Molecular Regulation And Photoelectric Properties Of Benzimidazole-based Ir(?)Complexes

Organic light-emitting diodes?OLEDs?have captured increasing attention in both industry and academia due to their promising merits of self-emitting ability,high efficiency,low power consumption,wide-viewing angle and flexibility.Phosphorescent Ir???complexes were employed successfully in OLEDs owing to their outstanding merits of high quantum efficiency,high thermal stability,relatively short excited state lifetime as well as wide color tenability.In order to improve electroluminescence?EL?efficiency,Ir???complexes were dispersed in proper host matrix at low concentration to avoid self-quenching effect.However,low doped concentration?< 10 wt%?is hard to precisely control,which prevents OLEDs from being reproduced for mass production processes.Moreover,the phase segregation is inevitable,further deteriorating the device stability and lifetime.Therefore,it is critical to construct high-concentration doped or nondoped OLEDs with excellent EL performance,which are rarely reported.In addition,the efficiency of OLEDs tends to decrease with increasing brightness,which has obvious negative impact on power consumption.To deal with these problems,one feasible solution is designing new Ir???complexes by using simple chemical structures,and then investigate structure-property relationship in details.In this thesis,a series of Ir???complexes were designed and synthesized by employing 1,2-diphenyl-1H-benzoimidazole derivatives as the cyclometalated ligands,which were functionalized by simple substituents to increase the steric hindrance and thus depress concentration quenching effect.Meanwhile,ancillary ligands equipped with different conjugated unit or functional groups were used to adjust photoelectronic properties of the Ir???complexes.A series of new Ir???complexes were synthesized and applied in high-concentration doped or nondoped OLEDs.The photophysical and EL performances of these complexes were studied and discussed in detail as follow:?1?For decreasing the concentration quenching effect,highly efficient Ir???complexes are designed and synthesized using PBI modified by simple methoxyl group as main ligand.Varying the position of methoxyl group on cyclometalated ligand and conjugation of ancillary ligand can tune photophysical and electronic properties.The monochromic OLED based on pOMe-Ir-PI at highly doped concentration exhibits excellent EL performance and negligible efficiency roll-off.The EL performance is much higher than those of commercial green Ir?ppy?₃ and Ir?ppy?₂acac.The WOLED based on pOMe-Ir-PI also achieves high EL performance.These results indicate that employing methoxyl group in Ir???complexes can increase steric hindrance and adjust molecular packing,and thus depress concentration quenching effect in solid states,which are favorable for constructing high-concentration monochromic and white OLEDs.?2?To develop highly efficient orange Ir???complexes is crucial for constructing B-O complementary color WOLEDs.We choose PBI modified with simple methoxyl group as the cyclometalated ligand to construct Ir???complexes,aiming to fabricate high-concentration devices.The highly conjugated and rigidity 2-benzo[d]imidazol-2-ylquinoline is adopted as the ancillary ligand for altering the emissions and decreasing nonradiative decays.High-concentration doped devices employing mOMe-Ir-BQ as dopant emitters achieves high EL performance and broad full-width at halfmaximum values over 100 nm,which is beneficial for constructing two-component WOLED with high quality.WOLED based on pOMe-Ir-BQ exhibits promising performance with the maximum CE of 31.8 cd A^-1,which is still keeping at 29.3 cd A^-1at high luminance,revealing low efficiency roll-off.More importantly,WOLEDs based on these complexes display extremely high color stability.?3?Constructiong simple structure nondoped devices with high EL performance is important for commercial application.Further investigation on relationship of molecular structure and EL performance is highly desired for materials design.We design and synthesize two Ir???complexes Me-Ir-FTZ and tBu-Ir-FTZ based on PBI ligands decorated with methyl and tert-butyl groups.The parent complex H-Ir-FTZ with non-substituent on PBI ligand has also been presented for a better comparison.Nondoped device employing tBu-Ir-FTZ as emitting layer shows the highest EL performance with maximum CE of 18.6 cd A^–1and PE of 16.2 lm W^–1,which is much higher than those of H-Ir-FTZ and Me-Ir-FTZ.These results herein suggest that introduction of the simple substituent into PBI ligand is an effective and feasible approach to develop highly efficient non-doped phosphors.?4?To further improve the EL performance of nondoped OLEDs,N-heterocyclic carbene?NHC?ligands with strong ?-donating ability are employed as ancillary ligand to increase the strength of metal-cabene antibonding orbital and depress nonradiative decay.The diphenylsulfone and diphenylphosphine oxide moieties with electron-withdrawing property are introduced to NHC ligands for improving electron injection/transporting abilities.Complex tBu-Ir-ISO is capable of transporting both electrons and holes with similar magnitude,and the nondoped device based on this complex achieves highest EL efficiency with the peak CE and power efficiency?PE?of 27.0 cd A^–1and 30.7 lm W^–1,which are higher than those of tBu-Ir-FTZ.These results herein suggest that introduction of NHC ligands functioned with electron-withdrawing groups can improve the emission efficiency and charge transporting ability of Ir???complexes,which are important for enhancing EL performance of nondoped devices.