Unveiling the Versatility of Coumarin-Integrated with\nPhenanthroline/Thiabendazole-Based Eu<sup>III</sup> Complexes for\nSmart LEDs, Vapoluminescence, and Bioimaging Applications

Narrow\nband red-emitting phosphors based on organo-Eu(III) complexes\nprove their energetic features with surprising performance in smart\nred/white LEDs, sensing, and biological fields. In this report, a\nseries of unique Eu(III) complexes have been synthesized with coumarin\nintegrated with a class of phenanthroline(Phen)/thiabendazole(TBZ)\nbased ancillary ligands and dibenzoyl methane (DBM)/2-theonyl trifluoroacetone\n(TTA) as an anionic ligand. The computational study reveals that the\nTBZ/Phen-based neutral ligands are superior energy harvesters to those\nother reported analogue neutral ligands. All the Eu-complexes demonstrated\noutstanding red emission due to electric dipole (ED) transition (<sup>5</sup>D<sub>0</sub> → <sup>7</sup>F<sub>2</sub>) in solid,\nsolution, and thin film with high quantum yield (QY). Theoretical\nanalysis (TD-DFT) and experimental findings describe that the energy\ntransfer (ET) from the ligand’s triplet level to the Eu(III)\nion is completely occurring. The Eu(III) complexes can potentially\nbe used to fabricate intense hybrid white and red LEDs. All of the\nfabricated red LEDs revealed high luminous efficiency of radiation\n(LER) values. The fabricated blue LED based hybrid white LEDs displayed\nremarkable performance with a low correlated color temperature (5634\nK), high color rendering index 88%, and CIE values (<i>x</i> = 0.33; <i>y</i> = 0.342) for 3Eu. By interaction with\nacid–base vapors, Eu-complexes displayed effectively alterable\non–off–on luminescence. Further, cellular imaging shows\nthat Eu-complexes can be a potential biomarker for cancer cell lines.