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.
Abstract2,2′‐Bis[bis(pentafluorophenyl)boryl]azobenzenes were synthesized. X‐ray crystallographic analysis exhibits a planar core structure of the azobenzene moiety, double intramolecular N–B coordination, and two tetracoordinate boron atoms. The 2,2′‐diborylazobenzenes show fluorescence emission with orange and red colors upon irradiation. The double N–B coordination causes redshifts in both the absorption and emission maxima and a decrease in the Stokes shifts relative to those of the 2‐borylazobenzene derivative. On the basis of the density functional theory calculations of the molecular orbitals of 2,2′‐diborylazobenzene, the π* orbital (LUMO) was found at a much lower energy level (–4.67 eV) than that in 2‐borylazobenzene (–3.70 eV). A reversible reduction wave was observed at a low reduction potential in the cyclic voltammograms of the 2,2′‐diborylazobenzenes. Single‐electron reduction of one of the 2,2′‐diborylazobenzenes generates an azobenzene radical anion, which was confirmed by an active ESR signal. The fluorescence was quenched by the reduction and recovered by air oxidation in this azobenzene.
A closer look into the function of diiron proteins, such as methane monooxygenase and ribonucleotide reductase, is provided by the crystal structure of a designed diiron protein (the picture shows the Fe environment). Cofactor rigidity may be a factor in O2 reactivity and a possible role of HisCεH⋅⋅⋅O hydrogen bonds in cofactor stabilization is implicated.
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