Theoretical Investigation Of Photoinduced Electron Transfer In Organic Systems

In this dissertation, the electronic structure and the kinetic problem of the intermolecular and intramolecular photoinduced electron transfer (PET) in the organic systems have been investigated at the higher calculated level, including the calculations of excited-state geometry and property, the evaluations of the kinetic parameters, and the discussion of photo-isomerization and dual-fluorescence phenomena; moreover, the solvent effects on the molecular structure, excited-state energy, and the spectrum have been taken into consideration. The dissertation is divided into three parts: The first part tells the PET theories, including fundamentals in the first charpter and kinetic theories in the second chapter; in the second part, we have investigated some specific PET systems using the methods of singlet configuration interaction (CIS) and time-dependent density functional theory (TDDFT), including explicitly the solvent effects. This part includes the charpters from the third to sixth; the third part, namely the seventh charpter, briefly introduces the theory of heterogeneous PET. The first chapter mainly includes following three sections: (i) time-dependent perturbation theory and quantum transition as well as electron excitation. We deduce 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 stand for the character of electron excitation; (ii) theoretical methods calculated the excited states. We mainly introduce the theoretical base of CIS, CASSCF and TDDFT, and the advantage and limitation of these methods in the excited-state calculations; (iii) briefly introduce the solvent effects on spectrum of photoinduced charge transfer process, mainly discuss the development of non-equilibrium solvation theory, as well as the main methods to treat the non-equilibrium solvation. The second chapter tells the kinetic theory of PET. We briefly introduce the classical kinetic process of PET and the steady-state treatment for it. PET is divided into adiabatic and nonadiabatic reactions based on the electron couple. Classical Marcus model is applied for most adiabatic reactions and quantum mechanical for most nonadiabatic ones. This chapter primarily discusses the two models and the calculated methods of the main parameters dominating the PET rate constant. In the last part, we introduce dual-fluorescence phenomena and the Lippert's explanation, and discuss the main hypotheses of explaining the dual-fluorescence phenomena, which are applied extensively in experiment and theoretical calculations. Because molecular recognition plays essential roles in biological systems and in man-made molecular devices, in the third chapter, we choose a molecular recognition system of tetracyanoethylenen??biphenylene (TCNEn??BP, n=0,1,2) to investigate its geometrical alteration and trichromic change following the change of oxidation states of TCNE. We firstly search the optimal structures of the TCNEn??BP systems at the level of MP2/6-31G*. Then, the CT absorption of the system is studied by CIS /6-31++G** method. Solvation energies and dipole moments of the ground and excited states are estimated by self-consistent reaction field (SCRF) method, and the correction of non-equilibrium solvation energies for the vertical transitions is made with Aguilar non-equilibrium solvation model. These calculations show that the geometry and spectrum of TCNE2??BP are completely different from those of the neutral and anion, which enable us to generate the redox-mediated recognition systems that have sufficiently different geometries and optical spectra allowing the individual states to be addressed. In the fourth chapter, we investigate the PET process of the model system of vitamin E-duroquinone (MVE-D) by the TDDFT method. The main aim is to studythe mechanism of the initial stage of antioxidant reaction of vitamin E from the theoretical aspect. The excited-state analyses and electronic coupling matrix element calculations indicate the decay from the LE state S4 to the CT state S2 corresponds to the dynamic process following photoexcitation of the duroquinone moiety in MVE-D, i.e., the initial stage of antioxidant reaction of vitamin E. The calculations of kinetic parameters show this decay is a low-barrier process and located in the Marcus'inverted regime. Since (n, π*) type-S1 state of the isolated DQ is regarded as a model for the peroxyl radical, the theoretical results in this chapter further support the view that the ET from the vitamin-E to the peroxyl radical occurs in the initial stage of antioxidant reaction of vitamin E. Photo-isomerization is the main aspect of the conformational relaxation dynamics of the excited electronic states, and occurs in the most structurally flexible molecules. In the fifth chapter, we choose benzil, a structurally flexible molecule, as the investigated object. Its stable ground-state geometry is present in the cis-skewed conformation and relaxes to trans-planar structure in the S1 and T1 states. The two isomers have been optimized using the DFT method both in gas phase and in solution, and the solvent effects on the conformation is discussed. The absorption and emission spectra are calculated applying the TDDFT method, and the spectral characteristics, solvent effects and stocks shifts are discussed. The calculated results are in agreement with the experimental values. Since the phenomenon of dual-fluorescence was discovered by Lippert et al 40 years ago in compounds such as 4DMAB-CN, the discussion and argument about the mechanism of dual-fluorescence went on for many years. Theoretical and experimental chemists put forwards many hypotheses to explain this phenomenon. Among them, the TICT model is supported by a large amount of experimental data, and numerous theoretical studies also confirm its validity. In the sixth chapter, we investigate the intramolecular charge transfer and solvent effects of dimethylaminobenzophenone (DMABP) by the TDDFT method. The solvent effects are described within the PCM model. Within the twisting ICT model, we constructthe potential energy curves of the two possible twisting motions of the excited states for the isolated and solvated systems. Our results show that both twisting motions lead to a charge-transfer state having a twisted structure and a large dipole moment as minimum on the potential energy curve of the first excited state. In gas phase, a crossing of a low barrier occurs between the LE and CT states. But in the polar solvent, the profile of the potential energy curve changes significantly, and the crossing between the two excited states disappears, and the CT state becomes the first excited state even at the non-twisted conformation. Fluorescence spectrum analysis indicates a large stokes shift between the absorption and fluorescence maxima; Computed oscillator strengths indicate the fluorescence is very weak; Fluorescence spectroscopic consequences of twisting only the dimethylamino group are similar to that of twisting the dimethylaminophenyl group. Our calculations including solvent effects show that the two twisting motions are possible to occur in the relaxation dynamics of the excited states, but the twisting of only the dimethylamino group may take place easier due to the lower TICT minimum and rotation barrier. This prediction seems contrary to the experimental suggestion. Due to the applications of the heterogeneous PET in AgBr photography and dye-sensitized solar cells as well as the proposed field of molecular device, it has been extensively investigated. The most striking difference between a solid state electron acceptor and a molecular one is the number of electronic acceptor states, which results in an ET mechanism completely different from the traditional Marcus-Levich-Jortner-Gerischer description. In the last chapter, we introduce the adiabatic limit and non-adiabatic limit of the heterogeneous ET, as well as the theoretical studies on the heterogeneous ET so far.