Effect of fluorination on the electronic structure and optical excitations of pi-conjugated molecules

Fluorination of pi-conjugated organic molecules is a strategy to obtain possible n-type conducting and air-stable materials due to the lowering of the frontier molecular orbitals (MOs) by the high electronegativity of fluorine. Nevertheless, the resulting optical gaps may be widened or narrowed, depending on the molecular backbone and/or the number and position of the fluorine atoms. The authors have performed time-dependent density functional theory calculations to address the subtle influence of fluorine substitution on the absolute MO energies and the subsequent impact on the optical transitions in homologous conjugated oligomers based on thiophene and acene units.