Non-Kekulé quinoidal azaacences m-A (1a,b) were synthesized and compared to their para- and ortho-quinodimethane analogues. m-A display high diradical characters (1b: y0 = 0.88) due to their meta-quinodimethane (m-QDM) topology. Electron paramagnetic (EPR), nuclear magnetic resonance (NMR) spectroscopies and supraquantum interference device (SQUID) measurements in combination with quantum-chemical calculations revealed singlet ground states for m-A with singlet-triplet gaps ΔEST (0.13-0.25 kcal mol-1) and thermally populated triplet states. These non-Kekulé structures are void of zwitterionic character and possess record high two-photon absorption cross sections over a broad spectral range in the near-infrared.
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Fuchs et al. (2024) studied this question.
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