Azaacene Diradicals Based on Non-Kekulé Meta-Quinodimethane with Large Two-Photon Cross-Sections in the Infrared Spectral Region

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: y0 = 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 ΔEST (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.