Novel “Potentially Antiaromatic”, Acidichromic Quinonediimines with Tunable Delocalization of Their 6π-Electron Subunits

We present a novel family of “potentially antiaromatic” alkyl-substituted p-benzoquinonediimine
pH-dependent chromophores. It appears from the structural data that these overall 12π-electron molecules
should be better considered as constituted by two chemically connected but electronically not conjugated
6π-electron subunits. Molecule 5 appears to be the first example of two separated, conjugated, and localized
6π-electron systems that can be tuned by reversible protonation to become delocalized. The mono- and
diprotonated derivatives have been characterized by spectroscopic methods and X-ray diffraction. These
systems develop supramolecular interactions in the solid state that clearly reflect the degree of protonation
and depend on the nature of the counterion. These compounds constitute new chromophores for which
the color can be tuned depending on the degree of protonation, going in solution from yellow for 5 to red
for 5·HCl and blue for 5·2HCl. Theoretical calculations have provided a deeper insight into the electronic
structure of these molecules and allowed an assignment of the experimental UV−vis spectra. The visible
and near-UV spectrum of the neutral and protonated benzoquinonediimines can be classically assigned
from the coupling of two 6π-electron polymethine units. TD-DFT calculations confirm the observed red
shift of the two lowest π → π* transitions of the benzoquinonediimines upon protonation and relate it to the
moderate energy lowering of the HOMO → LUMO transition induced by the delocalization of the polymethine
π system.