An exohedral fullerene is a derivative of a fullerene molecule where one or more atoms, ions, or molecules are covalently or non-covalently attached to the exterior surface of the fullerene cage. This term specifically distinguishes such derivatives from endohedral fullerenes, where guest atoms or molecules are encapsulated inside the fullerene cage, and from pristine, unfunctionalized fullerenes.
Structure and Bonding
The attachment of chemical species to the exterior of a fullerene typically occurs through various chemical reactions at the carbon atoms of the fullerene cage. This functionalization changes the hybridization of the carbon atoms involved from sp² to sp³, leading to modifications in the fullerene's electronic, geometric, and steric properties. Common types of exohedral functionalization include:
- Addition reactions: Atoms or groups (e.g., hydrogen, halogens, alkyl groups, aryl groups) can add across double bonds on the fullerene surface. Examples include fullerene hydrides (e.g., C₆₀H₃₆) and halogenated fullerenes (e.g., C₆₀F₁₈).
- Cycloaddition reactions: Reactions like the Diels-Alder reaction or 1,3-dipolar cycloadditions (e.g., Prato reaction with azomethine ylides) form new cyclic structures fused to the fullerene cage.
- Organometallic complexes: Metal atoms or complexes can bind to the fullerene surface, often through η²-coordination to a C=C bond, forming stable complexes.
- Polymerization: Fullerenes can be incorporated into polymer chains, either directly as monomers or as side groups, forming fullerene-containing polymers.
- Non-covalent interactions: While not forming new covalent bonds to the cage, supramolecular interactions (like π-π stacking, host-guest interactions, or van der Waals forces) also result in species associating with the exterior surface, affecting properties like solubility and assembly.
The functionalization often reduces the high symmetry of the pristine fullerene cage, leading to a range of isomers with distinct properties. The degree of functionalization can also vary, from mono-adducts to highly substituted derivatives.
Synthesis
Exohedral fullerenes are synthesized through a wide array of organic and inorganic chemical modification techniques. Direct reactions of fullerenes (such as C₆₀ or C₇₀) with appropriate reagents in solution are common. Key synthetic methodologies include:
- Radical additions: Using free radical initiators to add various groups to the fullerene surface.
- Nucleophilic additions: Reaction with nucleophiles, often followed by electrophilic quench.
- Electrophilic additions: Though less common due to the electron-rich nature of fullerenes.
- Transition metal catalysis: Employing metal catalysts to facilitate specific functionalization reactions.
- Click chemistry: Highly efficient and selective reactions used to attach complex moieties to pre-functionalized fullerenes.
Controlling the site-selectivity and degree of functionalization can be challenging, often requiring careful optimization of reaction conditions, and frequently leading to mixtures of regioisomers and compounds with varying numbers of addends.
Applications
The ability to tune the physical, chemical, and biological properties of fullerenes through exohedral functionalization makes them highly versatile for a diverse range of applications:
- Materials Science: As building blocks for novel polymers, hybrid materials, and composites with enhanced mechanical, electronic, or optical properties.
- Electronics: In organic photovoltaics (OPVs) as electron acceptors (e.g., PCBM derivatives), in organic field-effect transistors (OFETs), and other optoelectronic devices, due to their tunable electron affinity and energy levels.
- Biomedicine: For drug delivery (improving solubility, biocompatibility, and targeting of therapeutic agents), medical imaging, and photodynamic therapy (due to their ability to generate reactive oxygen species like singlet oxygen).
- Catalysis: As heterogeneous catalysts, catalyst supports, or components of molecular catalysts.
- Sensors: In chemical and biological sensing applications, leveraging changes in their electronic or optical properties upon interaction with analytes.
- Hydrogen Storage: Certain fullerene derivatives have been investigated for their potential in hydrogen storage applications.
The vast chemical space accessible through exohedral functionalization continues to drive research into new fullerene derivatives with tailored properties for emerging technologies.