Air stable and efficient rare earth Eu(ii) hydro-tris(pyrazolyl)borate complexes with tunable emission colors

Abstract

Luminescent rare earth Eu(II) complexes with a characteristic 5d–4f transition have shown potential in various applications. However, the exploration and further application of typical Eu(II) complexes are strongly impeded by their instability arising from the easily oxidation of Eu²⁺ ions. In this work, three new Eu(II) complexes bis(hydro-tris(3-phenyl-5-methylpyrazolyl)borate)europium(II) (Eu(Tp^Ph,Me)₂), bis(hydro-tris(3-phenylpyrazolyl)borate)europium(II) (Eu(Tp^Ph)₂), and bis(hydro-tris(3,5-diphenylpyrazolyl)borate)europium(II) (Eu(Tp^Ph2)₂) were designed and synthesized. Their crystal structures, photophysical properties and air stabilities were characterized and discussed. It is found that the introduction of the bulky phenyl group not only increases the air stability by sterically hindering the central Eu²⁺ ions, but also tunes the maximum emission wavelength by influencing the ligand field. This work provides rules for regulating the luminescence properties of Eu(II) complexes as well as design basis for achieving stable and efficient Eu(II) complexes.