| Organic light-emitting diodes?OLEDs?has attracted a lot of attention due to their many distinct advantages,such as high contrast ratio,wide viewing angle,self-luminescence,low driving voltage,flexibility,lightweight.Vast efforts have been made to apply them in television,lighting,wearable devices,etc.With the popularity of fold-screen mobile phones since the beginning of 2019,OLEDs with higher efficiency and better performance have undoubtedly become the common pursuit of academia and industry.1,3-di?9H-carbazol-9-yl?benzene?m CP?as a typical carbazole derivative,has many intrinsic properties of carbazole,such as high triplet energy level and excellent hole transporting properties.Therefore,it has been widely used in OLEDs as the host material,hole-transporting material and electron-blocking material.By modifying molecular structure of m CP,a great deal of works has been done to improve its thermal stability and electron transporting ability in order to abtain m CP-based host materials with excellent performance,which have made outstanding contributions to improving the efficiency and performance of OLEDs.However,there are few concerns about the energy loss caused by aggregation-caused quenching?ACQ?effect of m CP host molecules,thus decreasing EL performances.In 2001,Tang's group proposed a new concept of aggregation-induced emission?AIE?,which can fundamentally overcome the problem of the ACQ effect,and provide an effective strategy to solve the problem of energy loss caused by the aggregation of host molecules in OLEDs,so as to further improve the efficiency and performance of OLEDs.In chapter 2,based on a common strategy for ACQ-to-AIE transformation,we covalently integrated a typical AIE moiety of tetraphenylpyrazine?TPP?with the conventional host of m CP to generate a bipolar AIEgen of TPP-m CP.TPP-m CP exhibits better thermal and conformational stability and carrier capacity,and higher emission efficiency in the film state than those of m CP.More importantly,higher maxima power,current,and external quantum efficiencies of 104.2 lm W-1,89.5 cd A-1,and 28.7%,respectively,are achieved by devices with TPP-m CP as the host than those with m CP with the same guest phosphor of PO-01.Thus,this work not only generates a new AIE-active host of TPP-m CP with high device performance,but also provides a strategy to construct more effective hosts by decorating conventional ones with AIE units,which will potentially promote the development of OLEDs.In chapter 3,we happened to find that one of the intermediates in chapter 2,m CP-Br,exhibited obvious room temperature phosphorescence?RTP?phenomenon:upon the illumination of a 365 nm ultraviolet lamp,its as-prepared powder exhibits bright white light emission,while,after stopping the photo-excitation,it turns into a bright yellow phosphorescent emission.Therefore,we carefully studied the RTP properties of m CP-Br,which exhibits an ultralong lifetime of 200 ms with a quantum yield of 14%.And we discussed the possible mechanism that results in the RTP of m CP-Br by analyzing the single crystal structure of m CP-Br and m CP.In addition,we have found other unique properties of m CP-Br,such as resistance to crushing,water and oxygen,which means no extra procedures or treatments were needed to carefully capture the RTP-active crystals.These unique characteristics are of great value in promoting the practical application of pure organic room temperature phosphorescent materials. |
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