Dependence of the Lifetime upon the Excitation Energy and Intramolecular Energy Transfer Rates: The5D0EuIIIEmission Case

Triple-Decker Hexaazamacrocyclic Lanthanide(III) Complexes: Structure, Magnetic Properties, and Temperature-Dependent Luminescence

The reaction of fluoride anions with mononuclear rare-earth(III) complexes of the hexaazamacrocycle derived from 2,6-diformylpyridine and ethylenediamine affords trinuclear coordination compounds [Ln3L3(μ2-F)4(NO3)2](NO3)3. The X-ray crystal structures of these complexes show triplex cationic complexes where the three roughly parallel macrocyclic lanthanide(III) units are linked by bis-μ2-F bridges. The detailed analysis of the photophysical properties of the [Eu3L3(μ2-F)4(NO3)2](NO3)3·2H2O and [Tb3L3(μ2-F)4(NO3)2](NO3)3·3H2O complexes reveals different temperature dependence of luminescence intensity and luminescence decay time of the Eu(III) and Tb(III) derivatives. The spectra of mixed species of average composition [Eu1.5Tb1.5L3(μ2-F)4(NO3)2](NO3)3·3H2O are in accordance with the ratiometric luminescent thermometer behavior. Measurements of the direct-current (dc) magnetic susceptibility of the [Dy3L3(μ2-F)4(NO3)2](NO3)3·2H2O complex indicate possible ferromagnetic interactions between the Dy(III) ions. Alternating current (ac) susceptibility measurements of this complex indicate single-molecule magnet behavior in zero dc field with magnetic relaxation dominated by Orbach mechanism and an effective energy barrier Ueff = 12.3 cm-1 (17.7 K) with a pre-exponential relaxation time, τ0 of 7.3 × 10-6 s. A similar reaction of mononuclear macrocyclic complexes with a higher number of fluoride equivalents results in polymeric {[Ln3L3(μ2-F)5](NO3)4}n complexes. The X-ray crystal structure of the Nd(III) derivative of this type shows trinuclear units that are additionally linked by single fluoride bridges to form a linear coordination polymer.

Eu3+ and Tb3+ coordination compounds with phenyl-containing carbacylamidophosphates: comparison with selected Ln3+ β-diketonates

Materials based on Eu³⁺ and Tb³⁺ coordination compounds are of great interest due to their strong red and green luminescence. Appropriate selection of ligands plays a huge role in optimizing their photophysical properties. Another very helpful instrument for such optimization is theoretical modelling, which permits the prediction of the emissive properties of materials through intramolecular energy transfer analysis. The ligands that allow for achieving high efficiency of Eu³⁺ and Tb³⁺ emissions include carbacylamidophosphates (CAPh, HL). In this brief review, we summarize recent research for lanthanides CAPh-based coordination compounds of general formulas Cat[LnL]₄, [LnL₃Q] and [Ln(HL)₃(NO₃)₃], where Cat⁺ = Cs⁺, NEt4⁺, PPh₄ ⁺ and Q = 1,10-phenanthroline, 2,2-bipyridine or triphenylphosphine oxide, involving the use of thermal gravimetric analysis, X-ray analysis, and absorption and luminescence spectroscopy. We carried out a comparison with selected Ln³⁺ β-diketonates. Possibilities and developments of theoretical calculations on energy transfer rates are also presented.

PMMA or PVDF films blended with β-diketonate tetrakis EuIII or TbIII complexes used as downshifting coatings of near-UV LEDs

Luminescent Ln^III complexes incorporated in polymeric films exhibit narrow emission bands and absorption within the near-UV/blue spectral range, and enhanced phostability, which qualify them to be explored for solid-state lighting. Herein, (C₂₆H₅₆N)[Eu(dbm)₄] and Na[Tb(acac)₄], (C₂₆H₅₆N⁺ = didodecyldimethylammonium, dbm^- =1,3-diphenyl-1,3-propanedionate, acac^- = acetylacetonate), were dispersed in PMMA or PVDF films to protect them from degradation, and the obtained blends were applied as downshifting coatings on near-UV emitter LEDs. Upon such excitation, both Eu^III and Tb^III complexes emit red or green light with absolute emission quantum yields of 6.4 and 99%, respectively. The complex amount within films influences the photophysical parameters due to multiphotonic deactivation, and formation of agglomerates. For the PMMA-based LED prototypes, the Ln^III emission is well-observed while for the PVDF ones, only a poor Ln^III emission is detected due to their opacity. Therefore, the PMMA-based systems are better candidates to be used as luminescent coatings of near-UV LEDs for solid-state lighting.

Sensitization of lanthanide complexes through direct spin-forbidden singlet → triplet excitation

Ln^III complexes may display luminescence within the ultraviolet-visible-near-infrared spectral window and although they render bright emissions mainly due to the classical singlet-triplet-state-assisted ligand sensitization, which would be the photophysical parameters if they could be excited through direct spin-forbidden singlet → triplet transitions? Herein, we report the sensitization of Ln complexes through spin-forbidden S₀ → T transitions in a series of homobimetallic Eu^III, Tb^III, Er^III, and Yb^III complexes with halogen-substituted benzoate ligands. As halogens and Ln^III atomic numbers increase, intense singlet → triplet absorption/excitation bands and relative quantum yields up to 18% were achieved due to an increased spin-orbit coupling effect. Moreover, the near-UV-shifted excitation may enable application in luminescent solar concentrators where Yb^III near-infrared luminescence matches the maximum efficiency of the crystalline Si photovoltaic cell. Therefore, the spin-relaxed excitation channel provides new opportunities to improve the Ln^III complex luminescence and potential within the energy conversion field.

Seven-Coordinate Tb3+ Complexes with 90% Quantum Yields: High-Performance Examples of Combined Singlet- and Triplet-to-Tb3+ Energy-Transfer Pathways

Seven-coordinate, pentagonal-bipyramidal (PBP) complexes [Ln(bbpen)Cl] and [Ln(bbppn)Cl], in which Ln = Tb³⁺ (products I and II), Eu³⁺ (III and IV), and Gd³⁺ (V and VI), with bbpen^2- = N,N'-bis(2-oxidobenzyl)-N,N'-bis(pyridin-2-ylmethyl)ethylenediamine and bbppn^2- = N,N'-bis(2-oxidobenzyl)-N,N'-bis(pyridin-2-ylmethyl)-1,2-propanediamine, were synthesized and characterized by single-crystal X-ray diffraction analysis, alternating-current magnetic susceptibility measurements, and photoluminescence (steady-state and time-resolved) spectroscopy. Under a static magnetic field of 0.1 T, the Tb³⁺ complexes I and II revealed single-ion-magnet behavior. Also, upon excitation at 320 nm at 300 K, I and II presented very high absolute emission quantum yields (0.90 ± 0.09 and 0.92 ± 0.09, respectively), while the corresponding Eu³⁺ complexes III and IV showed no photoluminescence. Detailed theoretical calculations on the intramolecular energy-transfer rates for the Tb³⁺ products indicated that both singlet and triplet ligand excited states contribute efficiently to the overall emission performance. The expressive quantum yields, Q_Ln^L, measured for I and II in the solid state and a dichloromethane solution depend on the excitation wavelength, being higher at 320 nm. Such a dependence was rationalized by computing the intersystem crossing rates (W_ISC) and singlet fluorescence lifetimes (τ_S) related to the population dynamics of the S₁ and T₁ levels. Thin films of product II showed high air stability and photostability upon continuous UV illumination, which allowed their use as downshifting layers in a green light-emitting device (LED). The prototypes presented a luminous efficacy comparable with those found in commercial LED coatings, without requiring encapsulation or dispersion of II in host matrixes. The results indicate that the PBP environment determined by the ethylenediamine (en)-based ligands investigated in this work favors the outstanding optical properties in Tb³⁺ complexes. This work presents a comprehensive structural, chemical, and spectroscopic characterization of two Tb³⁺ complexes of mixed-donor, en-based ligands, focusing on their outstanding optical properties. They constitute good molecular examples in which both triplet and singlet excited states provide energy to the Tb³⁺ ion and lead to high values of Q_Ln^L.

High Eu3+ concentration quenching in Y3TaO7 solid solution for orange-reddish emission in photonics

We report the synthesis of a Y₃TaO₇ solid solution containing a high Eu³⁺ concentration (from 7 up to 50 mol%) and investigate how Eu³⁺ influences the Y₃TaO₇ crystallization process. To this end, we evaluate the Y₃TaO₇ structural features and photoluminescence properties after Eu³⁺ introduction into the Y₃TaO₇ lattice. The higher the Eu³⁺ ion concentration, the more stable the crystallization process of the Y₃TaO₇ phase seems to be. The Eu³⁺-containing Y₃TaO₇ displays intense orange-reddish, broad band emission because Eu³⁺ occupies different symmetry sites in the host and causes inhomogeneous broadening. Eu³⁺ emission quenching due to Eu³⁺ concentration is negligible up to 30 mol% and absolute quantum yield values of up to nearly 30% were obtained, making Eu³⁺-containing Y₃TaO₇ interesting materials for application as high-intensity emitters in photonics.

Eu³⁺ ions influence on the crystallization process of Y₃TaO₇ solid solution and this particular host has presented a high concentration quenching (30 mol%), displaying an intense orange-reddish emission, with color purity over 92.6%.

Luminescence Thermometry on the Route of the Mobile-Based Internet of Things (IoT): How Smart QR Codes Make It Real

Quick Response (QR) codes are a gateway to the Internet of things (IoT) due to the growing use of smartphones/mobile devices and its properties like fast and easy reading, capacity to store more information than that found in conventional codes, and versatility associated to the rapid and simplified access to information. Challenges encompass the enhancement of storage capacity limits and the evolution to a smart label for mobile devices decryption applications. Organic-inorganic hybrids with europium (Eu3+) and terbium (Tb3+) ions are processed as luminescent QR codes that are able to simultaneously double the storage capacity and sense temperature in real time using a photo taken with the charge-coupled device of a smartphone. The methodology based on the intensity of the red and green pixels of the photo yields a maximum relative sensitivity and minimum temperature uncertainty of the QR code sensor (293 K) of 5.14% · K-1 and 0.194 K, respectively. As an added benefit, the intriguing performance results from energy transfer involving the thermal coupling between the Tb3+-excited level (5D4) and the low-lying triplet states of organic ligands, being the first example of an intramolecular primary thermometer. A mobile app is developed to materialize the concept of temperature reading through luminescent QR codes.

Exploring the dual functionality of an ytterbium complex for luminescence thermometry and slow magnetic relaxation

We present a comprehensive investigation of the magnetic and optical properties of an ytterbium complex, which combines two desirable and practical features into a single molecular system. Based upon YbIII ions that promote near-infrared optical activity and a chemical backbone that is ideal for an in-depth understanding of the magnetic behaviour, we have designed a multifunctional opto-magnetic species that operates as a luminescent thermometer and as a single-molecule magnet (SMM). Our magnetic investigations, in conjunction with ab initio calculations, reveal one of the highest energy barriers reported for an YbIII-based complex. Moreover, we correlate this anisotropic barrier with the emission spectrum of the compound, wherein we provide a complete assignment of the energetic profile of the complex. Such studies lay the foundation for the design of exciting multi-faceted materials that are able to retain information at the single-molecule level and possess built-in thermal self-monitoring capabilities.

A luminescent polyoxoniobate lanthanide derivative {Eu3(H2O)9[Nb48O138(H2O)6]}27−

An Eu-containing polyoxoniobate (CN₃H₆)₇K₃H₁₇{Eu₃(H₂O)₉Nb₄₈O₁₃₈(H₂O)₆}·40H₂O is obtained under the protection of citric acid, which is a triangular cylindrical entity and displays red emission.

Red phosphor based on Eu3+-isoelectronically doped Ba2SiO4 obtained via sol–gel route for solid state lightning

Ba₂SiO₄:Eu³⁺ red phosphor with high quantum efficiency synthesized via sol–gel route as a candidate to white LEDs component is introduced. An approach for the 0–0 Eu³⁺ transition anomalous emission due to Eu³⁺–O²⁻ associates is also provided.

Excitation of magnetic dipole transitions at optical frequencies

We use the magnetic field distribution of an azimuthally polarized focused laser beam to excite a magnetic dipole transition in Eu^{3+} ions embedded in a Y2O3 nanoparticle. The absence of the electric field at the focus of an azimuthally polarized beam allows us to unambiguously demonstrate that the nanoparticle is excited by the magnetic dipole transition near 527.5 nm. When the laser wavelength is resonant with the magnetic dipole transition, the nanoparticle maps the local magnetic field distribution, whereas when the laser wavelength is resonant with an electric dipole transition, the nanoparticle is sensitive to the local electric field. Hence, by tuning the excitation wavelength, we can selectively excite magnetic or electric dipole transitions through optical fields.

Simultaneous realization of two approaches to white light in single-component phosphors

A novel single-component warm white light-emitting Sr(2)Ca(0.995)MoO(6): Sm(3+) (0.005) phosphor was synthesized by solid-state reaction. The photoluminescence excitation spectra ranging from 300 to 450 nm and 460 to 500 nm broadly are observed. Direct full-color warm white light [(x, y) = 0.3221, 0.3525] was realized in this single-phase phosphor with exposure to 380 nm UV light. When this phosphor is pumped by 466 nm radiation we obtained yellow emission with an intense red component, suggesting that this material is also competitive as a blue-pumped yellow phosphor. Thus two approaches to white light are realized simultaneously in Sm(3+) doped single-component phosphor for the first time. The quantum yield and the reliability of the as-synthesized phosphors for White LED applications were also investigated.

Ratiometric nanothermometer based on an emissive Ln3+-organic framework

Luminescent thermometers working at the nanoscale with high spatial resolution, where the conventional methods are ineffective, have emerged over the last couple of years as a very active field of research. Lanthanide-based materials are among the most versatile thermal probes used in luminescent nanothermometers. Here, nanorods of metal organic framework Tb0.99Eu0.01(BDC)1.5(H2O)2 (BDC = 1-4-benzendicarboxylate) have been prepared by the reverse microemulsion technique and characterized and their photoluminescence properties studied from room temperature to 318 K. Aqueous suspensions of these nanoparticles display an excellent performance as ratiometric luminescent nanothermometers in the physiological temperature (300-320 K) range.