1,8-Naphthyridinic fluorescent ‘turn-on’ and ‘turn-off’ chemosensors for detection of F− and Hg2+ ions mimicking INHIBIT molecular logic behaviour

Abstract

Two new receptors viz. 2-bromoacetamido-5,7-dimethyl-1,8-naphthyridine (1a) and 2-chloroacetamido-5,7-dimethyl-1,8-naphthyridine (1b) containing 1,8-naphthyridine as fluorophore unit have been designed and synthesised. These receptors exhibited very high selectivity for F⁻ ions among the various anions tested such as H₂PO₄⁻, OAc⁻, Cl⁻, Br⁻, I⁻, HSO₄⁻, PF₆⁻, ClO₄⁻, BF₄⁻ and NO₃⁻ owing to anion-induced deprotonation of the amide NH by F⁻ ions. Among the various cations tested (Na⁺, Ag⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺ and Mn³⁺), 1a exhibited selective detection of Hg²⁺ ions by binding with N1 and N8 atoms of the naphthyridine moiety. Receptor 1a has been explored as a dual-ion detector for F⁻ and Hg²⁺ ions in two contrasting modes (fluorescence ‘turn-on’ and ‘turn-off’) which is supported by spectroscopic and lifetime measurements. 1a exhibited lower detection limits for F⁻ (0.4 ppm) and Hg²⁺ (2.4 ppm) ions. These receptors were recovered from anionic species of 1a formed after addition of F⁻ ions by treating with protic solvents such as CH₃OH. The mode of binding was also confirmed by spectroscopic and DFT studies. The chemical inputs of F⁻ and Hg²⁺ ions generate an output which satisfies the condition of two-input INHIBIT logic operators.