Photostability of luminescent europium(III) hexafluoroacetylacetonates: Combined theoretical and experimental study

Abstract

Photostability is a critical problem during development of luminescent lanthanide matherials. In this work, four europium hexafluoroacetylacetonates has been investigated. The photostability of the compounds increased in the following sequence: K[Eu(Hfaa)4] – Rb[Eu(Hfaa)4] – Cs[Eu(Hfaa)4] – Eu(Hfaa)3∙2TPPO (Hfaa – hexafluoroacetylacetonate-anion, TPPO – triphenylphosphine oxide). Molecules in ground and excited states have been studied using the DFT/TDDFT/PBE0 method. Natural bond orbital (NBO) analysis has been carried out to understand the nature of different interactions responsible for the electron delocalization and the intramolecular charge transfer between the orbitals of ligands and Eu(III) ions. Quantum chemistry modeling has explained the reason of unique behavior of one of the coordinated Hfaa ligands, which noticeably distances from a central atom during photoexcitation. This ligand, on which HOMO is localized, has appeared to be the most photoreactive fragment of the complex and directly affect the photostability of the studied series of compounds. The maximum photostability in a number of compounds has been observed for a complex that has a TPPO ligand in its composition, which provides decreasing of HOMO's antibonding character during photoexcitation, unlike other complexes, in which this ligand is absent and the antibonding character HOMO rises. The results obtained can be used in the development of new promising optical materials with increased photostability.