Regiocontrolled Electrosynthesis of [60]Fullerene Bisadducts: Photovoltaic Performance and Crystal Structures of C60 o-Quinodimethane Bisadducts

Structure-Property Relationships for the Electronic Applications of Bis-Adduct Isomers of Phenyl-C61 Butyric Acid Methyl Ester

Higher adducts of a fullerene, such as the bis-adduct of PCBM (bis-PCBM), can be used to achieve shallower molecular orbital energy levels than, for example, PCBM or C60. Substituting the bis-adduct for the parent fullerene is useful to increase the open-circuit voltage of organic solar cells or achieve better energy alignment as electron transport layers in, for example, perovskite solar cells. However, bis-PCBM is usually synthesized as a mixture of structural isomers, which can lead to both energetic and morphological disorder, negatively affecting device performance. Here, we present a comprehensive study on the molecular properties of 19 pure bis-isomers of PCBM using a variety of characterization methods, including ultraviolet photoelectron spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, single crystal structure, and (time-dependent) density functional theory calculation. We find that the lowest unoccupied molecular orbital of such bis-isomers can be tuned to be up to 170 meV shallower than PCBM and up to 100 meV shallower than the mixture of unseparated isomers. The isolated bis-isomers also show an electron mobility in organic field-effect transistors of up to 4.5 × 10-2 cm2/(V s), which is an order of magnitude higher than that of the mixture of bis-isomers. These properties enable the fabrication of the highest performing bis-PCBM organic solar cell to date, with the best device showing a power conversion efficiency of 7.2%. Interestingly, we find that the crystallinity of bis-isomers correlates negatively with electron mobility and organic solar cell device performance, which we relate to their molecular symmetry, with a lower symmetry leading to more amorphous bis-isomers, less energetic disorder, and higher dimensional electron transport. This work demonstrates the potential of side chain engineering for optimizing the performance of fullerene-based organic electronic devices.

Mechanistic Studies of Regiocontrolled Bisaddition of Fullerenes Driven by Oriented External Electric Fields

The challenge of achieving regioselective multifunctionalization on highly symmetric C60 and C70 fullerenes persists as a significant hurdle. In this study, we present a novel approach involving the participation of an oriented external electric field (OEEF) to facilitate the regioselective formation of bisadducts in C60/C70 fullerenes. These products are obtained through consecutive Diels-Alder cycloaddition reactions. We constructed the field strength-barrier relationship and elucidated the OEEF-driven modulation mechanisms quantitatively. Leveraging the interplay between molecular dipoles and electric fields, the diverse reactions at distinct sites exhibit varying degrees of sensitivity to the applied electric fields, thereby leading to a pronounced regioselectivity in the bisaddition process. Our proposition suggests that the angle formed between the bonding direction (referred to as the reaction axis) and the external field can conveniently function as a predictive descriptor for the reactivity of different sites on the fullerene surface when subjected to electric fields.

Transition-metal-mediated benzylation of C60 with benzyl chlorides

Benzyl bromides have been widely used for fullerene functionalization. However, the use of benzyl chlorides, a more affordable but less reactive counterpart of benzyl bromides, has been rarely reported. Herein, a new metal-mediated benzylation of C₆₀ with benzyl chlorides is presented. In this method, with the combinatorial use of Mn powder and Cu(OAc)₂, various benzyl chloride derivatives could react with C₆₀ to afford 1,4-dibenzylated products in 12-53% yields. A mechanistic study by in situ visible near infrared (vis-NIR) spectroscopy and various control experiments suggests that, unlike the conventional anionic pathway that uses benzyl bromides, the transition-metal-mediated benzylation of C₆₀ with benzyl chlorides proceeds via a metal-mediated iterative single electron transfer process.

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.

cis-1 Isomers of tethered bismethano[70]fullerene as electron acceptors in organic photovoltaics

Isomer-controlled [70]fullerene bis-adducts can achieve high performance as electron-acceptors in organic photovoltaics (OPVs) because of their stronger absorption intensities than [60]fullerene derivatives, higher LUMO energy levels than mono-adducts, and less structural and energetic disorder than random isomer mixtures. Especially, attractive are cis-1 isomers that have the closest proximity of addends owing to their plausible more regular close packed structure. In this study, propylene-tethered cis-1 bismethano[70]fullerene with two methyl, ethyl, phenyl, or thienyl groups were rationally designed and prepared for the first time to investigate the OPV performances with an amorphous conjugated polymer donor (PCDTBT). The cis-1 products were found to be a mixture of two regioisomers, α-1-α and α-1-β as major and minor components, respectively. Among them, the cis-1 product with two ethyl groups (Et₂-cis-1-[70]PBC) showed the highest OPV performance, encouraging us to isolate its α-1-α isomer (Et₂-α-1-α-[70]PBC) by high-performance liquid chromatography. OPV devices based on Et₂-cis-1-[70]PBC and Et₂-α-1-α-[70]PBC with PCDTBT showed open-circuit voltages of 0.844 V and 0.864 V, respectively, which were higher than that of a device with typical [70]fullerene mono-adduct, [70]PCBM (0.831 V) with a lower LUMO level. However, the short-circuit current densities and resultant power conversion efficiencies of the devices with Et₂-cis-1-[70]PBC (9.24 mA cm⁻², 4.60%) and Et₂-α-1-α-[70]PBC (6.35 mA cm⁻², 3.25%) were lower than those of the device with [70]PCBM (10.8 mA cm⁻², 5.8%) due to their inferior charge collection efficiencies. The results obtained here reveal that cis-1 [70]fullerene bis-adducts do not guarantee better OPV performance and that further optimization of the substituent structures is necessary.

cis-1 Isomers of [70]fullerene bis-adducts were utilized as electron-acceptors in organic photovoltaic devices for the first time.