Independently Tuned Frontier Orbital Energy Levels of 1,3,4,6,9b-Pentaazaphenalene Derivatives by the Conjugation Effect

Herein reported are syntheses and the unique substituent effect in 1,3,4,6,9b-pentaazaphenalene (5AP) derivatives. We developed the new synthetic route via (N,N-pyridine-2,6-diyl)­bisamides, which enabled us to prepare a variety of substituted 5APs and to systematically investigate the substituent effects on the electronic structures of the multi-N-heterocyclic 5APs using UV–vis absorption spectroscopy, cyclic voltammetry, and density functional theory calculations. From the analyses, we found that the substituent effects regarding the electronic states of 5APs drastically varied depending on the substitution position. It was revealed that the conjugation effects from the substituents were specifically expressed either highest occupied molecular orbitals (HOMOs) or lowest unoccupied molecular orbitals (LUMOs) by altering the substitution position at the 5AP ring. As a result, small HOMO–LUMO gaps were accomplished only by elevating HOMOs or by lowering LUMOs. The intrinsically separated HOMO and LUMO of 5AP are the origin of this unique substituent effect. These results suggested that HOMOs and LUMOs can be independently tuned by the conjugation effects in 5APs. This selective tunability of either frontier orbital is a desired character for application of organic molecules as modern optoelectronic devices and advanced organic electronic materials.