A novel fluoride-sensing scaffold by a peculiar acid-promoted trimerization of 5,6-dihydroxyindole

An unusual rearranged trimer, 2-(2-amino-4,5-dihydroxybenzyl)-6,7-dihydroxy-3-(5,6-dihydroxyindol-3-yl)quinoline (1a), was obtained as the acetyl derivative (1b) by mild acid-promoted polymerization of 5,6-dihydroxyindole at pH 2. Compound 1b proved to be a selective fluoride-sensing compound, transducing F− binding into a distinct absorption at 414 nm and a marked fluorescence enhancement at 489 nm.