The Spectra Of Benzene Derivatives In The First Electronically Excited And Cationic States

Benzene derivatives have become one of most experimentally important model for studies of photophysical and photochemical properties,which play an crucial role in understanding photodissociation,proton transfer,photoisomerization,molecular recognition,photosynthetic,interstellar material,and so on.Therefore,spectroscopic study of benzene derivatives has always been an interesting subject in physical chemistry,atomic and molecular physics,astrophysics and biology.In the present thesis,we focus on four typical molecules of benzene derivatives: 3-fluorobenzonitrile,4-fluorobenzonitrile,4-isopropylphenol,3-methoxybenzonitrile.The resonance enhanced multiphoton ionization(REMPI)and mass analyzed threshold ionization spectroscopy(MATI)are applied to investigate the spectra of the four molecules in the first electronically excited S1 and cationic ground D0 states.On the one hand,we analyzed the vibrations of the excited and cationic states,the interaction between aromatic ring and substituted groups,and photo-induced structural changes.On the other hand,excitation and ionization energies and vibrational frequencies of these molecules can serve as the basis for further research of photophysical and photochemical processes.The major contents in this thesis consist of the following 3 parts:Firstly,we carried out the two-color resonant two photon ionization(2C-R2PI)and MATI spectroscopy to record the spectra of 3-fluorobenzonitrile and 4-fluorobenzonitrile in the S1 and D0 states.The excitation energies of two molecules are determined to be 35989 and 36616 cm-1,and the adiabatic ionization energies are 78873 and 78000 cm-1,respectively.The equilibrium geometries of two molecules in the S1 and D0 states are optimized by TD-B3 LYP and B3 LYP calculations,which indicated the expansion of aromatic ring in the S1?S0 electronic transition due to the exciation of ? electron,and the contraction of that in the ionization process resulting from the decrease of electronic density on the aromatic ring.For the transition energies of D0?S1,fluorine atom plays two different inductive effects: electron-withdrawing and electron-donating effects.Secondly,the spectra of 4-isopropylphenol(4IPP)in the S1 and D0 states are investigated.The ultraviolet-ultraviolet hole-burning(UV-UV HB)spectroscopy reveals the present of two rotamers of 4IPP,which are trans 4IPP and cis 4IPP.The excitation energies of S1?S0 transition are measured to be 35578 and 35593 cm-1,the ionization energies are 65331 and 65350 cm-1,respectively.In the Franck-Condon approximation,the vibrationally resolved electronic spectra of S1?S0 and D0?S1 transitions are simulated,and used to assign the vibronic modes.Most of active vibrations of two rotamers in the S1 and D0 states correspond to the in-plane ring bending as well as some active modes of isopropyl group.Comparing the excitation energy of 4IPP with that of phenol indicates that the hyperconjugation between isopropyl group and aromatic ring in the S1 state is stronger than in the S0 state.Thirdly,a technique named as three color UV-UV HB spectroscopy has been proposed.Two rotamers of 3-methoxybenzonitrile(3MBN)are distinctly found by the three color UV-UV HB spectroscopy.By comparing the energy difference between two rotamers in the S0 state,we can assign this two rotamers to trans 3MBN and cis 3MBN.The excitation energies of S1?S0 transition of both rotamers are measured to be 34191 and 34379 cm-1,the ionization energies are 70944 and 70952 cm-1,respectively,which are obtained by 2C-R2 PI and MATI spectroscopy.Based on the simulations of vibronic spectra of S1?S0 and D0?S1 transitions,the vibrations of two rotamers in the S1 and D0 states are assigned.Most of the observed vibrations belong to the totally symmetric a' modes,including the in-plane ring and methoxy group bending.Comparing the excitation and ionization energies of 3MBN with those of benzonitrile,anisole and meta-derivatives indicates that the transition energies are related to the properties of substituted groups.The innovations of this thesis:1.The precise adiabatic ionization energies and cationic spectra of 3-fluorobenzonitrile,4-fluorobenzonitrile,4-isopropylphenol,3-methoxybenzonitrile are measured by MATI spectroscopy for the first time.With the aid of the DFT calculations,we analyzed the geometrical changes of the four molecules on the S1 and D0 states.Based on the transitions energies of benzene derivatives,the interation between the substituted groups(F,C3H7,OCH3,CN)and aromatic ring and the effect of the groups on the excitation and ionization energies are discussed.2.The UV-UV HB spectroscopy reveals the present of two rotamers of 4IPP.We proposed a three color UV-UV HB spectroscopy,which is used to determine the numbers of rotamers of 3MBN.The vibronic transitions in the electronically excited spectrum from each rotamer are also distinguished.On the baisis of potential energy curve of 4IPP and 3MBN,the barrier of the conformational conversion is obtained.We demonstrated that the relative population between two rotamers of this two molecules does not change in the supersonic expansion process.3.In the Franck-Condon approximation,the vibronic spectra of 4IPP and 3MBN are simulated.The theoretical spectra are in good agreement with the experimental results,which realized the assignments of the vibrational bands in the experimental spectra.