A cationic iridium(III) complex using 2-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenol as its ancillary ligand: ESIPT-capability and application for “turn-off” chemiluminescent detection of Ni2+

Herein, a cationic iridium(III) complex [(ppy)₂Ir(IPP)]PF₆ (ppy: 2-phenylpyridine; IPP: 2-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenol) was synthesized. Under the excitation of 328 nm (λ_ex = 328 nm) UV light, this complex emitted bright yellow orange light (500 ∼ 700 nm, λ_em,max = 582 nm), the Stokes shift was up to 254 nm, mainly due to excited-state intramolecular proton transfer (ESIPT) occurring in its ancillary ligand IPP. [(ppy)₂Ir(IPP)]PF₆ was very sensitive to Ni²⁺, which was caused by the ESIPT being hindered by the coordination of [(ppy)₂Ir(IPP)]PF₆ with Ni²⁺. Its bright yellow orange light disappeared within 1 min when Ni²⁺ (1.0 equiv.) was added in its solution (in DMSO/H₂O, v/v = 999/1), therefore, it can be used for “turn-off” chemiluminescent detection of Ni²⁺. The recognition of [(ppy)₂Ir(IPP)]PF₆ to Ni²⁺ was not interfered by many common metal ions (such as Na⁺, K⁺, Li⁺, Mg²⁺, Al³⁺, Ca²⁺, Fe³⁺, Ag⁺, Zn²⁺, Hg²⁺, Co²⁺, Pb²⁺, Mn²⁺, Ba²⁺ or Cr³⁺), however, Cu²⁺ was an interfering metal ion, but its interference can be completely masked by thiourea. The detection limit was as low as 3.31 × 10⁻⁷ mol∙L^-1 (0.331 μM). The Job’s plot analysis revealed that the stoichiometry of [(ppy)₂Ir(IPP)]PF₆ with Ni²⁺ was 1:1, its detecting mechanism based on ESIPT effect was demonstrated by ¹H NMR spectra and density functional theory (DFT) calculation.