ULTRAFAST RELAXATION AND ELECTRON-TRANSFER IN OPTICALLY PREPARED SIMPLE ION-PAIRS

An extensive investigation of the ultrafast pump-probe spectroscopy of the charge-transfer band of the mesitylene/Br EDA complex and related complexes in various solvents and over a broad range of UV, visible, and near-IR probe wavelengths is described. The data allow for a measurement of the charge-recombination kinetics which exhibits a distribution of rate constants, including an initial very fast component (tau approximate to 1 ps), a factor of > 1000 slower component, and intermediate-rate processes. The nonexponential kinetics may be associated with a distribution of arene(+)/Br- acceptor geometries in the initially prepared state of the ion pair. The complex kinetics are discussed in terms of contemporary electron-transfer theory, which indicates that the most likely source of the dramatically nonexponential kinetics is an inner-sphere effect involving specific, configurationally dependent electronic interactions of the donor and acceptor. The reaction kinetics particularly at early times, is not strongly sensitive to solvent polarity or solvation dynamics, further supporting that mainly the donor/acceptor electronic interactions are dominating the kinetics. The data also reveal features due to the local transitions on the transient D+ species in the ion pair and evidence of vibrationally unrelaxed absorption due to the initially re-formed complex subsequent to charge recombination.