Photoluminescent Thermometer Based on a Phase-Transition Lanthanide Silicate with Unusual Structural Disorder

Evaluation of Point Group Symmetry in Lanthanide(III) Complexes: A New Implementation of a Continuous Symmetry Operation Measure with Autonomous Assignment of the Principal Axis

The structure of molecular systems dictates the physical properties, and symmetry is the determining factor for all electronic properties. This makes group theory a powerful tool in quantum mechanics to compute molecular properties. For inorganic compounds, the coordination geometry has been estimated as idealized polyhedra with high symmetry, which, through ligand field theory, provides predictive capabilities. However, real samples rarely have ideal symmetry, and although continuous shape measures (CShM) can be used to evaluate deviation from an ideal reference structure σideal, this often fails for lanthanide(III) complexes with high coordination numbers, no obvious choice of principal axes, and no obvious reference structure. In lanthanide complexes, the unique electronic structures and associated properties are intricately tied to the symmetry around the lanthanide center. Therefore, robust methodologies to evaluate and estimate point group symmetry are instrumental for building structure-property relationships. Here, we have demonstrated an algorithmic approach that orients a molecular structure Q in the best possible way to the symmetry axis of any given point group G and computes a deviation from the ideal symmetry σsym(G,Q). This approach does not compute the deviation from an ideal reference system, but the intrinsic deviation in the structure induced by symmetry operations. If the structure contains the symmetry operation, there is no deviation and σsym(G,Q) = 0. The σsym deviation is generated from all of the symmetry operation ÔS in a point group G using the most correct orientation of the sample structure in each group G. The best orientation is found by an algorithm that minimizes the orientation of the structure with respect to G. To demonstrate the methodology, we have investigated the structure and symmetry of 8- and 9-coordinated lanthanide(III) aqua complexes and correlated the luminescence from 3 europium(III) crystals to their actual symmetry. To document the methodology, the approach has been tested on 26 molecules with different symmetries. It was concluded that the method is robust and fully autonomous.

Ultrasensitive Colorimetric Luminescence Thermometry by Progressive Phase Transition

Luminescent materials that display quick spectral responses to thermal stimuli have attracted pervasive attention in sensing technologies. Herein, a programmable luminescence color switching in lanthanide-doped LiYO2 under thermal stimuli, based on deliberate control of the monoclinic (β) to tetragonal (α) phase transition in the crystal lattice, is reported. Specifically, a lanthanide-doping (Ln3+ ) approach to fine-tune the phase-transition temperature in a wide range from 294 to 359 K is developed. Accordingly, an array of Ln3+ -doped LiYO2 crystals that exhibit progressive phase transition, and thus sequential color switching at gradually increasing temperatures, is constructed. The tunable optical response to thermal stimuli is harnessed for colorimetric temperature indication and quantitative detection, demonstrating superior sensitivity and temperature resolution (Sr = 26.1% K-1 , δT = 0.008 K). The advances in controlling the phase-transition behavior of luminescent materials also offer exciting opportunities for high-performance personalized health monitoring.

Toward Magneto-Optical Cryogenic Thermometers with High Sensitivity: A Magnetic Circular Dichroism Based Thermometric Approach

Remote temperature probing at the cryogenic range is of utmost importance for the advancement of future quantum technologies. Despite the notable achievements in luminescent thermometers, accurately measuring temperatures below 10 K remains a challenging endeavor. In this study, we propose a novel magneto-optical thermometric approach based on the magnetic-circular dichroism (MCD) technique, which offers unprecedented capabilities for meticulous temperature variation analysis at cryogenic temperatures. The inherent temperature sensitivity of the MCD C-term, in conjunction with both positive and negative signals, enables highly sensitive magneto-optical temperature probing. Additionally, a groundbreaking relative thermal sensitivity value of 95.3 % K-1 at 2.54 K can be achieved using a mononuclear lanthanide complex, [[Ho(acac)3 (phen)], in the presence of a 0.25 T applied magnetic field and using a combination of multiparametric thermal read-out with multiple regression. These results unequivocally demonstrate the viability and effectiveness of our methodology for cryogenic temperature sensing.

Spotlight on Luminescence Thermometry: Basics, Challenges, and Cutting-Edge Applications

Luminescence (nano)thermometry is a remote sensing technique that relies on the temperature dependency of the luminescence features (e.g., bandshape, peak energy or intensity, and excited state lifetimes and risetimes) of a phosphor to measure temperature. This technique provides precise thermal readouts with superior spatial resolution in short acquisition times. Although luminescence thermometry is just starting to become a more mature subject, it exhibits enormous potential in several areas, e.g., optoelectronics, photonics, micro- and nanofluidics, and nanomedicine. This work reviews the latest trends in the field, including the establishment of a comprehensive theoretical background and standardized practices. The reliability, repeatability, and reproducibility of the technique are also discussed, along with the use of multiparametric analysis and artificial-intelligence algorithms to enhance thermal readouts. In addition, examples are provided to underscore the challenges that luminescence thermometry faces, alongside the need for a continuous search and design of new materials, experimental techniques, and analysis procedures to improve the competitiveness, accessibility, and popularity of the technology.

Desolvation-Induced Highly Symmetrical Terbium(III) Single-Molecule Magnet Exhibiting Luminescent Self-Monitoring of Temperature

A conjunction of Single-Molecule Magnet (SMM) behavior and luminescence thermometry is an emerging research line aiming at contactless read-out of temperature in future SMM-based devices. The shared working range between slow magnetic relaxation and the thermometric response is typically narrow or absent. We report TbIII -based emissive SMMs formed in a cyanido-bridged framework whose properties are governed by the reversible structural transformation from [TbIII (H2 O)2 ][CoIII (CN)6 ] ⋅ 2.7H2 O (1) to its dehydrated phase, TbIII [CoIII (CN)6 ] (2). The 8-coordinated complexes in 1 show the moderate SMM effect but it is enhanced for trigonal-prismatic TbIII complexes in 2, showing the SMM features up to 42 K. They are governed by the combination of QTM, Raman, and Orbach relaxation with the energy barrier of 594(18) cm-1 (854(26) K), one of the highest among the TbIII -based molecular nanomagnets. Both systems exhibit emission related to the f-f electronic transitions, with the temperature variations resulting in the optical thermometry below 100 K. The dehydration leads to a wide temperature overlap between the SMM behavior and thermometry, from 6 K to 42 K. These functionalities are further enriched after the magnetic dilution. The role of post-synthetic formation of high-symmetry TbIII complexes in achieving the SMM effect and hot-bands-based optical thermometry is discussed.

High-Efficiency Ce3+ Activated Orthorhombic Lanthanide Silicate Blue Phosphors for Plant Growth Lighting

Plant growth can be controlled and freed from natural environmental interference through indoor plant cultivation. Artificial light sources with better quality are required to promote indoor plant growth. In this study, we used a simple high-temperature solid-state reaction to synthesize high-efficiency Ce3+-activated NaGdSiO4 (NGSO) phosphors. X-ray diffraction and Rietveld refinement were performed to determine the detailed crystal structure of the NGSO:Ce3+ phosphors. The morphology of NGSO:Ce3+ and the elemental state of Ce3+ were measured and analyzed. Under near-ultraviolet (n-UV) light excitation, the Ce3+-activated NGSO phosphors exhibit a broad emission band from 375 to 500 nm, and their emission peaks are at approximately 401 nm. This asymmetrical blue emission band is caused by the spin-allowed 5d → 4f transition of Ce3+ and overlaps well with the blue absorption region of carotenoids and chlorophyll. The temperature-dependent luminescence spectra were utilized to assess the thermal stability of NGSO:Ce3+. The external quantum efficiency (EQE) was measured to be 60.91%, and the internal quantum efficiency (IQE) was measured to be 73.39%. A blue LED device assembled from the NGSO:Ce3+ phosphor has demonstrated the application potential in accelerating plant growth.

Thermally Triggered Multilevel Diffractive Optical Elements Tailored by Shape-Memory Polymers for Temperature History Sensors

The morphological transitions induced by external stimuli in shape-memory polymers (SMPs) can be exploited with the real-time response of far-field diffraction patterns in diffractive optical elements (DOEs). In this paper, we combine the temperature characteristics of SMPs and the display characteristics of DOEs to obtain an optical temperature sensing film where the temperature information is taken as a change of far-field diffraction images. This process was achieved by imprinting the micropatterns of the DOEs on the epoxy-based SMP film, which can be programmed to hold a temporary optical image and revert to its original image upon exposure to a specific temperature. Furthermore, the specific temperature at which the image transformation occurs can be customized by varying the chain flexibility of the SMP. Based on a range of transition points, by imprinting the desired combination of SMP-DOEs on a film, a sensor that can record and inform the temperature history is demonstrated. As for the feasible application of this technique, it can be used for the compact and reliable optical temperature indicators, which can be applied in temperature-sensitive industries such as food and pharmaceuticals.

Dual Emission in the Near-Infrared and Visible Regions from a Mixed Cyanido-Bridged EuIII/NdIII(4-OHpy)-CoIII Layered Material

Coordination polymers (CPs) with a dual emission spanning from the visible (vis) to near-infrared (NIR) regions of the electromagnetic spectrum are used for optical sensors, medical diagnostics, and telecommunication technologies. We herein report the synthesis, structural characterization, and optical response of heterometallic cyanido-bridged layered {[Eu_xNd_y(4-OHpy)₂(H₂O)₃][Co(CN)₆]} CPs, where 4-OHpy = 4-hydroxypyridine, with a multicolor emission profile across the vis and NIR regions. The crystals show an efficient energy transfer (ET) from the 4-OHpy ligand and the [Co(CN)₆] ions to the Eu³⁺ and Nd³⁺ ions, resulting in an enhanced photoluminescence (PL) efficiency. We study the ET with steady-state and time-resolved PL, reporting an ET between the Ln³⁺ centers. The excitation-dependent emission of the mixed Ln³⁺ CPs and the control over the PL lifetime yield new insights into the optoelectronic properties of these materials.

3-Pyridylacetic-Based Lanthanide Complexes Exhibiting Magnetic Entropy Changes, Single-Molecule Magnet, and Fluorescence

Four complexes from lanthanides, 3-pyridylacetate, and 1,10-phenanthroline, formulated as [Ln2(3-PAA)2(μ-Cl)2(phen)4](ClO4)2 [Ln = Gd(1), Dy(2), Eu(3), Tb(4), 3-PAA = 3-pyridylacetic acid, phen = 1,10-phenanthroline], were obtained. The four compounds were characterized by IR spectra, thermogravimetric analyses, powder X-ray diffraction, and single-crystal X-ray diffraction. Compounds 1-4 are isomorphous, and they have a dinuclear structure. Magnetic studies reveal that 1 shows the magnetocaloric effect with -ΔS m max = 19.03 J kg-1 K-1 at 2 K for ΔH = 5 T, and 2 displays a field-induced single-molecule magnet with U eff = 19.02 K. The photoluminescent spectra of 3 and 4 exhibit strong characteristic emission, which demonstrate that the ligand-to-EuIII/TbIII energy transfer is efficient.

Fluorescence Thermometer: Intermediation of the Fontal Temperature and Light

The rapid advance of thermal materials and fluorescence spectroscopy has extensively promoted micro-scale fluorescence thermometry development in recent years. Based on the advantages of fast response, high sensitivity, simple operation,...

Enhancing the Photoluminescence Property of Pr3+ Ions by Understanding the Polymorphous Influence of the K3Lu(PO4)2 Host

Adjusting the local coordination environment of lanthanide luminescent ions is a useful method to manipulate the relevant photoluminescence (PL) property. K₃Lu(PO₄)₂ is a phase-change material, and according to the stable temperature range from low to high, the related polymorphs are phase I [P2₁/m, coordination number (CN) of Lu³⁺ = 7], phase II (P2₁/m, CN = 6), and phase III (P3̅, CN = 6), respectively. Based on the temperature-dependent PL analysis of K₃Lu(PO₄)₂:Pr³⁺, we find that Pr³⁺ ions occupy the noninversion sites (C_s) in the two low-temperature phases but preferentially enter into the inversion ones (C_3i) in phase III. Compared to Pr³⁺-doped phase I (78 K), Pr³⁺ ions in phase III (300 K) manifest a weaker fluorescence intensity (170-fold lower). To enhance the room-temperature PL property of K₃Lu(PO₄)₂:Pr³⁺, a polymorphous adjustment strategy was proposed by the use of the ion-doping method. By introducing the Gd³⁺ ions into the lattice, Pr³⁺-doped phase I is successfully stabilized to room temperature, manifesting a 27-fold fluorescence increase in comparison to K₃Lu(PO₄)₂:Pr³⁺ (0.1 at. %). The finding discussed in this study highlights the significance of site engineering for luminescent ions and also presents the application value of phase-change hosts in the development of high-performance luminescent materials.

A new family of dinuclear lanthanide complexes exhibiting luminescence, magnetic entropy changes and single molecule magnet behaviors

Four isomorphic and dinuclear lanthanide complexes were synthesized. Complexes Eu^III and Tb^III exhibit strong emissions, while Gd^III shows the magnetocaloric effect and Dy^III displays a single-molecule magnet.

Luminescent Ratiometric Thermometers Based on a 4f-3d Grafted Covalent Organic Framework to Locally Measure Temperature Gradients During Catalytic Reactions

Covalent Organic Frameworks (COFs), an emerging class of crystalline porous materials, are a new type of support for grafting lanthanide ions (Ln³⁺ ), which can be employed as ratiometric luminescent thermometers. In this work we have shown that COFs co-grafted with lanthanide ions (Eu³⁺ , Tb³⁺ ) and Cu²⁺ (or potentially other d-metals) can synchronously be employed both as a nanothermometer and catalyst during a chemical reaction. The performance of the thermometer could be tuned by changing the grafted d-metal and solvent environment. As a proof of principle, the Glaser coupling reaction was investigated. We show that temperature can be precisely measured during the course of the catalytic reaction using luminescence thermometry. This concept could be potentially easily extended to other catalytic reactions by grafting other d-metal ions on the Ln@COF platform.

Organic Thermometers Based on Aggregation of Difluoroboron β-Diketonate Chromophores

We report several novel thermometers resulting from the temperature-induced aggregation of difluoroboron β-diketonate chromophores. These thermometers exhibit a much wider temperature-dependent fluorescence emission from 445 to 592 nm along with the color change from blue to red in a dilute chloroform solution. Spectroscopy measurements and theoretical calculations confirm that the thermochromic luminescence originates from the reversible change in the noncovalent intermolecular interactions and the abrupt volume shrinkage of the solvent at its melting point. The present work provides a new strategy for rationally designing high-performance thermometers having a wide emission property.

White-Light-Emitting AIE/Eu3+-Doped Ion Gel with Multistimuli-Responsive Properties

A white-light-emitting ion gel composed of a poly[(2-(4-vinylphenyl)ethene-1,1,2-triyl)tribenzene-b-ethylene glycol-b-(2-(4-vinylphenyl)ethene-1,1,2-triyl)tribenzene] aggregation-induced emission (AIE) network and a poly([2,2':6',2″-terpyridin]-4'-yl methacrylate-co-methyl methacrylate) Eu³⁺-doped network was fabricated via a solution mixing process. This ion gel exhibits special multistimuli-responsive properties, and it can change its luminescent color by changing pH, temperature, or the solvent. The unique color-changing property is attributed to the different luminescent mechanisms of the AIE/Eu³⁺-doped polymer networks. The former is affected by changes in its aggregation state, while the latter is controlled by the dynamic metal-ligand cross-linking bonds. Furthermore, owing to the interpenetrating networks formed by the two polymers, the hybrid gel has both good mechanical strength and flexibility. It may be used in the fields of sensors, probes, and light-emitting materials.

Solid-State Low-Temperature Thermoresponsive and Reversible Color Changes of Conjugated Polymer in Layered Structure: Beyond Infrared Thermography

Emergence of thermoresponsive and reversible color changes at low temperature is a challenging target. In general, it is not easy to induce sufficient dynamic motion of rigid molecules including chromophore at a lower temperature. The present work shows unusually low-temperature color-change properties originating from the dynamic motion of rigid conjugated polymer in solid state. The layered composites of polydiacetylene (PDA) and guest l-arginine (L-Arg) (PDA-(L-Arg)) exhibit temperature-responsive gradual color changes with reversibility in the range of 123-333 K in solid crystalline state. The dynamic properties are induced by gradual and reversible distortion of the π-conjugated main chain in response to temperature. The tuned flexibility of the layered structure facilitates motion of the rigid π-conjugated molecule at low temperature. The PDA-(L-Arg)-coated substrates are applied to visualization and quantification of 2D and 3D temperature distributions generated by cooling with liquid nitrogen. These thermographic devices afford to image lower temperature range than typical infrared thermography. The present work indicates potentials of layered architectures with tunable flexibility for emergence of dynamic properties.

Luminescent Thermochromic Silver Iodides as Wavelength-Dependent Thermometers

Luminescent thermochromic materials with a dramatic shift of emission band under different temperatures are highly desirable in temperature sensing fields. However, the design of the synthesis of such compounds remains a great challenge. In this work, two new luminescent thermochromic silver iodides, (emIm)Ag₃I₄ (1) and (emIm)Ag₂I₃ (2) (emIm = 1-ethyl-3-methyl imidazole), have been synthesized under solvothermal conditions. Compound 1 features a [Ag₃I₄]^- anionic layer, while compound 2 possesses an infinite [Ag₂I₃]^- chain structure, both of which are charge balanced by emIm⁺ cations. Particularly, they display luminescent thermochromism with a significant wavelength shift of emission maximum with temperature change. They represent rare examples of infinite layered or chain silver iodides that show luminescent thermochromism. Furthermore, the results indicate that compounds 1 and 2 are promising wavelength-dependent luminescent thermometers.

Pushing the Limit of Boltzmann Distribution in Cr3+-Doped CaHfO3 for Cryogenic Thermometry

Luminescence Boltzmann thermometry is one of the most reliable techniques used to locally probe temperature in a contactless mode. However, to date, there is no report on cryogenic thermometers based on the highly sensitive and reliable Boltzmann-based ⁴T₂ → ⁴A₂/²E → ⁴A₂ emission ratio of Cr³⁺. On the basis of structural information of the local HfO₆ octahedral site we demonstrated the potential of the CaHfO₃:Cr³⁺ system by combining deep theoretical and experimental investigation. The material exhibits simultaneous emission from both the ²E and ⁴T₂ excited states, following the Boltzmann law in a cryogenic temperature range of 40-150 K. The promising thermometric performance corroborates the potential of CaHfO₃:Cr³⁺ as a Boltzmann cryothermometer, being characterized by a high relative sensitivity (∼ 2%·K^-1 at 40 K) and exceptional thermal resolution (0.045-0.77 K in the 40-150 K range). Moreover, by exploiting the flexibility of the ⁴T₂-²E energy gap controlled by the crystal field of the local octahedral site, the design proposed herein could be expanded to develop new Cr³⁺-doped cryogenic thermometers.

Dual-emission fluorescent silicon nanoparticle-based nanothermometer for ratiometric detection of intracellular temperature in living cells

In this article, we present a kind of dual-emission fluorescent nanothermometer, which is made of europium (Eu3+)-doped silicon nanoparticles (Eu@SiNPs), allowing the detection of intracellular temperature in living cells with high accuracy. In particular, the presented SiNP-based thermometer features dual-emission fluorescence (blue (455 nm) and red (620 nm) emission), negligible toxicity (cell viability of treated cells remains above 90% during 24 h of treatment) and robust photostability in living cells (i.e., preserving >90% of fluorescence intensity after 45 min of continuous UV irradiation). More significantly, the fluorescence intensity of the Eu@SiNPs exhibits a linear ratiometric temperature response in a broad range from 25 to 70 °C. Taking advantage of these attractive merits, the Eu@SiNP-based nanothermometer is able to accurately (∼4.5% change per °C) determine dynamic changes in intracellular temperature in a quantitative and long-term (i.e., 30 min) manner.

A difluoroboron β-diketonate based thermometer with temperature-dependent emission wavelength

We report a series of difluoroboron β-diketonate fluorophores that manifest temperature-dependent emission wavelength accompanied by a change of fluorescence from green to orange. The distinct emission changes allow convenient and accurate measurements of temperature.

Rational design and synthesis of a stable pillar-layer NaI-organic framework as a multi-responsive luminescent sensor in aqueous solutions

A stable pillar-layer Na^I-organic framework, [Na₂(DCPB)∙(H₂O)₂]_n (namely 1), was rationally designed and synthesized by the assemble process of Na^I and 1,3-di(4'-carboxyl-phenyl)benzene (H₂DCPB). Benefiting from the luminescent property and high stability in water, the as-synthesized 1 is a potential multi-responsive luminescent sensor material toward Cr₂O₇^2-, Fe³⁺, and nitrofurazone (NFZ) in water. Ground 1 not only has the excellent detectability and selectivity but also possesses outstanding stability and circularity. The calculated K_sv values of 1 are 8.8×10³ for Fe³⁺, 9.9×10³ for Cr₂O₇^2-, and 2.7×10⁴ M^-1 for NFZ in aqueous solutions, respectively. Furthermore, 1 is able to accurately detect NFZ in real bovine serum samples through luminescence detection technology.

Highly Stretchable and Fast Self-Healing Luminescent Materials

Nowadays, the flourishing exploitation of multifunction luminescent materials with fast self-healing and superior mechanical features greatly broadens the scope for wide applications in optical and display devices, but this is still a formidably challenging task. Herein, we realize color-tunable luminescent materials functionalized with lanthanide ions (Ln³⁺) and terpyridine ligand coordination complexes that show highly stretchable and rapid self-healing performance, simultaneously broadening their application prospects both optically and mechanically. The multiple color emission, including visible and near-infrared luminescence, can be achieved by energy transfer from the coordinating terpyridine unit to Ln³⁺ via the so-called "antenna effect". The dynamic Ln-N coordination exhibits extreme stretchability and fast self-healing under ambient conditions. Of particular interest is that the healing process is not significantly affected by surface aging and atmospheric moisture. The multifunction materials open up a new pathway for future development of the next-generation wearable electronics including flexible and self-healable conductors.

Well-tuned white-light-emitting behaviours in multicenter-Ln polyoxometalate derivatives: A photoluminescence property and energy transfer pathway study

White light-emitting diodes (WLEDs) are of scientific significance in terms of their wide applications, and few uminescent materials based on white-light-emitting polyoxometalate (POM) derivatives have been reported till now. Herein, a series of organic chromophores modified POM derivatives [N(CH₃)₄]₃K₂Ln(C₇H₅O₂)(H₂O)₂(α-PW₁₁O₃₉)]·11H₂O (Ln³⁺ = Eu³⁺ (1), Tb³⁺ (2), Tm³⁺ (3), Lu³⁺ (4)) and multicenter-Ln analogues [N(CH₃)₄]₃K₂Eu_xTb_yTm_1-x-y(C₇H₅O₂)(H₂O)₂(α-PW₁₁O₃₉)·11H₂O (5-11) were synthesized successfully and were characterized by various physico-chemical analysis. The investigations indicate the white-light-emitting behavior can be well tuned by adjusting the molar ratio of Eu³⁺/Tb³⁺/Tm³⁺ = 0.06:0.10:0.84 in 9. The energy transfer process from organic benzoic and POM ligands to Eu³⁺, Tb³⁺ and Tm³⁺ emitting centers were detected through time-resolved emission spectroscopy (TRES) and the comparison of excitation of single-, double-, treble-Ln³⁺ mixed, indicating the energy can transfer from the photoexcitation O → M LMCT state of POM components and π → π* transition of organic ligand to sensitize the emissions of Ln³⁺ ions via intramolecular energy transition mechanism. The energy transfer between Eu³⁺ and Tb³⁺, Tm³⁺ and Eu³⁺, Tm³⁺ and Tb³⁺ ions also have been recorded and carefully studied by TRES and variations of Tm³⁺ luminescence lifetime in this context, and the results show a low-effectively process of energy transfer between Tm³⁺/Eu³⁺, Tm³⁺/Tb³⁺ ions and a relatively good energy transfer efficiency between Eu³⁺/Tb³⁺ ions.

High-Performance Pr3+-Doped Scandate Optical Thermometry: 200 K of Sensing Range with Relative Temperature Sensitivity above 2%·K-1

Nowadays, for lanthanide fluorescence thermometry, high relative temperature sensitivity (S_r) and wide sensing range are urgently required in practical applications. Herein, a Pr³⁺-doped scandate (Pr³⁺:CaSc₂O₄) luminescent thermometer is proposed, and the related crystal-field splitting of Pr³⁺ ions is systematically discussed. The 4f5d-4f and 4f-4f emissions of Pr³⁺ ions in CaSc₂O₄ basically present positive and negative correlation relationships with the increase in temperature, respectively. The different changing tendencies in relation to temperature for the two kinds of transitions are mainly derived from the effects of the thermally activated trap energy levels and the crossover process between 5d and 4f energy states of Pr³⁺ ions and are beneficial for the enhancement of relevant temperature sensitivities. The obtained experimental results manifest that Pr³⁺:CaSc₂O₄ owns a maximum relative temperature sensitivity S_r of 2.49%·K^-1 (at 390 K) and a low temperature uncertainty (around 0.1 K from 275 to 490 K). Moreover, it is also able to keep a relatively high S_r (not lower than 2%·K^-1) over a wide temperature sensing range (∼200 K), which is more excellent than those of reported luminescent thermometric materials (∼100 K). Hence, what discussed in this study might provide a useful design perspective for the exploration and development of high-performance luminescent thermometers with a wide applicable temperature range.

Ratiometric Luminescent Thermometers with a Customized Phase-Transition-Driven Fingerprint in Perovskite Oxides

The development of noncontact thermometers with self-control to specific temperatures to be used as control markers with an additional degree of reliability is a challenge in the field of thermal sensors. Herein, a strategy exploiting the wide tunability of an intrinsic feature of oxide perovskites such as the phase-transition temperature to design a new class of ratiometric luminescent thermometers is introduced. The structural and optical response to the thermal stimuli of LaGaO₃:Nd³⁺ system is used as a prototype to show the unprecedented opportunity to combine the processes of two different regimes in the same compound, leading to a reliable optical thermal sensor with an intrinsic tell-tale sign at specific temperatures. High relative sensitivity, low temperature uncertainty, and good reproducibility, together with the need for a single calibration curve irrespective of the phase-transition temperature and the doping effects, attest the goodness of the thermometric performances. This work demonstrates the control of the phase-transition (orthorhombic ↔ rhombohedral) temperature, T_c, of lanthanum gallate in the 400-700 K range by carefully doping the perovskite structure, as a proof of concept for the design of customized thermometers characterized by a spectral shape change acting as a self-fingerprint for the T_c.

Modulation of lanthanide luminescence via an electric field

The modulation of luminescence via external stimuli such as temperature, mechanical stress, hydrostatic pressure, as well as electric and/or magnetic fields, has witnessed great progress, enabled the disclosure of new principles and energy transfer pathways, and widened applications. However, investigations on the luminescence modulation of lanthanide ions doped in semiconductors via an applied electric field are still absent. Herein, for the first time, we have demonstrated the in situ, real-time, and reversible modulation of the luminescence of Eu³⁺ doped in SnO₂ nanocrystals by manipulating the recombination rate of photo-generated electrons and holes, and the accompanied energy transfer mode in terms of linear and quasi-sinusoidal, from semiconductor to lanthanide ions. Following the same principle, the modulation of near infrared responsive Er³⁺ in SnO₂ and the visible luminescence of perovskite nanocrystals is further realized. This study offers extra methodologies for luminescence modulation, in addition to those already reported for ferro- and/or piezoelectric-hosted luminescent materials.

Multifunctional Optical Thermometry Based on the Rare-Earth-Ions-Doped Up-/Down-Conversion Ba2TiGe2O8:Ln (Ln = Eu3+/ Er3+/ Ho3+/ Yb3+) Phosphors

The fabrication of a multifunctional sensor together with a widening temperature-sensing range is an essential challenge in optical thermometers especially for trivalent lanthanide-doped materials. Herein, we design a wide range, highly sensitive, and multifunctional thermometer by exploiting the emission spectrum of Eu³⁺ ions, and further detailed discussion has been made on the new temperature-sensing mechanism. The sensor can be operated between 358 and 548 K with a maximum relative sensitivity ( S_r) of 0.93% K^-1 at 358 K, which is higher than that of most temperature-sensing materials. A paramount superiority is that the calibration parameter can be directly calculated from the single Eu³⁺ emission spectrum, avoiding the demand of other calibrations, which realizes the coexistence of a simple structure and high precision. Furthermore, other up-conversion thermometers based on Er³⁺/Ho³⁺/Yb³⁺ co-doped Ba₂TiGe₂O₈ (BTG) phosphors as well as the down-conversion thermometer based on Eu³⁺-doped Ba₂TiGe₂O₈ (BTG:Eu³⁺) phosphor have been synthesized for comparison, and the results show that the novel thermometer (BTG:Eu³⁺) has a much higher sensitivity than that of the traditional thermometers (BTG:Er³⁺/Ho³⁺/Yb³⁺). In addition, the versatility of the phosphor (BTG:Eu³⁺) is simultaneously reflected in its applications to red phosphor for white-light emitting diodes (W-LEDs) and plant growth lamps. All of the results suggest that BTG:Eu³⁺ could be a good candidate with its highly sensitive S_r value for optical thermometry and as a safety sign in high-temperature environments.

Ferroelectric order driven Eu3+ photoluminescence in BaZrxTi1-xO3 perovskite

The ability to tune and enhance the properties of luminescent materials is essential for enlarging their application potential. Recently, the modulation of the photoluminescence emission of lanthanide-doped ferroelectric perovskites by applying an electric field has been reported. Herein, we show that the ferroelectric order and, more generally the polar order, has a direct effect on the photoluminescence of Eu3+ in the model BaZrxTi1-xO3 perovskite even in the absence of an external field. The dipole arrangement evolves with increasing x from long-range ferroelectric order to short-range order typical of relaxors until the non-polar paraelectric BaZrO3 is achieved. The cooperative polar interactions existing in the lattice (x < 1) promote the off-center displacement of the Eu3+ ion determining a change of the lanthanide site symmetry and, consequently, an abrupt variation of the photoluminescence emission with temperature. Each type of polar order is characterized by a distinct photoluminescence behaviour.

Li4SrCa(SiO4)2:Eu2+: A Potential Temperature Sensor with Unique Optical Thermometric Properties

Investigations on luminescence properties of lanthanide-activated phosphors are not only essential to understanding the fundamental structure-property relationship but also important to advancing the development and application of relevant research techniques. We report herein a promising optical thermometric material Eu²⁺-doped Li₄SrCa(SiO₄)₂ utilizing the different sensitivities of Eu_Sr²⁺ and Eu_Ca²⁺ emission intensities to temperature. A unique evolution of Eu²⁺ luminescence in the as-prepared sample is identified under the simultaneous action of UV illumination and thermal treatment. The maximum relative sensitivities are 2.87% K^-1 (at 440 K) and 1.51% K^-1 (at 460 K) for the as-prepared and illuminated samples, respectively. These temperature sensing features reflect a great potential of Eu²⁺-doped Li₄SrCa(SiO₄)₂ for applications in the optical thermometry field.

Three resorcin[4]arene-based lanthanide-coordination polymers with multifunctional photoluminescence sensing properties

By utilizing a novel octacarboxylate-functionalized resorcin[4]arene as organic linkers, three lanthanide-coordination polymers, namely, [(CH3)2NH2][Ln2(HL)(H2O)7]·2H2O (Ln = Tb (1), Eu (2) and Gd (3), H8L = 2,8,14,20-tetra-pentyl-4,6,10,12,16,18,22,24-octa-carboxymethoxy-resorcin[4]arene) have been solvothermally synthesized and structurally characterized. Isostructural 1-3 display unique two dimensional sandwich-based layers built with Ln3+ cations and bowl-shaped HL7- anions. Remarkably, 1 and 2 produce intensive green and red emissions respectively and long lifetimes thanks to the antenna effect of HL7- anions. The energy level testing of 3 indicates that the newly designed ligand H8L has a very efficient intersystem crossing process. More importantly, luminescent investigations reveal that 1 and 2 can selectively detect N,N'-dimethylformamide and Fe3+ ions with turn-on-type and turn-off-type responses, respectively.

An ultrastable metal–organic material emits efficient and broadband bluish white-light emission for luminescent thermometers

We discover a rare bluish white-light-emitting Sb³⁺-based coordination polymer with an unsually large Stokes shift of 230 nm (2.3 eV), ascribed to the assymetric–symmetric coordination shift of the Sb³⁺ centers.

A water-soluble TbIII complex as a temperature-sensitive luminescent probe

The water soluble [Tb(dipicCbz)3]3− (dipicCbz = 4-(9H-carbazol-9-yl-)pyridine-2,6-dicarboxylato) complex was isolated and evaluated as a temperature sensor in water. The 1:3 (TbIII:dipicCbz2−) stoichiometry in solution was confirmed by luminescence titration and high-resolution mass spectrometry. The quantum yield of sensitized emission is 3.8% ± 0.4% at 25.0 ± 0.1 °C, and the emission intensity depends on the temperature in the range of 5–70 °C with a relative thermal sensitivity of 3.4% °C−1 at 35 °C and temperature resolution < 0.01 °C in the range of 30–40 °C. The reversibility of this behavior was demonstrated for three heating–cooling cycles. Calculations of the energy gap between donor and acceptor show that the temperature dependence of the emission intensity is due to back-energy transfer from the Tb 5D4 excited state to the triplet and twisted intramolecular charge transfer (TICT) states of the dipicCbz. The assignment of one of the energy levels as a TICT state was confirmed by the temperature-dependent behavior of the phosphorescence band.

Near-Infrared Ratiometric Luminescent Thermometer Based on a New Lanthanide Silicate

A new lanthanide silicate system, Na₂ K[Ln₃ Si₆ O₁₈ ] (Ln=Lu, Yb/ Er, Lu/Eu, or Lu/Yb/Er), comprising microcrystals embedded in an amorphous siliceous matrix, obtained by sintering at 1373 K a Na₃ K[Ln₂ Si₆ O₁₇ ]⋅3 H₂ O nano-crystalline precursor, is reported. The crystal structure of these lanthanide silicates was solved from high-resolution synchrotron power X-ray diffraction data collected at 110 K, and further supported by ²⁹ Si MAS NMR and Eu³⁺ luminescence. The materials crystallize in the Pī triclinic centrosymmetric space group, exhibiting a dense framework consisting of hexameric [Si₆ O₁₈ ]^12- cyclosilicate units, and chains of two distinct {LnO₆ } octahedra. Na₂ K[(Lu_0.75 Yb_0.20 Er_0.05 )₃ Si₆ O₁₈ ] is the first example of a lanthanide silicate operative as a near-infrared ratiometric luminescent thermometer, with good sensitivity at cryogenic temperatures (<100 K). Upon excitation at 903 nm, the ratio between the ² F_7/2 →² F_5/2 (Yb³⁺ ) and ⁴ I_13/2 →⁴ I_15/2 (Er³⁺ ) emissions was used for sensing temperature in the 12-450 K range, reaching a maximum thermal sensitivity of 2.6 % K^-1 at 26.8 K.

The effect of ligand symmetry on the ratiometric luminescence characteristics of lanthanides

This work demonstrated that the ligand symmetry of europium(iii) complexes controls the ratiometric luminescence characteristics of lanthanides. Nona-coordinated europium(iii) complexes having unsymmetrical β-diketonate ligands (Cs) exhibit distinctive ratiometric spectral variations in the extremely narrow f-f transition bands over the temperature range from 253 to 323 K. In contrast, no such ratiometric change can be observed in a series of nona-coordinated europium(iii) complexes containing symmetrical β-diketonate ligands (C2v). The remarkable difference depending on the ligand symmetry (Csvs. C2v) suggests that the coordination rearrangement of β-diketonate in the complex causes ratiometric spectral variations in extremely narrow f-f transition bands, where two europium(iii) complex isomers exist in the solution equilibrium. A self-calibration method using dual iso-emissive points is reported, where self-calibration using the two emission intensities at the iso-emissive points reduces the coefficient of variation in luminescence thermometry.

Inherently Eu2+ /Eu3+ Codoped Sc2 O3 Nanoparticles as High-Performance Nanothermometers

Luminescent nanothermometers have shown competitive superiority for contactless and noninvasive temperature probing especially at the nanoscale. Herein, we report the inherently Eu²⁺ /Eu³⁺ codoped Sc₂ O₃ nanoparticles synthesized via a one-step and controllable thermolysis reaction where Eu³⁺ is in-situ reduced to Eu²⁺ by oleylamine. The stable luminescence emission of Eu³⁺ as internal standard and the sensitive response of Eu²⁺ emission to temperature as probe comprise a perfect ratiometric nanothermometer with wide-range temperature probing (77-267 K), high repeatability (>99.94%), and high relative sensitivity (3.06% K^-1 at 267 K). The in situ reduction of Eu³⁺ to Eu²⁺ ensures both uniform distribution in the crystal lattice and simultaneous response upon light excitation of Eu²⁺ /Eu³⁺ . To widen this concept, Tb³⁺ is codoped as additional internal reference for tunable temperature probing range.

Flexible and transparent films consisting of lanthanide complexes for ratiometric luminescence thermometry

Herein, a flexible and transparent film consisting Eu³⁺/Tb³⁺ lanthanide complexes and poly(methylmethacrylate) was constructed via solution casting method, and further developed as a ratiometric luminescent thermometer with an excellent linear response to temperature variation from 77 to 297 K. The thermometer displays higher photo- and thermostability than corresponding pure complexs. Based on that the emission intensity ratio of ⁵D₄ → ⁷F₅ transition (Tb³⁺) to ⁵D₀ → ⁷F₂ transition (Eu³⁺) can be linearly related to the temperature, the resulting thermometer is not only more reliable than single Eu³⁺(or Tb³⁺) material based on one emission, and but also has higher sensitivity than other types of luminescent thermometers. This work highlights the practical applications of luminescent films in temperature-sensing fields.

Tuning the sensitivity of lanthanide-activated NIR nanothermometers in the biological windows

Lanthanide-activated SrF₂ nanoparticles with a multishell architecture were investigated as optical thermometers in the biological windows. A ratiometric approach based on the relative changes in the intensities of different lanthanide (Nd³⁺ and Yb³⁺) NIR emissions was applied to investigate the thermometric properties of the nanoparticles. It was found that an appropriate doping with Er³⁺ ions can increase the thermometric properties of the Nd³⁺-Yb³⁺ coupled systems. In addition, a core containing Yb³⁺ and Tm³⁺ can generate light in the visible and UV regions upon near-infrared (NIR) laser excitation at 980 nm. The multishell structure combined with the rational choice of dopants proves to be particularly important to control and enhance the performance of nanoparticles as NIR nanothermometers.

Luminescent thermometer based on Eu3+ /Tb3+ -organic-functionalized mesoporous silica

In this work we investigate a mesoporous silica (MS) decorated with dipyridyl-pyridazine (dppz) ligands and further grafted with a mixture of Eu³⁺ /Tb³⁺ ions (28.45%:71.55%), which was investigated as a potential thermometer in the 10-360 K temperature range. The MS material was prepared employing a hetero Diels-Alder reaction: 3,6-di(2-pyridyl)-1,2,4,5-tetrazine was reacted with the double bonds of vinyl-silica (vSilica) followed by an oxidation procedure. We explore using the dppz-vSilica material to obtain visible emitting luminescent materials and for obtaining a luminescent thermometer when grafted with Eu³⁺ /Tb³⁺ ions. For the dppz-vSilica@Eu,Tb material absolute sensitivity S_a of 0.011 K^-1 (210 K) and relative sensitivity S_r of 1.32 %K^-1 (260 K) were calculated showing good sensing capability of the material. Upon temperature change from 10 K to 360 K the emission color of the material changed gradually from yellow to red.