Fluorometric Turn Off Detection of Cu²⁺ and Pd²⁺ Ions Using 1-(Imidazo[5,1-a]isoquinolin-3-yl)naphthalen-2-ol

The fused heterocycle 1-(imidazo[5,1-a]isoquinolin-3-yl)naphthalen-2-ol (LH) has been synthesized and characterized by spectroscopic methods. Probe LH upon irradiation with λ_ex = 336 nm exhibited strong fluorescence with λ_em = 437 nm in MeOH/HEPES buffer (5 mM, pH = 7.4, 2:8, v/v). Upon adding several metal ions, the fluorescence intensity was quenched only by Cu²⁺ and Pd²⁺ ions. This fluorescence turn-off response by Cu²⁺ and Pd²⁺ ions was observed even in the coexistence of other metal ions and anions. From the Job's plot and mass spectra analysis, a 2:1 ratio of LH:M²⁺ was obtained, and the composition [Cu(L)₂H₂O] and [Pd(L)₂] has been assigned. The detection limit for Cu²⁺ and Pd²⁺ ions was determined to be 0.24 and 0.35 μM, respectively, and workable pH window to be 4-10. The binding of LH to Cu²⁺ and Pd²⁺ resulted in red-shift in the UV-visible spectrum, which was supported by DFT/TDDFT calculations.