Design,Synthesis And Performance Of Triazatruxene-Based Luminescent Materials

Triazatruxene is a polycyclic aromatic hydrocarbon structure formed by three classical electron-donor carbazole units sharing a benzene ring.It has stronger electrondonating ability and larger π-conjugated plane than carbazole.At the same time,triazatruxene possesses wide optical bandgap,good hole mobility,and the highest occupied molecular orbital(HOMO)energy level close to that of PEDOT:PSS.These characteristics make it not only used as electron donor unit to construct thermally activated delayed fluorescence(TADF)materials,but also used as host materials for electroluminescent(EL)devices.However,most of the currently reported triazatruxene-based TADF materials are blue and green materials used for vacuumdeposited EL devices.There are relatively few deep-blue and red TADF emitters,and the host materials based on triazatruxene are rarely reported.In this paper,solutionprocessable green,red and deep blue TADF materials and host materials based on triazatruxene derivatives are designed and synthesized,and solution-processed EL devices are constructed.The main work contents are as follows:(1)Based on the strong electron donating ability of triazatruxene,two green TADF molecules,TAT-BP and TAT-2BP,are designed and synthesized by using triazatruxene as electron donor and benzophenone as electron acceptors.TAT-BP and TAT-2BP show green emission with obvious TADF effect and short delayed fluorescence lifetimes(0.79 and 0.54 μs).The maximum external quantum efficiency(EQE)of the TAT-BPbased non-doped EL device is 6.4%,and the efficiency roll-off is 10.9%at 1000 cd m2.The maximum EQE of the TAT-2BP-based non-doped device is 9.8%,and the efficiency roll-off is only 1.0%at 1000 cd m-2.Using triazatruxene as electron donor,phenanthroquinoxaline and difluorophenanthroquinoxaline as electron acceptors,two red TADF materials TAT-DBPZ and TAT-FDBPZ with donor-acceptor-donor structure are designed and synthesized.Due to the stronger intramolecular charge transfer,TATDBPZ and TAT-FDBPZ exhibit red emission and obvious TADF effect,and the photoluminescence quantum yield(PLQY)can be as high as 76%.The maximum EQE of the EL device using TAT-DBPZ as the emitter is 15.4%,the emission peak is 604 nm,and the color coordinate is(0.56,0.44).The maximum EQE of the device using TATFDBPZ as the luminescent material is 9.2%,the emission peak is 611 nm,and the color coordinate is(0.58,0.41).(2)A novel intramolecular-locked fused-ring electron donor unit(FTAT)is constructed by introducing two diphenylmethylene bridging units into diphenylsubstituted triazatruxene.Combining methyl(MBO),hydrogen(HBO),and fluorine substituted(FBO)rigid oxygen bridged boron acceptors,we design and synthesize a series of blue TADF materials(FTAT-MBO,FTAT-HBO,FTAT-FBO),which have been successfully applied in the solution-processed OLEDs.Compared with the diphenylsubstituted triazatruxene donor,FTAT exhibits a deeper HOMO energy level(-5.25 eV)and greater rigidity.Meanwhile,FTAT possesses high lowest triplet(Ti)energy level.The three compounds exhibit blue emission with emission peaks at 449 nm,452 nm and 483 nm and full-width at half maximum of 54 nm,56 nm and 64 nm,respectively.The deep blue EL devices based on FTAT-MBO and FTAT-HBO exhibit maximum EQE of 10.2%and 11.4%,emission peaks at 445 and 458 nm,and color coordinates of(0.15,0.08)and(0.15,0.12),respectively.Since FTAT-FBO has highest PLQY(90%)and shortest delayed fluorescence lifetime(1.8 μs),the sky-blue device based on FTAT-FBO achieves the maximum EQE of 17.5%,the emission peak is located at 473 nm,and the color coordinate is(0.15,0.25).(3)Based on the wide optical bandgap and high T1 energy level of triazatruxene,We design and synthesize three fused-ring triazatruxene-based host materials 1FTAT2Ph,2FTAT-1Ph and 3FTAT by introducing one to three diphenylmethylene bridging units into triphenyl-substituted triazatruxene(TAT-3Ph),respectively.Compared with TAT-3Ph,the introduction of diphenylmethylene bridging unit can not only enhance the molecular rigidity and improve the luminescence performance of the host molecules,but also increase the intermolecular distance,which is more conducive to suppress the aggregation and quenching of triplet excitons.The three molecules have high T1 energy level(≈2.8 eV),and the PLQYs are over 30%,which are significantly higher than that of TAT-3Ph(25.3%).Compared with 4,4’-N,N’-dicarbazolylbiphenyl(CBP),a commonly used host material in solution-processed EL devices,the three host molecules have higher HOMO levels(-5.2～-5.3 eV),matched well with the energy level of ITO/PEDOT:PSS,which can effectively reduce the hole injection barrier and turn-on voltage of devices.1FTAT-2Ph,2FTAT-1Ph and 3FTAT are all successfully applied to solution-processed EL devices,among which the device with 2FTAT-lPh as the host achieves the best device performance with a turn-on voltage of 2.8V,the maximum EQE of 20.9%,and the maximum power efficiency of 72.7 lm W-1.