NDLI: Indolo[2,3-<i>b</i>]carbazoles with tunable ground states: how Clar's aromatic sextet determines the singlet biradical character

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

Content Provider	Royal Society of Chemistry (RSC)
Author	Furukawa, Ko Zheng, Bin Wu, Jishan Kim, Dongho Huang, Kuo-Wei Sun, Zhe Zeng, Wangdong Luo, Ding Lee, Sangsu Webster, Richard D.
Copyright Year	2014
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.
Starting Page	4944
Ending Page	4952
Page Count	9
File Format	HTM / HTML PDF
ISSN	20416520
Volume Number	5
Issue Number	12
Journal	Chemical Science
DOI	10.1039/c4sc01843e
Language	English
Publisher	Royal Society of Chemistry
Access Restriction	Open
Subject Keyword	Fluorene Non-Kekulé molecule HOMO\/LUMO Radical centrism Redox Materials science Nuclear magnetic resonance spectroscopy Electronegativity Nitrogen Carbon Carbazole Isoelectronicity Electrochemistry Density functional theory Two-photon absorption Nuclear magnetic resonance Physical organic chemistry
Content Type	Text
Resource Type	Article
Subject	Chemistry

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

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

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