Cadmium–Furandicarboxylate Coordination Polymers Prepared with Different Types of Pyridyl Linkers: Synthesis, Divergent Dimensionalities, and Luminescence Study

Five new metal–organic frameworks (MOFs) have been synthesized by using cadmium ion and 2,5-furandicarboxylic acid in presence of a variety of bridging amine ligands, [Cd­(fdc)­(2,2′-bpy)­(H₂O)]_<i>n</i> (<b>1</b>), {[Cd­(fdc)­(pyz)­(H₂O)₂]­[Cd­(fdc)]­(H₂O)₂]·H₂O}_<i>n</i> (<b>2</b>), {[Cd­(fdc)­(4,4′-bpy)­(H₂O)₂]·EtOH}_<i>n</i> (<b>3</b>), [Cd­(fdc)­(1,2-bpe)­(H₂O)]_<i>n</i> (<b>4</b>), and [{Cd₂(fdc)₂(H₂O)₄}·(1,2-bpe)]_<i>n</i> (<b>5</b>), where fdc = 2,5-furandicarboxylic acid, 2,2′-bpy = 2,2′-bipyridyl, pyz = pyrazine, 4,4′-bpy = 4,4′-bipyridyl, 1,2-bpe = 1,2-di­(4-pyridyl)­ethylene. All the compounds were characterized by single-crystal X-ray analysis and show diversities in their structures. Compound <b>1</b> shows linear topology propagating along the crystallographic <i>b</i>-axis. Compound <b>2</b> shows supramolecular structure, where two types of 1D double chains (ladder type) are present. These chains propagate along the crystallographic <i>a</i>-axis and are tightly held with each other by strong hydrogen bonds. Compound <b>3</b> reveals a 1D + 1D → 2D polycatenated MOF, where four cadmium centers form a perfect square and these squares are further linked by the carboxylate ligand, forming a 1D tube. These tubes are interpenetrated with each other forming a polycatenated 3D MOF. Compound <b>4</b> also possesses a polycatenated MOF, but 1D sheets are polycatenated with each other forming the 1D + 1D → 3D MOF. Compound <b>5</b> is a 2D-based supramolecular 3D MOF, where 1,2-bpe ligands are entrapped within the layer of the 2D by strong hydrogen bonds and π···π interaction. Luminescence of all the compounds has been investigated

Cadmium–Furandicarboxylate Coordination Polymers Prepared with Different Types of Pyridyl Linkers: Synthesis, Divergent Dimensionalities, and Luminescence Study

Five new metal–organic frameworks (MOFs) have been synthesized by using cadmium ion and 2,5-furandicarboxylic acid in presence of a variety of bridging amine ligands, [Cd­(fdc)­(2,2′-bpy)­(H₂O)]_<i>n</i> (<b>1</b>), {[Cd­(fdc)­(pyz)­(H₂O)₂]­[Cd­(fdc)]­(H₂O)₂]·H₂O}_<i>n</i> (<b>2</b>), {[Cd­(fdc)­(4,4′-bpy)­(H₂O)₂]·EtOH}_<i>n</i> (<b>3</b>), [Cd­(fdc)­(1,2-bpe)­(H₂O)]_<i>n</i> (<b>4</b>), and [{Cd₂(fdc)₂(H₂O)₄}·(1,2-bpe)]_<i>n</i> (<b>5</b>), where fdc = 2,5-furandicarboxylic acid, 2,2′-bpy = 2,2′-bipyridyl, pyz = pyrazine, 4,4′-bpy = 4,4′-bipyridyl, 1,2-bpe = 1,2-di­(4-pyridyl)­ethylene. All the compounds were characterized by single-crystal X-ray analysis and show diversities in their structures. Compound <b>1</b> shows linear topology propagating along the crystallographic <i>b</i>-axis. Compound <b>2</b> shows supramolecular structure, where two types of 1D double chains (ladder type) are present. These chains propagate along the crystallographic <i>a</i>-axis and are tightly held with each other by strong hydrogen bonds. Compound <b>3</b> reveals a 1D + 1D → 2D polycatenated MOF, where four cadmium centers form a perfect square and these squares are further linked by the carboxylate ligand, forming a 1D tube. These tubes are interpenetrated with each other forming a polycatenated 3D MOF. Compound <b>4</b> also possesses a polycatenated MOF, but 1D sheets are polycatenated with each other forming the 1D + 1D → 3D MOF. Compound <b>5</b> is a 2D-based supramolecular 3D MOF, where 1,2-bpe ligands are entrapped within the layer of the 2D by strong hydrogen bonds and π···π interaction. Luminescence of all the compounds has been investigated

Upconverting Nanoparticles Working As Primary Thermometers In Different Media

In the past decade, noninvasive luminescent thermometry has become popular due to the limitations of traditional contact thermometers to operate at scales below 100 μm, as required by current demands in disparate areas. Generally, the calibration procedure requires an independent measurement of the temperature to convert the thermometric parameter (usually an intensity ratio) to temperature. A new calibration procedure is necessary whenever the thermometer operates in a different medium. However, recording multiple calibrations is a time-consuming task, and not always possible to perform, e.g., in living cells and in electronic devices. Typically, a unique calibration relation is assumed to be valid, independent of the medium, which is a bottleneck of the secondary luminescent thermometers developed up to now. Here we report a straightforward method to predict the temperature calibration curve of any upconverting thermometer based on two thermally coupled electronic levels independently of the medium, demonstrating that these systems are intrinsically primary thermometers. SrF₂:Yb/Er powder and water suspended nanoparticles were used as an illustrative example

Lamellar Salt-Doped Hybrids with Two Reversible Order/Disorder Phase Transitions

A lamellar bilayer hierarchically structured amide cross-linked alkyl/siloxane hybrid matrix (mono-amidosil, m-A(14)) was doped with a wide concentration range of potassium triflate (KCF₃SO₃), magnesium triflate (Mg­(CF₃SO₃)₂), and europium triflate (Eu­(CF₃SO₃)₃). In the K⁺-, Mg²⁺-, and Eu³⁺-based samples with <i>n</i> ≥ 5, 20, and 60 (where <i>n</i> is the molar ratio of amide CO groups per cation), respectively, the original lamellar structure of m-A(14) coexists with a new lamellar phase with lower interlamellar distance. The texture of the mono-amidosils doped with K⁺, Mg²⁺, and Eu³⁺ ions mimics cabbage leaves, foliated schist, and sea sponges, respectively. In the three series of materials, the cations bond to the oxygen atoms of the amide carbonyl groups. The amide–amide hydrogen-bonded array of m-A(14) is less perturbed by the inclusion of KCF₃SO₃ and Mg­(CF₃SO₃)₂ than by the incorporation of Eu­(CF₃SO₃)₃. The degree of ionic association is low for <i>n</i> ≥ 20. The cations coordinate to the oxygen atoms of the triflate ions, forming contact ion pairs at higher salt content. In the Mg­(CF₃SO₃)₂- and Eu­(CF₃SO₃)₃-containing materials with <i>n</i> = 5 and 10, respectively, crystalline salt is formed. The structural changes undergone by the alkyl chains of selected mono-amidosils in a heating/cooling cycle are reversible, are time-independent, and exhibit two distinct hysteresis domains, one associated with the order/disorder phase transition of the original lamellar bilayer structure of m-A(14) and the second one associated with the order/disorder phase transition of the new lamellar bilayer structure formed in the presence of the salts

Metal-Free Highly Luminescent Silica Nanoparticles

Stable, cost-effective, brightly luminescent, and metal-free organosilica nanoparticles (NPs) were prepared using the Stöber method without any thermal treatment above 318 K. The white-light photoluminescence results from a convolution of the emission originated in the NH₂ groups of the organosilane and oxygen defects in the silica network. The time-resolved emission spectra are red-shifted, relative to those acquired in the steady-state regime, pointing out that the NPs emission is governed by donor–acceptor (D<i>–</i>A) recombination mechanisms. Moreover, the increase of the corresponding lifetime values with the monitored wavelength further supports that the emission is governed by a recombination mechanism typical of a D<i>–</i>A pair attributed to an exceptionally broad inhomogeneous distribution of the emitting centers peculiar to silica-based NPs. These NPs exhibit the highest emission quantum yield value (0.15 ± 0.02) reported so far for organosilica biolabels without activator metals. Moreover, the emission spectra and the quantum yield values are quite stable over time showing no significant aging effects after exposure to the ambient environment for more than 1 year, stressing the potential of these NPs as metal-free biolabels

Ratiometric Nanothermometer Based on an Emissive Ln³⁺-Organic Framework

Luminescent thermometers working at the nanoscale with high spatial resolution, where the conventional methods are ineffective, have emerged over the last couple of years as a very active field of research. Lanthanide-based materials are among the most versatile thermal probes used in luminescent nanothermometers. Here, nanorods of metal organic framework Tb_0.99Eu_0.01(BDC)_1.5(H₂O)₂ (BDC = 1-4-benzendicarboxylate) have been prepared by the reverse microemulsion technique and characterized and their photoluminescence properties studied from room temperature to 318 K. Aqueous suspensions of these nanoparticles display an excellent performance as ratiometric luminescent nanothermometers in the physiological temperature (300–320 K) range

Modulating the Photoluminescence of Bridged Silsesquioxanes Incorporating Eu³⁺-Complexed <i>n</i>,<i>n</i>′-Diureido-2,2′-bipyridine Isomers: Application for Luminescent Solar Concentrators

Two new urea-bipyridine derived bridged organosilanes (<b>P5</b> and <b>P6</b>) have been synthesized and their hydrolysis–condensation under nucleophilic catalysis in the presence of Eu³⁺ salts led to luminescent bridged silsesquioxanes (<b>M5-Eu</b> and <b>M6-Eu</b>). An important loading of Eu³⁺ (up to 11%_w) can be obtained for the material based on the 6,6′-isomer. Indeed the photoluminescence properties of these materials, that have been investigated in depth (photoluminescence (PL), quantum yield, lifetimes), show a significantly different complexation mode of the Eu³⁺ ions for <b>M6-Eu</b>, compared with <b>M4-Eu</b> (obtained from the already-reported 4,4′-isomer) and <b>M5-Eu</b>. Moreover, <b>M6-Eu</b> exhibits the highest absolute emission quantum yield value (0.18 ± 0.02) among these three materials. The modification of the sol composition upon the addition of a malonamide derivative led to similar luminescent features but with an increased quantum yield (0.26 ± 0.03). In addition, <b>M6-Eu</b> can be processed as thin films by spin-coating on glass substrates, leading to plates coated by a thin layer (∼54 nm) of Eu³⁺-containing hybrid silica exhibiting one of the highest emission quantum yields reported so far for films of Eu³⁺-containing hybrids (0.34 ± 0.03) and an interesting potential as new luminescent solar concentrators (LSCs) with an optical conversion efficiency of ∼4%. The ratio between the light guided to the film edges and the one emitted by the surface of the film was quantified through the mapping of the intensity of the red pixels (in the RGB color model) from a film image. This quantification enabled a more accurate estimation of the transport losses due to the scattering of the emitted light in the film (0.40), thereby correcting the initial optical conversion efficiency to a value of 1.7%

Photo–Click Chemistry to Design Highly Efficient Lanthanide β‑Diketonate Complexes Stable under UV Irradiation

Europium (<i><b>t</b></i><b>-Eu</b>) and gadolinium (<i><b>t</b></i><b>-Gd</b>) β-diketonate complexes with photoactive <i>t</i>-bpete ligand, [Ln­(btfa)₃(<i>t</i>-bpete)­(MeOH)] (Ln = Eu, Gd), where btfa^– and <i>t</i>-bpete are 4,4,4-trifluoro-1-phenyl-1,3-butanedionate and <i>trans</i>-1,2-bis­(4-pyridyl)­ethylene, respectively, were synthesized, characterized by vibrational, absorption (reflectance) and photoluminescence spectroscopies and their crystal structure was determined using single-crystal X-ray diffraction. B3LYP calculations were performed to support the interpretation and rationalization of the experimental results. The complexes, under UV irradiation, do not display the typical photodegradation of the β-diketonate ligands exhibiting, in turn, an unprecedented photostability during, at least, 10 h. During UV-A exposure (>330 nm), the emission intensities of both complexes increase drastically (∼20 times), whereas for <i><b>t</b></i><b>-Eu</b> the emission quantum yield is enhanced at least 30-fold. A mechanism based on a photoclick trans-to-cis isomerization of both <i>t</i>- and <i>c</i>-bpete moieties was proposed to explain the abnormal photostability of these compounds, either in solid state or in solution. The experimental and computational results are consistent with a photostationary state involving the trans-to-cis isomerization of the bpete ligand under continuous UV-A exposure, which thus diverts the incident radiation from other deleterious photochemical or photophysical processes that cause the typical photobleaching behavior of chelate lanthanide complexes. This shielding mechanism could be extended to other ligands permitting the design of new lanthanide-based photostable systems under UV exposure for applications in lighting, sensing, and displays

High-Performance Near-Infrared Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) appear as candidates to enhance the performance of photovoltaic (PV) cells and contribute to reduce the size of PV systems, decreasing, therefore, the amount of material needed and thus the cost associated with energy conversion. One way to maximize the device performance is to explore near-infrared (NIR)-emitting centers, resonant with the maximum optical response of the most common Si-based PV cells. Nevertheless, very few examples in the literature demonstrate the feasibility of fabricating LSCs emitting in the NIR region. In this work, NIR-emitting LSCs are reported using silicon 2,3-naphthalocyanine bis­(trihexylsilyloxide) (SiNc or NIR775) immobilized in an organic–inorganic tri-ureasil matrix (t-U(5000)). The photophysical properties of the SiNc dye incorporated into the tri-ureasil host closely resembled those of SiNc in tetrahydrofuran solution (an absolute emission quantum yield of ∼0.17 and a fluorescence lifetime of ∼3.6 ns). The LSC coupled to a Si-based PV device revealed an optical conversion efficiency of ∼1.5%, which is among the largest values known in the literature for NIR-emitting LSCs. The LSCs were posteriorly coupled to a Si-based commercial PV cell, and the synergy between the t-U(5000) and SiNc molecules enabled an effective increase in the external quantum efficiency of PV cells, exceeding 20% in the SiNc absorption region

Bifunctional Mixed-Lanthanide Cyano-Bridged Coordination Polymers Ln_0.5Ln′_0.5(H₂O)₅[W(CN)₈] (Ln/Ln′ = Eu³⁺/Tb³⁺, Eu³⁺/Gd³⁺, Tb³⁺/Sm³⁺)

A new family of mixed-lanthanide cyano-bridged coordination polymers Ln_0.5Ln′_0.5(H₂O)₅[W­(CN)₈] (where Ln/Ln′ = Eu³⁺/Tb³⁺, Eu³⁺/Gd³⁺, and Tb³⁺/Sm³⁺) containing two lanthanide and one transition metal ions were obtained and characterized by X-ray diffraction, photoluminescence spectroscopy, magnetic analyses, and theoretical computation. These compounds are isotypical and crystallize in the tetragonal system <i>P</i>4<i>/nmm</i> forming two-dimensional grid-like networks. They present a magnetic ordering at low temperature and display the red Eu³⁺ (⁵D₀ → ⁷F_0–4) and green Tb³⁺ (⁵D₄ → ⁷F_6–2) characteristic photoluminescence. The Tb_0.5Eu_0.5(H₂O)₅[W­(CN)₈] compound presents therefore green and red emission and shows Tb³⁺-to-Eu³⁺ energy transfer