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

Abstract

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.