Molecular Design And Light-Emitting Properties Of High-Performance Ionic Iridium Complexes

Ionic iridium???complexes are emerging with great promise for organic light-emitting diodes?OLEDs?,owing to their unique features such as ease of molecular design,synthesis and purification,excellent photophysical properties,superior redox stability,and highly efficient emissions of virtually all the colors.On the basis of existing research results and problems,this paper develops a series of novel ionic iridium???complexes,discusses the physicochemical properties and investigates their applications in OLEDs.The main contents are as follows:Introduction of ligands with large-sized groups into the light-emitting coordinated iridium???cations can improve the photoluminescence quantum yields?PLQYs?,however,always enhances the intramolecular conjugation,leads to red shift and limits the development of highly efficient blue-emitting ionic iridium???complexes.This paper proposes a facile and feasible molecular design strategy to improve the PLQYs through negative counter-ion control.Introducing large-sized negative counter-ions effectively increases the ionic radii,reduces molecular aggregation and concentration quenching,thus improves the PLQYs.Here,a series of blue and blue-green light-emitting ionic iridium???complexes with pyrazole-type ancillary ligands and different-sized negative counter-ions is designed and synthesized.Their PLQYs in neat film are successively improved along with the increasing volume of negative counter-ions.Solution-processed single-layer OLEDs thereof are also investigated,achieving bluest emissions at 452 nm with a color coordinate of?0.18,0.23?,and blue-green emissions with a highest external quantum efficiency of 6.8%and maximum luminance of 14.2×10³ cd m^-2,among the best performance of solution-processed single-layer OLEDs based on ionic iridium???complexes.On the other hand,ionic iridium???complexes are seldom evaporable due to the ionic nature,severely restricting their applications in state-of-the-art OLEDs fabricated by vacuum evaporation deposition.This paper also demonstrates an effective and versatile molecular design strategy to develop sublimable ionic iridium???complexes by choosing negative counter-ions with large steric hindrance and well-dispersed charges,which remarkably reduce the ionic interaction,thus improve the volatility.Here,a series of sublimable ionic iridium???complexes with pyrazole,imidazole,dipyridine and oxadiazole derivatives as ancillary ligands is designed and synthesized,emitting from blue to red in solid states.Vacuum-evaporated OLEDs thereof also show desirable performance.The bluest emission of OLEDs based on ionic iridium???complexes is successfully obtained,with a shortest wavelength of 448 nm and color coordinate of only?0.16,0.13?.The maximum external quantum efficiency is 10.1,16.0,14.8 and 11.0%for blue-green,green,yellow and red light-emitting devices,respectively,and turn-on voltages are only 2.3-2.5 V,achieving the best performance of OLEDs based on ionic iridium???complexes.