Amorphous pyrano[3,2-c]quinoline-semicarbazone thin films switch from uniform to heterogeneous morphology and optical modulation upon copper coordination

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Thermally evaporated pyrano[3,2-c]quinoline semicarbazone (PQMHC) and its Cu(II) complex thin films were systematically investigated to elucidate the effect of metal coordination on their structural, morphological, photoluminescence, and optical properties. X-ray diffraction (XRD) revealed broad halos, confirming the amorphous nature of both films. Field-emission scanning electron microscopy (FE-SEM) showed that the PQMHC film possesses a relatively uniform surface composed of closely packed granular features, whereas Cu(II) incorporation induces a heterogeneous morphology with irregular clusters and elongated structures. Photoluminescence measurements revealed broad visible emission spanning 380-650 nm, with a pronounced red shift toward 520-560 nm upon Cu(II) complexation. Optical measurements showed reduced transmittance and enhanced reflectance following Cu(II) incorporation, accompanied by a decrease in the optical band gap from 2.87 ± 0.02 to 2.57 ± 0.01 eV. Furthermore, Miller's empirical rule was employed to estimate the third-order nonlinear susceptibility and nonlinear refractive index, both of which decreased upon Cu(II) coordination, indicating reduced calculated polarizability. Overall, Cu(II) coordination provides an effective means of tuning the structural, morphological, photoluminescence, and optical characteristics of PQMHC thin films, suggesting their potential for tunable optical and optoelectronic applications.

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