Phosphorescent Dimesitylboryl-Appended Iridium(III) Complex for Fluoride Anion Sensing

Abstract

Owing to the importance of fluoride anion (F−) in the treatment of osteoporosis and dental care, the recognition and sensing of fluoride anion is attracting attention. Many fluoride chemosensors containing (thio) ureas, amides, pyrrole, and imidazolium ring have been prepared to date. Those chemosensors usually form strong hydrogen bonds between their N-H hydrogens and fluoride anion. Recently, tri-coordinate organoboron compounds such as dimesitylboryl [B(Mes)2] compounds have been found to achieve highly efficient fluoride anion detection using specific Lewis acid-base interaction. Fluoride chemosensors based on tri-coordinate boron moiety such as B(Mes)2 bind with fluoride anion to afford strong B-F interaction, which can change the lowest excited state or block the intramolecular charge transfer. Thereby, the photophysical properties can be changed. However, the output signals of fluoride anion selective chemosensors have usually been absorption and/or fluorescence. Nowadays, phosphorescent chemosensors have also been developed for the detection of fluoride anions since the first phosphorescent fluoride sensor reported by Gabbai et al. Phosphorescent chemosensors are quite attractive because the relatively long life time of phosphorescence can eliminate the interference of the shorter-lived background fluorescence and/or scatterring light usually present in biological samples. Among the various types of phosphors, cyclometalated iridium(III) complexes that have been used, for example, as an efficient phosphorescent dopant in organic light emitting diodes (OLEDs) or as a phosphorescent chemosensor for the selective detection of mercuric ion have excellent photophysical and electrochemical properties. However, only a few fluoride anion chemosensors based on iridium(III) complex have been reported. In this paper, we report the synthesis of a novel phosphorescent iridium(III) complex, Ir(p-B-ppy)2picolinate (1), where p-B-ppy is 2-(4-(dimesitylboryl)phenyl)pyridine, and its application to the selective phosphorescent detection of fluoride anion. A multi-step synthetic pathway leading to Ir(p-B-ppy)2picolinate (1) is shown in Scheme 1. The main ligand (2), which was prepared by Suzuki coupling of 4-bromophenylboronic acid with 2-bromopyridine and then substitution of the bromo group with B(Mes)2 group via the procedure reported, was treated with iridium(III) chloride trihydrate in the mixed solvent of 2-ethoxyethanol and water to afford iridium chloride-bridged dimer 3, [(p-B-ppy)2IrCl]2. The iridium chloride-bridged dimer 3 was then treated with picolinic acid in 2-ethoxyethanol in the presence of sodium carbonate to afford Ir(p-B-ppy)2picolinate (1). The UV-vis and photoluminescence spectra of Ir(p-B-