Naphthalimide-decorated imino-phenol: supramolecular gelation and selective sensing of Fe3+ and Cu2+ ions under different experimental conditions

Abstract

Naphthalimide-based compound 1 with an imino-phenol functionality has been synthesized, characterized and employed in metal ion sensing in a sol–gel medium. Compound 1 forms stable red gels in DMF–H₂O (1 : 1, v/v) and DMSO–H₂O (1 : 1, v/v) with the minimum gelation concentrations of 8 mg ml⁻¹ and 10 mg ml⁻¹, respectively. Both the gels were thermo reversible and exhibited good viscoelastic characteristics. Importantly, while the compound 1 in DMF–H₂O was insensitive to any metal ion taken in the study, the DMF–H₂O gel which shows a highly cross-linked fibrous network was established as Fe³⁺ responsive over a series of other metal ions. The detection of Fe²⁺ was also possible in the presence of an oxidising agent. In CH₃CN or CH₃CN–H₂O (4 : 1, v/v) compound 1 shows selective sensing of Cu²⁺ with a color change. During interaction with Cu²⁺, the absorption and emission of 1 are significantly changed and distinguishable from other metal ions. The Cu²⁺ ensemble of 1 shows ‘turn on’ emission selectively in the presence of S²⁻ ions. The experimental findings have been explained by theoretical study using density functional theory.