Theoretical Investigation On The Light-Emitting Process For Molecular Aggregate

Molecular fluorescence undergoes drastic change upon aggregation.In general,molecular aggregate quenches luminescence.However,recently,it was found a class of molecules possess exotic aggregation-induced emission(AIE)behavior,implying potential applications in solid-state lighting,bio-sensing and imaging.There have been different explanations in literature.Understanding of AIE mechanism is crucial for further development of this field.In this dissertation,based on excited-state decay rate formalism coupled with quantum mechanics/molecular mechanics(QM/MM)calculations,we investigated the aggregation effects on the optical emission property of the 1,1,2,3,4,5-hexaphenylsilole(HPS).The aggregation induced blue-shifted emission is explained to be suppressed Stokes shift.We proposed to use the means of resonance Raman spectroscopy(RRS)and isotope effect(IE)to verify the AIE mechanism of restricted intramolecular nonradiative relaxation.Firstly,we investigated the photophysical properties of HPS through QM/MM computations with the vibration correlation function approach for excited-state radiative and nonradiative decay rates.We find from gas phase to solid state,the coplanar conjugation effect of ring at the 5-position collaborates with the restricted rotation effect of ring at the 2-position,which accelerates the radiative decay process and blocks the non-radiative decay channels,thus makes concerted efforts to enhance the solid state luminescence quantum efficiency.The calculated solid-phase absorption and emission optical spectra agree with the experiment.Secondly,we carried out computational investigations for the excited-state electronic structures of a series of AIE-active molecules exhibiting unusual blue-shifted emission from solution to solid state.We find that the Stokes shift(or reorganization energy)in aggregate is much less than that in solution,as evidenced by the restricted structural relaxation,excited-state planarization and low-frequency twists freezing,giving rise to the remarkable blue-shifted emission.The theoretical calculations based on such molecular picture are consistent with the experimental facts and well rationalize the exotic aggregation-induced blue-shifted emission phenomena.Thirdly,we proposed to use RRS to ‘visualize' the aggregation effects on the excited-state nonradiative decay channels and verify the AIE mechanism based on theoretical simulations.The restricted non-radiative relaxation pathways,as a consequence of aggregation in the AIE luminogen(AIEgen),distinctive from the non-AIEgen,could leave a spectroscopic signature of remarkable blue-shifted and depressed low-frequency peaks on RRS,because RRS amplitude is proportional to the mode's relaxation energy times frequency.Finally,we performed IE calculations on the excited-state nonradiative decay rates.We show that under the restriction of intramolecular motion mechanism,the IE is pronouncedly different when contrasting AIEgens and non-AIEgens.Thus,IE can be used to verify the AIE mechanism.Upon complete deuteration,the abnormal positive effect is found for AIEgens in solution while only the normal negative effect is observed for AIEgens in aggregates and non-AIEgens in solution and aggregation.The experimental results also well confirm the isotopic “jump” behaviors from solution to solid in the AIEgen.Further partial deuteration schemes for the 6-ring AIE analogues show the positional dependence.