Ionic Liquids: Not only Structurally but also Dynamically Heterogeneous

In recent years, the complex and heterogeneous structure of ionic liquids has been demonstrated; however, the consequences on the dynamics have remained elusive. Here, we use femtosecond IR spectroscopy to elucidate the local structural dynamics in protic alkylammonium‐based ionic liquids. The structural relaxation after an ultrafast temperature increase, following vibrational excitation and subsequent relaxation of the N‐D (or N‐H) stretching vibration, is found to vary substantially between the ionic and hydrophobic subdomains. The dynamics in the ionic domains are virtually unaffected by the alkyl chain length and is, therefore, decoupled from viscosity. Equilibration within the hydrophobic subdomains, as evident from the dynamics of the C‐H stretching vibration, is faster than that in the ionic domains and shows a remarkably low thermal activation. Size doesn't matter: The equilibration of the ionic ammonium group in protic ionic liquids after an ultrafast temperature jump depends on the anion but not on the size of the hydrophobic side chain. This equilibration of the ionic domains is approximately two times slower than restructuring within the hydrophobic alkyl domains, which demonstrates the dynamic heterogeneity in ionic liquids.