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

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 Br-2(-center dot) 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(-center dot) radicals was observed (where A(-) is a matrix anion), suggesting that solvated Br-center dot atoms do not occur in the ILs, as such a species would form three-electron sigma(2)sigma*(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(-center dot) radicals convert to X-2(center dot-) radicals in a reaction with the parent halide anion. Ramifications of these observations for photodegradation of ionic liquids are discussed.