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Abstract Non‐Kekulé quinoidal azaacences m‐A ( 1 a , b ) were synthesized and compared to their para ‐ and ortho ‐quinodimethane analogues. m‐A display high diradical characters ( 1 b : y 0 = 0.88) due to their meta ‐quinodimethane ( m ‐QDM) topology. Electron paramagnetic, nuclear magnetic resonance spectroscopies and supraquantum interference device measurements in combination with quantum‐chemical calculations revealed singlet ground states for m ‐ A with singlet‐triplet gaps ΔE ST (0.13–0.25 kcal mol −1 ) and thermally populated triplet states. These non‐Kekulé structures are over all void of zwitterionic character and possess record high two‐photon absorption cross sections over a broad spectral range in the near‐infrared.
Fuchs et al. (Tue,) studied this question.