| With the vigorous development of organic luminescent materials,our way of life was deeply affected by their applications.According to the luminescence mechanism,organic luminescent materials can be divided into fluorescent and phosphorescent materials.Only 25% singlet excitons can be used to emit light,however,the internal quantum efficiency of phosphorescent materials can reach 100% due to the strong spin orbit coupling.As a novel type of luminescent material,thermal activated delayed fluorescence(TADF)material is to use thermally activated triple exciton to achieve100% internal quantum efficiency by using reverse inter system crossing.or the light-emitting color,organic light-emitting materials can be divided into three primary colors: red,green and blue.Organic light emitting diode(OLED)made of red and green light-emitting materials have been mature and widely used,but blue light-emitting materials have not been well developed due to the inherent characteristics such as wide band gap and carrier annihilation.Therefore,how to achieve efficient and stable blue OLED is a hot research topic.As a common nitrogen heterocyclic compound,carbazole is cheap and easily obtained;which has high conjugation degree and good photochemical and thermal stability.Besides,it has good hole transport ability and exhibits high carrier mobility as an electron donor group.Because of many reaction sites in its structure,it is easy to modify the structure with functional groups and can also be well used in polymers.The derivatives of it exhibit appropriate wide gap and high triplet energy required by the host material.It is widely favored due to the blue emission in blue OLED.Trifluoromethyl is an unconjugated electron-withdrawing unit,which can provide a certain steric hindrance to regulate the intermolecular stacking mode.Hence,taking 3,6,9-tris(4-tert-butyl)phenyl-9H-carbazole(M1)as a reference,a series of star-shaped organic blue luminescent materials with donor-acceptor structure were synthesized by introducing different numbers of electron-withdrawing 3,5-bis(trifluoromethyl)phenyl into different positions of carbazole.The effects of the substitution position and number of electron-withdrawing group of 3,5-bis(trifluoromethyl)phenyl on their excited state properties were studied.The specific results are list as follows:In the first part,the effects of the substitution position of monosubstituted electron-withdrawing group of 3,5-bis(trifluoromethyl)phenyl on the excited state properties of carbazole based blue light materials were studied,3,5-bis(trifluoromethyl)phenyl were introduced into the 3 or 9 position of carbazole respectively through Ullmann coupling and Suzuki coupling reaction with M1 as a reference which contains solely 4-tert-butyl-phenyl trisubstitution.The luminescent materials 3,6-bis(4-tert-butyl-phenyl)-(3,5-bis(trifluoromethyl)phenyl)-9H-carbazole(M2)and 3-(3,5-bis(trifluoromethyl)phenyl)-6,9-bis(4-tert-butyl-phenyl)-9H-carbazole(M3)were obtained.The powders of M1 and M3 showed glass transition at 79 ℃ and 85 ℃ respectively,while M2 did not show obvious glass transition.The thermal mass loss of M2 and M3 are both 344 ℃ in powder state.The results showed that the thermal stability of M2 and M3 decreased after the introduction of 3,5-bis(trifluoromethyl)phenyl.In addition,three compounds have good solubility in different polar solvents.The emission peak of M1 in toluene is 380-397 nm,and the maximum emission peaks of M2 and M3 are 388 nm and 394 nm respectively,whose color coordinates of M1,M2 and M3 are(0.17,0.02),(0.19,0.03)and(0.21,0.05)respectively,indicating that the color of all luminous are blue.The optical energy gaps of M1,M2 and M3 measured in solution are 3.48 e V,3.32 e V and 3.54 e V and the highest occupied molecular orbital energy levels and the lowest unoccupied molecular orbital energy levels of M1,M2 and M3 are-5.53 /-2.05 e V,-5.57 /-2.25 e V and-5.73 /-2.19 e V respectively.Theoretical calculation showed that after changing the substitution position of 3,5-bis(trifluoromethyl)phenyl,the highest occupied molecular orbital is still distributed on carbazole,and the lowest unoccupied molecular orbital of compounds transferred from carbazole to the position of3,5-bis(trifluoromethyl)phenyl.The photoluminescence quantum yields of M1,M2 and M3 in toluene were 39%,47% and 34% respectively.The luminous efficiency of compounds is different with the substitution position of electron-withdrawing groups.In the second part,the substitution number and the position of 3,5-bis(trifluoromethyl)phenyl through Ullmann coupling and Suzuki coupling were further changed to study the effects on the luminescence properties of luminescent materials,and finally three compounds: 3-(4-tertbutyl)phenyl-6,9-(3,5-bis(trifluoromethyl)phenyl)-9H-carbazole(M4),3,6-bis(3,5-bis(trifluoromethyl)phenyl)-9-(4-(tert-butyl)phenyl)-9H-carbazole(M5)and 3,6,9-tris(3,5-bis(trifluoromethyl)phenyl)-9H-carbazole(M6)were obtained.The results show that after changing the substitution number of 3,5-bis(trifluoromethyl)phenyl,M5 has a crystallization peak at 187 ℃ in powder state,and the melting points of M4,M5 and M6 are 212 ℃,225 ℃ and 247 ℃ in the same state.Compared with M4 and M5,M6 shows a higher melting point after introducing more electronwithdrawing groups,indicating that trisubstituted compound can improve the melting point of the molecule.The maximum fluorescence emission peaks in toiuene of M4,M5 and M6 appear at 399 nm,427 nm and 386 nm respectively,the color coordinates are(0.17,0.03),(0.18,0.09)and(0.16,0.03)respectively,indicating that the color of all luminous are blue.The photoluminescence quantum yield of the three compounds in toluene solution are 49%,46% and 29% respectively.It shows that the compounds obtained by introducing double substituted electron-withdrawing groups at positions3,6 or 3,9 of carbazole can improve the luminescence efficiency,however,the trisubstituted electronwithdrawing groups reduce the luminescence efficiency. |
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