title: Intensely fluorescent azobenzenes: synthesis, crystal structures, effects of substituents, and application to fluorescent vital stain.
abstract: 2-[Bis(pentafluorophenyl)boryl]azobenzenes bearing hydrogen, methoxy, dimethylamino, trifluoromethyl, fluoro, n-butyl, and tert-butyldimethylsiloxy groups at the 4'-position or methoxy and bromo groups at the 4-position have been synthesized. The 4-bromo group of the 2-boryl-4-bromoazobenzene derivative was converted to phenyl and diphenylamino groups by palladium-catalyzed reactions. The absorption and fluorescence properties have been investigated using UV/Vis and fluorescence spectroscopy. The 2-borylazobenzenes emitted an intense green, yellow, and orange fluorescence, in marked contrast to the usual azobenzene fluorescence. The 4'-siloxy derivative showed the highest fluorescence quantum yield (0.90) among those reported for azobenzenes to date. The correlation between the substituent and the fluorescence properties was elucidated by studying the effect of the substituent on the relaxation process and from DFT and TD-DFT calculations. An electron-donating group at the 4'-position was found to be important for an intense emission. Application of fluorescent azobenzenes as a fluorescent vital stain for the visualization of living tissues was also investigated by microinjection into Xenopus embryos, suggesting these compounds are nontoxic towards embryos.
authors.affiliation: Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
authors.forename: Junro
authors.initials: J
authors.lastname: Yoshino
authors.forename: Akiko
authors.initials: A
authors.lastname: Furuta
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Kambe
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Itoi
authors.forename: Naokazu
authors.initials: N
authors.lastname: Kano
authors.forename: Takayuki
authors.initials: T
authors.lastname: Kawashima
authors.forename: Yuzuru
authors.initials: Y
authors.lastname: Ito
authors.forename: Makoto
authors.initials: M
authors.lastname: Asashima
mesh_terms: D000818:Animals; D001391:Azo Compounds / Q000138:chemical synthesis* / Q000737:chemistry / Q000633:toxicity; D001880:Boranes / Q000138:chemical synthesis* / Q000737:chemistry / Q000633:toxicity; D002384:Catalysis; D018360:Crystallography, X-Ray; D004625:Embryo, Nonmammalian / Q000187:drug effects; D005453:Fluorescence; D005456:Fluorescent Dyes / Q000138:chemical synthesis* / Q000737:chemistry / Q000633:toxicity; D008845:Microinjections; D008956:Models, Chemical; D015394:Molecular Structure; D010165:Palladium / Q000737:chemistry; D013050:Spectrometry, Fluorescence; D013329:Structure-Activity Relationship; D014981:Xenopus / Q000196:embryology
chemical_list: D001391:Azo Compounds; D001880:Boranes; D005456:Fluorescent Dyes; D010165:Palladium; C009850:azobenzene