Effect of microheterogeneity in room temperature ionic liquids on energy and electron transfer processes

Effect of microheterogeneity in room temperature ionic liquids on energy and electron transfer... •This paper presents the studies of triplet − triplet energy transfer (TET) and electron transfer reactions (ET) reactions involving long lived triplet states and radical anions, respectively, in conventional solvents, SDS micelle and RTIL solvents, namely, 1-alkyl-3-methyl-imidazoliumtetrafluoroborate ([CMIM][BF4], in which the chain length of the alkyl group is varied up to ten carbon atoms. Nanosecond laser flash photolysis and pulse radiolysis techniques have been used for these studies. Those diffusion controlled reactions occur in the stationary quenching regime. Strong dependence of the reaction rates on the non-ionic or ionic nature of the reactants elucidates the microscopic heterogeneity of the RTIL solvents, which becomes more significant as the alkyl chain length of the RTIL solvent becomes longer. Results of our MD simulations predict preferential organizations of the RTIL molecules, specifically with long alkyl chain length, to form nearly a micelle-like structure creating distinct regions with nonpolar and polar pockets. Reactant species is preferentially solubilized in a particular region depending on its polarity and its diffusional motion is dependent on the microviscosity of the region, in which it is solubilized.•In the cases, where both the reactants are neutral molecules, which prefer to stay inside the nonionic or nonpolar pockets, the diffusive motion of the reactants experiences much lower microscopic viscosity as compared to the bulk or macroscopic viscosity of the solution. Comparable values of TET and ET rate constants for these neutral donor − neutral acceptor pairs in the C10Mim and in SDS micelle suggest that the viscosity of the nonionic pocket of the C10Mim is nearly similar to that of the core region of SDS micelle, which has been reported to be about 24 cP.•However, if one of the two reactants is ionic and the other is nonionic, they reside at ionic and nonionic pockets of the RTIL solution, respectively, and the motion of the reactants occur through different regions of the RTIL solution. Therefore, the reactants are expected to experience the macroscopic viscosity of the solution and the reaction rates show a linear relationship with inverse of viscosity of the solution (Smoluchowski equation).•Thus this work truly establishes the microheterogeneous structure of the imidazolium cation based ionic liquids consisting of regions of different polarities as well as different micoviscosities. In addition, this work, for the first time, shows how the rate of the reactions may significantly be different for different kinds of reactants in RTIL because of its microheterogeneity and also the approximate value of the viscosity of the nonpolar region of RTIL could be estimated. Considering these aspects the paper merits its publication in JPCPB A: Chemistry. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Photochemistry and Photobiology A: Chemistry Elsevier

Effect of microheterogeneity in room temperature ionic liquids on energy and electron transfer processes

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Publisher
Elsevier
Copyright
Copyright © 2017 Elsevier B.V.
ISSN
1010-6030
eISSN
1873-2666
D.O.I.
10.1016/j.jphotochem.2017.12.014
Publisher site
See Article on Publisher Site

Abstract

•This paper presents the studies of triplet − triplet energy transfer (TET) and electron transfer reactions (ET) reactions involving long lived triplet states and radical anions, respectively, in conventional solvents, SDS micelle and RTIL solvents, namely, 1-alkyl-3-methyl-imidazoliumtetrafluoroborate ([CMIM][BF4], in which the chain length of the alkyl group is varied up to ten carbon atoms. Nanosecond laser flash photolysis and pulse radiolysis techniques have been used for these studies. Those diffusion controlled reactions occur in the stationary quenching regime. Strong dependence of the reaction rates on the non-ionic or ionic nature of the reactants elucidates the microscopic heterogeneity of the RTIL solvents, which becomes more significant as the alkyl chain length of the RTIL solvent becomes longer. Results of our MD simulations predict preferential organizations of the RTIL molecules, specifically with long alkyl chain length, to form nearly a micelle-like structure creating distinct regions with nonpolar and polar pockets. Reactant species is preferentially solubilized in a particular region depending on its polarity and its diffusional motion is dependent on the microviscosity of the region, in which it is solubilized.•In the cases, where both the reactants are neutral molecules, which prefer to stay inside the nonionic or nonpolar pockets, the diffusive motion of the reactants experiences much lower microscopic viscosity as compared to the bulk or macroscopic viscosity of the solution. Comparable values of TET and ET rate constants for these neutral donor − neutral acceptor pairs in the C10Mim and in SDS micelle suggest that the viscosity of the nonionic pocket of the C10Mim is nearly similar to that of the core region of SDS micelle, which has been reported to be about 24 cP.•However, if one of the two reactants is ionic and the other is nonionic, they reside at ionic and nonionic pockets of the RTIL solution, respectively, and the motion of the reactants occur through different regions of the RTIL solution. Therefore, the reactants are expected to experience the macroscopic viscosity of the solution and the reaction rates show a linear relationship with inverse of viscosity of the solution (Smoluchowski equation).•Thus this work truly establishes the microheterogeneous structure of the imidazolium cation based ionic liquids consisting of regions of different polarities as well as different micoviscosities. In addition, this work, for the first time, shows how the rate of the reactions may significantly be different for different kinds of reactants in RTIL because of its microheterogeneity and also the approximate value of the viscosity of the nonpolar region of RTIL could be estimated. Considering these aspects the paper merits its publication in JPCPB A: Chemistry.

Journal

Journal of Photochemistry and Photobiology A: ChemistryElsevier

Published: Apr 1, 2018

References

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