Study On Iridium Complexes Modified By Steric Hindrance Groups And Their Photoelectric Properties

Organic light-emitting diodes（OLEDs）are widely used in flexible displays due to their advantages of spontaneous emission and excellent color quality.Iridium complexes can attain high phosphorescence efficiency,but triplet-triplet annihilation（TTA）and triplet-polaron annihilation（TPA）would occur,resulting in change of emitting wavelength and severe device efficiency roll-off.It is necessary to design and prepare complexes that are insensitive to doping concentration.Therefore,through the introduction of different steric hindrance groups to the ligands,this study regulates the emitting properties and reduces the concentration quenching of complexes.Four aspects of works have been done:（1）Electronic-deficient pyrimidine-based ligands can improve the electronic transmission of iridium complexes,therefore,using the pyrimidine derivative mfp as the main ligand,with the 2-pyrazole pyridine derivative pz as auxiliary ligand,the iridium complex Ir（mfp）₂pz was synthesized,in order to increase the density of steric hindrance,the tert-butyl and pinene were introducing to the pyridine ring of pz respectively,and complexes Ir（mfp）₂tpz and Ir（mfp）₂ppz were obtained.The three complexes have similar photoluminescence（PL）wavelengths of 507-509 nm with high photoluminescence quantum efficiencies（PLQYs）of 69-97%,the efficiency increases with the steric hindrance,among which Ir（mfp）₂ppz with pinene group has the highest PLQY of 97%.Their spin-coated electroluminescence（EL）devices achieve the high maximum luminance(L_max)of 43596,34840 and 32766 cd m^-2,respectively.Due to the larger steric hindrance of rigid pinene group,the doped device D3A（Ir（mfp）₂ppz）shows the highest efficiencies of 39.6 cd A^-1,13.1%with the small roll-off of 4.2%at high luminance of 1000 cd m^-2,and the device efficiencies fluctuate slightly even at high doping concentration,showing that the rigid pinene group can improve the luminous efficiency of iridium complexes and the device performance efficiently;（2）Due to the excellent steric effect of pinene,different pull-push electronic substituents were introduced into cyclometallated benzene rings of phenylpyridine-based iridium complex containing pinene group,adjusting the wavelengths and carrier transport performance and adding the density of steric hindrance,and iridium complexes Ir（CF₃mp）₃,Ir（d CF₃mp）₃,Ir（tpmp）₃ and Ir（Clmp）₂（taz）were obtained.The PL wavelength of reference complex Ir（mppy）₃ is 506 nm,the wavelengths of complexes Ir（CF₃mp）₃,Ir（d CF₃mp）₃ and Ir（Clmp）₂（taz）with pull-electron substituents blue shift to 478-500 nm,and the wavelength of complex Ir（tpmp）₃ complex with the push-electron substituent red shifts to 512 nm.As the complex Ir（CF₃mp）₃contains the CF₃ group which possesses both steric resistance and electron transport performance,its PLQY reaches up to 78%,and its doped-device D4 shows the highest efficiencies of 63.4 cd A^-1,28.0%.As its PLQY maintains at 48%at neat powder,the efficiencies of its non-doped device N4 achieve 15.9 cd A^-1,4.5%.Therefore,further increasing the steric resistance density of pinene-based iridium complexes and adjusting the carrier transport performance can significantly inhibit the concentration quenching of iridium complexes and improve the device performance;（3）The triptycene was introduced into the structure of pyridazine/phthalazine-based ligands to inhibit the intermolecular interaction and adjust the photoelectric performance of luminescent materials,and the CF₃ group which can improve the coordination activity,steric hindrance and electronic transport performance of materials was also introduced,and the pyridazine-based tricyclic iridium complex Ir（mtp）₃ and phthalazine-based tricyclic iridium complexes Ir（mdtp）₃,Ir（odtp）₃,Ir（dptp）₃ and Ir（tmdtp）₃ were obtained by their ligands.The single crystal diffraction results show that the benzene rings of pyridazine-based complex Ir（mtp）₃ have face-to-faceπ···πstacking,while only weak interaction is observed for phthalazine-based complexes,indicating that triptycene group is more efficient to reduce the intermolecular interaction of phthalazine-based iridium complexes.The PL wavelength of pyridazine-based complex Ir（mtp）₃ is 521 nm,and the wavelengths of the phthalazine complexes red shift to 597-608 nm due to the enhanced conjugation effect of ligands.All complexes have high PLQYs（74%-89%）and good thermal stabilities（T_d=364.5-451.0℃）.The doped device D7efficiencies of pyrazine-based complex Ir（mtp）₃ are 48.5 cd A^-1,17.1%,and the device D8 efficiencies of phthalazine-based complex Ir（mdtp）₃ reach up to 46.4 cd A^-1,27.5%,which are the highest efficiencies among phthalazine-based complexes’devices at present.When the brightness reaches 1000 cd m^-2,the efficiency roll-off of device D8 is only 14.9%,which is lower than that of device D7（18.7%）.Moreover,the efficiency roll-off of non-doped device N8（Ir（mdtp）₃）is also very small（8.5%）,which is significantly lower than that（36.1%）of device N7（Ir（mtp）₃）.The results show that tricyclic iridium complexes with good stability,high luminescence efficiency and excellent device performance can be obtained by the introduction of triptycene group,which have great application potential in OLED fields;（4）Bis-（tridentate）iridium complexes are easy to be synthesized and have high luminescence efficiency,but they are more prone to molecular accumulation than tri-（bisdentate）iridium complexes.In order to inhibit concentration quenching,the triptycene group was introduced and complex Ir（tpz）₂ was obtained.In order to increase steric resistance density and enhance carrier transport,alkyl and 3,5-bis（carbazolyl）phenyl were introduced to obtain complexes Ir（mtpz）₂ and Ir（Cztpz）₂,respectively.The PLQY of the reference complex Ir（pz）₂ is only 57%,and the PLQYs of these new complexes are increased to 69-78%.High PLQYs（18-35%）of these new complexes at neat powder are also obtained,which are higher than that of complex Ir（pz）₂（3%）.The efficiencies of reference doped-device D13（Ir（pz）₂）are 52.2 cd A^-1,9.8%,the efficiencies of the device D11（Ir（tpz）₂）reach 80.2 cd A^-1,12.0%,the maximum current efficiency(CE_max)of D11 is 53.6%higher than that of device D13.The efficiencies of the reference non-doped device N13（Ir（pz）₂）are only 3.2 cd A^-1,0.7%,while that of the device N11（Ir（tpz）₂）remaine at 14.7 cd A^-1,4.3%.These results indicate that the triptycene group is also very effective in improving the luminescence efficiency and reducing the concentration quenching of bis-（tridentate）iridium complexes.