Photoinduced Charge Separation in the Carbon Nano-Onion C60@C240

The double-shell fullerene C60@C240 formed by inclusion of C60 into C240 is the smallest stable carbon nano-onion. In this article, we analyze in detail the character of the excited states of C60@C240 in terms of exciton localization and charge transfer between the inner and outer shells. The unique structure of the buckyonion leads to a large electrostatic stabilization of charge-separated (CS) states in the C60@C240. As a result, the CS states C60(+)@C240(-) lie in the same region of the electronic spectrum (2.4-2.6 eV) as strongly absorbing locally excited states and, therefore, can be effectively populated. The CS states C60(-)@C240(+) are found to be 0.5 eV higher in energy than the CS states C60(+)@C240(-). Unlike the situation observed in donor-acceptor systems, the energies of the CS states in C60@C240 do not practically depend on the environment polarity. This leads to exceptionally small reorganization energies for electron transfer between the shells. Electronic couplings for photoinduced charge-separation and charge-recombination processes are calculated. The absolute rate of the formation of the CS state C60(+)@C240(-) is estimated at ∼4 ps(-1). The electronic features found in C60@C240 are likely to be shared by other carbon nano-onions.