Design,Synthesis And Optoelectronic Properties Of Phosphorescent Iridium Complexes

Organic light-emitting diodes(OLEDs)have drawn tremendous attention from both academia and industry over the last two decades because of their unique applications in full-color flat-panel displays and solid-state lighting.Nowadays,phosphorescent materials are becoming ideal for fabricating highly efficient OLEDs since phosphors using Ir(?),Pt(?),or Os(?)can harvest both singlet(25%)and triplet(75%)excitons through intersystem crossing(ISC),and therefore elevate the internal quantum efficiency to 100%.Particularly,d6 octahedral Ir(?)complexes are regarded as the most promising phosphors due to their relatively short triplet lifetimes,high quantum yields,tunable emission color,and splendid thermal and electrochemical stability.In this thesis,a series of Ir(?)complexes were synthesized,and their properties were studied.Finally,a series of high-performance OLEDs based on these luminescent materials were achieved.In chapter 1,historic development of OLEDs,the basic principles of organic electroluminescence,device structures and device parameters are outlined.Then the various functional materials in OLED are introduced,in particular,the progress of iridium complexes are reviewed.In chapter 2,two deep-red emitting iridium complexes,(TP-BQ)2Ir(acac)and(TPA-BQ)2Ir(acac),with 6-phenanthridine derivatives as cyclometalating ligands were designed and synthesized.The introduction of a phenanthridine moiety enlarges the ? conjugation and causes the peak emissions to red-shift to ca.660 nn.Meanwhile,the bulky.ligands sufficiently protect the emissive core from the intermolecular interaction to decrease triplet-triplet annihilation(TTA).The solution-processed devices with a single emissive layer using(TPA-BQ)21r(acac)as the triplet emitter achieve a maximum external quantum efficiency(EQE)of 5.2%with the emission peak at 682 nm.In chapter 3,three novel heterolepic Ir(?)complexes,namely(ptq)2Ir(acac),(ttq)2Ir(acac)and(tptq)2Ir(acac),were designed and synthesized by utilizing extended?-conjugated thieno[3,2-c]quinoline derivatives as cyclometalating ligands.These complexes display intense red phosphorescence emission in the range of 610-620 nm.Accordingly,a vacuum-deposited red OLED based on(ptq)2Ir(acac)shows the highest EQE of 22.9%with Commission Internationale de L'Eclairage(CIE)coordinates of(0.61,0.36).In particular,the device exhibits an extraordinary low efficiency roll-off of 7.8%at a luminance of 1000 cd m-2.In chapter 4,the introduction of a pyrimidine moiety lowers the lowest unoccupied molecular orbital(LUMO)energy levels and consequently narrows the HOMO-LUMO(HOMO = highest occupied molecular orbital)energy gaps to obtain orange/red emissions.As a result,a series of phosphorescent iridium complexes,namely,PMD-Ir-1,PMD-Ir-2,PMD-Ir-3,and PMD-Ir-4,with emission peaks in the range of 578-614 nm are obtained.Notably,OLEDs utilizing PMD-Ir-2 and PMD-Ir-3 as emitters exhibit orange and red emissions with maxima EQEs up to 24.5%and 27.6%,respectively.In chapter 5,a series of highly efficient phosphorescent iridium emitters with 3-(2,4-difluorophenyl)-6-methylpyridazine(dfppdz)as cyclometalated ligands have been prepared via facile synthetic routes.Their photophysical and electrochemical.properties have been studied and further confirmed by the density functional theory(DFT)calculations.By altering the ancillary ligands,these complexes exhibited tunable emission colors with diversified photoluminescence quantum yields in the range of 0.72-0.92.OLEDs utilizing these phosphors as emitters achieved maxima EQEs of 23.1%,28.7%,22.7%,and 24.5%together with CIE coordinates of(0.24,0.58),(0.30,0.62),(0.36,0.61),and(0.38,0.60),respectively.In chapter 6,highly emissive thermally activated delayed fluorescence(TADF)moiety 4-((4-(9,9-dimethyl-9,10-dihydroacridine)phenyl)sulfonyl)phenyl(-DPS-DMAC)functioned as energy donor was grafted to an iridium complex bis[2-(4-(tert-butyl)phenyl)-N,N'-diphenylquinazolin-4-amine](3-hydroxypicolinic acid)iridium(III)(Ir-OH)with enhanced steric hindrance by a long alkyl chain,for the first time,to simultaneously manage intra-/intermolecular Forster resonance energy-transfer to the iridium unit,and suppressed phase segregation in the host-guest system.A dramatic improvement of 30%in photoluminescence quantum yield and an obvious reduction of 44%in exciton lifetime were simultaneously guaranteed via TADF functionalized phosphor.Bright red solution-processed OLED with a maximum EQE of 24.5%was demonstrated by further energy transfer assisted by 10-(4-((4-(hexyloxy)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine(hexODPS-DMAC)featuring TADF characteristics.In addition,the doped and non-doped devices exhibited the electroluminescent peak wavelength of 610 and 640 nm,respectively,accompanied with the CIE coordinates of(0.61,0.39)and(0.67,0.33).In chapter 7,two prototyped yellow-emitting dendrimers,namely Y-Ir-1 and Y-Ir-2,with a 4,6-diphenylpyrimidine-based cyclometalating iridium complex as the emissive core and TADF derivatives as outer dendrons were designed and synthesized.The TADF units not only efficiently reduce the intermolecular interactions but also contribute to enhanced either Forster to the emissive core.Consequently,significantly reduced exciton loss and enhanced phosphorescent emission are simultaneously realized.As a result,a maximum EQE of 10.9%was achieved in the nondoped solution-processed OLED using Y-Ir-2 as emitters.Furthermore,the doped devices based on Y-Ir-1 achieved a maximum EQE as high as 19.7%.For the first time,TADF dendronized iridium dendrimers are managed for high-performance OLEDs,which proves the diversity and versatility of the compounds.