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 457 nm which is considerably blue-shifted compared to the archetypal blue emitter FIrpic (λmax 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 460 nm, CIEx,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.
Highlights
The p-tolylsulfonyl and methylsulfonyl groups both blue shift the emission to the same extent,[29] and methoxy substituents can lead to a blue shift depending on their position on the ppy ligand.[36,37]
We report the new mesityl substituted pyridylpyrazole ligand 9 which achieves the desired aims of increasing the solubility of the complexes 12 and 13, in comparison with 14 and 15, thereby facilitating film formation by solution processes without affecting the emission colour
To summarise, replacing the picolinate ancillary ligand of the heteroleptic iridium complexes 1 and 2 with a pyridylpyrazolebased ligand affords the new complexes 12–15 with blue shifted emission. This is ascribed to a reduction of electron density around the metal centre which results in a lowering of the energy of the highest occupied molecular orbital (HOMO) of the complexes, as observed in the solution electrochemical data. (TD-)Density functional (DFT) calculations are in excellent agreement with the observed photophysical and electrochemical properties of the complexes
Summary
Pyridylpyrazole N^N ligands combined with sulfonyl-functionalised cyclometalating ligands for blue-emitting iridium(III) complexes and solution-processable PhOLEDs†. 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 additional mesityl substituent on the pyridylpyrazole ligand of 12 and 13 enhances the solubility of the complexes facilitating thin film formation by solution processing. The most blue-shifted electroluminescence (λEmLax 460 nm, CIEx,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
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