Synthesis And Characterization Of Alternating Polymer Blue Light Materials Based On Benzophenone And Carbazole Derivatives

Organic light-emitting diodes based on thermally activated delayed fluorescent materials are developing rapidly.Their theoretical 100%internal quantum efficiency breaks through the upper limit of traditional fluorescent organic light-emitting diodes and overcomes the disadvantages of high pollution and high cost of the second generation phosphorescent organic light-emitting diodes.Compared with red and green light materials,there are still deficiencies in the design and synthesis of thermally activated delayed blue light materials with high color purity,efficiency and stability.Because strong intramolecular charge transfer is very important to reduce the energy gap between the lowest singlet exciton and the lowest triplet exciton and promote the reverse intersystem crossing to produce thermally activated delayed fluorescence,but it often leads to the red-shift of luminescence and is not easy to achieve blue light.Therefore,to develop highly efficient and stable organic thermally activated delayed blue light materials,in this paper,a series of polycarbazole-alternating and main-chain-broken blue light polymers were synthesized by Suzuki coupling reaction using benzophenone derivatives as electron-withdrawing group and carbazole as an electron-donating group.The monomers and polymers’photoelectric properties such as thermal stability,photophysical properties,electrochemical energy levels,and crystal structure were comparatively studied.The specific research work is as follows:In the first part,to completely compare the heavy atom effect of bromine,the connection mode between benzophenone and carbazole,the electron-withdrawing effect of trifluoromethyl,and the effect of polymerization on the comprehensive luminescence properties of the obtained organic materials,firstly,we used benzophenone as the electron acceptor and introduced 3,6-dibromocarbazole as the electron donor at the 2 or 4 positions of benzophenone to synthesize two Compounds with donor-acceptor structures,2-（3,6-dibromo-9H-carbazole）benzophenone（O-2Br Cz BP）and 4-（3,6-dibromo-9H-carbazole）benzophenone（P-2Br Cz BP）by the Ullmann reaction.Then,based on O-2Br Cz BP,electron-withdrawing trifluoromethyl was further introduced into benzophenone’s 3 or 5 positions,respectively.Compounds 2-（3,6-dibromo-9H-carbazol）-3-（trifluoromethyl）benzophenone（3-CF₃-2Br Cz BP）and 2-（3,6-dibromo-9H-carbazol）-5-（trifluoromethyl）benzophenone（5-CF₃-2Br Cz BP）were synthesized.On this basis,a bromine-free compound 2-（9H-carbazole）-5-（trifluoromethyl）benzophenone（5-CF₃-Cz BP）was also synthesized.Finally,a series of side-chain polycarbazole luminescent materials P₁,P₂,P₃ and P₄ were synthesized by Suzuki coupling polymerization with O-2Br Cz BP,P-2Br Cz BP,5-CF₃-2Br Cz BP,3-CF₃-2Br Cz BP and 9-decyl-3,6-bis（4,4,5,5-tetramethyl-1,3,2-dioxaboheterocyclopentane）-9H carbazole（M₀）as monomers.The crystal forms of O-2Br Cz BP,5-CF₃-2Br Cz BP,3-CF₃-2Br Cz BP and 5-CF₃-Cz BP are monoclinic,monoclinic,triclinic and orthorhombic,respectively,and the corresponding space groups are P2₁/c,P2₁/n,P-1 and P2₁2₁2₁,respectively.The results indicated that the introduction of electron-withdrawing trifluoromethyl and bromine atoms significantly changed the crystalline properties of the resulting compounds.The maximum emission wavelengths of compounds O-2Br Cz BP,P-2Br Cz BP,5-CF₃-2Br Cz BP,3-CF₃-2Br Cz BP and 5-CF₃-Cz BP in toluene are 421 nm,393 nm,436 nm,443 nm and 451 nm,respectively.They all achieved blue light and the results indicated that the introduction of 3,6-dibromocarbazole into the 4 positions of benzophenone resulted in a blue shift of the luminescence of the compounds.The 5%mass heat loss temperatures of the powders of polymer P₁,P₂,P₃ and P₄ are 397℃,394℃,310℃and 252℃,respectively.The glass transition temperatures of the powders of polymer P₁,P₂ and P₃ are 180℃,163℃and 119℃,respectively.The results indicated that the introduction of trifluoromethyl produces a large steric hindrance,resulting in poor thermal stability of polymer P₃ and P₄.The maximum emission wavelengths of polymers P₁,P₂,P₃ and P₄ in toluene are 500 nm,476 nm,522 nm and 528nm,respectively,and the corresponding color coordinates are（0.26,0.42）,（0.20,0.29）,（0.33,0.52）and（0.34,0.53）,P₂ achieved blue light compared to the other three polymers.The photoluminescence quantum yields of the powders of polymer P₁,P₂,P₃ and P₄ are 7%,5%,6%and 3%,respectively.Compared with P₁,the introduction of electron-withdrawing trifluoromethyl reduced the photoluminescence quantum yields of powders of polymer P₃ and P₄ by 1%and 4%,respectively.The highest occupied molecular orbital energy levels of the film of polymer P₁,P₂,P₃ and P₄ are similar,and the lowest unoccupied molecular orbital energy levels are-2.43 e V,-2.48 e V,-2.35 e V and-2.02 e V,respectively,indicating that the introduction of electron-withdrawing trifluoromethyl at3 positions of benzophenone leads to the increase of the lowest unoccupied molecular orbital energy level and improves the electron injection barrier.In the second part,two compounds with donor-acceptor-donor structure,4,4′-bis（3-bromo-9H-carbazole）benzophenone（P-2Cz Br BP）and 2,6-bis（3-bromo-9H-carbazole）benzophenone（O-2Cz Br BP）,were synthesized by introducing 3-bromocarbazole at 4,4′or 2,6 positions of benzophenone.Two main-chain-broken polycarbazole luminescent materials P₅ and P₆ were obtained by Suzuki coupling polymerization with P-2Cz Br BP,O-2Cz Br BP and M₀ as monomers,respectively.The effects of benzophenone substitution mode on the comprehensive photoelectric properties of small molecules and polymers were systematically studied.The maximum emission wavelengths of compounds P-2Cz Br BP and O-2Cz Br BP in toluene are 432 nm and 506 nm,respectively.P-2Cz Br BP realized blue emission with color coordinates of（0.16,0.10）.The emission color of O-2Cz Br BP is close to green and the color coordinates are（0.26,0.42）.The delayed fluorescence lifetime of O-2Cz Br BP is 8.59μs,showing thermally activated delayed fluorescent characteristics.The characterization results showed that the 5%mass heat loss temperatures of powders of polymer P₅ and P₆ are 439℃and 405℃and the glass transition temperatures are 211℃and 122℃,respectively.Polymer P₅ shows better thermal stability.The reason for the poor thermal stability of P₆ is that the carbazole group in its monomer replaces the same benzene ring to produce a large steric hindrance,resulting in poor thermal stability.The highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of the films of polymer P₅ and P₆ are-5.29/-2.44 e V and-5.28/-2.33 e V,respectively.The maximum emission wavelengths of polymers P₅ and P₆ in toluene are 480 nm and 527 nm,respectively,and the corresponding color coordinates are（0.21,0.33）and（0.35,0.50）.The photoluminescence quantum yields of the powder of polymer P₅ and P₆were 11%and 5%,respectively.Based on experimental results,it can be found that the small molecule and polymer obtained by introducing 3-bromocarbazole into the 4,4′positions of benzophenone both emit blue light,and the polymer has higher stability and photoluminescence quantum yields.The small molecule and polymer obtained by introducing 3-bromocarbazole into the 2,6 positions of benzophenone both emit green light,which may be caused by the space charge transfer between benzophenone and carbazole.