Theoretical investigations of oligo(phenylene ethylene) molecular wire: Effects from substituents and external electric field

Theoretical investigation of the substituent effect on the molecular wire, oligo(phenylene ethynylene) (OPE), has been performed with density functional calculations by considering the influence from the external electric field (EF). Compared to the electron-donating –NH₂ group, the electron-withdrawing –NO₂ group plays more important roles, such as increase of molecular dipole moment, decrease of LUMO–HOMO gap, and localization of LUMO. Both the geometric and electronic structures of the model molecular wires are sensitive to the external EF. In particularly, the –NO₂ substituted OPE yields obvious asymmetrical evolutions of both the frontier molecular orbital energies and their spatial distribution, which could be used to intuitively interpret the asymmetrical current–voltage behaviors of molecules.