Thermally Activated Delayed Fluorescent Emitters With Through Space Charge Transfer Based On Triarylboron/Carbazole Hybrids

Nowadays,organic light emitting diodes（OLEDs）have attracted inscreasing attention owing to their potentials in the application of flexible flat-panel displays.The development of the emitter of OLEDs has undergone a process of fluorescence,phosphorescence and thermally activated delayed fluorescence（TADF）.Compared with conventional fluorescent emitters,TADF emitters can utilize both singlet and triplet excitons,which can improve the luminescent performance of the emitters.What is more,compared with phosphors,TADF emitters can significantly reduce the cost,because they do not need to use the noble metal.The most important strategy for efficient TADF emitters is to minimize the overlap degree of frontier molecular orbitals（FMOs）and thus obtain small energy gaps between S₁ and T₁(（35）E_STs),which can help emitters to possess efficient RISC process and improve the utilization of triplet excitons.Using triarylboron as the electron acceptor and carbazole as the electron donor to construct an emitter with through bond charge transfer（TBCT）,namely TB-3Cz.The planar and small structure of carbazole made it impossible for TB-3Cz to achieve twisted D-A configuration and the enough separation of FMOs,resulting in a large（35）E_ST（0.28 eV）of TB-3Cz,which meant triplet excitons of TB-3Cz was not able to upconvert to S₁ via RISC and thus TB-3Cz did not possess TADF characteristic.TB-3Cz could not utilize triplet excitons,which led to its poor luminescent performance（the PLQY of TB-3Cz was only 42.4%）.By changing the linking method between donors and the acceptor,an isomer,oB-3Cz was constructed.The ortho-linked structure could enlarge the dihedral angle between donors and acceptors,and greatly separate the FMOs of oB-3Cz,and thus a small（35）E_STs（0.05 eV）as well as efficient RISC process were obtained,which made oB-3Cz a TADF molecule.The utilization of triplet excitons had a positive effect on the luminescent performance of oB-3Cz（the PLQY of TB-3Cz was 70.0%）.By using phenyl as a linker,oB-2Cz and oB-2t Cz with ortho-linked D-A-D structure were constructed,in which diarylboron and carbazole（or tert-butyl carbazole）were used as the donor and acceptor,respectively.The photoluminescent emission of oB-2Cz and oB-2t Cz peaked 482 and 495 nm,respectively.oB-2Cz and oB-2t Cz had small D-A spatial distances（3.39-3.63（?）for oB-2Cz and 3.35-3.63（?）for oB-2t Cz）,which enabled the charge to transfer from donors to the acceptor through space.Moreover,the ortho-linked D-A-D configuration made oB-2Cz and oB-2t Cz to be more rigid,which could suppress the non-radiative decay and thus improved the luminescent performance（PLQYs of oB-2Cz and oB-2t Cz were 92.9%and 96.3%,respectively）.Employing oB-2Cz and oB-2t Cz as the emissive guest of OLED devices,the maximum EQEs were 28.1%and 27.5%with CIE of（0.18,0.37）and（0.22,0.49）,respectively.