Molecular Modification,synthesis And Optoelectronic Properties Of Nitrogen-containing Benzoheterocyclic Compounds Based On Tris(2,4,6-trichlorophenyl)methyl Radical

With the development of organic light-emitting diodes(OLEDs)in the past 30 years,abundant luminescent materials have been designed and synthesized.All of these molecules can be classified into four systems though the variety of their molecular structures: 1.The traditional materials of fluorescence using the singlet excitons;2.The phosphorescent materials using the triplet excitons;3.The new type of fluorescent materials via transforming the triplet excitons into singlet excitons which include TTA,TADF and HLCT.4.The materials of triplet-polaron-interaction(TPI)which enhances the singlet excitons via the interaction between triplet excitons and polarons.Different from all the above materials,in 2015,our group creatively proposed a novel mechanism based on the doublet excitons,and firstly achieved the highly efficient deep red/near-infrared electroluminescence using an open-shell molecule,TTM-1Cz(adduct of TTM and carbazole),as the emitter.For an open-shell molecule of the ground state,there is only one electron in the sinly occupied molecular orbital(SOMO).When it is excited to the higher level orbital,the SOMO is empty,transition back of the excited electron to the SOMO is spin allowed.Thus,the upper limit of internal quantum efficiency(IQE)of open-shell molecule-based OLEDs can reach 100% theoretically.However,the kind for air-stable radicals is limited,and even rarer for the luminescent radicals at room temperature.On the other hand,the emission bands of the luminescent radicals are almost in the deep red/near-infrared region.Therefore,it is not only meaningful but also challenging to enrich the material system and to modulate the emission band to a wider wavelength range.And there are still a number of fundamental works needed to be implemented.In this thesis,we carried out the following two-part study based on neutral stable tris(2,4,6-trichlorophenyl)methyl(TTM)radical derivatives:1.Molecular modification,Synthesis and Optoelectronic Properties of open-shell molecules based on an electron-donating unit indole and TTM radicalA nitrogen-containing heterocyclic donor,indole,was selected as substituent group in this chapter.We also notice that the lack of a benzene leads to the decrease of conjugated effect,though the slightly enhancement of donating ability compared to carbazole.We synthesized TTM-1ID and TTM-2ID with different number of groups by attaching indole to p-carbon of the TTM center.Both of them showed properties of intra-molecular charge transfer(ICT)with emission of red-shift related to TTM and hypsochromic shift related to TTM-1Cz.The electroluminescence(EL)spectra centered at ~ 650 nm and were almost unchanged with the increase of voltage.The CIE coordinate were(0.649,0.306)and(0.667,0.301)respectively,which were close to the National Television System Committee(NTSC)standard.The maximum external quantum effiency(EQE)of TTM-1ID is 1.8% and 2.4% for TTM-2ID.Particular for the device based on TTM-2ID,the formation ratio of doublet excitons is even up to 100%.However,the low fluorescent quantum yields in doped films limited the further improvement of device performance.2.Molecular modification,Synthesis and Optoelectronic Properties of open-shell molecules based on an electron-withdrawing unit benzimidazole and TTM radicalLuminescent materials with both high quantum yield and an enhancement for formation ratio of excitons are needed for excellent device performance.In this chapter,we chose a nitrogen-containing heterocyclic acceptor,benzimidazole,as substitute and obtained two novel open-shell molecules by incorporating it to the p-carbon of TTM.Further hypsochromic shift of emitting and an enhancement of quantum yields in films were achieved via suppressing the ICT properties of TTM-2Bi and TTM-3Bi.Both devices exhibited outstanding stabilities and high-voltage-durable characteristics.The device based on TTM-2Bi exhibited better performance with a maximum EQE of 5.4% and a formation ratio of doublet excitons of 69%,relatively,4.1% and 57% for TTM-3Bi,respectively.The EQE for TTM-2Bi is even superior to the 5 % theoretical upper limit in conventional fluorescence devices.