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 (Φ = 0.49–0.74 for 1 and Φ = 0.71–0.92 for 2) and show a positive solvatochromism. Upon fluoride binding to the boron center, they exhibit a ratiometric fluorescence response (Δλem = 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 π–π* 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.