Modulating transport properties in N-alkyl-4-cyanopyridinium ionic liquids through formation of liquid charge transfer complexes

Journal article, 2026

The impact of liquid charge-transfer complex formation between nitrile-functionalised N-alkyl-4-cyanopyridinium bis(trifluoromethylsulfonyl)imide ionic liquids ([Cn⁴CNPy][NTf2], n = 1–4) and 1-methylnaphthalene on conductivity and viscosity of the liquids has been investigated. These ionic liquids exhibit higher viscosities and lower conductivities than N-butylpyridinium bis(trifluoromethylsulfonyl)imide, confirming the role of the nitrile group in strengthening ion association as previously identified by Hardacre et al. (Phys. Chem. Chem. Phys., 2010, 12, 1842). Strikingly, [C2⁴CNPy][NTf2] displays anomalous viscosity–conductivity behaviour, deviating from homologous trends. Formation of charge transfer complex liquids with one equivalent of 1-methylnaphthalene suppresses these anomalies, producing convergent transport profiles and revealing how π–π interactions reshape nanoscale structure and dynamics. These findings provide new insight into tailoring ionic liquid properties through molecular design and controlled charge-transfer interactions.