A Comparison of Electron-Transfer Dynamics in Ionic Liquids and Neutral Solvents

Abstract

The effect of ionic liquids on photoinduced electron-transfer reactions in a donor−bridge−acceptor system is examined for two ionic liquid solvents, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide and tributylmethylammonium bis(trifluoromethylsulfonyl)amide. The results are compared with those for the same system in methanol and acetonitrile solution. Electron-transfer rates were measured using time-resolved fluorescence quenching for the donor− bridge−acceptor system comprising a 1-N,1-N-dimethylbenzene-1,4-diamine donor, a proline bridge, and a coumarin 343 acceptor. The photoinduced electron-transfer processes are in the inverted regime (−ΔG > λ) in all four solvents, with driving forces of −1.6 to −1.9 eV and estimated reorganization energies of about 1.0 eV. The observed electron-transfer kinetics have broadly distributed rates that are generally slower in the ionic liquids compared to the neutral solvents, which also have narrower rate distributions. To describe the broad distributions of electron-transfer kinetics, we use two different models: a distribution of exponential lifetimes and a discrete sum of exponential lifetimes. Analysis of the donor−acceptor electronic coupling shows that for ionic liquids this intramolecular electron-transfer reaction should be treated using a solvent-controlled electron-transfer model.

Electron-Transfer Dynamics for a Donor–Bridge–Acceptor Complex in Ionic Liquids

“…11,[33][34][35][36] In several cases, it was found that the reactions in ILs exhibit distributed dynamics and are often solventcontrolled. 11,36 Previously, Lee, et al, compared ET dynamics in ionic liquids and neutral solvents by investigation of a D-B-A complex consisting of DMPD as electron donor connected to the excited-state acceptor coumarin 343 (C343) by a proline bridge. Using time-resolved fluorescence, the excited state dynamics of DMPD-Pro 1 -C343 (P1 ) was studied in the neutral solvents acetonitrile and methanol, and in two ILs based on the bis(trifluoromethylsulfonyl)amide (NTf − 2 ) anion.…”

“…From the 16 starting conformations, 8 distinct conformations corresponding to local energy minima were obtained. The five lowest-energy conformations and energies relative to the lowestenergy conformer are shown in Figure 6.Results and DiscussionLee, et al, characterized the dynamics of electron transfer from DMPD to C343 in a D-B-A system utilizing a single proline bridge (DMPD-Pro-C343, P1 ) 36. In this system, only a single low-energy conformation was obtained using geometry optimization methods…”

“…All fitting methods were identical for the experiments on the P1 samples by Lee, et al, who provided a more detailed description of these models and their implementations 36. …”

Photoinduced Bimolecular Electron Transfer from Cyano Anions in Ionic Liquids

“…9−14 Time-resolved fluorescence measurements of rates on picosecond time scales demonstrate significant heterogeneities in rates of intramolecular charge shifts. 15,16 Much of the heterogeneity in IL solvation dynamics arises because the viscosities are typically 1−3 orders of magnitude larger than for typical neutral solvents. 8 Bimolecular electron-transfer rates in ILs are not well described by simple rate theories.…”

“…In addition to the sums-of-exponentials modeling, we also applied a distributed lifetime model based on a quadratic programming algorithm, as discussed previously. 16,46 We have applied both methods of analysis to the full set of data for concentration-dependent fluorescence quenching of 9,10-DCNA by the five cyano anion ILs. The distributed exponential analysis of 9,10-DCNA quenching by C(CN) 3 − in Im 2,1 + / NTf 2 − is given in the Supporting Information.…”