Theoretical Investigation Of Photoinduced Electron Transfer In Para-aminobenzophenone

This work includes two chapters: In the first chapter we introduce the fundamental theories of photoinduced electron transfer. Firstly some important photochemical principles and concepts are reviewed. Then we introduce the famous Fermi's golden rule from the time-dependent perturbation theory. Applying this rule, we discuss the light absorption and emission, and obtain the physical parameters of transition possibility, transition electronic dipole moment and oscillator strength et al., which strand for the character of electron excitation. For the calculation of electronic excited states, we mainly introduce three common computational methods (CASSCF, CIS and TDDFT), and TDDFT are used for the calculations in the second chapter.In the second part, we investigate the excited states of para-aminobenzophenone in both gas and solution by time-dependent density functional theory, and compare the calculated results with experimental spectra, and present qualitative explanation for the triplet yields and the photoreduction reactivity of p-ABP in different solvents.Singh et al. investigated the excited states of p-ABP by absorption spectra, fluorescence spectra and phosphorescence spectra respectively, and found that the largest absorption spectra showed obvious redshift as the polarity of the solvent increases, which tells that the S1 state has strong intramolecular charge transfer (ICT) character. The geometry of the ground-state molecule is optimized by B3LYP/6-31G*. Frequency analysis is performed to confirm the stability of the optimized geometry. The absorption spectra of p-ABP are calculated by means of TDDFT method in gas phase and in solution at B3LYP/6-311+G** level. Ground state geometry is used throughout all the calculations of the excited states. TDDFT results in gas show that S1 is produced by n→π? transition on the carbonyl group, while S2 is produced by charge transfer from the amino-substituted aromatic ring to the carbonyl acceptor group. In polar solvents, the energy level sequence changes. S1 is calculated to be charge transfer (CT) state, and S2 is calculated to be local excited (LE) state. It is found that the CT state undergoes a redshift and the LE state undergoes a blueshift as the polarity of the solvent increases.In this work we also investigate the relationship between the electronic configurations of the excited states and the photoreduction reactivity of p-ABP. In the past century hydrogen-abstraction reactions from photoexcited molecules were studied intensively by theoretical and experimental methods. The rate, efficiency and mechanism of this reaction have been shown to depend on the electronic configuration of the lowest triplet excited state to a great extent, and the electronic configuration of excited state have been shown to depend on the solvent environment to a great extent. Hence, we investigate the electronic configurations of the excited states in various solvents. With the theoretically calculated results and the El Sayed rule, we can qualitatively predict the triplet yields and explain some experimental phenomena. In addition, the results show that the lowest triplet states of p-ABP haveππ? configuration in almost all kinds of solvents. As a result, unlike most aromatic carbonyl compounds, p-ABP could hardly abstract hydrogen atom in almost all kinds of solvents. However, the energy gap between theππ? and nπ? triplet states in cyclohexane is smaller than that in polar solvents. Hence, the lowest triplet state in cyclohexane may have partial nπ? character and could weakly abstract hydrogen atom from hydrogen donor substrates.