Azacalixquinarenes: From Canonical to (Poly-)Zwitterionic Macrocycles

Azacalixquinarenes,
a new family of macrocycles composed of diaminobenzoquinone
diimine units linked by dinitrobenzene rings, are synthesized by selective
oxidation of the parent azacalixarenes. Crystallographic analyses
of two compounds demonstrated the presence of canonical (uncharged)
and zwitterionic quinones within a single structure. The electron-withdrawing
nature of the dinitrobenzene moieties can trigger the intramolecular
H-transfer that generates zwitterionic ground-state quinones. The
nature of the N-substituents and the polarity of
the solvent have a crucial impact on the equilibrium between the canonical
and zwitterionic forms that present distinct optical and electrochemical
properties. Thus, within [4]- and [6]-membered macrocycles, poly-zwitterionic
structures can be reached, as demonstrated experimentally and theoretically
using first-principle approaches.