Synthesis And Characterization Of Solution Processable Thermally Activated Delayed Blue Fluorescence Materials Based On 1,3,5-Triazine And Fluorene Derivatives

In the past thirty years,organic light emitting diodes?OLEDs?have been extensively studied in laboratories and factories due to their potential applications in the next generation of flat panel displays and solid-state lighting technologies.However,the first generation of OLED based on fluorescent materials can only harvest the singlet excitons and the internal quantum efficiency can not break the limit of 25%.Second-generation OLED based on rare heavy-metal phosphorescent materials can harvest both singlet and triplet excitons for light emission,and then achieve 100%internal quantum efficiency through the enhanced spin-orbit coupling.However,the high cost of phosphorescent materials and unknown toxicity have limited their further development.In the past ten years,third-generation OLED based on pure organic thermally activated delayed fluorescence materials with external quantum efficiencies of more than 20%can achieve 100%internal quantum efficiency through up-conversion.The electron transportability of conjugated compounds can be improved by using the 1,3,5-triazine derivatives due to their high electronic affinity.In addition,as electron donating groups,fluorene derivatives can impart hole transporting properties to conjugated compounds.Moreover,it also has high luminous efficiency,wide energy gap,and high triplet energy level.The main topic of this paper is to chemically bond the 1,3,5-triazine derivatives with fluorene derivatives to produce excellent organic light-emitting materials with high luminous efficiency,wide energy gap,and high triplet energy level.The chemical structure of the material is characterized by using ¹H NMR,¹³C NMR,MALDI-TOF-MS and GPC.DSC and TGA are used to determine the thermal stability of the material,and the ultraviolet absorption,photoluminescent spectra and low temperature phosphorescent spectra are used to characterize the photophysical properties of the material.The CV is also used to measure the energy levels of the HOMO and LUMO of the obtained material.Their device performance was also investigated to further correlate the light-emitting properties with the topology-varied molecular structure.Three new bipolar derivatives based on 1,3,5-triazine and fluorene moieties,namely FTRZ,pTFTRZ and mTFTRZ were designed,synthesized and reported as hosts for thermally activated delayed fluorescence OLED.Their thermal,photophysical and electrochemical properties as well as device performance were thoroughly investigated to correlate the optoelectronic properties with the topology-varied molecular structure.All the materials possessed high thermal stability with the thermal decomposition above 400°C and the glass transition temperatures of pTFTRZ and mTFTRZ are 103 and 120°C,respectively.The optical band gaps of compound FTRZ,pTFTRZ and mTFTRZ in toluene solution are 3.24,3.29 and 3.24 eV,and their triplet energy levels are 3.04,3.11 and 3.05 eV,respectively.Due to the?-?interaction between the 1,3,5-triazine planes,the compounds FTRZ,pTFTRZ and mTFTRZ form excimers in the thin film state,and their fluorescence spectra are clearly red-shifted.The electroluminescent properties of OLED using FTRZ,pTFTRZandmTFTRZasthehostsand2,4,5,6-tetrakis?carbazol-9-yl?-1,3-dicyanobenzene as guest emitter were investigated.The green OLED of compound FTRZ as host material shows a peak emission at 510 nm with a maximum current efficiency of 6.7 cd/A,a maximum external quantum efficiency of 2.07%and a maximum brightness of 35718 cd/m²,which is much better than those of the same devices hosted by compounds pTFTRZ and mTFTRZ.The red phosphorescent organic light-emitting diode hosted by compound FTRZ,pTFTRZ and mTFTRZ with bis?2-phenylpyridine??acetylacetonato?iridium?III?as the guest exhibit the red emission spectra peaking at 630 nm.The turn-on voltage,maximum brightness and maximum current efficiency of the red OLED hosted by compounds FTRZ,pTFTRZ and mTFTRZ are 5.1,4.6 and 4.1 V;4630,6255 and 6016 cd/m²;1.1,1.3 and1.4 cd/A,respectively.In order to study the influence of different substituents at the 9-carbon of the fluorene unit on the photophysical properties of organic light-emitting materials based on polyfluorene,we use the1,3,5-triazine and fluorene derivatives as monomers to obtain the polymer 1,2 and 3 through Suzuki cross-coupling reaction.The thermal decomposition of polymer 1,2 and 3 are 274,318 and401°C and the glass transition temperatures of polymer 1,2 and 3 are 91,120 and 139°C,respectively.The maximum absorption peaks of polymer 1,2 and 3 in toluene solution are 380 nm,and the maximum fluorescence emission peaks in toluene solution of them are 435 nm.As the volume of the substituents on the 9-carbon of the fluorene unit increased,the full width at half maxima of the fluorescence spectra decreased gradually in the turn of the polymer 1,2 and 3.The triplet energy levels of polymer 1,2 and 3 are 2.82,2.81 and 2.97 eV and the energy gaps of them in toluene solution are 2.95,2.95 and 2.91 eV respectively.The singlet-triplet energy splites for polymer 1,2 and 3 are 0.32,0.32 and 0.15 eV,respectively.The HOMO energy levels of polymer 1,2 and 3 are-5.72,-5.95 and-5.96 eV and the LUMO energy levels are-2.70,-2.39 and-2.34 eV.The root mean-square?rms?roughness of the films of polymer 1,2 and 3 are 5.28 and 2.49 and0.387 nm,respectively.After annealed at 200°C for 10 min,the root mean-square?rms?roughness of the films of polymer 1,2 and 3 increased from 5.28,2.49 and 0.387 nm to 18.7,7.24 and 2.33nm,and the stability of the film fluorescence spectra gradually increased either.The wide-angle X-ray diffraction results of the polymer 1,2 and 3 powders showed that the order of the powder of polymer 3 located between the polymer 1 and 2,and the alkoxybenzene substituent on the side chain of the polymer 2 helped to increase the order diversity of the powder.All the fluorescence materials based on 1,3,5-triazine and fluorene derivatives mentioned in this paper show excellent thermal stability,high triplet energy levels,wide energy gaps and good solubility in common organic solvents.The wide application prospects of the fluorescence materials in the preparation of solution-processable high-efficiency OLED make them to play an important role in promoting the development of OLED.