Exploring the effect of fluorine radicals as substituents in β-diketones on the luminescence properties of Eu3+ complexes

Abstract

For decades, researchers have utilized fluorination of ligand molecules in trivalent lanthanide ion (Ln³⁺) complexes as the chemical approach to suppress non-radiative relaxation processes. In this study, we provide a comprehensive analysis of the photophysical properties of europium Eu³⁺ acetylacetonate complexes bearing methyl (CH₃) and trifluoromethyl (CF₃) substituents. Our findings reveal that fluorination of the β-diketone ligand exerts a multifaceted influence on its photophysical properties. Sequential replacement of CH₃ with CF₃ lowers the energies of both singlet and triplet excited states, thereby enhancing energy transfer efficiency from the ligand to the metal ion. Moreover, the replacement of CH₃ with CF₃ fundamentally alters the nature of the charge transfer state and significantly suppresses non-radiative vibrational relaxation. It leads to a remarkable increase in both intrinsic and overall luminescence quantum yield by an order of magnitude. This study establishes a foundation for designing highly luminescent Ln³⁺ complexes by unveiling previously unexplored characteristics of charge transfer states.