New strategy for lowering the energy levels of one frontier molecular orbital in conjugated molecules and polymers based on Aza-substitution at the isolated HOMO or LUMO

A new method to control the electronic states of conjugated polymers without extending the pi-conjugated system was developed. In the distribution of frontier molecular orbitals (FMOs), if one FMO was distributed on skeletal carbon, we defined each FMO as the isolated highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO). By replacing carbon with nitrogen (aza-substitution) at the isolated FMO, we found that the energy level of one FMO can be efficiently and selectively lowered. Based on this principle, we obtained blue and near-infrared emitting materials. Moreover, aggregation-induced emission (AIE) and/or crystallization-induced emission (CIE)-active complexes with longer wavelength emission were obtained through similar protocols. Furthermore, solid-state luminescent polymers were obtained. Various types of conjugated polymers with near-infrared (NIR) luminescent properties were also developed through selectively lowering the LUMO level by aza-substitution. In this review, we explain that aza-substitution in isolated FMOs is a new method for controlling the electronic state of conjugated polymers.