Construction And Optoelectronic Properties Of The Multiple Resonance Typed Aza-arenes With Narrow-band Emission

Multiple resonance thermally activated delayed fluorescence（MR-TADF）materials have attracted great attention from domestic and foreign scholars in recent years due to their narrow full width at half maximum（FWHM）and efficient luminous.At present,MR-TADF materials achieve high-efficiency narrowband emission in the full color gamut of visible light,which has become a new growth point in the field of organic light-emitting diodes（OLEDs）.However,the relationship between the molecular structure of MR-TADF materials and narrowband emission is still elusive,and precise regulation of high-efficiency narrowband emission lacks theoretical guidance;More critically,most MR-TADF materials have a serious efficiency roll-off phenomenon in OLEDs,inducing the great challenges of high-quality development of MR-TADF materials and their corresponding OLEDs.To this end,this research has investigated the molecular construction and properties of MR-TADF materials from three approaches,such as acceptor unit modulation,donor unit modulation,and circularly polarized luminescence（CPL）coupling.By the design of MR-TADF materials in three approaches mentioned above and investigation the origin of narrowband emission,we attempt to construct highly efficient narrow-band violet luminescence materials and circularly polarized multiple resonance thermal activated delayed fluorescence（CPMR-TADF）materials with triple characteristics of efficient luminescence,circularly polarized luminescence and narrowband emission.Furthermore,these systemic studies can reveal their luminescence mechanisms and broaden their application fields.The main research contents and findings of this essay are as follows:Firstly,we designed and synthesized a class of polycyclic aromatic hydrocarbon luminescent materials,Cz CN,Cz Spa CN,Cz P5O and Cz AO based on the weak electron-donating group carbazole and different acceptor units（cyano,sulfone and carbonyl）,and studied the effects of acceptor unit and substitution position on the luminescent properties of polycyclic aromatic hydrocarbon luminescent materials.This approach proposed the design law of high efficient narrowband violet luminescent materials,revealed the source of narrowband emission of the MR-TADF materials.This research found:（1）All these polycyclic aromatic hydrocarbon luminescent materials with rigid framework achieved narrowband emission.Among them,Cz CN and Cz Spa CN based on the weak electron-withdrawing unit of cyano group,and Cz P5O based on the weak electron-withdrawing unit of sulfone group exhibited violet emission,and Cz AO based on the strong electron withdrawing unit of carbonyl group exhibited blue emission.Therefore,incorporating weak electron donating/withdrawing units into the fused rigid structure is an effective strategy to construct narrow FWHM violet materials;（2）Theoretical calculations indicate that Cz CN and Cz Spa CN are not MR-TADF materials,but they still exhibit extremely narrow FWHM emission.The emission peaks/FWHM of their toluene solution are 379/13 nm and 362/22 nm,respectively,indicating that the planar rigid structure is a determinant of the narrowband emission;（3）Through the solvation effect,Lippert-Mataga model and theoretical calculation to study the Cz Spa CN luminescent material,it is further found that the introduction of spirofluorene groups can effectively reduce the conjugation degree of the molecule and reinforce the separation of electrons and holes,so that the Cz Spa CN luminescent material has hybrid local and charge transfer（HLCT）characteristics,and efficient emission of deeper violet light with a photoluminescence quantum yield（PLQY）of 85%in toluene solution;（4）Cz P5O and Cz AO are MR-TADF materials,and their luminescence peaks/FWHMs in toluene solution are 377/29 nm and 451/36 nm,respectively,indicating that regulating the structure and substitution position of the receptor units can tailor the properties and FWHM of luminescence.Secondly,we designed and synthesized a class of nitrogen/carbonyl MR-TADF materials,Cz AO,MQAO,QPXO and QPO based on the acceptor unit carbonyl and different donor units（carbazole,acridine,phenoxazine and phenothiazine）,and investigated the effects of donor unit structure and charge transfer（CT）effect on the luminescence performance of nitrogen/carbonyl MR-TADF materials,particularly the narrowband emission,and analyzed the dominant factors of regulating the narrowband emission.This research found:（1）With the increase of the electrical reinforcement of the giving unit（carbazole,acridine,phenxazine and phenothiazine）,the light-emitting peak FWHM of CZAO,MQAO,QPXO and QPO toluene is increasing,from CZAO 431/36 nm increased to 501/86nm of QPO;（2）Stokes shift,solvation effect and theoretical calculations further confirmed that the strength sequence of the intramolecular CT effect was Cz AO<MQAO<QPXO<QPO,which is consistent with the increasing order of FWHM;（3）The order of molecular twist degree of single crystal structure analysis is Cz AO<QPXO<MQAO<QPO,which is the constant with the molecular planarity parameter and Huang-Rhys factor obtained by theoretical calculation,but the order of change with FWHM is different.It can be inferred that the CT effect is the dominant factor in regulating the FWHM of MR molecules;（4）With the enhancement of molecular CT effect,although the FWHM increases,its singlet and triplet energy gap(ΔE_ST)is significantly reduced,and the TADF performance is critically enhanced.Among them,the maximum external quantum efficiency(EQE_max)of the OLED with QPO as the emitter and m CPCN as the host reached 15.27%.Thirdly,we designed and synthesized a class of boron/nitrogen helicene-type MR-TADF materials,BN1,BN2,BN3,BN4 and BN5derived from DABNA-1 with different donor units（diphenylamine,acridine,phenoxazine and phenothiazine）using an asymmetric peripheral locking strategy.Furthermore,we analyzed the effect of the asymmetric donor unit peripheral locking strategy on the separation of spirochiral materials,investigated the effect of heavy atom effect on the luminescence properties,and active participation of helical centers in frontier molecular orbitals on the chiral optical properties.This research found:（1）The peripheral locking strategy of the asymmetric donor unit can effectively increase the helical curvature and racemization barrier of the molecule,and realize the chiral separation of the spirochiral materials BN4 and BN5;（2）The introduction of heavy atomic sulfur can effectively reduce theΔE_STand increase the rate of inverse system crossing（RISC）,so that sulfur-containing molecules（BN3,BN4 and BN5）exhibit a more pronounced TADF properties in the same series of compounds;（3）When the absolute configuration of BN4 and BN5 enantiomers are dissolved in toluene solution,circular dichroism（CD）test shows that it has a large Cotton effect at the lowest electronic transition state（400-500 nm）,and the CPL test also exhibits an obvious mirror symmetry relationship;（4）When the absolute configuration BN4 and BN5 enantiomers were doped into m CPCN films at lower concentrations,they still showed obvious Cotton effect;（5）Solution-processed OLEDs based on BN4 and BN5 enantiomers exhibit excellent electroluminescence performance.The maximum external quantum efficiencies were 20.6%/19.0%and 22.0%/26.5%,and the electroluminescence asymmetric factors g _EL were+3.7×10^-3/-3.1×10^-3 and+1.9×10^-3/-1.6×10^-3,respectively.