Green synthesis of zinc metal complex incorporated PVA composite films with tunable optical band gap for photonic applications

This study presents a rapid and eco-friendly strategy for synthesizing zinc (Zn²⁺) complexes using polyphenol-rich black tea extract (BTS) as a natural chelating agent and their incorporation into polyvinyl alcohol (PVA) to develop tunable optical composite films. FESEM analysis reveals that the incorporation of Zn–metal complexes disrupt the fibrillar morphology of semi-crystalline PVA, indicating significant microstructural modification. FTIR results confirm the coordination interaction between Zn²⁺ ions and polyphenol ligands, as well as strong interfacial bonding within the polymer matrix. The introduction of these green-synthesized complexes leads to a pronounced enhancement in optical properties, with the optical absorption edge decreasing from 6.24 eV to 1.21 eV. This behavior is associated with the formation of localized electronic states and increased charge carrier density, as supported by refractive index and dielectric analyses. Optical parameters, including oscillator and dispersion energies, were evaluated using the Wemple–DiDomenico model, while band-gap characteristics were determined using Tauc analysis. Overall, this work demonstrates a sustainable coordination-driven approach for engineering polymer composites with tunable optical properties, offering strong potential for photonic and optoelectronic applications.