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

Abstract

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.