A 1,8-naphthalimide–pyridoxal conjugate as a supramolecular gelator for colorimetric read out of F− ions in solution, gel and solid states

Abstract

A naphthalimide–pyridoxal conjugate 1 has been designed and synthesized. Compound 1 forms a stable greenish yellow colored gel in DMSO : H₂O (8 : 1 v/v). Rheological study reveals that the gel is mechanically strong (G′> G′′) over a wide range of applied strains. The morphology of the gel as determined by FESEM shows a highly cross-linked fibrous network. The gel is anion-responsive and is selectively transformed into a sol with a color change from greenish yellow to deep blue only in the presence of F⁻ among other anions. In CH₃CN, compound 1 was also sensitive to basic anions such as F⁻ and AcO⁻ ions. In solution, F⁻ was differentiated from AcO⁻ through a color change. While the yellow colored solution of 1 in acetonitrile was changed into deep blue in the presence of F⁻, AcO⁻ ions gave a faint blue coloration. A similar colorimetric differentiation of F⁻ from AcO⁻ has been possible in CH₃CN by a reusable Schiff base-linked Merrifield resin 1a or 1b.