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.