Transition-metal-containing moieties can be combined with fullerene cages in a number of different ways either endohedrally, encased within the fullerene cage, or exohedrally, attached to the cage exterior.

Organometallic fullerene derivatives in which the transition metal is coordinated directly to the C–C bond of the exterior of the fullerene cage can be easily synthesized from transition-metal-containing precursors and pristine fullerenes. The range of metals that can be coordinated include the majority of first-, second-, and third-row transition metals. In these systems, strong electronic coupling between the fullerene cage and the metal center is observed in both the ground and excited states. However, the fullerene–metal distance in these systems and the geometry of the metal center cannot be adjusted, and hence, it is difficult to tune the properties of the resulting systems. Depending on the nature of the transition-metal center, these compounds exhibit interesting photophysical properties, and are capable of forming charge-separated states upon photoexcitation which are however short-lived (lifetimes in the picosecond range). This is attributed to strong electronic coupling between the fullerene cage and the metal center and therefore efficient charge recombination processes. On the contrary, complexes containing spatially separated ferrocene moieties with no ground-state electronic coupling exhibit competitive energy transfer processes as well as charge transfer, and decay via ferrocene triplet-state formation rather than through radical ion pairs.

Coordinating transition metals to a metal-binding group attached to the fullerene cage via covalent or noncovalent interactions is the most versatile approach and enables the whole of the transition block of metals to be attached to fullerenes. The geometry of the metal center can be precisely controlled by the choice of the metal-binding group, and the fullerene–metal distance can be adjusted by changing the size and shape of the linker. These systems do not exhibit any ground-state electronic interactions between the fullerene cage and the metal center; however, strong coupling is observed in the excited state, which results in the formation of long-lived charge-separated states upon photoexcitation and makes these systems ideal candidates for applications in photovoltaic devices. The photophysical properties of these compounds depend strongly on the nature of the transition-metal-containing moiety as well as the distance from the fullerene cage and the presence of additional chromophores or electron donors in the molecule. Bidentate and tridentate ruthenium and rhenium complexes attached to the fullerene cage exhibit mainly energy transfer processes upon photoexcitation with limited examples of charge-separated-state formation. On the contrary, tetradentate metalloporphyrin derivatives exhibit formation of charge-separated states upon photoexcitation with lifetimes in the picosecond range for complexes with a short fullerene–metalloporphyrin distance and in the nanosecond range for complexes with longer fullerene–metalloporphyrin distances. Incorporation of additional electron donors or chromophores results in long charge-separated-state lifetimes in the microsecond range.

Combining fullerene cages and transition-metal-containing molecules using van der Waals or electrostatic forces can be utilized to obtain various weakly bound cocrystallates and inclusion host–guest complexes. The stability of these systems is however limited as they are often found to be air sensitive and in the case of cocrystallates can only exist as solid-state materials. These compounds exhibit weak or no ground-state electronic interactions, with the properties being generally a superposition of the properties of the individual components. Some of these complexes show evidence of weak excited-state interactions resulting in fluorescence quenching of the metal-containing unit by the fullerene cage, but no evidence of charge transfer processes has been observed. This strategy does however lead to magnetic solid-state fullerene-containing structures which exhibit good photoconductivity and might find application in electronic devices.

To summarize, the most versatile approach to create fullerene–transition-metal arrays is attaching a metal-binding group to the exterior of the fullerene. It enables the formation of robust, stable, and soluble systems in which the fullerene-to-metal distance can be adjusted by the choice of a suitable linker, and the geometry of the metal center can be precisely controlled by the choice of the metal-binding group. However, only a limited number of metal-binding groups have been attached to the fullerene cage. These have mainly consisted of N-donor ligands such as pyridine, bipyridine, terpyridine, or porphyrin derivatives. Each of them can specifically bind a small number of transition metals. Currently, no universal fullerene-based metal receptor exists that can bind a wide range of different transition metals in an identical fashion. The most promising systems to date, porphyrin- and phthalocyanine-functionalized fullerenes, have shown potential in this area; however, they are difficult to synthesize as they require preparation and separation of asymmetric precursors.

The ability to systematically vary the metal center within fullerene–transition-metal arrays while retaining the same fullerene-based framework will allow the light-absorbing, redox, and consequently photoexcited-state properties of these systems to be investigated and optimized for use in photovoltaic devices. Additionally, the unique properties of the fullerene cage, such as the ability to reversibly accept electrons and a high affinity for a variety of carbon nanostructures, will allow the formation of well-ordered 2D and 1D arrays of spin-active metal–fullerene centers. These novel functional nanoscale materials have potential to find use in a wide range of applications, including molecular electronics and quantum computing.

In conclusion, fullerene–transition-metal complexes are an interesting class of compounds that have evolved significantly over the past decade. These compounds possess unique properties combining the advantages of both the carbon-based nanosized fullerene cage and transition metals, and as such, they can find applications in various fields, including catalysis, molecular electronics, and photovoltaics.