Engineering Ligand Spatial Arrangement Enabled Broad-Spectrum White Light Emission mimicking sunlight from Donor-Acceptor Europium(III) Complexes

Abstract

Commercial white LEDs emit high blue light, which contributes to their high brightness, but has detrimental effects on the human body. The development of sunlight-mimicking white-light-emitting single-molecule europium (Eu) complexes represents a significant advancement in the field of solid-state lighting, offering the potential for high CRI and exceptional LER values in white light-emitting diodes (WLEDs). However, synthesis of such complexes remains a formidable challenge due to the inherent difficulty in achieving balanced emissions from a single molecular entity across the visible spectrum. In this investigation, we have embarked on a strategic design and synthesis of novel triphenylamine-phenanthroimidazole based ancillary ligands, constructed upon a donor-acceptor (D-A) architecture. These ligands are meticulously engineered by incorporating a phenyl or substituted phenyl spacer to facilitate unrestricted and restricted electronic conjugation and fine-tune the dynamics of energy transfer between the Lanthanide ion and the organic antenna. The ligands displayed greenish-blue emission characteristics in both solution as well as solid state. Leveraging computational modelling, we have demonstrated that the energy levels in the excited states of these organic antennas are deliberately mismatched with the characteristic emissive level of the Europium ion. This strategic mismatch is hypothesized to create multiple energy transfer pathways, potentially leading to the simultaneous population of 5D0 energy level of Eu(III), thus enabling the desired broadband white light emission. These complexes show dual emission behaviour and were used to fabricate white LEDs with CIE color coordinates (0.31, 0.34) and (0.26, 0.28), with good CRI values of 87% and 89% for the complexes EU1 and Eu2, respectively.