Enhancing flame retardancy and optical functionality in multifunctional devices through advanced design of phosphorus-containing copoly(imide-oxadiazole)s

The combination of flame retardance and optical functionality in phosphorus-containing copoly(imide-oxadiazole)s offers a remarkable opportunity for the development of advanced and multifunctional devices, including organic light-emitting diodes, sensors, and photovoltaics. In this context, a series of phosphorus-containing copoly(imide-oxadiazole)s were synthesized through a polycondensation reaction. This involved an aromatic commercial dianhydride, 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride), a diamine with phosphorus in the main and side chains, and various aromatic diamines with 1,3,4-oxadiazole rings. To achieve this, a solution imidization method was employed, which effectively converted the poly(amic acid) intermediates into the corresponding polyimides with high efficiency. The chemical structure and thermal properties of the resulting polymers were investigated. All polymers exhibited high thermal stability. Moreover, the absorption bands observed in dichloromethane, N,N-dimethylformamide, methanol, and N-methyl-2-pyrrolidone exhibited weak intensities, appearing as shoulders, with maximum values around 270, 303, and 310 nm. In most solvents, the photoluminescence spectra of the polymers showed a single emission band. However, one of the polymers exhibited two emission bands due to pendant groups that disrupted the molecular structure. The fluorescence of these derivatives was influenced by the polarity and proticity of the solvents. Furthermore, they exhibited large Stokes shifts, indicating extended conjugation and the potential to maximize luminescence efficiency. Protonation and deprotonation of polyimides alter their optical properties. These findings provide insights for the design of materials with improved optical performance.