Study of γ radiation-induced colored products on [Cnmim][NTf2] ionic liquids

Ionic liquids 1-alkyl-3-methylbis(trifluoromethylsulfonyl)imide ([Cnmim][NTf2]) are promising separating reagents in the nuclear fuel cycle. Their chemical structure changes slightly when exposed to γ radiation, but the apparent darkening of [Cnmim][NTf2] occurs at low dose. This radiation-induced darkening of [Cnmim][NTf2] should be investigated further. To understand the effect of radiation on [Cnmim][NTf2], we systematically studied the influences of the length of the C(1)-alkyl chain and substitution of C(2)-H on the UV-Vis spectra of irradiated [Cnmim][NTf2]. Fluorescence and mass spectra of [Cnmim][NTf2] allowed the possible colored products to be determined. The darkening of [Cnmim][NTf2] is more obvious as the length of the C(1)-alkyl chain and absorbed dose increase, but it is weakened effectively after methylation at the C(2)-position of the imidazole ring. The dominant colored products are possibly imidazolium cations containing double bonds, dimers of imidazolium cations and fluorinated imidazolium compounds. Imidazolium cations could aggregate through π-π stacking after γ-irradiation, and the associated states may also play a role in darkening these ionic liquids. © Editorial office of Acta Physico-Chimica Sinica.