Synthesis And Optoelectronic Properties Of Red Thermally Activated Delayed Fluorescence Materials Derived From Phenanthroline Acceptors

Over the past thirty years,organic light-emitting diodes(OLEDs)have been continuously updated and recently emerged as a new display technology to be widely used in small-and medium-size electronic products,such as smart phones and watches.Luminescent materials for OLEDs have evolved rapidly from the first generation of traditional fluorescent materials,to the second generation of heavy-metal based phosphorescent materials,and to the third generation of luminescent materials.Among them,thermally activated delayed fluorescence(TADF)emitters have been regarded as the most promising new generation emitters because of the theoretical 100%excitons utilization efficiency,purely organic structure,and low cost.During the past decade,blue and green TADF emitters and the related devices have achieved considerable progresses,yet red TADF emitters have lagged behind.According to the energy gap law,the narrow energy gap between the excited state and the ground state for red TADF molecules normally induces severe non-radiative transition processes and thus result in low emissive efficiency.To overcome this problem,this thesis designed and synthesized three phenanthroline-based acceptor moieties with a large conjugated plane and rigid structure to suppress non-radiative transition process,and then constructed several high-efficiency red TADF emitter to exhibit good photo-and electroluminescent performance.In chapter 1,we firstly summarized the development history and research status of OLEDs and then we mainly introduced the research progress of small-molecule red TADF emitters.Finally,we presented the main research ideas of this thesis.In chapter 2,we designed and synthesized two red TADF molecules,3,6?R and2,7?R,based on the cyanobenzene-substituted dipyrido[3,2-a:2?,3?-c]phenazine acceptor and triphenylamine donor.Both compounds in the doped film had high photoluminescence quantum efficiencies of 80?86%,significant TADF characteristics and short delayed fluorescence lifetimes(about 1?s).The doped device using 3,6?R as the guest emitter exhibited a red emission peaking at 619 nm together with the CIE coordinates of(0.62,0.38),and achieved good electroluminescence performance with the maximum external quantum efficiency of 12.0%,the maximum current efficiency of 15.3 cd A^-1,and the maximum power efficiency of 12.0 lm W^-1.In chapter 3,we used the cyano unit to replace the cyanobenzene unit with the aim to increases the planarity and electron-withdrawing ability of the acceptor moiety.Combining with the same triphenylamine donor,we designed and synthesized two red TADF molecules,TD-3,6-DCN and TD-2,7-DCN.Both target molecules obtained asignificantly red-shifted emissions,small single-triplet energy gaps,high photoluminescence quantum efficiencies,distinct TADF characteristics and short delayed fluorescence lifetimes of less than 1?s.Both compounds exhibited high photoluminescence quantum efficiencies(94?100%)in the m CP host,but their poor film-forming ability made the corresponding solution-processed devices have low external quantum efficiency of 6.9%.To further optimize the device performance,we selected a TADF emitter of DMAC-DP-Cz as the sensitizing host.The corresponding optimized solution-processed OLEDs based on TD-3,6-DCN and TD-2,7-DCN emitters delivered red emissions peaking at 636 nm and 632 nm,respectively and achieved maximum external quantum efficiencies of 16.6%and 12.9%,respectively.These efficiencies are among one of the highest performances for solution-processed red TADF OLEDs with similar color gamut reported so far.In chapter 4,we constructed two fluorescent molecules,ANPPT-TPA and ANPPT-DMAC,using an acenaphtho[1',2':5,6]pyrazino[2,3-f][1,10]phenanthroline acceptor combined with triphenylamine or 9,9-dimethyl-9,10-dihydroacridine donor units,.ANPPT-DMAC exhibited good thermal stability,significant TADF characteristics and high photoluminescence quantum efficiency.The ANPPT-DMAC-based device displayed an orange-red electroluminescence emission peaking at 596 nm and delivered a maximum external quantum efficiency of 12.9%.