Sustainable methylcellulose composites with enhanced amorphous phase and giant optical bandgap reduction via rosemary-derived green copper complexes

Abstract Tailor-made biopolymer composites with high-performance and sustainable optoelectronic properties are crucial towards the next generation of flexible electronics. In the current study, an eco-friendly Cu(II)–rosemary organometallic complex was prepared using thermal aqueous extraction of the bioactive polyphenols present in rosemary (Rosmarinus officinalis) leaves and embedded in a matrix of the host polymer methylcellulose (MC) through a solution-casting technique. Coordination of Cu2 + ions with polyphenolic functional groups was supported by infrared Fourier transform (FTIR) spectroscopy and the strong intermolecular hydrogen bond interactions in the MC matrix were also demonstrated. The crystallinity to the predominant amorphous phase was confirmed by X-ray diffraction (XRD). The optical band gap of pure MC was measured by spectroscopic analysis to be 6.13 eV, which reduced to 0.76 eV at the maximum loading of the complex. This is due to the ligand to metal charge transfer (LMCT), localised d–d transitions and higher structural disorder (increased Urbach energy, from 0.74 eV to 1.63 eV). For the same material, the modelling by Wemple–DiDomenico and Drude–Lorentz resulted in a considerable rise of the static refractive index (1.14–2.38), high-frequency dielectric constant (1.35–7.99), and free-carrier concentration (1.03 × 10 26 m − 3 to 6.88 × 10 27 m − 3 ), with a low optical resistivity of 0.00082 Ω⋅m. Importantly, the films showed an enormous increase in the third-order NLO susceptibility (χ (3) from 6.39 × 10 − 17 to 3.17 × 10 − 12 ) and NLO refractive index (n 2 from 3.67 × 10 − 13 to 7.42 × 10 − 11 ). The results have shown that the MC/Cu(II)–rosemary composite membranes are very efficient, environmentally friendly and can be used for creating flexible photonic devices, UV shielding and all-optical switching devices.