2013 Volume 86 Issue 4 Pages 445-451
The excited electronic states and optical absorption spectra of coronene (C24H12), hexa-peri-hexabenzocoronene (HBC) (C42H18), and circumcoronene (C54H18) were studied using the symmetry-adapted cluster-configuration interaction (SAC-CI) method. For coronene and HBC, the SAC-CI calculations reproduced the experimental spectra well and predicted optically forbidden excited states. For HBC, the symmetry lowering enhanced the intensity of the S2 state that corresponds to the p-band, and the SAC-CI calculation predicted the existence of the second and third optically allowed states around the β-band region near 4.0 eV. For circumcoronene, the SAC-CI calculation predicted a strong absorption of the β-band in the visible light region. The mechanisms of energy splitting for the HOMO–LUMO transition were investigated. Electron correlation was the most important factor for the energy splitting between the lowest and the next-lowest states. Configuration interaction with single excitations (CIS) calculations could not correctly predict the relative energies of these states in coronene and circumcoronene. For HBC, on the other hand, the CIS calculation provided the same energy order as the SAC-CI calculation.
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