The diffusion, structural relaxation, and fragility of [VIO2+][Tf2N-]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) ([VIO2+][Tf2N-](2)) ionic liquid within 400-800 K has been exploited by molecular dynamics simulations. [VIO2+][Tf2N-](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. [VIO2+][Tf2N-](2) behaves as a fragile glass former. The decoupling of diffusion and relaxation is observed in [VIO2+] but not in [Tf2N-]. The variation of dynamics with temperature is attributed to the time differences of the persistence in ion cage and exchange out of ion cage.