Unravelling the optical properties and self-assembly behavior of ciprofloxacin in ionic liquid environments: probing the role of cationic residues and counter anions

This study investigated the interaction of ciprofloxacin (CIP) with three different ionic liquids (ILs) featuring distinct cationic residues (pyrrolidinium, IL₁, vs. imidazolium, IL₂, with bis(trifluoromethylsulfonyl)imide as the counter anion) and counter anions bis(trifluoromethylsulfonyl)imide, IL₂, vs. hexafluorophosphate, IL₃, with imidazolium as the cationic residue) in an aqueous environment. A series of spectroscopic studies have been performed to elucidate the role of ILs in the optical properties as well as aggregation behavior of CIP. The fluorescence quenching experiment indicated that the IL with a pyrrolidinium residue showed stronger binding with CIP, while bis(trifluoromethylsulfonyl)imide was the preferred anion. These quenching effects might be attributed to complex formation mediated by charge-pair and cation–π interactions, along with hydrogen bonding. The Stern–Volmer analysis confirmed a static quenching mechanism, with binding constants (K_b) reflecting the stronger affinity of IL₁ due to the hydrophobic butyl group and the flexible pyrrolidinium cation, resulting in the formation of larger aggregates. In contrast, the imidazolium residue in IL₂ facilitated π–π and hydrogen-bond interactions, disrupting CIP aggregation and resulting in smaller clusters. The polarizable nature of bis(trifluoromethylsulfonyl)imide along with its hydrogen bond-accepting ability enabled stronger binding of ILs containing this anion to CIP compared to hexafluorophosphate-containing ILs. Further studies indicated that pH 6 is optimum for CIP–IL interactions, where CIP remained in its zwitterionic form. Increased temperature and ionic strength diminished the quenching efficiency, consistent with the reduced stability of CIP–IL complexes under such conditions.