Thermal property of an aggregation-induced emission fluorophore that forms metal-ligand complexes with Zn(ClO₄)₂ of salicylaldehyde azine-functionalized polybenzoxazine

In this report, we designed a new and simple salicylaldehyde azine-functionalized benzoxazine (azine-BZ) monomer via Mannich condensation reaction of aniline and paraformaldehyde with 1,2-bis(2,4-dihydroxybenzylidene)hydrazine in 1,4-dioxane. Compared with 3-phenyl-3,4-dihydro-2H-benzoxazine monomer (263 °C), the maximum exothermic peak of azine-BZ shifted to a lower temperature (213 °C) based on differential scanning calorimetry (DSC) analyses because of the basicity of the phenolic group (OH) in the ortho position and the azine groups. Blending azine-BZ with different weight ratios of zinc perchlorate [Zn(ClO₄)₂] to form benzoxazine/zinc ion complexes not only affected the thermal properties based on thermogravimetric analysis (TGA) due to physical crosslinking through metal-ligand interactions but also expedited the ring-opening polymerization, decreasing the curing temperature from 213 to 184 °C (at 10 wt% Zn²⁺). Based on the fluorescence results, the azine-BZ and azine-BZ/Zn(ClO₄)₂ complexes were non-emissive in a THF solution. Their fluorescence increased gradually upon the addition of water. Interestingly, both the pure azine-BZ and Zn(ClO₄)₂-blended complex still emitted light after thermal curing at 150 °C, as determined through photoluminescence measurements, indicating that the azine group could act as a probe of the curing behavior of the benzoxazine monomer, as well as a fluorescent chemosensor for Zn²⁺ and, possibly, other transition metal ions through a metal-ligand charge transfer mechanism.