Excited State Regulation And Optoelectronic Properties Of Aza-Aromatic Donor-Acceptor Luminescent Materials

Organic electroluminescent device(OLED)has become a relatively mature industrialization technology.In recent years,China's OLED has made rapid progress and gradually catches up with leading countries both in basic research and industrialization.However,it is the most fundamental solution that developing a series of independent intellectual property materials,methods and technologies to breakthrough technology blockade and dominate the market competition.Pure organic fluorescent materials take the advantages of low cost,clear structure,easy synthesis and simple processing comparing to the widely used metal-containing phosphorescent materials.Nevertheless,they suffer from electrogenerated exciton utilization limitation by the spin statistics rule:only 25%of singlet exciton can be luminous,while 75%of the triplet excitons are wasted.Therefore,four practicable mechanisms have been proposed to realise high exciton utilization,such as thermally activated delayed fluorescence(TADF),triplet-triplet annihilation(TTA),hybrid locally-emissive and charge-transfer(HLCT)and doublet.In which,HLCT materials with "hot" exciton channel can harvest 100%excitons through high-lying reverse intersystem crossing(RISC).Thus,in principle,this kind of material can be possible to achieve fast response,non-doped,high efficiency and high stability OLED.However,there is still no clear idea of how to construct HLCT states,especially the molecular design of "hot" exciton channels.In this thesis,we developed a series of donor-acceptor(D-A)materials based on "hot" characteristic aza-aromatic aiming to explore the construction strategy of HLCT state and the mechanism of "hot" exciton,and eventually,achieve high performance organic electroluminescent materials.Theoretical calculations combined with experiments were conducted to reveal the relationship of molecular structure(including electron-donating ability,substituted site,twist angle,type of acceptor,spin-orbit coupling,etc.),excited state properties and electroluminescence performance of materials.The research contents in this thesis are summarized as follows:(1)Acridine,possessing large energy gap of Ti-T2,was chosen as acceptor.Through enhancing electron-donating ability of donor,we tailored hybridization status of excited state with gradually reduced CT energy level.The photophysical experiments combined with theoretical calculation clearly revealed the interaction between the intrinsic LE and CT state.The quasi-equivalent HLCT material 4-(acridin-1-yl)-N,N-diphenylaniline(TPA-1AC)achieved the best photoelectric performance.Furthermore,we enhanced the degree of hybridization and LE component by optimizing the substituted site.Finally,the photoelectric properties of molecules have been improved again in 4-(acridin-3-yl)-N,N-diphenylaniline(TPA-3AC).These results indicate that the quasi-equivalent hybridization status is an effective way to improve the photoelectric properties for aza-aromatic based D-A materials.(2)We expanded the structure of nitrogen heterocyclic acceptor to adjust quantum efficiency,color and single triplet spin-orbit coupling(SOC)from the aspects of conjugate degree,planarity,energy level,etc.According to the optimized results in last chapter,we chose triphenylamine as donor to construct four D-A small molecules.Benefitting from the HLCT state,they all achieved relatively high photoluminescence(PL)efficiency(>80%).We systematically analyzed the effects of nitrogen heterocyclic acceptor moiety on excited state efficiency,color and device performance by comparing theoretical calculations,photophysical and device properties.Due to the effective suppression of non-radiative transition,the quasi-equivalent molecule 4-(dibenzo[a,c]phenazin-11-yl)-N,N1-diphenylaniline(TPA-DPPZ)achieved the best performance.In addition,we also studied the excited state characteristics of the acceptor chromophore,and found that specific n??*transition can effectively enhance the intersystem SOC.(3)Dibenzo[a,c]phenazine(DPPZ)powder exhibited a special single-molecule white light emission at room temperature,which is composed of fluorescence(S i)and dual room temperature phosphorescence(T1,T2).Temperature-dependent PL spectra excluded the T1?T2 thermally activated reversed inner conversion mechanism in DPPZ.Further,theoretical calculations illustrated that the nitrogen heteroatoms enhanced the spin orbit coupling between single and triplet,resulting in intersystem crossing channels of S1-T1,S1-T2 and S2-T2,which was confirmed by excitation spectra at different wavelengths of emission.Simultaneously,the T2 room temperature phosphorescence indicated the large number of stable T2 exciton populations,providing the possibility for the "hot" channel of T2?S1.(4)The independent T1 and T2 excitons allow large energy gap of these two states,which is more effective to construct high-lying RISC channel.Therefore,we have further optimized different D-A molecules to achieve high-performance "hot"exciton luminescent materials.We systematically regulated the type of excited states:the LE dominated excited state for 11-(4-(9H-carbazol-9-yl)phenyl)dibenzo[a,c]phenazine(CZP-DPPZ),the HLCT state for TPA-DPPZ and the CT dominated state with TADF characteristics for 10-(dibenzo[a,c]phenazin-11-y1)-10H-phenoxazine(PXZ-DPPZ).Then,we analyzed the properties of these optoelectronic materials with different mechanisms based on DPPZ as acceptor,through comparing their theoretical calculations,photophysical and device properties.Further,the exciton utilization was obviously improved after adjusting the substitution site and number of donors.The external quantum efficiencies of 4,4',4"-(dibenzo[a,c]phenazine-3,6,11-triyl)tris(N,N-diphenylaniline)(TTPA-DPPZ)and PXZ-DPPZ in doped devices are more than 5%.In fact,the exciton utilization of the materials all exceed 25%in different degree,which can be attributed to the effective high-energy reverse intersystem crossing channel of the acceptor moiety.