Quinoidal Azaacenes: 99 % Diradical Character

Quinoidal azaacenes with almost pure diradical character ( y= 0.95 to y= 0.99) were synthesized. All compounds exhibit paramagnetic behavior investigated by EPR and NMR spectroscopy, and SQUID measurements, revealing thermally populated triplet states with an extremely low‐energy gap Δ E ST′ of 0.58 to 1.0 kcal mol −1 . The species are persistent in solution (half‐life≈14–21 h) and in the solid state they are stable for weeks.