Ultrafast intermolecular electron transfer from orthomethoxyaniline to excited coumarin dyes

Ultrafast intermolecular electron transfer (ET) from orthomethoxyaniline (orthoanisidine, ANS) to a number of excited (S1) 4-trifluoromethyl-1,2-benzopyrones (coumarins) having differently substituted 7-amino group has been investigated by femtosecond fluorescence up-conversion technique. The ET dynamics in the present systems are nonsingle-exponential and occur faster than the diffusive solvation dynamics. The ET rates are largely dependent on the nature of the substituents at the 7-amino group of the coumarins. This dependence is well correlated with the free energy changes (ΔG0) for the ET reactions. The ET dynamics become slower on using deuterated ANS as the donor, where the amino group hydrogens of ANS are substituted by deuterium. The deuterium isotope effect, however, gradually reduces as the ET dynamics becomes faster. Conventional ET theories can not explain all the observations. The results are explained on the basis of the two-dimensional ET model, which considers the solvent coordinate and th...