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 457 nm which is considerably blue-shifted compared to the archetypal blue emitter FIrpic (λ 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, CIE 0.15, 0.21) is obtained for an OLED containing complex 12 and mCP, with a brightness of 5400 cd m at 10 V which is high for PhOLEDs with similar blue CIE coordinates using a solution-processed emitter layer.

Quantum-chemical studies of homoleptic iridium(III) complexes in OLEDs: fac versus mer isomers

“…It is known that frontier molecular orbitals (FMO) of complex ground state S 0 are related to its spectral properties [86]. Emission color of iridium(III) complexes can be adjusted by changing their HOMO-LUMO bandgap, which can be achieved on the course of ligand functionalization with electron-donating and electronwithdrawing substituents [86], and values of HOMO-LUMO gaps predicted for Ir(III) complexes by DFT methods showed surprisingly good correlation with the experimentally recorded values of energies of emitted photons even in the case of phosphorescence, see for example [21,22,[83][84][85][87][88][89][90]. Contour plots of frontier orbitals of both [Ir(bzq) 3 ] isomers are depicted in Fig.…”

Design and Synthesis of Heteroleptic Iridium(III) Phosphors for Efficient Organic Light-Emitting Devices

“…In order to develop the phosphorescent emitters that are compatible with both solution and dry processes, the molecular design concepts are outlined as follows. First, a pyrazole-based chelation motif, which significantly enhances the solubility in the common organic solvents without compromising the thermal stability during evaporation, 23 Next, an electron-withdrawing chlorine atom (-Cl), an electron-donating methyl group (-CH 3 ), and a relatively electroneutral hydrogen atom (-H), are bound to the phenyl tail (see Scheme 1), corresponding to the L1, L2, and L3 ancillary ligands used in this report, respectively. Such variation of ligand structure is anticipated to modulate the electron density around the Ir(III) center, which may allow us to tune the photophysical and electrochemical properties of Ir(III) complexes.…”

Blue Phosphorescence with High Quantum Efficiency Engaging the Trifluoromethylsulfonyl Group to Iridium Phenylpyridine Complexes

“…For example, the most popular strategy to obtain blue-emitting phosphorescent Ir complex has been set up by incorporating an electron-withdrawing group (EWG) at the phenyl moiety of ppy ligand, as shown in Scheme . The EWG can stabilize the highest occupied molecular orbital (HOMO), resulting in an increased bandgap of Ir complexes. ,, Several EWGs, such as fluorine (−F), − trifluoromethyl (−CF 3 ), ,,− nitro (−NO 2 ), − cyano (−CN), − and sulfonyl (−SO 2 R) − have been used to induce such blue emissions from Ir­(III) complexes.…”