Fused-ring molecules based on B ← N and imide units with low energy levels

Abstract

The general strategy to design n-type organic semiconductors is to introduce electron-withdrawing moieties, such as sp²-hybridized N atoms, cyano groups, imide units, a boron–nitrogen coordination bond (B ← N), and halogen substituents, to π-conjugated skeletons. In this work, for the first time, we report n-type polycyclic aromatic hydrocarbons (PAHs) incorporated with both B ← N and imide units. By changing the positions of the two B ← N units, we synthesize a centrosymmetric PAH molecule and an axisymmetric PAH molecule. Both the molecules have a planar configuration, which is beneficial for close π-stacking in the solid state, and thus for charge transport. The centrosymmetric molecule has a net dipole moment of zero, while the axisymmetric molecule has a net dipole moment of 3.20 Debye with intramolecular charge transfer characteristics. The two isomeric molecules exhibit deep-lying LUMO and HOMO energy levels of −4.04 eV/−5.78 eV and −4.12 eV/−5.95 eV, respectively, which are much lower than those of n-type molecules containing only two B ← N units or two imide units reported in the literature studies.