Investigation progresses of rare earth complexes as emitters or sensitizers in organic light-emitting diodes

Near-Infrared Ytterbium Complexes Based on Polycyclic Aromatic Dicarboxylate Ligands and the Solution-Processed NIR OLED with Irradiance up to 110,284 μW/m2

Since the O-H and N-H oscillators of solvent molecules attached to ytterbium ion (Yb3+) and C-H oscillators existing in the inner coordination sphere of Yb3+ would quench the excited energy of Yb(III), which leads to low quantum yields (QYs) of Yb(III) complexes, we aimed to design a ligand that could block solvent molecules and C-H oscillators out of the first coordination sphere of Yb3+. Herein, a series of novel polycyclic aromatic dicarboxylate ligands are designed and synthesized to effectively protect Yb3+ from solvent molecules and efficiently sensitize Yb3+ luminescence, while the cost and sophistication of the synthesis are satisfactory. Therein, [Yb(MO-DPyPDA)2](DIEA) exhibited a considerable QY of 5.20% and a long luminescent lifetime of 102 μs in CD3OD. The single-crystal structure demonstrates that there are no solvent molecules and C-H oscillators existing in the inner coordination sphere of Yb3+, which is conducive to alleviating the quenching effect. Meanwhile, we also carried out experiments to verify that it was thermodynamically feasible for ligands to sensitize the luminescence of center ion through internal redox processes. Moreover, several groups of near-infrared organic light-emitting diodes based on [Yb(DTFM-DPyPDA)2](DIEA) were fabricated based on the solution-processing method, and the highest irradiance of 110,284 μW/m2 was realized by optimizing the device structure.

Unveiling mechanistic insights and applications of aggregation-enhanced emission (AEE)-active polynuclear transition metal complexes

Aggregation-enhanced emission (AEE) in polynuclear transition metal complexes (PTMCs) represents a major advancement in luminescent materials, overcoming the limitations of aggregation-caused quenching (ACQ) in traditional systems. Unlike conventional materials that suffer from quenching, AEE-active PTMCs exhibit enhanced luminescence in the aggregated state, driven by mechanisms such as restricted molecular motion, π-π stacking, and metal-metal interactions. These properties make PTMCs highly versatile for applications including chemical sensing, bioimaging, photodynamic therapy (PDT), optoelectronics (e.g., OLEDs, WOLEDs, and LEDs), and security technologies (e.g., anti-counterfeiting inks). They enable the sensitive detection of pollutants, facilitate high-performance bioimaging, and enhance the efficiency of energy devices. However, PTMCs face several challenges, including complex synthesis, limited thermal and photostability, solubility issues, and environmental and toxicity concerns. Additionally, high production costs, instability in different media, and the need for optimized energy transfer efficiency must be addressed to enhance their practical performance. This review explores the mechanisms behind AEE in PTMCs and discusses strategies for overcoming these challenges, including ligand engineering, hybrid material development, and sustainable synthesis methods. It also highlights their potential in advancing energy-efficient technologies, precision therapeutics, and secure communication systems, contributing to a more sustainable and innovative future.

Organic-Inorganic Hybrid Perovskite-Like Indium Chloride with Strong Red Emission

Low-dimensional organic-inorganic hybrid metal halide materials have attracted widespread attention due to their excellent and tunable photoelectric properties. However, the low intrinsic photoluminescence quantum yields (PLQYs) limit their further applications in optoelectronic devices. Here, we report the synthesis of lead-free zero-dimensional hybrid organic-inorganic indium chloride crystals, (FA)3InCl6: xSb3+, with strong red-light emission through controlled Sb3+ doping. The optimal composition, (FA)3InCl6: 20.16% Sb3+, exhibits PLQY up to 30% and emits red broadband light centered at 690 nm. The photoluminescence enhancement of the doped samples was investigated by combining temperature-dependent and wavelength-dependent photoluminescence spectra, revealing the self-trapped exciton (STE) recombination process. The clear elucidation of the self-trapped exciton complexation process has provided a solid theoretical basis for the further optimization of the material properties, which is of great significance for the development of new red light-emitting materials. Far-red light-emitting phosphor-converted LED devices have been constructed with these materials and demonstrate stable and efficient red-light emission at various voltages, exhibiting superior photoluminescence stability. This study highlights the potential of Sb3+-doped metal halides to achieve tunable broadband emission and demonstrates the great potential of these metal halide single crystals for indoor plant lighting, infrared imaging, photodynamic therapy and wound healing.

Fabrication, spectroscopic and photofunctional studies of layered lutetium-dysprosium hydroxides

The spectroscopic studies of layered rare-earth hydroxides (LRHs) have aroused great interests owing to the unique features of combing rare-earth ions with diverse anions. In this work, some anions were incorporated into layered lutetium-dysprosium hydroxides (LLuH:Dy) by the hydrothermal and ion exchange process, in which Dy3+ ion acted as the emission center. The results manifest that the assemblies possess clearly lamellar structure and flake shape, and emit the characteristic yellow and blue light of Dy3+ under ultraviolet excitation. The inorganic WO42- intercalated LLuH:Dy shows enhanced luminescence under the excitation with 280 nm which originates from the O-W charge transfer band. The more fantastic is that the assembly produces bright cyan light due to the appropriate yellow/blue light intensity ratio, and can transform into transparent and flexible film by mixing with certain polymer. Moreover, the intercalation of organic chromophore benzenetetracarboxylic anion into LLuH:Dy also greatly promotes the Dy3+ luminescence, and can easily form the nanosheet colloid through combining with surfactant dodecyl sulfonate. The colloid is very stable at ambient temperature and displays excellent selectivity, sensitivity and accuracy for detecting metal Co2+ in aqueous media, constructing a superior fluorescence sensor for Co2+ with a detection limit of 2.65 × 10-7 mol/L. This work expands the photofunctional performances of LRHs in the form of transparent film and colloidal state.

Enhancement of Sm3+ Luminescence in Sol-Gel Silica Matrix: Effect of Ligands Phenyl Phosphinic Acid and Trioctylphosphine Oxide

Sol-gel silica matrices singly doped with Sm3+ and co-doped with ligands phenyl phosphinic acid (PPIA) and trioctylphosphine oxide (TOPO) were fabricated and studied for their structural and spectroscopic behaviour. Structural studies were done by x-ray diffraction (XRD) and Fourier transform infra-red (FTIR) absorption analysis whereas spectroscopic behaviour was studied by ultraviolet - visible (UV-Vis) absorption, photoluminescence (PL) excitation, emission and time-correlated decay analyses. XRD studies exhibit the amorphous nature of the samples and FTIR studies corroborate the presence of the ligands in the silica matrix. UV-Vis absorption and PL studies show significant enhancement in the radiative parameters in ligand co-doped samples compared to singly doped Sm3+ sample. Judd Ofelt parameters estimated from the absorption spectra possess higher values compared to popular hosts; revealing higher asymmetry and covalency around the active Sm3+ ions. An enhancement in PL intensity by 50.38 times and yield by 52.57 times with the co-doping of the ligands PPIA and TOPO is reported. These enhancement in the radiative output in ligand co-doped samples is attributed to the modification of the silica network with the incorporation of the ligands, energy transfer from ligands to nearby Sm3+ ions as well as due to the shielding of Sm3+ ions from the OH quenchers in the silica matrix. The improved PL behaviour especially corresponding to 4G5∕2→6H9∕2 transitions occurring at 643 nm suggests the potential of PPIA and TOPO co-doped Sm3+ silica matrix for diverge optical applications.

Efficient Organic Light-Emitting Diodes Obtained by Introducing Gadolinium (Gd) Complexes Based on Pyrazolone Derivative Ligands as Hole Trappers

The utilization of lanthanide (Ln) complexes in the realm of organic light-emitting diodes (OLEDs) has garnered extensive interest, particularly in their role as luminescent materials or electron trappers. A series of gadolinium (Gd) complexes with energy levels of high HOMO/LUMO and different triplet state energies were designed and synthesized by introducing substituents with different electronic effects onto the pyrazolone derivative ligands. Subsequently, these complexes were precisely purified by vacuum sublimation and codoped into the light-emitting layer (EML) of the OLEDs. This process was facilitated through the well-matched HOMO/LUMO levels and triplet energies among various functional materials. Consequently, the maximum external quantum efficiencies of blue, red, and green OLEDs were simultaneously enhanced with the ratios of 119%, 28%, and 71%, respectively. This improvement can be credited to the introduction of Gd(III) complex molecules within EMLs, which helps to capture excess holes and improve carriers' balance.

Synthesis and fluorescence properties of europium complex functionalized fiberglass paper

The development of novel rare earth fluorescent materials and the exploration of their applications have consistently been focal points of research in the fields of materials science and chemistry. In this work, a novel rare earth composite material with good photo-fluorescence properties and self-supporting has been prepared via a simple ultrasonic solvent reaction method. Initially, the Phen moieties is immobilized onto the surface of a self-supporting fiberglass paper using ICPTES, followed by the coordination of Eu(TTA)3 moieties with Phen moieties through a convenient ultrasonic solvent reaction. The resulting GF-Phen-Eu(TTA)3 has been characterized using FTIR, UV-Vis DRS, fluorescence measurements, and so on. The results indicate that the composite material exhibits strong fluorescent emission and presents a vivid red color under ultraviolet light. Further research has shown that the fluorescence of GF-Phen-Eu(TTA)3 strips demonstrated a pronounced quenching effect in response to some transition metal ions (1 mM). Hence, the rare earth composite materials presented here can be utilized not only for the production of optical materials, but also for the development of fluorescence sensing strips.

Excellent Electroluminescent Property of Eu3+-Induced Polystyrene-co-poly(acrylic acid) Aggregates (EIPAs) in Polymeric Light-Emitting Diodes

Eu3+-induced polystyrene-co-poly(acrylic acid) aggregates (EIPAs) were synthesized using a self-assembly approach, and their structures and photophysical characteristics were examined to achieve effective monochromatic red emission in polymer light-emitting diodes (PLEDs). By adjusting the monomer ratio in RAFT polymerization, the size of Eu3+-induced block copolymer nanoaggregates can be regulated, thereby modulating the luminescence intensity. High-performance bilayer polymer light-emitting devices were fabricated using poly(9,9-dioctylfluorene) (PFO) and 2-(tert-butylphenyl)-5-biphenylyl-1,3,4-oxadiazole (PBD) as the host matrix, with EIPAs as the guest dopant. The devices exhibited narrow red emission at 615 nm with a full width at half-maximum (fwhm) of 15 nm across doping concentrations of 1, 3, 5, and 10 wt %. At a doping concentration of 3 wt %, the device achieved a maximum brightness of 1864.48 cd/m2 at 193.82 mA/cm2 and an external quantum efficiency of 3.20% at a current density of 3.5 mA/cm2. These results indicate that incorporating polystyrene-co-poly(acrylic acid) with Eu3+ complexes enhances the excitation and emission intensity, as well as the structural stability of the emitting layer in PLEDs, thereby improving the device performance.

Structural metamorphosis and photophysical properties of thermostable nano- and microcrystalline lanthanide polymer with flexible coordination chains

Luminescent lanthanide coordination polymer crystals (LCPCs) represent an area of growing interest in materials chemistry owing to their unique and tailorable functional properties. The LCPCs provide a high level of structural tunability, including size- and morphology-dependent properties; therefore, they are promising materials for next-generation phosphors in a wide range of applications such as light emitting diodes. Here, by controlling the morphology of thermostable europium coordination polymer crystals, [Eu(hfa)₃(dpbp)]_n, hfa: hexafluoroacetylacetonate and dpbp:4,4'-bis(diphenyl　phosphoryl) biphenyl), we realized a novel red phosphor with narrow linewidth emission (FWHM = 7.8 nm). The obtained luminescent LCPCs with unique structures were characterized by X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), dynamic light scattering (DLS) and thermogravimetric analysis. Among, them, size tunable crystalline polymer spheres were found to have high internal quantum efficiency (ex., IQE = 79%) and highly thermostability (>300°C), and to exhibit dispersibility in PMMA media. The obtained results on the structural tunability of these materials can be used for the development of synthesis techniques for nanoscale materials based on crystalline lanthanide-based coordination phosphors.

A Novel Near-Infrared Ytterbium Complex [Yb(DPPDA)2](DIPEA) with Φ = 0.46% and τobs = 105 μs

The luminescent performances of near-infrared (NIR) lanthanide (Ln) complexes were restricted greatly by vibration quenching of X-H (X = C, N, O) oscillators, which are usually contained in ligands and solvents. Encapsulating Ln³⁺ into a cavity of coordination atoms is a feasible method of alleviating this quenching effect. In this work, a novel ytterbium complex [Yb(DPPDA)₂](DIPEA) coordinated with 4,7-diphenyl-1,10-phenanthroline-2,9-dicarboxylic acid (DPPDA) was synthesized and characterized by FT-IR, ESI-MS and elemental analysis. Under the excitation of 335 nm light, [Yb(DPPDA)₂](DIPEA) showed two emission peaks at 975 and 1011 nm, respectively, which were assigned to the characteristic ²F_5/2 → ²F_7/2 transition of Yb³⁺. Meanwhile, this ytterbium complex exhibited a plausible absolute quantum yield of 0.46% and a luminescent lifetime of 105 μs in CD₃OD solution. In particular, its intrinsic quantum yield was calculated to be 12.5%, and this considerably high value was attributed to the near-zero solvent molecules bound to Yb³⁺ and the absence of X-H oscillators in the first coordination sphere. Based on experimental results, we further proposed that the sensitized luminescence of [Yb(DPPDA)₂](DIPEA) occurred via an internal redox mechanism instead of an energy transfer process.

OLED Structure Optimization for Pure and Efficient NIR Electroluminescence of Nd3+ Complexes Bearing Fluorinated 1,3-Diketones

NIR emitting OLEDs (organic light-emitting diodes) with high photoluminescence quantum yields were developed on the basis of fluorinated 1,3-diketonate coordination compounds of the Nd³⁺ ion. Both thermal evaporation and spin-coating techniques were successfully employed for active layer deposition resulting in electroluminescence quantum yields up to 1.38·10^-2%. Blueish-green emission from exciplex and electroplax formations was almost suppressed with the topology optimization of the cell.

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]