A novel pyridine-functionalized naphthoxazine: thermal stability, proton and metal ion-responsive optical sensing, and biological activity with DFT validation

Abstract Designing multifunctional polymers that combine thermal stability, selective optical response, and biological activity remains a key challenge in smart materials development. Herein, we report a novel pyridine-functionalized naphthoxazine monomer (Py-NZ) synthesized via a one-pot Mannich condensation and fully characterized by 1D/2D NMR, FTIR, and HRMS. DFT calculations confirmed the optimized geometry, showing excellent agreement with experimental data (RMSD = 0.018 Å; mean IR error = 0.43%), validating its extended π-conjugated structure. Thermogravimetric analysis of the cured Poly(Py-NZ) revealed outstanding thermal stability, with a char yield of 69% at 800 °C and a limiting oxygen index (LOI) of 45.1, indicating strong flame-retardant potential. UV–Vis and fluorescence spectroscopy demonstrated that metal coordination significantly modifies the optical behavior. Among the tested metal ions, Zn²⁺ induced a marked fluorescence enhancement due to its d¹⁰ configuration, which suppresses nonradiative decay and favors radiative emission. Py-NZ also exhibited strong acid-responsive fluorescence: high HCl concentrations quenched emission via protonation-induced nonradiative pathways, while lower acidity restored intensity, confirming its dual-responsive chemosensing capability for both metal ions and protons. Biological evaluation showed moderate antibacterial activity against Staphylococcus aureus and Escherichia coli , with the protonated form exhibiting enhanced activity against Gram-positive bacteria (MIC = 15.6 µg/mL). In DPPH antioxidant assays, all compounds displayed dose-dependent reductive activity comparable to ascorbic acid, with the Zn²⁺-coordinated complex showing the highest potency of nanomolarity radical scavenging affinities (IC₅₀ = 0.15 ± 0.01 vs. 0.041 ± 0.0062 for ascorbic acid). Overall, this work establishes a versatile platform for naphthoxazine-based multifunctional materials with potential applications in flame-retardant coatings, optical sensing, and biomedical fields.