Abstract
The first isoindigo (bi)radicals were obtained by proton coupled oxidation of their 4‐hydroxyaryl substituted precursors. Optical and magnetic spectroscopic studies revealed a singlet open‐shell biradicaloid electronic ground state for the bisphenoxyl‐isoindigo (<s2>=1.20) with a small singlet–triplet energy gap of 0.065 eV and a large biradical character of y=0.79 that was corroborated by temperature‐dependent EPR spectroscopy and quantum chemical calculations. The concept of kinetic blocking of the radical centers and delocalization of spin density into the electron‐withdrawing chromophore core of isoindigo offers an entry into a new class of exceptionally stable open‐shell functional materials based on organic colorants.
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