The Design And Synthesis Of Triazine And Imidazole Based Organic Materials For Organic Light-emitting Diodes

Organic light-emitting diodes（OLEDs）have very important applications in the current flat-panel displays and solid-state lightings due to the advantages of self-illuminating,light weight,super thin,low energy consumption,fast response,high contrast and the potential to realize flexible displays.However,OLEDs still confronted with some challenges on the issues of high cost,poor device stabilities and so on.To develop the organic functional materials with low cost,high device efficiency and operational stability are the determining factors to solve these key challenges in OLEDs application,such as,blue fluorescent materials and electron-transport materials.Blue emitter can not only effectively decrease the power consumption of the devices,but also can be utilized to generate emission of other colors by energy transfer to suitable emissive dopant in full-color displays.Blue PhOLEDs typically have shorter lifetime and sharper efficiency roll-off at high brightness.Therefore,FlOLEDs typically are used for most high-efficiency blue OLEDs.Among,non-doped blue fluorescent emitters show great advantage in OLEDs application for simplify the device structure and reduce fabricating cost.D-A structure molecular design has been proven to be an effective strategy to obtain highly efficient non-doped blue OLED due to the low charge injection barrier and balanced charge transfer in the emissive layer.However,D-A structure molecular employing conjugated acceptor unit with donor unit will lead to bathochromic shifts of their photoluminescence（PL）and electroluminescence（EL）.Besides,organic electron-transport materials is the essential element to determine the performance and stability of OLEDs for promoting electron injection and transporting and blocking excition and hole.While a variety of electron-transport organic small-molecule materials have been reported to afford high OLED efficiencies and/or low efficiency roll-offs,there appear only a few having being investigated for the purpose of operational stability,which will hinder its application.This thesis mainly focuses on the design and synthesis of small-molecule blue-emitting materials and electron-transport materials based on the electron-deficient imidazolyl and triazinyl moieties.The main findings are shown below:（1）Phenanthroimidazole as one of the ideal building blocks for non-doped blue fluorescent emitters,which possesses bipolar character,high PLQY,deep blue emitter and thermal stability.Three D-A blue emitting compounds PPI-2FPh,PPI-PO and PPI-TRZ have been constructed by attaching electron-withdrawing groups such as difluorobenzene,diphenylphosphine oxide and 1,3,5-triazine to the weak electron-donating phenanthroimidazole moiety,respectively.The fluorescent spectra of PPI-2FPh and PPI-PO exhibit peaks at 424 nm and 448 nm,respectively.However,the fluorescent spectrum of PPI-TRZ shows a red-shifted peak at 482 nm for the strong electron-accepting ability of 1,3,5-triazine unit.The OLEDs that contained these non-doped emitters exhibit attractive EL performance.The PPI-2FPh and PPI-PO OLEDs exhibit blue emission with a maximal external quantum efficiency(EQE_max)of5.8%and 4.2%,current efficiency(LE_max)of 5.3 cd A^-1 and 4.5 cd A^-1,power efficiency(PE_max)of 5.5 lm W^-1 and 4.3 lm W^-1 and CIE coordinates of（0.15,0.10）and（0.15,0.11）,respectively.On the other hand,the PPI-TRZ OLED shows sky blue emission with LE_max,PE_max,EQE_max of 12.2 cd A^-1,12.2 lm W^-1,6.1%and CIE（0.17,0.28）.（2）Electron-transport materials ET-1 and ET-2 were synthesized by combining benzimidazolyl and 1,3,5-triazinyl moieties.The red phosphorescent OLEDs that contained the electron-transport layer ET-1:50%wt Liq produced LE,PE and EQE of 15.4 cd A^-1,11.0 lm W^-1and 10.7%@1000 cd m^-2 with a primitive lifetime t₉₀≈240 h@1000 cd m^-2 under constant current driving vs.15.0 cd A^-1,10.7 lm W^-1 and 9.4%with t₉₀≈178 h for ET-2:50%wt Liq.（3）To further explore the new electron-transport materials with operational stability and low-cost of synthesis,a simple small-molecule compound 3-（3-（4,6-diphenyl-1,3,5-triazin-2-yl）phenyl）-1,10-phenanthroline（TRZ-m-Phen）is presented through appending triphenyltriazine to 1,10-phenanthroline.It possesses a high T_g of 112°C,and appropriate LUMO/HOMO levels of ca.-3.0/-6.5 eV.We show that n-doping TRZ-m-Phen with 8-hydroxyquinolatolithium（Liq,50%wt）provides enhanced electron mobility of 5.2×10^-6-5.8×10^-5 cm² V^-1 s^-1 at E=2-5×10⁵ V cm^-1.The TRZ-m-Phen:50%wt Liq OLED produced a long lifetime with t₉₆>255 h@1000 cd m^-2 for the red phosphorescent OLEDs.For green phosphorescent OLEDs,at a luminance of 1000 cd m^–2,LE=55.1 cd A^–1,PE=57.7 lm W^–1;moreover,the initial luminance of 1000 cd/m² showed little decay after being driven under a constant current for ca.330 h.In addition,we have developed TADF emitters CO-1 and CO-2 with a high PL efficiency in the solid state.Spiro[acridine-9,9’-fluorene]and spiro[acridine-9,9’-xanthene]were chosen as the steric electron-donor units,while benzophenone as the acceptor.CO-1 and CO-2 have a very small singlet and triplet energy gapΔE_STT of 0.04 eV.The PL spectra in film show maxima around 504 nm with a lifetime of⁰.5μs.The PLQY of the non-doped films is as high as about60-70%.