Structural and optical properties of new yellow emitting iridium(iii) complexes and their application as an active layer component in white organic light-emitting diodes

Abstract

Iridium(iii) complexes containing two 2-(3-fluorophenyl)-4-methylpyridine and one branched alkyl side group on the 2,2'-bipyridine ligand were synthesized and characterized by UV-Visible, fluorescence, FTIR, NMR spectroscopies, thermal gravimetric analysis (TGA) and cyclic voltammetry. The complexes containing 4-dihexylmetyl-4'-heptyl-2,2'-bipyridine, 4,4'-bis(3-ethylheptyl)-2,2'-bipyridine and 4-(3-ethylheptyl)-4'-methyl 2,2'-bipyridine are named as CS137, CS138 and CS139, respectively, and exhibit a yellow emission band at around 590 nm. The photoluminescence quantum yields are in the range of 0.63-0.72 in chlorobenzene. The TGA curves of the complexes present high decomposition temperatures above 280 °C. The HOMO and LUMO energy levels are in the range of -5.36-(-5.43) eV and -3.21-(-3.27) eV, respectively. Solution processed organic light-emitting diodes are fabricated by the use of these complexes as dopants at various concentrations in the poly(N-vinyl carbazole: 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene host. Almost no electroluminescence could be obtained from the blend containing CS139 whereas those of CS137 and CS138 resulted in white light. Commission Internationale de L'Eclairage (CIE) chromaticity coordinates shifted to the yellow region with the increase in complex ratio in the blend. The obtained device characteristics are attributed to the positive effects of higher steric hindrance in CS137 and CS138. © the Partner Organisations 2014.