Transient Exciplex Formation Electron Transfer Mechanism

Michael G Kuzmin,Elena V Dolotova,Irina V Soboleva

doi:10.1155/2011/813987

Michael G Kuzmin, Elena V Dolotova + Show 1 more

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https://doi.org/10.1155/2011/813987

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Abstract

Transient exciplex formation mechanism of excited-state electron transfer reactions is analyzed in terms of experimental data on thermodynamics and kinetics of exciplex formation and decay. Experimental profiles of free energy, enthalpy, and entropy for transient exciplex formation and decay are considered for several electron transfer reactions in various solvents. Strong electronic coupling in contact pairs of reactants causes substantial decrease of activation energy relative to that for conventional long-range ET mechanism, especially for endergonic reactions, and provides the possibility for medium reorganization concatenated to gradual charge shift in contrast to conventional preliminary medium and reactants reorganization. Experimental criteria for transient exciplex formation (concatenated) mechanism of excited-state electron transfer are considered. Available experimental data show that this mechanism dominates for endergonic ET reactions and provides a natural explanation for a lot of known paradoxes of ET reactions.

Highlights

IntroductionElectron transfer (ET) reactions are known to proceed at different distances between reactant molecules in loose reactant pairs (long range ET or outer sphere reaction) as well as in tight pairs (contact ET or inner sphere reaction) [1,2,3,4]
Electron transfer (ET) reactions are known to proceed at different distances between reactant molecules in loose reactant pairs as well as in tight pairs [1,2,3,4]
The goal of this paper is to demonstrate that many kinds of excited-state ET reactions follow the transient exciplex mechanism rather than onestep long-range mechanism

Summary

Introduction

Electron transfer (ET) reactions are known to proceed at different distances between reactant molecules in loose reactant pairs (long range ET or outer sphere reaction) as well as in tight pairs (contact ET or inner sphere reaction) [1,2,3,4]. The goal of this paper is to demonstrate that many kinds of excited-state ET reactions follow the transient exciplex (contact) mechanism rather than onestep long-range mechanism In these cases adiabatic ET occurs, strong electronic coupling promotes medium and reactants reorganization consistent with gradual charge shift, and single-step Marcus and radiationless transition theories fail. Provides considerable contribution into ΔGEx∗ because of small energy gap ΔGET∗ = 0±0.1 < VAD ≈ 0.2 eV We present experimental values for activation barriers of exciplex conversion into solvent-separated radical ion pairs (SSRIP) (ΔG‡CRIP → SSRIP) and its dissociation into free radical ions (ΔG‡SSRIP → FRI) for more exergonic (ΔGET∗ ≈ −0.5 eV) ET reaction of 1, 2, 4, 5-tetracyanobenzene and various methylbenzenes. One can see that for such reaction transient exciplex (contact) mechanism has significantly smaller activation energy relative to preliminary reorganization (long range) mechanism

Transient Exciplex Decay

Competition of Different ET Mechanisms

Main Features and Paradoxes of Excited-State ET Reactions

Conclusion