Electronic properties of acenes: Oligomer to polymer structure

Abstract

The conformations and electronic structures of long oligoacenes [${\mathrm{C}}_{2}{\mathrm{H}}_{2}{({\mathrm{C}}_{4}{\mathrm{H}}_{2})}_{n}$, $20\ensuremath{\leqslant}n\ensuremath{\leqslant}23$] and polyacene $[{({\mathrm{C}}_{8}{\mathrm{H}}_{4})}_{x}]$ were theoretically investigated through density functional theory adopting the hybrid B3LYP/6-31G(d) functional. The long oligoacenes present a cis conformation and solitonlike distortions along the chain. The defective regions having uniform bond lengths produce localized states on the top of the oligomer valence band. The spontaneous creation of bond alternation defects leads to high-spin magnetic ground states. The nonmagnetic state of polyacene presents a Peierls-distorted trans conformation which is lower in energy by a few meV per unit cell from the symmetric (nonalternating) state. The lowest-energy structure is predicted to present a cis pattern of bonds with alternation defects and a triplet state per unit cell.