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) 2 Ir(IPP)]PF 6 (ppy: 2-phenylpyridine; IPP: 2-(1 H -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) 2 Ir(IPP)]PF 6 was very sensitive to Ni 2+ , which was caused by the ESIPT being hindered by the coordination of [(ppy) 2 Ir(IPP)]PF 6 with Ni 2+ . Its bright yellow orange light disappeared within 1 min when Ni 2+ (1.0 equiv.) was added in its solution (in DMSO/H 2 O, v/v = 999/1), therefore, it can be used for “turn-off” chemiluminescent detection of Ni 2+ . The recognition of [(ppy) 2 Ir(IPP)]PF 6 to Ni 2+ was not interfered by many common metal ions (such as Na + , K + , Li + , Mg 2+ , Al 3+ , Ca 2+ , Fe 3+ , Ag + , Zn 2+ , Hg 2+ , Co 2+ , Pb 2+ , Mn 2+ , Ba 2+ or Cr 3+ ), however, Cu 2+ 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) 2 Ir(IPP)]PF 6 with Ni 2+ was 1:1, its detecting mechanism based on ESIPT effect was demonstrated by 1 H NMR spectra and density functional theory (DFT) calculation.