Substituent effect and deuterium isotope effect of ultrafast intermolecular electron transfer: Coumarin in electron-donating solvent

Intermolecular electron transfer (ET), which occurs faster than solvation dynamics, has been investigated using the fluorescence up-conversion technique. The ultrafast ET processes have been observed from the electron-donating solvents to the excited coumarin dyes. In this work we have mainly focused our attention on the roles of the substitution of electron-donating solvent molecules in the ET dynamics. We have used aniline, N-monoalkylanilines, and N,N-dialkylanilines as electron-donating solvents and five 7-amino coumarins as acceptor molecules. For these systems the free energy gaps have been estimated from the cyclic voltammetry measurement and the steady-state absorption and fluorescence measurements. The experimental results indicate that the ET rate depends largely on the substituent groups of the solvent molecules. In N,N-dialkylanilines the ET rate gets smaller as the size of the alkyl substituent group becomes larger. For N-monoalkylanilines, however, the ET dynamics are not changed by the different alkyl substituent groups. In many donor-acceptor combinations we recognized that the ET rate is much faster than the solvation time. We simulated the results by the two-dimensional ET model with solvent and nuclear coordinates and found that the N-alkyl substituent effect on the ET rate appears to be mainly due to the changes in the electronic matrix element. We have also investigated the deuterium isotope effect on the ET dynamics using N-deuterated-N-monoalkyl anilines as donor solvents. For the fastest ET the isotope effect is hardly observable, whereas the effect is quite large (similar to 20%) for slower ET, The deuterium isotope effect seems to mainly come from the change of stabilization energies in intermolecular hydrogen-bonding interaction by deuteration. The extent of deuterium isotope effect on ET is similar for all N-monoalkylanilines used. This result indicates that the size of the alkyl groups does not affect much the hydrogen-bonding interaction.