Pulse Radiolysis and Computational Studies on a Pyrrolidinium Dicyanamide Ionic Liquid: Detection of the Dimer Radical Anion

Abstract

A pulse radiolysis study on pyrrolidinium cation based ionic liquids is presented herein. Time-resolved absorption spectra for 1-methyl-1-propylpyrrolidinium dicyanamide (DCA) at 500 ns after the electron pulse show broad absorption bands at wavelengths below 440 nm and at 640 nm. In pyrrolidinium bis(trifluoromethylsulfonyl)imide (NTf) and tris(perfluoroethyl)trifluorophosphate (FAP) ILs, the transient absorption below 440 nm is much weaker. The absorption at 500 ns, which increases with wavelength from 500 nm to beyond 800 nm, was assigned to the tail of the solvated electron NIR absorption spectrum, since it disappears in the presence of NO. In the DCA IL, the presence of a reducing species was confirmed by the formation of pyrene radical anion. The difference in the transient species in the case of the DCA IL compared to other two ILs should be due to the anion, with cations being similar. In pseudohalide ILs such as DCA, radicals are formed by direct hole trapping by the anion (X + h → X), followed by addition to the parent anion. Prediction of the UV/vis absorption spectra of the dimer radical anion by computational calculation supports the experimental results. The oxidizing efficiency of (DCA) and its reduction potential ( E) have been determined.

Formation of intramolecular dimer radical ions of diphenyl sulfones

“…Similar charge delocalization through transannular interaction has been reported in radical anion of 1,1-diphenyl methane in γ-irradiated glassy matrix at 77 K 20 . Therefore, we found that the dimer radical anions of DPs produced in a liquid at RT are different from those previously reported, which were species with stacked phenyl rings and strong electron-withdrawing substituents [21][22][23][24] .…”

“…Dimer radical anions of aromatic molecules have been mainly formed in rigid matrixes [15][16][17][18][19][20] . However, some recent studies have reported the formation of dimer radical anions of some limited molecules in the liquid phase at room temperature (RT) [21][22][23][24][25][26] . The dimer radical anions observed at RT are classified into two types.…”

“…One is the σ-type, which is formed between the radical anion of phenylethynyl compounds and the neutral molecule 25,26 . The other is the π-type, in which phenyl rings with electron-withdrawing groups, such as CN-and F-substituents, overlap [21][22][23][24] . This is similar to the π-π stacking in dimer radical cations.…”

Photodetachment and Electron Dynamics in 1-Butyl-1-methyl-pyrrolidinium Dicyanamide

“…This is the same time scale commonly associated with the earliest events observed in dipolar solvation, and it is assumed to involve the same inertial solvent reorganization . The experimental results do not rule out the possibility of localization on ions or the formation of radical dimers . At the same time, they do not provide direct evidence for these channels and indicate that if there is significant localization on ions such states are likely a short-lived intermediate en route to localization on a cavity.…”

“…The excited electron is initially delocalized over a number of neighboring molecules. From this state, the electron can relax via one of three general pathwaysthe electron can geminately recombine with the hole, localize onto an ion, or localize onto a cavity in the liquid. , We will refer to this cavity electron as a “free” electron. Following initial separation from the parent ion, the free electron undergoes solvation on femtosecond, picosecond, and nanosecond time scales. ,, As the electron localizes and solvates, it can undergo a significant, correlated loss in reactivity. , …”