Radiolysis behavior of ionic liquids of thiocyanate salts

Complete text of publication follows. Hydration and reactions of Eu(III) ion and electron in hydrophobic ionic liquids (IL) by adding a small amount of water have been studied by using the laser-induced fluorescence spectroscopy and pulse radiolysis. In the radiolysis of ILs, the ionization sites and the presence of geminate radical formed together with electron are not clear. The radiolytic reactions in hydrophilic ILs containing thiocyanate ion as a constituent, which reacts with OH in aqueous solutions, were studied to understand the formation and reaction of products in ILs. ILs were synthesized by the combination of several anions and cations in acetone or water: thiocyanate bromide (BrSCN) was used as anion source, while lithium bis(trifluoromethanesulfonyl)imide (Litfsi = LiN(SO2CF3)2) was used for the comparison; chlorides of imidazolium (C4mim+: 1-butyl-3-methylimidazolium, C2mim+: 1-ethyl-3- methyl imidazolium) and ammonium (demma+: N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium) were mainly used as cation sources. The samples were irradiated by electron pulses (28 MeV, 8 ns) from L-band LINAC at ISIR, Osaka Univ. to observe transient absorption of electrons and radicals formed in the samples. In the imidazolium salt of [C4mim][tfsi], the transient absorption originated from C4mim+ was observed, while neither electron nor radical originated from tfsi- observed. On the other hand, the absorption ascribed to dimer radical anion of (SCN)2- was observed in [C4mim][SCN] as well as that from C4mim+. The rapid formation of (SCN)2-, irrespective of the high viscosity of ILs, could be explained by the direct dimerization between SCN radical formed in ILs and its surrounding SCN-, not via their diffusive encounter. In order to elucidate the formation yield and pathway of SCN radical, the ammonium salts containing two anions, i.e., the mixtures of [demma][SCN] and [demma][tfsi] were prepared. The initial absorbance of (SCN)2- increased with increasing the fraction of [demma][SCN] in the mixed ILs, where any radicals originated from demma+ were not observed. This suggests that one of the ionization sites of ILs is SCN- to generate a pair of electron and SCN radical.