Multifunctional Aliphatic Diamine-Based Polynaphthoxazines: From Synthesis and Polymerization to Bioactivity and Photoluminescence

A series of aliphatic diamine-based naphthoxazine monomersdesignated as NZ-EDA, NZ-PDA, and NZ-HDAwere successfully synthesized via a one-pot Mannich condensation reaction involving 2-naphthol, paraformaldehyde, and three different aliphatic diamines: 1,2-diaminoethane, 1,3-diaminopropane, and 1,6-diaminohexane, respectively. Structural confirmation was achieved using FTIR and advanced NMR techniques (¹H, ¹³C, and DEPT-135). Unlike conventional naphthoxazine systems, the synthesized monomers exhibited markedly lower polymerization temperatures and a broader processing window, thereby enhancing their practical utility. Among the resulting polymers, poly-(NZ-PDA) exhibited the highest thermal stability, with a 10% weight-loss temperature (<i>T</i> _d10) of 367 °C. Photoluminescence analysis revealed intense blue emission across all monomers and their polymers, thanks to the molecular packing effects within the crystal structures, consistent with previous research findings. Morphological investigations via SEM showed that the NZ-EDA, NZ-PDA, and NZ-HDA monomers formed distinct spherical flower-like nanocrystals. The XRD results indicated that the average crystallite sizes of these monomers were 26, 15, and 20 nm, respectively. Their polymers exhibited a characteristic broad peak at 2θ ≈ 22.5°, indicating the formation of an amorphous structure that aligns with expectations. Biological assessments revealed that NZ-EDA and NZ-HDA exhibited potent anticandidal activities, with minimum inhibitory concentration values of 2 μg/mL against <i>Candida albicans</i> and 7.8 μg/mL against <i>Candida parapsilosis</i>, respectively, but with low antibacterial activity. In comparison to substantially antioxidative vitamin C, NZ-PDA> NZ-EDA > NZ-HDA had the appreciable descending order of antioxidative DPPH radical scavenging capacities (with IC₅₀ values 4.9 < 9.4 < 57.8 μM, respectively), but conversely, a considerable ascending order of antimicrobial properties. Any inferences of dual antioxidative-antimicrobial efficacies could not be established.