Understanding the nature of quinoidal and zwitterionic states in carbazole-based diradicals.

We report two carbazole-based diradicals, out of which the m-isomer shows a large diradical character y(0) (0.89) and a small singlet-triplet energy gap Delta ES-T (-0.98 kcal mol(-1)), whereas the p-isomer exhibits smaller y(0) (0.79) but a much larger Delta ES-T (-6.16 kcal mol(-1)). DFT calculations reveal that this tendency is also suitable for nitrogen and carbon-centered diradicals.