Molecular Design And Excited State Dynamics Modulation Of Novel Purely Organic Red Emitters

Since organic light-emitting diode（OLED）was creatively invented by Chinese American Dr.Tang in 1987,OLED has undergone in-depth research for more than 30 years,and has occupied a place in the field of small and medium-sized commercial displays nowadays and the development trend of OLED is in the ascendant.The mainstream luminescence mechanism of OLED has developed from the first generation of conventional fluorescence（CF）materials and the second generation of heavy metal complex phosphorescence materials to the third generation of thermally activated delayed fluorescence（TADF）materials.At present,the emitters used for commercial displays are conventional fluorescence materials for blue and phosphorescence materials for red and green one,respectively.Unfortunately,the price of phosphorescence materials is expensive,and most of the patent rights of phosphorescence materials belong to the Universal Display Corporation in the United States,which can easily become a key technology that is"stuck".Therefore,the development of a new generation of purely organic light-emitting materials with domestic independent intellectual property rights is imminent.As one of the essential primary colors for full color displays,the development of red emitters is far behind blue and green ones.Since the red emitters has an intrinsic smaller emission energy,so the photoluminescence quantum yield of the red emitters is restricted by the"energy-gap law".That is,due to the vibrational coupling between the low vibrational energy level of the first singlet excited state（S₁）or the first triplet excited state（T₁）of the materials with the high-energy vibrational energy level of the ground state（S₀）is aggravated,the non-radiative transition rate increases exponentially with the decrease of luminescent energy,and so the photoluminescence quantum yield decreases with the red-shift of the emission.Therefore,this thesis will focus on the development of a new generation of pure organic red light-emitting materials and realizing high-efficiency,low-efficiency roll-off red OLED devices by regulating the excited state processes of emitters.Starting from the design of donor-acceptor（D-A）type molecules,this thesis realizes high-efficiency,low-efficiency roll-off red OLED devices by means of the conjugatedπbridge design,excited state processes and energy transfer modulation.Quantum chemical calculations,molecular dynamics calculations,and photophysical exciton dynamics were used to analyze the potential factors affecting the device performance of emitters.In fact,research works are carried out mainly from the following aspects:First of all,in order to develop highly efficient deep-red to near-infrared organic light-emitting materials,three emitters were designed and synthesized based on difluoro-benzothiadiazole（BTDF）as the acceptor（A）core and triphenylamine as the donor（D）.Then,by introducing thiophene as a conjugatedπbridge into the molecular framework,the emission peak of red emitter was red-shifted 67 nm without reducing the photoluminescence quantum yield of the materials.In terms of material properties,the large frontier molecular orbitals overlap of these emitters was brought about by the small steric hindrance of thiophene,so a photoluminescence quantum yield up to 83%was maintained even the emission peak at 625nm.Photophysical characterizations and density functional theory calculations indicated that the three emitters were belonged to"hot exciton"materials,and a maximum external quantum efficiency(EQE_max)of 5.75%was achieved based on these emitters.Secondly,11H-indeno[1,2-b]quinoxalin-11-one（IQ）,a novel electron acceptor was developed and used for the construction of two red light-emitting isomers,by adjusting the attachment position of the triphenylamine donor unit,switch between CF and TADF emission mechanisms based on the isomers was realized for the first time.The electroluminescent devices based on IQ-o TPA and IQ-p TPA as emitters obtained the EQE_max of 20.6 and 3.5%with the emission peaks of 604 and 642 nm,respectively.The analysis of the experimental results shows that the energy difference,energy level arrangement,molecular rigidity induced the difference of non-radiative decay of triplet state and spin coupling effect（SOC）of the materials have a great influence on the triplet exciton up-conversion process of red emitters.Therefore,adjusting the molecular configuration of materials is an effective molecular design strategy to tune the performance of electroluminescent devices,which proposes a feasible design concept for obtaining molecules with high exciton utilization in future.In addition,Dexter energy transfer（DET）from the triplet state of the TADF assistant host to the triplet state of the CF guest is the primary energy loss process in hyperfluorescence OLEDs.Therefore,a new TADF assistant host FPXZ-DBPZ was obtained by introducing an electronically inert phenyl fluorenyl terminal substituent into the TADF assistant host PXZ-DBPZ with a one-step reaction,which reduced the DET process from the TADF assistant host to the CF guest by increasing the intermolecular distance between the TADF assistant host and the CF guest.The inhibitory effect of phenyl fluorenyl substituents on the DET process in hyperfluorescence OLEDs was investigated by photophysical characterization,theoretical calculations,device fabrication and Monte Carlo simulations.EQE_max of 18.1%was obtained for the hyperfluorescence device based on TADF assistant host FPXZ-DBPZ with the CIE coordinates of（0.61,0.38）,which is one of the highest efficiencies among the current red hyperfluorescence devices,and point out a new design direction for the next step to realize high-efficiency,low-efficiency roll-off OLED devices.Finally,to solve the issue of efficiency roll-off in red OLEDs,multiple efficient reverse intersystem crossing processes are realized by constructing a double TADF exciplex system formed by TADF donor material and TADF acceptor material for the first time.Compared with single TADF donor material or TADF acceptor material,the reverse intersystem crossing rate of double TADF exciplex is 3 times and 6 times higher than that of single donor or acceptor TADF materials,which can suppress the efficiency roll-off trouble in red light-emitting devices to a great extent.Actually,a EQE_max of 10.6%was obtained for the device based on double TADF exciplex with an emission peak of 604 nm.In addition,in order to improve the photoluminescence quantum yield of the double TADF exciplex system,the traditional fluorescent material DBP was introduced into the exciplex system as the luminescent guest.As a result,EQE_max of 11.8%was obtained for the exciplex sensitized fluorescence device with an emission peak of 626 nm,which are the highest efficiency among the red OLEDs based on exciplex and exciplex sensitized fluorescence.