60BA), which can offer a significantly higher open-circuit voltage (Voc) than monoadducts, has become the research focus as electron acceptor materials in polymer solar cells (PSCs) in recent years. However, despite its popularity, IC60BA have always been applied in PSCs as mixture of several regioisomers and the nature of this mixture has never been fully investigated and understood. Herein, for the first time, 12 major regioisomers of IC60BA were isolated and a full investigation was carried out with respect to their structure, abundance, solubility and their corresponding photovoltaic performance. The results show that the PSCs based on these regioisomeric structures present very diverse PCE and their photovoltaic performance was dramatically affected not only by the relative indene positions but also by the steric orientation of the two indene groups. Electrochemical studies further revealed that the effect of energetic disorder inside the IC60BA regioisomers on their photovoltaic performance is insignificant when applied in PSCs. However, the steric structures and solubility of the regioisomers were found to have significant impact on the morphology and bulk properties of the active layer of PSCs, which give rise to very different PCE of devices based on IC60BA regioisomers with different structures.

Clusterfullerenes with variable carbon cages have been extensively studied in recent years. However, despite all these efforts, C74 cage-based clusterfullerene remains a missing piece of the puzzle. Herein, we show that single-crystal X-ray crystallographic analysis unambiguously assigns the previously reported dimetallofullerene Sc2@C76 to a novel carbide clusterfullerene, Sc2C2@D3h(14246)-C74, the first experimentally proven clusterfullerene with a C74 cage. In addition, Sc2C2@D3h(14246)-C74 was charaterized by mass spectrometry, ultraviolet–visible–near-infrared absorption spectroscopy, 45Sc nuclear magnetic resonance, and cyclic voltammetry. Comparative studies of the motion of the carbide cluster in Sc2C2@D3h(14246)-C74 and Sc2C2@C2n (n = 40−44) revealed that a combination of factors, involving both the shape and size of the cage, is crucial in dictating the cluster motion. Moreover, structural studies of D3h(14246)-C74 revealed that it can be easily converted to Cs(10528)-C72 and Td(19151)-C76 cages via C2 desertion/insertion and Stone–Wales transformation. This suggests that D3h(14246)-C74 might play an important role in the growth pathway of clusterfullerenes.

For perovskite solar cells (Pero-SCs), one of the key issues with respect to the power conversion efficiency (PCE) is the morphology control of the perovskite thin-films. In this study, an easily-accessible additive polyethylenimine (PEI) is utilized to tune the morphology of CH3NH3PbI3−xClx. With addition of 1.00 wt% of PEI, the smoothness and crystallinity of the perovskite were greatly improved, which were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). A summit PCE of 14.07% was achieved for the p-i-n type Pero-SC, indicating a 26% increase compared to those of the devices without the additive. Both photoluminescence (PL) and alternating current impedance spectroscopy (ACIS) analyses confirm the efficiency results after the addition of PEI. This study provides a low-cost polymer additive candidate for tuning the morphology of perovskite thin-films, and might be a new clue for the mass production of Pero-SCs.

TiO2 is widely used in perovskite solar cells (Pero-SCs), but its low electrical conductivity remains a drawback for application in electron transport layer (ETL). To overcome this problem, an easily accessible hydroxylated fullerene, fullerenol, was employed herein as ETL modified on ITO in n-i-p type (ITO as cathode) Pero-SCs for the first time. The results showed that the insertion of a single layer of fullerenol between perovskite and TiO2 dramatically facilitates the charge transportation and decreases the interfacial resistance. As a consequence, the device performance was greatly improved, and a higher power conversion efficiency of 14.69% was achieved, which is ∼17.5% enhancement compared with that (12.50%) of the control device without the fullerenol interlayer. This work provides a new candidate of interfacial engineering for facilitating the electron transportation in Pero-SCs.

It has been proposed that the fullerene formation mechanism involves either a top-down or bottom-up pathway. Despite different starting points, both mechanisms approve that particular fullerenes or metallofullerenes are formed through a consecutive stepwise process involving Stone–Wales transformations (SWTs) and C2 losses or additions. However, the formation pathway has seldomly been defined at the atomic level due to the missing-link fullerenes. Herein, we present the isolation and crystallographic characterization of two isomeric clusterfullerenes Sc2O@C2v(3)-C78 and Sc2O@D3h(5)-C78, which are closely related via a single-step Stone–Wales (SW) transformation. More importantly, these novel Sc2O@C78 isomers represent the key links in a well-defined formation pathway for the majority of solvent-extractable clusterfullerenes Sc2O@C2n (n = 38–41), providing molecular structural evidence for the less confirmed fullerene formation mechanism. Furthermore, DFT calculations reveal a SWT with a notably low activation barrier for these Sc2O@C78 isomers, which may rationalize the established fullerene formation pathway. Additional characterizations demonstrate that these Sc2O@C78 isomers feature different energy bandgaps and electrochemical behaviors, indicating the impact of SW defects on the energetic and electrochemical characteristics of metallofullerenes.

By introducing CO2 as the oxygen source during the arcing process, a new isomer of Sc2O@C82, Sc2O@C3v(8)-C82, previously investigated only by computational studies, was discovered and characterized by mass spectrometry, UV–vis–NIR absorption spectroscopy, cyclic voltammetry, 45Sc NMR, density functional theory (DFT) calculations, and single-crystal X-ray diffraction. The crystallographic analysis unambiguously elucidated that the cage symmetry was assigned to C3v(8) and suggests that Sc2O cluster is disordered inside the cage. The comparative studies of crystallographic data further reveal that the Sc1–O–Sc2 angle is in the range of 131.0–148.9°, much larger than that of the Sc2S@C3v(8)-C82, demonstrating a significant flexibility of dimetallic clusters inside the cages. The electrochemical studies show that the electrochemical gap of Sc2O@C3v(8)-C82 is 1.71 eV, the largest among those of the oxide cluster fullerenes (OCFs) reported so far, well correlated with its rich abundance in the reaction mixture of OCF synthesis. Moreover, the comparative electrochemical studies suggest that both the dimetallic clusters and the cage structures have major influences on the electronic structures of the cluster fullerenes. Computational studies show that the cluster can rotate and change the Sc–O–Sc angle easily at rather low temperature.

Besides the conventional D5h(8149)-C70 fullerene, there are a large number of C70 isomers that violate the isolated pentagon rule (IPR). However, these non-IPR C70 fullerenes have been less investigated owing to their low stabilities or high reactivities. In this study, we report for the first time the X-ray structure of an unconventional endohedral C70 fullerene, Sc2O@C2(7892)-C70. The combined study of geometrical analysis and computation further reveals the ionic and covalent interactions between the cluster and the cage, both of which contribute to the stabilization of this non-IPR C70 fullerene. In addition, a close structural relationship between the non-IPR C2(7892)-C70 and the IPR D5h(8149)-C70 has been demonstrated, which might provide an alternative explanation of the formation of non-IPR fullerenes.

•A series of novel dihydrobenzofuran-C60 bisadducts were synthesized by a facile method and utilized as acceptor materials in polymer solar cells.•Device performances were significantly improved by using the alkyl chain substituted dihydrobenzofuran-C60 bisadducts (Cn-BFCBA) as acceptors.•When n = 4, i.e. tert-butyl as substituent (C4-BFCBA), PCE of the corresponding device reached 3.40%, which is comparable to that of the device based on PC61BM.A series of novel dihydrobenzofuran-C60 bisadducts, denoted by BFCBA (without side chain substituent) and Cn-BFCBA (n represents the number of carbon atoms on side chain substituent, and varies as 1–5), were synthesized by a facile method and utilized as acceptor materials in polymer solar cells (PSCs). These fullerene bisadducts were found to possess similar optical, electrochemical properties but the PSCs based on them as acceptors present very diverse photovoltaic performance as the variation of n. Compared to the PSC based on BFCBA, device performances were significantly improved by the application of alkyl chain substituted dihydrobenzofuran-C60 bisadducts (Cn-BFCBA). In particular, when n = 4, i.e. tert-butyl used as substituent, PCE of the corresponding device reached to a summit of 3.40%. This result is comparable to that of the device based on PC61BM as acceptor. The morphology studied by AFM and electron mobility investigated by the space charge limited current (SCLC) method further revealed that C4-BFCBA shows a reasonably well miscibility with P3HT and a significantly higher electron mobility. These properties might account for its optimal photovoltaic performance among these series of the fullerene bisadducts. For comparison, dihydrobenzofuran-C60 monoadduct and trisadduct based on tert-butyl substituents were also synthesized. The corresponding PSC device test results showed that C4-BFCBA present much better photovoltaic performance than its corresponding monoadduct (C4-BFCMA) and trisadduct (C4-BFCTA) as well as BFCBA.

The trimetallic oxide clusterfullerene (OCF) Sc3O@C80 has been obtained with rather high abundance in the raw soot. Most of the formed product, however, remained nonextracted in the soot so that only a small amount of it was isolated and purified. The tiny quantity of pure product acquired made only possible characterization by UV–vis-NIR spectroscopy. DFT computations predict Sc3O@Ih(7)-C80 to be the isolated isomer and provide further information about the electronic structure and other (magnetic and electrochemical) properties of this singular OCF. Significant spin density on the endohedral Sc ions and in cavea redox processes are two main features of Sc3O@Ih(7)-C80, which is isoelectronic to the anion of the prototypical nitride Sc3N@Ih(7)-C80. Polymerization is predicted to be a favored process that could explain the very low yields obtained once the product is purified.

A new oxide cluster fullerene, Sc2O@C2v(5)-C80, has been isolated and characterized by mass spectrometry, UV–vis–NIR absorption spectroscopy, cyclic voltammetry, 45Sc NMR, DFT calculations, and single crystal X-ray diffraction. The crystallographic analysis unambiguously elucidated that the cage symmetry was assigned to C2v(5)-C80 and suggests that the Sc2O cluster is ordered inside the cage. The crystallographic data further reveals that the Sc1–O–Sc2 angle is much larger than that found in Sc2O@Td(19151)-C76 but almost comparable to that in Sc2O@Cs(6)-C82, suggesting that the endohedral Sc2O unit is flexible and can display large variation in the Sc–O–Sc angle, which depends on the size and shape of the cage. Computational studies show that there is a formal transfer of four electrons from the Sc2O unit to the C80 cage, i.e., (Sc2O)4+@(C80)4–, and the HOMO and LUMO are mainly localized on the C80 framework. Moreover, thermal and entropic effects are seen to be relevant in the isomer selection. Comparative studies between the recently reported Sc2C2@C2v(5)-C80 and Sc2O@C2v(5)-C80 reveal that, despite their close structural resemblance, subtle differences exist on the crystal structures, and the clusters exert notable impact on their spectroscopic properties as well as interactions between the clusters and corresponding cages.

An extensive family of oxide cluster fullerenes (OCFs) Sc2O@C2n (n = 35–47) has been facilely produced for the first time by introducing CO2 as the oxygen source. Among this family, Sc2O@C70 was identified as the smallest OCF and therefore isolated and characterized by mass spectrometry, 45Sc nuclear magnetic resonance, UV–vis–near-infrared absorption spectroscopy, cyclic voltammetry, and density functional theory calculations. The combined experimental and computational studies reveal a non-isolated pentagon rule isomer Sc2O@C2(7892)–C70 with reversible oxidative behavior and lower bandgap relative to that of Sc2S@C2(7892)–C70, demonstrating a typical example of unexplored OCF and underlining its cluster-dependent electronic properties.

Besides the conventional D5h(8149)-C70 fullerene, there are a large number of C70 isomers that violate the isolated pentagon rule (IPR). However, these non-IPR C70 fullerenes have been less investigated owing to their low stabilities or high reactivities. In this study, we report for the first time the X-ray structure of an unconventional endohedral C70 fullerene, Sc2O@C2(7892)-C70. The combined study of geometrical analysis and computation further reveals the ionic and covalent interactions between the cluster and the cage, both of which contribute to the stabilization of this non-IPR C70 fullerene. In addition, a close structural relationship between the non-IPR C2(7892)-C70 and the IPR D5h(8149)-C70 has been demonstrated, which might provide an alternative explanation of the formation of non-IPR fullerenes.

Indene-C60 bisadduct (IC60BA), which can offer a significantly higher open-circuit voltage (Voc) than monoadducts, has become the research focus as electron acceptor materials in polymer solar cells (PSCs) in recent years. However, despite its popularity, IC60BA have always been applied in PSCs as mixture of several regioisomers and the nature of this mixture has never been fully investigated and understood. Herein, for the first time, 12 major regioisomers of IC60BA were isolated and a full investigation was carried out with respect to their structure, abundance, solubility and their corresponding photovoltaic performance. The results show that the PSCs based on these regioisomeric structures present very diverse PCE and their photovoltaic performance was dramatically affected not only by the relative indene positions but also by the steric orientation of the two indene groups. Electrochemical studies further revealed that the effect of energetic disorder inside the IC60BA regioisomers on their photovoltaic performance is insignificant when applied in PSCs. However, the steric structures and solubility of the regioisomers were found to have significant impact on the morphology and bulk properties of the active layer of PSCs, which give rise to very different PCE of devices based on IC60BA regioisomers with different structures.