Dibenzoheptazethrene Isomers with Different Biradical Characters: An Exercise of Clar’s Aromatic Sextet Rule in Singlet Biradicaloids

<i>Clar’s aromatic sextet rule</i> has been widely used for the prediction of the reactivity and stability of polycyclic aromatic hydrocarbons with a <i>closed-shell</i> electronic configuration. Recent advances in <i>open-shell</i> biradicaloids have shown that the number of aromatic sextet rings plays an important role in determination of their ground states. In order to test the validity of this rule in singlet biradicaloids, the two soluble and stable dibenzoheptazethrene isomers <b>DBHZ1</b> and <b>DBHZ2</b> were prepared by different synthetic approaches and isolated in crystalline form. These two molecules have different numbers of aromatic sextet rings in their respective biradical resonance forms and thus are expected to exhibit varied singlet biradical character. This assumption was verified by different experimental methods, including nuclear magnetic resonance (NMR), electron spin resonance (ESR), superconducting quantum interference device (SQUID), steady-state and transient absorption spectroscopy (TA), and X-ray crystallographic analysis, assisted by unrestricted symmetry-broken density functional theory (DFT) calculations. <b>DBHZ2</b>, with more aromatic sextet rings in the biradical form, was demonstrated to possess greater biradical character than <b>DBHZ1</b>; as a result, <b>DBHZ2</b> exhibited an intense one-photon absorption (OPA) in the near-infrared region (λ_abs^max <i>=</i> 804 nm) and a large two-photon absorption (TPA) cross-section (σ⁽²⁾_max = 2800 GM at 1600 nm). This investigation together with previous studies indicates that Clar’s aromatic sextet rule can be further extended to the singlet biradicaloids to predict their ground states and singlet biradical characters