Synthesis and selective colorimetric detection of iodide ion by novel 1,5-naphthyridine-based conjugated polymers

A highly selective colorematric detection of iodie ion based on 1,5-naphthyridine-based conjugated polymers (CPs) is reported. The synthesis of these new conjugated copolymers poly(2-(2-methoxyphenoxy)-6-(3-methoxyphenoxy)-1,5-naphthyridine)-p-bisdodecyloxyphenylene vinylenes) (P1) and poly(2-(2-methoxyphenoxy)-6-(3-methoxyphenoxy)-1,5-naphthyridine)-p-bis-2-(ethyl-hexyloxy)-phenylene vinylenes) (P2) was achieved by the Horner-Emmons polymerization reaction. These polymers were characterized by ¹H NMR, FT-IR and UV–vis spectroscopy and their bandgaps (P1 = 2.35 eV; P2 = 2.25 eV) were determined by cyclic voltammetric measurements. In colorimetric sensing, both these polymers can selectively detect iodide ion in a mixture of water and THF in the presence of a wide range of competing ions including F⁻, Cl⁻, Br⁻, NO₃ ⁻, CH₃COO⁻, SO₄ ⁻², SO₃ ⁻², S₂O₃ ⁻² and CN⁻ and exhibit high sensitivity with LOD of 6.5 and 4.5 ppm, respectively. In the sensing protocol, the addition of aqueous solution of iodide salts to the colorless solution of polymers resulted in bare-eye color change from colorless to yellow, along with concomitant changes in the UV–vis spectra. The time dependent and concentration based studies revealed that P2 was more sensitive and quick in response as compared to P1.