Ultrafast Electron Transfer Dynamics in Micellar Media Using Surfactant as the Intrinsic Electron Acceptor

Abstract

Ultrafast photoinduced intermolecular electron transfer (ET) dynamics involving 7-aminocoumarin derivatives as electron donor and pyridinium moiety of surfactant molecules in cetylpyridinium chloride (CPC) micelle as electron acceptor has been investigated to understand the role of separation and orientation of reactants on micellar ET reactions. Unlike in noninteracting micelles (like Triton-X-100, sodium dodecyl sulfate, dodecyltrimethylammonium bromide, etc.), where surfactant-separated donor-acceptor pairs are understood to give the ultrafast ET component with the shortest time constant in the range of approximately 4 ps, in CPC micelles with pyridinium moiety as the intrinsic acceptor the ultrafast ET component is found to be in the subpicosecond time scale (of around 240 fs). This time scale is very similar to the values reported in the cases of ultrafast ET reactions involving coumarin dyes in electron-donating solvents. The ultrafast ET times in CPC micelles are significantly faster than the diffusive solvation dynamics in the micellar media. Correlation of the observed ET rates in the present cases with the free-energy changes of the reactions shows the inverse-bell-shaped correlation, predicted by Marcus ET theory. Interestingly, the onset of the Marcus inversion appears at a relatively lower exergonicity, which is attributed to the nonequilibrium solvent configuration during the ultrafast ET reaction, as envisaged from two-dimensional ET (2DET) model. Along with the ultrafast ET component, there are also slower ET components in these systems, which are attributed to those close-contact donor-acceptor populations in the micelles that have relatively weaker electronic coupling due to improper orientation of the interacting donor-acceptor pairs. The present results suggest that, along with the shifting of Marcus inversion at lower exergonicity, the ET rates can also be maximized in a micellar media by using surfactant molecule as an intrinsic reactant.

Tuning of electron transfer reactions in pluronic–surfactant supramolecular assemblies

“…The observed results indicate that there is a strong quenching for the excited C343 dye by the DMAN quencher and the quenching in the present systems does not involve any excited state complex or exciplex formation. [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.…”

“…[10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][80][81][82][83][84] In micellar media, the reactant molecules are preferentially solubilized within a very small region of the microhetrogeneous systems. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] Therefore, even under the conditions where the bulk estimate for the quencher concentration apparently seems quite low, the actual localized concentration of the quencher can be significantly high such that a good fraction of the fluorophore molecules will already be in close proximity with the quenchers at the moment of their excitation. [10][11][12][13][14][15][16][17][18][19][20][21]…”

“…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.…”

“…[1][2][3][4][5][6][7] Due to such importance, ET reactions in various microheterogeneous assemblies have been investigated very extensively with the ultimate aim of achieving control over the dynamics, energetics and mechanism of the ET processes. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] In microheterogeneous media, the degree of hydration (i.e. micropolarity) not only determines the rate of the solvent relaxation process but also plays an important role in determining the microviscosity for the reaction environment.…”

“…While the solvent relaxation rate would directly determine the contribution of solvent reorganization energy towards the free energy of activation for the ET reaction in such media, the microviscosity and the reactant entanglement with surfactant chains in such systems will in effect control the diffusion of the reactants and also the effective separation among the reacting donor-acceptor (D-A) pairs. [12][13][14][25][26][27][28] Logically the above effects in relation to the ET reactions can be adjusted to a significant extent by applying suitable strategies that help in modulating the microenvironments of the micellar systems. Two important approaches in this regard can be: (i) addition of an electrolyte in the micellar solution and (ii) changing temperature or pressure of the solution.…”