Deuterium Isotope Effect on Ultrafast Intermolecular Electron Transfer

Abstract

Deuterium isotope effects on ultrafast intermolecular electron transfer (ET) from electron-donating solvents such as aniline (AN) and N,N-dimethylaniline (DMA) to a number of excited (S 1 ) coumarin dyes have been 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 the N-methyl group hydrogens by deuterium (DMA-d 6 ). Using deuterated coumarins, where the 7-amino group hydrogens are substituted by deuterium, we do not observe any change in the ET dynamics. The effect of isotopic substitution of AN on the ET dynamics has been discussed in terms of the several ET parameters. Cyclic voltammetric measurements indicated that the isotopic substitution of the solvent AN causes a reduction 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 present results also correlate well with the theoretical predictions of the two-dimensional ET model.

Ultrafast Dynamics in Biological Systems and in Nano-Confined Environments

“…51 Yoshihara and co-workers first reported that in liquid DMA (N,N-dimethylaniline) PET exhibits a component faster than the solvation time. [52][53][54] Interestingly, even for ultrafast PET, they observed a bell-shaped dependence of PET rate on free-energy change (ÁG) and thus detected the so called Marcus inverted region. [52][53][54] Bagchi and Gayathri developed a non-Markovian model and correlated the highly non-exponential ET process with the highly non-exponential solvation dynamics.…”

“…[52][53][54] Interestingly, even for ultrafast PET, they observed a bell-shaped dependence of PET rate on free-energy change (ÁG) and thus detected the so called Marcus inverted region. [52][53][54] Bagchi and Gayathri developed a non-Markovian model and correlated the highly non-exponential ET process with the highly non-exponential solvation dynamics. 55 They noted that in most solvents, the solvation dynamics consist of an ultrafast sub-picosecond inertial component and a slower component in tens of picosecond.…”

“…They showed that 30-40% solvent relaxation is enough to bring about the ultrafast electron transfer observed by Yoshihara and co-workers. [52][53][54] Barbara and Olson proposed a modified model which includes a classical low frequency vibration, a classical solvent coordinate (X), and a high frequency quantum mode. 56 In a seminal paper, Sumi and Marcus introduced an irreversible stochastic model of ET with a wide sink term.…”

Photoinduced intermolecular electron transfer between Coumarin dyes and electron donating solvents in cetyltrimethylammonium bromide (CTAB) micelles: evidence for Marcus inverted region

“…In an ultrafast intermolecular electron transfer process the donor and acceptors are in contact to each other and the diffusion is not necessary here. ET in this case is much faster than the solvation dynamics [11][12][13]. On the other hand under diffusive condition in the homogeneous medium [26,27] the donor and acceptor are separated by solvent molecules and the diffusion processes control the ET rate.…”

“…Both theoretical and experimental investigations have been carried out on the dynamical aspect of the electron transfer (ET) [3][4][5][6][7][8][9][10][11][12][13]. Most of these investigations, so far have been carried out are either in neat solvent where the solvent acts as a donor or under diffusive condition where the solvent is non-interacting and reactants have to diffuse before ET takes place [11][12][13]. While ET processes in the homogeneous medium are well known, only a limited works have been performed in organized media such as micelles [14][15][16][17].…”