Study on a novel emitter based on rhenium complex: Synthesis, molecular structure, photophysical feature and electroluminescence performance

In the present work, we report a diamine ligand of 2-(4-chlorophenyl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (CPO) which owns a large conjugation plane and an electron-pulling moiety of 1,3,4-oxadiazole group. Its corresponding Re(I) complex of Re(CO)3(CPO)Br is also synthesized and studied in detail, including single crystal structure, electronic structure, photophysical performance, thermal stability and electrochemical property. Experimental data suggest that Re(I) localizes in the center of a distorted octahedral coordination sphere, and the π-π attraction between CPO ligands makes Re(CO)3(CPO)Br molecules take a highly-ordered arrangement. Upon photoexcitation, Re(CO)3(CPO)Br shows a yellow emission peaking at 540 nm with quantum yield of 0.13 and excited state lifetime on microsecond scale. Theoretical calculation then points out that the emission comes from the triplet excited state of ML&LLCT, where ML&LLCT means metal-to-ligand-charge-transfer and ligand-to-ligand-charge-transfer. Further experimental result suggests that Re(CO)3(CPO)Br owns a high decomposition temperature of 300 °C and a band gap of 2.27 eV with HOMO and LUMO energy levels of −5.39 eV and −3.12 eV, respectively. The optimal electroluminescence device using Re(CO)3(CPO)Br as the emissive dopant renders an emission peaking at ∼555 nm with maximum brightness of 3520 cd/m2 and maximum current efficiency of 7.2 cd/A.