Thermally Activated Delayed Fluorescence Materials Based On Spiro-acceptors

As the third generation organic electroluminescent material,thermally activated delayed fluorescence(TADF)materials can achieve 100%internal quantum efficiency(IQE)in electric light-emittiing devices because their triplet excitons can be up-converted into singlet excitons through reverse intersystem crossing(RISC).Compared with traditional fluorescence and phosphorescence materials,they have the higher device efficiency and the lower production cost,and so they have attracted all world's attention.Currently,the problem that need to be solved urgently for them is efficiency roll-off.And the study found that increasing the reverse intersystem crossing rate can shorten the lifetime of delayed fluorescence and help to suppress the efficiency roll-off;the spiro structure can block the extension of the conjugated surface and inhibit the interaction of the frontier orbitals between the molecules.At the same time,it can also enhance the rigidity of the molecular framework and suppress the vibrational relaxation of the excited states,thereby playing the role of narrowing spectrum.Considering that current TADF materials mainly use spiro-structured donors and the acceptors containing spiro structure are rarely used,we hope to use spiro-structured acceptors to construct TADF materials for futher explore.In chapter ?,we used boron-containing spirofluoren boroxanthene unit as acceptor and tert-butylbenzene as the?bridge to construct two TADF materials of o-CZPh-B and o-2CZ-B by connecting carbazole on the ortho-position side of the acceptor and phenyl or carbazole on the other side,respectively.The o-CZPh-B and o-2CZ-B show PL peakings at 443 nm and 445 nm in toluene.Due to the dihedral angles between the plane of?bridge and boroxanthene or carbazole exceed 50°,the highest occupied molecular orbital(HOMO)and the lowest unoccupied molecular orbital(LUMO)are mainly distributed on the donor and acceptor moieties,respectively,which lead to the decrease in the HOMO-LUMO overlap and?E_ST.Meanwhile,multiple triplet excited states close to S₁ state provide more channels for RISC and improve the RISC rate of triplet excitons.Finally,the delayed fluorescence lifetime is shortened to the ns level.Compared with microsecond lifetime,it is more rarely.This has great reference value for suppressing the efficiency roll-off.In chapter ?,in order to have more exploration on the spiro-acceptor,we constructed new spiro-acceptor unit,spirofluorene acridantone,based on carbonyl-spirofluorene acridine structure.And the compounds of sNO-N and sNO-NOO were synthesized,where triphenylamine and bridged triphenylamine were connected as the donors,respectively.Influenced by the steric hindrance of isopropyl group in the acceptor,the torsion angles between the donors and acceptors are close to80°which lead to no overlap between HOMO and LUMO and weak intramolecular charge transfer process from donor to acceptor.Nonetheless,the presence of CT state component in the S₁ state lead?E_ST to decrease to 0.25 e V and the materials to show TADF feature with delayed fluorescence lifetimes are 175?s and 123?s.It can be said that we have successfully explored two TADF materials based on novel spiro-structured acceptor.`As luminescent materials,sNO-N and sNO-NOO were used to fabricate OLEDs,obtaining the electroluminescence peaks at 460 nm and 464 nm and the maximum external quantum efficiency of 1.1%and 1.6%.In this paper,we synthesized novel TADF materials by using boron-containing spiro-acceptor and carbonyl-containing spiro-acceptor,respectively.We found:o-CZPh-B and o-2CZ-B with boron-containing spiro-acceptor have multi-channel reverse intersystem crossing process,their delayed fluorescence lifetime was shortened to ns level and this provides more design considerations for reducing the efficiency roll-off;the molecules sNO-N and sNO-NOO constructed by linking the spirofluorene acridantone acceptor with different donors possess TADF feature,this provides more ideas for the construction of spiro-acceptors and their application in TADF materials.