Photo- and Radiation-Chemistry of Halide Anions in Ionic Liquids

Abstract

One- and two- photon excitation of halide anions (X(-)) in polar molecular solvents results in electron detachment from the dissociative charge-transfer-to-solvent state; this reaction yields a solvated halide atom and a solvated electron. How do such photoreactions proceed in ionic liquid (IL) solvents? Matrix isolation electron paramagnetic resonance (EPR) spectroscopy has been used to answer this question for photoreactions of bromide in aliphatic (1-butyl-1-methylpyrrolidinium) and aromatic (1-alkyl-3-methyl-imidazolium) ionic liquids. In both classes of ILs, the photoreaction (both 1- and 2-photon) yields bromine atoms that promptly abstract hydrogen from the alkyl chains of the IL cation; only in concentrated bromide solutions (containing >5-10 mol % bromide) does Br2(-•) formation compete with this reaction. In two-photon excitation, the 2-imidazolyl radical generated via the charge transfer promptly eliminates the alkyl arm. These photolytic reactions can be contrasted with radiolysis of the same ILs, in which large yield of BrA(-•) radicals was observed (where A(-) is a matrix anion), suggesting that solvated Br(•) atoms do not occur in the ILs, as such a species would form three-electron σ(2)σ(*1) bonds with anions present in the IL. It is suggested that chlorine and bromine atoms abstract hydrogen faster than they form such radicals, even at cryogenic temperatures, whereas iodine mainly forms such bound radicals. These XA(-•) radicals convert to X2(•-) radicals in a reaction with the parent halide anion. Ramifications of these observations for photodegradation of ionic liquids are discussed.

Gamma and heavy ion radiolysis of ionic liquids: A comparative study

“…[21][22][23][24] Several research groups have investigated the radiation stability of different hydrophobic ILs using various methodologies. [18,[25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] An assessment of the radiation effects of ,  and  on imidazolium cation based hydrophilic ILs show less than 1 % degradation of the media even when exposed to a dose of 400 kGy, which suggests very high radiation stability for these ILs. [25] Some studies reported changes in the physical properties such as density, viscosity, conductivity, surface tension and refraction index after -radiolysis.…”

“…[43][44][45][46][47] Shkrob et al have investigated radical generated species by EPR spectroscopy, showing different degradation pathways for aromatic and aliphatic ILs. [18,[35][36][37][38][39][40][41][42] Degradation pathways have been proposed on the basis of final product identification using spectrometric techniques such as NMR, electrospray ionization mass spectrometry (ESI-MS) and liquid chromatography coupled with ESI-MS. [26,27] The anion effect on the stability of imidazolium based ILs has been reported using spectrometric techniques. [29] All of these studies suggest that many radiolysis products are formed from ILs, but in small quantities, and that the mechanisms are difficult to quantify.…”

Understanding the Photoexcitation of Room Temperature Ionic Liquids

“…They studied photoreactions, where bromine atoms are being formed after photoexcitation that abstract hydrogen from alkyl chains of the cation. Therefore, no solvated bromine radicals in RTILs could be formed [74] …”

Ultrafast transient absorption spectrum of the room temperature Ionic liquid 1-hexyl-3-methylimidazolium bromide: Confounding effects of photo-degradation

“…In a recent study, Shkrob and Wishart (Shkrob et al 2013;Wishart and Shkrob 2009) used a combination of computational chemistry methods, low-temperature electron spin resonance and TA spectroscopy to study electron localization in imidazolium-based ILs. They propose that the electron localization on the imidazolium cation produces a gauche dimer radical cation (C 2 +• ) via reaction of the BMIm • radical with a BMIm + cation, and that these species should absorb in the near-IR and the visible to near-UV regions.…”

“…Furthermore, they go on to show that the TA kinetics observed at a single wavelength strongly depend upon the scan rate, suggesting possible contributions from stable photo-products. (Brands, Chandrasekhar, and Unterreiner 2007) In earlier work (Shkrob et al 2013) part of our team observed that in dilute solutions of C n MIm + /Brin C n MIm + /NTf 2 -, the released halogen atom very rapidly abstracts H from the aliphatic arms of the IL cations, forming HBr, the dimer radical,…”