Photoinduced Electron Transfer in Ionic Media

Abstract

The goal of this research was to explore the use of room temperature ionic liquids (RTILs) and ionic liquids crystals (ILCs) as reaction media for photoinduced electron transfer (PET) processes.

Photoinduced electron transfer in two room-temperature ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) and 1-octyl-3-methylimidazolium hexafluorophosphate (OMIM-PF6) has been investigated using steady-state fluorescence quenching of 9,10-dicyanoanthracene (DCA) with a series of single electron donors. From these fluorescence quenching rates, reorganization energy values and kdiff values can be derived from a Rehm-Weller analysis. In many cases, these fluorescence quenching reactions occur at rates larger than what would be expected based on the Smoluchowski equation. In addition, reorganization energy values of 10.1 kcal/mol and 16.3 kcal/mol for BMIM-PF6 and OMIM-PF6, respectively, have been determined.

The dynamics of electron transfer reactions in butyl pyridinium bis(trifluoromethanesulfonyl)imide (BuPyr-NTf2) and other solvents have been explored using laser flash photolysis. In these experiments, benzophenone (BP), duroquinone (DQ), and 9-cyanoanthracene (9CA) were used as excited-state acceptors, 1,4-diazabicyclo[2.2.2]octane (DABCO) and hexamethylbenzene (HMB) were used as ground-state donors, and methyl viologen (MV2+) was used as a probe molecule. Analysis of kinetic and spectroscopic data from these experiments shows that electron transfer from photoreduced acceptors to the probe occurs via one or more solvent ions in cases where the acceptor anion radical has a reduction potential that is more negative than the solvent ions (BP- and 9CA- in BuPyr-NTf2). Mediated electron transfer was demonstrated to significantly enhance quantum efficiencies of photoinduced electron transfer in cases where back electron transfer would otherwise predominate.

In addition to RTILs, a pyridinium and imidazolium ILC were used as reaction media in LFP experiments. In these experiments, BP and a pyrromethene dye were used as excited-state acceptors, DABCO and DMB were used as ground-state donors, and MV2+ was used as a probe molecule. In these experiments it was determined that the pyridinium ILC was able to mediate electron transfer with both BP and PM such that quantum efficiencies (Φ) using both acceptors are improved significantly. Additionally, a rate enhancement for PET is observed using the pyridinium ILC as opposed to the imidazolium ILC for the BP system.