Efficient sky-blue and blue-green light-emitting electrochemical cells based on cationic iridium complexes using 1,2,4-triazole-pyridine as the ancillary ligand with cyanogen group in alkyl chain

Abstract

Sky-blue and blue-green light-emitting cationic iridium (III) complexes with triazole-pyridine as the ancillary ligand, with formula [Ir(dfppy) 2(trzpy-cn)](PF 6) ( C1), [Ir(mppy) 2(trzpy-cn)](PF 6) ( C2), [Ir(dfppy) 2(trzpy-hx)](PF 6) ( C3) and [Ir(mppy) 2(trzpy-hx)](PF 6) ( C4) were synthesized, where mppy, trzpy-cn, trzpy-hx are 2-methylphenylpyridine, 2,2-dimethyl-6-(3-methyl-5-(pyridin-2-yl)-4H-1,2,4-triazol-4-yl)hexanenitrile, and 2-(5-methyl-4-octyl-4H-1,2,4-triazol-3-yl)pyridine, respectively. C1/C3 and C2/C4 emit sky-blue and blue-green light peaked at 458 and 484 nm, respectively. Single layer light-emitting electrochemical cells (LECs) based on C1 and C2 show luminous efficiencies (LE) of 2.6 and 6.8 cd/A (corresponding to an external quantum efficiency (EQE) of 2.7 and 2.8%), with CIE coordinates of (0.21, 0.33) and (0.31, 0.53), respectively. As compared with the devices from C3 and C4, the device efficiencies based on C1 and C2 are found to be significantly enhanced by 10 times and 4 times, respectively, owing to the incorporation of cyanogen group in the alkyl chain. The results demonstrate that using the triazole-pyridine as ancillary ligand with a cyanogen group in the side alkyl chain are effective strategies to tune the emission into blue region and to improve the efficiency of the cationic iridium complex.