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Photo-Induced Bimolecular Electron Transfer in Ionic Liquids: Cationic Electron Donors Boning Wu, Min Liang, Nicole Zmich, Jasmine L. Hatcher, Sharon I LallRamnarine, James F. Wishart, Mark Maroncelli, and Edward W. Castner, Jr. J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b12542 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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The Journal of Physical Chemistry

Photo-Induced Bimolecular Electron Transfer in Ionic Liquids: Cationic Electron Donors Boning Wu,†,# Min Liang,†,@ Nicole Zmich,‡ Jasmine Hatcher,§,k Sharon I. Lall-Ramnarine,¶ James F. Wishart,‡ Mark Maroncelli,∗,⊥ and Edward W. Castner, Jr.∗,† †Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, New Jersey 08854, United States ‡Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973, United States ¶Department of Chemistry, Queensborough Community College, City University of New York, Bayside, New York 11364, United States §The Graduate Center of CUNY, 365 Fifth Ave., New York, New York, 10016, United States kHunter College, CUNY, 695 Park Avenue, New York, New York 10065, United States ⊥Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States #Present address: Department of Chemistry, Stanford University, Stanford, California 94305, United States @Present Address: Agios Pharmaceuticals, 88 Sidney St, Cambridge, Massachusetts 02139, United States E-mail: [email protected]; [email protected]

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Abstract Recently we reported a systematic study of photo-induced electron-transfer reactions in ionic liquid solvents using neutral and anionic electron donors and a series of cyano-substituted anthracene acceptors (J. Am. Chem. Soc. 2017, 139, 14568). Herein, we report complementary results for a cationic class of 1-alkyl-4-dimethylaminopyridinium (1-alkyl DMAP) electron donors. The reductive quenching of cyanosubstituted anthracene fluorophores by these cationic quenchers is studied in solutions of acetonitrile and the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Varying the length of the alkyl chain permits tuning of the quencher diffusivities in solution. The observed quenching kinetics are interpreted using a diffusionreaction analysis. Together with results from the prior study, these results show that the intrinsic electron-transfer rate constant does not depend on the quencher charge in this family of reactions.

Introduction Recent research into the mechanisms of electron-transfer reactions in ionic liquid solvents has focused on how the transport properties of the reactants and the solvation properties of the ionic liquids (ILs) determine the reaction kinetics. 1–5 The present work involves an extension of our recent investigations of the reductive quenching of a series of cyanoanthracene fluorophores by an array of neutral and anionic electron donors. 1 In that prior work, the measured differences in the electron donor diffusivities correlated with the observed quenching rates, which were interpreted based on an analysis using the Marcus electron transfer theory. 1,6 Applications of ionic liquids in energy areas 7–9 require a thorough understanding of the electron transfer kinetics in these ionic solvents. Therefore, several studies have reported either intramolecular 10–14 or intermolecular 2,3,5,15,16 electron transfer reactions using ionic liquids as solvents. Solvents influence bimolecular electron transfer reactions in several ways. 2


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They affect the diffusion coefficients, 1 solvation dynamics, 17 and the solvation free energies of the reactants. 18 Ionic liquids represent a unique class of solvents with a broad range of physical properties, and the equilibrium and dynamical aspects of solvation in these solvents are complex. 19–21,21,22 The local electric fields present in ILs, 23 the local structures caused by ionic ordering, 24–26 their high viscosities, 2,3 the intermediate range ordering due to nanodomain aggregation 27,28 and the hydrogen bonding 29,30 in ionic liquids all have significant influence on chemical processes. Thus, reaction dynamics in IL solvents are more complex than in conventional organic solvents, which typically lack free charges and have low to moderate viscosities. Diffusion of reactants frequently determines the rates and mechanisms of bimolecular reactions in solution. 31 In many low-viscosity solvents, the diffusion-limited electron transfer rate constant can be calculated with reasonable accuracy using the simple hydrodynamic theory based on the Smoluchowski and Stokes-Einstein equations: 3,16

kd =

8kB T 3η

(1)

where kB is Boltzmann’s constant, T is the absolute temperature, and η the shear viscosity. The observed electron transfer quenching rate constants in ionic liquids are often 10 to 100 fold larger than the predictions from Eq. 1, 15,16 due to the contribution to the electron transfer rate from the transient regime (often referred to as the ‘static quenching’ limit) as well as underestimation of the diffusivities of neutral species in ionic liquid solutions by the Stokes-Einstein equation. 1–5 In several previous studies, we systematically measured the rate constants of bimolecular photo-induced electron transfer reactions by means of fluorescence quenching experiments. 1,3,5 A series of photo-excited cyano-anthracene fluorophores served as electron acceptors while both neutral 1,3 and anionic 1,5 quenchers served as electron donors (Fig. 1). We compared these reactions in the neutral solvent CH3 CN and two ionic liquids of widely differ− ing viscosities (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, Im+ 2,1 / NTf2 ,

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− and trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide, P+ 14,6,6,6 / NTf2 ). One

of the main conclusions of these studies was that faster diffusion of neutral species in ionic liquids leads to faster quenching compared to anionic donors, but once the correct diffusivities of donors and acceptors are accounted for, there is no obvious distinction between the intrinsic electron transfer rates of anionic and neutral donors. Deviations from the predictions of the Marcus theory also lead us to suggest that local relative motions of fluorophores and quenchers limits the intrinsic electron transfer rate at the donor-acceptor contact distance. 1 The present study extends these prior investigations to a set of cationic quenchers based on the 1-alkyl-4-dimethylaminopyridinium (DMAP) cations shown in Fig. 1. DMAP-based ILs are a subset of pyridinium ILs that have significant potential for antibacterial, 32 catalysis 33 and energy applications. 34 There are reports on the use of this family of ILs for heat transfer, 34 electrochemical degradation 35 and anti-aging stabilizers. 36 Of interest here is the fact that DMAP cations can reductively quench the excited states of cyano-substituted anthracene fluorophores. The idea that these cations should be moderately strong reductants can be understood if we consider that 1-alkyl-DMAP cations are isoelectronic to 1-alkyl-4dimethylaminoanilines, which are a well known family of strong electron donors. 3,37 We can systematically vary the self-diffusion coefficients of these donors by varying the length of the DMAP alkyl chain, which enables us to further explore the effect of diffusion on electron transfer kinetics in ionic liquids. The photo-acceptors used in this work include 9,10-dicyanoanthracene (DCNA), 2,9,10tricyanoanthracene (TrCNA) and 2,6,9,10-tetracyanoanthracene (TCNA), shown in Fig. 1. Increasing the number of cyano groups on these fluorophores increases the driving force for electron transfer. 38 The fluorescence quenching of DCNA, TrCNA and TCNA is measured − in solutions of acetonitrile (CH3 CN) and in the ionic liquid Im+ 2,1 / NTf2 . Measurements in − the P+ 14,6,6,6 / NTf2 IL are not reported here because the reaction was too slow to enable

extraction of reliable rate constants from fluorescence quenching.

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Figure 1: Electron acceptors (fluorophores), electron donors (quenchers) and solvents used in this study. The cationic, anionic and neutral quenchers are shown in green, red and blue, respectively. The compounds in the dashed green box are used in this work, while others were discussed in Ref. 1.

The data collected here are first subjected to a Stern-Volmer analysis and then to more complete treatments in terms of diffusion-reaction models, which enables separation of the effects of varying diffusivities and intrinsic electron transfer rates on the reactions. 39–50 The present results, when combined with prior data on neutral and anionic quenchers, show that there are no special effects of quencher charge in these reactions.

Methods Materials DCNA was purchased from TCI-America and recrystallized before use from a mixture of pyridine and acetonitrile. 51 TrCNA and TCNA were gifts from Prof. Dr. Eric Vauthey of

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the University of Geneva. The DMAP quenchers were added in the form of ionic liquids − + by pairing with the NTf− 2 anion. The synthesis and purification of C4 DMAP / NTf2 , 52,53 C8 DMAP+ / NTf− and that for C10 DMAP+ / NTf− 2 were previously reported, 2 is described

in the Supporting Information. Each of the three DMAP ILs was colorless when used for the photolysis studies. Anhydrous CH3 CN was purchased from Sigma-Aldrich and stored under argon. Ultra− high purity (>99.5%) Im+ 2,1 / NTf2 was purchased from IoLiTec, checked for colored impu-

rities, and then dried under vacuum (2×10−2 mbar) at room temperature for 48 h before use. Steady-state fluorescence and time correlated single photon counting (TCSPC) lifetimes of the fluorophores in solutions were measured at different quencher concentrations using the same procedure described previously. 1 Maximum quencher concentrations were in the range 0.05 – 0.2 M. Steady state fluorescence spectra were recorded using a Spex Fluoromax-3 fluorometer, and the time-resolved fluorescence decays measured using the TCSPC facilities described previously. 13,54 The full time windows for acquisition of the TCSPC transients were set to 120 ns or 160 ns, and the temporal instrument response under these conditions was 0.15–0.2 ns FWHM, respectively. − The self-diffusion coefficients of the DMAP ionic liquids in CH3 CN and Im+ 2,1 / NTf2 were

measured by Pulse-Gradient Spin-Echo (PG-SE) NMR experiments using the DBPPSTE pulse sequence, 55 as described previously. 1,5 The oxidation potentials of the DMAP cations, C4 DMAP+ , C8 DMAP+ and C10 DMAP+ were measured in CH3 CN solutions using cyclic voltammetry. The details of the electrochemistry experiments are provided in the Supporting Information. All solutions were prepared inside an argon glovebox, where the water and oxygen levels were below 0.1 and 0.4 ppm, respectively. In the fluorescence experiments, the temperature was regulated to 298±0.2 K. Temperatures for the PG-SE NMR experiments were set to 298±0.1 K.

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Results and Discussion Oxidation Potentials of DMAP Quenchers − − + + The oxidation potentials of C4 DMAP+ / NTf− 2 , C8 DMAP / NTf2 and C10 DMAP / NTf2 in

CH3 CN were measured to be -1.86, -1.88 and -1.88 V vs. SCE, respectively (see Supporting Information, Section 4). The oxidation reactions occur on the DMAP cations, since the 5 NTf− 2 anions and CH3 CN solvent are not redox active in this potential range. The oxidation

potentials for these cations are much more negative compared to homologous neutral donors like N,N-dimethylaniline (∼-0.7 V vs. SCE), 3 which makes these cations weaker electron donors than the isoelectronic neutral molecules. However, their potentials are similar to those for some other weak anionic electron donors such as dicyanamide that we studied previously (∼-1.7 V vs. SCE). 1,5

Stern-Volmer Analysis Steady-state and time-resolved fluorescence quenching data were analyzed using a simple Stern-Volmer type analysis. The steady-state Stern-Volmer plots of all the fluorophore/ quencher/ solvent systems are shown in Fig. 2. For each fluorophore in both solvents, the curves for C4 DMAP+ , C8 DMAP+ and C10 DMAP+ are nearly coincident, implying almost identical quenching rates, despite the three DMAP cations having side chains of different lengths, and hence different diffusivities. The low oxidation potentials of these cationic quenchers lead to low driving forces for these reactions. For this reason, the quenching reactions are mostly limited by their intrinsic electron transfer rates, which are expected to be nearly identical for the three donors, rather than being limited by their diffusivities. For each DMAP quencher, the quenching rate increases with increasing fluorophore reduction potential. This last observation suggests that it is reasonable to assign the fluorescence quenching by DMAP cations to electron transfer.

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+

CH3CN +

-

+

-

-

Im2,1 /NTf2

C4DMAP / NTf2

8

C8DMAP / NTf2 +

3

-

C10DMAP / NTf2

6 4

2

2

TCNA

TCNA 1

0.00

0.01

0.02

0.03

0.00

0.05

0.10

0.15

8 2.0

6

I0 / I

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4

1.5

2

TRCNA

TRCNA 1.0

0.00

0.01

0.02

0.03

0.04

0.00

0.04

0.08

0.12

0.16

2.0 1.2

1.5

1.1

DCNA

DCNA

1.0

1.0 0.00

0.02

0.04

0.06

0.08

0.00

0.04

0.08

0.12

0.16

[Q] (M)

Figure 2: Steady-state Stern-Volmer plots showing the quenching of TCNA, TrCNA and − DCNA by C4 DMAP+ , C8 DMAP+ and C10 DMAP+ in CH3 CN (left) and Im+ 2,1 / NTf2 (right). I and I0 are the quenched and unquenched fluorescence intensities and [Q] is the quencher concentration.

Since the Stern-Volmer plots are sometimes nonlinear, they have been fit to a quadratic function of [Q] and the quenching rate constant kq was determined from the slopes of these fits at [Q]=0.05 M, as described previously. 1,3 The resulting values of kq are plotted in Fig. 3 as green dots and listed in Table 1. In Fig. 3 we also plot the kq values determined for quenching of cyano-anthracenes by neutral and anionic quenchers. 1,5 The latter data

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are plotted using blue symbols for neutral and red symbols for anionic donors. The data points from the cationic DMAP quenchers appear to be consistent with the data previously reported. Thus, Fig. 3 suggests that the same electron transfer mechanism underlies all of these quenching reactions. Table 1: Reaction free energies ∆G0 (eV) and steady-state quenching rate constants kq (× 109 M−1 s−1 ) of the photoreactions. ∆G0 values are calculated using Eqs. 1 and 2 in Section 4 of the Supporting Information.

Fluorophore

Quencher

TCNA TCNA TCNA TrCNA TrCNA TrCNA DCNA DCNA DCNA

C4 DMAP+ C8 DMAP+ C10 DMAP+ C4 DMAP+ C8 DMAP+ C10 DMAP+ C4 DMAP+ C8 DMAP+ C10 DMAP+

CH3 CN ∆G0 kq

− Im+ 2,1 / NTf2 ∆G0 kq

-0.51 -0.49 -0.49 -0.33 -0.31 -0.31 -0.04 -0.02 -0.02

-0.71 -0.67 -0.67 -0.53 -0.49 -0.49 -0.24 -0.20 -0.20

11.9 11.4 11.1 10.5 9.5 9.4 0.63 0.58 0.59

0.61 0.63 0.66 0.45 0.47 0.51 0.11 0.11 0.13

Figure 3: Plots of steady-state quenching rate constants kq vs. free energies ∆G0 in CH3 − CN (left) and ionic liquid Im+ 2,1 / NTf2 (right). The cationic, neutral and anionic donors are represented by green, blue and red symbols, respectively. Neutral and anionic data are taken from Ref. 1. The dashed lines represent the diffusion-limited rate constants predicted using Eq. 1. 56 9



Diffusion-reaction models As described in detail in Ref. 1, time-resolved and steady-state quenching data of the type collected here can be fit to diffusion-reaction models using a least-squares approach by varying only a few model parameters. Such diffusion-reaction models provide a time-resolved F-Q pair distribution p(r, t) based on a spherically symmetric diffusion-reaction equation. 31,47,57,58 The solution of the equation depends primarily on two functions, the distance-dependent reaction rate, κ(r), and the equilibrium F-Q radial distribution function, g(r). We fit the new DMAP data collected here using the same κ(r) and g(r) models described in detail in our prior work. 1 As before, we assess the extent to which a given model is able to represent the observed data using a goodness-of-fit metric ρ ≥ 1, with values ρ < 3 indicating a reasonable fit. Here we employ two models, termed the extended sink (ES) and classical Marcus (CM) models, which were previously shown to provide good performance in fitting quenching data in ionic liquid solvents. 1 Both models approximate the real system by assuming spherical symmetry, and employ the approximate solution of the spherical reaction-diffusion problem proposed by Dudko and Szabo. 59 They also both account for the non-spherical nature of the experimental systems in a crude manner, by distinguishing between the intrinsic reaction rate κ(r) within a first solvation shell and in the remaining space. The ES model assumes a constant empirical value for κ(r) in the first shell whereas the CM model adopts the distance dependence expected by most electron transfer theories in both regions.

The Extended Sink (ES) Model In the extended sink model, κ(r) = κ0 in the first solvation shell, for which the spatial extent is defined by a variable parameter r1 . Beyond the first solvation shell, κ(r) is assumed to decrease exponentially with r (Eq. 16 in the Supporting Information). A model in which no reaction occurs beyond the first shell also fits the present data reasonably, but a slightly larger value of the fit quality parameter ρ is obtained. 10


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Figure 4: Representative fits to the TCSPC data for the fluorescence quenching of TCNA acceptors by C4 DMAP+ donors using the diffusion-reaction models. The top two graphs are − for CH3 CN, while the lower two are for Im+ 2,1 /NTf2 . The left and right panels graph the same data with a log-linear presentation (left) and linear-log plots (right). There are three adjustable parameters used to fit the DMAP data with the extended sink model. These are the relative self-diffusion coefficient of the donor and acceptor D, the size of the first solvation shell r1 , and the reaction rate constant in the first shell κ0 . The extended sink model provides good fits to the DMAP quenching data; in all cases ρ < 2.5. Representative fits of TCNA quenching by C4 DMAP+ are shown in Fig. 4. The complete 11



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set of such figures for all other quenchers and photoacceptors is provided in the Supporting Information. The output parameters from fits to the extended sink model are compiled in Table S4 and plotted in Fig. 5. In the left panels of Fig. 5, the net diffusion coefficients providing best fits to the quenching data (Dfit ) are compared to the estimated diffusivities (Dest ) obtained by summing the measured DMAP diffusion coefficients (Table S1) and the estimated diffusivities for the fluorophores. 1 We again plot the present results (green points) together with the analogous prior results on neutral (blue) and anionic (red) quenchers from Ref. 1. The DMAP data shows a strong correlation between Dfit and Dest in the ionic liquid (R2 =0.94). In CH3 CN, where the fits are not as closely constrained by the data, the correlation is weaker, (R2 =0.73). Nevertheless, Df it is within ±30% of Dest in nearly all cases. In the right panel of Fig. 5, we plot the initial reaction rate kq (0) versus the reaction free energy ∆G0 , where kq (0) is defined as: 50

kq (0) = 4π

Z

∞

κ(r)g(r)r2 dr .

(2)

0

The new data for the cationic DMAP quenchers exhibit the same overall trend of kq (0) with ∆G0 previously established by the anionic and neutral quenching data. (The broad scatter of the CH3 CN data reflect the fact that κ0 is poorly constrained by the data.) Similar to the Stern-Volmer analysis, this more detailed extended sink model analysis suggests a common quenching mechanism that is largely uncorrelated with quencher charge.

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Figure 5: Results from the extended sink model. Left: Dfit , diffusion coefficients from fits vs. Dest , diffusivities from measurements or best estimates. The dashed and dotted lines indicate equality and ±30% deviation, respectively. Right: k(0) vs. ∆G0 (est) for cationic (green), anionic (red) and neutral (blue) quenchers. The red and blue data points are from Ref. 1.

The Classical Marcus (CM) Model In the classical Marcus model, the distance-dependent electron-transfer rate κ(r) is calculated using the combination of the classical non-adiabatic Marcus equation and the solventcontrolled adiabatic treatment by Zusman. 60–62 In this model, two parameters are adjusted to fit the fluorescence quenching data, the relative diffusion coefficient D and the reaction free energy ∆G0 . Another important parameter for calculating κ(r) in the classical Marcus model is the electronic coupling matrix element, HDA and its distance dependence. We attempted to calculate HDA (r) using the same time-dependent DFT method used in Ref. 1. However, for all of the fluorophore-DMAP pairs examined, either in vacuum or using a polarized continuum solvent model, the first excited state of the encounter complex was always the locally excited state, with the charge transfer states having much larger energies. This failure suggests that such calculations do not provide sufficient accuracy when the reaction

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free energy is small. For fitting the present data to the Marcus model, we assumed that the values for HDA (r) = V0 exp (−β1 (r − r0 )/2) should be very similar to those previously used for the isoelectronic N,N-dimethyl-p-toluidine (DMPT) so we choose the the contact value V0 = 900 cm−1 and the decay constant β1 = 0.65 Å−1 (see Eq. 20 in the Supporting Information). A final free parameter, the solvation time constant, is set to 0.5 ps in CH3 CN − 1 and 35 ns in Im+ 2,1 / NTf2 , based on our previous study.

The classical Marcus model also afforded good quality fits to the DMAP fluorescence quenching data, as shown in Fig. 4. The two fit parameters from the Marcus model are compared in the graphs shown in Fig. 6. The left panels compares Dfit to Dest , as done in Fig. 5 for the ES model. In each case, the values of Dfit obtained from fits to the ES and CM models are nearly the same and need no further comment. In the right panels, the fitted reaction free energy ∆G0 (fit) is plotted versus the estimated free energy ∆G0 (est). For cationic donors, ∆G0 (fit) values are very close to ∆G0 (est) values. Since the reaction free energies are only weakly negative, the DMAP quenching reactions all fall in the normal Marcus region.

Figure 6: Results from analysis using the classical Marcus model. Left: Dfit , diffusion coefficients from fits vs. Dest , diffusivities from measurements or best estimates; right: ∆G0 (fit) vs. ∆G0 (est) for cationic (green), anionic (red) and neutral (blue) quenchers.

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One of the main results of Ref. 1 was related to the inability of the CM model to adequately fit all of data in ionic liquids using best estimates for ∆G0 . This difficulty is illustrated by the systematic deviation between ∆G0 (fit) and ∆G0 (est) at larger driving − force (-∆G0 ) seen in the Im+ 2,1 / NTf2 data Fig. 6. Using the observed dependence of kq (0)

on ∆G0 (est) shown in Figure 7, as well as other evidence, we conjectured that slow dynamics in ionic liquid solvents limits the rate of electron transfer in a manner not accounted for in the classical Marcus approaches, perhaps by limiting reactants within an encounter complex in a manner unimportant in high-fluidity conventional solvents. As illustrated in Figure 7, the data collected using the DMAP quenchers strengthen the evidence accumulated using neutral and anionic quenchers.

− Figure 7: kq (0) determined from the classical Marcus fits in CH3 CN (left) and Im+ 2,1 / NTf2 (right). The green, red, and blue dots are for cationic, anionic and neutral quenchers, respectively. The curves are the predictions of a semi-classical ET model, 63,64 using V0 =886 cm−1 , β1 =0.51 Å−1 , λin =0.18 eV (solid) or 0.08 eV (dashed) and the values of τs indicated. The main observation to be made about these plots is that the CM or this semi-classical variant provide reasonable descriptions of quenching in CH3 CN but not in the ionic liquid.

Conclusions In this work, we used time-resolved and steady-state fluorescence quenching experiments to measure the kinetics of photo-induced electron transfer between a series of three cationic 15



DMAP donors and three cyano-substituted anthracene acceptors. The three DMAP donors, C4 DMAP+ , C8 DMAP+ and C10 DMAP+ , have measured self-diffusion coefficients that vary − with overall molecular size, ranging from 3.8 x 10−11 m2 s−1 for C4 DMAP+ in Im+ 2,1 / NTf2

to 2.3 x 10−11 m2 s−1 for C10 DMAP+ in the same IL solvent. The three DMAP quenchers have very similar oxidation potentials of -1.88 V, making them about 1.2 V less favorable as reductants than the analogous neutral 1-alkyl-N,N’-dimethylanilines. We note that although the 1-alkyl DMAP family of cations are isoelectronic to the latter quenchers, the extra 1.2 V is simply a result of oxidizing a cation instead of a neutral. Analysis of the data collected here, using either simple Stern-Volmer treatments or more appropriate reaction-diffusion modeling, shows that these cationic quenchers follow the same general trends established by the neutral and anionic quenchers studied previously. Thus, we conclude that these weakly exergonic reactions occur via the same electron transfer mechanism operative in the other quenchers and that no apparent distinction exists as a function of quencher charge. A reasonable account of all of the data collected thus far is provided by spherical reaction-diffusion descriptions based on either the heuristic extended sink model or the classical Marcus model of electron transfer. By adding data at low driving force to the prior data set, the data collected with these cationic DMAP donors strengthen the case made in Ref. 1, which suggested that motions of reactants within encounter pairs might limit electron transfer rates in ionic liquids.

Acknowledgement This research was funded by a collaborative grant from the U.S. Department of Energy, Office of Basic Sciences, Division of Chemical Sciences, Geosciences, under contract nos. DE-SC0001780 (Rutgers), DE-SC0008640 (Penn State) and DE-SC0012704 (BNL: JFW, NZ and JH). We thank Prof. dr. Eric Vauthey for the gifts of TrCNA and TCNA fluorophores. At Rutgers, we thank Mr. Letao Yang for help with cyclic voltammetry experiments and

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Prof. KiBum Lee for loan of the BASi electrochemical analyzer, and Dr. Nagarajan Murali for assistance with the PG-SE NMR diffusivity measurements.

Supporting Information Available The absorption spectra, synthesis of DMAP ionic liquids, measurements or estimates of diffusion coefficients, reaction free energies or reorganization energies, detailed descriptions on diffusion-reaction models and the results of multi-exponential TCSPC fits are provided in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org/.

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