Replacing a nitrogen heteroatom in a molecular Eu complex with a phenyl moiety facilitated sunlight-like emissions for sustainable white LEDs and non-contact thermometers

Abstract

To enhance the quality of white light (mimicking sunlight-like emission) for solid-state lighting (SSL) applications, a novel imidazolyl-bipyridyl ligand was designed as an ancillary ligand to achieve multi-coloured emissions in a europium(III) complex. Structural engineering in the case of the ligand was crafted by replacing a nitrogen heteroatom with a phenyl moiety to shift the broad green emission (λ = 553 nm) into a narrow blue emission (λ = 460 nm). This blue emission of ligand, when combined with europium(III) using thenoyltrifluoroacetone (TTA) and dibenzoylmethane (DBM) as anionic ligands, helped to achieve white light emissions on excitation with a near-UV LED (λ_ex = 390 nm), which had a great advantage of obtaining a white colour emission without any blue LED (λ_ex = 460 nm). Density functional theory (DFT) calculations were performed to theoretically investigate the energy transfer process from the ligand to the emitting level (⁵D₀) of Eu³⁺ in Eu complexes. These europium(III) complexes exhibited a unique dual-emissive behaviour (multi-coloured emissions), including white emissions across various solvents. The Eu(III) complexes have shown excellent properties in a white LED, with Commission Internationale de l’Éclairage (CIE) coordinates (x = 0.30, y = 0.34), and serve as a functional luminescent thermometer, displaying a colour change with the increase in temperature.