Theoretical Design Of Thermally Activated Delayed Fluorescence Blue Emitters Under Several Structural Paradigms

In view of the great potential of high-performance organic light-emitting diodes(OLEDs)in modern technologies such as digital displays and solid-state lighting,a great deal of research interest has been focused on the design and development of OLED related materials(including luminescent materials,charge transport materials,electrode materials,light extraction materials,packaging materials,etc.).The functioning of OLEDs rely on the electroluminescence phenomenon to efficiently convert electrical energy into light energy.Luminescent material,as one of the most important materials for OLEDs,the relationship between its structure and properties has been intensively investigated.Since Tang and Van Slyke made the first practical OLED in 1987,the performance of the device has been greatly improved.Apart from factors such as the device structure and the optimization of the inter-faces between the material layers,the improvement should be mainly attributed to the shift from traditional pure fluorescent materials to phosphorescent materials(e.g.,heavy transi-tion metal organic complexes).The heavy metal atoms in the center of the phosphorescent material can exhibit strong spin-orbit coupling,which facilitates intersystem crossing be-tween singlet and triplet states.By using a phosphorescent material,both singlet excitons and triplet excitons can decay radiatively,thereby greatly improving the(internal or external)quantum efficiency of the device.After years of continuous improvement,the performance of OLEDs based on red or green phosphorescent materials has already been very good.However,relatively speaking,stable high-performance blue phosphorescent materials are still scarce(hard to obtain).The use of phosphorescent materials to generate blue lumines-cence has several disadvantages:heavy transition metals(e.g.,Ir,Pt,etc.)are very scarce and expensive;complexes are not easy to synthesize;the available material space is relative-ly small;short-wavelength phosphorescent materials are easily quenched;Phosphorescence is naturally more red than fluorescence;and the monochromaticity of phosphorescence e-mission is relatively poor.Since the 1960s,although the phenomenon of delayed fluorescence of some molecules has been observed,it was not until 2012 that attention was paid to using it as an efficient emitter in OLED devices.There are mainly two types of thermally activated delayed fluores-cent(TADF)materials that are widely explored.One is a pure organic aromatic compound and the other is a multinuclear metal organic complex.They all have very small singlet-triplet energy gaps.Under normal temperature conditions,lower energy triplet excitons are upconverted to higher energy singlet excitons due to thermal activation.The light is completely emitted from the emissive singlet state back to the ground state.Owing to the outstanding work of C.Adachi and his collaborators,TADF materials are being considered as third-generation electroluminescent materials with great potential to be used in modern OLEDs to replace expensive rare noble metal organic complexes based phosphorescent ma-terials(so-called second-generation luminescent materials).To achieve this goal,it is undoubtedly important to develop high-efficiency and inex-pensive TADF materials.Therefore,it is necessary to deeply understand the relationship between the structure and properties of TADF emitters.In a few years,TADF OLEDs have undergone substantial improvements in their luminescent materials and device effi-ciencies.Despite this,the development of highly stable,deep-blue,solution-processible OLEDs TADF emitters is still very challenging and important for the upgrade of this tech-nology.Needless to say,quantum chemistry calculations play an increasingly critical role in understanding and improving TADF materials.This study is based on quantum chem-istry calculations and attempts to design the blue TADF emitters under several structural paradigms.The entire dissertation is arranged as follows:In chapter 1,we introduces the basic concepts involved in the research,including OLED and TADF.In chapter 2,we describes the basic quantum chemistry theory involved in the research,including Hartree-Fock method(HF),density functional theory(DFT),range-separation hy-brid DFT(RSH DFT),and time-dependent DFT(TD-DFT).In chapter 3,under the Donor-π-bridge-Accceptor(D-π-A)structural paradigm,we explore the potential of NB-type compounds as blue TADF materials,focusing on the effect of different connection patterns in the structure on the electrooptic properties.There,we systematically studied the effect of different π-bridge,connection pattern,steric hindrance,nitrogen-containing donors and boron-containing acceptors on the electrooptical properties of NB-type electron asymmetric compounds within traditional D-pi.-A frameworks.Four types of NB molecules were studied:D-π-A,D-X1-π-A,D-π-X1-A,and D-X1-π-X2-A.Through the analysis of the energetic properties(especially the singlet-triplet energy gap,and the first excitation energy),along with other electronic and optical properties,our study shows that the interaction mode D-π-X1-A,the modified D-π-A system with rigidly fixed acceptor and relatively free donor,can serve as a valuable molecular design pattern for new blue TADF emitters.Specifically,our calculations predict that ARD-BZN-2CMe2-PYN and its relatives may have good potential as TADF emitters.In chapter 4,under the Donor-Acceptor-Donor(D-A-D)structural paradigm,we ex-plore the possibility of using benzazasiline and acridine as electron donors to combine with rigid acceptors for designing blue TADF emitters.There,we systematically studied on the fitness of two tricyclic donors(acridine and benzazasiline)interacting with five acceptors(triazine or triphenylborane-based)within the donor-acceptor-donor(D-A-D)framework for applications in organic light-emitting diodes(OLEDs)as TADF emitters.Through the analysis of computed data,we found that:by incorporating into the D-A-D frameworks and adopting an appropriate donor/acceptor combination,the studied compounds can bear rather small singlet-triplet gaps of～0.02 eV,and the emission energy can span a large space in(deep-)blue region(2.62～3.13 eV);the molecular symmetry,in addition to the character of donors,control the properties of excited states,and acridine is more easily to be excited compared to benzazasiline.The symmetric setup is more suitable than asymmetric coun-terpart for optimal optoelectric properties.The study suggests that benzazasiline donor can serve as a versatile structural unit to combine with triphenylborane-based acceptor cores for constructing(deep-)blue D-A-D TADF emitters.In chapter 5,under the Donor-Carborane-Donor structure paradigm,we explore the possibility of using highly-fluorinated closo-carborane clusters as electron acceptors for constructing(deep-)blue TADF emitters with excellent stability.There,we systematical-ly studied the effect of the size of closo-carborane cages,the extent of fluorination on the cages,and different donors on electrooptic properties of seemingly Donor-Acceptor-Donor framework compounds.Through the analysis of computed data,we found that both the size of the cage and the extent of fluorination on the cage have great influence on the electro-optical properties of the system.Structural control causes the observed evolution of frontier molecular orbitals,and this evolution leads to the evolution of electrooptic properties.In short,only the Donor-Carborane-Donor system,which is highly close to a fully fluorinat-ed large cage compound,combined with a suitable donor,is likely to meet the minimum requirements of TADF material.Although the emission characteristics of related systems are still relatively poor,they should constitute the starting point for further optimization to create new high-performance(deep-)blue TADF emitters with excellent stability.In chapter 6,under the Donor-Acceptor-(AnchoringGroup)2(DA-(AG)2)structural paradigm,we explore the possibility of using benzazasiline and acridine as electron donors combining with diazines and triphenylborane-based cores acceptors to construct blue TADF linkers for solution-processible OLEDs.There,we systematically studied the combination of four tricyclic electron donors with five electron acceptors to form DDonor-Acceptor-(AnchoringGroup)2(DA-(AG)2,where AG is para-benzoic acid),framework compounds.They were evaluated for their potential as TADF linkers for solution-processible OLEDs.Through the analysis of computed data,we found that only part of the D-A-(AG)2 framc-work formed by the first two donors(acridine and benzazasiline)can exhibit the appropriate blue-light TADF emitter potential.This study also highlights the importance of maintaining proper steric hindrance in the design of TADF emitters,and suggests that acridine(or ben-zazasiline)donor can serve as a versatile structural unit to combine with diazine(or rigid triphenylborane)acceptor,the constructed D-A-(AG)2 framework compounds should have potential as blue TADF linkers for Metal-Organic Framework based solution-processible OLEDs.