Photoluminescent And Electroluminescent Properties Of Novel Tris-Heteroleptic Ir(Ⅲ)-Complexes With Near-Infrared Emission

In recent years,near-infrared organic/polymer light-emitting diodes（NIR-OLEDs/PLEDs）have attracted extensive attention in applications of data storage,bioimaging,optogenetics,biosensing,optical communications,remote sensing and night-vision display.Accordingly,the development of organic near-infrared emissive materials with excellent photo-luminescent properties has become one of the most energetic and emergent topics in the emitting material fields so far.Of those,concerted efforts have been devoted to cyclometalated iridium（III）complexes forwards desirable NIR-OLEDs/PLEDs,due to the advantages of structural stability,six-coordinate octahedral configuration,color-tunability within the NIR region,theoretically 100%internal quantum efficiency and relatively short excited state lifetime,etc.Nonetheless,as constrained by the so-called“energy-gap law”,where with the increase of the emitting wavelength in the NIR regime,the quantum efficiency has to be decreased exponentially,and thus,it is quite challenging to develop ideal organic NIR-emission materials and the resultant NIR-OLEDs/PLEDs with high external quantum efficiency,weak efficiency roll-off and low-cost.In this perspective,besides an unaddressed issue on the opposition and contradiction between red-shifted wavelength and reduced luminescence efficiency,the molecular design of the NIR-emitting cyclometalated iridium（III）complexes is so limited in homoleptic or and bis-heteroleptic structural form.Aiming at resolving the above scientific problems,a conceptual molecule-designed idea of C₁-symmetric tris-heteroleptic cyclometalated iridium（III）complexes is proposed in this thesis.Through the strategy,it is not only beneficial to the improvement of their NIR-emissive properties,but also the NIR-emission with both high color-purity(λ_em>750 nm)and large quantum efficiency can also be achieved just through the increase ofπ-conjugation of one specific C^N ligand.At the same time,their NIR-PLEDs devices with both high external quantum efficiency and low efficiency-roll-off,especially suitable for low cost,large area and scalable product,are successfully realized.The main research contents and results of this thesis are as follows:Firstly,the nitrogen and sulfur heterocyclic ligand Hiqbt（1-（benzo[b]thiophen-2-yl）-isoquinoline）as the C^N¹ cyclometalated main ligand,and Hacac（acetylacetone）as the O^O-type ancillary ligand,and one of the three different cyclometallic ligands（Hd Fppy（2-（2,4-Difluorophenyl）pyridine）/Hppy（2-Phenylpyridine）/Hdpqx（2,3-Diphenylquinoxalin-e））were introduced as the C^N² cyclometalated ligand,respectively,were adopted to afford three novel[Ir（C^N¹）（C^N²）（O^O）]-type C₁-symmetric tris-heteroleptic iridium（III）complexes[Ir（iqbt）（d Fppy）（acac）]（1）,[Ir（iqbt）（ppy）（acac）]（2）and[Ir（iqbt）（dpqx）（acac）]（3）,correspondingly.The results indicated that iridium（III）complexes 1-3 exhibited excellent NIR-emissive efficiency(Φ_PL=0.18 for 1(λ_em=703 nm),0.26 for 2(λ_em=715 nm)or 0.28 for 3(λ_em=707 nm)),owing to the strengthened ³MLCT contribution（MLCT=metal-to-ligand charge transfer）from the C₁-symmetry and tris-heteroleptic structure.Moreover,the quantitative molecular orientation determination of their doped emitting layer EMLs-1/2/3 of NIR-PLED-1/2/3 reveals that the preferential horizontal orientation of the transition dipole moment is beneficial to the enhancement of the light out-coupling efficiency of their NIR-PLEDs.Furthermore,NIR-PLED-2 showed the highest external quantum yield(η_EQE^Max=5.30%)and negligible efficiency roll-off（<2%）compared with the reported NIR-PLEDs based on Ir（III）-complexes in that NIR gamut(λ_EL=715 nm).This study demonstrates that C₁-symmetric[Ir（C^N¹）（C^N²）（O^O）]tris-heteroleptic iridium（III）complexes should provide a new platform to NIR-emitting materials for low-cost and large-area and scalable NIR-PLEDs.Secondly,using Hiqbt as the C^N¹ main ligand,Hppy as the C^N² ligand,and picolinic acid Hpic（2-picolinic acid）or its bromine substituted derivative Br-Hpic（4-bromo-2-picolinic acid）as the N^O ancillary ligand,two novel C₁-symmetric[Ir（C^N¹）（C^N²）（N^O）]-tris-heteroleptic Ir（III）-complexes[Ir（iqbt）（ppy）（pic）]（4）and[Ir（iqbt）（ppy）（Br-pic）]（5）with efficient NIR-emission were molecularly designed,respectively.Among those,large TDM（transition dipole moment）and strengthened ³MLCT（metal-to-ligand charge transfer）effects were established to confirm their good NIR phosphorescent efficiency(Φ_PL=0.27 for 4(λ_em=698 nm);0.21 for 5(λ_em=696 nm)).Meanwhile,the cyclometalated iridium（III）complexes 4 and 5 also had short excited state lifetimes（τ=0.34～0.35μs）,excellent thermal stability and electrochemical performance.Moreover,in their doped EMLs（emitting layers）,the preferential horizontal orientation of the transition dipole moment also enabled NIR-PLED-4/5 to exhibit attractive device performance((η_EQE^Max=3.1～4.7%,λ_EL=698 nm,and efficiency-roll-off below 5%).Thirdly,using Hiqbt as the C^N¹ main ligand,Hppy as the C^N² ligand,and picolinic acid derivative hpa（3-Hydroxy-2-pyridinecarboxylic acid）as the N^O or O^N ancillary ligand,the C₁-symmetric[Ir（C^N¹）（C^N²）（N^O/O^N）]-tris-heteroleptic iridium（III）complexes[Ir（iqbt）（ppy）（hpa）]（6a and 6b）characteristic of geometric isomerization were successfully constructed,respectively.Particularly,the relationship between their isomeric structure and photo/electroluminescent property was studied in depth.The isomers 6a and6b exhibit excellent thermal stability(ΔT_5%=312?C and 316?C)and NIR-emitting properties(λ_em=697～698 nm;τ=0.33～0.38μs;Φ_PL=0.27～0.33).Meanwhile,the doped NIR-PLED-6A/6B was fabricated,from which,NIR-PLED-6A showed more superior electroluminescent property(V_on=6.1 V,J^Max=191.5 m A?cm^-2,R^Max=1952.9 m W?sr^-1?m^-2andη_EQE^Max=3.6%).At last,using the more rigid Hdpbq（Hdpbq=2,3-diphenylbenzo[g]quinoxaline）and Hiqbt as C^N¹ and C^N² ligands,respectively,the N-heterocyclic carbene-based ligand pmi（pmi=2-phenylimidazole）as the C^C-type ancillary ligand,the bis-heteroleptic iridium（III）complexes[Ir（dpbq）₂（pmi）]（7）,[Ir（iqbt）₂（pmi）]（8）,and C₁-symmetric tris-heteroleptic iridium（III）complex[Ir（dpbq）（iqbt）（pmi）]（9）were successfully obtained.Complex 8exhibits deep-red emission(λ_em=680 nm),which with certain residual visible light is comparable to those in the research contents 1-3.Interestingly,complexes 7 and 9 are typically high color-purity NIR-emitters with the emissive wavelengths of 772 nm and 776nm,respectively.Using PVK-OXD7 as the host material and high color-purity NIR-emitting7 or 9 as the dopant,their NIR-PLED-I and NIR-PLED-II characteristic of high color-purity NIR(λ_em>750 nm)electroluminescence,were reliably realized,respectively.Noticeably,the device performance of NIR-PLED-II based on C_1-symmetric tris-heteroleptic iridium（III）metal complex 9 is as follows:V_on=5.2 V,J^Max=102 m A?cm^-2,R^Max=861 m W?sr^-1?m^-2 andη_EQE^Max=1.85%.To the best of our knowledge,theη_EQE^Max of NIR-PLED-II is the highest among those of high color-purity NIR-OLEDs/PLEDs based on iridium（III）complexes,and it is also the first example of high-color-purity NIR-OLEDs/PLEDs from C₁-symmetric tris-heteroleptic iridium（III）complexes.