The diffusion, structural relaxation, and fragility of [VIO 2+ ][Tf 2 N − ] 2 ionic liquid

Viologen-based ionic liquids can form various microscopic structures at different temperatures. In this work, the dynamics of dimethyl-viologen bis-(tetrafluoroborate) ([VIO 2+ ][Tf 2 N − ] 2 ) ionic liquid within 400–800 K has been exploited by molecular dynamics simulations. [VIO 2+ ][Tf 2 N − ] 2 exhibits a supercooled liquid analogous diffusion and structural relaxation even at temperature T = 400 K, and behaves more like simple liquids as temperature increases. The variation of the diffusion constant and structural relaxation time with temperature follows a super-Arrhenius law; both can be fitted by two Arrhenius laws with a crossover temperature or fitted by a VFT law. [VIO 2+ ][Tf 2 N − ] 2 behaves as a fragile glass former. The decoupling of diffusion and relaxation is observed in [VIO 2+ ] but not in [Tf 2 N − ]. The variation of dynamics with temperature is attributed to the time differences of the persistence in ion cage and exchange out of ion cage.