Bimolecular Electron Transfer in Ionic Liquids: Are Reaction Rates Anomalously High?

Abstract

Steady-state and picosecond time-resolved emission spectroscopy are used to monitor the bimolecular electron transfer reaction between the electron acceptor 9,10-dicyanoanthracene in its S(1) state and the donor N,N-dimethylaniline in a variety of ionic liquids and several conventional solvents. Detailed study of this quenching reaction was undertaken in order to better understand why rates reported for similar diffusion-limited reactions in ionic liquids sometimes appear much higher than expected given the viscous nature of these liquids. Consistent with previous studies, Stern-Volmer analyses of steady-state and lifetime data provide effective quenching rate constants k(q), which are often 10-100-fold larger than simple predictions for diffusion-limited rate constants k(D) in ionic liquids. Similar departures from k(D) are also observed in conventional organic solvents having comparably high viscosities, indicating that this behavior is not unique to ionic liquids. A more complete analysis of the quenching data using a model combining approximate solution of the spherically symmetric diffusion equation with a Marcus-type description of electron transfer reveals the reasons for frequent observation of k(q) ≫ k(D). The primary cause is that the high viscosities typical of ionic liquids emphasize the transient component of diffusion-limited reactions, which renders the interpretation of rate constants derived from Stern-Volmer analyses ambiguous. Using a more appropriate description of the quenching process enables satisfactory fits of data in both ionic liquid and conventional solvents using a single set of physically reasonable electron transfer parameters. Doing so requires diffusion coefficients in ionic liquids to exceed hydrodynamic predictions by significant factors, typically in the range of 3-10. Direct, NMR measurements of solute diffusion confirm this enhanced diffusion in ionic liquids.

Fluorescence quenching of 7-amino-4-methylcoumarin by different TEMPO derivatives

“…On the basis of these results we can state that it is highly probable that the bimolecular process is diffusion-limited, caused by the increase of rate constants of non-radiative transitions, such as an internal conversion or an intersystem crossing. Time-resolved fluorescence measurements report the fraction of excited molecules that are quenched prior to an emission as: f ðsÞ q ¼ 1 À s s 0 [31,32]. The value of f ðsÞ q for 7-amino-4-methylcoumarin and 0.008 M TEMPO-4-amino-4-carboxylic acid is 0.28 while for the same concentration of other quenchers studied it is in the range 0.19-0.20.…”

Study of Diffusion-Assisted Bimolecular Electron Transfer Reactions: CdSe/ZnS Core–Shell Quantum Dot Acts as an Efficient Electron Donor and Acceptor

“…9 In a very recent work, Maroncelli and his co-workers used a spherically symmetric diffusion equation within a framework of a Marcus-type description of ET reactions to study PET from DMA to a photo excited 9,10-dicyanoanthracene molecule in various ionic liquid solvents. 19 Their analysis, resembling to encounter theory, showed an apparently faster bimolecular ET rate within the high viscous solvents compared to the ET rate in low viscous solvents is due to the viscosity effect of the high viscous solvents. A high viscous solvent mostly appreciates the static regime only where ET rate is faster.…”

“…Conversely, low viscous solvents appreciate mostly the stationary regime where ET rate diminishes. 6,9,19 Study of PET in inorganic semiconductors is important for improving their wide spread applications in modern devices where ET is an integral part of it. Metal chalcogenide (e.g., CdSe, CdS, PbSe, PbS etc.)…”

Single-Molecule Electrochemical Gating in Ionic Liquids

“…79,80 For the purpose of our approximation, in eq 7 a is set at half the distance between the nitrogen atoms in the redox active moiety of the 6pTTF6 molecular bridge. This produces a λ solv value of ∼0.46 eV.…”