Influence of electronic and steric effects of substituted ligands coordinated to Ir(iii) complexes on the solution processed OLED properties

Orange and green phosphorescent heteroleptic iridium complexes 1 and 2 [(1): iridium(iii)bis[2-(5′-benzylsulfonyl)phenylpyridinato-N,C2′](2,4-decanedionate) and (2): iridium(iii)bis[2-(5′-benzylsulfonyl-3′,6′-difluoro)phenylpyridinato-N,C2′](2,4-decanedionate)] are used as dopant emitters in solution processed organic light-emitting diodes. These emitters bear one sterically hindered 2,4-decanedionate and two phenylpyridine (ppy) ligands functionalized with electron withdrawing benzylsulfonyl groups and fluorine atoms. The steric and electronic effects of such substituents on the photophysical properties and performance of devices of 1 and 2 are investigated by making comparison with the green emitting reference phosphor Ir(ppy)2(acac) [iridium(iii)bis(2-phenylpyridinato-N,C2′)-acetylacetonate]. In particular, the functionalization of ppy ligands with the benzylsulfonyl group enhances the photoluminescence quantum yield (Φ) and red shifts the emission of complex 1 with respect to Ir(ppy)2(acac). Further substitution of the same ligands with two fluorine atoms in 2 restores the green emission of the reference complex, increasing its Φ. Hence, the combination of the two kinds of substituents represents a suitable functionalization pattern to increase the photoluminescence efficiency of 2vs. the unfunctionalized Ir(ppy)2(acac). Interestingly, a fully solution processed phosphorescent light-emitting device (PHOLED) made using complex 2 as an emitter and with properly tailored architecture, which includes an electron transporting layer of a PEG substituted polyfluorene (PEG: poly(ethylene glycol)), exhibits high external emission quantum efficiency (EQE up to 12%) and a high luminous efficiency (LE up to 24.2 cd A-1). We attribute such a high performance to the bulky effect of both benzylsulfonyl groups in ppy and of the alkyl chain in the β-diketonate ligand, as well as to the enhanced electron mobility induced by fluorine atoms. This device outperforms the control diode based on Ir(ppy)2(acac) (EQE 5.5% and LE 17 cd A-1) and approaches the best efficiencies achieved so far for green Ir(iii) complex based PHOLEDs.