White light-emissive europium complexes and their versatile applications

Abstract

Dual-emissive single-component white light-emissive luminogens are promising color convertors for white light-emitting diodes. In this context, a new class of bipolar phenanthroimidazole-based (N1 functionalization with Ph, mCF₃, and pCF₃) ancillary ligands and their efficient β-diketonate Eu^III complexes have been successfully designed, synthesized, and characterized. Their photophysical and electrochemical properties have also been investigated and their versatile applications explored in white LEDs and temperature sensors. All ligands showed ultra-deep-blue emission with CIE value y < 0.1. Solvent-dependent photoluminescence studies of Eu-complexes revealed extremely pure white emission (confirmed CIE color coordinates x = 0.33, y = 0.33). In contrast, Eu-complexes showed pure red emission (due to electric dipole transitions) with CIE value x = 0.66, y = 0.33. The observed tunable emission indicates that the ligand-to-Eu ion energy transfer is partial, and the energy transfer mechanism for all Eu-complexes was proposed based on combined experimental and theoretical studies (DFT, TD-DFT). The absolute quantum yield of the mCF₃-based Eu-complex was 75.9%. The red LEDs were fabricated with Eu-complexes conjugated with a near-UV LED and the fabricated LED showed superior performance (among all, the 2Eu-based LED showed a luminous efficiency of radiation (LER) value of 264 lm W⁻¹). In addition, presently studied Eu-complexes can be explored for ratiometric temperature applications based on dual emissive characteristics, as well as temperature-dependent PL studies.