Synthesis And Properties Of Space Charge Transfer Thermally Activated Delayed Fluorescent Materials Based On Spiro[triphenylamine-9,9?-fluorene]

In the past decade,the development of organic light-emitting diodes(OLEDs)was reshaped by thermally activated delayed fluorescence(TADF)which can fully utilize both singlet/triplet manifold excitons through reverse intersystem crossing(RISC)and remove heavy metal by using environmentally benign pure organic molecules.Now,the cutting-edge TADF materials can exceed 30% external quantum efficiency(EQE)in electrolumi-nescent devices,but these highly efficient TADF materials are basically limited to donor(D)–?–acceptor(A)molecular architecture.This type of molecular design,first proposed by Adachi and co-workers for TADF,can tune their D or/and A groups,geometries,and steric hindrance between them to generate twisted induced charge-transfer-type emission.The twisted dihedral angle between D and A units can minimize the singlet–triplet splitting energy(?EST)for fast RISC,but the resulted TADF OLEDs still need to be significantly improved,notably in terms of efficiency roll-off at high brightness and concentration quenching because of the?-? intermolecular interactions in the solid-state.Another way to achieve TADF is to use D/A complex,in which the D/A blocks are spatially isolated,but their forming exciplexes are far less efficient than D–?–A analogs,and the resulting OLEDs also display severe efficiency roll-off.Recently,researchers conceptually consider that the intramolecular noncovalent interaction between D/A units in face-to-face alignment could be a new option to realize TADF.Constructing TADF materials in this unconjugated way has the potential to combine the small ?EST value with substantial transition dipole and achieve high luminescent efficiency.These two electron-rich and electron-poor?-systems need to be held close in space to form homoconjugation.In this regard,dozens of molecular frameworks have been employed but the performances remain around 10–20% EQE,similar to those of D/A complexes.The problem may probably account for the D/A geometry not only the D/A distance.In other words,short D/A distance is essential for TADF but cannot ensure high efficiency which is more challenged and determinant for material and device performance.In addition,restriction of intramolecular motion should also be taken into account in regard to enhancing photoluminescence quantum yield(PLQY).However,in previously reported structures,it is noted that both D and A units are extended in single bonds,indicating the molecular conrotatory motion cannot be thoroughly restricted.Even though these D/A units exist in a short distance and the excitation process is completed,the emission energy may be lost in free rotation.In the second chapter,we designed and synthesized an efficient sky-blue TADF material STF-TPZ-Me with a space charge transfer mechanism based on the steric hindrance effect.The key to the design includes not only the short D/A distance,but also the The fixation of the spatial configuration.We use the fluorene unit as a rigid linking group,introduce triphenylamine as an electron donating group at the C9 position of fluorene,and connect two? units through a sp3 hybridized carbon atom,so that the planes where the two ? units are located are mutually vertical.For the acceptor,the rigid planar structure brought about by the strong hydrogen bonding of 2,4,6-triphenyl-1,3,5 triazine(TPZ)is considered to facilitate space charge transfer,reduce nonradiative decay,and Enhanced PLQY.Therefore,it was introduced into the C1 position of fluorene as an electron withdrawing group.Finally,considering that the acceptor TPZ part is still connected to the fluorene unit through a single bond,resulting in its free rotation and cannot be fully inhibited,we introduced a sterically hindered methyl group at its ortho position to enhance its steric confinement effect,confine the D/A cells to tight coplanar arrangements.As a comparison,we replaced the sterically hindered group in the ortho position of the acceptor TPZ with a flexible phenyl group,and designed and synthesized the molecule STF-TPZ-Ph.The flexibility of the phenyl group made it not only unable to play a steric hindrance role,but even It exacerbates the configurational change and non-radiative energy loss of the excited state,thereby reducing the RISC rate and PLQY of the material.Through single crystal analysis and theoretical calculation,it was found that compared with STF-TPZ-Ph,STF-TPZ-Me has a shorter D/A distance,stronger steric ?-? interaction and smaller excited state configuration changes,as we expected.In addition,the delayed lifetime and PLQY of STF-TPZ-Ph and STF-TPZ-Me in 20 wt% doped films(with DPEPO as the host material)are: 5.5 ?s\3.8 ?s and 78 %\97 %,respectively.Consistent with our predictions,STF-TPZ-Me has significantly shorter delayed lifetime and higher PLQY than the material STF-TPZ-Ph.This proves that the molecular design strategy of this work is very successful and provides a new idea for the development of TADF materials.In the third chapter,based on the previous chapter,we designed and synthesized a novel TSCT-TADF material STF-DBP-Me by controlling the rigidity of the material.Among them,we still use spirofluorene-triphenylamine-methyl(STF-Me)as the bridging unit(methylfluorene part)and donor(triphenylamine part),and replace the acceptor with dibenzothiophene(DBP)to connect to the C1 position of fluorene.The rigid structure of DBP acceptors facilitates space charge transfer,reduces molecular nonradiative decay,and enhances PLQY,and also facilitates the formation of tightly packed D/A structures to facilitate their intramolecular charge transfer(ICT).For comparative purposes,we also constructed the flexible molecule STF-DPS-Me with diphenylsulfone(DPS)as the acceptor,which has a less rotationally constrained DPS acceptor that not only enhances the nonradiative decay of the material,but also enables D\A is farther away.As we expected,the flexible molecule STF-DPS-Me with DPS acceptor did not exhibit TADF properties,and the PLQY in the 30 wt% doped film(CBP as host material)was only 11%.In contrast,the molecule STF-DBP-Me exhibits pronounced AIE and TADF properties and a PLQY as high as 61%.This work proves that the luminous efficiency and photophysical properties of materials can be directly affected by the regulation of molecular rigid structure.