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

被引：35

作者：

Luo, Ding ^{[1
]}

Lee, Sangsu ^{[2
,3
]}

Zheng, Bin ^{[4
,5
]}

Sun, Zhe ^{[1
]}

Zeng, Wangdong ^{[1
]}

Huang, Kuo-Wei ^{[4
,5
]}

Furukawa, Ko ^{[6
]}

Kim, Dongho ^{[2
,3
]}

Webster, Richard D. ^{[4
,7
]}

Wu, Jishan ^{[1
,8
]}

机构：

[1] Natl Univ Singapore, Dept Chem, Singapore 117543, Singapore

[2] Yonsei Univ, Spectroscopy Lab Funct Elect Syst, Seoul 120749, South Korea

[3] Yonsei Univ, Dept Chem, Seoul 120749, South Korea

[4] KAUST, KAUST Catalysis Ctr, Thuwal 239556900, Saudi Arabia

[5] KAUST, Div Phys Sci & Engn, Thuwal 239556900, Saudi Arabia

[6] Niigata Univ, Inst Res Promot, Ctr Instrumental Anal, Nishi Ku, Niigata 9502181, Japan

[7] Nanyang Technol Univ, Sch Phys & Math Sci, Div Chem & Biol Chem, Singapore 637371, Singapore

[8] ASTAR, Inst Mat Res & Engn, Singapore 117602, Singapore

来源：

CHEMICAL SCIENCE | 2014年 / 5卷 / 12期

基金：

新加坡国家研究基金会;

关键词：

EXTENDED P-QUINODIMETHANES; OPEN-SHELL; CLOSED-SHELL; 2-PHOTON ABSORPTION; FACILE SYNTHESIS; HYDROCARBON; CHEMISTRY; PHOTOPHYSICS; DERIVATIVES; CRYSTAL;

D O I：

10.1039/c4sc01843e

中图分类号：

O6 [化学];

学科分类号：

0703 ;

摘要：

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 (y(0) = 0.269) and a small singlet-triplet energy gap (Delta E-S-T congruent to -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.

引用

页码：4944 / 4952

页数：9