Synthesis And Properties Of Multiple Resonance Delayed Fluorescence Materials Based On Carbonyl/Nitrogen Fused Ring Structure

Thermally activated delayed fluorescence（TADF）materials,known as the third generation materials of organic light-emitting diode（OLED）,have been developed rapidly in recent years.By reducing the energy gap(ΔE_ST)between the lowest excited singlet state（S₁）and triplet state（T₁）,TADF luminescent materials can promote the conversion of excitons from triplet state to radiant singlet state（RISC）by absorbing ambient heat,thus achieving a theoretical 100%exciton utilization rate and greatly improving the performance of OLED.Therefore,the main strategy for designing TADF molecules is to reduce theΔE_STof molecules.In molecular design,the highest occupied molecular orbital（HOMO）and the lowest unoccuated molecular orbital（LUMO）are usually distributed in different parts of the molecule to reduce the electron exchange energy.For example,the addition ofπ-bridges between donor-π-receptor（D-π-A）molecules enables the formation of distorted conformations upon excitation,leading to efficient separation of HOMO（mainly electron donor）and LUMO（mainly electron acceptor）and inducing effective charge transfer（CT）.However,structural relaxation between the ground state and torsional excited state inevitably leads to large Stokes shifts and wide CT emission peaks,which are extremely unfavorable for high-resolution display.In addition,TADF molecules generally have conjugated structures with large planes,which are easy to produce aggregation-caused quenching（ACQ）effect,which limits the development and practical application of such materials.Therefore,although TADF materials develop rapidly,most TADF materials still have room for further study on the structure and properties of receptors.Therefore,it has become the research goal to develop TADF materials with novel structure and good properties.Multiple resonance-thermally activated delayed fluorescence materials have certain potential in terms of structure and properties,so they have been widely concerned since their appearance.The carbonyl/triangene structure is a rigid structure with multiple resonance properties,strong electron absorption ability and room temperature phosphorescent properties,which can be used in the synthesis of luminous materials.In this paper,from the perspective of molecular design,the carbonyl/triangene structure as the acceptor structure,by introducing tert-butylbenzene as the peripheral group and phenyltriphenylamine and tert-butyltriphenylamine as the donor to increase molecular length and improve molecular level dipole orientation strategy to improve the external quantum efficiency of the device,and reduce the intermolecular quenching effect.A series of TADF molecules with excellent performance were constructed by introducing quinoline acridine dione and di-tert-butylindoline carbazole as rigid donors to reduce the vibration coupling and excited state structural relaxation between the ground state and excited state,and to improve the color purity.Their electrochemical properties,photophysical properties and electroluminescence properties were systematically studied.The relationship between their structure and performance is discussed in detail.The details are as follows::（1）By utilizing the advantage of carbonyl/triangene structure with more carbonyl groups,the n→πtransition is promoted,the spin orbit coupling（SOC）of the molecule is enhanced,the RISC process is promoted,and the TADF property of the molecule is enhanced,which is used as the acceptor structure.PTPA-TOAT and TTPA-TOAT are synthesized by introducing phenyltriphenylamine and tert-butyl triphenylamine as electron donors.There is a certain torsion Angle between phenyltriphenylamine and tert-butyl triphenylamine donors and the acceptor plane,which makes the molecules no longer maintain the planar configuration.Meanwhile,the steric hindrance of donor molecules is large.To some extent,the quenching effect caused by molecular aggregation can be effectively inhibited.Most importantly,the addition of donor increased the length of the molecule and increased the optical extraction efficiency,withΘ//reaching 81%and 86%in the doping films,which was beneficial to obtain higher off-device quantum efficiency.For TTPA-TOAT,the external quantum efficiency reaches 32%,while the maximum luminance(L_max),maximum current efficiency(CE_max)and maximum power efficiency(PE_max)are 17520 cd m^-2,101.4 cd A^-1 and 93.63 lm W^-1,respectively,and the roll off of efficiency is 48%.（2）Using carbonyl/triangene structure as electron acceptor,rigid donor quinoline acridine dione and di-tert-butylindoline carbazole were connected to construct narrow spectrum luminescent materials using traditional D-A structure.For ITCz-TOAT,when the doping concentration is 1 wt%,the device performance reaches the optimum,the luminescence peak position is 491 nm,and the blue light emission is presented.The specific performance is as follows:The turn-on voltage(V_on),the maximum brightness(L_max),the maximum current efficiency(CE_max),the maximum power efficiency(PE_max)and the maximum external quantum efficiency(EQE_max)are 3.2 V,13010 cd m^-2,50.6 cd A^-1,51.7lm W^-1 and 24.5%,respectively.Moreover,a narrow spectrum was maintained,with a full width at half maximum of 50 nm and CIE coordinates of（0.167,0.426）,and a good color purity was achieved.This design strategy,on the one hand,both electron donor and electron acceptor have rigid structures,which inhibits the relaxation of excited states and improves the color purity of materials.On the other hand,by adjusting the intensity of the rigid electron donor,the light color of the material can be adjusted while the narrow-spectrum emission of the material is maintained.It can be said that it is an efficient method to construct narrow-spectrum luminescent materials.