Synthesis And Application Of Thermally Activated Delayed Fluorescence Materials Based On Pyrimidine Dericatives

As the core material of the third-generation of organic light-emitting diodes(OLEDs),thermally activated delayed fluorescence(TADF)materials have been widely attracted attention because they can achieve 100% internal quantum efficiency without the use of heavy metals.However,the current understanding of TADF materials in terms of molecular structure design and luminescence mechanism is still not perfect.Most existing design theories only provide a basic framework.In actual research,the specific system usually encounters a situation beyond the theoretical framework,especially the large difference in their efficiency.Therefore,it is necessary to study the TADF material of the specific system on the basis of following the existing design theory.At present,many organic light-emitting materials containing pyrimidine and benzopyrimidine have been reported,but their device efficiency still keep low,especially the maximum external quantum efficiency(EQE)of the device of benzopyrimidine-based emitters is only 1.58%.In order to further improve their luminescent properties and study their design rules,this thesis uses pyrimidine and benzopyrimidine as the acceptor unit of donor-acceptor(D-A)type TADF materials to systematically study the molecular structure and the relationship between device performance.In addition,combining theoretical calculation to assist design and feedback,explore the effective ways and laws for molecular design optimization of such materials,and finally obtain a series of new high-efficiency TADF materials.The specific research contents are as follows:Using the pyrimidines with different substituents as acceptor units and 10H-spiro[acridine-9,9'-fluorene](SPAc)as donor unit,we designed and synthesized three D-A-D type of blue highly-efficiency TADF materials with large twisted D-A-D structure,2SPAc-HPM,2SPAc-PPM and 2SPAc-MPM.They have successfully achieved the small singlet-triplet energy gaps(?EST)of 0.16-0.19 e V and the effective reverse intersystem crossing(r ISC),and therefore exhibit high photoluminescence quantum yields of 82.2-97.0%.2SPAc-HPM-,2SPAc-MPM-,and 2SPAc-PPM-based OLED devices have the emission peaks at 479-489 nm and have achieved high EQEs of 25.6%,24.3%,and 31.5%,respectively.Among them,2SPAc-PPM is also one of the few blue TADF emitter whose EQE exceeds 30.0%,and has reached the best level of similar pyrimidine-based TADF emitters.Based on the emitter of p-2SPAc-PPM(the above mentioned 2SPAc-PPM),we utilized the optimization of molecular structure to concurrently achieve high efficiency,luminous brightness,and suppress efficiency roll-off in the OLED devices.Using 2-phenylpyrimidine(PPM)as the acceptor unit and SPAc as the donor unit,two new D-A-D type of TADF emitters,m-2SPAc-PPM and o-2SPAc-PPM,were constructed by changing their connection positions.Unlike the previous para-links,these two emitters are connected through the meta-links and the ortho-links,respectively,resuting in the larger twisted structure than p-2SPAc-PPM.Therefore,both m-2SPAc-PPM and o-2SPAc-PPM exhibit smaller ?EST of 0.10 and 0.03 e V,and larger r ISC rates up to 9.10 × 106 s-1 and 3.55 × 107 s-1,respectively.Applied them to OLED devices,it was found that o-2SPAc-PPM-based OLED achieved a maximum EQE of 24.8% and a maximum luminous brightness of 24200 cd/m2.Compared with p-2SPAc-PPM-based OLED,o-2SPAc-PPM-based OLED has smaller efficiency roll-off,and its maximum EQE is greater than 10.0% even at a high brightness of 10000 cd/m2.We obtained a simple benzopyrimidine acceptor unit by fusing a benzene ring at the 5,6-position of pyrimidine unit and extending its ? conjugation length.It also was combined with phenoxazine donor unit to obtain four new type of TADF emitters,4HQ-PXZ,4PQ-PXZ,2HQ-PXZ,and 2PQ-PXZ.Their energy level gaps between the charge transfer singlet state(1CT)and the local triplet state(3LEA)of the benzopyrimidine acceptor unit are 0.09-0.22 e V,played a key role for their TADF character.At the same time,four materials doped in the CBP host achieved fluorescence quantum yields as high as 67.5-81.0%.Moreimportantly,Their OLED devices exhibit the high EQEs of 16.0-20.5%,which is 10-13 times higher than the reported benzopyrimidine-based light-emitting materials(?1.58%).Single pure organic molecular white light emitters(SPOMWLE),2PQ-PTZ and 4PQ-PTZ with dual configurations were designed and synthesized by combining the phenothiazine donor unit with quasi-axial and quasi-equatorial configuration features with the benzopyrimidine acceptor unit.Analysis by single-crystal X-ray diffraction,the quasi-axis and quasi-equatorial conformational isomers coexisted at a precise ratio of 1: 1 in 2PQ-PTZ crystals was found.Two different configurations of 2PQ-PTZ provide ideal flexibility for white light emission.Based on this feature,it was applied to light emitting diodes(LEDs),and obtained white LEDs with CIE(0.32,0.34)and color rendering index(CRI)of 89.3.In addition,the white OLED based on 2PQ-PTZ exhibits a maximum EQE of 10.1%,which is the highest performance among the reported SPOMWLE-OLEDs.At the same time,in order to make full use of the TADF characteristics of the quasi-equatorial conformer,a high-efficiency orange TADF OLED based on 2PQ-PTZ was obtained with an EQE of 25.0% by increasing the doping concentration.