Novel Mechanism Study On Aggregation-Induced Emission Phenomenon

In recent years,the importance of aggregation-induced emission（AIE）in basic research and practical application has attracted wide attention from many researchers in academia and industry.Understanding the internal mechanism of AIE is the key to design new structure of AIE materials and developing new uses of AIE molecules.Because of the diversity of organic molecules and the complexity of the luminescence process,the luminescence mechanism has a strong systemic dependence on the material system.Of the many explanations,the restriction of intramolecular motion（RIM）and the restriction of non-radiative decay path are currently applicable to most AIE systems.However,there are still some unexplained AIE phenomena.The fundamental reason is that the existing mechanism is based on the premise that the emitting state is transition dipole allowed“bright state”.Therefore,when the emitting state is a transition dipole forbidden“dark state”,the above mechanisms are not reasonable.Based on this,this paper studied the transformation between dark and bright excited states of molecules in different environments and proposed some new mechanisms of AIE systems,such as the vibronic coupling induced emission from dark state,which provided a new perspective for comprehensive understanding of AIE phenomenon and a new way for structural design.In this dissertation,model molecules like derivatives of[1,2,4]Thiadiazole[4,3-a]pyridine（TZP）,Squaraines（SQ）and Phenazine（PZ）have been investigated based on excited-state decay rate formalism coupled with quantum mechanics/molecular mechanics（QM/MM）methods,to explore the nature of AIE systems which contains dark emission state.Firstly,the photo-physical process of derivatives of TZP named TZPP and TZPE in both solution and solid phase have been investigated.Theoretical calculation results show that the C=N double bond of TZPP is prone to isomerization over a small energy barrier,from local excited（LE）state to charge transfer（CT）state in solution.While in solid state,the isomerization process is prevented by the surrounding environmental molecules,and the exciton is stabilized in the bright LE state,which leads to enhanced emission in solid state.However,the ethyl group in TZPE molecule is not involved in molecular orbital conjugation,both emission of this derivative in solution state and solid state came from bright LE state.This new mechanism provides a reference for the design and synthesis of new AIE molecules.Secondly,theoretical calculation is used to study the crystallization-induced enhanced emission（CIEE）phenomenon of a squaraine derivative called SQ1.The calculation results show that the nature of SQ1 at S₁ state reverse from transition dipole forbidden（n+σ,π^*）to transition dipole allowed（π,π^*）when it come from solution to solid state.As we all know,the transition type（n+σ,π^*）is forbidden because of symmetry rules.This change mainly come from the structure change of carbon skeleton of squaraine and the carboxyl group.After doping CHCI₃,the resulting eutectic get stronger intramolecular interactions which can make the allowed transition（π,π^*）more stable and further improve thee fluorescence efficiency.This new mechanism can furtherly guide the synthesis of AIE-active molecules.And lastly,Herzberg-Teller effect has been taken into consideration in the calculation of a model symmetric system named DPh PZ,which is a classic derivative of PZ.Since the experimental spectra of both solution and solid phase of DPh PZ showed fluorescence emission,while the theoretical calculation results of DPh PZ at S₁under Franck-Condon approximation turned out to be dark state.It means that the contribution of the vibronic coupling cannot be ignored in the photo-physical process of this transition dipole forbidden system.And the calculation results showed that the spectra and radiative transition rates with HT approximation are highly consistent with the experimental data,and base on this the AEE phenomenon of DPh PZ can be understood by the restriction of its non-radiative process from solution to solid.This calculation method can be applied to other transition dipole forbidden systems,and provides reference for the research of functional solid-state luminescent molecules.