Abnormally high oscillator strengths of the graphene nanoribbons electronic spectrum quantum chemistry calculations

Armchair-edged narrow graphene nanoribbons (GNRs) are modelled by semi-empirical Hartree–Fock based quantum chemistry method ZINDO/S-CI. Electronic transitions with abnormally high oscillator strengths of over 200 are found in long GNRs (over 150 hexagonal carbon rings). We argue that this high optical absorption is caused by the structure of molecular orbitals and by the system size, and not by the configuration interaction.