Photoluminescence properties of a novel cyclometalated iridium(III) complex with coumarin-boronate and its recognition of hydrogen peroxide
A novel neutral iridium(III) complex-based phosphorescent probe (Ir-2) for hydrogen peroxide (H2O2) has been designed and synthesized by incorporating a benzeneboronic acid pinacol ester (bpe) moiety into 3-(benzothiazol-2-yl)-7-hydroxy-coumarin (Bthc) as a cyclometalated ligand (Bthc–bpe). The photophysical behavior of Ir-2 was investigated by UV-Vis absorption spectroscopy, photoluminescence spectroscopy, and quantum mechanical calculations. The absorption spectra of the complex Ir-2 are dominated by the cyclometalated ligand; thus it shows an intense absorption band in the visible region at 460 nm with a molar extinction coefficient (ε) of about 3 × 104 M−1 cm−1, which is rarely found for typical polypyridine iridium(III) complexes. The complex Ir-2 displays efficient phosphorescent emission at 560 nm at room temperature originating from a mixed triplet metal-to-ligand charge-transfer (3MLCT, dπ(Ir) → π* (Bthc–bpe)) and triplet intraligand (3ILCT, π–π* (Bthc–bpe)) excited states as suggested by the DFT computational studies. Upon reaction with H2O2, the complex displays an emission decrease induced by an intense intermolecular aggregation due to the cleavage of the bulky benzeneboronic acid pinacol ester substituent, indicating that Ir-2 could act as an ON–OFF-type phosphorescent probe for H2O2. Additionally, selectivity studies reveal that the complex Ir-2 possesses high selectivity toward H2O2 over other reactive oxygen species (ROS).