Semiempirical study on the absorption spectra of the coronene-like molecular models of graphene quantum dots

Polycyclic aromatic hydrocarbons of the general formula (C6nH6n)-H-2 (coronene family) were used as molecular models of graphene quantum dots (GQDs). Absorption spectra of the model compounds were calculated by ZINDO/S method. The S-o -> S-1 transition energy (E-1) was found to decrease with n as E-1 = 4.75 x n(-0.633) eV. This transition is forbidden in symmetric compounds but 'switches on' upon symmetry breaking. The energy of the first bright optical peak (E-br) was found to decrease with n as E-br = 6.31 x n(-0.6) eV. The data obtained corroborate the earlier finding that the size-independent optical properties of GQDs are determined by relatively small isolated sp(2) clusters separated by sp(3) (oxygen-contained) 'defects' rather than the whole (corrupted) graphene sheets; such nanoparticles actually are not quantum dots. GQDs of pure (without defects) graphene sheets with fully pi-conjugated sp(2) systems should exhibit size-dependent optical properties due to the quantum confinement effect. (C) 2018 Elsevier B.V. All rights reserved.