Fluorine-free, blue-emitting cationic iridium complexes with a phenyl-triazole type cyclometalating ligand: Synthesis, characterizations and their use for efficient organic light-emitting diodes

Fluorine-free, blue-emitting cationic iridium complexes with good blue-color purity and high phosphorescent efficiencies are still rare. Here we report efficient fluorine-free, blue-emitting cationic iridium complexes with 3,4,5-triphenyl-4H-1,2,4-triazole (tPhTAZ) as the cyclometalating ligand and 2-(1H-pyrazol-1-yl)pyridine (pzpy), 4-dimethylamino-2-(1H-pyrazol-1-yl)pyridine (dmapzpy) and 3,3 '-methylenebis(1-methyl-1H-imidazole-3-ium2-ide) (bi-cb) as the ancillary ligand, namely, [Ir(tPhTAZ)2(pzpy)]PF6 (1), [Ir(tPhTAZ)2(dmapzpy)]PF6 (2) and [Ir(tPhTAZ)2(bi-cb)]PF6 (3). In solution, complexes 1-3 afford blue-emission peaked at around 462 and 487 nm. In doped films, complexes 1-3 afford similar blue emission with high phosphorescent efficiencies at 0.77, 0.85 and 0.67, respectively. Electrochemical measurements show that complexes 1-3 exhibit similar oxidation potentials but gradually cathodically-shifted reduction potentials. Theoretical calculations reveal that the emission of complexes 1-3 originates from mixed 3MLCT (Ir -> tPhTAZ)/3 pi-pi* (tPhTAZ-centered) states, nearby which there exist different non-radiative states (one charge-transfer and two metal-centered (3MC) states for complex 1, two 3MC states for complex 2 and one 3MC state for complex 3). These non-radiative states influence differently on the non-radiative deactivations of the emitting triplet states. Solution-processed double-layer OLEDs using complexes 1 and 2 as dopants afford sky-blue electroluminescence with emission peaks at around 470 and 497 nm and current efficiencies of up to 24.5 cd A-1.

Automated summary

This study introduces efficient fluorine-free, blue-emitting cationic iridium complexes for OLED applications. The complexes exhibit high phosphorescent efficiencies and pure blue color in both solution and doped films. They show potential for use in optoelectronic devices due to their high-performance blue emission properties.