Indolo[2,3-b]carbazoles with tunable ground states: How Clar's aromatic sextet determines the singlet biradical character

Abstract
Polycyclic hydrocarbons (PHs) with a singlet biradical ground state have recently attracted extensive interest in physical organic chemistry and materials science. Replacing the carbon radical center in the open-shell PHs with a more electronegative nitrogen atom is expected to result in the more stable aminyl radical. In this work, two kinetically blocked stable/persistent derivatives (1 and 2) of indolo[2,3-b]carbazole, an isoelectronic structure of the known indeno[2,1-b]fluorene, were synthesized and showed different ground states. Based on variable-temperature NMR/ESR measurements and density functional theory calculations, it was found that the indolo[2,3-b]carbazole derivative 1 is a persistent singlet biradical in the ground state with a moderate biradical character (y0 = 0.269) and a small singlet-triplet energy gap (ΔES-T ≅ -1.78 kcal mol-1), while the more extended dibenzo-indolo[2,3-b]carbazole 2 exhibits a quinoidal closed-shell ground state. The difference can be explained by considering the number of aromatic sextet rings gained from the closed-shell to the open-shell biradical resonance form, that is to say, two for compound 1 and one for compound 2, which determines their different biradical characters. The optical and electronic properties of 2 and the corresponding aromatic precursors were investigated by one-photon absorption, transient absorption and two-photon absorption (TPA) spectroscopies and electrochemistry. Amphoteric redox behaviour, a short excited lifetime and a moderate TPA cross section were observed for 2, which can be correlated to its antiaromaticity and small biradical character. Compound 2 showed high reactivity to protic solvents due to its extremely low-lying LUMO energy level. Unusual oxidative dimerization was also observed for the unblocked dihydro-indolo[2,3-b]carbazole precursors 6 and 11. Our studies shed light on the rational design of persistent aminyl biradicals with tunable properties in the future. This journal is

Citation
Luo, D., Lee, S., Zheng, B., Sun, Z., Zeng, W., Huang, K.-W., … Wu, J. (2014). Indolo[2,3-b]carbazoles with tunable ground states: how Clar’s aromatic sextet determines the singlet biradical character. Chem. Sci., 5(12), 4944–4952. doi:10.1039/c4sc01843e

Acknowledgements
J. W. acknowledges the financial support from MOE Tier 2 grants (MOE2011-T2-2-130, MOE2014-T2-1-080), MINDEF-NUS JPP Grant (12-02-05), and A*STAR JCO grant (1431AFG100). The work at Yonsei Univ. was supported by Mid-career Researcher Program (2010-0029668) and Global Research Laboratory (2013K1A1A2A02050183) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (Information and Communication Technologies) and Future Planning. K.-W. H. acknowledges the financial support from KAUST. We thank Dr Tan Geok-Kheng for crystallographic analysis.

Publisher
Royal Society of Chemistry (RSC)

Journal
Chem. Sci.

DOI
10.1039/c4sc01843e

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