Synthesis And Properties Characterization Of Thermally Activated Delayed Fluorescence Materials Based On Pyrimidine And Fused-donors

Organic light-emitting diodes(OLEDs),by virtue of their self-emission,solid-state,flexibility,wide color gamut and so on,have rapidly been penetrating into the display and lighting markets.Third-generation emitters of the purely organic thermally activated delayed fluorescence(TADF)molecule with the capability of converting triplet(T₁)into singlet(S₁)excitons by reverse intersystem crossing(RISC),achieving 100%internal quantum efficiencies(IQEs),have been an active area of research.However,the donors in the molecular design are very limited and the efficiency roll off is serious.In this dissertation,multiple TADF molecule were designed and synthesized based on pyrimidine and fused-donors.The CH×××N positions in the acceptors of pyrimidine–pyridine hybrids,the substituted positions of donors at the TPPM(2,4,6-triphenylpyrimidine)acceptor and the fused topologies in fused donors were studied,respectively,which confirmed the molecular vibration relaxation and intermolecular packing were weakened by the bulky and rigid fused-donors.Finally,highly efficient TADF OLEDs with low efficiency roll-off have been fabricated.The details were listed as follows:(1)The peripheral 4-position phenyls of TPPM were substituted with 2-pyridyl.The additional intramolecular CH×××N hydrogen bonding enhanced the PLQYs of the delayed components for DPAc-4Py PM.As a result,the rate of reverse intersystem crossing(k _RISC)of DPAc-4Py PM realized nearly 5 and 10 fold improvements compared with two other contrast molecules.Its corresponding devices achieved a maximum EQE(EQE_max)of 24.3%and efficiency roll-off was the slowest.Then the linking topologies of the carbazole-benzofuran/benzothiophene fused-donors of BFCz(5H-benzofuro[3,2-c]carbazole)and BTCz(5H-benzo[4,5]thieno[3,2-c]carbazole)with TPPM acceptor were optimized.Finally,the compound with the para-substitution of BFCz at 4-phenyl of TPPM performed the best integrated nature.(2)The fused-donor of 34AcCz was fabricated by fusing carbazole and acridine.Linking34AcCz with optimal position at TPPM and TPTrz(2,4,6-triphenyl-1,3,5-triazine)acceptors,two TADF emitters with AIE characteristics were designed and synthesized.34AcCz-PM with the more prominent AIE characteristic possessed a higher PLQY and shorter delayed lifetime.The 34AcCz-PM-based doped device demonstrated the EQE_max of22.6%,which remained at 22.0%and 20.1%at the luminance of 1000 cd/m² and 5000 cd/m²,respectively.The 34AcCz-PM-based nondoped device provided the EQE_max of 14.1%.The performances of above devices outperformed that of 34AcCz-Trz.(3)Tha carbazole-acridine hybrid donors,12AcCz and 23AcCz,further fabricated.Two emitters of 12AcCz-PM and 23AcCz-PM are constructed by attaching at TPPM with optimal position.The large steric hindrance of 12AcCz-PM between the donor and acceptor compels Ac of 12AcCz to be crooked,which endowed it with a relatively planar quasi-axial conformation and no TADF feature.The steric hindrance of 23AcCz-PM was insufficient to induce 23AcCz crooked.The orthogonal structure rendered 23AcCz-PM with a quasi-equatorial(E)conformation and efficient TADF behavior.An EQE_max of 26.1%for the23AcCz-PM-based doped device was realized,and EQEs can still remain to be 24.8%and20.7%at at 1000 cd/m² and 5000 cd/m²,respectively.Compared with the reference devices of 34AcCz-PM,23AcCz-PM-based doped and non-doped devices presented EQE_max of23.1%and 17.6%,which outperformed that of 34AcCz-PM.(4)The acridine-benzofuran/benzothiophene-fused donors of 34Bx Ac(Benzofuran,x=F:34BFAc;Benzothiophene x=T:34BTAc)were designed and synthesized.Combining the TPPM with optimal position using the donors of 34Bx Ac and the reported 12Bx Ac,12Bx Ac-PM and 34Bx Ac-PM were synthesized.Compared with 12Bx Ac-PM,the intramolecular steric hindrance of 34Bx Ac-PM was small,the performance was not sensitive to the heteroatom species(O or S)and their electronic cloud of HOMO–LUMO was spatially extended better.As a result,34Bx Ac-PM possessed the smaller(35)E_ST,shorter decay lifetime and higher PLQY.Finally,the 12BFAc-PM and 12BTAc-PM-based doped device shown the EQE_max of 12.9%and 25.3%,respectively.However,the EQE roll-offs of12Bx Ac-PM-based devices were very serious.The 34BFAc-PM and 34BTAc-PM-based device realized an EQE_max of 27.7%and 25.6%,respectively.EQEs remained to be 19.6%and 20.0%at 5000 cd/m²,respectively.