Rare Earth Complexes with 5d-4f Transition: New Emitters in Organic Light-Emitting Diodes

Microwave Synthesis and Luminescence Efficiencies in Mixed-Ligand Europium Complexes

The microwave-assisted methodology is now extended and fine-tuned for the synthesis of mixed-ligand europium complexes with an average reaction time of 12 min. Overall, 14 different complexes were synthesized to improve luminescence using our previously proposed strategy to boost luminescence through ligand diversification, specifically by applying it to quaternary europium complexes with at least one DBM (1,3-diphenylpropane-1,3-dionate) ligand. DBM is a strong absorbant of UV radiation that can dissipate energy through nonradiative channels; thus, it is a useful molecular scaffold for sunblockers and cosmetics. Accordingly, the following luminescent tetrakis and quaternary complexes were prepared: K[Eu(DBM)4], K[Eu(β)4], K[Eu(DBM)3(β)], K[Eu(DBM)2(β)2], K[Eu(DBM)2(β)(β')], and the fully mixed complex K[Eu(DBM)(BTFA)(TTA)(HFAC)], where β can be either BTFA (4,4,4-trifluoro-1-phenylbutane-1,3-dionate), TTA (4,4,4-trifluoro-1-(2-thienyl)butane-1,3-dionate), or HFAC (1,1,1,5,5,5-hexafluoropentane-2,4-dionate). For all the complexes, luminescence experiments were performed in chloroform and acetone solutions. Our findings confirm that mixed-ligand complexes exhibit superior quantum efficiencies compared to the average of their homoleptic counterparts. The presence of DBM in the complexes tends to dramatically increase the nonradiative decay rates of the solutions. Finally, we present formulae that provide a detailed understanding of the distinctive roles of each ligand and their relevant interactions in luminescence.

Ligand Effects on the Emission Characteristics of Molecular Eu(II) Luminescence Thermometers

Discrete molecular organometallic europium(II) complexes are promising functional materials due to their ability to behave as highly sensitive band-shift luminescence thermometers. Furthering our understanding of the design principles salient to the emission behavior of such systems is important for developing them in this emerging application. To this end, a series of pseudo-C4v-symmetric organometallic europium(II) complexes bearing systematically varying ligand sets were synthesized and characterized to probe the influence of subtle structural modification on their optical properties. Opto-structural correlation analyses via variable-temperature single-crystal X-ray diffraction and photoluminescence spectroscopy reveal a remarkable variability in properties among structurally similar complexes and a convoluted dependence of the emission characteristics on the stereoelectronic properties of the ligands. A few factors of particular influence are nevertheless identified, including the distance between the europium(II) ion and the basal plane of the square-pyramidal coordination polyhedron, the presence of pendant electron density that might further interact with the excited-state 5d orbitals, and, qualitatively, the metal-ligand flexibility of the construct. These results help to elucidate principles that govern the luminescence properties of organometallic europium(II) complexes with an eye to enabling the rational design of high-performance luminescence thermometers of this genre.

Enhanced luminescence of Eu3+ in LaAl2B4O10 via energy transfer from Dy3+ doping

In this study, an investigation was conducted on the structural and photoluminescence (PL) characteristics of LaAl2B4O10 (LAB) phosphors initially incorporated with Dy3+ and Eu3+ ions. Subsequently, the impact of varying Eu3+ concentration while maintaining a constant Dy3+ concentration was examined. Structural characterization was performed using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and energy-dispersive X-ray spectroscopy (EDS). XRD analysis confirmed the effective embedding of both dopants into the hexagonal framework of the LAB. The PL emission spectra revealed characteristic emissions of Dy3+ (blue and yellow) and Eu3+ (red) ions. The optimized dopant concentrations of both Dy3+ and Eu3+ were observed to be 3 wt%. The dominant mechanism for concentration quenching in doped LAB phosphors was determined to be the electric dipole-dipole interaction. Co-doping with Eu3+ led to a substantial decrease in Dy3+ emission intensity (∼0.18-fold) while enhancing Eu3+ emission intensity (∼3.72-fold). The critical energy transfer distance (RC = 11.64 Å) and the analysis based on the Dexter theory confirmed that the energy transfer mechanism corresponds to dipole-dipole interaction. The color purities and correlated color temperatures (CCT) were estimated, suggesting the potential of these phosphors for warm white and red lighting applications, respectively. The observed energy transfer and luminescence properties, along with the structural and compositional characterization, highlight the promising potential of LAB:Dy3+/Eu3+ co-doped phosphors for advanced lighting and display technologies.

Fast and Robust Modeling of Lanthanide and Actinide Complexes, Biomolecules, and Molecular Crystals with the Extended GFN-FF Model

Lanthanides (Ln) and actinides (An) have recently become important tools in biomedical and materials science. However, the development of computational methods able to describe such elements in various environments has not kept up with the pace of the field. Addressing this challenge, this work introduces and showcases an extension of the GFN-FF to An alongside a reparameterization for Ln. This development fills a gap for fast computational methods that are out-of-the-box applicable to large f-element-containing systems with thousands of atoms. We discuss the reparameterization of the charge model and the covalent topology setup and showcase the model through various applications: Molecular dynamics simulations, optimization of Ln-containing biomolecules, and optimizations of several periodic structures. With the presented improvements, GFN-FF is a powerful method that routinely delivers robust and accurate geometries for large Ln/An systems with thousands of atoms.

[Nd(NTA)2·H2O]3- complex with high-efficiency emission in NIR region

The distinctive photophysical characteristics possessed by lanthanides, including europium, neodymium, and ytterbium, render them adaptable molecular tools for studying biological systems. Specifically, their enduring photoluminescence, precise emission spectra, and significant Stokes shifts allow for experiments not achievable with organic fluorophores or fluorescent proteins. Moreover, the capacity of these metal ions for luminescence resonance energy transfer and photon upconversion extends the potential applications of lanthanide probes even further. In this research, a new [Nd(NTA)2·H2O]3- complex was synthesized and its optical properties were assessed using practical characterization techniques such as UV-Vis absorption, photoluminescence, and FTIR. It was discovered that when the sample was excited by a 357 nm wavelength, it emitted a strong line at 1076 nm with a full-width at half maximum (FWHM) of 10 nm, a phenomenon not previously documented. The Judd-Ofelt theory and its intensity parameters were utilized in a theoretical approach to determine the fluorescence branching ratio and the radiative lifetime of the [Nd(NTA)2·H2O]3- complex. The absorption and luminescence spectra were then analyzed accordingly. Experimental findings validated the potential applications of the prepared sample in bioimaging.

Electrohydrodynamically Printed d-f Transition Cerium(III) Complex

The d-f transition rare earth complexes have recently emerged as a promising candidate for display applications due to the parity-allowed transition, high photoluminescence quantum yield (PLQY), short excited lifetime, and tunable emissions. Besides, inkjet printing has been regarded as an important technique for realizing full-color display. However, inkjet-printed d-f transition rare earth complexes have not been investigated. Herein, for the first time, we explored d-f transition cerium(III) complex 2-Me as the luminescent material by inkjet printing. With 1,2-dichlorobenzene as solvent and polystyrene as an additive, 2-Me film exhibits a similar emission peak and excited-state lifetime with 2-Me powder and a high PLQY of 45%, demonstrating the excellent stability of 2-Me ink. Finally, we suppressed the coffee ring effect and prepared the first inkjet-printed pattern ''HUST'' composed of d-f transition rare earth complex ink with uniform blue fluorescence. Our pioneering work provides a promising alternative for inkjet printing inks.

Air- and Water-Stable Heteroleptic Copper (I) Complexes Bearing Bis(indazol-1-yl)methane Ligands: Synthesis, Characterisation, and Computational Studies

A series of four novel heteroleptic Cu(I) complexes, bearing bis(1H-indazol-1-yl)methane analogues as N,N ligands and DPEPhos as the P,P ligand, were synthesised in high yields under mild conditions and characterised by spectroscopic and spectrometric techniques. In addition, the position of the carboxymethyl substituent in the complexes and its effect on the electrochemical and photophysical behaviour was evaluated. As expected, the homoleptic copper (I) complexes with the N,N ligands showed air instability. In contrast, the obtained heteroleptic complexes were air- and water-stable in solid and solution. All complexes displayed green-yellow luminescence in CH2Cl2 at room temperature due to ligand-centred (LC) phosphorescence in the case of the Cu(I) complex with an unsubstituted N,N ligand and metal-to-ligand charge transfer (MLCT) phosphorescence for the carboxymethyl-substituted complexes. Interestingly, proper substitution of the bis(1H-indazol-1-yl)methane ligand enabled the achievement of a remarkable luminescent yield (2.5%) in solution, showcasing the great potential of this novel class of copper(I) complexes for potential applications in luminescent devices and/or photocatalysis.

Electrospinning Preparation, Structure, and Properties of Fe3O4/Tb(acac)3phen/Polystyrene Bifunctional Microfibers

Compared to single functional materials, multifunctional materials with magnetism and luminescence are more attractive and promising; Thus, it has become an important subject. In our work, bifunctional Fe₃O₄/Tb(acac)₃phen/polystyrene) microfibers with magnetic and luminescent properties (acac: acetylacetone, phen: 1,10-phenanthroline) were synthesized by simple electrospinning process. The doping of Fe₃O₄ and Tb(acac)₃phen made the fiber diameter larger. The surface of pure polystyrene microfibers and microfibers doped only with Fe₃O₄ nanoparticles were chapped similar to bark, whereas the surface of the microfibers was smoother after doping with Tb(acac)₃phen complexes. The luminescent properties of the composite microfibers were systematically studied in contrast to pure Tb(acac)₃phen complexes, including excitation and emission spectra, fluorescence dynamics, and the temperature dependence of intensity. Compared with the pure complexes, the thermal activation energy and thermal stability of composite microfiber was significantly improved, and the luminescence of the unit mass of Tb(acac)₃phen complexes in composite microfibers was stronger than that in pure Tb(acac)₃phen complexes. The magnetic properties of the composite microfibers were also investigated using hysteresis loops, and an interesting experimental phenomenon was found that the saturation magnetization of the composite microfibers gradually increased with the increase in the doping proportion of terbium complexes.

Vacuum-deposited Rb3CeI6 for deep-blue-light-emitting diodes

Recently, perovskite light-emitting diodes (PeLEDs) have exhibited outstanding performance in next-generation high-definition display applications. However, compared with green and red PeLEDs, the development of efficient and stable blue PeLEDs to meet the requirement for a wide color gamut has been a challenge. Herein, we vacuum thermally deposited a film of the lead-free rare earth halide Rb₃CeI₆, which shows deep blue emission with peaks at 427 nm and 468 nm. Due to the parity-allowed 5d-4f transition of Ce(III), the excited-state lifetime is as short as 22.3 ns (427 nm) and 25 ns (468 nm), respectively. The photoluminescence quantum yield (PLQY) is optimized to 51% by regulating the nucleation and growth of Rb₃CeI₆ grains. In a prototype rare earth light-emitting diode (ReLED) device, a thin insulating Al₂O₃ layer (5 nm) is inserted between the electron transport layer (ETL) and the emitting layer (EML, Rb₃CeI₆) to balance the carriers and reduce the dark current. The device shows a maximum luminance and EQE of 98 cd m^-2 and 0.67%, respectively, and the electroluminescence (EL) spectrum maintains stability with changes in the operating voltage. In addition, the corresponding CIE coordinate is (0.15, 0.06), which closely matches the Rec. 2020 standard (0.131, 0.046).

Rare Earth Complexes of Europium(II) and Substituted Bis(pyrazolyl)borates with High Photoluminescence Efficiency

Rare earth europium(II) complexes based on d-f transition luminescence have characteristics of broad emission spectra, tunable emission colors and short excited state lifetimes, showing great potential in display, lighting and other fields. In this work, four complexes of Eu(II) and bis(pyrazolyl)borate ligands, where pyrazolyl stands for pyrazolyl, 3-methylpyrazolyl, 3,5-dimethylpyrazolyl or 3-trifluoromethylpyrazole, were designed and synthesized. Due to the varied steric hindrance of the ligands, different numbers of solvent molecules (tetrahydrofuran) are participated to saturate the coordination structure. These complexes showed blue-green to yellow emissions with maximum wavelength in the range of 490-560 nm, and short excited state lifetimes of 30-540 ns. Among them, the highest photoluminescence quantum yield can reach 100%. In addition, when the complexes were heated under vacuum or nitrogen atmosphere, they finally transformed into the complexes of Eu(II) and corresponding tri(pyrazolyl)borate ligands and sublimated away.

Special Issue: Rare earth luminescent materials

This special issue covers a series of cutting-edge works on exploring novel rare earth luminescent materials and their applications in lighting, display, information storage, sensing, and bioimaging as well as therapy. [Image: see text]

Inorganic Lanthanide Compounds with f-d Transition: From Materials to Electroluminescence Devices

With the rapid development of the panel display market, demand for efficient light emitters as active layers in electroluminescence (EL) devices has significantly increased. Luminescent inorganic lanthanide compounds (ILCs) with a characteristic f-d transition are particularly preferred for EL devices because of their high photoluminescent quantum yield, short excited-state lifetime, tunable emission spectra, and high thermal stability. In this Perspective, we first present an overview of inorganic lanthanide compounds with an emphasis on the mechanisms and characteristics of f-d emission. Then, the comprehensive advances of lanthanide element-doped inorganic compounds for EL study in recent decades are summarized. Moreover, the recent progress in directly employing ILCs for EL applications and rational improvement strategies in EL performance are highlighted. Last, we summarize the current challenges and opportunities of ILC-based EL devices as well as future improvement directions.