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 ΔEST′ 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. Best of two worlds: Lateral fusion of azaacenes onto a quinoidal system furnishes stable diradicals with a radical character (y) of up to 0.99. All compounds exhibit paramagnetic behavior as determined by EPR and NMR spectroscopy and SQUID measurements, revealing thermally populated triplet states with an extremely low‐energy gap ΔEST of 0.58 to 1.0 kcal mol−1. The species are persistent in solution (half‐life≈14–21 h), and they are stable for weeks in the solid state.