Synthesis and Photoluminescence Properties of Quinoline-Derived Thiosemicarbazone Lanthanide Complexes and Their Polymer-Embedded Stability

Thiosemicarbazone QMPC, (quinolin-2-ylmethylene)pyrrolidine-1-carbothiohydrazide, and its lanthanide complexes (Ln = Nd3+, Gd3+, Yb3+) were synthesized and fully characterized. 1H and 13C NMR spectra showed resonances in line with the suggested structural framework of the ligand. The FT-IR spectral analysis distinctly verified the presence of characteristic −C═N and −N–C═S functional groups, confirming the successful coordination of the ligand to the metal center. The precise molecular makeup of the synthesized lanthanide complexes is strongly supported by the excellent correlation between the calculated and experimentally observed m/z values in the HRMS spectra. Furthermore, the EPR spectral feature characterized by a g∥ > g⊥ revealed a distorted coordination environment around the lanthanide ion, highlighting the predominant coordinate nature of the metal–ligand bonding and the structural asymmetry within the complex. Photoluminescence studies revealed ligand-centered emissions along with characteristic lanthanide transitions, such as 4F3/2 → 4IJ (J = 9/2, 11/2, 13/2) for Nd3+, 6P7/2 → 8S7/2 for Gd3+, and 2F5/2 → 2F7/2 for Yb3+. Luminescence decay analyses showed that the QMPC–Yb3+ complex possessed the longest average lifetime (6.55 ns), while in a PMMA matrix, the QMPC–Gd3+ composite achieved the highest lifetime value of 5.27 ns. The enhanced luminescence on coordination and sustainability of the same along with thermal stability in polymer matrix validates its application in optoelectronic and photonic devices. The DFT and TD-DFT calculations supported the experimental observations and provided insights into the excited-state properties.