Fluoreno[2,3‑<i>b</i>]fluorene vs Indeno[2,1‑<i>b</i>]fluorene: Unusual Relationship between the Number of π Electrons and Excitation Energy in <i>m</i>‑Quinodimethane-Type Singlet Diradicaloids

The dimesityl derivative of fluoreno­[2,3-b]­fluorene (6b) was synthesized and its structure and physical properties were investigated to elucidate the effects of its enhanced open-shell character, which was predicted theoretically in comparison with the smaller congener indeno­[2,1-b]­fluorene (5b). All structural and physical properties are in accordance with the theoretical predictions and can be interpreted in terms of the resonance contributors. The most remarkable spectroscopic property is the larger excitation energy of 24π-electron hydrocarbon 6b than that of 20π-electron system 5b in their lowest energy absorption bands of electronic spectra, a trend that is contrary to the well-known feature for common, alternant aromatic hydrocarbons. The theoretical basis of this unusual behavior was elucidated on the basis of the balance between the diradical character, exchange integral, and HOMO–LUMO gap and was confirmed by a complete-active-space configuration–interaction method with two electrons in two orbitals for the corresponding parent hydrocarbons 5a and 6a.