Experimental And Theoretical Investigation On Intramolecular Excited State Ultrafast Dynamics In Liquid Phase System

Intramolecular excited state ultrafast dynamics has been a hot issue in the field of atomic and molecular physics. As we all know, most of the natural environment and chemical phenomena of life activities are carried out in a liquid phase system. It is very important to research the complex molecule system in liquid phase for understanding the mechanism of basic photophysical, photochemical processes in nature. It is worth noting that many important photophysical and photochemical processes involving excited states of molecules, such as excited state proton transfer, charge transfer, etc. in biological molecules. In this thesis, we applied the femtosecond transient absorption spectroscopic technique combining by time-dependent density functional theory to investigate the intramolecular excited state ultrafast dynamics processes of several molecular systems in liquid phase. The main contents are summarized as follows:(1) Time-dependent density functional theory method was performed to investigate the excited state intramolecular proton tranfer(ESIPT) process of salicylaldehyde(SA). We demonstrated that the ESIPT process happens in the first electronic excited state(S1 state), which proved the speculation of experimental phenomena of SA molecule in cyclohexane(CHX) solvent performed by Stock. And we revealed that the special single fluorescence characteristic of the SA molecule is derived from the unique stable structure in the S1 state. Futhermore, the results presented here suggested that the electronegativity increasement of the carbonyl oxygen due to the intramolecular charge redistribution in the S1 state induces the ESIPT of SA.(2) We investigated the photophysical properties of two molecules, 1-hydroxypyrene-2-carbaldehyde(HP) and 1-methoxypyrene-2-carbaldehyde(MP), using ultrafast transient absorption spectroscopic technique and density functional theory/time-dependent density functional theory(DFT/TDDFT). HP and MP have similar geometric conformations but exhibit entirely different photophysical properties upon excitation into the S1 state. In contrast to traditional excited state intramolecular proton transfer(ESIPT) in the molecules that exhibit either single or dual fluorescence, HP has an unusual non-fluorescent property. Combined with the theoretical calculation, we demonstrated that the ultrafast ESIPT process occurs and is followed by an intersystem crossing(ISC) process which induces the non-fluorescent property of HP. In contrast to HP, MP undergoes only one process, which was attributed to interactions between solute and solvent.(3) We investigated the characterization of photophysical properties of coumarin 510(C510), especially the TICT, with direct use of experimental high pressure transient absorption and theoretical density functional theory/time-dependent density functional theory(DFT/TDDFT). We found that C510 undergoes a twisted intramolecular charge transfer(TICT) process in the S1 state. Unlike conventional rotation rate slowing down with the increase of viscosity coefficient, we observed obviously the opposite result with the increase of solvent viscosity by high pressure. Specifically, the pressure obviously accelerates the TICT process. Our results suggested that such abnormal acceleration of TICT is attributed to the reverse rotation angle. In addition, we found that the high pressure also reduces fluorescence lifetime which is due to the decreasement of energy gap between S0 and S1 state induced by pressure.