Ultrafast intermolecular electron transfer in coumarin-hydrazine system

Ultrafast intermolecular electron transfer (ET) from electron-donating solvents to excited coumarin dyes has been investigated by the femtosecond fluorescence up-conversion technique. In this work we have used hydrazine derivatives as an electron-donating solvent to investigate the effects of very low oxidation potentials of electron-donating solvents on the ET dynamics. The ET rate with the aromatic hydrazines is much faster (sub-picoseconds) than that with the aliphatic hydrazine (a few tens of picoseconds), although the oxidation potential of the aliphatic hydrazine is lower than that of the aromatic hydrazine. However, the coumarin-hydrazine systems used in this study are not in the so-called Marcus inverted region. The results are discussed in light of the theoretical prediction from the two-dimensional ET model. We find that the electronic matrix element should be largely different between the aromatic and aliphatic hydrazine cases, which may result from the different molecular orbitals of these molecules. (C) 1998 Published by Elsevier Science B.V. All rights reserved.