Sustainable green-synthesized chitosan/red barberry dye doped polymer with structural disorder and tuning optical band gap for flexible optoelectronics

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The development of sustainable optically active materials from natural polymers is important for emerging flexible optoelectronic, photonic, and functional coating applications; however, the intrinsically wide bandgap and limited optical response of pristine chitosan restrict its direct use in such applications. In this study, chitosan-based composite films were prepared through a green functionalization approach using red barberry fruit extract as a natural dye dopant at different concentrations. X-ray diffraction (XRD) analysis showed that pristine chitosan exhibited a semicrystalline structure, while increasing barberry-dye incorporation progressively reduced the diffraction intensity and broadened the characteristic chitosan peaks, indicating disruption of the original molecular ordering and an increase in the amorphous fraction. Fourier-transform infrared (FTIR) spectroscopy further revealed changes in the O–H/N–H, amide/NH₂, and C–O/C–O–C regions, confirming intermolecular interactions between chitosan functional groups and barberry constituents, mainly through hydrogen bonding and dipole–dipole interactions without formation of new covalent bonds. These structural modifications produced pronounced changes in the optical properties. UV–Visible absorption analysis and the absorption spectrum fitting method revealed reductions in the direct and indirect optical bandgaps from 5.31 to 2.67 eV and from 5.10 to 1.89 eV, respectively, accompanied by an increase in Urbach energy from 0.59 to 0.67 eV. The static refractive index increased from 1.124 to 1.549, while optical modeling indicated an increase in carrier concentration from 0.269 × 10²⁷ to 5.821 × 10²⁷ m⁻³. In addition, the modified films exhibited reduced thermal emissivity and enhanced volume energy-loss-function response. Overall, the concentration-dependent structural and optical modifications demonstrate that green incorporation of red barberry extract can effectively tailor the properties of chitosan films, highlighting their potential for flexible optoelectronic, photonic, and energy-saving coating applications.