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

Clar’s aromatic sextet
rule has been widely
used for the prediction of the reactivity and stability of polycyclic
aromatic hydrocarbons with a closed-shell electronic
configuration. Recent advances in open-shell 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 DBHZ1 and DBHZ2 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. DBHZ2, with more aromatic sextet rings in the biradical form,
was demonstrated to possess greater biradical character than DBHZ1; as a result, DBHZ2 exhibited an intense
one-photon absorption (OPA) in the near-infrared region (λabsmax = 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.