| With the development of technology,people’s needs have become more and more targeted.Organic light-emitting materials are concerned by researchers in many industries and fields due to their wide variety and functional diversity.These organic luminescent materials with different properties and functions have been applied to different fields.Although scientists have reported countless organic luminescent materials,there are not many studies on compounds that have multiple luminescence modes at the same time.Therefore,the relationship between the structure of the material and the light-emitting performance is still unclear,and meeting the needs of the rapid development of science and technology is still a challenge.The design of organic luminescent materials with the same groups and different molecular structures is undoubtedly of great help to investigate the mechanism of multifunctional organic luminescent materials that we have studied.As a very typical donor material,carbazole has a rigid structure and strong electron-donating ability,its own luminescent properties are very excellent,and it is continuously used as an electron donor group by various organic luminescent materials.Benzophenone contains both the n-type orbital component contributed by the carbonyl group and the π-type orbital component contributed by the aromatic ring,and has a very flexible and rotatable aromatic ring structure,which is usually used as a receptor unit to develop multifunctional organic light-emitting materials.Therefore,this thesis designed and synthesized six compounds based on the derivatives of these two molecules.By comparing the effects of different substitution positions and numbers and heavy atoms on the luminescence performance of the compound in single molecule and aggregated state,a series of multi-functional organic luminescent materials with different luminescence are finally obtained.Firstly,using o-Cz BP as a reference,three derivatives of m-Cz BP,o-2Cz BP and 2o-Cz BP based on carbazole and benzophenone were synthesized.The results show that the absorption and emission spectra of m-Cz BP,o-2Cz BP and 2o-Cz BP in dichloromethane solution are similar to oCz BP.The maximum emission peaks of m-Cz BP,o-2Cz BP and 2o-Cz BP in dichloromethane solution were 501 nm,347 nm and 561 nm,respectively.The luminescence of m-Cz BP is similar to the o-Cz BP and the p-Cz BP.Their powders and crystals have a very obvious room temperature phosphorescence phenomenon,and the afterglow of 2-3 s can be observed with the naked eye.However,the powder and crystal of o-2Cz BP,an ortho-disubstituted compound,can only see a short afterglow.The powder of 2o-Cz BP with two o-Cz BP groups has no afterglow and its crystals cannot be obtained,but its powder has thermally activated delayed fluorescence characteristics and the relative fluorescence efficiency is as high as 39%.Their spectra in solvents with different ratios of water and tetrahydrofuran indicate that the dual emission phenomenon of m-Cz BP and o-2Cz BP,and the two emission peaks showed a competitive relationship.m-Cz BP,o-2Cz BP and 2o-Cz BP all have the characteristics of aggregation-induced emission,but no aggregation-induced emission is found in o-Cz BP.Through theoretical calculation,the electron cloud distribution and excited state energy level of the highest occupied molecular orbit and the lowest unoccupied molecular orbit optimized by the three in the gas phase state are obtained.In m-Cz BP,o-2Cz BP,2o-Cz BP and oCz BP,the space charge transfer phenomenon between the two benzene rings of benzophenone exists.In o-2Cz BP,the space charge transfer phenomenon between the carbazole group and the benzophenone group also found in o-Cz BP was also observed.In summary,the ortho-substituted compounds have a folded configuration,can achieve space charge transfer,and have room temperature phosphorescence and thermally activated delayed fluorescence characteristics.The metasubstituted compounds have no space charge transfer between carbazole and benzophenone,and have no thermally activated delayed fluorescence properties.When the substituent group is added,the interaction between the carbazole planes is destroyed,the crystallinity is reduced,and the nonradiative transition is not well suppressed,so the life is reduced.After coupling the two o-Cz BP together,the non-radiative transition rate is higher,the room temperature phosphorescence characteristic disappears,and only the thermally activated delayed fluorescence characteristic.These compounds with different substitution positions and numbers that show different light-emitting modes prove that the molecular structure has a very important influence on the luminescence of a single molecule or in an aggregated state.The folded configuration is also an effective strategy for developing multifunctional organic luminescent materials.Based on the compounds in the previous chapter,the third chapter of this thesis synthesized three bromine-containing compounds o-Br Cz BP,o-2Br Cz BP and m-2Br Cz BP.We studied the effect of heavy atoms on the structure and luminescence properties of organic luminescent materials.The results show that the absorption and emission spectra of these three compounds in dichloromethane solution are almost completely consistent.But in the solid state,the maximum emission peak of the crystal of o-Br Cz BP is at 449 nm,and after melting and cooling in the air,the emission peak is redshifted to 476 nm.The crystal of o-2Br Cz BP also has the same phenomenon,its emission peak is red-shifted from 451 nm to 490 nm.No similar phenomenon was observed in m-2Br Cz BP,but m-2Br Cz BP showed dual emission behavior,and its powder had two emission peaks at 443 nm and 570 nm.The introduction of heavy atoms has also reduced the luminescence lifetime of these compounds,and the efficiency has also decreased.In addition,there are a large number of C-H…Br bonds in the crystals of these compounds.The joint action of these bonds and the π-π bond between the carbazole groups makes them obtain excellent crystallinity and promote the process of charge transfer between molecules.We have also proved this through theoretical calculations.However,heavy atoms also increase the dihedral angle and spacing between the carbazole and benzophenone of o-Br Cz BP and o-2Br Cz BP,weakening the space charge transfer between them,reducing efficiency and life.In summary,by adding bromine atoms,not only can the compound’s orbital coupling properties be improved,but also the intermolecular C-H…Br bonds in the aggregate state can be increased.This makes their molecular configuration in the aggregated state very different from the single molecular state,and can also improve the crystallinity of the ortho compound,which is also a very effective design strategy for multifunctional organic luminescent materials.By comparing these compounds,we can find: The planar structure of carbazole can increase the rigidity and molecular aggregation ability of the materials.Benzophenone can increase the charge transfer path of the molecule and obtain multiple light-emitting methods.Different substitution positions and numbers have greatly changed the topological structure and aggregation state structure of molecular,so that molecular have intramolecular charge transfer,intramolecular space charge transfer and intermolecular charge transfer,so that it has obtained a variety of luminescent pathway.In addition to increasing the rate of intersystem hopping,bromine atoms can also change the molecular topology and aggregation state structure,and strong intermolecular halogen bonds also make their single molecule and aggregation state light emission methods very different.In summary,the several organic luminescent materials based on carbonyl-modified carbazole derivatives involved in this article have multiple ways of emitting light,and have also obtained some reasonable and very effective molecular design strategies through a lot of research.This also provides ideas for continued research on new multi-functional organic luminescent materials. |
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