New Idea for Narrowing an Energy Gap by Selective Perturbation of One Frontier Molecular Orbital

In this manuscript, we will illustrate a new idea for narrowing energy gaps between frontier molecular orbitals (FMOs) by selectively perturbing the levels of highest occupied molecular orbitals (HOMOs) and/or lowest unoccupied molecular orbitals (LUMOs). Initially, the basic concept of the isolated FMOs is explained by employing pentaazaphenalene (5AP) derivatives. It was found that electronic structures of the isolated LUMO of 5AP can be preserved even when incorporated into polymer chains if the connecting points are separated from the isolated LUMO. The mechanism and their unique electronic properties are summarized. Next, conversely, isolated HOMO and LUMO can be perturbed by conjugation effects independently. On the basis of this fact, near infrared (NIR)-absorbing molecules can be obtained by selectively elevating HOMO and lowering LUMO energy levels. We also mention strategies for enhancing luminescent properties of 5AP derivatives. Finally, we demonstrate that the isolated LUMO can be found in commodity luminescent dyes, such as boron dipyrromethene (BODIPY). By selectively lowering the LUMO energy level through the aza-substitution at the skeletal carbon where the isolated LUMO is distributed, NIR-emissive polymers can be obtained. Versatility of the isolated FMOs for obtaining optoelectronic organic materials is explained in this review. Near-infrared (NIR)-absorbing and emissive materials have attracted attention as a platform for constructing advanced optoelectronic materials. In this review, we will illustrate the new idea for narrowing energy gaps between frontier molecular orbitals (FMOs), which are essential electronic structures for constructing NIR materials, by selectively perturbing the levels of one of FMOs.