ULTRAFAST ELECTRON-TRANSFER IN OPTICALLY PREPARED SIMPLE ION-PAIRS - THE MECHANISM AT HIGH DONOR CONCENTRATION

The ultrafast spectroscopy of the charge-transfer band of the benzene/Br atom electron donor-acceptor EDA complex and related complexes of various alkylated benzenes with Br atoms (arene/Br) have been investigated in several solvents at high donor concentrations. The excited state of the complex, a contact ion pair, is observed to decay (by charge recombination) with non-first-order kinetics on several time scales. Various aspects of the transient spectroscopy are investigated in detail. The previously described enhancement of the recombination rate at high donor concentration is observed to be general. Even donors with sterically bulky substituents show the rate enhancement at high concentration, precluding dimer cations as intermediates. The paper also reports observations of coherent vibrational motion in the mesitylene/Br complex ground state, initiated by impulsive excitation of the charge-transfer band. Finally, ab initio calculations of the computationally accessible complex benzene/Cl are reported, indicating that the equilibrium geometry has C-s symmetry with the Cl atom positioned over the C-C bond, i.e., a pi complex. A sigma complex of comparable energy is also observed. The various results are discussed in relation to the charge-recombination mechanism of the contact ion pair of the arene/Br complexes.