Naphthothiophene-rhodamine conjugates as selective sensors for Fe3+ ions

Abstract

Naphthothiophene derivatives are widely used in organic electronics, and in this work, we investigate their potential as part of a “turn-on” sensor system based on rhodamine-naphthothiophene (RNT and RNDT) scaffolds. The photophysical properties of the sensors were studied using ultraviolet-visible (UV–Vis) and fluorescence spectroscopies in a MeOH/water solution (4:1 v/v, pH 7.0). The Schiff base sensors displayed selectivity for Fe³⁺ over other metal ions, with significant changes in both absorption and emission. Upon Fe³⁺ binding, the color shifted from colorless to pink-orange, accompanied by a 70- and 11-fold enhancement in emission within seconds. The detection limits were 0.78 ppb for RNT and 0.79 ppb for RNDT. The Fe³⁺ ions triggered the opening of the spirocyclic ring in the rhodamine scaffold by coordinating with the N-imine and O‑carbonyl groups, forming the first complex. Further addition of Fe³⁺ led to a decrease in photophysical response, indicating a “turn-off” effect and the formation of a second complex. This second complex is thought to result from the interaction between Fe³⁺, sulfur of thiophene, and nitrogen of the amide. The binding stoichiometry of both compounds was determined to be 1:2, confirmed by binding fit analysis and mass spectrometry spectra.