Electrofluorochromic Device Based on a Redox-Active Europium(III) Complex
Yuna Kim, Hitomi Ohmagari, Akira Saso, Nobuyuki Tamaoki, Miki Hasegawa
Abstract
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.