Stabilization of excited states of fullerenes

Abstract

Exohedral derivatives of С60 (Ih), С76(D2), С80(D5d) и С84 (D3d) fullerenes containing an all-trans-[N]annulene ring (N = 18, 20, 22), encircling the molecules along the equator are studied at density functional theory (DFT/B3LYP) within the 6–31 G(d) basis set. We found that the highest occupied molecular orbital (HOMO) and the lowest unoccupied (LUMO) molecular orbital of fullerenes, which comprise aromatic all-trans-[N]annulene rings (N follows Hückel's rule) are doubly degenerate. In addition, the electron density of frontier orbitals is almost completely localized on the atoms of the all-trans-[N]annulene rings. DFT calculations revealed that in the ground state of the considered molecules, the Mulliken electron density on the all-trans-[N]annulene atoms does not correspond to the Hückel aromaticity rule. The absorption of light is mostly associated with transitions of an electron from the highest occupied orbital into the lowest unoccupied orbital. The photoinduced electronic transitions lead to the formation of a conjugated p-electrons system in the all-trans-[N]annulene cycle, which, according to the structural criteria and the number of p-electrons, obeys to the Hückel's rule “4n + 2". This stabilizing effect, leading to the aromatic character of all-trans-[N]annulene, causes an increase in the lifetime of charge-separated excited states.