Study On Morphology-dependent Photooxidation In Carbazole Luminescent Materials

As a typical representative of organic semiconductor materials,fluorene materials have a wide range of applications in organic light-emitting diodes,transistors,solar cells,and lasers.As the basic building block of fluorene materials,the fluorene unit has the characteristics of high fluorescence quantum efficiency,easy functionalization and solution processing,which has attracted wide attention from scientists.However,the 9-position carbon easily forms fluorenone defects under light irradiation,which seriously hinders the application and development of fluorene materials in the field of optoelectronics.In addition,aggregation defects,hydroxyl defects and bending defects are also problems faced by fluorene materials.Therefore,scientists have proposed a variety of solutions to these problems,including steric functionalization,design of polyaromatic hydrocarbon nanopolymers,and hyperbranching.However,there are still difficulties in designing and synthesizing highly stable and deep blue luminescent fluorene materials,further clarifying the structure-activity relationship,and fine-tuning their self-assembly morphology.In 2019,Xie Linghai's research team was inspired by the enhanced emission capability of planarized?-phase polyfluorene,and established a rule for organic two-dimensional nano-assembly of attraction-repulsion(A₂R₁)fluorene-based molecules.In addition,through comparison with amorphous film,crystal-induced luminescence or/and enhanced stability(CLoSE)was observed in A₂R₁ racemate,indicating that the behavior of excitons can be affected by regulating aggregates and conformational planarization.Under this background,we introduced fluorene unit into synthons,designed and synthesized methoxy synthon(MCs),and studied the self-assembly behavior and photophysical properties of different aggregation states.On this basis,we used synthons through Friedel-Crafts reaction to further assemble drawing hands grids(DHGs),explored the influence of substituent types on the stereoselectivity of drawing hands grids,and changed the reaction time,reaction concentration,the type of acid catalyst and solvent optimize the reaction conditions of the methoxy drawing hands grids.Compared with synthons,the drawing hands grids has greater steric hindrance and a more stable macrocyclic structure frame.Therefore,we used the synthons and drawing hands grids with different substituents as models to further compare their stability.First,the methoxy synthon was designed and synthesized,and its self-assembly behavior and photophysical properties of different aggregation states were studied.We synthesized the methoxy synthon(2-(9-ethyl-9H-carbazol-2-yl)-9-(4-methoxyphenyl)-9H-9-alcohol)through the Grignard reaction and the Suzuki coupling reaction.X-ray single crystal diffraction,NMR and MADLI-TOF-MS were used to confirm the structure of the methoxy synthon.Finally,the self-assembly behavior and photophysical properties of different aggregation states were explored.The results show that the methoxy synthon can be grown under different solvent conditions to obtain two single crystals of MC1 and MC2,and both of them are slip-type stacking.However,compared with MC2,the stacking of MC1 is tighter and its supramolecular forces are distributed on the fluorene and carbazole groups,while the carbazole group of MC2 has no supramolecular force.Subsequently,band-like and flaky crystallites were obtained by the method of reprecipitation.The X-ray powder diffraction test results of the microcrystals correspond well with the standard spectrum peaks obtained by the single crystal software simulation,and when the two microcrystals of MC1 and MC2 are annealed at 100^oC,it is found that:the crystal form of MC2 will change to MC1 after annealing for 2 h.The quantum efficiency of the dilute solution,thin film and crystalline state of the methoxy synthon was tested,and the results showed:the PLQY values of MCs solution,MCs molecular film,MC1 and MC2 were 36.4%,5.3%,9.6%and 19.3%,respectively.In summary,methoxy synthon have polymorphic properties and can be transformed under heating conditions;compared with amorphous films,methoxy synthon have the property of crystal-induced luminescence enhancement.Then,explore the effect of substituent types on the stereoselectivity of the drawing hands grids,and optimize the reaction conditions of the methoxy drawing hands grids by changing the reaction time,reaction concentration,acid catalyst and solvent type.The study found that when the substituent is a methoxy group,multiple drawing hands grids are formed,and the main product are binary cis-trans drawing hands grids,the yield can reach 89%.And it has high stereoselectivity,among which the yield rate of binary cis drawing hands grids is 69.7%;When the substituent is a methyl group,only the cis-multiple drawing hands grids are generated.Although it has high stereoselectivity,the yield is low,and the yield of the binary drawing hands grids is 20%;When the substituent is bromine,the reaction generates multiple cis-trans drawing hands grids,and the stereoselectivity is low.The reaction conditions of the methoxy drawing hands grids were optimized,and the results showed that when the acid catalyst is BF₃OEt₂(10 eq),the solvent is DCM(0.005M),and the temperature is 20^oC,the binary drawing hands grids have the highest yield.In addition,the yield of drawing hands grids(meso-CH₃O-DHGs)will decrease with the extension of reaction time,and the yield of binary cis drawing hands grids(rac-CH₃O-DHGs)will increase.At 1 h,the characteristic peaks of the binary trans drawing hands grids on the hydrogen spectrum of the reaction solution basically disappeared.In summary,compared with the substituents of methyl and bromine,when the substituent is methoxy,the stereoselectivity and yield of the ring closure reaction are higher,and it can be controlled by changing the reaction time.Its stereoselectivity,that is,extending the reaction time,the binary trans drawing hands grids(kinetic product)will be transformed into the binary cis drawing hands grids(thermodynamic product).Finally,on the basis of the work of the first two chapters,the stability of synthons and drawing hands grids with different substituents is further studied.The results of comparing the stability of methoxy synthon in different aggregation states show that,the emission of the molecular film will appear as a green band with the prolongation of illumination(ultraviolet light),and the green index will increase with the prolongation of the illumination time.Also,the molecular solution will generate unknown substances after 4 hours of illumination;The microcrystalline film spectra of MC1 and MC2 have no green band under light conditions,and the spectra of methoxy synthon in different aggregation states have no obvious green band under heating and humid conditions.The stability of the methyl synthon molecular film was studied,and it was found that the molecular film was unstable under the irradiation of light,and the green band was generated.The stability of the methoxy binary cis-trans drawing hands grids was studied,and it was found that the molecular film did not have obvious green band generation under the conditions of light,heating and humidity,but the TLC dot plate and MALDI-TOF-MS results show that the molecular films of methoxy cis-trans drawing hands grids are unstable under light conditions and unknown substances are formed.The stability of the methyl binary cis drawing hands grids was studied,and it was found that its molecular film generates tthe green band under the irradiation of light.In summary,the luminescence and stability of the methoxy synthon in the crystalline state are enhanced compared with amorphous film.The increase in steric hindrance through ring closure does not increase the stability of the drawing hands grids.However,compared with the methoxy cis-trans drawing hands grids,the stability of the molecular film under light irradiation is poor.