Competitive structural and cooperative dynamical heterogeneities of hydrogen bonding and π-type interactions in imidazolium bis(oxalato)borate and polymers

Delicate intermolecular features like hydrogen bonding (HB) and π-type interactions play pivotal roles in stabilizing molecular structures in ionic liquids (IL) bearing ring planes and HB sites. The nature of these interactions is distinct depending on specific ion types. HB and π-type interactions simultaneously occur in ILs consisting of imidazolium cations coupled with small anions. However, both HB and π-type interactions are considerably weakened when imidazolium cations are associated with large anions. The trade-off between HB and π-type interactions becomes more complicated if anions are featured with ring planes. We performed first-principles and atomistic simulations to explore HB and π-type associations in imidazolium bis(oxalato)borate ILs. Preferential HBs and π-type coordinations among ring planes coexist in ILs. Lengthening alkyl chains in cations leads to a substantial increase in HB strength but decrease in π-π stacking stability between ring structures, indicating a competitive structural feature between HB and π-type associations. A cooperative character is observed in HB dynamics and in reorientations of rings with lengthening alkyl chains in cations. We will explore additional structural and dynamical heterogeneities in polymerized ILs in our ongoing work.