Thermally Activated Delayed Fluorescence Carbazole Derivatives With Rotation-Restricted Receptor

Carbazole is a commonly used functional group in photoelectric materials and is often used as an electron donor unit in thermally induced delayed fluorescence(TADF)materials.Single carbazole only exhibits weak donor properties and has a small torsion angle(46-51°)between the receptor attached to its 9-position and it,making it prone to torsional vibration.Therefore,it is necessary to introduce electron-donating groups onto the 3,6-position or 2,7-position of the carbazole or a methyl group or other steric hindrance groups onto the ortho position attached to the receptor to obtain effective TADF emission.On the basis of the above research,in this paper,we propose a material design strategy for the introduction of aromatic groups onto the 1,8-position of carbazole.It can increase the torsion angle between the donor/acceptor,restrict the rotation of the acceptor and improve the TADF properties of the carbazole derivatives and the stability of the C-N single bond between the donor/acceptor with the help of steric effect and ?-? interaction.The main contents are as follows:1?The 3,6-di-tert-butylcarbazole attaching triphenyltriazine at its 9-position as the model molecule TC.Three carbazole derivatives:PTC,PAPTC and BPAPTC were synthesized by introducing phenyl,diphenylaminophenyl and bis(4-tert-butylphenyl)aminophenyl to the 1,8-position of carbazole of TC,respectively.Single crystal data show that the torsion angle between carbazole and the acceptor increase to about 70° after introducing aryl group.In PAPTC and BPAPTC,there are multiple ?-?interactions between the arylamine groups at the 1,8-position of carbazole and the triazine receptors,forming a face-to-face sandwich structure of donor-acceptor-donor.Compared with TC and PTC which are no TADF properties,PAPTC and BPAPTC have higher thermal stability and show obvious TADF characteristics with through space charge transfer(TSCT).The singlet/triplet energy level differences(?ESTS)of PAPTC and BPAPTC are 0.07 and 0.06 eV,the delayed lifetimes are 610 and 617 ns,the photoluminescence quantum efficiencies(PLQYs)are 78%and 90%,respectively.In the organic light emitting diodes(OLEDs)prepared by solution-processed method,the emission peaks of PAPTC and BPAPTC are both 520 nm,and the maximum external quantum efficiencies(EQEs)reach 17.4%and 24.3%,respectively.The EQEs of PAPTC and BPAPTC can still stay at 11.6%and 19.8%at 3000 cd/m2.2?In order to understand the effect of sandwich structure on the properties of TADF,two pairs of compounds,BPAPTC/mBPAPTC and BPAMTC/mBPAMTC,were designed and synthesized.BPAPTC and BPAMTC contain two bis(4-tert-butylphenyl)amine donors and a triazine acceptor,which are connected at the 1,8-position and 9-position of carbazole,respectively,forming a sandwich structure.There is only one bis(4-tert-butylphenyl)amine donor and one triazine receptor in mBPAPTC and mBPAMTC,which are connected at the 1-position and 9-position of carbazole,respectively,forming an open sandwich structure.There exists strong spatial conjugation between donor and acceptor in all molecules,which can produce effective TADF emission.The photophysical properties and electrochemical behavior of each pair of compounds are similar,for example,the emission peaks of BPAPTC/mBPAPTC and BPAMTC/mBPAMTC are 519/510 nm and 492/491 nm,the PLQYs are 90/90%and 63/69%,respectively.However,there are significant differences in their electroluminescence performance.The devices of BPAPTC and BPAMTC achieve the maximum EQEs of 23.3%and 14.7%,while the devices of mBPAPTC and mBPAMTC only show the maximum EQEs of 17.8%and 9.5%,respectively.As comparison,the compound MPAPTC in which the tert-butyl groups on the amine donor is substituted by methoxyl groups emits yellow light with a maximum EQE of 9.1%.3?Three carbazole derivatives,AcPTC,PxPTC and PtPTC were synthesized by replacing diphenylamine donor with cyclic amine donor dihydroacridine,phenoxazine or phenothiazine.The single crystal data of AcPTC shows that there are also multiple ?-? interactions between the cyclic donor and acceptor,forming the sandwich structure of the edge-to-face configuration.The compounds also exhibit TADF emission with TSCT properties,and the maximum PLQY reach to 73%.Due to the large steric hindrance of the donors,the electron cloud of the highest occupied orbital(HOMO)is prevented from diffusing to the carbazole,so it can be determined that the TADF of the three compounds is only derived from the TSCT transition.The?ESTS of the compounds are all less than 0.05 eV,and the proportions of delayed fluorescence components are all higher than 80%.The OLED devices prepared by solution-processed method exhibit sky-blue emission to yellow emission,and the maximum EQEs are 10.0%,11%and 5.6%,respectively,with small efficiency roll-offs.4?In order to obtain blue-emitting carbazole derivatives,the compounds BPAPmC and BPAPPmC were synthesized by exploiting pyrimidine or 4,6-diphenylpyrimidine with weak electron-drawing ability as the acceptor.The emission peak of BPAPmC is 462 nm,but its ?EST is as high as 0.3 eV,and only exhibit the prompt fluorescence emission with a lifetime of 19 ns.The emission peak of BPAPPmC is red-shifted to 491 nm,its ?EST is 0.12 eV and show a delayed emission component with a lifetime of 14?s.In the OLED device with same structure,the emission wavelength of BPAPPmC is 499 nm and the EQE is 4.4%.The emission peak of BPAPMC is blue-shifted to 437 nm,but the EQE is only 0.28%.Then a pair of compounds mBPABTC and BPABTC were synthesized by using 4,6-ditert-butyltriazine as acceptor,while one or two bis(4-tert-butylphenyl)amine donors were fixed on the 1-position or 1,8-position of carbazole.Compared with BPAPTC in which phenyltriazine as the receptor,the emission peaks of mBPABTC and BPABTC are blue-shifted to about 483 nm,the PLQYs decrease to about 40%,the delayed lifetimes are 4.3 ?s and 6.2 ?s,respectively.The OLED emission peaks of mBPABTC and BPABTC are both around 475 nm,and the EQE of BPABTC is 4.5%,which is higher than the 3.4%of mBPABTC.5?A series of carbazole derivatives with sandwich structure were synthesized by using benzophenone and its derivatives as the acceptor while 9,9-dimethyl-9,10-dihydroacridine or bis(4-tert-butylphenyl)aminophenyl as the donors.The ?ESTS of all molecules are less than 0.1 eV,so they have obvious TADF properties.With the successive changes of the donor/acceptor intensity,the emission wavelength covers from 487 nm to 653 nm,achieving full-spectrum emission,and the delayed lifetimes are between 478 ns and 3.2 ?s.The electroluminescence performance of the compounds were preliminarily characterized.The OLED of green compound achieves a maximum EQE of 14.5%,but the long-wavelength compounds have poor electroluminescence performance,with the EQEs of less than 1%.