Design,Synthesis And Electroluminescence Of Thermally Activated Delayed Fluorescence Emitters Based On Acridone Acceptors

In recent years,organic light-emitting diodes（OLEDs）display technologies have been developed rapidly and applied successfully in our daily life.The importance of light-emitting materials in OLEDs is self-evident.As the third-generation OLED emitters,the purely organic thermally activated delayed fluorescence（TADF）materials have attracted more and more attentions because of the merits of high efficiencies like phosphorescent materials and low cost like the traditional fluorescent ones.However,the current TADF materials still can’t be used for practical applications owing to many fundamental problems.For example,it is impossible to increase the rate constant of radiation（K_R）and reduce the singlet-triplet energy splitting(ΔE_ST)in traditional molecular design methods by increasing molecular twist angles.In addition,the blue TADF emitters,especially deep blue（CIE_y≤0.10）and pure blue(CIE_x+y<0.30)ones,still lag behind other two colors among the RGB.This dissertation is devoted to develop high-performance TADF emitters using acridone（AD）as electron acceptor,which is characterized by n-π*transition and high rigidity.A series of AD based novel TADF materials with emitting color covering from deep blue to yellow have been designed and synthesized.The study was focused on tune the energy alignment of singlet and triplet excited states by appropriate molecular design strategies and thus facilitate the RISC and TADF performance.The innovative molecular design method to simutaneously improve K_R and K_RISC has been proposed.The specific contents are as follows:（1）A series of new blue TADF emitters,i.e.,2,7-DCz-AD,2,7-Dt Bu Cz-AD,3,6-DCz-AD and 3,6-Dt Bu Cz-AD,were designed and synthesized using acridone（AD）as acceptor and carbazole（Cz）or 3,6-di-tert-butyl-9H-carbazole（t Bu-Cz）as donors.Single crystal analyses confirmed the sp² hybrided feature of the N atom in AD ring and the universal high rigidity of AD acceptor,which is mainly responsible for the narrow spectra and high K_R for all the AD emitters.The regioisomeric effect study verified that the attachment of donors at 3,6-sites of AD ring dramatically enhanced TADF ratio in comparison with the 2,7-site isomer.With slightly increasing donor strength and enlarge HOMO conjufation in 3,6-Dt Bu Cz-AD as compared to 3,6-DCz-AD,ΔE_ST was reduced and spin orbital coupling（SOC）was enhanced,resulting in K_R>10⁸ s^-1 and K_RISC>10⁴ s^-1.The deep-blue OLED of 3,6-Dt Bu Cz-AD realized the EQE_max of 17.88%,CIE（0.15,0.08）,and a FWHM of 53 nm,which are among the best performance for deep-blue TADF-OLEDs so far.（2）To further maintain the“trade-off”between K_R and RISC process,five deep blue or pure blue TADF emitters 3,6-DCz-AD,3,6-DPh Cz-AD,3,6-Dt Bu Ph Cz-AD,3,6-DDPh Cz-AD,3,6-DDt Bu Ph Cz-AD and 3,6-DDPy Cz-AD were designed and synthesized.By selecting Cz as the same donor skeleton,E_g of these emitters were almost kept.While the ¹CT（S₁）was stepwisely pulled down by gradually increasing donor strength via increasing the number of the peripheral groups on Cz.At the same time,³CT energy was also reduced to get close to the ³LE（T₁）state,resulting in smallerΔE_ST andΔE_TT and finally K_R>10⁸ s^-1 and K_RISC>10⁵ s^-1.Pure-blue OLED of 3,6-DDPh Cz-AD exhibited EQE_max of 17.35%with CIE（0.15,0.11）.（3）With dimethylacridine（DMAC）as moderate donors,a series of sky-blue TADF emitters were developed,including one DMAC containing 3-DMAC-AD,3-DMAC-6-Me-AD,3-DMAC-6-CF₃-AD and two DMAC containing 3,6-DMAC-AD and 3,6-DMAC-AD-CF₃.The planar conformation and high steric hindrance of DMAC with enhanced donor strength as compared to Cz led to extremely low or almost zeroΔE_ST for these DMAC-AD emitters.The energy level difference among the¹CT,³CT and ³LE states,as well as intermolecular interactions were simultaneously manipulated,generating both K_RISC and K_R in the order of 10⁶s^-1.3-DMAC-6-CF₃-AD achieved EQE_max of 21.61%and 14.68%in doped and non-doped sky-blue OLEDs.The peripheral CF₃ group in 3,6-DMAC-AD-CF₃ decreased intermolecular interactions,thus improved efficiency stability with efficiency roll-off of only 5.1%from the EQE_maxof 21.64%even at the high brightness of 1000 cd m^-2.（4）With phenoxazine（PXZ）as donors,a series of yellow TADF emitters 3-PXZ-AD,3-PXZ-6-Me-AD and 3,6-DPXZ-AD were developed.Owing to strong electron-donating ability and high steric hindrance of PXZ ring,the energetically close-lying excited states（i.e.,¹CT≈³CT≈³LE）were achieved,resulting in simutaneously high K_R and K_RISC.3-PXZ-6-Me-AD achieved EQE_max of 23.28%and 17.59%in doped and non-doped yellow devices.Since intermolecular through-space charge transfer state（TSCT）was involved,multichannel RISC process occurred for 3,6-DPXZ-AD,leading to K_R=1.4×10⁷ s^-1and K_RISC=1.1×10⁶ s^-1.The OLED of 3,6-DPXZ-AD exhibited an EQE_max of 30.6%,being the highest efficiency for acridone based TADF materials reported so far.