Theoretical investigations of the substituent effect on the electronic and charge transport properties of butterfly molecules

Pyrene is one kind of interesting polycyclic aromatic hydrocarbon with high charge carrier mobility and chemical stability. The electronic and charge transport properties of six butterfly shaped tetraaryl pyrenes with different electron-donating and electron-withdrawing substituents are explored based on density functional theory. The effects of substituents on the molecular structure, reorganization energy, molecular orbitals, ionization energies, electron affinities, crystal packing, transfer integrals, and charge carrier mobilities are analyzed in detail to clarify the structure-property relationships of the studied molecules. The different crystal packing motifs and diverse intermolecular interactions of the investigated molecules lead to obviously different transfer integrals when different substituents are introduced into the central π-system. The reorganization energies and the frontier molecular orbitals are also modulated by the substituents. These factors result in drastically different charge carrier mobilities of molecules 1 to 6. The hole and electron mobilities of molecule 1 are on the same order of magnitude (μ_h,ave./μ_e,ave. is equal to 2.3) due to the balanced transfer integrals, which could be used as an ambipolar semiconductor. Molecule 6 with the thienyl moiety possesses the highest hole mobility due to the largest transfer integrals of holes originating from the strong intermolecular interactions, and may be used as a p-type semiconductor. While the introduced difluorophenyl groups of molecule 4 lead to a lower LUMO level and effective transfer integrals of electrons, achieving an electron mobility of up to 0.01 cm² V^-1 s^-1, and showing promising application as an n-type semiconductor.