A solvent-dependent fluorogenic probe containing julolidine for Cr(III) and Cu(II)

A julolidine-containing naphthylamine probe (EW1) was prepared for sensing specific metal ions. Probe EW1 afforded a broad low fluorescence at 500-600 nm in acetonitrile, but gave a relatively stronger fluorescence at 540 nm in methanol. EW1 showed a selective turn-on emission for Cr³⁺ in CH₃CN. Upon treatment with Cr³⁺, a strong emission was observed at 515 nm, but other metal ions, such as K⁺, Ag⁺, Ca²⁺, Zn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Cd²⁺, Hg²⁺, Fe²⁺, and Fe³⁺, failed to induce such an emission band. A clean isosbestic point appeared at 405 nm in the UV-vis titration with Cr³⁺. The emission intensity of EW1 was enhanced with incremental addition of Cr³⁺, indicating the formation of a 1:1 complex of the probe EW1 and chromium ion. However, EW1 showed a selective turn-off response with Cu²⁺ in CH₃OH. The emission band of EW1 at 540 nm was quenched only with Cu²⁺ and not with other control metal ions. The absorption band of EW1 at 410 nm progressively decreased, and a new absorption at 390 nm appeared upon progressive addition of Cu²⁺. All spectroscopic titration experiments and ESI mass data provided evidence for the formation of a 1:1 complex between EW1 and Cu²⁺.