Ratiometric fluorescence sensing of fluoride ions by triarylborane-phenanthroimidazole conjugates

Triarylborane-phenanthroimidazole conjugates 1 and 2 linked by a biphenylene and phenylene group, respectively, were prepared and characterized. Both compounds are highly fluorescent ( Phi = 0.49-0.74 for 1 and Phi = 0.71-0.92 for 2) and show a positive solvatochromism. Upon fluoride binding to the boron center, they exhibit a ratiometric fluorescence response (Delta lambda(em) = 84 nm for 1 and 63 nm for 2 in acetone) accompanying a vivid emission color change. Photophysical data and TD-DFT studies suggest that intramolecular charge transfer (ICT) transition from a phenanthroimidazole donor to a borane acceptor in the neutral 1 is switched to pi-pi* transition of phenanthroimidazole moiety after fluoride binding. Due to the greater elevation of LUMO level by fluoride complexation, the resulting fluoride adduct [ 1F]fluoresces at the apparently blue-shifted region. (C) 2012 Elsevier B.V. All rights reserved.