Abstract
In this thesis, we used an ultrafast time-resolved fluorescence (TRFL) spectrometer implemented by optical Kerr gating (OKG) and density functional theory calculations to study the electron transfer dynamics in some substituted benzene-tetracyanoethylene (SBZ-TCNE) complexes (SBZ = Chlorobenzene, Fluorobenzene, Benzonitrile, p-Tolunitrile, p-Chlorobenzonitrile) in two solvents (CH2Cl2, CCl4) with different polarities. We used a femtosecond laser to excite the SBZ-TCNE complexes to the charge-transfer (CT) states, and obtained the information from their absorption, fluorescence and TRFL spectra. The analysis of the total fluorescence intensity function P(t), which described the temporal evolution of the population of the excited states and the transition dipole moments, revealed the relaxations of the complexes such as the charge recombination (CR). We found different decay behaviors of the complexes excited states in two solvents. The fastest components of the complexes are in the similar time scale (< 0.2 ps), which is assigned to CT2→CT1 transition, and the slowest one is identified as the CR. The CR time constants of PhCl-TCNE, PhF-TCNE, PhCN-TCNE in the CH2Cl2 are 45, 21, and 28 ps. The CR time constants of PhCl-TCNE, PhF-TCNE, PhCN-TCNE, p-MePhCN-TCNE and p-ClPhCN-TCNE are 580, 470, 140, 210 and 280 ps, respectively. We found that the CR rates of complexes in the polar solvent are faster than in the nonpolar solvent which is mostly due to solvation effects. Surprisingly, the relationship between CR time constants and -∆G0 does not completely obey the Marcus theory and exhibits a double-inversion behavior. We used the intersecting state model (ISM) to explain this unexpected behavior. This model proposed that ultraexothermic reactions accompany large changes in structures which can affect the reorganization energy, and reaction barrier, resulting in the observed double-inversion behavior.