Inside-Out/Outside-In Tunability in Nanosized Lanthanide-Based Molecular Cluster-Aggregates: Modulating the Luminescence Thermometry Performance via Composition Control

Doping Gd16 nanoclusters for expanded optical properties and thermometry applications

Lanthanide metal clusters are composed of rigid multinuclear metal cores encapsulated by organic ligands, which have become one of the most interesting research frontiers because of their fantastic architecture, intriguing physical and chemical properties, and potential applications. However, very little attention has been paid to exploring their potential as highly efficient optical materials. Gd16 clusters are a new cluster structure that has a rich and varied coordination environment, which is highly conducive to doping and thus controlling luminescence and luminescence color modulation. We achieved green emission by doping Tb3+ ions and red emission by doping Eu3+ ions in the Gd16 cluster structure. Meanwhile, we achieved red-orange-yellow color-tunable luminescence by controlling the composition of Tb3+ and Eu3+ ions. Studies on the PL properties show that Gd16 clusters as the host can be used for doping and efficiently photosensitizing Tb3+ ions and Eu3+ ions. The existence of energy transfer from the ligand to Tb3+ ions and Eu3+ ions in the co-doped Ln16 clusters was sufficiently demonstrated by time-resolved photoluminescence spectroscopy tests, and the energy transfer efficiency in the clusters was calculated. Furthermore, the temperature-dependent photoluminescence properties of these clusters were investigated to determine their potential as luminescent thermometers.

Local Environment-Modulated f-f Transition in Unit-Cell-Sized Lanthanide Ultrathin Nanostructures

The regulation of the f-f transition is the basis of utilizing the abundant optical properties of lanthanide (Ln), of which the key is to modulate the local environment of Ln ions. Here, we constructed Eu(III)-based unit-cell-sized ultrathin nanowires (UCNWs) with red luminescence and polymer-like behavior, which appears as an ideal carrier for regulating f-f transition. The f-f transition of Eu(III) in UCNWs could be precisely regulated through various ligands. It is the unusual surface states that make the UCNWs exhibit greater electric dipole strength and better sensitivity to various ligands compared with the carefully constructed ultrathin nanosheets. In addition, the possibility of regulating f-f transition in UCNWs through energy transfer and a high entropy strategy was also revealed. Finally, a temperature-dependent universal fluorescent ink was prepared based on UCNWs, which provides ideas for intelligent flexible fluorescent materials.

Pyrazine Dicarboxylic Acid and Phosphite-Bridging Lanthanide-Incorporated Tellurotungstates and Their Fluorescence Performances

Two pyrazine dicarboxylic acid and phosphite-bridging lanthanide-incorporated tellurotungstates [H2N(CH3)2]12 Na4[Ln4(H2O)2(H2PDBA)2(HPO3)2W6O10][B-α-TeW8O31]4 · 70H2O [Ln = Eu3+ (1), Tb3+ (2); H2PDBA = 2,5-pyrazine dicarboxylic acid) were prepared, which contain four [B-α-TeW8O31]10- subunits and a deca-nuclear heterometallic [Ln(H2O)2(HPO3)2 (H2PDBA)2(W3O5)2]24+ cluster. Strikingly, two H2PDBA ligands connect two equivalent {W3Eu2O5(H2O)(B-α-TeW8O31)2(HPIIIO3)}8- moieties to form the polyanion skeleton, while the phosphite plays a bridging role in joining two lanthanide centers in the {W3Eu2O5(H2O)(B-α-TeW8O31)2(HPIIIO3)}8- moiety. In addition to the fluorescence (FL) properties of 1 and 2 at room temperature, their temperature-dependent FL properties were also investigated. In 80-298 K, FL intensities of 1 and 2 decrease as temperature increases, and their maximum relative sensitivities (Sr) are 3.70 and 1.99% K-1, whereas the minimum temperature uncertainties (δT) are 1.25 and 1.18 K for 1 and 2. In 298-973 K, upon increasing temperature, FL intensities of 1 and 2 initially rise to their maxima at 373 K and subsequently decrease. This is because samples of 1 and 2 undergo dehydration together with amorphization below 473 K and decomposition above this temperature. This work lays a foundation for the development for luminescent thermometers based on lanthanide-incorporated polyoxometalates.

Distortion Engineering: A Strategy to Modulate Molecular Upconversion with Molecular Cluster-Aggregates

The rational engineering of molecules is a powerful chemistry tool of pivotal importance in the fields of molecular magnetism and luminescence. Hence, systems that can be modulated via molecular engineering and composition control are expected to present extra versatility regarding the tunability of their properties. This is the case with molecular cluster aggregates (MCAs), high nuclearity molecular compounds. Herein, we demonstrate how the union of both strategies, namely, composition control and molecular engineering, can be employed to enhance molecular upconversion in MCAs. This was achieved by doping a {Gd8Er2Yb10} MCA with CeIII ions. By replacement of the optically silent GdIII ions with CeIII, the upconversion mechanism is modified due to CeIII-mediated cross-relaxation. In addition to this effect, we could also engineer the degree of metal site distortion due to the larger size of CeIII ions, relaxing the selection rules and impacting the upconversion quantum yield and luminescent thermometry. Opto-structural correlations demonstrate that the presented molecular engineering strategy can be used to enhance the performance of molecular upconverters.

Dual-signalled magneto-optical barcodes with lanthanide-based molecular cluster-aggregates

A proof-of-concept for magneto-optical barcodes is demonstrated for the first time. The dual-signalled spectrum observed via magnetic circular dichroism spectroscopy can be used to develop anti-counterfeiting materials with extra layers of security when compared with the widely studied luminescent barcodes.

Intra-cluster energy transfer editing in a dual-emitting system to tap into lifetime thermometry

The impact of composition control and energy transfer on luminescence thermometry was investigated in a TbIII/EuIII dual-emitting molecular cluster-aggregate, known as {Ln20}. The study of lifetime dynamics sheds new light on how one can take advantage of rational planning to enhance thermometric performance and gaining insights into intriguing optical properties.

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.

Composition Control in Molecular Cluster-Aggregates: A Toolbox for Optical Output Tunability via Energy Transfer Pathways

Composition control is a powerful tool for obtaining high-performance lanthanide (Ln) luminescent materials with adjustable optical outputs. This strategy is well-established for hierarchically structured nanoparticles, but it is rarely applied to molecular compounds due to the limited number of metal centers within a single unit. In this work, we present a series of molecular cluster-aggregates (MCAs) with an icosanuclear core {Ln2Eu2Tb16} (Ln = Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, Tm, and Yb) in which we explore composition control, akin to nanoparticles, to modulate the optical output. More specifically, we target to understand how the presence of a third LnIII doping ion would impact the well-known TbIII → EuIII energy transfer and the ratiometric optical thermometry performance based on the TbIII/EuIII pair. Photophysical properties at room and at varying temperatures were investigated. Based on experimental data and well-established intrinsic features, such as spin-orbit coupling strength and LnIII 4f energy levels' structure, we discuss the possible luminescent processes present in each MCA and provide insight into qualitative trends that can be rationally correlated throughout the series.

[α-AsW9O33]9- bridged hexagonal clusters of Ln(III) showing field induced SMM behavior: experimental and theoretical insight

Polyoxometalates (POM), as inorganic polydentate oxygen donors, provide binding opportunities for oxophilic lanthanide metal centers to construct novel Ln-substituted POM materials with exciting structures and attractive properties. Herein, we have reported four arsenotungstate [α-AsW₉O₃₃]^9- based lanthanide-containing polyoxometalates [Cs_xK_36-x{Ln₆(H₂O)₁₂(α-AsW₉O₃₃)₆}]·yH₂O (Ln = Er (1), Gd (2), Ho (3), and Tb (4)), which are synthesized in an alkaline medium. Complexes 1-3 are the dimeric structures of [Ln₃(H₂O)₆(α-AsW₉O₃₃)₃]^18- polyanions, whereas complex 4 is a hexamer of the polyanion [Tb (H₂O)₂(α-AsW₉O₃₃)]^6- as a building unit. In all the complexes, [α-AsW₉O₃₃]^9- units are staggered up and down and give rise to the chair conformation, where one [α-AsW₉O₃₃]^9- unit bridges two Ln(III) centers through four μ₂-oxygen and two terminal oxygen atoms, resulting in the hexagonal arrangement of lanthanides. The dynamic magnetic measurement indicates that only complex 1 exhibits slow relaxation of magnetization with an applied dc field (1500 Oe). To gain insight into the slow relaxation of magnetization in complex 1, the ligand-field parameters and the splitting of the ground-state multiplet of the Er(III) ions have been estimated. The ab initio calculation results confirm that the ground state wave function of these molecules (1, 3, and 4) is mainly composed of a mixture of m_J states, and the non-axial crystal field (CF) terms are more predominant than the axial CF term. The solid-state fluorescence spectra of 1-4 reveal that the photoexcitation O → M ligand-to-metal charge-transfer (LMCT) of arsenotungstate fragments is effectively quenched due to the spatial coordination environment around the Ln(III) ion.

Lanthanide molecular cluster-aggregates as the next generation of optical materials

In this perspective, we provide an overview of the recent achievements in luminescent lanthanide-based molecular cluster-aggregates (MCAs) and illustrate why MCAs can be seen as the next generation of highly efficient optical materials. MCAs are high nuclearity compounds composed of rigid multinuclear metal cores encapsulated by organic ligands. The combination of high nuclearity and molecular structure makes MCAs an ideal class of compounds that can unify the properties of traditional nanoparticles and small molecules. By bridging the gap between both domains, MCAs intrinsically retain unique features with tremendous impacts on their optical properties. Although homometallic luminescent MCAs have been extensively studied since the late 1990s, it was only recently that heterometallic luminescent MCAs were pioneered as tunable luminescent materials. These heterometallic systems have shown tremendous impacts in areas such as anti-counterfeiting materials, luminescent thermometry, and molecular upconversion, thus representing a new generation of lanthanide-based optical materials.

Boosting the sensitivity with time-gated luminescence thermometry using a nanosized molecular cluster aggregate

Luminescence thermometry with trivalent lanthanide ions is a promising avenue for contactless temperature probing. The area has been growing exponentially for the last two decades, and its viability has been successfully demonstrated in various research domains. However, moving from laboratory equipment to real-life applications remains a challenging task. One of the reasons is the possibility of a background luminescence from the probing device or probed environment. To tackle this issue, we elegantly incorporate a rarely explored thermometric approach called time-gated luminescence thermometry (TGLT). Furthermore, we demonstrate an enhanced relative sensitivity through this innovative approach and a path to move toward practical application.

Platonic and Archimedean solids in discrete metal-containing clusters

Metal-containing clusters have attracted increasing attention over the past 2-3 decades. This intense interest can be attributed to the fact that these discrete metal aggregates, whose atomically precise structures are resolved by single-crystal X-ray diffraction (SCXRD), often possess intriguing geometrical features (high symmetry, aesthetically pleasing shapes and architectures) and fascinating physical properties, providing invaluable opportunities for the intersection of different disciplines including chemistry, physics, mathematical geometry and materials science. In this review, we attempt to reinterpret and connect these fascinating clusters from the perspective of Platonic and Archimedean solid characteristics, focusing on highly symmetrical and complex metal-containing (metal = Al, Ti, V, Mo, W, U, Mn, Fe, Co, Ni, Pd, Pt, Cu, Ag, Au, lanthanoids (Ln), and actinoids) high-nuclearity clusters, including metal-oxo/hydroxide/chalcogenide clusters and metal clusters (with metal-metal binding) protected by surface organic ligands, such as thiolate, phosphine, alkynyl, carbonyl and nitrogen/oxygen donor ligands. Furthermore, we present the symmetrical beauty of metal cluster structures and the geometrical similarity of different types of clusters and provide a large number of examples to show how to accurately describe the metal clusters from the perspective of highly symmetrical polyhedra. Finally, knowledge and further insights into the design and synthesis of unknown metal clusters are put forward by summarizing these "star" molecules.

Bifunctional Temperature and Oxygen Dual Probe Based on Anthracene and Europium Complex Luminescence

In this work, we synthesized a polydimethylsiloxane membrane containing two emitter groups chemically attached to the membrane structure. For this, we attached the anthracene group and the [Eu(bzac)₃] complex as blue and red emitters, respectively, in the matrix via hydrosilylation reactions. The synthesized membrane can be used as a bifunctional temperature and oxygen ratiometric optical probe by analyzing the effects that temperature changes and oxygen levels produce on the ratio of anthracene and europium(III) emission components. As a temperature probe, the system is operational in the 203-323 K range, with an observed maximum relative sensitivity of 2.06% K^-1 at 290 K and temperature uncertainties below 0.1 K over all the operational range. As an oxygen probe, we evaluated the ratiometric response at 25, 30, 35, and 40 °C. These results show an interesting approach to obtaining bifunctional ratiometric optical probes and also suggest the presence of an anthracene → europium(III) energy transfer, even though there is no chemical bonding between species.

Phonon-assisted molecular upconversion in a holmium(III)-based molecular cluster-aggregate

Upconversion (UC) is a fascinating process in which higher energy photons can be emitted from excitation by lower energy photons. The current challenge remains in downscaling and effectively achieving upconversion with lanthanide ions at the molecular scale. Here, using a rationally designed molecular cluster-aggregate (MCA), we demonstrate for the first time Ho^III ion molecular upconversion. The synthesized MCA exhibits identifiable Ho^III green and red UC emissions with a uniquely enhanced red to green ratio as well as a conventional near-infrared (NIR) emission. A combined rigid spherical cluster core with reduced molecular vibrations, ideally matched donor and acceptor excited levels via a phonon-assisted mechanism, led to an upconversion quantum yield of 5.24 × 10^-6%.

Luminescent Color-Adjustable Europium and Terbium Co-Doped Strontium Molybdate Phosphors Synthesized at Room Temperature Applied to Flexible Composite for LED Filter

In this study, terbium and europium rare-earth ions were single-doped and co-doped to synthesized SoMoO4 phosphor at room temperature. The samples prepared synthesized crystalline SrMoO4 powder by the co-precipitation. Samples had a tetragonal structure in XRD analysis and d(112) spacing was changed by rare-earth doping. As the amount of rare earth added increased, a secondary phase appeared, and the structure changed. The synthesized SrMoO4:Tb3+ phosphors showed a green light emission at 544 nm under 287 nm, SrMoO4:Eu3+ phosphors showed a red light emission at 613 nm under 290 nm, and SrMoO4:[Eu3+]/[Tb3+] phosphor showed a yellow-white light emission at 544 and 613 nm when excited at 287 nm. The synthesized phosphor exhibited a change in green and red luminescence intensity based on the amount of Eu3+ doped and showed strong red luminescence as the Eu3+ doping increased. To use the SrMoO4:[Eu3+]/[Tb3+] phosphor with these characteristics in an LED color filter, a flexible composite prepared by mixing with PDMS showed green, red, and yellow-white emission under a UV-lamp.

Visible and near-infrared emitting heterotrimetallic lanthanide-aluminum-sodium 12-metallacrown-4 compounds: discrete monomers and dimers

The luminescence properties of two types of heterotrimetallic aluminum-lanthanide-sodium 12-metallacrown-4 compounds are presented here, LnNa(ben)₄[12-MC_{Al(III)N(shi)}-4] (LnAl4Na) and {LnNa[12-MC_{Al(III)N(shi)}-4]}₂(iph)₄ (Ln2Al8Na2), where Ln = Gd^III, Tb^III, Er^III, and Yb^III, MC is metallacrown, ben^- is benzoate, shi^3- is salicylhydroximate, and iph^2- is isophthalate. The aluminum-lanthanide-sodium metallacrowns formed with benzoate are discrete monomers while, upon replacement of the benzoate with the dicarboxylate isophthalate, two individual metallacrowns can be joined to form a dimer. In the solid state, the terbium version of each structure type displays emission in the visible region, and the erbium and ytterbium complexes emit in the near-infrared. The luminescence lifetimes (τ_obs) and quantum yields have been collected under ligand excitation (QLLn) for both LnAl4Na monomers and Ln2Al8Na2 dimers. Several of these values tend to be shorter (luminescence lifetimes) and smaller (quantum yields) than the corresponding values recorded for the structurally similar gallium-lanthanide monomer and dimer 12-MC-4 molecules. However, the quantum yield value recorded for the visible emitting Tb2Al8Na2 dimer, 43.9%, is the highest value observed in the solid state to date for a Tb^III based metallacrown.

Covert MOF-Based Photoluminescent Tags via Tunable Linker Energetics

Optical anticounterfeiting tags utilize the photoluminescent properties of materials to encode unique patterns, enabling identification and validation of important items and assets. These tags must combine optical complexity with ease of production and authentication to both prevent counterfeiting and to remain practical for widespread use. Metal-organic frameworks (MOFs) based on polynuclear, rare earth clusters are ideal materials platforms for this purpose, combining fine control over structure and composition, with tunable, complex energy transfer mechanisms via both linker and metal components. Here we report the design and synthesis of a set of heterometallic MOFs based on combinations of Eu, Nd, and Yb with the tetratopic linker 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene. The energetics of this linker facilitate the intentional concealment of the visible emissions from Eu while retaining the infrared emissions of Nd and Yb, creating an optical tag with multiple covert elements. Unique to the materials system reported herein, we document the occurrence of a previously not observed 11-metal cluster correlated with the presence of Yb in the MOFs, coexisting with a commonly encountered 9-metal cluster. We demonstrate the utility of these materials as intricate optical tags with both rapid and in-depth screening techniques, utilizing orthogonal identifiers across composition, emission spectra, and emission decay dynamics. This work highlights the important effect of linker selection in controlling the resulting photoluminescent properties in MOFs and opens an avenue for the targeted design of highly complex, multifunctional optical tags.

Lanthanide-MOFs as Multifunctional luminescence Sensors

Five isostructural lanthanide metal-organic frameworks, [Ln(BDPO)(H2O)4] (Ln= Eu for CUST-623, Tb for CUST-624, Gd for CUST-625, Dy for CUST-626, Sm for CUST-627, BDPO = N, N' bis (3,5 - dicarboxyphenyl)-oxalamide)...