We report imidazole-linked quinoidal constitutional isomers with aryl substituents connected at the 2,2'- (I1a) and 4,4'-positions (I2a-I2c), enabling systematic modulation of diradical character through positional isomerism. Despite identical molecular compositions, I1a adopts a weak open-shell diradical character (diradical character, y0 ≈ 0.2), whereas the 4,4'-linked isomers I2a-I2c exhibit pronounced open-shell diradical character (y0 ≥ 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.
Venkatareddy et al. (Mon,) studied this question.
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