Fine tuning the steric hindrance of the Eu(<sc>ii</sc>) tris(pyrazolyl)borate complex for a blue organic light-emitting diode

Eu(ii) complexes with characteristic d-f transitions demonstrate their potential applications in organic light-emitting diodes (OLEDs) due to their advantages including short excited-state lifetimes, 100% theoretical exciton utilization efficiency and tunable emission colors. Thermally stable and efficient Eu(ii) complexes are necessary for blue OLEDs, which are still the bottleneck as compared to red and green OLEDs. A blue-emitting Eu(ii) complex can be obtained by appropriate ligand design because the 5d energy levels are sensitive to the external environments. Herein, a series of Eu(ii) complexes based on tris(pyrazolyl)borate ligands were designed and synthesized. By carefully adjusting the steric hindrance, a thermally stable sky-blue emission Eu(ii) complex bis[hydrotris(3-tert-butylpyrazolyl)borate]europium(ii) (Eu-tBu) was obtained by employing the bulky tert-butyl group. The sky-blue OLED based on Eu-tBu was fabricated, showing a maximum external quantum efficiency of 15.7%, a maximum luminance of 52 240 cd m(-2), and Commission Internationale del Eclairage coordinates of (0.13, 0.27) at 1000 cd m(-2).