All‐Fullerene Electron Donor–Acceptor Conjugates

Facilitating Electron Transfer by Resizing Cyclocarbon Acceptor from C18 to C16

Recent advances in synthetic methods, combined with tip-induced on-surface chemistry, have enabled the formation of numerous cyclocarbon molecules. Here, we investigate computationally the experimentally studied C16 and C18 molecules as well as their van der Waals (vdW) complexes with several typical donor and acceptor molecules. Our results demonstrate a remarkable electron-withdrawing ability of cyclocarbon molecules. The vdW complexes of C16 and C18 exhibit a thermodynamically favorable photoinduced electron transfer (ET) from the donor partner to the cyclocarbons that occurs on a picosecond time scale. The lower reorganization energy of C16 compared to C18 leads to a significant acceleration of the ET reactions.

Recent advances in supramolecular fullerene chemistry

Fullerene chemistry has come a long way since 1990, when the first bulk production of C60 was reported. In the past decade, progress in supramolecular chemistry has opened some remarkable and previously unexpected opportunities regarding the selective (multiple) functionalization of fullerenes and their (self)assembly into larger structures and frameworks. The purpose of this review article is to provide a comprehensive overview of these recent developments. We describe how macrocycles and cages that bind strongly to C60 can be used to block undesired addition patterns and thus allow the selective preparation of single-isomer addition products. We also discuss how the emergence of highly shape-persistent macrocycles has opened opportunities for the study of photoactive fullerene dyads and triads as well as the preparation of mechanically interlocked compounds. The preparation of two- or three-dimensional fullerene materials is another research area that has seen remarkable progress over the past few years. Due to the rapidly decreasing price of C60 and C70, we believe that these achievements will translate into all fields where fullerenes have traditionally (third-generation solar cells) and more recently been applied (catalysis, spintronics).

Fullerene-Functionalized Halogen-Bonding Heteroditopic Hosts for Ion-Pair Recognition

Despite their hydrophobic surfaces with localized π-holes and rigid well-defined architectures providing a scaffold for preorganizing binding motifs, fullerenes remain unexplored as potential supramolecular host platforms for the recognition of anions. Herein, we present the first example of the rational design, synthesis, and unique recognition properties of novel fullerene-functionalized halogen-bonding (XB) heteroditopic ion-pair receptors containing cation and anion binding domains spatially separated by C60. Fullerene spatial separation of the XB donors and the crown ether complexed potassium cation resulted in a rare example of an artificial receptor containing two anion binding sites with opposing preferences for hard and soft halides. Importantly, the incorporation of the C60 motif into the heteroditopic receptor structure has a significant effect on the halide binding selectivity, which is further amplified upon K+ cation binding. The potassium cation complexed fullerene-based receptors exhibit enhanced selectivity for the soft polarizable iodide ion which is assisted by the C60 scaffold preorganizing the potent XB-based binding domains, anion-π interactions, and the exceptional polarizability of the fullerene moiety, as evidenced from DFT calculations. These observations serve to highlight the unique properties of fullerene surfaces for proximal charged guest binding with potential applications in construction of selective molecular sensors and modulating the properties of solar cell devices.

Aromaticity controls the excited-state properties of host-guest complexes of nanohoops

π-Conjugated organic molecules have exciting applications as materials for batteries, solar cells, light-emitting diodes, etc. Among these systems, antiaromatic compounds are of particular interest because of their smaller HOMO-LUMO energy gap compared to aromatic compounds. A small HOMO-LUMO gap is expected to facilitate charge transfer in the systems. Here we report the ground and excited-state properties of two model nanohoops that are nitrogen-doped analogs of recently synthesized [4]cyclodibenzopentalenes - tetramers of benzene-fused aromatic 1,4-dihydropyrrolo[3,2-b]pyrrole ([4]DHPP) and antiaromatic pyrrolo[3,2-b]pyrrole ([4]PP). Their complexes with C60 fullerene show different behavior upon photoexcitation, depending on the degree of aromaticity. [4]DHPP acts as an electron donor, whereas [4]PP is a stronger electron acceptor than C60. The ultrafast charge separation combined with the slow charge recombination that we found for [4]PP⊃C60 indicates a long lifetime of the charge transfer state.

Structurally Defined Water-Soluble Metallofullerene Derivatives towards Biomedical Applications

Endohedral metallofullerenes (EMFs) are excellent carriers of rare-earth element (REE) ions in biomedical applications because they preclude the release of toxic metal ions. However, existing approaches to synthesize water-soluble EMF derivatives yield mixtures that inhibit precise drug design. Here we report the synthesis of metallobuckytrio (MBT), a three-buckyball system, as a modular platform to develop structurally defined water-soluble EMF derivatives with ligands by choice. Demonstrated with PEG ligands, the resulting water-soluble MBTs show superb biocompatibility. The Gd MBTs exhibit superior T₁ relaxivity than typical Gd complexes, potentially superseding current clinical MRI contrast agents in both safety and efficiency. The Lu MBTs generated reactive oxygen species upon light irradiation, showing promise as photosensitizers. With their modular nature to incorporate other ligands, we anticipate the MBT platform to open new paths towards bio-specific REE drugs.

Unexpected Formation of Pyrazoline-Fused Metallofullerenes from the Multicomponent Cascade Reaction of Sc3N@Ih-C80 with Tetrazines, Water, and Oxygen

The multicomponent cascade reaction of Sc₃N@I_h-C₈₀ with 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazines, water, and oxygen unexpectedly affords the pyrazoline-fused Sc₃N@I_h-C₈₀ derivatives. The obtained endohedral metallofullerene derivatives have been characterized by various spectral means, and their structures have been unambiguously established by single-crystal X-ray crystallography. A possible reaction mechanism via a complicated sequence of Diels-Alder reaction, retro Diels-Alder reaction, hydration, rearrangement, and dehydrogenation processes is proposed for the formation of pyrazoline-fused metallofullerenes.

Successive Diels-Alder Cycloadditions of Cyclopentadiene to [10]CPP⊃C60: A Computational Study

Fullerenes have potential applications in many fields. To reach their full potential, fullerenes have to be functionalized. One of the most common reactions used to functionalize fullerenes is the Diels–Alder cycloaddition. In this case, it is important to control the regioselectivity of the cycloaddition during the formation of higher adducts. In C₆₀, successive Diels–Alder cycloadditions lead to the T_h-symmetric hexakisadduct. In this work, we explore computationally using density functional theory (DFT) how the presence of a [10]cycloparaphenylene ring encapsulating C₆₀ ([10]CPP⊃C₆₀) affects the regioselectivity of multiple additions to C₆₀. Our results show that the presence of the [10]CPP ring changes the preferred sites of cycloaddition compared to free C₆₀ and leads to the formation of the tetrakisadduct. Somewhat surprisingly, our calculations predict formation of this particular tetrakisadduct to be more favored in [10]CPP⊃C₆₀ than in free C₆₀.

Predictive Catalysis in Olefin Metathesis with Ru-based Catalysts with Annulated C60 Fullerenes in the N-heterocyclic Carbenes

Predictive catalysis must be the tool that does not replace experiments, but acts as a selective agent, so that synthetic strategies of maximum profitability are used in the laboratory in a surgical way. Here, nanotechnology has been used in olefin metathesis from homogeneous Ru-NHC catalysts, specifically annulating a C₆₀ fullerene to the NHC ligand. Based on results with the C₆₀ in the backbone, a sterile change with respect to the catalysis of the metal center, an attempt has been made to bring C₆₀ closer to the metal, by attaching it to one of the two C-N bonds of the imidazole group of the SIMes (1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) ligand (reference NHC ligand of the 2^nd generation Grubbs catalysts) to increase the steric pressure of C₆₀ in the first sphere of reactivity of the metal. The DFT calculated thermodynamics and the kinetics of SIMes-derived systems show that they are efficient catalysts for olefin metathesis.

A Trapezoidal Octacyanoquinoid Acceptor Forms Solution and Surface Products by Antiparallel Shape Fitting with Conformational Dipole Momentum Switch

A new compound (1) formed by two antiparallelly disposed tetracyano thienoquinoidal units has been synthesized and studied by electrochemistry, UV/Vis-NIR, IR, EPR, and transient spectroscopy. Self-assembly of 1 on a Au(111) surface has been investigated by scanning tunneling microscopy. Experiments have been rationalized by quantum chemical calculations. 1 exhibits a unique charge distribution in its anionic form, with a gradient of charge yielding a neat molecular in-plane electric dipole momentum, which transforms out-of-plane after surface deposition due to twisted→folded conformational change and to partial charge transfer from Au(111). Intermolecular van der Waals interactions and antiparallel trapezoidal shape fitting lead to the formation of an optimal dense on Au(111) two-dimensional assembly of 1.

Evaluation of charge-transfer rates in fullerene-based donor-acceptor dyads with different density functional approximations

The shift towards renewable energy is one of the main challenges of this generation. Dye-sensitized solar cells (DSSCs), based on donor-acceptor architectures, can help in this transition as they present excellent photovoltaic efficiencies yet cheap and simple manufacturing. For molecular heterojunction DSSCs, donor-acceptor pairs are linked in a covalent manner, which facilitates their tailoring and rational design. Nevertheless, reliable computational characterization of charge transfer rate constants (kCT) is needed to speed this development process up. In this context, the performance of time-dependent density functional theory for the calculation of kCT values in donor-acceptor fullerene-based dyads has not been benchmarked yet. Herein, we present a detailed analysis on the performance of seven well-known density functional approximations (DFAs) for this type of system, focusing on several parameters such as the reorganization energies (λ), electronic couplings (VDA), and Gibbs energies (ΔG0CT), as well as the final rate constants. The amount of exact exchange at short range (SR) and long range (LR) electron-electron distances (and the transition from the SR to LR) turned out to be key for the success of the prediction. The tuning of these parameters improves significantly the performance of current DFAs.

Aromaticity Survival in Hydrofullerenes: The Case of C66 H4 with Its π-Aromatic Circuits

The isolated-pentagon rule (IPR) is a determining structural feature that accounts for hollow fullerene stabilization and properties related to C_n (n≥60) cages. The recent characterization of an unprecedented non-IPR hydrofullerene, C_2v C₆₆ H₄ , bearing two heptagons with adjacent fused-pentagon motifs, largely dismisses this feature. Herein, employing DFT calculations, the ¹³ C NMR spectroscopy and aromatic behavior of C_2v C₆₆ H₄ are explored. The results show the presence of three π-aromatic circuits at the bottom boat section of C₆₆ H₄ , indicating the unique features of this hydrofullerene in comparison to those of pristine C₆₀ . In addition, under specific orientations of the external field, certain π-aromatic circuits are enabled, resulting in a more aromatic fullerene than that of C₆₀ , but lower than that of the spherical aromatic C₆₀ ^6- fulleride. Notably, under a field aligned with the saturated carbon atoms, nonaromatic characteristics are exposed. This reveals that spherical-like cages can involve a complex magnetic response that heavily depends on the orientation of the applied field.

Bingel-Hirsch Addition of Diethyl Bromomalonate to Ion-Encapsulated Fullerenes M@C60 (M=Ø, Li+, Na+, K+, Mg2+, Ca2+, and Cl-)

In the last 30 years, fullerene-based materials have become popular building blocks for devices with a broad range of applications. Among fullerene derivatives, endohedral metallofullerenes (EMFs, M@C_x) have been widely studied owing to their unique properties and reactivity. For real applications, fullerenes and EMFs must be exohedrally functionalized. It has been shown that encapsulated metal cations facilitate the Diels-Alder reaction in fullerenes. Herein, the Bingel-Hirsch (BH) addition of ethyl bromomalonate over a series of ion-encapsulated M@C₆₀ (M=Ø, Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, and Cl^-; Ø@C₆₀ stands for C₆₀ without any endohedral metal) is quantum mechanically explored to analyze the effect of these ions on the BH addition. The results show that the incarcerated ion has a very important effect on the kinetics and thermodynamics of this reaction. Among the systems studied, K⁺@C₆₀ is the one that leads to the fastest BH reaction, whereas the slowest reaction is given by Cl^-@C₆₀.

Photoinduced electron transfer in nanotube⊃C70 inclusion complexes: phenine vs. nanographene nanotubes

In this work, we computationally study the photoinduced electron transfer in fullerene inclusion complexes of two phenine nanotubes pre-pNT⊃C70 and pNT⊃C70 and their nanographene analog [4]CHBC⊃C70. Charge separation is shown to efficiently occur in [4]CHBC⊃C70. In contrast, the electron transfer process between the host and guest units in the pre-pNT⊃C70 and pNT⊃C70 complexes is blocked by the structural changes incorporated in the nanographene framework.

Electron Transfer in a Li+-Doped Zn-Porphyrin-[10]CPP⊃Fullerene Junction and Charge-Separated Bands with Opposite Response to Polar Environments

Recently synthesized porphyrin-cycloparaphenylene (ZnP-[10]CPP) junction is a powerful platform to develop useful organic photovoltaic devices. In this work, we computationally study photoinduced electron transfer processes in the supramolecular complex ZnP-[10]CPP⊃C₆₀ and its Li⁺-doped derivative. The most striking finding is charge-separated (CS) bands in ZnP-[10]CPP⊃Li⁺@C₆₀ with opposite response to solvent polarity. Besides CS bands that demonstrate a bathochromic shift, there exist CS transitions showing a rarely observed hypsochromic shift. The rates of energy transfer, charge separation, and charge recombination in the supramolecular complexes are computed by using the semiclassical approach. These estimates suggest that the both types of CS states can be efficiently populated in polar media by decay of locally excited states.

Covalent Functionalization of Single-Walled Carbon Nanotubes by the Bingel Reaction for Building Charge-Transfer Complexes

Functionalization of nanotubes with donor and acceptor partners by the Bingel reaction leads to the formation of charge-transfer dyads, which can operate in organic photovoltaic devices. In this work, we theoretically examine the mechanism of the Bingel reaction for the (6,5)-chiral, (5,5)-armchair, and (9,0)-zigzag single-walled carbon nanotubes (SWCNTs), and demonstrate that the reaction is regioselective and takes place at the perpendicular position of (6,5)- and (5,5)-SWCNTs, and the oblique position of (9,0)-SWCNT. Further, we design computationally the donor-acceptor complexes based on (6,5)-SWCNT coupled with partners of different electronic nature. Analysis of their excited states reveals that efficient photoinduced charge transfer can be achieved in the complexes with π-extended analogue of tetrathiafulvalene (exTTF), zinc tetraphenylporphyrin (ZnTPP), and tetracyanoanthraquinodimethane (TCAQ). The solvent can significantly affect the population of the charge-separated states. Our calculations show that electron transfer (ET) occurs in the normal Marcus regime on a sub-nanosecond time scale in the complexes with exTTF and ZnTPP, and in the inverted Marcus regime on a picosecond time scale in the case of the TCAQ derivative. The ET rate is found to be not very sensitive to the degree of functionalization of the nanotube.

Light-Controlled Regioselective Synthesis of Fullerene Bis-Adducts

Multi-functionalization and isomer-purity of fullerenes are crucial tasks for the development of their chemistry in various fields. In both current main approaches-tether-directed covalent functionalization and supramolecular masks-the control of regioselectivity requires multi-step synthetic procedures to prepare the desired tether or mask. Herein, we describe light-responsive tethers, containing an azobenzene photoswitch and two malonate groups, in the double cyclopropanation of [60]fullerene. The formation of the bis-adducts and their spectroscopic and photochemical properties, as well as the effect of azobenzene photoswitching on the regiochemistry of the bis-addition, have been studied. The behavior of the tethers depends on the geometry of the connection between the photoactive core and the malonate moieties. One tether lead to a strikingly different adduct distribution for the E and Z isomers, indicating that the covalent bis-functionalization of C₆₀ can be controlled by light.

Electrochemical regioselective alkylations of a [60]fulleroindoline with bulky alkyl bromides

Electrochemical alkylations of a [60]fulleroindoline with different bulky alkyl bromides exhibit different reaction behaviors. The hydroalkylation and dialkylation of the electrochemically generated dianionic [60]fulleroindoline with bulky 2,4,6-tris(bromomethyl)mesitylene give rise to 1,2,3,16-adducts. In comparison, the hydroalkylation of the dianionic [60]fulleroindoline with bulkier diphenylbromomethane still affords a 1,2,3,16-adduct, while the corresponding dialkylation provides a sterically favoured 1,4,9,12-adduct, which is scarcely investigated, as the major product along with the isomeric 1,2,3,16-adduct as the minor product. The structures of these products have been determined by spectroscopic data and single-crystal X-ray diffraction analysis. A plausible reaction mechanism has been proposed to explain the formation of the observed products.

Alternative Access to Cyclopentafullerenes through the Reaction of [60]Fullerene with Aldehydes and Secondary Amines

A series of cyclopentafullerenes have been synthesized in high stereoselectivity by the thermal reaction of [60]fullerene with aldehydes and secondary amines. Both α,β-unsaturated aldehydes and saturated aldehydes can be utilized to synthesize cyclopentafullerenes as the cis isomers. The possible reaction mechanisms for the formation of cyclopentafullerenes are proposed on the basis of the experimental results.

Palladium-catalyzed synthesis of [60]fullerene-fused furochromenones and further electrochemical functionalization

The palladium-catalyzed heteroannulation of [60]fullerene with 4-hydroxycoumarins affords [60]fullerene-fused furochromenones, which can be further derivatized via an electrochemical method to synthesize 1,2,3,4-adducts.

Photoinduced electron transfer and unusual environmental effects in fullerene–Zn-porphyrin–BODIPY triads

Molecular arrays containing donor–acceptor sites and antenna molecules are promising candidates for organic photovoltaic devices.