Study On The Mechanism Of Organic Room Temperature Phosphorescence And Aggregation-Induced Emission Of Halogenated Benzophenone And Its Derivatives

At present,small organic light-emitting materials,as emerging light-emitting materials,have attracted the attention of many researchers.Compared with inorganic materials and metal complexes,pure organic small molecules have the advantages of simple synthesis,low toxicity and high modifiability.However,there are still many problems to be solved in the research of pure organic small molecule luminescence:(1)traditional fluorescent small molecules possess polycyclic rings,and these polycyclic structures contribute to the good planarity of organic small molecules,this benefits the luminescence of organic fluorescent molecules in solution.However,too good planarity and high-conjugated structures are unfavorable for solid-state fluorescence.In the solid state,strong inter-molecular interactions exist in these small molecules due to close molecular packing.It is easy to nduce energy transfer and electron transfer owing to the formation of excimer and exciplex.The proportion of excitation-state energy attenuation through non-radiative pathways will increase significantly,leading to fluorescence quenching.The unique phenomenon of aggregation-induced emission(AIE)effectively avoids the problem of aggregation-causing quenching of organic fluorescent molecules.Organic molecules with aggregation-induced emission can rotate or vibrate or even flip in solution state,but the motion of these groups in the aggregated state is restricted,so the AIE molecule can acquire good fluorescence emission when excited.The discovery of the AIE phenomenon has greatly promoted the application prospects of small organic molecules,so it is urgent to develop more functional AIE functional materials.(2)Room-temperature phosphorescence emission is more difficult to obtain than small organic molecule fluorescence emission.Analyzed from the principle of luminescence,singlet excitons will be first generated after being excited by external light or electricity.After the exciton energy is released in the form of fluorescence,the molecules return to the ground state.and only a small part of the singlet excitons will be converted to the triplet state This part of the exciton returns to the ground state and emits phosphorescence.The triplet exciton has a long lifetime,making the phosphorescence easy to quench when it is interfered by the exterior environment such as oxygen and water.Quantum chemical calculations tell us that in theory,only25%of the excitons will be converted into light energy via the fluorescence emission pathway when electrically excited.Therefore,it is very valuable to study that what means can effectively enhance singlet-triplet conversion so as to improve phosphorescence emission.The specific research content and conclusions are as follows:(1)A series of dihalogenated derivatives were prepared,mostly using lithiation reaction,and oxidizing secondary alcohol to carbonyl under the action of pyridinium dichromate,some target products are required to make appropriate adjustments to the reaction conditions according to the of the reaction extent.The crystals of three symmetrically substituted products were successfully prepared by interfacial crystallization,and the luminescence properties of these three target products in different states(as-synthesized powder,crystal,after grinding,and thin-film)were characterized,and the mechanism was discussed in detail.The crystals of the three substituted products have obvious luminescence difference under hand-held UV lamp irradiation.The para-substituted products emit blue light(425 nm,450 nm,484 nm),and both the meta-position(530 nm)and ortho-position products(490 nm)emit green fluorescence.Immediately after grinding the three crystals,as the degree of grinding deepens,the phosphorescence intensity tends to decrease.XRD characterization also indicated that the grinding process destroyed the crystal structure,indicating that all the three compounds have the property of crystallization-induced phosphorescence.At the same time,we conducted thermal stability tests on the three crystal products.The initial weight loss temperature of the three crystals was 99°C,indicating that the three crystals were relatively stable at room temperature under normal pressure.(2)In order to further explore the luminescence mechanism and the effect of different bromination sites on the difference in phosphorescence,we selected meta-molecules and para-molecules for crystal structure analysis.Firstly,considering the effect of heavy atom interaction on the phosphorescence properties,we compared the heavy atom interaction between the bromine atom and the carbonyl oxygen between the two molecules,and found that the heavy atom interaction between the two was similar(5.7(?)-5.8(?))The effect of heavy atoms is not the main reason causing the phosphorescence between difference.At the same time,we observed that the two have different unit cell conformations,resulting in different intermolecular effect within the crystal,and the?-?effect and?-?effect near the benzene rings of the meta-molecule is stronger,and this may be the cause of the red-shift of meta-molecule phosphorescence.In order to seek theoretical support,combined with the crystal structure data,we performed density-functional theory on the three molecules.The theoretical calculations show that the energy levels of meta-molecules are discrete more,and the HOMO and LUMO energy levels are also in accordance with electrochemical experiments.These data indicate that the intramolecular charge transfer process(ICT)is stronger,which also causes the wavelength of the meta-molecule to be red-shifted.(3)Using the Mc Murry coupling reaction,a small molecule AIE fluorescent probe TPE-2BC of double crown ether modified tetraphenylethylene(TPE)was prepared,and its responsiveness to various metal ions was explored.The maximum absorption peak wavelength of TPE-2BC is 340 nm.The fluorescence spectrum of the target molecule is tested under an excitation wavelength of 340 nm.The results show that TPE-2BC has obvious AIE properties,indicating that the modification of crown ether has not affected TPE's AIE properties.We conducted concentration gradient tests on different metal ions for solution systems with different water contents.The final result shows that in a solvent system with a H₂O/THF ratio of 9:1,the fluorescence tends to decrease as the Ba²⁺concentration increases.We controlled other conditions and tested the responsiveness of TPE-2BC to different metal ions,and the results showed that it had specific response to Ba²⁺only.The thermal decomposition temperature of 200°C also indicates that it has good thermal stability.