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Title: Indolo [2,3-b] carbazoles with tunable ground states : how Clar's aromatic sextet determines the singlet biradical character
Authors: Luo, Ding
Lee, Sangsu
Zheng, Bin
Sun, Zhe
Zeng, Wangdong
Huang, Kuo-Wei
Furukawa, Ko
Kim, Dongho
Webster, Richard D.
Wu, Jishan
Keywords: Ground State
Polycyclic Aromatic Hydrocarbons
Issue Date: 2014
Source: 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. Chemical Science, 5(12), 4944-4952. doi:10.1039/C4SC01843E
Series/Report no.: Chemical Science
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.
ISSN: 2041-6520
DOI: 10.1039/C4SC01843E
Fulltext Permission: none
Fulltext Availability: No Fulltext
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