Photoluminescence properties of a novel cyclometalated iridium(iii) complex with coumarin-boronate and its recognition of hydrogen peroxide

Abstract

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).