Underestimated Color Centers: Defects as Useful Reducing Agents in Lanthanide-Activated Luminescent Materials

Achieving Ultra-Broadband Sunlight-Like Emission in Single-Phase Phosphors: The Interplay of Structure and Luminescence

The quest for artificial light sources mimicking sunlight has been a long-standing endeavor, particularly for applications in anticounterfeiting, agriculture, and color hue detection. Conventional sunlight simulators are often cost-prohibitive and bulky. Therefore, the development of a series of single-phase phosphors Ca9LiMg1-xAl2x/3(PO4)7:0.1Eu2+ (x = 0-0.75) with sunlight-like emission represents a welcome step towards compact and economical light source alternatives. The phosphors are obtained by an original heterovalent substitution method and emit a broad spectrum   spanning from violet to deep red. Notably, the phosphor with x = 0.5 exhibits an impressive full width at half-maximum of 330 nm. A synergistic interplay of experimental investigations and theory unveils the mechanism behind sunlight-like emission due to the local structural perturbations introduced by the heterovalent substitution of Al3+ for Mg2+, leading to a varied distribution of Eu2+ within the lattice. Subsequent characterization of a series of organic dyes combining absorption spectroscopy with convolutional neural network analysis convincingly demonstrates the potential of this phosphor in portable photodetection devices. Broad-spectrum light source testing empowers the model to precisely differentiate dye patterns. This points to the phosphor being ideal for mimicking sunlight. Beyond this demonstrated application, the phosphor's utility is envisioned in other relevant domains, including visible light communication and smart agriculture.

Impact of 1,10-Phenanthroline-Induced Intermediate Valence on the Luminesence of Divalent Europium Halides

Starting from EuX₂ (X = Cl, Br, I), we systematically investigated a variety of divalent europium complexes containing bidentate 1,10-phenanthroline (Phen) ligands. Depending on the Eu/Phen ratio, mono-, di-, and polynuclear complexes are formed, with the latter yielding one-dimensional _∞¹[EuBr₂(phen)] chains. Seven new divalent europium complexes, [Eu(phen)₄(H₂O)]Br₂·2MeCN, [Eu(phen)₄]I₂·1.7Tol, [EuBr(phen)₃]₂Br₂·4MeCN, [EuCl₂(phen)₂]₂·2MeCN, [EuBr₂(phen)₂]₂, [EuI₂(phen)₂]₂, and [EuBr₂(phen)]_x, are presented in this work. All species show remarkable optical properties based on a partial electron transfer from the Eu^II center to the Phen ligand. The photophysical characterization is further supported by electrochemistry studies in order to describe the intermediate valence of the Eu center.

Rare Earth Starting Materials and Methodologies for Synthetic Chemistry

The number of rare earth (RE) starting materials used in synthesis is staggering, ranging from simple binary metal-halide salts to borohydrides and "designer reagents" such as alkyl and organoaluminate complexes. This review collates the most important starting materials used in RE synthetic chemistry, including essential information on their preparations and uses in modern synthetic methodologies. The review is divided by starting material category and supporting ligands (i.e., metals as synthetic precursors, halides, borohydrides, nitrogen donors, oxygen donors, triflates, and organometallic reagents), and in each section relevant synthetic methodologies and applications are discussed.

Recent Developments in Lanthanide-Doped Alkaline Earth Aluminate Phosphors with Enhanced and Long-Persistent Luminescence

Lanthanide-activated alkaline earth aluminate phosphors are excellent luminescent materials that are designed to overcome the limitations of conventional sulfide-based phosphors. The increasing research attention on these phosphors over the past decade has led to a drastic improvement in their phosphorescence efficiencies and resulted in a wide variety of phosphorescence colors, which can facilitate applications in various areas. This review article discusses the development of lanthanide-activated alkaline earth aluminate phosphors with a focus on the various synthesis methods, persistent luminescence mechanisms, activator and coactivator effects, and the effects of compositions. Particular attention has been devoted to alkaline earth aluminate phosphors that are extensively used, such as strontium-, calcium-, and barium-based aluminates. The role of lanthanide ions as activators and coactivators in phosphorescence emissions was also emphasized. Finally, we address recent techniques involving nanomaterial engineering that have also produced lanthanide-activated alkaline earth aluminate phosphors with long-persistent luminescence.

Underestimated Color Centers: Defects as Useful Reducing Agents in Lanthanide-Activated Luminescent Materials

Inorganic hosts, such as SrB4 O7 or certain nitrides, intrinsically stabilize Eu2+ even when the dopant is an Eu3+ -based precursor and reducing conditions are not employed in the synthesis. Although this concept is well known in the synthesis of phosphorescent materials, the mechanistic details are scarcely understood. Herein, we demonstrate that trapped charge carriers, such as color centers, can also act as redox partners to stabilize certain oxidation states of activators. Eu-activated CsMgCl3 and CsMgBr3 are used as examples. Upon doping with EuCl3 and in the absence of reducing conditions during the synthesis, dominant cyan or green luminescence from Eu2+ ions was observed. Photoluminescence spectroscopy at 10 K revealed that the reduction is correlated to color centers localized at defects. Although defects are typically undesired in phosphors, we have shown that their role may be underestimated and they could be used on purpose in the preparation of selected inorganic phosphors.