Full Picture of Energy Transfer in a Trivalent Europium Complex with Bidentate β-Diketonate Ligand

Trivalent europium (Eu(III)) complexes emit narrow-band luminescence as a result of energy transfer following the photoexcitation of antenna ligands. Bidentate β-diketonates are typically used as antenna ligands; however, their entire energy transfer mechanism to Eu(III) remains unknown. We used time-resolved photoluminescence spectroscopy and femtosecond transient absorption spectroscopy to map the complete intramolecular energy transfer process in the [Eu(hfa)3(TPPO)2] (hfa = hexafluoroacetylacetonate, TPPO = triphenylphosphine oxide) complex; hfa is a β-diketonate antenna ligand. Our analysis provides a "full picture" of energy transfer, tracing each step from initial photoexcitation to final relaxation: intersystem crossing in the ligands, energy transfer from the ligands to the metal center, and multiple internal conversion of Eu(III). We demonstrated that the direct bonds of the bidentate ligands enabled a quick and near-unity-efficiency energy transfer from the triplet state of the ligand to the 5D2 state of Eu(III). We also revealed that the loss of the ligand-centered intersystem crossing process reduces the overall sensitization efficiency. Our findings provide a foundation for the rational design of advanced luminescent materials, paving the way for advances in lanthanide-based luminescence research.

Micro-Inclusion Engineering via Sc Incompatibility for Luminescence and Photoconversion Control in Ce3+-Doped Tb3Al5-xScxO12 Garnet

Aluminum garnets display exceptional adaptability in incorporating mismatching elements, thereby facilitating the synthesis of novel materials with tailored properties. This study explored Ce3+-doped Tb3Al5-xScxO12 crystals (where x ranges from 0.5 to 3.0), revealing a novel approach to control luminescence and photoconversion through atomic size mismatch engineering. Raman spectroscopy confirmed the coexistence of garnet and perovskite phases, with Sc substitution significantly influencing the garnet lattice and induced A1g mode softening up to Sc concentration x = 2.0. The Sc atoms controlled sub-eutectic inclusion formation, creating efficient light scattering centers and unveiling a compositional threshold for octahedral site saturation. This modulation enabled the control of energy transfer dynamics between Ce3+ and Tb3+ ions, enhancing luminescence and mitigating quenching. The Sc admixing process regulated luminous efficacy (LE), color rendering index (CRI), and correlated color temperature (CCT), with adjustments in CRI from 68 to 84 and CCT from 3545 K to 12,958 K. The Ce3+-doped Tb3Al5-xScxO12 crystal (where x = 2.0) achieved the highest LE of 114.6 lm/W and emitted light at a CCT of 4942 K, similar to daylight white. This approach enables the design and development of functional materials with tailored optical properties applicable to lighting technology, persistent phosphors, scintillators, and storage phosphors.

Photoluminescence lifetime stability studies of β-diketonate europium complexes based phenanthroline derivatives in poly(methyl methacrylate) films

In this work, five phenanthroline derivatives substituted with different methyl groups have been selected to synthesize β-diketonate-based europium complexes to check the influence of the substitutions on the degradation effect of those complexes in poly(methyl methacrylate) (PMMA) films. The photophysical properties of Eu(III) complexes, including absorbance, excitation, and emission have been carefully investigated in solution, solid-state, and doped in PMMA film. In all these states, the complexes exhibit an impressive red emission at 614 nm with a high photoluminescence quantum yield of up to 85 %. The films have been exposed under outdoor, indoor, and dark storage stability lifetime conditions for 1200 hours. The photoluminescence measurements recorded every 400, 800, and 1200 hours demonstrated that the film containing europium complex with phenanthroline ligand substituted by a high number of methyl groups (Eu(TTA)3L5) showed good photoluminescent stability in indoor and dark conditions, and exhibited better resistance to degradation in outdoor conditions compared to other complexes. This study has proved that phenanthroline ligands could be tuned chemically leading to better stability of those types of complexes in films which can be end-used for future stable optoelectronic devices such as luminescent solar concentrators.

Deep Red Photoluminescence from Cr3+ in Fluorine-Doped Lithium Aluminate Host Material

Deep red phosphors have attracted much attention for their applications in lighting, medical diagnosis, health monitoring, agriculture, etc. A new phosphor host material based on fluorine-doped lithium aluminate (ALFO) was proposed and deep red emission from Cr3+ in this host material was demonstrated. Cr3+ in ALFO was excited by blue (~410 nm) and green (~570 nm) rays and covered the deep red to near-infrared region from 650 nm to 900 nm with peaks around 700 nm. ALFO was a fluorine-doped form of the spinel-type compound LiAl5O8 with slightly Li-richer compositions. The composition depended on the preparation conditions, and the contents of Li and F tended to decrease with preparation temperature, such as Al4.69Li1.31F0.28O7.55 at 1100 °C, Al4.73Li1.27F0.17O7.65 at 1200 °C, and Al4.83Li1.17F0.10O7.78 at 1300 °C. The Rietveld analysis revealed that ALFO and LiAl5O8 were isostructural with respect to the spinel-type lattice and in a disorder-order relationship in the arrangement of Li+ and Al3+. The emission peak of Cr3+ in LiAl5O8 resided at 716 nm, while Cr3+ in ALFO showed a rather broad doublet peak with the tops at 708 nm and 716 nm when prepared at 1200 °C. The broad emission peak indicated that the local environment around Cr3+ in ALFO was distorted, which was also supported by electron spin resonance spectra, suggesting that the local environment around Cr3+ in ALFO was more inhomogeneous than expected from the diffraction-based structural analysis. It was demonstrated that even a small amount of dopant (in this case fluorine) could affect the local environment around luminescent centers, and thus the luminescence properties.

Strong Circularly Polarized Luminescence at 1550 nm from Enantiopure Molecular Erbium Complexes

Circularly polarized luminescence (CPL) in two subregions of the near-infrared (NIR) has been achieved. By leveraging the rigidity and diminishing detrimental vibrations of the heterobimetallic binolate complexes of erbium [(Binol)₃ErNa₃], species exhibiting an exceptionally high dissymmetry factor (|g_lum |) of 0.47 at 1550 nm were obtained. These erbium complexes are the first reported examples of CPL observed beyond 1200 nm. Analogous complexes of ytterbium and neodymium also exhibited strong CPL (|g_lum| = 0.17, 0.05, respectively) in a higher energy NIR window (800-1200 nm). All complexes exhibit high quantum yields (Er: 0.58%, Yb: 17%, Nd: 9.3%) and high B_CPL values (Er: 57 M^-1 cm^-1, Yb: 379 M^-1 cm^-1, Nd: 29 M^-1 cm^-1). Because of their strong CPL emission in the telecom band (1550 nm), biologically relevant NIR emission window (800-1100 nm), and synthetic versatility, the complexes reported here could permit further promising developments in quantum communication technologies and biologically relevant sensors.

Photocatalysis, terahertz time domain spectroscopy and weak interactions of six polyoxometalate-based lanthanide phosphine oxide complexes

Using triphenylphosphine oxide (OPPh3) or tetraethyl ethylenebisphosphonate (L) as ligands, phosphomolybdic acid hydrate as the anion template, six new lanthanide complexes [Nd(OPPh3)4(H2O)3](PMo12O40)∙4CH3CN (1a), [Ln(OPPh3)4(H2O)3](PMo12O40)∙4C2H5OH (2a-4a) (Ln = Dy, Ho, Er),...

Bright photo- and triboluminescence of centrosymmetric Eu(iii) and Tb(iii) complexes with phosphine oxides containing azaheterocycles

Novel mononuclear Eu(iii) and Tb(iii) complexes based on phosphine oxides bearing pyridine, pyrimidine or pyrazine moieties show bright photo- and triboluminescence under ambient conditions.

Dual Energy Transfer Pathways from an Antenna Ligand to Lanthanide Ion in Trivalent Europium Complexes with Phosphine-Oxide Bridges

Trivalent europium (Eu³⁺) complexes are attractive materials for luminescence applications if energy transfer from antenna ligands to the lanthanide ion is efficient. However, the microscopic mechanisms of the transfer remain elusive, and fundamental physical chemistry questions still require answers. We track the energy transfer processes in a luminescent complex Eu(hfa)₃(DPPTO)₂ (hfa, hexafluoroacetylacetonate; DPPTO, 2-diphenylphosphoryltriphenylene) using time-resolved photoluminescence spectroscopy. In addition to the conventional energy transfer pathway through the T₁ state of the ligands, we discovered ultrafast energy transfer pathway directly from the singlet excited states of the ligands to the ⁵D₁ state of Eu³⁺. The short time scale of the energy transfer (3 ns, 200 ns) results in its high photoluminescence quantum yield. The discovery of the distinct energy transfer pathways from a single chromophore is important for establishing design strategies of luminescent complexes.

Photo-physical, theoretical and photo-cytotoxic evaluation of a new class of lanthanide(iii)–curcumin/diketone complexes for PDT application

Improved ISC in La(iii) complex of curcumin, on activation with visible light, has resulted in high yield of ¹O₂ in HeLa/MCF-7 cells, leading to the oxidative stress which was responsible for remarkable caspase 3/7-dependent apoptotic photocytotoxicity.

Combinatorial design of multimeric chelating peptoids for selective metal coordination

The combinatorial synthesis of a new library of tetrameric peptoid ligands is introduced, enabling coordination and characterization of f-block metals.

Current methods for metal chelation are generally based on multidentate organic ligands, which are generated through cumbersome multistep synthetic processes that lack flexibility for systematically varying metal-binding motifs. Octadentate ligands incorporating hydroxypyridinone or catecholamide binding moieties onto a spermine scaffold are known to display some of the highest affinities towards f-elements. Enhancing binding affinity for specific lanthanide or actinide ions however, necessitates ligand architectures that allow for modular and high throughput synthesis. Here we introduce a high-throughput combinatorial library of 16 tetrameric N-substituted glycine oligomers (peptoids) containing hydroxypyridinone or catecholamide chelating units linked via an ethylenediamine bridge and, for comparison, we also synthesized the corresponding mixed ligands derived from the spermine scaffold: 3,4,3-LI(1,2-HOPO)₂(CAM)₂ and 3,4,3-LI(CAM)₂(1,2-HOPO)₂. Coordination-based luminescence studies were carried out with Eu³⁺ and Tb³⁺ to begin probing the properties of the new ligand architecture and revealed higher sensitization efficiency with the spermine scaffold as well as different spectroscopic features among the structural peptoid isomers. Solution thermodynamic properties of selected ligands revealed different coordination properties between the spermine and peptoid analogues with Eu³⁺ stability constants log β₁₁₀ ranging from 28.88 ± 3.45 to 43.97 ± 0.49. The general synthetic strategy presented here paves the way for precision design of new specific and versatile ligands, with a variety of applications tailored towards the use of f-elements, including separations, optical device optimization, and pharmaceutical development.

The Coordination and Luminescence of the Eu(III) Complexes with the Polymers (PMMA, PVP)

The rare earth complexes and the polymers can be made into composite nanofibers through electrospinning. The fluorescence intensity of these fiber composites is much higher than that of the rare earth complexes. By changing the mixed proportion of polymethyl methacrylate (PMMA) and complexes, nanofiber materials were prepared. Then, by measuring their fluorescence intensity, it is found that the carbonyl bond of PMMA may have coordinated with the rare earth ions and enhanced the luminescence intensity of them. Then, a series of experiments were designed to study their coordination and luminescence mechanism. The coordination mechanism of the polymers with carbonyl groups and the rare earth complexes was explained by Eu(TFT)₃(TPPO), Eu(TFT)₃(TPPO)₂, Eu(PFP)₃(TPPO), Eu(PFP)₃(TPPO)₂, and polyvinyl pyrrolidone (PVP) dissolved in chloroform solution, where TFT means 2-(2,2,2-trifluoroethyl)-1-tetralone, PFP means 2-(2,2,3,3,3-Pentafluoropro-panoyl)-3,4-dihydronaphthalen-1(2H)-one and TPPO means phosphine oxide. The coordination of PVP and the rare earth complexes in solution was studied, and it was found that the fluorine atoms of the ligand had a significant impact on the aggregation-induced effect of the composites. The electron transitioned in the polymers and the complexes were enhanced greatly by the coordination. The colors of emission light could be adjusted by the coordination of the polymers and the rare earth complexes.

LMCT facilitated room temperature phosphorescence and energy transfer in substituted thiophenolates of Gd and Yb

To obtain luminescent lanthanide complexes with a low energy LMCT state the 2-(2'-mercaptophenyl)benzothiazolates, Ln(SSN)₃, and 2-(2'-mercaptophenyl)benzoxazolates, Ln₂(OSN)₆ (Ln = Gd, Yb), were synthesized by the reaction of amides Ln[N(SiMe₃)₂]₃ with respective thiophenols. Ytterbium complexes were structurally characterized by X-ray diffraction analysis. Cyclic voltammetry revealed that the deprotonated mercaptophenyl ligands have significantly lower oxidation potentials than their phenoxy analogues and some β-diketones. The photophysical properties of Gd and Yb compounds were studied both in solution and in the solid state. The fluorescence spectra of the compounds in solution display the bands of the keto and enol forms of the ligands. No energy transfer from the organic part to Yb³⁺ has been detected in solutions of both Yb complexes, whereas in solids an intense metal-centered emission in the near infrared region was observed. The solid Gd compounds exhibited room temperature phosphorescence caused by unusually efficient intersystem crossing facilitated by the essentially reducing properties of OSN and SSN ligands. To explain the sensitization process occurring in solids Yb₂(OSN)₆ and Yb(SSN)₃ a specific non-resonant energy transfer mechanism via a ligand to metal charge transfer state has been proposed. Based on the Yb derivatives, NIR-emitting OLEDs with 860 μW cm^-2 maximal irradiance were obtained. Their Gd counterparts showed bright electrophosphorescence (up to 1350 cd m^-2) in the devices containing doped emission layers.

A near-infrared organic light-emitting diode based on an Yb(iii) complex synthesized by vacuum co-deposition

Yb(DBM)₃(DPEPO), an emitter, was directly synthesized on a substrate by the vacuum co-deposition of the precursor Yb(DBM)₃(H₂O)₂ and the ligand DPEPO.

Photoluminescence and magnetic analysis of a family of lanthanide(iii) complexes based on diclofenac

A family of ten isostructural complexes based on the diclofenac ligand exhibiting interesting magnetic and luminescence properties has been prepared.

PMMA-supported hybrid materials doped with highly near-infrared (NIR) luminescent complexes [Zn(L1)(Py)Ln(L2)3] (Ln = Nd, Yb or Er)

Based on the tris-β-diketonate hetero-binuclear complexes [Zn(L1)(Py)Ln(L2)3] (Ln = Nd, Yb or Er) with good NIR luminescent properties, a series of PMMA-supported doping hybrid materials PMMA@[Zn(L1)(Py)Ln(L2)3] exhibit improved properties.