(Invited) Helicate Europium Complexes for the Design of Efficient Luminescence and Electro-Luminescence Devices

Abstract

Molecular structure in metal complexes enhances their electronic structure and physicochemical properties such as luminescence. It is well known that the number of coordination of a lanthanide ion is various from six to twelve. Here, we will introduce a series of helicate europium (Eu) complexes which having hexadentate ligand with chelate effect to keep the coordination of its aromatic ligand as photo-antenna to induce ff emissions of Eu.[1] The Eu complex (EuL) has a neutral ligand consisted in two bipyridine moieties bridged by ethylenediamine and two nitrate ions, and takes a cationic complex. The molecular structure succeeded to form the efficient energy donor behavior from the measurement of electro-static potential by the synchrotron single crystal X-ray analysis (SPring-8, BL01B1), because the bipyridine moiety has kept the excited state like a terpyridine moiety. The luminescence band of Eu was observed at 616 nm with relatively high luminescent quantum yields (QY) by UV excitation. The complex is also soluble in ionic liquids (IL) when the same counter anions were used in both the complex and IL such as PF6 -.[2] Luminescence and electronic absorption bands position of EuL in IL were same. Interestingly the QY of the complex drastically increased in IL because of the high viscosity. IL was also useful for the application for electrochemistry as a medium. Actually, there are several papers of Eu complexes were reported to explain the possibility of luminescence color change by the redox reaction of Eu ion[3]. If Eu ion takes divalent state, the luminescence color will be shifted at blue, and can be switch the color between red and blue by electrochemistry. In the case of EuL in IL, we adopted the solution to electronic cells and found the luminescence intensity changing at around +2.0 and -1.8 V [4]. It exhibites the reversibility with keeping the possible luminescence intensity. Thus, the stable halicate structure and cationic state of EuL act as a electrochemical active behavior in IL.Author acknowledges Professor Y. Kim, Hokkaido University for the collaboration of electrochemistry in the ionic liquids.[1] (a)M. Hasegawa, et al., New J. Chem., 2014, 38, 1225-1234; (b) M. Hasegawa, et al., ChemPlusChem, 2020, 85, 294-300; (c) M. Hasegawa, et al., Photochem. Photobiol. Sci., 2020, DOI: 10.1039/d0pp00069h, (d) M. Hasegawa & H. Ohmagari, Chem. Lett., 2020, 49, 845-854.[2] Y. Hasegawa, M. Hasegawa, et al., Molecules, 2018, 23, 55.[3] (a)K. Binnemans, Chem Rev, 2007, 107(6), 2592–2614; (b) M. Mukaigawa & H. Ohno, Solid State Ionics, 1998, 113-115, 439-442.[4] Y. Kim & M. Hasegawa, et al., 2020, DOI:10.1021/acsami.0c13765. Figure 1