Electrofluorochromic Device Based on a Redox-Active Europium(III) Complex

Electrofluorochromism owing to redox reactions on the center europium (Eu) ion in ionic liquids is examined for the helicate complexes (abbreviated as EuL) with a hexadentate pyridine derivative. Typical electrofluorochromism requires extra electroactive units complementing intra- or intermolecular energy transfer to quench fluorophores. Herein, an unprecedentedly simplified electrofluorochromic system overcoming such issues is demonstrated by utilizing reversible electrochemistry of EuL between Eu³⁺ and Eu²⁺, which accompanies large emission transition. A three-electrode electrochemical switching device is facilely prepared with an ionic liquid [BMIM][PF₆] and EuL mixture. Benefiting from the stable helical coordinated structure of the ligand in [BMIM][PF₆], highly enhanced red fluorescence of EuL with small quantity (≤1 wt %) is utilized. Rapid response and large contrast of luminescence are achieved: the emission is drastically quenched at the reduced state (Eu²⁺) and it is successfully restored by subsequent oxidation (Eu³⁺). The reversible fluctuation of excitation and emission spectra of an electrofluorochromic device is achieved in the potential window within ±2 V. The device affords excellent optoelectric properties in terms of well-controlled luminescence switching depending on the applied potentials and its durability. This work paves an efficient and smart way toward Eu luminescence control in optoelectronic devices.