Synthesis of indano[60]fullerene thioketone and its application in organic solar cells

Evaporable indano[60]fullerene ketone (FIDO) was converted to indano[60]fullerene thioketone (FIDS) in high yield by using Lawesson's reagent. Three compounds with different substituents in para position were successfully converted to the corresponding thioketones, showing that the reaction tolerates compounds with electron-donating and electron-withdrawing substituents. Computational studies with density functional theory revealed the unique vibrations of the thioketone group in FIDS. The molecular structure of FIDS was confirmed by single-crystal X-ray analysis. Bulk heterojunction organic solar cells using three evaporable fullerene derivatives (FIDO, FIDS, C60) as electron-acceptors were compared, and the open-circuit voltage with FIDS was 0.16 V higher than that with C60.

Enhanced reactivity of Li+@C60 toward thermal [2 + 2] cycloaddition by encapsulated Li+ Lewis acid

Lithium ion-endohedral fullerene (Li+@C60), a member of the burgeoning family of ion-endohedral fullerenes, holds substantial promise for diverse applications owing to its distinctive ionic properties. Despite the high demand for precise property tuning through chemical modification, there have been only a few reports detailing synthetic protocols for the derivatization of this novel material. In this study, we report the synthesis of Li+@C60 derivatives via the thermal [2 + 2] cycloaddition reaction of styrene derivatives, achieving significantly higher yields of monofunctionalized Li+@C60 compared to previously reported reactions. Furthermore, by combining experimental and theoretical approaches, we clarified the range of applicable substrates for the thermal [2 + 2] cycloaddition of Li+@C60, highlighting the expanded scope of this straightforward and selective functionalization method.

Copper-Mediated Radical-Induced Ring-Opening Relay Cascade Carboannulation Reaction of [60]Fullerene with Cyclobutanone Oxime Esters: Access to [60]Fullerene-Fused Cyclopentanes

An unexpected copper-mediated radical-induced ring-opening relay cascade carboannulation reaction of [60]fullerene with cyclobutanone oxime esters is presented for the preparation of various Cl-/Br-incorporated [60]fullerene-fused cyclopentanes. The unique relay cascade transformation uses inexpensive copper salts as promoters and halogen sources and features simple redox-neutral conditions and a broad substrate scope, providing a practical access to a class of novel five-membered carbocycle-fused fullerenes.

Transition-Metal-Free Domino Reaction of [60]Fullerene, Indole, and DMSO/HCl: One-Pot Access to Diverse N-Substituted [60]Fulleroindole Derivatives

An unprecedented multicomponent domino reaction of [60]fullerene, indole, and DMSO/HCl has been developed for the one-pot efficient synthesis of diverse N-substituted [60]fulleroindole derivatives. This methodology features simple operation, low cost, and transition-metal-circumvented and good functional group tolerance in indole.

Palladium-catalyzed decarboxylative [2 + 3] cyclocarbonylation reactions of [60]fullerene: selective synthesis of [60]fullerene-fused 3-vinylcyclopentan-4-ones and cyclopentane-4-carbaldehydes

A new palladium-catalyzed decarboxylative strategy has been developed toward direct cyclocarbonylation of [60]fullerene, selectively furnishing novel [60]fullerene-fused 3-vinylcyclopentan-4-ones and cyclopentane-4-carbaldehydes.

Palladium-catalyzed three-component 1,4-aminoarylation of [60]fullerene with aryl iodides and N-methoxysulfonamides, and further transformations

The palladium-catalyzed three-component 1,4-aminoarylation of [60]fullerene afforded 1,4-(aryl)(sulfonamide)[60]fullerenes, of which the sulfonamide group could be replaced by a (hetero)aryl, malonate ester or allyl group in the presence of FeCl3.

Metal-Free-Catalyzed Three-Component [2+2+2] Annulation Reaction of [60]Fullerene, Ketones, and Indoles: Access to Diverse [60]Fullerene-Fused 1,2-Tetrahydrocarbazoles

The first example of metal-free-catalyzed multicomponent annulation reaction of [60]fullerene has been developed for concise and efficient construction of novel [60]fullerene-fused 1,2-tetrahydrocarbazoles. Using inexpensive and readily available I₂ as a catalyst, [60]fullerene, ketones, and indoles undergo a formal [2+2+2] annulation process to conveniently assemble diverse 1,2-tetrahydrocarbazoles. Mechanistic studies indicate that this reaction proceeds through I₂-promoted generation of a 3-vinylindole structure with the characteristics of a conjugated diene followed by cycloaddition to [60]fullerene.

An "Umpolung Relay" Strategy: One-Pot, Twice Polarity Inversion Cascade Synthesis of Diversified [60]Fulleroindoles

An "umpolung relay" strategy, which includes an one-pot, twice polarity inversion cascade of C₆₀ via carbanion and carbocation polarity reversed relay pathway, has been developed for the synthesis of a diverse range of novel [60]fulleroindole derivatives.

Palladium-catalyzed domino spirocyclization of [60]fullerene: synthesis of diverse [60]fullerene-fused spiro[4,5]/[5,5] derivatives

Herein a new, general and practical method for the spirocyclization of [60]fullerene through a palladium-catalyzed domino Heck/C-H activation reaction is presented. A wide range of novel [60]fullerene-fused spirocyclic derivatives can be easily and flexibly synthesized with a broad substrate scope and excellent functional-group tolerance. A plausible mechanism involving an alkyl Pd(ii) species as a key intermediate has been proposed.

Sonochemical synthesis of novel C60 fullerene 1,4-oxathiane derivative through the intermediate fullerene radical anion

We have discovered a selective and efficient method for the synthesis of previously unknown 1,9-(1',4'-oxathiano)-1,9-dihydro-(C₆₀-I_h)[5,6]fullerene, a compound with the direct attachment of the sulfur atom to the fullerene core. The method is based on the reaction of C₆₀ with 1,2-hydroxythiols in the presence of inorganic bases in air under ultrasonication. The significance of ultrasound has been exemplified with the comparative conventional methods. The title compound has been identified with mass-(MALDI TOF/TOF), one- and two-dimensional NMR ¹H and ¹³C (COSY, HSQC, HMBS), IR, UV-Vis spectroscopic techniques. Using the direct EPR method, we have detected radical anion C₆₀^- (g = 2.0046 and ΔH = 2 G) as a key reaction intermediate of the sonochemical reaction. Based on the experimental results and quantum chemical calculations, we have proposed a mechanism for the conversion of C₆₀ and 2-mercaptoethanol to the C₆₀-1,4-oxathiane adduct.

Iron-Catalyzed Redox-Neutral Radical Cascade Reaction of [60]Fullerene with γ,δ-Unsaturated Oxime Esters: Preparation of Free (N-H) Pyrrolidino[2',3':1,2]fullerenes

Herein an unprecedented iron(II)-catalyzed redox-neutral radical cascade reaction of [60]fullerene with γ,δ-unsaturated oxime esters is reported for the preparation of novel free (N-H) pyrrolidino[2',3':1,2]fullerenes. The transformation undergoes an intramolecular cyclization/intermolecular cyclization/oxidation/hydrolysis cascade, and features simple operation, broad substrate scope/high functional group compatibility as well as suitable for scale-up synthesis, providing a facile and practical access to a range of free pyrrolidino[2',3':1,2]fullerenes.

Potassium salt promoted regioselective three-component coupling synthesis of 1,4-asymmetrical [60]fullerene bisadducts with superior electron transport properties

An efficient one-pot three-component domino coupling reaction of phenols, C₆₀, and bromoalkanes was developed, resulting in the highly regioselective synthesis of 1,4-asymmetrical C₆₀ bisadducts.

Highly Selective Synthesis of Tetrahydronaphthaleno[60]fullerenes via Fullerene-Cation-Mediated Intramolecular Cyclization

A high-yielding protocol to construct six-membered carbon rings on fullerene is presented. This methodology with in situ fullerene-cation-mediated intramolecular cyclization provides high selectivity and efficient access to six-membered tetrahydronaphthaleno[60]fullerenes with a remarkable functional group tolerance and excellent yields. Furthermore, high solubilities of tetrahydronaphthaleno[60]fullerenes are reported.

Copper-Catalyzed N-H/C-H Sequential Relay Oxidative Radical Carboannulation: Construction of Diversely Substituted [60]Fullerene-Fused Tetrahydrocyclopenta[b]indoles

Reported herein is a new copper-catalyzed N-H/C-H sequential relay oxidative radical carboannulation of [60]fullerene with C2-functionalized free indoles for the direct construction of novel [60]fullerene-fused tetrahydrocyclopenta[b]indoles. The transformation shows high regioselectivity and atom economy, broad substrate scope, and good functional group tolerance, providing an efficient and practical approach to access diversely substituted fullerene-fused polycyclic derivatives from simple hydrocarbons.

Highly soluble C2v-symmetrical fullerene derivatives: efficient synthesis, characterization, and electrochemical study

Through an efficient octa-substitution reaction, octabromofullerene (C₆₀Br₈) was reacted with alcohols or anisole in the presence of silver triflate to produce octaalkoxy and octaaryl fullerenes, respectively, in up to 79% yield with retention of C_2v-symmetry.

Synthesis of (MeO)2Bn2C70: Regiochemistry of 2-fold Additions to C70 with Addends That Are Preferential for Ortho Addition and Capable of Para Addition

Three C₇₀ tetraadducts, 2,10-(MeO)₂-5,9-Bn₂C₇₀ (6), 1,56-Bn₂-2,57-(MeO)₂C₇₀ (7, 2 o'clock isomer), and 1,41-Bn₂-2,58-(MeO)₂C₇₀ (8, 12 o'clock isomer), were obtained from the reaction of C₇₀ with MeO^- and BnBr (benzyl bromide). The structures of 6-8 were resolved via single-crystal X-ray diffraction and spectroscopic characterizations. Computational calculations on the electrophilic Fukui functions f_k⁺, the stability of reaction intermediates, and activation barriers for the key processes of the reaction were performed to rationalize the regioselectivity of the reaction. A conversion of the 5 and 12 o'clock intermediates to the 2 o'clock intermediate was proposed to account for the regioselectivity related to the 2-fold additions at the two distinctive polar regions of C₇₀. Electrochemical study showed a similar electron deficiency for the 2 and 12 o'clock isomers, while the 2,5,9,10-tetraadduct was more electron deficient with respect to the 2 and 12 o'clock isomers.

Octaalkoxyfullerenes: Widely LUMO-Tunable C2 v-Symmetric Fullerene Derivatives

C_{2 v}-Symmetric octaalkoxyfullerenes, C₆₀(OR)₈ (R = CH₃, C₂H₅, CH₂CF₃), were synthesized by reacting octabromofullerene with the corresponding alcohols in the presence of AgBF₄. The reactions occurred with no change in the addition pattern, and the compounds were unambiguously characterized by NMR spectroscopy and X-ray structure analysis. Electrochemical measurements revealed not only that these derivatives have stable redox properties but also that their LUMO levels can be tuned over a very wide range.

Structural Identification of 19 Purified Isomers of the OPV Acceptor Material bisPCBM by 13C NMR and UV-Vis Absorption Spectroscopy and High-Performance Liquid Chromatography

The molecular structures of 19 purified isomers of bis-phenyl-C₆₂-butyric acid methyl ester were identified by a combination of ¹³C NMR and UV-vis absorption spectroscopies and high-performance liquid chromatography (HPLC) retention time analysis. All 19 isomers are dicyclopropafullerenes (none are homofullerenes). There were seven isomers with C₁ molecular point-group symmetry, four with C _s, six with C₂, one with C_{2 v}, and one with C_{2 h} symmetry. The C_{2 h}, C_{2 v}, and all five nonequatorial C₁ isomers were unambiguously assigned to their respective HPLC fractions. For the other 12 isomers, the ¹³C NMR and UV-vis spectra placed them in six groups of two same-symmetry isomers. On the basis of the widely spaced HPLC retention times of the two isomers within each of these six groups, and the empirical inverse correlation between retention time and addend spacing, each isomer was assigned to its corresponding HPLC fraction. In addition, the missing trans-1 isomer was found, purified, and characterized.

Copper-Catalyzed Aerobic Oxidative Reaction of C60 with Aliphatic Primary Amines and CS2

A novel type of fullerene derivatives, [60]fullerothiazolidinethiones (2), were obtained from the copper-catalyzed aerobic oxidative reaction of C₆₀ with aliphatic primary amines and CS₂ in 4:1 v/v DMF and o-DCB. The obtained products were fully characterized with the X-ray single-crystal diffraction and spectroscopic methods. Control experiment with maleic anhydride, an analogue to C₆₀, also afforded thiazolidinethione product, but via a mechanism different from that of C₆₀ judging from the structure difference between the two types of thiazolidinethione compounds, demonstrating the unique reactivity of C₆₀.

Photochemical site-selective synthesis of [70]methanofullerenes

Methanofullerenes such as the well-known [70]PCBM are commonly synthesized under harsh conditions to obtain the product as a mixture of site-isomers (namely α, β and minor γ) due to the D_5h symmetry of the C₇₀ cage. We report the first site-selective synthesis of [70]methanofullerenes under light irradiation and low temperatures, thus avoiding time-consuming and highly expensive HPLC separations. Pure major site-isomers α-[70]PCBM and α-[70]DPM have been thus efficiently prepared including the crystal structure of 5b. Photovoltaic preliminary results revealed a slightly beneficial performance for α-pure [70]PCBM site-isomer devices.

Dependence of the Electrochemical Redox Properties of Fullerenes on Ionic Liquids

Control of the electrochemical properties of fullerenes via the chemical modification approach has attracted considerable attention. However, surface modification of fullerene cages with various functional groups can lead to the destruction of their original structures. Herein, we report a simple approach for controlling the electrochemical properties of fullerene thin films formed on Au(111) electrodes in various ionic liquids (ILs). A total of eight reversible redox couples for six reductive and two oxidative processes were observed for fullerenes in ammonium- and pyrrolidinium-based ILs. The redox potentials, the differences between two successive redox potentials, the average values of these potential differences for fullerene reduction, and the electrochemical band gaps of fullerene films in various ILs were found to depend on the cation and its alkyl chain length, the anion, and the chemical structure of the fullerene. Highly charged C₇₀ anions were reduced more easily than C₆₀ anions. An increase in the alkyl chain length of the cation led to an increase in the average potential difference between two successive redox potentials for fullerene reduction. The results indicate that the electrochemical band gaps of fullerenes can be manipulated using ILs with appropriate anions, which can be determined based on the size of the anion and the charge distribution.

Tightly Bound Double-Caged [60]Fullerene Derivatives with Enhanced Solubility: Structural Features and Application in Solar Cells

A series of novel highly soluble double-caged [60]fullerene derivatives were prepared by means of lithium-salt-assisted [2+3] cycloaddition. The bispheric molecules feature rigid linking of the fullerene spheres through a four-membered cycle and a pyrrolizidine bridge with an ester function CO₂ R (R=n-decyl, n-octadecyl, benzyl, and n-butyl; compounds 1 a-d, respectively), as demonstrated by NMR spectroscopy and X-ray diffraction. Cyclic voltammetry studies revealed three closely overlapping pairs of reversible peaks owing to consecutive one-electron reductions of fullerene cages, as well as an irreversible oxidation peak attributed to abstraction of an electron from the nitrogen lone-electron pair. Owing to charge delocalization over both carbon cages, compounds 1 a-d are characterized by upshifted energies of frontier molecular orbitals, a narrowed bandgap, and reduced electron-transfer reorganization energy relative to pristine C₆₀ . Neat thin films of the n-decyl compound 1 a demonstrated electron mobility of (1.3±0.4)×10^-3  cm²  V^-1  s^-1 , which was comparable to phenyl-C₆₁ -butyric acid methyl ester (PCBM) and thus potentially advantageous for organic solar cells (OSC). Application of 1 in OSC allowed a twofold increase in the power conversion efficiencies of as-cast poly(3-hexylthiophene-2,5-diyl) (P3HT)/1 devices relative to the as-cast P3HT/PCBM ones. This is attributed to the good solubility of 1 and their enhanced charge-transport properties - both intramolecular, owing to tightly linked fullerene cages, and intermolecular, owing to the large number of close contacts between the neighboring double-caged molecules. Test P3HT/1 OSCs demonstrated power-conversion efficiencies up to 2.6 % (1 a). Surprisingly low optimal content of double-caged fullerene acceptor 1 in the photoactive layer (≈30 wt %) favored better light harvesting and carrier transport owing to the greater content of P3HT and its higher degree of crystallinity.

Isomer-Pure Bis-PCBM-Assisted Crystal Engineering of Perovskite Solar Cells Showing Excellent Efficiency and Stability

A fullerene derivative (α-bis-PCBM) is purified from an as-produced bis-phenyl-C₆₁ -butyric acid methyl ester (bis-[60]PCBM) isomer mixture by preparative peak-recycling, high-performance liquid chromatography, and is employed as a templating agent for solution processing of metal halide perovskite films via an antisolvent method. The resulting α-bis-PCBM-containing perovskite solar cells achieve better stability, efficiency, and reproducibility when compared with analogous cells containing PCBM. α-bis-PCBM fills the vacancies and grain boundaries of the perovskite film, enhancing the crystallization of perovskites and addressing the issue of slow electron extraction. In addition, α-bis-PCBM resists the ingression of moisture and passivates voids or pinholes generated in the hole-transporting layer. As a result, a power conversion efficiency (PCE) of 20.8% is obtained, compared with 19.9% by PCBM, and is accompanied by excellent stability under heat and simulated sunlight. The PCE of unsealed devices dropped by less than 10% in ambient air (40% RH) after 44 d at 65 °C, and by 4% after 600 h under continuous full-sun illumination and maximum power point tracking, respectively.

Purification and electronic characterisation of 18 isomers of the OPV acceptor material bis-[60]PCBM

The as-produced isomer mixture of the organic photovoltaic device acceptor material bis-[60]PCBM has been purified into its constituents by peak-recycling HPLC, and those individual isomers were characterised by UV-Vis absorption spectroscopy and cyclic voltammetry.

Multifunctionalization of C70 at the two polar regions with a high regioselectivity via oxazolination and benzylation reactions

C₇₀ multiadducts with a novel 1,2,3,4,41,56,57,58-configuration were prepared via oxazolination and benzylation reactions.

Manganese powder promoted highly efficient and selective synthesis of fullerene mono- and biscycloadducts at room temperature

Discovery of an efficient, practical, and flexible synthetic method to produce various important electron acceptors for low-cost organic photovoltaics (OPVs) is highly desirable. Although the most commonly used acceptor materials, such as PC₆₁BM, PC₇₁BM, IC₆₀BA, bisPC₆₁BM have been proved to be promising for the OPVs, they are still very expensive mainly due to their low production yields and limited synthetic methods. Herein, we report an unprecedented and innovative synthetic method of a variety of fullerene mono- and biscycloadducts by using manganese powder as a promotor. The reaction of fullerenes with various dibromides proceeds efficiently and selectively under very mild conditions to give the corresponding cycloadducts in good to excellent yields. The combination of manganese power with DMSO additive is crucial for the successful implementation of the present cycloaddition. Notably, the standard OPV acceptors, such as PCBMs, have been obtained in extraordinarily high yields, which cannot be achieved under the previously reported methods.

Stereochemistry of spiro-acetalized [60]fullerenes: how the exo and endo stereoisomers influence organic solar cell performance

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.

Increased efficiency in small molecule organic solar cells through the use of a 56-π electron acceptor--methano indene fullerene

Organic solar cells (OSCs) offer the possibility of harnessing the sun's ubiquitous energy in a low-cost, environmentally friendly and renewable manner. OSCs based on small molecule semiconductors (SMOSCs)--have made a substantial improvement in recent years and are now achieving power conversion efficiencies (PCEs) that match those achieved for polymer:fullerene OSCs. To date, all efficient SMOSCs have relied on the same fullerene acceptor, PCBM, in order to achieve high performance. The use of PCBM however, is unfavourable due to its low lying LUMO level, which limits the open-circuit voltage (VOC). Alternative fullerene derivatives with higher lying LUMOs are thus required to improve the VOC. The challenge, however, is to prevent the typical concomitant decrease in the short circuit current density (JSC) when using a higher LUMO fullerene. In this communication, we address the issue by applying methano indene fullerene, MIF, a bis-functionalised C60 fullerene that has a LUMO level 140 mV higher than PCBM, in solution processed SMOSCs with a well known small molecule donor, DPP(TBFu)2. MIF-based devices show an improved VOC of 140 mV over PC61BM and only a small decrease in the JSC, with the PCE increasing to 5.1% (vs. 4.5% for PC61BM).

Ni-Catalyzed direct 1,4-difunctionalization of [60]fullerene with benzyl bromides

A new Ni-catalyzed direct 1,4-difunctionalization of [60]fullerene with various benzyl bromides has been developed, leading to the 1,4-bisadducts in good to high yields and high 1,4-selectivities.

NBS-promoted oxidation of fullerene monoradicals leading to regioselective 1,4-difunctional fullerenes

We have demonstrated that NBS is able to promote the oxidation of fullerene monoradicals to form 1,4-difunctional fullerenes. The singly bonded fullerene dimers were used as fullerene monoradical precursors, which produced various 1,4-fullerenes with a wide range of functional groups in good to high yields with high regioselectivity in terms of cosolvents and nucleophiles.

Hiding and recovering electrons in a dimetallic endohedral fullerene: air-stable products from radical additions

Fullerenyl radicals can be generated by addition of a free radical to a fullerene surface, by nucleophilic addition followed by one-electron oxidation, or by thermal dissociation of singly bonded fullerene dimers. However, fullerenyl radicals are usually very reactive and generally cannot be isolated. On the contrary, we have found that the reactions of the dimetallic endofullerenes, La2@Ih-C80 and La2@D5h-C80, with 3-chloro-5,6-diphenyltriazine resulted in mono-addition of the triazinyl radical to the fullerene cages to yield isolable fullerenyl radicals. The unusual stability of these fullerenyl radicals arises from the confinement of the unpaired electron to an internal, metal-metal bonding orbital. Accordingly, the fullerene cage protects the radical center from other reactive species. Furthermore, we demonstrate that the fullerenyl radical adduct of La2@Ih-C80 reacts with toluene to afford additional benzylation. Interestingly, the benzylated derivative is diamagnetic in solution, while it forms a paramagnetic dimer when crystallized.