A triphenylamine-functionalized fluorescent organic polymer as a turn-on fluorescent sensor for Fe³⁺ ion with high sensitivity and selectivity

A triphenylamine-functionalized A₃B₂-type covalent organic polymer (TPA-COP) was synthesized by the palladium-catalyzed Sonogashira polycondensation reaction from tris(4-ethynylphenyl)amine and 4-bromo-N-(4-bromo-2-methoxybenzylidene)-2-methoxyaniline. The obtained polymer possessing amorphous structure and good thermal stability exhibited strong yellow–green luminescence with an emission band at 498 nm. It is worth noting that the polymer also shows selective turn-on fluorescence response to Fe³⁺ ion over a variety of other competing metal ions such as Na⁺, K⁺, Ca²⁺, Mg²⁺, Ba²⁺, Mn²⁺, Cr³⁺, Cu²⁺, Ni²⁺, Zn²⁺, Co²⁺, Al³⁺. The fluorescent detection limit of TPA-COP sensor for Fe³⁺ in solution was 4.3 × 10⁻⁷ M. Furthermore, the proposed sensing mechanism should result from complexation between tridentate-coordinated Schiff base sites and Fe³⁺ ion, which was further investigated by fluorescence titration, ¹H NMR and IR measures, together with fluorescence lifetime analysis.