Experimental And Theoretical Study Of Fluorescence Enhancement Based On Restriction Of Excited-state Twisted Intramolecular Charge Transfer Process

Excited-state twisted intramolecular charge transfer(TICT)process is one of the front issues in the field of atomic and molecular physics.The molecules with TICT property have exhibited potential application value in fluorescence probes,bioimaging and organic light-emitting diodes.However,some molecules with TICT property are generally emit weak fluorescence due to the nonradiative transition of TICT state dominates in deactivation process.Thus,restraining such TICT process is the key to enhance luminescent property of molecules with TICT process.In this thesis,the effect of external condition on TICT process and fluorescence intensity were investigated by femtosecond transient absorption spectroscopy and time-dependent density functional theory(TDDFT)method.The detailed research contents as follows:(1)Femtosecond transient absorption spectroscopy was employed to investigate solvent viscosity effects on TICT process and fluorescence intensity for Thioflavin T(Th T).By analyzing global fitting results of Th T in different viscosity solvent and optimized geometric structure of Th T by TDDFT method,it is found that TICT process of Th T intramolecular C-C bond twist between benzothiazole and aniline group is suppressed with increasing solvent viscosity.Thus,nonradiative deactivation process of Th T in high viscosity solvent is hindered that induces fluorescence intensity and fluorescence quantum yield increase.Moreover,the measured rotation corresponding time of Th T in low viscosity solvent fits well with the calculated result by Stokes-Einstein-Debye(SED)equation.The fraction SED equation is appropriate for Th T in relatively high viscosity solvents.(2)DFT/TDDFT method was employed to investigate the effect of intermolecular hydrogen bond interaction between solute and solvent on excited-state TICT process and fluorescence intensity of Coumarin-1(C1).Base on optimized geometric structures,it is found that compared to C1 in acetonitrile(ACN)solvent,TICT process exists in C1-methanol(Me OH)complex because of intermolecular hydrogen bond interaction between C1 and Me OH.The analysis of noncovalent interaction and infrared spectra,we demonstrated that the strength of intermolecular hydrogen bond between C1 and Me OH increases in the excited-state.The occurrence of intramolecular charge transfer process for both C1 in ACN solvent and C1-Me OH complex was demonstrated.Frontier molecular orbitals confirmed that the low charge couple degree of C1-Me OH complex in TICT state induces TICT state nonradiative transition,the fluorescence intensity of C1 in Me OH solvent decreases compared with C1 in ACN solvent.(3)Femtosecond transient absorption spectroscopy combined with high-pressure technology were carried out to study pressure effects on TICT process and aggregation-induced emission(AIE)intensity of Tris[4-(diethylamino)phenyl]amine(TDAPA).The measured fluorescence spectra revealed that the AIE intensity of TDAPA in THF solvent under high pressure significantly increased compared to the AIE intensity of TDAPA in THF/water mixtures.By analyzing high-pressure Raman spectra and high-pressure femtosecond transient absorption spectra have successfully revealed the physical mechanism of AIE intensity increases phenomenon for TDAPA under pressure.The stretching vibration of C-N bond and TICT process for TDAPA are effectively restrained under pressure that induces AIE intensity of TDAPA increases significantly under pressure.(4)DFT/TDDFT method was employed to study how excited-state intramolecular proton transfer(ESIPT)process,TICT process and fluorescence intensity for benzimidazole derivative(BIDOP)are influenced by solvent polarity.The optimized geometric structure,noncovalent interaction and infrared vibration spectra revealed that the intramolecular hydrogen bond strength increases in excited-state for BIDOP.By comparing the hydrogen bond strength of BIDOP in cyclohexane(CHX)and THF solvent,it is found that excited-state intramolecular hydrogen bond strength of BIDOP decreases with increases solvent polarity.Analyzing potential energy surface results demonstrated that ESIPT process exists in CHX solvent,both ESIPT and TICT process exist in relatively high polarity THF solvent for BIDOP in excited-state.The low polarity solvent can effectively inhibit the occurrence of TICT process for BIDOP that enhances fluorescence intensity of BIDOP.