Effect Of Intermolecular Hydrogen Bonding And Dispersion Interactions On Excited State Dynamics Of Oxygen-containing Aromatic Compounds

The excited state dynamics of molecules is a discipline that studies the microscopic mechanism of chemical reactions on the molecular scale and plays an important role in the fields of photoreaction,molecular biology,materials science and environmental science.Intermolecular hydrogen bonding and dispersion interactions can significantly affect the excited state dynamics of molecules,such as enhancing internal conversion and other non-radiation processes,facilitating charge transfer processes,quenched or enhanced fluorescence.The research on the effects of intermolecular hydrogen bonding and dispersion interactions on the excited state dynamics of molecules has been a hot spot in the field of photophysics and photochemistry.In this thesis,the effects of hydrogen bonding and dispersion forces on the ultrafast dynamic process of dibenzofuran and diphenyl ether respectively are investigated with combined use of femtosecond time-resolved transient absorption spectroscopy and quantum chemical calculations.The thesis can be summarized as the following two parts(1)The effect of hydrogen bonding on the ultrafast dynamics of dibenzofuran was studied.Dibenzofuran is excited to the S2 state by excitation at 267 nm,and the S2 state of dibenzofuran decays to the Si state after an internal conversion process of about 10 ps.Then the S1 state undergoes a vibration energy relaxation(VER)process with a time scale of tens of ps,and finally relaxes to the triplet state through intersystem crossing on the time scale of nanoseconds.In protic solvents methanol,ethanol and isobutanol,hydrogen bonds are formed in dibenzofuran molecules with solvents.For the solvent with larger Kamlet-Taft parameter ? the hydrogen bond is stronger.As hydrogen bonding connects the molecules to the solvent,the intermolecular energy transfer process is favored.As a result,the time constant of VER process is shortened with the increase of ?.In addition,in methanol,ethanol and isobutanol solvents,hydrogen-bonded complex is formed between dibenzofuran and solvents through hydrogen bonding,while the formation of a new intermediate state S*was observed in methanol and ethanol,indicating that the formation of hydrogen-bonded complexes is related to the intermediate state S*.The final experimental results prove that the hydrogen bonding has a significant effect on the excited state dynamics of dibenzofuran,which not only enhances the vibration energy relaxation process,but also provides an extra relaxation channel to the intermediate state S*(2)The effect of dispersion forces on the excited state dynamics of diphenyl ether was studied.In aprotic solvents cyclohexane and 1,4-dioxane,only dispersion interactions exist.Diphenyl ether molecule is excited to the S1 state upon excitation at 267nm,and then the vibration energy transfer process occurs with a time scale about 1 ps.Subsequently,the S1 state decays to the ground state by internal conversion with a lifetime about 50 ps or relaxes via fluorescence and intersystem crossing in a time scale of nanoseconds.The investigation indicates that the intersystem crossing process is exclusively affected by solvent polarity which is suppressed as the polarity increases In protic solvents methanol and t-butanol,the OH-? and OH-O hydrogen bonded complexes are formed in the interplay of dispersion interactions and hydrogen bonds.The structural preference for the hydrogen bonded complex is affected by intermolecular dispersion force.Compared with the dynamics after excitation above mentioned,an additional process from OH-O to OH-? geometry rearrangement occurs in the relaxation processes of the S1 state in protic solvents in the first 1 ps.Considering that only the OH-? structure is observed in the S1 state in methanol and the excited state dynamics is same as that in the aprotic solvent,the OH-? structure has little effect on the excited state dynamics of diphenyl ether.In contrast,OH-O structure still exists in the S1 state in t-butanol with a small amount,and the lifetime of internal conversion is significantly shorter than that in methanol,indicating that the OH-O structure facilitates internal conversion.These experimental results suggest that the OH-O structure is preferred for the hydrogen bonded complex in the protic solvent with large dispersion force and facilitates the internal conversion process.