Abstract
Ultrafast dynamics of substituted benzene-TCNE complexes in two solvents (CH2Cl2, CCl4) was investigated in ultrafast time-resolved fluorescene (TRFL) spectrometer implemented by optical Kerr gating (OKG). We used femtosecond laser to excite the non-methyl substituted benzene-TCNE complexes to CT states, and the resulting TRFL spectra were measured. We used the total fluorescene (P(t)) to analyze the result which reveal complexes relaxations associated with charge recombination(CR). We found different decay behaviors in different solvents. The fastest component in the similar time scale (<0.2 ps) for three complexes (chlorobenzene(PhCl)-TCNE、fluorobenzene(PhF)-TCNE、benzonitrile(PhCN)-TCNE) is assigned to CT2→CT1 transition and the slowest component is ascribed to CR. The CR time constant in CH2Cl2 for PhCl-TCNE、PhF-TCNE、PhCN-TCNE are 45, 21 and29 ps, respectively. The CR time constant in CCl4 for PhCl-TCNE、PhF-TCNE、Tolunitrile-TCNE、PhCN-TCNE、4-chlorobenzonitrile-TCNE are 580, 470, 210, 140 and 280 ps, respectively. We concluded that no matter which solvents we used, CR time constants of methylbenzene-TCNE complexes are consisted with the behavior in the Marcus inverted region. However, the others violate the behavior. We use the intersecting state model (ISM) which accounts for structural relaxation of complex that the Marcus theory doesn’t consider to explain this unexpected behavior.