4-Hydroxybenzaldehyde derived Schiff base gelators: case of the sustainability or rupturing of imine bonds towards the selective sensing of Ag+ and Hg2+ ions via sol–gel methodology

Cholesterol appended 4-hydroxybenzaldehyde derived Schiff bases 1–4 were designed and synthesized. They are explored for the naked-eye detection of metal ions using sol–gel methodology that involved the imine bonds remaining intact or rupturing. All the compounds form a gel under different conditions with different morphologies. The gel of 1 in CHCl3/MeOH undergoes a change to a sol in the presence of copper (Cu2+), iron (Fe3+) and mercury (Hg2+) ions because of imine bond hydrolysis and exhibits poor selectivity in the sensing process. The Hg2+ ion is distinguished from the others because the gel-to-sol conversion only takes a short time. However, while the benzimidazole-based Schiff base 2 shows a gel-to-sol transformation selectively in the presence of Ag+ ions, the benzothiazole-based Schiff base 3 forms a gel selectively in the presence of Ag+ ions without showing any rupturing of the imine bonds. The Ag+ ion-induced sol-to-gel or gel-to-sol transformation is tunable upon the addition of tetrabutylammonium chloride (TBACl). The gel 2 is relatively fragile and weak in comparison to Ag+-3 gel and this was established using a rheological study. The tripodal Schiff base 4 in DMF/H2O underwent imine bond hydrolysis in the presence of Hg2+ ions to give the aldehyde 5 which forms an instant gel under the conditions described and validates the visual sensing of Hg2+ ions.