Synthesis And Structure-properties Correlation Of Blue Fluorescence Isomer Emitters Based On Rigid Diazine-bridged Carbazole Frameworks

Organic Light-Emitting Diodes?OLED?is considered the most competitive next-generation technology for flat panel displays and solid-state lighting sources.Due to their advantages of low driving voltage,high luminous efficiency,fast response,flexible display,ultra-thin and so on,they have a very wide range of application prospects in the fields of flat panel display and solid illumination.The synthesis of highly efficient and stable blue light materials is still one of the focuses determining the further development of OLED.Intramolecular charge transfer type light-emitting materials are widely used in improving the carrier transporting,regulation of energy levels of luminescent materials and synthesis of thermally activated delayed fluorescence?TADF?materials.Our research group has been long focusing on the exploitation of new organic electroluminescent blue light materials for high-tech applications.To the best of our knowledge,there is rare report about the structure-properties correlation of blue fluorescence isomer emitters with an intramolecular charge transfer structure.In this report,a series of donor-acceptor?D-A?isomer emitters based on rigid diazine-bridged carbazole were designed and synthesized.The measurements of cyclic voltammetry?CV?,ultraviolet-photoluminescent spectra and device applications are studied to illustrate the structure-properties correlation of the materials in depth.In the first part,four compounds named 3,6-bis?4-tert-butyl-benzene?-9-yl-pyrimidine-9H-carbazole?M1?,3,6-bis?4-tert-butyl-phenyl?-9-yl-5-pyrimidine-9H-carbazole?M2?,3,9-bis?4-tetra-tert-butyl-benzene?-6-yl-2-pyrimidine-9H-carbazole?M3?and3,9-bis?4-tert-butyl-benzene?-6-yl-5-pyrimidine-9H-carbazole?M4?were obtained by the classical Suzuki coupling reaction and Walman coupling reaction.All four compounds have good solubility in common organic solvents.Their maximum emission wavelengths in toluene solution(10^-5 mol/L)are 453,434,402,405 nm,and all were blue light emission.The CT features are not obvious and can reduce exciton quenching of the luminescent layer in the doped OLED device,which is consistent with the relatively large absolute fluorescence quantum efficiency results of M1-M4 in toluene solution.It was found that the four materials did not show obvious glass transition temperature;the thermal decomposition temperatures of materials M1-M4 were 289,313,243,and321°C,respectively.The energy gaps of M1-M4 according to the initial UV absorption in the toluene solution were 3.63,3.72,3.46,and 3.54 eV.The four compounds have an aggregation-induced emission effect in a mixed solvent of tetrahydrofuran and water.According to cyclic voltammetry,the highest occupied molecular orbital energy levels of molecules M1,M2,M3and M4 were-5.59,-5.67,-5.66,-5.69 eV.Similarly,the lowest unoccupied molecular orbital energy levels of M1,M2,M3 and M4 were-2.29,-2.34,-2.36,and-2.33 eV.The electrochemical energy gap of M1-M4 are 3.66,3.44,3.42 and 3.48 eV,which were basically consistent with the optical band gap obtained by the UV absorption spectra of the four compounds in a toluene solution.It was indicated that the presence of the diazine isomer as an electron-withdrawing group could indeed adjust the photophysical properties of the blue light emitting material in a wide range.Based on the work in the first chapter,three novel blue fluorescence isomer emitters,which were2,5-bis?6,9-bis?4-?tert-butyl?phenyl?-9H-carbazol-3-yl?pyrazine?TCz-3PA-TCz?,9-?5-?6,9-bis?4-?tert-butyl?phenyl?-9H-carbazol-3-yl?pyrazin-2-yl?-3,6-bis?4-?tert-butyl?phenyl?-9H-carbazole?TCz-3,9PA-TCz?and 2,5-bis?3,6-bis?4-?tert-butyl?phenyl?-9H-carbazol-9-yl?pyrazine?TCz-9PA-TCz?,based on rigid pyrazine-bridged carbazole frameworks with a donor-accepter-donor structure were synthesized and characterized.Through thermal measuring,the glass transition temperatures of TCz-3PA-TCz,TCz-3,9PA-TCz and TCz-9PA-TCz were 99,101and 100?,and the decomposition temperatures corresponding to 5%weight loss were 200,393and 171°C for TCz-3PA-TCz,TCz-3,9PA-TCz,and TCz-9PA-TCz,respectively,which illustrated that TCz-3,9PA-TCz showed much higher thermal stability than those of TCz-3PA-TCz and TCz-9PA-TCz.The maximum photoluminescence emission wavelengths of TCz-3PA-TCz,TCz-3,9PA-TCz,and TCz-9PA-TCz in toluene solution were 423,435,454 nm,and their optical band gaps in the same state were 2.94,2.90,and 2.74 eV.Unfortunately,their blue light photoluminescence emissions were aggregation-induced quenching in a mixture solvent of water and tetrahydrofuran.The absolute photoluminescent quantum yields of TCz-3PA-TCz,TCz-3,9PA-TCz and TCz-9PA-TCz doped in 4,4?-Bis?9-carbazolyl?-1,1?-biphenyl with a weight of5%were 10.75%,4.46%,and 16.70%.According to cyclic voltammetry curves,the highest occupied molecular orbital energy levels of TCz-3PA-TCz,TCz-3,9PA-TCz and TCz-9PA-TCz were-6.10,-6.09,and-5.81 eV,and the lowest unoccupied molecular orbital energy levels of TCz-3PA-TCz,TCz-3,9PA-TCz and TCz-9PA-TCz were-3.19,-3.36,and-3.21 eV.To further correlate the electroluminescent performance with the chemical structure of isomer emitters,two sets of OLEDs were fabricated by spin coating and thermal deposition in vacuum.The driving voltage,maximum external quantum efficiency,maximum current efficiency,maximum power efficiency and maximum luminance of doped OLEDs based on TCz-9PA-TCz fabricated by spin coating were 4.0 V,2.67%,4.12 cd A^-1,2.46 lm W^-11 and 8052 cd m^-2,which were much better than those based on TCz-3PA-TCz and TCz-3,9PA-TCz.Furthermore,the doped OLEDs with TCz-3,9PA-TCz as dopant emitter fabricated by thermal deposition in vacuum showed driving voltage of 3.5 V,maximum luminance of 6369.67 cd m²,maximum external quantum efficiency3.85%at a brightness of 100 cd m^-2 and obviously reduced efficiency roll-off.