Fullerenes: multitask components in molecular machinery

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).

An Organic Molecular Nanobarrel that Hosts and Solubilizes C60

Organic cages have gained increasing attention in recent years as molecular hosts and porous materials. Among these, barrel-shaped cages or molecular nanobarrels are promising systems to encapsulate large hosts as they possess windows of the same size as their internal cavity. However, these systems have received little attention and remain practically unexplored despite their potential. Herein, we report the design and synthesis of a new trigonal prismatic organic nanobarrel with two large triangular windows with a diameter of 12.7 Å optimal for the encapsulation of C₆₀ . Remarkably, this organic nanobarrel shows a high affinity for C₆₀ in solvents in which C₆₀ is virtually insoluble, providing stable solutions of C₆₀ .

Direct Photocatalyzed Hydrogen Atom Transfer (HAT) for Aliphatic C-H Bonds Elaboration

Direct photocatalyzed hydrogen atom transfer (d-HAT) can be considered a method of choice for the elaboration of aliphatic C–H bonds. In this manifold, a photocatalyst (PC_HAT) exploits the energy of a photon to trigger the homolytic cleavage of such bonds in organic compounds. Selective C–H bond elaboration may be achieved by a judicious choice of the hydrogen abstractor (key parameters are the electronic character and the molecular structure), as well as reaction additives. Different are the classes of PCs_HAT available, including aromatic ketones, xanthene dyes (Eosin Y), polyoxometalates, uranyl salts, a metal-oxo porphyrin and a tris(amino)cyclopropenium radical dication. The processes (mainly C–C bond formation) are in most cases carried out under mild conditions with the help of visible light. The aim of this review is to offer a comprehensive survey of the synthetic applications of photocatalyzed d-HAT.

Mechanically Interlocked Nitrogenated Nanographenes

Herein, we report the synthesis of mechanically interlocked nitrogenated nanographenes. These systems have been obtained by clipping different tetralactam macrocycles around a 1.9 nm dumbbell-shaped nitrogenated nanographene. Thermal, optoelectronic, and electrochemical characterization of the different mechanically interlocked nanographenes evidence enhanced thermal and photochemical stability, and also absorption and emission properties that vary with the structure of the macrocycle.

π-Concave Hosts for Curved Carbon Nanomaterials

Carbon nanomaterials have been at the forefront of nanotechnology since its inception. At the heart of this research are the curved carbon nanomaterial families: fullerenes and carbon nanotubes. While both have incredible properties that have been capitalized upon in a wide variety of applications, there is an aspect that is not commonly exploited by nanoscientists and organic chemists alike: the interaction of curved carbon nanomaterials with curved organic small molecules. By taking advantage of these interactions, new avenues are opened for the use of fullerenes and carbon nanotubes.

Stable and Biocompatible Monodispersion of C60 in Water by Peptides

The lack of solubility in water and the formation of aggregates hamper many opportunities for technological exploitation of C₆₀. Here, different peptides were designed and synthesized with the aim of monomolecular dispersion of C₆₀ in water. Phenylalanines were used as recognizing moieties, able to interact with C₆₀ through π-π stacking, while a varying number of glycines were used as spacers, to connect the two terminal phenylalanines. The best performance in the dispersion of C₆₀ was obtained with the FGGGF peptidic nanotweezer at a pH of 12. A full characterization of this adduct was carried out. The peptides disperse C₆₀ in water with high efficiency, and the solutions are stable for months both in pure water and in physiological environments. NMR measurements demonstrated the ability of the peptides to interact with C₆₀. AFM measurements showed that C₆₀ is monodispersed. Electrospray ionization mass spectrometry determined a stoichiometry of C₆₀@(FGGGF)₄. Molecular dynamics simulations showed that the peptides assemble around the C₆₀ cage, like a candy in its paper wrapper, creating a supramolecular host able to accept C₆₀ in the cavity. The peptide-wrapped C₆₀ is fully biocompatible and the C₆₀ "dark toxicity" is eliminated. C₆₀@(FGGGF)₄ shows visible light-induced reactive oxygen species (ROS) generation at physiological saline concentrations and reduction of the HeLa cell viability in response to visible light irradiation.

Cyclodextrin-based ordered rotaxane-monolayers at gold surfaces

Complexation-to-deaggregation effect of cyclodextrin was applied to achieve ordered functional monolayers on a gold surface.

Diamide-based fullerosteroidal and disteroidal [2]rotaxanes: solvent-induced macrocycle translocation and/or unthreading

Two hydrogen bonded rotaxanes: a template directed synthesis, detailed characterization and shuttling/unthreading processes.

Unveiling the nature of supramolecular crown ether-C60 interactions

A series of exTTF-(crown ether)₂ receptors, designed to host C₆₀, has been prepared. The size of the crown ether and the nature of the heteroatoms have been systematically changed to fine tune the association constants. Electrochemical measurements and transient absorption spectroscopy assisted in corroborating charge transfer in the ground state and in the excited state, leading to the formation of radical ion pairs featuring lifetimes in the range from 12 to 21 ps. To rationalize the nature of the exTTF-(crown ether)₂·C₆₀ stabilizing interactions, theoretical calculations have been carried out, suggesting a synergetic interplay of donor-acceptor, π-π, n-π and CH···π interactions, which is the basis for the affinity of our novel receptors towards C₆₀.

Tuneable dynamics of a scandium nitride cluster inside an Ih-C80 cage

The internal clusters in metallofullerenes usually exhibit certain motion that is potentially usable in molecular gyroscopes and nano-machines. Based on (45)Sc NMR, the motion of the scandium nitride cluster within the C80 cage was investigated via varying the temperature and modifying the cage, and by changing the cluster size.

Fulleropyrrolidine molecular dumbbells act as multi-electron-acceptor triads. Spectroscopic, electrochemical, computational and morphological characterizations

Three novel fulleropyrrolidine dumbbells consisting three electron acceptor moieties joined by alkyl linker displaying tunable electrical and morphological properties were synthesized and characterized.

Fullerene-stoppered bistable rotaxanes

This chapter aims at giving an overview of the different types of fullerene-stoppered bistable rotaxanes, their switching mechanism and their potential applications with a special focus on fullerene-driven molecular shuttles.

Polarizability as a landmark property for fullerene chemistry and materials science

The review summarizes data on dipole polarizability of fullerenes and their derivatives, covering the most widespread classes of fullerene-containing molecules (fullerenes, fullerene exohedral derivatives, fullerene dimers, endofullerenes, fullerene ions, and derivatives with ionic bonds).

Fluorescence modulation in tribranched switchable [4]rotaxanes

Two novel tribranched [4]rotaxanes with a 1,3,5-triphenylene core and three rotaxane arms have been designed, synthesized, and characterized by (1)H and (13)C NMR spectroscopies and HR-ESI mass spectrometry. [4]Rotaxanes 1 and 2 each possess the same three-armed skeleton. Each arm incorporates two distinguishable binding sites for a dibenzo[24]crown-8 ring, namely a dibenzylammonium site and an N-methyltriazolium site, and is terminated by a 4-morpholino-naphthalimide fluorophore as a stopper. [4]Rotaxane 1 has three di-ferrocene-functionalized dibenzo[24]crown-8 rings whereas 2 has three simple dibenzo[24]crown-8 rings interlocked with the thread component. Uniform shuttling motions of the three macrocycles in both 1 and 2 can be driven by external acid-base stimuli, which were confirmed by (1)H NMR spectroscopy. However, [4]rotaxanes 1 and 2 show distinct modes of fluorescence modulation in response to external acid-base stimuli. [4]Rotaxane 1 exhibits a remarkable fluorescence decrease in response to the addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a base, which can displace the ferrocene-functionalized macrocycle from the dibenzylammonium station to the N-methyltriazolium station. In contrast, the fluorescence intensity of [4]rotaxane 2 showed an enhancement with the addition of DBU. Time-resolved fluorescence measurements have been performed. The different photoinduced electron-transfer processes responsible for the fluorescence changes in the two molecular systems are discussed. Topological structures of this kind have significant potential for the design and construction of large and complex assemblies with controllable functions.

Soccer goes BOXing: synthetic access to novel [6:0] hexakis[(bisoxazolinyl)methano]fullerenes

The syntheses of [6:0] hexakis[(bisoxazolinyl)methano]fullerenes are presented. Two derivatives could be directly obtained using conditions developed by the Sun group. For the remaining products, a two stage protocol had to be developed. All compounds we obtained in synthetically useful scales and were purified via column chromatography with standard achiral phase. These new fullerene adducts bear six metal-chelation sites which are aligned in the three orthogonal space directions and are disposed on a completely rigid scaffold. First experiments indicate that the generation of six-fold metal-complexes is possible with these structures. This makes them very appealing as ligands in asymmetric catalysis and as building blocks in higher supra-molecular assemblies.

Getting tubed: mechanical bond in endohedral derivatives of carbon nanotubes?

We present a brief overview of some of the most prominent examples of encapsulation of molecules inside carbon nanotubes. We then relate them to mechanically interlocked molecules, and in particular rotaxanes, by examining the most prominent features of the mechanical bond (topology, dynamic properties, and stability) and comparing them to those of endohedral derivatives of nanotubes. Our analysis shows that there is a surprisingly clear link between these two apparently disparate species.

Water-dispersible fullerene aggregates as a targeted anticancer prodrug with both chemo- and photodynamic therapeutic actions

Prodrug therapy is one strategy to deliver anticancer drugs in a less reactive manner to reduce nonspecific cytotoxicity. A new multifunctional anticancer prodrug system based on water-dispersible fullerene (C60) aggregates is introduced; this prodrug system demonstrates active targeting, pH-responsive chemotherapy, and photodynamic therapeutic (PDT) properties. Incorporating (via a cleavable bond) an anticancer drug, which is doxorubicin (DOX) in this study, and a targeting ligand (folic acid) onto fullerene while maintaining an overall size of approximately 135 nm produces a more specific anticancer prodrug. This prodrug can enter folate receptor (FR)-positive cancer cells and kill the cells via intracellular release of the active drug form. Moreover, the fullerene aggregate carrier exhibits PDT action; the cytotoxicity of the system towards FR-positive cancer cells is increased in response to light irradiation. As the DOX drug molecules are conjugated onto fullerene, the DOX fluorescence is significantly quenched by the strong electron-accepting capability of fullerene. The fluorescence restores upon release from fullerene, so this fluorescence quenching-restoring feature can be used to track intracellular DOX release. The combined effect of chemotherapy and PDT increases the therapeutic efficacy of the DOX-fullerene aggregate prodrug. This study provides useful insights into designing and improving the applicability of fullerene for other targeted cancer prodrug systems.

Non-destructive inhibition of metallofullerenol Gd@C(82)(OH)(22) on WW domain: implication on signal transduction pathway

Endohedral metallofullerenol Gd@C₈₂(OH)₂₂ has recently been shown to effectively inhibit tumor growth; however, its potential adverse bioeffects remain to be understood before its wider applications. Here, we present our study on the interaction between Gd@C₈₂(OH)₂₂ and WW domain, a representative protein domain involved in signaling and regulatory pathway, using all-atom explicit solvent molecular dynamics simulations. We find that Gd@C₈₂(OH)₂₂ has an intrinsic binding preference to the binding groove, particularly the key signature residues Y28 and W39. In its binding competition with the native ligand PRM, Gd@C₈₂(OH)₂₂ is shown to easily win the competition over PRM in occupying the active site, implying that Gd@C₈₂(OH)₂₂ can impose a potential inhibitory effect on the WW domain. Further analyses with binding free energy landscapes reveal that Gd@C₈₂(OH)₂₂ can not only directly block the binding site of the WW domain, but also effectively distract the PRM from its native binding pocket.