A boron-substituted azobenzene, (E)-[2-(4-methoxyphenylazo)phenyl]bis(pentafluorophenyl)borane, presented the most intense fluorescence among the azobenzene derivatives.

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.

Photoisomerization of a catecholborane bearing a 2-(phenylazo)phenyl group with an N-B dative bond caused photoswitching of the coordination number of boron between 3 and 4. The Lewis acidity of the catecholborane was switched by photoirradiation, and the complexation ability of the (E)- and the (Z)-isomers of the catecholborane with pyridine differs by more than a factor of 300. [reaction: see text]

N-Aryl, N-alkyl, N-alkoxy, and N-amino derivatives of 2-[bis(pentafluorophenyl)boryl]benzylideneamine were synthesized by the condensation reactions of 2-[bis(pentafluorophenyl)boryl]benzaldehyde with the corresponding amines. Their structures were investigated by NMR and X-ray crystallographic analysis. Their properties were investigated by UV-vis and fluorescence spectroscopy. The boryl-substituted N-arylimines show blue or green fluorescence in hexane at room temperature, and their fluorescence efficiency is much higher than that of N-benzylideneaniline. In particular, the boryl-substituted N-(4-dimethylaminophenyl)imine showed strong green emissions with at least 7000 times higher fluorescence quantum yield (0.73) compared with that of N-benzylideneaniline. The boryl-substituted N-(1-indolyl)- and N-(9-carbazolyl)imines showed dual emissions, one of which was assignable as arising from the lowest singlet excited state and the other from the local excited state of the substituent on the imine nitrogen. The fluorescent properties of the boryl-substituted N-butyl- and N-methoxyimines were also investigated. Reactions of the N-arylimine derivatives with cyanide ion gave the corresponding cyanide adducts and quenched the fluorescence, indicating that these 2-[bis(pentafluorophenyl)boryl]benzylideneamine derivatives have a potential as a cyanide ion sensor.

Azobenzenes are constituents of the commonly and widely used azo dyes. Many dyes, except for the azo dyes, have been utilized for fluorescent materials. However, there are only a few fluorescent azobenzene derivatives and their fluorescence efficiencies are quite low. The current perspective provides an account of the fluorescent azobenzenes and aromatic aldimines featuring an N-B interaction. Incorporation of the intramolecular N-B interaction by using the bis(pentafluorophenyl)boryl group makes the azobenzenes and aromatic aldimines fluorescent with a range of colours. Some of them fluoresce with extraordinarily high fluorescence quantum yields. Their synthesis, structures, fluorescence properties, and applications are discussed.