Abstract
We employed a broadband ultrafast time-resolved fluorescence (TRFL) spectrometer implemented by the optical Kerr gating (OKG) to study the charge transfer (CT) state dynamics of methyl-substituted benzene-tetracyanoethylene (MBZ-TCNE, MBZ = benzene, toluene and p-xylene) in two solvents (CH2Cl2, CCl4) of diffrernt polarities. The CT-state of the MBZ-TCNE complexes are reached via femtosecond laser excitation, and the observed TRFL spectra reveal CT-state relaxation dynamics from the initial CT-state to charge recombination (CR). We used a total fluorescence intensity function P(t) to describe the excited-state population and transition dipole moment evolution with time during CT-state relaxation. The three complexes studied in two solvents exhibit a very fast initial decay component of similar time scale (< 0.2 ps), which can be assigned to CT2→CT1 internal conversion. The CR time constants for p-XY-TCNE (λex = 383 nm), TOL-TCNE ( nm) and BZ-TCNE ( nm) are 0.6, 7 and 29 ps in CH2Cl2. The equilibrium CR time constants for p-XY-TCNE (λex = 383 nm), TOL-TCNE ( nm) and BZ-TCNE ( nm) are 280, 850 and 150 ps in CCl4. The CR rate constants of MBZ-TCNE in CH2Cl2 are in line with the expectation for the Marcus inverted region, but CR rates in CCl4 do not agree with Marcus inverted region. The unexpected reversed -G0 dependence of the equibrium CR rates in CCl4 can be explained by the intersecting state model (ISM), which takes structural relaxation of complexes into account. Finally, CR rates of the CT-state of these MBZ-TCNE complexes are much faster in polar solvents due to the strong polar solvation.