Theoretical Study On Photoinduced Intermolecular Electron Transfer Of Conjugate Systems

Photoinduced electron transfer (ET) is one of the most pivotal processes in photosynthesis, organic reactions and optoelectronic devices. In this work, we intended to study the mechanism of photoinduced intermolecular ET reaction, including the estimation of the solvation energy in charge transfer (CT) transitions,and investigation of the kinetics of photoinduced ET.The interaction energy between electron donor and acceptor was investigated using second order M011er-PIesset method, and the counterpoise method was adopted to consider the correction of basis set superposition error for the interaction energy. The electronically excited states of electron donor-acceptor (EDA) complexes and ion pair complexes were investigated by means of several popular quantum chemical methods, the configuration interaction singles (CIS), the complete active space self-consistent field (CASSCF) method, and the time-dependent density functional (TDDFT) calculation. Comparison of the CIS results with those by CASSCF and TDDFT shows that CIS method is a proper choice in calculating low-lying charge-balanced excited state and excited charge transfer (CT) states in large π-conjugated systems. Applying the natural bond orbital population analysis and molecular orbital analysis, we investigated the electron population of ground and excited states and the transition properties. Theoretical calculations indicated that the photoexcitation of charge-balanced EDA complexes may lead to CT absorptions, which arise from π-π* excitations. Some theoretical evidences were provided for the experimental observations so that no CT absorption was not observed in DCA-DUR complex. Our results revealed that CT state of EDA complex can be formed directly through the photoexcitation of EDA complex, whereas the CT state can be produced by the decay of locally excited (LE) state. The geometries of ion pair complexes were optimized using CIS method in this work.Invoking the continuum model, we investigated the solvent effect on CTabsorption, local excitation and CT emission in polar solvents. Equilibrium solvation energies of the ground and excited states of EDA complex were calculated by using the self-consistent reaction field (SCRF) method. Based on the treatment of non-equilibrium theory, solvation energy corrections for the vertical transitions were evaluated. The solvation shifts of CT absorption and CT emission in the polar solvent, compared with those in the gas phase, were estimated. Explicit hydrogen bonding effect of solvent molecules on the CT absorptions was considered in this work, using supermolecular model.According to the generalized Mulliken-Hush (GMH) model, electronic coupling matrix elements in the decay processes of LE states and those in the charge recombination (CR) processes of contact ion pair (CIP) complex were evaluated. Based on the Golden rule, discussions on the probabilities of different CR processes of CIP complex were made. The solvent effect on the CR processes of CIP complex was considered as well.