Lithium-encapsulated [60]fullerene Li@C60, namely, lithium-ion-encapsulated [60]fullerene radical anion Li+@C60˙−, was synthesised by electrochemical reduction of lithium-ion-encapsulated [60]fullerene trifluoromethanesulfonylimide salt [Li+@C60](TFSI−). The product was fully characterised by UV-vis-NIR absorption and ESR spectroscopy as well as single-crystal X-ray analysis for the co-crystal with nickel octaethylporphyrin. In solution Li@C60 exists as a monomer form dominantly, while in the crystal state it forms a dimer (Li@C60–Li@C60) through coupling of the C60 radical anion cage. These structural features were supported by DFT calculations at the M06-2X/6-31G(d) level of theory.

We report a systematic study of organic photovoltaic cells using novel spiro-acetalized and (thio)acetalized [60]fullerene monoadducts bearing five- to seven-membered rings. One of these compounds had power-conversion efficiencies of 5.8% with PTB7 and 4.0% with poly(3-hexylthiophene); the latter is comparable to that of the commonly used [6,6]-phenyl-C61-butyric acid methyl ester. We investigated the precise factor that governs the device performance by examining the solubility, space-charge-limited current mobility balance of holes and electrons, morphology, and dark current. Single-crystal X-ray diffraction highlighted the key role of compact and flexible spiro ring folding in the rim space in fullerene packing.

Fullerene bis-adducts are increasingly being studied to gain a high open circuit voltage (Voc) in bulk heterojunction organic photovoltaics (OPVs). We designed and synthesized homo and hetero bis-adduct [60]fullerenes by combining fused cyclohexanone or a five-membered spiro-acetalized unit (SAF5) with 1,2-dihydromethano (CH2), indene, or [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). These new eight 56π fullerenes showed a rational rise of the lowest unoccupied molecular orbital (LUMO). We perform a systematic study on the electrochemical property, solubility, morphology, and space-charge-limited current (SCLC) mobility. The best power conversion efficiency (PCE) of 4.43% (average, 4.36%) with the Voc of 0.80 V was obtained for poly(3-hexylthiophene) (P3HT) blended with SAF5/indene hetero bis-adduct, which is a marked advancement in PCE compared to the 0.9% of SAF5 monoadduct. More importantly, we elucidate an important role of mobility balance between hole and electron that correlates with the device PCEs. Besides, an empirical equation to extrapolate the solubilities of hetero bis-adducts is proposed on the basis of those of counter monoadducts. Our work offers a guide to mitigate barriers for exploring a large number of hetero bis-adduct fullerenes for efficient OPVs.Keywords: bulk heterojunction; fullerene bis-adduct; mobility; organic photovoltaic; poly(3-hexylthiophene); regioisomers; solubility; space-charge-limited current;

Exploiting bis-addition products of fullerenes is a rational way to improve the efficiency of bulk heterojunction-type organic photovoltaic cells (OPV); however, this design inherently produces regio- and stereoisomers that may impair the ultimate performance and fabrication reproducibility. Here, we report unprecedented exo and endo stereoisomers of the spiro-acetalized [60]fullerene monoadduct with methyl- or phenyl-substituted 1,3-dioxane (SAF6). Although there is no chiral carbon in either the reagent or the fullerene, equatorial (eq) rather than axial (ax) isomers are selectively produced at an exo-eq:endo-eq ratio of approximately 1:1 and can be easily separated using silica gel column chromatography. Nuclear Overhauser effect measurements identified the conformations of the straight exo isomer and bent endo isomer. We discuss the origin of stereoselectivity, the anomeric effect, intermolecular ordering in the film state, and the performance of poly(3-hexylthiophene):substituted SAF6 OPV devices. Despite their identical optical and electrochemical properties, their solubilities and space-charge limited current mobilities are largely influenced by the stereoisomers, which leads to variation in the OPV efficiency. This study emphasizes the importance of fullerene stereochemistry for understanding the relationship between stereochemical structures and device output.Keywords: bulk heterojunction; conformation; organic photovoltaic; stereoisomer; stereoselectivity;

We studied the kinetics of the Diels–Alder reaction of Li+-encapsulated [60]fullerene with 1,3-cyclohexadiene and characterized the obtained product, [Li+@C60(C6H8)](PF6–). Compared with empty C60, Li+@C60 reacted 2400-fold faster at 303 K, a rate enhancement that corresponds to lowering the activation energy by 24.2 kJ mol–1. The enhanced Diels–Alder reaction rate was well explained by DFT calculation at the M06-2X/6-31G(d) level of theory considering the reactant complex with dispersion corrections. The calculated activation energies for empty C60 and Li+@C60 (65.2 and 43.6 kJ mol–1, respectively) agreed fairly well with the experimentally obtained values (67.4 and 44.0 kJ mol–1, respectively). According to the calculation, the lowering of the transition state energy by Li+ encapsulation was associated with stabilization of the reactant complex (by 14.1 kJ mol–1) and the [4 + 2] product (by 5.9 kJ mol–1) through favorable frontier molecular orbital interactions. The encapsulated Li+ ion catalyzed the Diels–Alder reaction by lowering the LUMO of Li+@C60. This is the first detailed report on the kinetics of a Diels–Alder reaction catalyzed by an encapsulated Lewis acid catalyst rather than one coordinated to a heteroatom in the dienophile.

Metal encapsulation into a cage and chemical modification on the outer surface of fullerenes endow them with some unique characteristic properties. Although the derivatization of endohedral fullerenes holds promise for producing novel new nano-carbon materials, there are few reports about such compounds. Herein, we report the synthesis of lithium encapsulated fullerenol Li+@C60O−(OH)7 using a fuming sulfuric acid method from [Li+@C60](PF6−) and characterization of its structure by IR, NMR, FAB mass spectroscopy, and elemental analysis. The hydroxylation of [Li+@C60](PF6−) is site-selective to preferentially give a single isomer (ca. 70%) with two minor isomers in marked contrast to the reaction of empty C60. We conclude from the analysis of radical species produced in the reaction of a C60 cage with fuming sulfuric acid that this unusual site-selective hydroxylation is caused by the lower HOMO level of Li+@C60 than that of empty C60. Furthermore, our results clearly indicate that the internal lithium cation is interacted with the introduced hydroxyl groups, and thus the properties of endohedral fullerenes can be controlled by the external modification of a fullerene cage.

The ionic conductivity of [Li+@C60](PF6−) was measured in o-dichlorobenzene, and found to be higher than that of TBA+PF6−. Electrochemical reduction of [Li+@C60](PF6−) without any supporting electrolyte gave the monovalent radical anion Li+@C60˙−, as confirmed by the characteristic ESR signal and NIR absorption band.

Acetalized [60]fullerenes were synthesized from cyclohexanone-fused [60]fullerene, which was facilely prepared by Diels–Alder reaction of C60 with silylether diene, on treatment of various aliphatic alcohols under TiCl4 catalyst. The spiro-cyclic acetalized [60]fullerenes having five, six, and seven-membered rings were also synthesized by using the corresponding diols under the same condition. The slightly raised reduction potentials Ered (∼0.04 V) relative to those of PCBM were observed by cyclic voltammetry measurement, depending on the identity of alkyl group/chain. The noncyclic acetalized [60]fullerenes showed lower thermal stability up to 200 °C, while the cyclic ones exhibited the drastically improved thermal stability up to 350 °C under nitrogen atmosphere. The acid-catalyzed hydrolysis easily removed the acetal moiety quantitatively, resulting in a considerable change of solvent solubility of the fullerene.Acetalized [60]fullerenes were synthesized from cyclohexanone-fused [60]fullerene, which was facilely prepared by Diels–Alder reaction of C60 with silylether diene, on treatment of various aliphatic alcohols under TiCl4 catalyst. The spiro-cyclic acetalized [60]fullerenes having 5-, 6-, and 7-membered ring were also synthesized by using the corresponding diols under the same condition. The slightly raised reduction potentials Ered (∼0.04 V) relative to that of PCBM were observed by cyclic voltammetry measurement, depending on the identity of alkyl group/chain. The noncyclic acetalized [60]fullerenes showed lower thermal stability up to 200 °C, while the cyclic ones exhibited the drastically improved thermal stability up to 350 °C under nitrogen atmosphere. The acid-catalyzed hydrolysis easily removed the acetal moiety quantitatively, resulting in the considerable change of solvent solubility of the fullerene.

Due to the spherical shape with a diameter of ca. 1 nm, the aggregation behaviour of fullerene C60 is very interesting in view of the possible formation of magic number particle in a similar manner as metal cluster in gas phase. Herein, we report for the first time the magic number aggregation behaviours of polyhydroxylated fullerenol C60(OH)36 in water–alcohol (methanol, ethanol and 1-propanol) binary solution with increasing alcohol component. The diameters of particle were ca. 6–8 nm depending on the alcohol used. The particle sizes were precisely measured by the novel-induced grating method which is superior for the particle-size measurement in single-nano region (1–10 nm). The magic number cluster was also detected by scanning probe microscopy observation. However, such aggregation behaviours were not found in DMSO–alcohol system or for the use of less hydroxylated C60(OH)10.

A water-soluble polyhydroxylated fullerene, i.e. a fullerenol, with 44 hydroxyl groups and 8 secondary bound water molecules, C60(OH)44·8H2O (estimated average structure), has been synthesized in a facile one step reaction from pristine C60 by hydroxylation with hydrogen peroxide in the presence of a phase-transfer catalyst, tetra-n-butylammonium hydroxide (TBAH), under organic/aqueous bilayer conditions. The fullerenol exhibited high water solubility, up to 64.9 mg/mL, under neutral (pH = 7) conditions. Dynamic light-scattering (DLS) analysis showed a narrow particle size distribution, of 1–2 nm, indicating that the fullerenol had high dispersion properties in water. The results of particle size analyses, which both focused on a single nanoregion and were conducted using a novel induced grating (IG) method and a scanning probe microscope (SPM), were consistent with the DLS results. A plausible reaction mechanism, which includes fullerene oxide intermediates detected by liquid chromatography-mass spectrometry (LC-MS), has been proposed.

We report a BF3-mediated novel dehydrative cycloaddition reaction of benzoquinones with stilbene oxides to afford 2,3-diaryl-5-hydroxybenzofurans and 2,3-diaryl-5-hydroxydihydrobenzofurans in good combined yields. No change in the products or the yield is observed when using diphenylacetaldehyde and cis-stilbene oxide instead of trans-stilbene oxides. When stilbene oxide is reacted with hydroquinone instead of the corresponding benzoquinone under the same conditions, dihydrobenzofuran is isolated in high yield. On the basis of these results, we propose the following possible reaction mechanism: stilbene oxide is converted to a phenonium ion (the key intermediate), which undergoes nucleophilic attack by benzoquinone or the simultaneously generated hydroquinone and subsequent dehydrative intramolecular cyclization to afford benzofuran or dihydrobenzofuran, respectively.

Using a novel hydrogen peroxide heating method, we synthesized milky white, water-soluble polyhydroxylated fullerenes (fullerenols) with 36–40 hydroxyl groups (estimated average) along with 8–9 secondary bound water molecules. The fullerenols exhibited high water solubility up to 58.9 mg/mL in a neutral (pH = 7) condition. Dynamic light scattering analysis showed a high dispersion property, to give a narrow particle size distribution within 0.7–2.0 nm.Keywords: fullerene; fullerenol; hydroxyl group; nanoparticle; water solubility

The introduction of pin-up oxygen on C60, such as in the oxidized fullerenes C60O and C60On, induced noticeable increase in the antioxidant activity as compared to pristine C60. The water-soluble inclusion complexes of fullerenes C60O and C60Onreacted with linoleic acid peroxyl radical 1.7 and 2.4 times faster, respectively.

Lithium-encapsulated [60]fullerene Li@C60, namely, lithium-ion-encapsulated [60]fullerene radical anion Li+@C60˙−, was synthesised by electrochemical reduction of lithium-ion-encapsulated [60]fullerene trifluoromethanesulfonylimide salt [Li+@C60](TFSI−). The product was fully characterised by UV-vis-NIR absorption and ESR spectroscopy as well as single-crystal X-ray analysis for the co-crystal with nickel octaethylporphyrin. In solution Li@C60 exists as a monomer form dominantly, while in the crystal state it forms a dimer (Li@C60–Li@C60) through coupling of the C60 radical anion cage. These structural features were supported by DFT calculations at the M06-2X/6-31G(d) level of theory.

We report a systematic study of organic photovoltaic cells using novel spiro-acetalized and (thio)acetalized [60]fullerene monoadducts bearing five- to seven-membered rings. One of these compounds had power-conversion efficiencies of 5.8% with PTB7 and 4.0% with poly(3-hexylthiophene); the latter is comparable to that of the commonly used [6,6]-phenyl-C61-butyric acid methyl ester. We investigated the precise factor that governs the device performance by examining the solubility, space-charge-limited current mobility balance of holes and electrons, morphology, and dark current. Single-crystal X-ray diffraction highlighted the key role of compact and flexible spiro ring folding in the rim space in fullerene packing.

The ionic conductivity of [Li+@C60](PF6−) was measured in o-dichlorobenzene, and found to be higher than that of TBA+PF6−. Electrochemical reduction of [Li+@C60](PF6−) without any supporting electrolyte gave the monovalent radical anion Li+@C60˙−, as confirmed by the characteristic ESR signal and NIR absorption band.