Cyano disubstituted tetrabenzoindeno[2,1- a ]fluorene: open-shell or closed-shell?

Organic diradicaloids have lately emerged as potential spintronic materials. We report the unprecedented synthesis of a near-IR absorbing indeno[2,1- a ]fluorene derivative that displays remarkably low LUMO (-4.15 eV) and a small HOMO-LUMO gap (0.85 eV). NMR/EPR studies indicated its open-shell diradical property, which was supported by DFT calculations while suggesting a 30% diradical character and a small singlet (S)-triplet (T) gap (-2.52 kcal mol -1 ). A large bond length alternation of the as-indacene core for its single-crystals indicated a quinoidal contribution with greater antiaromaticity, which is in line with the small diradical character despite showing a small S-T gap.