Azulenoisoindigo: A Building Block for π-Functional Materials with Reversible Redox Behavior and Proton Responsiveness

Azulene, one of representative nonbenzenoid aromatic hydrocarbons, exhibits unique molecular structure and distinctive physical and chemical properties. Herein, an azulene-based isoindigo analogue, azulenoisoindigo (AzII) is designed and synthesized, which has a twisted molecular backbone and R/S-isomers in single crystals. Interestingly, AzII shows the characteristics of both isoindigo and azulene, such as completely reversible redox behavior and reversible proton responsiveness. UV-vis-NIR, ¹H NMR and electron paramagnetic resonance (EPR) measurements were carried out to get insights into the possible mechanism of the proton-responsive property of AzII. The results demonstrated that only one azulenyl moiety of molecule of AzII was protonated and deprotonated, and the protonated AzII can be further oxidized to form azulenium cation radicals.