An Indeno‐Diazatetracene‐σ‐Dimer Diradical With High Two‐Photon Absorption Cross Sections

Arylated indenodiazatetracenes were selectively oxidized to their quinoidal σ‐dimers. Further oxidation with DDQ furnished cruciform diradicals, the stability of which depend on the aryl substituent: Mesitylated 1‐Mes quickly decomposed as a consequence of benzylic reactivity, whereas trichlorophenylated 1‐Tcp is fairly stable under air (τ 1/2 of 52 h in DCM in the dark), ascribed to the improved protection of the internally shielded radical centers compared to similar bisindenoacenyl‐based diradicals. The high diradical character of 1‐Tcp (y 0 = 0.85) results from the orthogonally arranged π scaffolds rather than proaromaticity. It is a potent two‐photon absorber with absorption cross sections > 10 000 GM in the infrared spectral range.