6-Aminocoumarin-derived Schiff base gelators: aggregation and sensing of CN−, Fe3+, Cu2+ and CO2 under different conditions†

Abstract

Herein, we report the synthesis, characterization, supramolecular gelation and multiple applications of 6-aminocoumarin-derived Schiff bases 1 and 2. Both Schiff bases underwent gelation in DMF–H₂O (2 : 1, v/v), DMSO–H₂O (2 : 1, v/v) and dioxane–H₂O (2 : 1, v/v) involving weak forces. Furthermore, the gels were stable and exhibited good viscoelastic properties. The storage modulus (G′) of each gel was considerably higher than its loss modulus (G′′). The higher value of the crossover point and lower value of tan δ for the gel of Schiff base 2 compared to the gel of Schiff base 1 demonstrated the better gelation behaviour of 2 than that of 1 in DMF–H₂O (2 : 1, v/v). Further, iodo-analogue 2 exhibited cross-linked helical morphology, whereas non-iodo analogue 1 exhibited long chain fibrous morphology, as observed via FESEM. These differences in morphology and viscoelastic behaviors were attributed to the iodo group present in 2, which influenced its aggregation involving halogen bonding. To demonstrate their application, the DMF–H₂O (2 : 1, v/v) gels of both 1 and 2 recognized CN⁻ over a series of other anions by exhibiting a gel-to-sol phase change. Besides anion sensing, gels 1 and 2 selectively detected Fe³⁺ and Cu²⁺ ions over other metal ions via a gel-to-gel colour change. Finally, CN⁻-treated solutions of 1 and 2 allowed the successful detection of CO₂ by the naked eye. Moreover, the detection was possible using a test-kit method.