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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
ABSTRACT - Internet Electron. J. Mol. Des. February 2003, Volume 2, Number 2, 86-95

Partial Structures of C60 Responsible for Its Lowest Electronic Absorption Band: Corannulene or Triphenylene?
Tai-ichi Shibuya, Susumu Narita, Yasushi Nomura, and Tetsuo Morikawa
Internet Electron. J. Mol. Des. 2003, 2, 86-95

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Abstract:
Corannulene is widely accepted as a particularly important fragment of the fullerene C60. We consider that corannulene alone cannot explain the origin of the lowest-allowed electronic absorption band of C60. Here we clarify what else is needed other than corannulene, and propose triphenylene as a more important partial structure of C60 for the electronic absorption. Full SECI calculations are made in the semiempirical CNDO/S approximation. We first observe how the degenerated MO-energy levels of C60 split as the molecular symmetry Ih breaks into two different elongated shapes, D5d with its C5-axis through a pair of corannulene parts and D3d with its C3-axis through a pair of triphenylene parts. Components of the molecular orbitals of C60 that play principal roles in the lowest-allowed electronic absorption are all found in the vicinity of the HOMO-LUMO gap of triphenylene, whereas some part is missing in corannulene. The lowest-allowed electronic transitions of larger triphenylene derivatives tend to converge to the lowest absorption band of C60. A pair of triphenylene molecules separated by the diameter of C60 in the staggered conformation (D3d) makes a good model system to explain the origin of the lowest-allowed absorption band of C60. As the C60 molecule has 20 equivalent 6-membered rings, there are ten choices for such a pair. On the other hand, a pair of corannulene molecules in the staggered conformation (D5d) lacks one component of the HOMO of C60, which we have found in the encircled cis-polyene of ten ethylene units at the equatorial belt of C60.

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