Regulation And Utilization Of Triplet Excitons In Organic Luminescent Materials

Organic luminescent materials are promising for next-generation flexible optoelectronics.Especially for triplet excitons in organic molecules,due to their long lifetime and spin multiplicity,organic room temperature phosphorescence（RTP）and thermally activated delayed fluorescence（TADF）materials derived from triplet excitons have great potential in developing a new generation of organic optoelectronic materials and biomedical agents.In the past few years,substantial progress has been made in the development of RTP and TADF materials based on molecular structure design and aggregation behavior modulation.However,due to the high sensitivity and complexity of triplet exciton behavior,its luminescence process and mechanism are not fully understood,which limits its further application.The studies in this dissertation are focused on the regulation of excited state process and the effective utilization of triplet exciton.Thermally activated delayed fluorescence lasers and pure organic phosphorescent light-emitting diodes have been developed.The main contents are described as follows:1.Controll of excited-state dynamics plays a central role in tuning room-temperature phosphorescence（RTP）and thermally activated delayed fluorescence（TADF）emissions,but remaining challenging in the exploration of organic luminescent materials（OLMs）.Herein,we demonstrated the regulation of TADF and RTP emissions of a boron difluorideβ-acetylnaphthalene chelate（βCBF₂）by controlling the excited-state dynamics via its J-and H-aggregation states.Two crystalline polymorphs emitting green and red light have been controllably obtained.Although both belong to the monoclinic system,the green and red crystals are dominated by J-and H-aggregation respectively,due to different molecular packing arrangements.Experimental and theoretical studies show that J-aggregation not only significantly reduces the energy gap between the lowest singlet and triplet excited states for the realization of ultra-fast reverse intersystem crossing（RISC）process,but also enhances the radiative decay of singlet,together leading to TADF.The H-aggregation accelerates the ISC and meanwhile suppresses the radiative decay of singlet,helping to stabilize the triplet exciton for RTP.2.The typical separation of the highest occupied and the lowest unoccupied molecular orbitals（HOMO and LUMO）of thermally activated delayed fluorescent molecules leads to charge transfer（CT）states,which degrades the oscillator strength of emission transition（f）and may sacrifice high solid-state photoluminescence quantum yield（PLQY）,together limiting its application in organic solid-state lasers（OSSLs）.Here,the TADF molecule-DCz BF₂ was designed and synthesized,which has the characteristics of aggregation induced emission（AIE）and local excited（LE）state characteristics.The unique LE dominated emission and AIE feature of TADF molecular leads to the approaching 50%PLQY in microwires.The regenerated singlet produced by reverse intersystem crossing（RISC）process is conducive to population inversion,combined with the optical feedback provided by the microwire FP-type resonant micro-cavity,we have achieved 465 nm multimode lasers in microwires with a low threshold of 3.74μJ/cm² and a high optical gain of 870 cm^-1.3.Metal-free organic phosphorescent materials are promising alternatives to the organometallic counterparts predominantly adopted in organic light-emitting diodes（OLED）due to their low cost,chemical stability,and large molecular design window.A pure organic molecule 3,6-bis（tert butyl）carbazole boron difluoride（Dt Cz BF₂）with RTP features was developed.It is found that the increase of intersystem crossing channels caused by molecular aggregation enhances the phosphorescence effect.The photoluminescence quantum yield of 53%was achieved when doped in 1,3-bis（9h-carbazole-9-yl）benzene（m CP）host material.Sky blue emitting OLED was achieved by a solution spin coating method using phosphorescent molecules as a light-emitting layer.The maximum external quantum efficiency is as high as 18.63%,the brightness is 5188 cd/m² and the power efficiency is 37.47 lm/W.