Pyridylpyrazole N^N ligands combined with sulfonyl-functionalised cyclometalating ligands for blue-emitting iridium(iii) complexes and solution-processable PhOLEDs

Abstract

A series of blue iridium(III) complexes (12–15) comprising sulfonyl-functionalised phenylpyridyl cyclometalating ligands and pyridylpyrazole N^N ligands are reported, with an X-ray crystal structure obtained for 12. The complexes are highly emissive with photoluminescence quantum yields of 0.52–0.70 in dichloromethane solutions: two of the complexes (12 and 14) show emissions at λ^PL_max 457 nm which is considerably blue-shifted compared to the archetypal blue emitter FIrpic (λ_max 468 nm). The short excited state lifetimes (1.8–3.3 μs) and spectral profiles are consistent with phosphorescence from a mixture of ligand-centred and MLCT excited states. Density functional (DFT) and time dependent DFT (TD-DFT) calculations are in agreement with the electrochemical properties and the blue phosphorescence of the complexes. The additional mesityl substituent on the pyridylpyrazole ligand of 12 and 13 enhances the solubility of the complexes facilitating thin film formation by solution processing. Phosphorescent organic light-emitting diodes (PhOLEDs) have been fabricated using 12 or 13 in a solution-processed single-emitting layer using either poly(vinylcarbazole) (PVK) or 1,3-bis(N-carbazolyl)benzene (mCP) as host. The most blue-shifted electroluminescence (λ^EL_max 460 nm, CIE_x,y 0.15, 0.21) is obtained for an OLED containing complex 12 and mCP, with a brightness of 5400 cd m⁻² at 10 V which is high for PhOLEDs with similar blue CIE coordinates using a solution-processed emitter layer.