Pyrrole-Fused Azacoronenes with Various Redox States

Polycyclic aromatic hydrocarbons represented by hexabenzocoronenes (HBC) have attracted a great deal of attention from materials science. Among them, nitrogen-containing π-system have potential advantages since such heteroatoms can tune the electronic nature without modifying the structure. Given these expectations, we have reported a novel nitrogen-containing π-system, i.e., hexapyrrolohexaazacoronene (HPHAC). To investigate the effect of the interior nitrogen atoms, we have recently synthesized a series of pyrrole-fused azacoronenes, where some pyrrole rings of HPHAC were substituted with phenyl rings. In all cases, they showed multiple oxidation states due to the interior nitrogen atoms and various resonance forms.

The synthesis was successively performed by S_NAr reaction with fluoroiodobenzenes, Suzuki-Miyaura coupling, and oxidative Scholl reaction. As characteristic features, cyclic voltammograms exhibit up to three reversible oxidation peaks having similar first two oxidation potentials and differences between second and third oxidation potentials become smaller in order of decreasing the number of pyrroles. Chemical oxidations by monitoring the absorption spectra revealed that these compounds had different electronic states in their oxidation states. Recently, we have further designed and synthesized a new family that has two carbonyl groups directly introduced to the π-system. It is expected that the resonance forms observed for the oxidation states of previous azacoronenes become possible in the neutral state. As the result, new ones exhibit significantly different physical properties.

In the presentation, synthesis and optical and electronic properties of azacoronenes will be discussed.