Narrow band red emitting europium complexes and their application in smart white LEDs and vapoluminescent sensors

Abstract

Narrow band red emitting phosphors play a vital role in high performance smart white LEDs. In this context, a series of new Eu(III) complexes have been synthesized with neutral ligands (C1-functionalised phenanthro-imidazole-based ancillary ligands substituted with phenyl moieties (m-CF₃, p-CF₃, p-CH₃ and 1-naphthyl and 2-naphthyl group)) and dibenzoylmethane (DBM) as an anionic ligand. All the newly synthesized Eu-complexes showed extremely narrow band red emission due to the electric dipole transition (⁵D₀–⁷F₂, both in solid and thin film) with high quantum efficiency. A combined experimental and theoretical study indicates that the energy transfer from the ligand to the metal ion is complete. A temperature dependent photoluminescence (PL) study in solution reveals that the red emission is retained (only the ED (⁵D₀–⁷F₂) intensity decreases with increasing temperature). Red LEDs and hybrid white LEDs were fabricated by using a near UV LED chip with europium complexes and a near UV/blue LED conjugated with a yellow organic dye and europium complex mixture, respectively. Hybrid white LEDs (Eu(DBM)₃Phen-pCN-pCF₃ + yellow organic dye) showed superior CRI (84%) and CIE (x = 0.36, y = 0.39) values (near UV based) and CRI (82%) and CIE (x = 0.34, y = 0.36) values (blue LED based). In addition, the studied Eu-complexes herein showed excellent reversible on–off–on luminescence behavior with exposure to acid–base vapours. Detailed spectroscopic investigation reveals that the protonation of the Eu-complexes (ligands) plays a key role in the on–off–on luminescence.