Synthesis And Properties Of Bipolar Organic Small Molecule Luminescent Materials Based On Imidazole Units

Organic light-emitting diodes(OLED)have attracted wide attention due to the features of viewing angle of more than 160 degrees,low driving voltage,low cost,simple manufacturing process,flexible display of OLED products.More importantly,OLED are made of organic materials and the light color is highly tunable.In the development of OLED,fluorescent materials are first used in OLED.Doping them into the appropriate host materials can improve the performance of OLED devices.Therefore,the development of high efficiency fluorescent materials has become an important direction in the field of organic electroluminescence.This thesis is devoted to the synthesis of bipolar organic luminescent small molecular materials with excellent properties.The specific research contents are as the follows:In the first system of the thesis,using single and double branched phenanthrimidazole as the acceptor and phenothiazine as the donor,three donor-acceptor(D-A)and acceptor-donor-acceptor(A-D-A)type of phenanthroimidazole-phenothiazine derivatives,PI-PTZ,PPI-PTZ and PPI-PTZ-PPI,were designed and synthesized.First,the structures of the compounds were confirmed by ¹H NMR,¹³C NMR and HRMS,and the thermal stability,photophysical and electrochemical properties of the three compounds were further studied systematically.Moreover,the vacuum-deposited devices of the compounds were fabricated,and their electroluminescent performances were discussed in detail.The compounds PI-PTZ,PPI-PTZ and PPI-PTZ-PPI were used as the light-emitting guest materials,and the multilayer devices with a structure of ITO/NPB(30 nm)/TBADN:compound PI-PTZ or PPI-PTZ or PPI-PTZ-PPI(x wt%,30 nm)/TPBi(30 nm)/Liq(2 nm)/Al(100 nm)were fabricated by the vacuum-deposited method.Among them,when the doping concentration was 10 wt%,the device based on the compound PPI-PTZ-PPI exhibited the best performances with a maximum luminance(L_max)of 30180 cd/m²,a maximum current efficiency(CE_max)of 9.19cd/A,a maximum power efficiency(PE_max)of 9.76 lm/W and a maximum external quantum efficiency(EQE_max)of 4%.In the second system of this thesis,four organic light-emitting small molecules(Py-ICZ-Py,PI-ICZ-PI,BI-ICZ-BI and IM-ICZ-IM)were synthesized by using indolocarbazole(ICZ)as donor and different imidazole units as the acceptors.The thermal stability,electrochemical and photophysical properties of the compounds were discussed systematically.Based on their above properties,the vacuum-deposited devices of the compounds were fabricated and their electroluminescent properties were investigated.The compounds Py-ICZ-Py,PI-ICZ-PI,BI-ICZ-BI and IM-ICZ-IM were used as the luminescent guest materials,and the multilayer devices with a structure of ITO/NPB(30 nm)/TBADN:Compound Py-ICZ-Py or PI-ICZ-PI or BI-ICZ-BI or IM-ICZ-IM(x wt%,30 nm)/TPBi(30nm)/Liq(2 nm)/Al(100 nm)were fabricated by vacuum-processed method.Among them,the doped device based on the compound PI-ICZ-PI at 7 wt%doping concentration showed the best device performances with the turn-on voltage of 3.0 V,the maximum brightness of3973 cd/m²,the maximum current efficiency of 3.76 cd/A,the maximum power efficiency of3.01 lm/W,and the maximum external quantum efficiency of 2.64%.In the third system of this thesis,three organic luminescent small molecules(EB-PTZ,EB-PTZ-EB and EB-AE)were synthesized using phenothiazine as the donor,anthracene as the chromophore and diphenylimidazole as the acceptor.And their thermal stability,electrochemical and photophysical properties were discussed systematically.The maximum absorption bands of the three compounds EB-PTZ,EB-PTZ-EB and EB-AE were at 361,364and 385 nm,respectively,and their maximum emission peaks were at 458,474 and 455 nm.Their fluorescence quantum yields were measured at room temperature.Their lifetimes were8.06 ns,8.23 ns,and 7.94 ns,respectively,and their photoluminescence quantum yields were18.49%,28.92%,and 47.65%,respectively.The photoluminescence spectra of the compounds EB-PTZ-EB and EB-AE were highly dependent on the polarity of solvents.As the polarity of solvent increases,the photoluminescence spectra of the compound EB-PTZ-EB and EB-AE were red-shifted.The electrochemical properties of the compounds were studied by cyclic voltammetry(CV),and the HOMO energy levels of the compounds EB-PTZ,EB-PTZ-EB and EB-AE were calculated to be-5.285 e V,-5.355 e V,-5.435 e V,and their LUMO energy levels were-2.435 e V,-2.535 e V and-2.585 e V,respectively.The thermal decomposition temperatures of the compounds EB-PTZ,EB-PTZ-EB and EB-AE were377?,344?,342?,and their glass transition temperatures were 226?,204?,202?.Based on their above properties,the vacuum-deposited devices of the compounds were fabricated and their electroluminescent properties were investigated.The compounds EB-PTZ,EB-PTZ-EB and EB-AE were used as the luminescent guest materials,and the multilayer devices with a structure of ITO/PEDOT:PSS(30 nm)/m CP:EB-PTZ(25 nm)/TPBi(35 nm)/Liq(2 nm)/Al(150 nm)were fabricated by vacuum-processed method.Among them,the doped device based on the compound EB-PTZ-EB at 15 wt% doping concentration exhibited the best device performances with the turn-on voltage of 5.8 V,the maximum brightness of 648 cd/m²,the maximum current efficiency of 2.45 cd/A,the maximum power efficiency of 1.20 lm/W and the maximum external quantum efficiency of 1.48%.