DEUTERIUM-ISOTOPE EFFECT ON ULTRAFAST INTERMOLECULAR ELECTRON-TRANSFER

Deuterium isotope effects on ultrafast intermolecular electron transfe r (ET) from electron-donating solvents such as aniline (AN) and N,N-di methylaniline (DMA) to a number of excited (SI) coumarin dyes have bee n investigated by a femtosecond fluorescence up-conversion technique. We carry out investigations using both normal and deuterated solvents and normal and deuterated coumarin dyes. With perdeuterated AN (AN-d(7 )) or amino-deuterated AN (AN-d(2)), we observe almost the same extent of reduction in the ET rate constants as in normal AN. In the case of DMA, we do not observe any isotope effect upon substituting all of th e N-methyl group hydrogens by deuterium (DMA-d(6)). Using deuterated c oumarins, where the 7-amino group hydrogens are substituted by deuteri um, we do not observe any change in the ET dynamics. The effect of iso topic substitution of AN on the ET dynamics has been discussed in term s of the several ET parameters. Cyclic voltammetric measurements indic ated that the isotopic substitution of the solvent AN causes a reducti on in the driving force for the ET processes. It is inferred that the differences in the driving force for the normal and the deuterated AN as the donor solvent mainly arise from the solvent structural effects, caused by the intermolecular hydrogen bonding in this solvent. The pr esent results also correlate well with the theoretical predictions of the two-dimensional ET model.