White Light Emissive Eu(III) Complexes through Ligand Engineering and their Applications in Cool Near Ultraviolet White Light Emitting Diodes and Thermometer.

ABSTRACT

In pursuit of enhancing white light quality for solid-state lighting (SSL) applications, an attempt has been made to design novel imidazo-bipyridyl ligands as an ancillary ligand to obtain multiple emissions (mimic sunlight) in the Eu-complex. By strategically modifying the phenanthroline core with imidazo-bipyridyl incorporation with 1 or 2-Napthyl groups at the C1 position, the excitation spectral line is successfully shifted from Ultraviolet (UV) to near UV/visible spectrum (where the LED emission occurs). The ligands showed greenish blue emission in solid and solution. Density Functional Theory (DFT) calculations were utilized to understand the energy transfer processes from ligand to Eu ion in the Eu complexes. The analysis revealed that the energy transfer is incomplete, primarily attributed to the proximity of triplet state energy levels to the resonance level of Eu(III) ions as reflected in solvatochromism. These complexes exhibit a unique dual emissive behavior (emitting multi-color) including white light across various solvents. These complexes hold great promise as single-component white light-emissive materials, with potential applications in white light-emitting diodes (WLED). The fabricated white LED showed an excellent color rendering index (CRI ~93 %). Beyond lighting, this distinctive property opens avenues for temperature sensing ([Eu(DBM)31-Naph] shows the highest sensitivity of Sr=10.97 %, and [Eu(DBM)32-Naph] shows the highest sensitivity of Sr=5.5 % at 333 K) and vapoluminescent (acid-base on-off-on luminescence) studies. This research pioneers the development of these complexes as potential single-component materials for superior white LEDs, underlining their multifaceted utility in cutting-edge lighting and sensing technologies.