Dynamic solvent effects on electron transfer rates in the inverted regime: Ultrafast studies on the betaines

Abstract

Transient pump–probe measurements have been made on betaine-30, the solvent polarity probe. Betaine-30 is composed of organic donor and acceptor groups, D+A−⇄kethνDA (excited state). The transient data has been analyzed to measure ket, the electron transfer (et) rate constant, in polar solvents for this inverted regime et. The observed rate constant ket,obs in various polar solvents is as much as a factor of 108 larger than is predicted by the standard, classical, implementation of the theory of Sumi and Marcus, using Kjaer and Ulstrup’s estimates for necessary parameters. In contrast, a quantum treatment of the vibrational modes by Jortner and Bixon, which includes the dynamic solvent effect, predicts that the rate should be approximately ket,JB≂1/〈τs〉. This is close to the experimental results (≂1012 s−1) in typical polar aprotic solvents. However, in highly viscous, slowly relaxing solvents, ket,JB≪ket,obs. This is taken as evidence that the efficient coupling modes for the et in these environments is not the ‘‘conventional solvent coordinate.’’ It is shown that a hybrid of the Jortner/Bixon and Sumi/Marcus theories can physically account for the results. The kinetic behavior of betaine-30 in slowly relaxing solvents is shown to be in the ‘‘diffusionless limit’’ of the hybrid, generalized Sumi–Marcus/Jortner–Bixon picture.

Two-Dimensional Reaction Free Energy Surfaces of Catalytic Reaction: Effects of Protein Conformational Dynamics on Enzyme Catalysis

“…This is the limit where the catalysis reaction is so much evolved (or optimized) that the protein conformational dynamics totally determines the enzyme reaction dynamics, which is similar to the solvent-controlled electron transfer discussed by Hynes, 46 Barbara, 50 and Zusman 51 et al, among many others. Numerical results for a specific enzyme case will be presented in the following section.…”

Ultrafast intermolecular electron transfer from orthomethoxyaniline to excited coumarin dyes

“…[18][19][20][21][22] ͑3͔͒. A number of intramolecular ET systems have also been discovered by Barbara and co-workers [35][36][37][38][39][40][41][42][43] where ET occurs faster than the diffusive solvation rates. Since the contemporary ET theories can not explain these results, a twodimensional ET ͑2D-ET͒ model [44][45][46] is generally invoked to account the ultrafast nature of these ET reactions, where a solvent and an intramolecular coordinate is considered in a noncoupled manner to assist the ET process.…”

The effect of excitation pulse carrier frequency on ultrafast charge recombination dynamics of excited donor–acceptor complexes

“…The time resolution of modern femtosecond laser spectroscopy allows the real-time investigation of the dynamics of electron transfer (ET) reactions, which are substantially faster than solvent relaxation [1][2][3][4][5][6][7][8][9][10][11][12]. The typical oscillations reflecting coherent wave-packet motion have been also observed in ET reactions dynamics [5,7,8].…”