Enhancing Luminescence Efficiency of Solvated Europium(III) via Electronic-Vibrational Decoupling: An Unconventional Aggregation-Induced Emission System.

Abstract

Europium(III) (Eu³⁺) ions are renowned for their exceptional photophysical properties, making them invaluable in applications such as energy-efficient lighting, display technologies, and advanced laser systems. However, transitioning Eu³⁺ from solid-state matrices to solution-based environments typically results in a significant decline in luminescence efficiency due to strong vibrational coupling and dynamic coordination interactions with solvents. These issues have hindered the broader application of rare earth ions in solution-based technologies such as biological imaging probes and optical sensors. Herein, we report an innovative electronic-vibrational decoupling (EVD) strategy aimed at minimizing nonradiative decay pathways in rare earth ions. Through systematic modulation of the solvent environment─including replacing water with <i>N</i>,<i>N</i>-dimethylformamide (DMF), tuning temperature, and employing deuterated solvents─we demonstrate that the photoluminescence quantum yield (Φ_PLQY) of Eu³⁺ solutions can be enhanced dramatically from around 2% in H₂O to over 80% in deuterated DMF. The suppression of nonradiative decay pathways is corroborated by significant increases in emission intensity, prolonged luminescence lifetimes, and marked shifts in the <i>I</i>₆₁₆/<i>I</i>₅₉₁ intensity ratio, an established indicator of coordination symmetry. Furthermore, our study reveals that the unconventional aggregation-induced emission (AIE) phenomenon in Eu³⁺ solvents is governed by EVD rather than by restrictions of the intramolecular motion (RIM) mechanism found in organic systems. This work highlights the interplay between solvent vibrations and rare earth photophysics, establishing a robust framework for developing high-performance, solution-based rare earth luminescent materials.