Investigation On The Ultrafast Dynamics Of Excited-state Intramolecular Charge Transfer Process In Dye Molecules

The excited-state intramolecular charge transfer(ICT)process has always been one of the hot issues in the field of atomic and molecular physics.It is very important to investigate the ultrafast dynamics of the ICT process for understanding the photophysical mechanism of the ICT process.It has important application value and development prospect in the organic optical materials,probe molecules,solar energy storage and other fields.In our thesis,femtosecond transient absorption spectroscopy and time-dependent density functional theory were carried out to study the excited-state ICT process of several dye molecules.The specific research contents are summarized as follows:(1)Density functional theory and time-dependent density functional theory(DFT/TDDFT)methods were employed to investigate the effects of intermolecular hydrogen bonding(H-bonding)on the ICT process of coumarin 340(C340)in tetrahydrofuran(THF)solvent.The plotted electrostatic potential and optimized geometric structures vividly indicated that the intermolecular H-bonding can be formed between the C340 and the THF solvent.The analysis of frontier molecular orbitals(MOs)and the differences in electronegativity confirmed the occurrence of ICT process in C340 and hydrogen-bonded complex C340-THF upon photoexcitation.Meanwhile,the intermolecular H-bonding in C340-THF was proved to be significantly strengthened in the excited state.We demonstrated that the ICT process in C340-THF can be facilitated effectively by the excited-state intermolecular H-bonding strengthening.(2)The ICT process of LD 490 were investigated in different hydrogen-bond donating(a)solvents using the femtosecond transient absorption spectra and DFT/TDDFT methods.Experimentally,we obtained two relax processes,which was attributed to the intramolecular vibrational relaxation(IVR)and the ICT process,respectively.It shows that with the increasing of solvent's ? capacity,the IVR process becomes slower,while ICT process becomes much faster.Combining with TDDFT calculations,we demonstrated that the enhancement of ? scale substantially strengthens the excited-state intermolecular H-bonding interactions and consequently facilitates the ICT process of LD 490.(3)The effect of pressure on the TICT process of coumarin 7(C7)in acetonitrile(ACN)solvent was studied by the means of in situ high-pressure fluorescence spectroscopy and transient absorption spectroscopy.The results show that C7 molecule emits a single fluorescence peak in the liquid ACN solvent.Combining with the theoretical calculation,we demonstrated that the single fluorescence comes from the ICT state of C7,while the new appeared peak is derived from an intermediate TICT state.When the ACN transformed to the solid phase,a new fluorescence peak appears.We proposed the crystallization of ACN solvent induced by pressure restricts the free rotation of aminoethyl in C7 molecule,and a new TICT state with fluorescent emission forms.We confirmed that high pressure can successfully open up the channel from “dark” TICT state to “visible” TICT state of C7.Furthermore,high pressure ultrafast spectroscopy revealed the TICT process can be effectively regulated by the pressureinduced liquid-solid phase transition of ACN solvent.(4)DFT/TDDFT methods were performed to investigate the ICT coupled with excited-state double proton transfer(ESDPT)process of 2-(1H-pyrazol-5-yl)pyridine(PP)in propanol(1-PrOH)solvent.Intermolecular hydrogen-bonded complex(PPP)was demonstrated to be formed by the analysis of molecular configuration and IR spectra.We proved that PPP complex can undergo ESDPT process in the excited-state.The results of MOs and transition density matrix reveal that the occurrence of ICT process in PPP provides a good driving force for the ESDPT process.