Modulation of Open-Shell Diradical Character of Imidazole Quinoidal Systems

We report imidazole-linked quinoidal constitutional isomers with aryl substituents connected at the 2,2'- (<b>I</b>_<b>1a</b>) and 4,4'-positions (<b>I</b>_<b>2a</b>-<b>I</b>_<b>2c</b>), enabling systematic modulation of diradical character through positional isomerism. Despite identical molecular compositions, <b>I</b>_<b>1a</b> adopts a weak open-shell diradical character (diradical character, <i>y</i>₀ ≈ 0.2), whereas the 4,4'-linked isomers <b>I</b>_<b>2a</b>-<b>I</b>_<b>2c</b> exhibit pronounced open-shell diradical character (<i>y</i>₀ ≥ 0.5). This difference arises from enhanced π-electron delocalization between the quinoidal core and aryl substituents in the 4,4'-connected framework. Variable-temperature ¹H NMR, EPR spectroscopy, and DFT calculations confirmed the open-shell nature. All isomers exhibit excellent solid-state stability under ambient conditions for over one year. These findings establish positional isomerism as an effective strategy for tuning and stabilizing open-shell diradical systems.