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

Abstract

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 (λabs(max) = 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.

Modern zethrene chemistry

“…21 To further elaborate the fundamental structure-diradical character-physical property relationships in open-shell singlet diradicaloids, two laterally extended dibenzozethrene isomers, 1,2:9,10-dibenzoheptazethrene (isomer 1) and 5,6:13,14-dibenzoheptazethrene (isomer 2) were investigated and their respective derivatives 39 and 40 were synthesized (Scheme 7). 22 Compound 39 was prepared from the dialdehyde intermediate 41, while compound 40 was synthesized from the corresponding quinone. The ground-state and diradical characters of these molecules were carefully studied by both experimental and theoretical methods.…”

Recent Advances in Organic Radicals and Their Magnetism

“…Wu et al have subsequently reported the two stable dibenzoheptazethrene derivatives 143 and 144 (Chart 9) and their biradical characters [158]. The biradical characters were examined by X-ray crystallographic analysis, spectroscopic measurements, and theoretical studies.…”

Zethrene and Dibenzozethrene: Masked Biradical Molecules?

“…All substituted compounds, 1 a, 8 a, and 8 b, are ESR inert, even at 300 K, thus manifesting their closed-shell properties. Similar to 4, [3] 5, [4] and quarteranthene, [14] both 1 c and 8 c exhibited featureless broad signals centered at g = 2.0026 and 2.0005, respectively ( Figure 2 and the Supporting Information), and DM s =AE 2 transitions were not observed. These spectral features were caused by the weak spin-spin dipole interaction within the molecules, the long-distance separation between two radicals, and the extended spin-delocalization.…”

“…and their pextended derivatives, including the substituted heptazethrenes 2 [2] /3, [3] octazethrenes 4, [3] and dibenzoheptazethrenes 5 and 6, [4] which have remarkable chemical stability to contribute a non-KekulØ structure, bear two formal radical centers, and hence are well-suited to investigations of their biradical properties. [5] The compounds 2, 4, and 5 have been experimentally determined to have a significant open-shell biradical character, and exhibit unique optical and magnetic properties.…”