New field for inorganics and polymer composite fabrication with modulate optical band gaps based on green synthesized metal complex using low cost ligands available in drinking tea dyes

Green synthesis routes for the design of functional polymer composites are increasingly important for sustainable optoelectronic applications. In this work, a chromium-based metal complex (CrMC) was synthesized using polyphenolic ligands of black tea. The CrMC particles were incorporated into a poly(vinyl alcohol) (PVA) matrix to tailor its optical and electronic properties. The main aim of this study was to develop an eco-friendly polymer composite with a controllable optical band gap using a low-cost, natural ligand source. The CrMC-doped PVA films were fabricated via a solution casting method and the PVA composite films were investigated using Fourier-transform infrared spectroscopy (FTIR), UV–Vis spectroscopy, and field-emission scanning electron microscopy (FESEM), alongside comprehensive optical and dielectric analyses. FTIR results confirmed successful coordination between Cr³⁺ ions, tea-derived ligands, and PVA functional groups, while FESEM images revealed homogeneous dispersion of the metal complex within the polymer matrix. UV–Vis analysis showed a pronounced red shift in the absorption edge with increasing CrMC content, leading to a systematic reduction in the optical band gap from 5.90 eV for pristine PVA to 2.18 eV for PVACr3. Additional optical parameters, including absorption coefficient, skin depth, refractive index, dielectric loss, surface and volume energy loss functions (SELF and VELF), and optical conductivity, demonstrated enhanced photon absorption, increased electronic transitions, and improved charge transport behavior in the doped films. Moreover, CrMC incorporation reduced sheet resistance and thermal emissivity while significantly improving the figure of merit (φ_film). Overall, this study demonstrates a sustainable and effective strategy for engineering low-band-gap polymer composites using naturally derived metal complexes, highlighting their strong potential for optoelectronic, photonic, and energy-efficient coating applications.