Synthesis And Properties Of Thermally Activated Delayed Fluorescence Materials Based On The Electron Acceptors Of Xanthone Derivatives

Thermally activated delayed fluorescence（TADF）materials have gradually replaced phosphorescent materials as the core of the third-generation display technology due to their simple synthesis,free of exspensive metals（such as iridium and platinum）,full spectrum coverage,and 100%exciton utilization.Although the external quantum efficiency（EQE）of TADF-OLED can exceed 20%in the entire visible spectrum,it still inevitably faces some problems.First,when TADF materials were employed as emitters in OLEDs,the doped concentration of TADF molecules must be at a relatively high level（≥5%）to satisfy the need of Dexter energy transfer（short-range electron-exchange energy transfer）from host to TADF molecules.At this doped concentration level the TADF molecules should not adopt mono-dispersed state and are in aggregated states.Therefore,the effect of aggregation or supramolecular structures on the photophysical properties of TADF molecules should be addressed.Unfortunately,there are still few reports on the supramolecular structure of TADF materials until now.Second,white organic light-emitting device（WOLED）based on one single organic molecule exhibits many advantages such as a simple fabrication process,low cost,improved stability,as well as no color aging,which is a research hotspot that urgently needs to break through.Considering the low EQE（≤5%）of single-component fluorescent molecule based WOLED,an emitter with TADF property is regarded as an ideal solution.However,it is still challenging whether under photo-or electro-luminescent conditions.Thirdly,for TADF-OLED,the electrically generated triplet excitons contribute to emission through reverse intersystem crossing（RISC）process,which usually takes a relatively long time.Therefore,in emitting layer（EML）accumulation of T₁ excitons is unavoidable,leading to intense T₁-T₁ annihilation（TTA）and S₁-T₁ annihilation（STA）.The TTA and STA can result in remarkable efficiency roll-off.Therefore,reasonable supramolecular and device structure design is needed to achieve high efficiency and stable TADF-OLED.And so on.In this thesis,we carried out the following research using xanthone（XO）and its derivatives as electron acceptors to build a series of new TADF materials.1,In chapter II,a novel TADF molecule 3-（10H-phenoxazin-10-yl）-9H-xanthen-9-one（3-PXZ-XO）,featuring a D-A configuration with xanthone（XO）as the acceptor and phenoxazine（PXZ）as the donor,was easily synthesized.Three kinds of 3-PXZ-XO based crystals A,B and C with different TADF properties were obtained in different ways.The three crystals display obviously different emission maxima(λ_em,max:535 nm for A,555 nm for B and 576 nm for C),photoluminescence（PL）quantum yields（Φ:51%for A,28%for B and 39%for C）and delayed life times of excited states(τ_TADF:914 ns for A,774 ns for B and 994 ns for C).The supramolecular structure analyses revealed that in A,B and C there are different intermolecularπ-πstacking interaction modes between the adjacent donor planes or acceptor planes.The different TADF properties of the three polymorphs are mainly attributed to their different supramolecular structures.Appropriate non-covalent interactions（such asπ-π,hydrogen bonds,and CH···π）and supramolecular structures can strongly enhance TADF property of organic solids.2,In chapter III,a unique molecule of 3-（diphenylamino）-9H-xanthen-9-one（3-DPH-XO）,which was found to exhibit bright white-light emission in the solid state,was obtained by coupling diphenylamine（DPH）with XO.Detailed studies on theoretical calculations,supramolecular structures and photoluminescence properties demonstrated that the white-light emission of 3-DPH-XO powder was attributed to the spontaneous formation of the mixture of polymorphs（A,B and C）.Polymorphs B and C with the acceptor···acceptor stacking feature showed the TADF characteristics,indicating that appropriate non-covalent interactions could enhance the reverse intersystem crossing（RISC）process and thus lead to delayed fluorescence,further confirming the conclusion of Chapter II.Additionally,polymorph B with extended weak intermolecularπ-πinteractions also showed the special dual fluorescence emissive properties.This discovery provides an effective strategy for the design of new white-light emitting single molecules as well as TADF materials.3,In chapter IV,by extending theπ-skeleton of the XO acceptor or introducing Br atom with relatively bulky structure and electron-withdrawing properties,we successfully obatained two novel TADF molecules 10-DPH-BXO and 3-DPH-6-Br-XO,which were found to exhibit dual fluorescence in the crystalline state（supramolecular aggregates）.Theoretical calculations,crystal analysis,and photophysical tests show that they all have extendedπ-stacked supramolecular structures;Moreover,the dual emissions of 10-DPH-BXO crystals（aggregates）came from the radiative transitions of the S₁ and S₂ states of the supramolecular aggregates,respectively.In 10-DPH-BXO crystals,the“supramolecular aggregate emission（SAE）”is stimulated as a whole for emission.While the dual emissions of 3-DPH-6-Br-XO were derived from the two S₁ states of the monomer（TADF-1）and supramolecular aggregates（TADF-2）,respectively.TADF-OLEDs fabricated with 10-DPH-BXO and 3-DPH-6-Br-XO as luminescent materials all showed bright white-light emissions.Among them,by altering the doping concentration of 10-DPH-BXO,highly efficient WOLEDs covering the entire visible spectrum could be achieved.The best maxima EQE values in the cool white,near white and warm white regions are 8.13%,4.89%and 3.55%,respectively,ranking among the highest levels reported in single organic compound based WOLED with similar EL spectra.Warm white light could be obtained in the relatively high doping concentrations（15 wt%to 30 wt%）of 3-DPH-6-Br-XO,giving an ever highest EQE of 14.85%for single-component fluorescent molecules based WOLED.This study indicates that reasonable molecular and supramolecular structure regulation is an effective method to construct efficient single organic compound based WOLED.4,In chapter V,4H-chromen-4-one（CMO,with a reducedπ-conjugation）and phenoxazine（PXZ）were selected as the electron A and D moieties,respectively,to construct a target TADF molecule PXZ-CMO.By dispersing the emitter into different hosts,devices G1（15 wt%PXZ-CMO:MCP）and G2（20 wt%PXZ-CMO:DPEPO）with the same configuration were then carefully fabricated,which showed similar maximum EQE/CE of12.1%/38.2 cd A^-1 and 11.8%/33.1 cd A^-1 with CIE coordinates of（0.28,0.55）and（0.30,0.48）,respectively.Despite the severe efficiency roll-off in device G2 with only 6.4%EQE remaining at a luminance of 1000 cd m^-2,a remarkably reduced efficiency roll-off was achieved in device G1,retaining EQE as high as 10.4%at the same luminance of 1000 cd m^-2.The study of photophysical properties shows that the excellent device performance with reduced roll-off in device G1 should result from the dual delayed fluorescence in the emitting layer.We believe that our present study provides an efficient approach to achieve the high-efficiency TADF-OLED with low efficiency roll-offs.In summary,a series of conjugated organic molecules with D-A structural properties have been designed and synthesized based on the electron acceptors of xanthone derivatives in the method of molecular and supramolecular structure regulation.The intrinsic relationship between the excited state characteristics and the molecular,supramolecular structure is systematically explained.White light emissions could be skillfully obtained based on a single TADF molecule in both photo-and electro-induced conditions.Upon a kind of TADF molecules are doped into a suitable host,we successfully develop an effective strategy to reduce the efficiency roll-off of TADF-OLED based on the dual delayed fluorescence in the emitting layer.