5,10-Dimesityldiindeno[1,2-a:2',1'-i]phenanthrene: a stable biradicaloid derived from Chichibabin's hydrocarbon.

A diindenophenanthrene biradicaloid, formally derived from Chichibabin's hydrocarbon, is obtained in a short, scalable synthesis. The present system is electron-rich and devoid of conjugated substituents, and still exhibits very good stability under ambient conditions. The introduction of the diindeno[1,2-a:2',1'-i] phenanthrene ring framework results in a singlet biradicaloid system with an easily accessible triplet state (ΔE S-T = -1.30 kcal mol-1) and a small electronic bandgap (1.39 V). The stability limits of the title hydrocarbon were explored systematically in the solid state, to reveal an unusual thermally initiated hydrogen-scrambling oligomerization process.