Luminescent Single-Molecule Magnets as Dual Magneto-Optical Molecular Thermometers.

Luminescent thermometry allows the remote detection of the temperature and holds great potential in future technological applications in which conventional systems could not operate. Complementary approaches to measuring the temperature aiming to enhance the thermal sensitivity would however represent a decisive step forward. For the first time, we demonstrate the proof-of-concept that luminescence thermometry could be associated with a complementary temperature readout related to a different property. Namely, we propose to take advantage of the temperature dependence of both magnetic (canonical susceptibility and relaxation time) and luminescence features (emission intensity) found in Single-Molecule Magnets (SMM) to develop original dual magneto-optical molecular thermometers to conciliate high-performance SMM and Boltzmann-type luminescence thermometry. We highlight this integrative approach to concurrent luminescent and magnetic thermometry using an air-stable benchmark SMM [Dy(bbpen)Cl] (H2 bbpen=N,N'-bis(2-hydroxybenzyl)-N,N'-bis(2-methylpyridyl)ethyl-enediamine)) exhibiting Dy3+ luminescence. The synergy between multiparametric magneto-optical readouts and multiple linear regression makes possible a 10-fold improvement in the relative thermal sensitivity of the thermometer over the whole temperature range, compared with the values obtained with the single optical or magnetic devices.

Tunable Optical Molecular Thermometers Based on Metallacrowns.

The effect of ligands' energy levels on thermal dependence of lanthanide emission was examined to create new molecular nanothermometers. A series of Ln2Ga8L8'L8″ metallacrowns (shorthand Ln2L8'), where Ln = Gd3+, Tb3+, or Sm3+ (H3L' = salicylhydroxamic acid (H3shi), 5-methylsalicylhydroxamic acid (H3mshi), 5-methoxysalicylhydroxamic acid (H3moshi), and 3-hydroxy-2-naphthohydroxamic acid (H3nha)) and H2L″ = isophthalic acid (H2iph), was synthesized and characterized. Within the series, ligand-centered singlet state (S1) energy levels ranged from 23,300 to 27,800 cm-1, while triplet (T1) energy levels ranged from 18,150 to 21,980 cm-1. We demonstrated that the difference between T1 levels and relevant energies of the excited 4G5/2 level of Sm3+ (17,800 cm-1) and 5D4 level of Tb3+ (20,400 cm-1) is the major parameter controlling thermal dependence of the emission intensity via the back energy transfer mechanism. However, when the energy difference between S1 and T1 levels is small (below 3760 cm-1), the S1 → T1 intersystem crossing (and its reverse, S1 ← T1) mechanism contributes to the thermal behavior of metallacrowns. Both mechanisms affect Ln3+-centered room-temperature quantum yields with values ranging from 2.07(6)% to 31.2(2)% for Tb2L8' and from 0.0267(7)% to 2.27(5)% for Sm2L8'. The maximal thermal dependence varies over a wide thermal range (ca. 150-350 K) based on energy gaps between relevant ligand-based and lanthanide-based electronic states. By mixing Tb2moshi8' with Sm2moshi8' in a 1:1 ratio, an optical thermometer with a relative thermal sensitivity larger than 3%/K at 225 K was created. Other temperature ranges are also accessible with this approach.

Lanthanide Luminescence Thermometry and Slow Magnetic Relaxation in 3-D Polycyanidometallate-Based Materials.

Two three-dimensional (3-D) polycyanidometallate-based luminescent thermometers with the general formula {Ln4Co4(CN)24(4-benpyo)17(H2O)·7H2O}n Ln = (Dy(III)(1), Eu(III)(2)), based on the red-emissive diamagnetic linker [Co(CN)6]3- and the bulky pyridine derivative that possesses the N-oxide moiety, 4-benzyloxy-pyridine N-oxide (benpyo), were prepared for the first time. The structure of compound 1 has been determined by single-crystal X-ray crystallography while the purity and structure of 2 have been confirmed by CHN, Fourier transform infrared spectroscopy (FT-IR), and powder X-ray diffraction (PXRD) analysis. Magnetic AC susceptibility measurements at zero field show no single-molecule magnet (SMM) behavior indicating fast relaxation operating in 1. Upon application of an optimal field of 2 kOe, the SMM character of compound 1 is revealed while the τ(Τ) can be reproduced solely considering the Raman process τ-1 = CTn with C = 7.0901(3) s-1 K-n and n = 3.58(1), indicating that a high density of low-lying states and optical as well as acoustic phonons play a major role in the relaxation mechanism. Micron-sized superconducting quantum interference device (µ-SQUID) loops show a very narrow opening in agreement with the AC susceptibility studies and complete active space self-consistent field (CASSCF) calculations. The interaction operating between the Dy(III) ions was quantified from CASSCF calculations. Good agreement is found by fitting the experimental DC χMΤ(Τ) and M(H), employing the Lines model, with JLines = -0.087 cm-1 (-0.125 K). The excitation spectra of compound 2 are used for temperature sensing in the 25-325 nm range with a maximum relative thermal sensitivity, Sr = 0.6% K-1 at 325 K, whereas compound 1 operates as a luminescent thermometer based on its emission features in the temperature range of 16-350 K with Sr ≈ 2.3% K-1 at 240 K.

Dynamics of the Energy Transfer Process in Eu(III) Complexes Containing Polydentate Ligands Based on Pyridine, Quinoline, and Isoquinoline as Chromophoric Antennae.

In this work, we investigated from a theoretical point of view the dynamics of the energy transfer process from the ligand to Eu(III) ion for 12 isomeric species originating from six different complexes differing by nature of the ligand and the total charge. The cationic complexes present the general formula [Eu(L)(H2O)2]+ (where L = bpcd2- = N,N'-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane N,N'-diacetate; bQcd2- = N,N'-bis(2-quinolinmethyl)-trans-1,2-diaminocyclohexane N,N'-diacetate; and bisoQcd2- = N,N'-bis(2-isoquinolinmethyl)-trans-1,2-diaminocyclohexane N,N'-diacetate), while the neutral complexes present the Eu(L)(H2O)2 formula (where L = PyC3A3- = N-picolyl-N,N',N'-trans-1,2-cyclohexylenediaminetriacetate; QC3A3- = N-quinolyl-N,N',N'-trans-1,2-cyclohexylenediaminetriacetate; and isoQC3A3- = N-isoquinolyl-N,N',N'-trans-1,2-cyclohexylenediaminetriacetate). Time-dependent density functional theory (TD-DFT) calculations provided the energy of the ligand excited donor states, distances between donor and acceptor orbitals involved in the energy transfer mechanism (RL), spin-orbit coupling matrix elements, and excited-state reorganization energies. The intramolecular energy transfer (IET) rates for both singlet-triplet intersystem crossing and ligand-to-metal (and vice versa) involving a multitude of ligand and Eu(III) levels and the theoretical overall quantum yields (ϕovl) were calculated (the latter for the first time without the introduction of experimental parameters). This was achieved using a blend of DFT, Judd-Ofelt theory, IET theory, and rate equation modeling. Thanks to this study, for each isomeric species, the most efficient IET process feeding the Eu(III) excited state, its related physical mechanism (exchange interaction), and the reasons for a better or worse overall energy transfer efficiency (ηsens) in the different complexes were determined. The spectroscopically measured ϕovl values are in good agreement with the ones obtained theoretically in this work.

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 bbpen2- = N,N'-bis(2-oxidobenzyl)-N,N'-bis(pyridin-2-ylmethyl)ethylenediamine and bbppn2- = 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, QLnL, 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 (WISC) and singlet fluorescence lifetimes (τS) 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 QLnL.

Thermal enhancement of upconversion emission in nanocrystals: a comprehensive summary.

Luminescence thermal stability is a major figure of merit of lanthanide-doped nanoparticles playing an essential role in determining their potential applications in advanced optics. Unfortunately, considering the intensification of multiple electron-vibration interactions as temperature increases, luminescence thermal quenching of lanthanide-doped materials is generally considered to be inevitable. Recently, the emergence of thermally enhanced upconversion luminescence in lanthanide-doped nanoparticles seemed to challenge this stereotype, and the research on this topic rapidly aroused wide attention. While considerable efforts have been made to explore the origin of this phenomenon, the key mechanism of luminescence enhancement is still under debate. Here, to sort out the context of this intriguing finding, the reported results on this exciting topic are reviewed, and the corresponding enhancement mechanisms as proposed by different researchers are summarized. Detailed analyses are provided to evaluate the contribution of the most believed "surface-attached moisture desorption" process on the overall luminescence enhancement of lanthanide-doped nanoparticles at elevated temperatures. The impacts of other surface-related processes and shell passivation on the luminescence behaviour of the lanthanide-doped materials are also elaborated. Lack of standardization in the reported data and the absence of important experimental information, which greatly hinders the cross-checking and reanalysis of the results, is emphasized as well. On the foundation of these discussions, it is realized that the thermal-induced luminescence enhancement is a form of recovery process against the strong luminescence quenching in the system, and the enhancement degree is closely associated with the extent of luminescence loss induced by various quenching effects beforehand.

Inert Shell Effect on the Quantum Yield of Neodymium-Doped Near-Infrared Nanoparticles: The Necessary Shield in an Aqueous Dispersion.

Lanthanide-doped nanoparticles (LnNPs) are versatile near-infrared (NIR) emitting nanoprobes that have led to their growing interest for use in biomedicine-related imaging. Toward the brightest LnNPs, high photoluminescence quantum yield (PLQY) values are attained by implementing core/shell engineering, particularly with an optically inert shell. In this work, a thorough investigation is performed to quantify how an outer inert shell maintains the PLQY of Nd3+-doped LnNPs dispersed in an aqueous environment. Three relevant quantitative findings affecting the PLQY of Nd3+-doped LnNPs are identified: (i) the PLQY of core LnNPs is improved 3-fold upon inert shell coating; (ii) PLQY decreases with increasing Nd3+ doping despite the inert shell; and (iii) solvent quenching has a major influence on the PLQY of the LnNPs, though it is relatively lessened for high Nd3+ doping. Overall, we shed new light on the impact of the LnNP architecture on the NIR emission, as well as on the quenching effects caused by doping concentration and solvent molecules.

Exploring Single-Nanoparticle Dynamics at High Temperature by Optical Tweezers.

The experimental determination of the velocity of a colloidal nanoparticle (vNP) has recently became a hot topic. The thermal dependence of vNP is still left to be explored although it is a valuable source of information allowing, for instance, the discernment between ballistic and diffusive regimes. Optical tweezers (OTs) constitute a tool especially useful for the experimental determination of vNP although they have only been capable of determining it at room temperature. In this work, we demonstrate that it is possible to determine the temperature dependence of the diffusive velocity of a single colloidal nanoparticle by analyzing the temperature dependence of optical forces. The comparison between experimental results and theoretical predictions allowed us to discover the impact that the anomalous temperature dependence of water properties has on the dynamics of colloidal nanoparticles in this temperature range.

Real-Time Intracellular Temperature Imaging Using Lanthanide-Bearing Polymeric Micelles.

Measurement of thermogenesis in individual cells is a remarkable challenge due to the complexity of the biochemical environment (such as pH and ionic strength) and to the rapid and yet not well-understood heat transfer mechanisms throughout the cell. Here, we present a unique system for intracellular temperature mapping in a fluorescence microscope (uncertainty of 0.2 K) using rationally designed luminescent Ln3+-bearing polymeric micellar probes (Ln = Sm, Eu) incubated in breast cancer MDA-MB468 cells. Two-dimensional (2D) thermal images recorded increasing the temperature of the cells culture medium between 296 and 304 K shows inhomogeneous intracellular temperature progressions up to ∼20 degrees and subcellular gradients of ∼5 degrees between the nucleolus and the rest of the cell, illustrating the thermogenic activity of the different organelles and highlighting the potential of this tool to study intracellular processes.

[Ga3+ 8 Sm3+ 2 , Ga3+ 8 Tb3+ 2 ] Metallacrowns are Highly Promising Ratiometric Luminescent Molecular Nanothermometers Operating at Physiologically Relevant Temperatures.

Nanothermometry is the study of temperature at the submicron scale with a broad range of potential applications, such as cellular studies or electronics. Molecular luminescent-based nanothermometers offer a non-contact means to record these temperatures with high spatial resolution and thermal sensitivity. A luminescent-based molecular thermometer comprised of visible-emitting Ga3+ /Tb3+ and Ga3+ /Sm3+ metallacrowns (MCs) achieved remarkable relative thermal sensitivity associated with very low temperature uncertainty of Sr =1.9 % K-1 and δT<0.045 K, respectively, at 328 K, as an aqueous suspension of polystyrene nanobeads loaded with the corresponding MCs. To date, they are the ratiometric molecular nanothermometers offering the highest level of sensitivity in the physiologically relevant temperature range.

Developing Luminescent Ratiometric Thermometers Based on a Covalent Organic Framework (COF).

Covalent Organic Frameworks (COFs), an emerging class of crystalline porous materials, are proposed as a new type of support for grafting lanthanide ions (Ln3+ ) and employing these hybrid materials as ratiometric luminescent thermometers. A TpBpy-COF-prepared from 1,3,5-triformylphloroglucinol (Tp) and 2,2'-bipyridine-5,5'-diamine (Bpy) grafted with Eu/Tb and Dy acetylacetone (acac) complexes can be successfully used as a luminescent thermometer in the 10-360 K (Eu) and 280-440 K (Tb) ranges with good sensing properties (thermal sensitivity up to 1.403 % K-1 , temperature uncertainty δT<1 K above 110 K). For the Eu/Tb systems, we observe an unusual and rarely reported behavior, that is, no thermal quenching of the Tb3+ emission, a result of the absence of ion-to-ligand/host energy back-transfer. The LnCOF materials proposed here could be a new class of materials employed for temperature-sensing applications following up on the well-known luminescent metal-organic framework thermometers.

Gd- and Eu-Loaded Iron Oxide@Silica Core-Shell Nanocomposites as Trimodal Contrast Agents for Magnetic Resonance Imaging and Optical Imaging.

Superparamagnetic maghemite core-porous silica shell nanoparticles, γ-Fe2O3@SiO2 (FS), with 50 nm diameter and a 10 nm core, impregnated with paramagnetic complexes b-Ln ([Ln(btfa)3(H2O)2]) (where btfa = 4,4,4-trifluoro-l-phenyl-1,3-butanedione and Ln = Gd, Eu, and Gd/Eu), performing as promising trimodal T1-T2 MRI and optical imaging contrast agents, are reported. These nanosystems exhibit a high dispersion stability in water and no observable cytotoxic effects, witnessed by intracellular ATP levels. The structure and superparamagnetic properties of the maghemite core nanocrystals are preserved upon imbedding the b-Ln complexes in the shell. Hela cells efficiently and swiftly internalize the NPs into the cytosol, with no observable cytotoxicity below a concentration of 62.5 µg mL-1. These nanosystems perform better than the free b-Gd complex as T1 (positive) contrast agents in cellular pellets, while their performance as T2 (negative) contrast agents is similar to the FS. Embedding of the b-Eu complex in the silica pores endows the nanoparticles with strong luminescence properties. The impregnation of gadolinium and europium complexes in a 1:1 ratio afforded a trimodal nanoplatform performing as a luminescent probe and a double T1 and T2 MRI contrast agent even more efficient than b-Gd used on its own, as observed in cell-labeled imaging experiments and MRI cell pellets.

Promoting a Significant Increase in the Photoluminescence Quantum Yield of Terbium(III) Complexes by Ligand Modification.

Two discrete mononuclear complexes, [Tb(bbpen)(NO3)] (I) and [Tb(bbppn)(NO3)] (II), for which H2bbpen = N,N'-bis(2-hydroxybenzyl)-N,N'-bis(pyridin-2-ylmethyl)ethylenediamine and H2bbppn = N,N'-bis(2-hydroxylbenzyl)-N,N'-bis(pyridin-2-ylmethyl)-1,2-propanediamine, were synthesized and characterized by FTIR, Raman, and photoluminescence (PL, steady-state and time-resolved modes) spectroscopy. The attachment of a methyl group to the ethylenediamine portion of the ligand backbone differentiates II from I and acts as a determining feature to both the structural and optical properties of the former. The single-crystal X-ray structure of H2bbppn is described here for the first time, while that of complex II has been redetermined in the monoclinic C2 space group in light of new diffraction data. In II, selective crystallization leads to spontaneous resolution of enantiomeric molecules in different crystals. Absolute emission quantum yields (ϕ) and luminescence excited-state lifetimes (at room temperature and 11 K) were measured for both complexes. Despite their similar molecular structures, I and II exhibit remarkably different ϕ values of 21 ± 2% and 67 ± 7%, respectively, under UV excitation at room temperature. Results of quantum-mechanical (DFT and TD-DFT) calculations and experimental PL measurements also performed for H2bbpen and H2bbppn confirmed that both ligands are suitable to work as "antennas" for TbIII. Considering the 5D4 lifetime profiles and the significantly higher absolute quantum yield of II, it appears that thermally active nonradiative pathways present in I are minimized in II due to differences in the conformation of the ethylenediamine bridge.

Infrared and Raman spectroscopy of non-conventional hydrogen bonding between N,N'-disubstituted urea and thiourea groups: a combined experimental and theoretical investigation.

The variety of H bond (HB) interactions is a source of inspiration for bottom-up molecular engineering through self-aggregation. Non-conventional intermolecular HBs between N,N'-disubstituted urea and thiourea are studied in detail by vibrational spectroscopies and ab initio calculations. Raman and IR mode assignments are given. We show that it is possible to study selectively the different intermolecular bifurcated intra- and inter-dimer HBs with the two types of HB acceptors. Through the ab initio calculation, the thioamide I mode, a specific marker of N-HS[double bond, length as m-dash]C HB interactions, is unambiguously identified.

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 Pi 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[(Lu0.75 Yb0.20 Er0.05 )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 F7/2 →2 F5/2 (Yb3+ ) and 4 I13/2 →4 I15/2 (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.

Tethering Luminescent Thermometry and Plasmonics: Light Manipulation to Assess Real-Time Thermal Flow in Nanoarchitectures.

The past decade has seen significant progresses in the ability to fabricate new mesoporous thin films with highly controlled pore systems and emerging applications in sensing, electrical and thermal isolation, microfluidics, solar cells engineering, energy storage, and catalysis. Heat management at the micro- and nanoscale is a key issue in most of these applications, requiring a complete thermal characterization of the films that is commonly performed using electrical methods. Here, plasmonic-induced heating (through Au NPs) is combined with Tb3+/Eu3+ luminescence thermometry to measure the thermal conductivity of silica and titania mesoporous nanolayers. This innovative method yields values in accord with those measured by the evasive and destructive conventional 3ω-electrical method, simultaneously overcoming their main limitations, for example, a mandatory deposition of additional isolating and metal layers over the films and the previous knowledge of the thermal contact resistance between the heating and the mesoporous layers.

Dual-Property Supramolecular H-Bonded 15-Crown-5 Ln(III) Chains: Joint Magneto-Luminescence and ab Initio Studies.

Two complexes comprising 9-coordinate capped square antiprismatic [Ln(NO3)3(OH2)2(MeOH)] units [Ln(III) = Dy 6; Tb 7] are reported in which the metal complexes are hydrogen-bonded to 15C5 (15-crown-5) macrocycles to form supramolecular chains, {[Ln(NO3)3(OH2)2(MeOH)]·(15C5)}n. Alternating current magnetic susceptibility measurements supported by ab initio studies show field-induced SMM (single-molecule magnet) behavior for 6, but rapid relaxation of the magnetization for 7 because of the presence of dominant quantum tunneling processes as evidenced by the presence of a significant calculated tunnel splitting within the ground-state multiplet. Modeling the high-resolution emission spectra for 6 afforded energies of 37 ± 5 and 28 ± 5 cm-1 for the first-excited-state Stark sublevels of the two crystallographically independent Dy1 and Dy2 ions, in excellent agreement with the calculated values of 31 and 21 cm-1 for ΔE1 derived from ab initio studies.

Lanthanide Organic Framework Luminescent Thermometers.

Metal-organic frameworks (MOFs) are excellent platforms for engineering luminescence properties as their building blocks, metal ions, linkers, and guest ions or molecules, are all potential sources of light emission. Temperature is one of the most important physical properties affecting the dynamics and viability of natural and engineered systems. Because the luminescence of certain lanthanide-bearing MOFs changes considerably with temperature, in the last few years, these materials have been explored as optical thermometers, especially in temperature sensing based on the intensity ratios of two separate electronic transitions. This review discusses the main concepts and ideas assisting the design of such ratiometric thermometers, and identifies the main challenges presented to this nascent field: develop nanothermometers for bio-applications and nanomedicine; understand the energy transfer mechanisms determining the thermal sensitivity; achieve effective primary thermometers; realize multifunctional nanothermometers; integrate Ln3+ -based thermometers in commercial products.

Photoluminescent thermometer based on a phase-transition lanthanide silicate with unusual structural disorder.

The hydrothermal synthesis of the novel Na[LnSiO4] (Ln = Gd, Eu, Tb) disordered orthorhombic system is reported. At 100 K, and above, these materials are best described in the centrosymmetric orthorhombic Pnma space group. At lower temperatures (structure solved at 30 K) the unit cell changes to body-centered with Imma symmetry. The materials exhibit unique photophysical properties, arising from both, this phase transformation, and the disorder of the Ln(3+) ions, located at a site with D2d point symmetry. Na[(Gd0.8Eu0.1Tb0.1)SiO4] is an unprecedented case of a luminescent ratiometric thermometer based on a very stable silicate matrix. Moreover, it is the first example of an optical thermometer whose performance (viz., excellent sensitivity at cryogenic temperatures <100 K) is determined mainly by a structural transition, opening up new opportunities for designing such devices.

Diffusive random laser modes under a spatiotemporal scope.

At present the prediction and characterization of the emission output of a diffusive random laser remains a challenge, despite the variety of investigated materials and theoretical interpretations given up to now. Here, a new mode selection method, based on spatial filtering and ultrafast detection, which allows to separate individual lasing modes and follow their temporal evolution is presented. In particular, the work explores the random laser behavior of a ground powder of an organic-inorganic hybrid compound based on Rhodamine B incorporated into a di-ureasil host. The experimental approach gives direct access to the mode structure and dynamics, shows clear modal relaxation oscillations, and illustrates the lasing modes stochastic behavior of this diffusive scattering system. The effect of the excitation energy on its modal density is also investigated. Finally, imaging measurements reveal the dominant role of diffusion over amplification processes in this kind of unconventional lasers.