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 measu rement of the charge-recombination kinetics which exhibits a distribut ion of rate constants, including an initial very fast component (tau a pproximate to 1 ps), a factor of > 1000 slower component, and intermed iate-rate processes. The nonexponential kinetics may be associated wit h 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 t he most likely source of the dramatically nonexponential kinetics is a n inner-sphere effect involving specific, configurationally dependent electronic interactions of the donor and acceptor. The reaction kineti cs particularly at early times, is not strongly sensitive to solvent p olarity or solvation dynamics, further supporting that mainly the dono r/acceptor electronic interactions are dominating the kinetics. The da ta also reveal features due to the local transitions on the transient D+ species in the ion pair and evidence of vibrationally unrelaxed abs orption due to the initially re-formed complex subsequent to charge re combination.