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 v arious alkylated benzenes with Br atoms (arene/Br) have been investiga ted in several solvents at high donor concentrations. The excited stat e of the complex, a contact ion pair, is observed to decay (by charge recombination) with non-first-order kinetics on several time scales. V arious aspects of the transient spectroscopy are investigated in detai l. The previously described enhancement of the recombination rate at h igh donor concentration is observed to be general. Even donors with st erically bulky substituents show the rate enhancement at high concentr ation, precluding dimer cations as intermediates. The paper also repor ts observations of coherent vibrational motion in the mesitylene/Br co mplex ground state, initiated by impulsive excitation of the charge-tr ansfer band. Finally, ab initio calculations of the computationally ac cessible complex benzene/Cl are reported, indicating that the equilibr ium 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 als o observed. The various results are discussed in relation to the charg e-recombination mechanism of the contact ion pair of the arene/Br comp lexes.