A Fluorescent Sensor for Cu²⁺ Ion with High Selectivity and Sensitivity Based on ICT and PET

A novel fluorescent sensor (L) based on 1,8-naphthalic anhydride has been developed which can selectively detect Cu²⁺ in CH₃CN medium over other metal ions at 408 nm in the fluorescence spectra. When Cu²⁺ was added into L, L showed fluorescent turn-off by coordinating with Cu²⁺. A fresh absorption band was found at the position of 290 nm as was a red-shifted absorption band from 356 nm to 376 nm in UV-vis spectra which might be attributed to the intramolecular charge transfer (ICT). Meanwhile, L-Cu²⁺ showed fluorescence quenching via photoinduced electron transfer (PET). The complexation ratio was proposed to be 1:1 which was determined by Job's plot, fluorescence titration and ¹H NMR titration. The detection limit was 9.1 × 10^-8 mol·L^-1, a satisfying level to detect Cu²⁺ in the micromolar scale. Corresponding molecular geometries, orbital energies and electron contributions of sensor L were calculated by the DMol3 program package using the density functional theory.