Ultrafast Time-resolved Spectroscopyfor The Photoinduced Electron Transfer Processes In Dye Molecular Systems

Photoinduced electron transfer (PIET) from electron donor to electron acceptor after photoexcitation is a crucial process in many photochemical, photophysical and photobiological reactions. The importance and complexity of PIET reactions have led many researchers to exploit the fundamental chemical and physical mechanisms of these processes in simplified model systems and complicated natural systems. The aim of these works is to reveal the PIET reaction processes so as to design photovoltaic systems for conversing solar energy into chemical potential. However, several key aspects of these processes remain to be intensively investigated. For example, the rate and yield of the photogenerated electrons and molecular cations at the interface of different material systems are significantly different, and the physical mechanisms of these phenomena are poorly understood. The molecular structure of dye and polarity of solvent play an important role in the typical photochemical reactions in liquid phase. Therefore, the excited-state dynamics and intermolecular hydrogen bonding interactions for different organic molecules (dyes) in nonreactive solvents, the PIET reaction dynamics of the molecules in reactive solvents, and the the PIET reaction dynamics and molecular structure dependances in dye-sensitized TiO2nanoparticles systems were extensively investigated by the ultrafast time-resolved spectroscopy techniques in this thesis. The results offer important insights into the design of the new photovoltaic materials, the optimization of device configurations, and the selection of solvents.First, using Coumarin343(C343) as a model molecule, we investigated the excited-state dynamics and solvent effects of C343in different nonreactive solvents (nonelectron donor solvents) using steady-state absorption, fluorescence, and time-resolved fluorescence spectroscopy. It was found that the absorption red shifts slightly with the solvent polarity (π*), and the fluorescence red shifts nearly linearly with increasing solvent polarity parameters f(ε,n). These results can be ascribed to the solvent effects on charge distribution that corresponds to the changes of the configuration on the excited state. The dipole moment (12.73D) of the lowest excited state was determined from solvatochromic measurements and quantum chemical calculation, and consistent results were obtained. The theoretical studies indicate that the significant change of the dipole moment on the lowest excited state compared to the ground state is attributed to the changes of the charge distribution and bond order, and the intramolecular charge transfer. The time-resolved fluorescence reveal that the fluorescence lifetimes increase nearly linearly with increasing solvent polarity parameters (ε-1)/(ε+2) from3.09ns in toluene to4.45ns in water. It was found that the fluorescence lifetimes for C343in hydrogen-bonding solvents are much longer than those in non-hydrogen-bonding solvents because the intermolecular hydrogen bonding interactions change the molecular to a more stable configuration on the excited state. It can therefore be concluded that the nonreactive solvents with stronger polarity are helpful to lengthen the fluorescence lifetime of C343.Second, the ultrafast photoinduced intermolecular electron transfer (PIIMET) reaction dynamics of Rhodmine6G (Rh6G+)(and Rhodamine101, Rh101+) in reactive solvents were investigated by using of femtosecond time-resolved multiplex transient grating (MTG) and transient absorption (TA) spectroscopies, as well as off-resonance Raman spectroscopy and theoretical calculations. The energy level schemes of the ultrafast PIIMET processes in Rh6G+/DEA and Rh101+/DEA systems were established, and the mechanisms of the PIET reaction dynamics were analyzed. The steady off-resonance Raman spectrum of Rh101+/DEA system found that the1594cm-1mode is significantly enhanced, and the1620cm-1mode undergoes a visible red-shift of26cm-1with respect to the same mode in uncomplexed Rh101+. This is because the C=C stretching vibrations are more sensitive to ET compared with the C-C stretching modes, and the ground state partial electron transfer produces the complex Rh101+/DEA and changes the vibrational structure of the C=C stretching vibrations. The physical mechanism and corresponding time constants of the PIIMET processes were obtained by using of the ultrafast time-resolved spectroscopy techniques. The ultrafast photoinduced intermolecular forward ET (FET) from DEA to the excited state radical cation Rh6G+*(Rh101+*) occurs on a time scale of FET=220~320fs (FET=420~560fs). The backward ET (BET) occurs in the inverted region with a time constant of BET=22.8~42.3ps (BET=46.2~51.4ps). The intramolecular vibrational relaxation (IVR) process in the charge transfer state (CTS) occurs with the time constant of IVR=1.1~6.9ps (IVR=2.8~5.4ps). However, the PIET process does not present in nonreactive solvent, and then the excited-state lifetime of the dye molecule (on the timescale of nanosecond) is much longer than that in reactive solvent (electron donor solvents).Finally, the characteristics of the photoinduced interfacial electron transfer (PIIFET) and the molecular structural effects were extensively studied in the5(6)-carboxyfluorescein (5(6)CFL), C343, and alizarin (Alz) sensitized TiO2nanoparticles systems by using of the absorption, fluorescence, Raman, and time-resolved fluorescence spectra. The absorption and fluorescence spectra red shift, and the C=C, C-O, and C=O stretching vibrations enhance in the dye-sensitized TiO2nanoparticles systems compared with the pure dye systems. The time constants of BET in the C343/TiO2and5(6)CFL/TiO2systems were determined by the time resolved fluorescence spectroscopy. BET take place on the time scale of BET=31ps for C343/TiO2system, and around BET1=37ps (stongly coupled interaction) and BET2=478ps (less stongly coupled interaction) for5(6)CF/TiO2system. Comparisons of the spectroscopic properties among5(6)CFL, C343and Alz sensitized TiO2nanoparticles, it is found that the rate constants of PIIFET depend on the chromophores of dye molecules and the types of binding between chromophores and TiO2nanoparticles, and the rigid binding structure increases the rate constants of ET.