Synthesis And Properties Of Single-Molecule White Emitting Materials Based On The Nitrogen Heterocyclic And Carbazole Derivatives

Organic room temperature phosphorescence(RTP)materials had become a research hotspot in organic luminescence field owing to its internal quantum efficiency of 100% by utilizing triplet excitons in theoretically.But there were rare reports about organic RTP materials used as a white single-molecule emitter in organic light-emitting diodes(OLEDs).The molecular design law of organic RTP materials to achieve single-molecule white emission wasn't clear.In this article,a series of RTP materials based on carbazole and nitrogen heterocycles was designed and synthesized.The RTP emission was realized by improving the molecular spin-orbit coupling constant and promoting the intersystem crossing rate between singlet and triplet states.At the same time,the single-molecule white-emitting materials and their OLEDs were obtained by utilization of double emissions with short wavelength fluorescence and long wavelength phosphorescence.The research contents and conclusions were listed as follows:Using 3,6-bis(pyrimidin-2-yl)-carbazole and 3,6-bis(benzopyrimidin-2-yl)-carbazole as the core,phenyl,benzyl and bromobenzyl groups as the N-substituents of carbazole,a series of small molecules CBM,CPM,CBBM and CBK was designed and synthesized.Comprehensive experimental and theoretical studies had been carried out to investigate their structure–property relationships.The results showed 1)All the molecules exhibited double emission which came from the same excited state transition in their solid film state.The transient lifetime spectra further indicated that the double emission was caused by their intrinsic fluorescence and excimer fluorescence.2)CBK's film exhibited standard white emission with a color coordinate(0.32,0.33)under photoexcitation.And its single-layer non-doped OLEDs exhibited cold white emission with a color coordinate(0.26,0.33)and the maximum external quantum efficiency was 1.35%.3)CBM and CPM exhibited RTP emission with a phosphorescence lifetime of 105.67 and 2.64 ms in their crystal states,respectively.Interestingly,the single-layer non-doped OLEDs based on CPM emitter exhibited cold white emission a color coordinate(0.23,0.30)and the maximum external quantum efficiency was 1.72%.4)Transient EL spectra revealed that fluorescence and phosphorescence mechanism of the OLEDs based on CBM and CPM materials.Using 3,6-bis(pyrazin-2-yl)-carbazole and 3,6-bis(6-methylpyrazin-2-yl)-carbazole as core,and phenyl,benzyl,and benzyl bromide groups as the substituent of the N atom of carbazole,a series of small molecules CBP,CPP,CBBP and CBMP were designed and synthesized.Comprehensive experimental and theoretical studies had been carried out to investigate their structure–property relationships.The results showed 1)Four molecules also exhibited dual emission in their film states.The emissive mechanism was the same as that of the luminescent pyrimidine-carbazolebased small-molecules.2)Four molecules exhibited broad-spectrum luminescence,realizing quasi-white emission.A series of small molecules CB5 M,CP5M and CBB5 M based on 3,6-bis(pyrimidin-5-yl)-carbazole were designed and synthesized by using 3,6-bis(pyrimidin-5-yl)-carbazole as core and phenyl,benzyl and bromobenzyl as substituents of carbazole N atom.Comprehensive experimental and theoretical studies had been carried out to investigate their structure–property relationships.The research results showed that three molecules exhibited intrinsic fluorescence emission only in tetrahydrofuran solution with a PLQY of 18.07%,17.84%,and 13.20%,respectively.But CP5 M exhibited a broad-spectrum emission in the film state.Using carbazole synthesized in the laboratory,the resulting N-benzyl-3,6-bis(pyrimidine-2-yl)-carbazole(Lab-CBM)was prepared,which had the same molecular structure as the CBM synthesized by commercial carbazole.The luminescent properties of Lab-CBM and CBM were compared.The effects of different carbazole source on the luminescent properties were studied.The results showed that the source of carbazole did had a great influence on the RTP performance.Lab-CBM didn't show obvious RTP emission,meanwhile it exhibited poorer electroluminescence performance than CBM in OLEDs with the maximum external quantum efficiency of 3.03%.