ortho-Halogen functionalized N-squaraines: structure–property relationship and dual-mode colorimetric and fluorometric sulfide ion detection

Abstract

Symmetric ortho-halogen derivatives of anilinium N-squaraines were synthesized and characterized using ¹H-NMR, mass, FT-IR, and single-crystal X-ray diffraction techniques. The effect of halogen substituents (–F, –Cl, –Br, –I) on the optoelectronic and electrochemical properties of N-squaraines has been thoroughly investigated. Additionally, the theoretical calculations demonstrated that the ortho functionalization slightly lowers the HOMO–LUMO energy band gap, which aligns with the optical band gap. Moreover, the solid-state photophysical characterization revealed that the photo-excited state remains a singlet, even in the presence of heavy atoms like bromine and iodine. The solid-state fluorescence emission was also significantly higher than in the solution state, with the quantum yields soaring up to 24%. Further, the two acidic binding sites in the synthesized compounds 2–5 were evaluated for anion sensing. The o-halo-derivatives act as selective dual-mode colorimetric and “Turn-On” fluorometric chemosensors for sulfide anions, with the solution changing from colorless to yellow and a four-fold enhanced emission intensity. Furthermore, adding acid makes the solution turn colorless again, as investigated in detail using the o-chloro-derivative. The chemosensor displayed good reversibility for up to seven cycles and demonstrated applications in molecular logic gates.