Controlling the Melting Temperatures and Ion Dynamics of Carboxy-Functionalized Ionic Liquids

We measured the ²H spectra of carboxy-functionalized ionic liquids [Py-(CH₂)_n-COOD][NTf₂] with alkyl chains n = 1,2,4,5,6,7,9 by means of NMR solid-state spectroscopy. The change from the broad Pake patterns in the glassy state to the narrow lines in the liquid with increasing temperatures indicated the glass-liquid phase transition and allowed the determination of the melting temperatures T_m of these unique ILs characterized by the formation of doubly hydrogen-bonded cationic dimers (c⁺═c⁺) present in the gas, as well as in the condensed phases. The melting occurs at 248 K, only 30 K above the glass transition temperature T_g. The T_m/T_g ratio is 1.13 and thus significantly smaller than the reported values for H-bonded molecular liquids such as water and alcohols. The small ratio is usually referred to the dynamical heterogeneities in the liquid material, which is illustrated by the simultaneously observed solid and liquid NMR spectra. Additionally, we measured the NMR relaxation times T₁ and T₂ that could be modeled by a slow isotropic and a fast anisotropic motion. The maximum between the two T₁ minima coincides with the observed melting temperatures. The microscopic ILs viscosity reflected by the cation dynamics reaches optimal values for n > 4. Our results show that the doubly H-bonded carboxy-functionalized ILs provide low melting temperatures and thus might be suitable for electrolytes.