Dibenzoheptazethrene isomers with different biradical characters: An exercise of clar's aromatic sextet rule in singlet biradicaloids

Zhe Sun, Sangsu Lee, Kyu Hyung Park, Xiaojian Zhu, Wenhua Zhang, Bin Zheng, Pan Hu, Zebing Zeng, Soumyajit Das, Yuan Li, Chunyan Chi, Run Wei Li, Kuo Wei Huang, Jun Ding, Dongho Kim, Jishan Wu

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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.

Original languageEnglish
Pages (from-to)18229-18236
Number of pages8
JournalJournal of the American Chemical Society
Volume135
Issue number48
DOIs
Publication statusPublished - 2013 Dec 4

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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    Sun, Z., Lee, S., Park, K. H., Zhu, X., Zhang, W., Zheng, B., Hu, P., Zeng, Z., Das, S., Li, Y., Chi, C., Li, R. W., Huang, K. W., Ding, J., Kim, D., & Wu, J. (2013). Dibenzoheptazethrene isomers with different biradical characters: An exercise of clar's aromatic sextet rule in singlet biradicaloids. Journal of the American Chemical Society, 135(48), 18229-18236. https://doi.org/10.1021/ja410279j