A step forward in unraveling the lanthanide discrimination puzzle: structure–selectivity relationship based on phenanthroline diimide ligands towards europium and terbium detection in water

Abstract

The substantial expansion of technology and geopolitical needs necessitate the search for innovation and alternatives in the supply and recycling of lanthanides. As a prerequisite, lanthanide sensing proves invaluable for identifying lanthanides and locating high-value streams before resorting to laborious extraction processes. Equally important, this selective sensing can potentially contribute to environmental management and optimization of extraction processes. Presently, there are no systematic guidelines for designing highly selective sensing and extraction ligands. In this contribution, we present the successful demonstration of two isomeric phenanthroline diimides capable of selectively sensing europium(III) and terbium(III) in aqueous solutions. The limits of detection (LOD) for both cations were measured to be 15 nM and 2.95 μM, respectively, with superior anti-inference properties with other lanthanides. Data fitting from ratiometric titrations revealed distinct solution coordination species evolutions even though coordination sites were identical for the two isomerides. Close examination of photophysical and electrochemical characteristics, together with DFT calculations, revealed that subtle structural modifications influenced ligand solubilities (intermolecular interaction), the nature of emission states, and molecular energy levels. These ligand structural modifications ultimately led to selective sensing of Eu(III) and Tb(III) in aqueous media. We believe that this work contributes to a fundamental understanding of the selective photoluminescent sensing of rare earth elements (REEs) and represents the first investigation into Eu(III)/Tb(III) sensing and discrimination in water.