Aerial Oxidation of Protonated Aromatic Amines. Isolation, X‑ray Structure, and Redox and Spectral Characteristics of N- Containing Dyes

This work reports the results of our investigation on
the aerial
oxidation of aromatic amines that are promoted by protic acid. While
primary aromatic amines produce substituted phenazines as major products, N-phenyl-o-phenylenediamine produces polycyclic
aromatic heterocycles like azaacene and secondary and tertiary amines
give exclusively the dyes containing a triphenylmethane moiety. Isolation
of the compounds and the effects of substitutions on the aromatic
rings have been investigated. In this context, plausible reaction
steps that are involved have been discussed. Single-crystal X-ray
structure analyses of the representative compounds are solved to authenticate
their formation. In almost every case, a high degree of delocalization
of electron was noted. The compounds have been characterized thoroughly
and show rich spectral properties. For example, the phenazine molecules
exhibited absorption peaks between 475 and 605 nm because of the charge-transfer
transition from the amine and tricyclopyrazine moiety. Their acidochromic
and solvatochromic behaviors, which are supported by theoretical calculations,
are investigated. The polycyclic azacene molecule exhibits strong
absorption in the visible region and fluoresces with high quantum
yield. The phenazine dyes undergo a quasi-reversible reduction at
a low cathodic potential that varies linearly as a function of Hammett’s
constant.