Synthesis And Properties Of Nitrogen-containing Heterocyclic Star-shaped Luminescent Materials

Organic Light-Emitting Diodes(OLEDs)have been attracted widespread attention due to its outstanding advantages such as self-luminescence,low energy consumption,good flexibility and high response,and have been studied and commercialized.Star-shaped compounds have been attracted much attention due to their improved solution processability,self-assembly properties,and interesting electrochemical and charge transport behaviors.Among them,the donor-acceptor(D-A)molecules were often used to construct organic field-effect transistors(OFETs)and bulk heterojunction solar cells.However,there were relatively few applications of such molecules in organic light emitting diodes(OLEDs).Due to the dispersive branch structure,the star-shaped D-A molecules can reduce the intermolecular interaction in the solid state,thereby enhancing the solid-state emission and having good film-forming properties.In addition,the star-shaped D-A molecules could replace polymers to apply in OLEDs and could overcome the problem of insufficient repeatability of the device performances caused by different molecular weight,purity and polydispersity of polymers.As a structural functional unit for building star-shaped molecules,triazatruxene(TAT)was widely used in the field of organic optoelectronics.This thesis focused on the synthesis and properties of the triazatruxene(TAT)organic luminescent materials.D-?-A bipolar star-shaped molecules with three-branched structure were constructed by grafting imidazole derivatives as terminal groups on different substitution sites of triazatruxene(TAT).The specific research content was divided into the following three parts:In the first part,two bipolar star-shaped luminescent materials were synthesized by grafting the phenanthrimidazole units as the terminal groups on the 3,8,13-position and2,7,12-position of the triazatruxene(TAT)core,named 3,8,13-TPI-TAT and 2,7,12-TPI-TAT respectively.Due to the special symmetrical structure,the triazatruxene(TAT)core has a unique rigid structure,?-conjugated plane and excellent hole transport properties.Introducing phenanthroimidazole units as the terminal groups to construct the bipolar star-shaped molecules containing the triazatruxene(TAT)core,not only the?-?stacking of the phenanthrimidazole unit was weakened,but also the aggregation fluorescence quenching(ACQ)phenomenon caused by the?-conjugated plane of the triazatruxene(TAT)compound was weakened.The fluorescence emission peaks of the compounds 3,8,13-TPI-TAT and2,7,12-TPI-TAT in solution were located at 440 nm and 468 nm,respectively,showing blue emission.The HOMO and LUMO energy levels of the compounds 3,8,13-TPI-TAT and2,7,12-TPI-TAT were calculated by using cyclic voltammetry.The HOMO and LUMO energy levels of the compound 3,8,13-TPI-TAT were-5.34 e V and-2.42 e V,respectively.The HOMO and LUMO energy levels of the compound 2,7,12-TPI-TAT were-5.30 e V and-2.54 e V,respectively.The electron cloud distribution and the molecular conformation of the two compounds were calculated by density functional theory.The thermal decomposition temperature of the compound 3,8,13-TPI-TAT was 461?,and the thermal decomposition temperature of the compound 2,7,12-TPI-TAT was 450?.The multilayer devices with a structure of ITO/PEDOT:PSS(45 nm)/PVK:PBD(7:3,wt%):TAT compound(x wt%)(30nm)/TPBi(20 nm)/Liq(2 nm)/Al(150 nm)were fabricated by the vacuum-deposited method,the device based on the compound 2,7,12-TPI-TAT showed better device performance.In the case of 100%doping(non-doping),L_max was 2943 cd/m²,CE_max was 3.42 cd/A,PE_max was1.68 lm/W and EQE_max was 1.93%.The non-doped device based on the compound3,8,13-TPI-TAT exhibited L_max of 784 cd/m²,CE_max of 2.17 cd/A,PE_maxof 0.94 lm/W and EQE_max of 1.25%.In the second part,considering that the large conjugated plane of the branched group may affect the luminescence properties of the compound,two bipolar star-shaped organic light-emitting materials with small conjugated plane benzimidazole units as the terminal groups were synthesized,named 3,8,13-TBI-TAT and 2,7,12-TBI-TAT.The photophysical and electrochemical properties,thermal stabilities and electroluminescence performance of the compounds 3,8,13-TBI-TAT and 2,7,12-TBI-TAT were studied.The multilayer devices with a structure of ITO/PEDOT:PSS(45 nm)/PVK:PBD(7:3,wt%):TAT compound(x wt%)(30 nm)/TPBi(20 nm)/Liq(2 nm)/Al(150 nm)were fabricated by the vacuum-deposited method,the device fabricated from the compound 2,7,12-TBI-TAT showed better device performance.In the case of 100%doping(and non-doping),the device based on the compound 2,7,12-TBI-TAT exhibited L_max of 10075 cd/m²,CE_max of 5.35 cd/A,PE_max of 3.16lm/W,and EQE_max of 2.25%,while the device based on the compound 3,8,13-TBI-TAT showed L_max of 639 cd/m²,CE_max of 1.33 cd/A,PE_max of 0.52 lm/W,and EQE_max of 1.25%.In the third part,in order to further reduce the self-conjuged plane of the terminal imidazole units,two bipolar star-shaped organic luminescent materials 3,8,13-TDPI-TAT and2,7,12-TDPI-TAT with diphenylimidazole units as the terminal groups were synthesized.The photophysical,thermal and electrochemical properties of the two compounds were studied.The fluorescence emission peaks of the compounds 3,8,13-TDPI-TAT and 2,7,12-TDPI-TAT in solution were at 423 nm and 453 nm,respectively,showing deep-blue and blue emission,respectively.The HOMO and LUMO energy levels of 3,8,13-TDPI-TAT and2,7,12-TDPI-TAT were calculated by using cyclic voltammetry.The HOMO and LUMO energy levels of the compound 3,8,13-TDPI-TAT were-5.23 e V and-2.31 e V,respectively.The HOMO and LUMO energy levels of the compound 2,7,12-TDPI-TAT were-5.27 e V and-2.52 e V,respectively.The electron cloud distribution and the molecular conformation of the two compounds were calculated by density functional theory.The thermal decomposition temperature of the compound 3,8,13-TDPI-TAT was 307?,and the thermal decomposition temperature of the compound 2,7,12-TDPI-TAT was 429?.The multilayer devices with the same structure were prepared by the same method as in the part 1 and 2,and the device based on the compound 2,7,12-TDPI-TAT showed better device performance.In the case of 100%doping(non-doping),the device based on the compound 2,7,12-TDPI-TAT exhibited L_max of678 cd/m²,CE_max of 2.52 cd/A,PE_max of 1.58 lm/W,and EQE_max of 1.50%,while the device based on the compound 3,8,13-TDPI-TAT showed Lmax of 500 cd/m²,CE_max of 1.10 cd/A,PE_max of 0.50 lm/W,and EQE_max of 1.25%.