Electron Transfer in a Flexible, Tethered Donor−Acceptor Pair:  The Influence of Solute Conformation on Solvent-Dependent Free Energies

Abstract

ADMA (1-(9-anthryl)-3-(4-dimethylanilino)propane) undergoes charge transfer following excitation of the anthryl moiety and forms an exciplex. Two mechanisms of charge transfer have been identified in previous work, and the operative mechanism depends on the polarity of the solvent. These are referred to as the nonpolar and the polar mechanisms. In polar solvents the charge transfer is rapid and occurs in an extended conformation followed by folding to form the exciplex. In hydrocarbons the exciplex formation rate is much slower and requires the donor and acceptor moieties to attain the correct geometry prior to charge transfer. Prior to the present work, it has been assumed that the charge transfer mechanism in hydrocarbons is diffusion controlled. In this work we demonstrate that charge transfer in nonpolar solvents is an activated process. We measure the rate of charge transfer in ethers (solvents of modest polarity with ) 3-4.3) and compare these to charge transfer rates in alkanes having similar viscosity. Whereas alkane charge transfer rates are well correlated to a viscosity power law, the rates in ethers are accelerated. We calculate the solvent-dependent driving force for the reaction using two different models, and the results also allow us to calculate the reaction reorganization energies. These are then used to estimate the activation barrier for the reaction, which demonstrate that the reaction is not encounter controlled. Analysis of the results demonstrates that solvent stabilization of the product state accelerates the charge transfer rate, in accord with Marcus theory, for solvents with dielectric constants between 2 and ∼4. Solvents with dielectric constants between 4 and 5 exhibit additional acceleration of the charge transfer reaction due to solvent dependence of the distance at which charge transfer occurs. This reflects a transition between the nonpolar and polar mechanisms.

Dynamical Solvent Control of Electron Transfer in a Flexible, Tethered Donor−Acceptor Pair

“…We will now estimate values of the parameters in eq 1 for ADMA charge transfer in polar solvents. In a recent paper, we calculated solvent-dependent values of λ and Δ G ct ε , the free energy of electron transfer, for the ADMA charge-transfer reaction in the folded conformation as a function of solvent dielectric constant using continuum models. The values for Δ G † were determined from the Marcus equation, where λ 0 ε = λ v + λ s ε is the sum of the inner sphere (λ v ) and solvent (λ s ε ) contributions to the reorganization energy.…”

“…Estimates of the activation barriers to unfolding and charge transfer in nonpolar solvents suggest that the folding rate may be as much as a factor of 3 larger than the chargetransfer rate determined from the viscosity power law. 72 Second, in the polar mechanism, the charged LH state will also experience dielectric friction that will reduce the value of k 2 . These two phenomena have opposing effects on the difference between the folding rate and the charge-transfer rate determined from the viscosity power law.…”

“…First, it must be recognized that ADMA may undergo several folding−unfolding cycles before electron transfer occurs in nonpolar solvents, and therefore the viscosity power law only provides a lower limit on the folding rate constant. Estimates of the activation barriers to unfolding and charge transfer in nonpolar solvents suggest that the folding rate may be as much as a factor of 3 larger than the charge-transfer rate determined from the viscosity power law . Second, in the polar mechanism, the charged LH state will also experience dielectric friction that will reduce the value of k 2 .…”

“…These two phenomena have opposing effects on the difference between the folding rate and the charge-transfer rate determined from the viscosity power law. Thus the viscosity power law, may provide a reasonable approximation for k 2 . Values of k η based on eq 6 are tabulated in Table , with typical values in the range of 10 8 s -1 .…”

Ultrafast Dynamics of Nonequilibrium Electron Transfer in Photoinduced Redox Cycle: Solvent Mediation and Conformation Flexibility

“…These reactions were usually studied by changing one parameter while the other two parameters were fixed. For example, with the designed donor and acceptor, we can alter a series of distances ( r ) in a solvent or change different solvents (λ) with a fixed distance. − One extensively studied system is the “C-clamp” structure and particular solvent mediation, i.e., enhanced coupling J , was observed in the cleft of this C-shaped donor–spacer–acceptor configuration. − These dynamics usually follow a single exponential behavior with a defined ET rate dictated by the three parameters. However, when ET dynamics occur ultrafast on the similar time scales of solvent relaxation, the ET reactions are in nonequilibrium with local environments and the rates thus show significantly heterogeneous behaviors as predicted by theory − and observed in experiment. − This is particularly true for the recent observations in proteins. − When the linker between D and A is flexible and the distance varies on the similar time scales of ET, the ET dynamics must also be heterogeneous.…”