Organic Electroluminescent Materials Based On Rigid Boron-containing Groups

Wide viewing angle,self-luminous,low power consumption,fast response and other excellent performance make organic light-emitting diodes（OLEDs）become one of the most competitive choices in the future display technology,and its huge application potential in commercial field has gradually been stimulated.After decades of development,OLEDs of various light-emitting colors have achieved success.The most basic starting point for developing high-efficient OLEDs is to obtain high internal quantum efficiency（IQE）.Organic luminescent materials based on thermally activated delayed fluorescence（TADF）mechanism reported in recent years can upconvert non-luminescent triplet excitons into singlet states that can radiate fluorescence through the reverse intersystem crossing resulting in 100%IQE.This type of luminescent material does not contain rare metals and is low in cost.Generally,a small singlet-triplet energy difference(△E_ST)is realized through a single molecule or an exciplex to open a channel for reverse intersystem crossing to realize TADF nature.The low cost and high-efficient performance make OLEDs based on the TADF mechanism rapidly develop into the research hotspot of the next generation of display technology.After nearly 10 years of research,TADF-OLEDs are expected to replace traditional fluorescent and phosphorescent OLEDs to officially enter the commercial field,but there are still some areas that still have research value,such as:high-performance deep blue and red TADF-OLEDs are relatively lacking;There are few related researches on non-doped TADF-OLEDs that can simplify the device manufacturing process.Based on the above considerations,this article focuses on the following aspects through some boron-containing organic light-emitting materials:In ChapterⅡ,we have connected two large hindered acridine derivatives,diphenylacridine（DPAC）and spirofluorene acridine（s AC）to boraxanthracene acceptors through diisopropylbenzene,and successfully synthesized two D-π-A compounds,DPAC-B and s AC-B.Theoretical calculation results show that in the ground state,both molecules have a very distorted geometric configuration,and the protecting group extending from the sp³ hybridized carbon atom on the donor can not only effectively inhibit the vibration and rotation of the molecule,but also Inhibit the interaction between adjacent luminous bodies and weaken the concentration quenching effect.The overlapping area of the highest occupied orbital（HOMO）and the lowest unoccupied orbital（LUMO）is small,and a small singlet triplet energy level difference(△E_ST)can be achieved.Both molecules exhibited blue emission in toluene solution（DPAC-B is 476 nm,FWHM is 0.37 e V;s AC-B is 448 nm,FWHM is 0.33 e V）,and have a higher fluorescence quantum yield（PLQY）,DPAC-B is 0.75,s AC-B is 0.94.The electroluminescence（EL）peak positions of the deep blue devices developed based on DPAC-B and s AC-B are all located at 436 nm,and the color coordinates are（0.159,0.055）,（0.166,0.066）,which are close to the standard blue CIE（0.131,0.046）of BT.2020.Moreover,the maximum external quantum efficiency（EQE）is 15.3%and 16.2%,respectively.In ChapterⅢ,we modified the boron-containing acceptor in Chapter 2 and introduced a rigid fluorene group to obtain a rigid large sterically hindered boron-containing acceptor,spirofluorene boronanthracene.And through diisopropylbenzene connected with DPAC and s AC donor,successfully synthesized two D-π-A compounds,DPAC-s B and s AC-s B.According to the theoretical calculation results,these two molecules also have very distorted geometric configurations;there are protecting groups at both ends of the molecule,which can protect the HOMO and LUMO orbitals at the same time,and can further inhibit the vibration and rotation of the molecules,which is conducive to realization High PLQY and high color purity.In addition,a small HOMO/LUMO overlap area will result in a small△E_ST.Both exhibited blue emission in the toluene solution.The photoluminescence peaks of DPAC-s B and s AC-s B were located at 473 nm and 453 nm,FWHM are 0.33 e V and 0.31 e V,respectively,and the PLQY was as high as 0.91 and 0.95.Doped deep blue OLEDs devices based on DPAC-s B and s AC-s B show the maximum EQE is as high as 22.5%and 25.4%,respectively.At 1000 cd m^-2,13.6%and 20.0%of EQE are still maintained,and the CIE are close to the standard blue CIE（0.131,0.046）of BT.2020.DPAC-s B is（0.154,0.055）and s AC-s B is（0.151,0.058）.The non-doped deep blue device developed by s AC-s B,the maximum EQE is as high as 22.5%,and at 1000cd m^-2,it also maintains a high EQE of 14.8%.It is worth noting that the CIE coordinates of this non-doped device are（0.155,0.089）,which is very close to the standard blue CIE coordinates established by NTSC.The results show that the blue TADF material developed based on this type of boron-containing acceptor is one of the most efficient materials among the blue TADF materials reported so far.In ChapterⅣ,in order to explore the application potential of the rigid boron-containing receptor developed in Chapter 3 in blue-green/green OLEDs,phenoxazine（PXZ）and phenothiazine（PTZ）were combined through diisopropylbenzene.Connected with this rigid large steric boron-containing acceptor,two compounds with D-π-A structure,PXZ-s B and PTZ-sB,were successfully synthesized.The theoretical calculation results and single crystals show that both molecules possess a twisted geometric configuration,it induces a smaller HOMO/LUMO overlap area to achieve a small△E_ST.In the toluene solution,the two compounds have green emission（peak positions are 503 nm and 522 nm,FWHM are 0.34 e V and 0.37e V,respectively）.The doped blue-green OLEDs device based on PXZ-s B shows the maximum EQE of as high as 25.9%,and at 1000 cd m^-2,the EQE is still as high as 22.8%,and the efficiency roll-off is relatively small.The maximum EQE of the non-doped green device developed based on PXZ-s B is 15.8%.When the brightness is increased to 1000 cd m-2,the EQE drops slightly to 14.6%.This result shows that this kind of rigid boron-containing acceptor also has great potential for the development of efficient blue-green/green OLEDs devices.In Chapter Ⅴ,we used the isomerization reaction of indole to introduce the 3H-indole unit with a deeper LUMO energy level into the π-conjugated system with both rigidity and steric hindrance,and successfully constructed two four-coordinate boron complexes,t Cz-B and t DPA-B,with red emission.Combining theoretical calculations and single crystal structure,it can be known that the two tertiary butyl groups on the periphery of the molecule and the two benzenes on the boron atom can serve as steric hindrance groups,extending the distance between two adjacent molecules to inhibit the interaction between molecules.It can be doped with a slightly higher concentration to reduce the luminescence of the host material and increase the red-light saturation in the OLEDs devices.When an exciplex with TADF properties composed of 3P-T2T and TCTA is used as the co-host of the light-emitting layer,the maximum EQE of the device based on t DPA-B is as high as 10.2%,which is twice the EQE limit of traditional small-molecule fluorescent OLEDs.Not only that,the electroluminescence peak position of the device is 612 nm,and the higher doping concentration significantly inhibits the emission of the excimer complex,and the red electroluminescence with CIE coordinates（0.620,0.371）is also obtained,which is very close to standard red CIE of（0.67,0.33）established by NTSC.