Fluoride-induced spectral modulation in a 1,8-naphthalimide-squaric ester amide conjugate: a spectroscopic and mechanistic study

Fluoride-induced spectral modulation of a 1,8-naphthalimide–squaric ester amide conjugate (Nap-SQ) was investigated using UV–visible and ¹H NMR spectroscopy in DMSO. The receptor exhibits highly selective absorption changes in the presence of fluoride (F⁻) compared to other common anions such as Cl⁻, Br⁻, I⁻, AcO⁻, H₂PO₄⁻, NO₃⁻, SCN⁻, BF₄⁻, and HSO₄⁻. ¹H NMR titration studies indicate that the sensing mechanism involves initial hydrogen bonding between squaramide N–H groups and F⁻, followed by deprotonation at higher fluoride concentrations, resulting in the formation of [HF₂]⁻ species. Control experiments with OH⁻ further confirm the contribution of an acid–base interaction in aprotic media. Competitive experiments reveal that fluoride sensing is largely retained in the presence of most anions, with slight interference observed from SCN⁻. The spectral response is reversible, as the original absorption profile is restored upon addition of methanol, highlighting its potential applicability in sensing systems.