Observation of the Marcus Inverted Region for Bimolecular Photoinduced Electron-Transfer Reactions in Viscous Media

Abstract

The general observation of Marcus inverted region (MIR) for bimolecular electron-transfer (ET) reactions in different viscous media, e.g., micelles, reverse micelles, vesicles, ionic liquids, DNA scaffold, etc. has been doubted in some recent publications arguing limitations in Stern-Volmer (SV) analysis to account for the static and transient stages of quenching in these slow diffusing media. Thus, following a theoretical treatment based on a spherically symmetric diffusion equation coupled with conventional Marcus ET description, it has been suggested that the MIR observed in viscous media arises due to the inadequate consideration of different quenching regimes and also due to the differential excited-state lifetimes of the fluorophores used than a genuine one (J. Am. Chem. Soc. 2012, 134, 11396). However, the overall treatment in this study is severely compromised by setting the minimum solvent reorganization energy (λs) to ∼0.96 eV while fitting the experimental data, which unambiguously suggests that the inversion in ET rate will never appear in the exergonicity (-ΔG(0)) range of 0.16 to 0.71 eV, as is the case for the studied ET systems. Besides, the applicability of the conventional Marcus ET model (instead of Sumi-Marcus two-dimensional ET model) in such extremely viscous media with exceptionally slow solvent response is highly debatable and perhaps is the main cause of the failure in fitting the experimental data quite satisfactorily. In the present study involving ultrafast ET quenching for coumarin derivatives by dimethylaniline donor in viscous ionic liquid media, we demonstrate clear MIR for the intrinsic ET rates, directly obtained from the ultrafast decay components of 1-10 ps, a time scale in which diffusion of reactants is negligible and the ET rates are either faster than or, at the most, competitive with the solvent reorganization. The appearance of MIR at ΔG(0) ∼ -0.5 eV, significantly lower than expected from the λs value, further substantiate the nonapplicability of conventional ET description but certainly advocate for the applicability of the Sumi-Marcus two-dimensional ET model in such media. Moreover, no obvious correlation has experimentally been observed between the excited-state lifetimes of the coumarin derivatives and the ET rates for a large number of dyes used in the present study. On the basis of the present results and drawing inferences from reported literatures in viscous media, we conclude that not only is the appearance of MIR very genuine but also the mechanistic model necessary to account the observed facts for the bimolecular ET reactions in a viscous medium is the two-dimensional ET description, which deals with an extremely slow relaxing solvent coordinate and a fast relaxing intramolecular coordinate to describe the ET reactions.

Tuning of electron transfer reactions in pluronic–surfactant supramolecular assemblies

“…To have a quantitative estimate of this effect, the TR fluorescence quenching results were systematically correlated with the changing DMAN concentrations at different CTAC to pluronic molar ratios in the two mixed micellar systems studied. Since the quenching interaction in the close contact donor-acceptor pairs occurs with an unusually fast rate, 13,14,24,68 much faster than the time resolution of the present TCSPC setup (B30 ps), unlike in the SS fluorescence quenching, the reductions in the hti values observed in the present TR studies will not have any significant contribution from the ultrafast (or static) quenching components. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]68 With this assumption, the observed changes in the hti values as a function of DMAN concentrations in each of the studied micellar media (pure pluronic or CTAC-pluronic micelles) were correlated following the widely employed Stern-Volmer relation for TR quenching studies, 26,27,70,71 as given by the following equation.…”

“…[10][11][12][13][14][15][16][17][18][19][20]27 To be mentioned that in the literature there are extensive reports, suggesting and demonstrating experimentally the involvement of photoinduced ET (PET) from ground state amine donors to excited coumarin acceptors as the mechanism for the strong fluorescence quenching observed in the coumarin-amine systems. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]67,68,[76][77][78][79] Drawing an analogy, thus, we attribute the observed fluorescence quenching in the present C343-DMAN system is due to the PET process.…”

“…The versatility of the coumarin-amine systems in carrying out efficient ET reactions and thus to understand many inside of the mechanisms and dynamics of these reactions has been well documented in the literature. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]28,30,67,68 In the present study, we expect that in these mixed micelles the location of the anionic C343 will be gradually changed on changing the surfactant to pluronic ratios and accordingly there will be a modulation in the observed ET rate.…”

Halide Photoredox Chemistry

“…Focusing on photoinduced electron-transfer chemistry, many studies utilized neutral donor–acceptor pairs, ,, while others have probed the work term in cases where at least one or both of the reactants are charged. ,− In many cases, the work terms can be safely ignored as their magnitude is less than 2 kT (∼50 mV). ,, In other cases where the solvent dielectric constant is small and/or the charge, z , is high, ω should be considered. − This is often the case for halide redox chemistry in organic solvents.…”

“…352,354−356 In many cases, the work terms can be safely ignored as their magnitude is less than 2kT (∼50 mV). 354,357,358 In other cases where the solvent dielectric constant is small and/or the charge, z, is high, ω should be considered. 359−361 This is often the case for halide redox chemistry in organic solvents.…”