Photophysical and Primary Self-Referencing Thermometric Properties of Europium Hydrogen-Bonded Triazine Frameworks.

Lanthanide hydrogen-bonded organic frameworks (LnHOFs) are recently emerging as a novel versatile class of multicomponent luminescent materials with promising potential applications in optics and photonics. Trivalent europium (Eu3+) incorporated polymeric hydrogen-bonded triazine frameworks (PHTF:Eu) have been successfully obtained via a facile and low-cost thermal pyrolysis route. The PHTF:Eu material shows a porous frame structure principally composed of isocyanuric acid and ammelide as a minor constituent. Intense red luminescence with high colour-purity from Eu3+ is obtained by exciting over a broad absorption band peaked at 300 nm either at room or low temperature. The triazine-based host works as excellent optical antenna towards Eu3+, yielding ~42% sensitization efficiency (ηsens) and an intrinsic quantum yield of Eu3+ emission (ΦEu) as high as ~46%. Temperature-dependent emission studies show that PHTF:Eu displays relatively high optical stability at elevated temperatures in comparison to traditional inorganic phosphors. The retrieved activation energy of 89 meV indicates that thermal quenching mechanisms are attributed to the intrinsic energy level structure of the metal-triazine assembly, possibly via a thermally activated back transfer to ligand triplet or CT states. Finally, by using an innovative approach based on excitation spectra, we demonstrate that PHTF:Eu can work as a universal primary self-referencing thermometer based on a single-emitting center with excellent relative sensitivity in the cryogenic temperature range.

Luminescent PMMA Films and PMMA@SiO2 Nanoparticles with Embedded Ln3+ Complexes for Highly Sensitive Optical Thermometers in the Physiological Temperature Range*.

In recent years, luminescent materials doped with Ln3+ ions have attracted much attention for their application as optical thermometers based on both downshifting and upconversion processes. This study presents research done on the development of highly sensitive optical thermometers in the physiological temperature range based on poly(methyl methacrylate) (PMMA) films doped with two series of visible Ln3+ complexes (Ln3+ =Tb3+ , Eu3+ , and Sm3+ ) and SiO2 nanoparticles (NPs) coated with these PMMA films. The best performing PMMA film doped with Tb3+ and Eu3+ complexes was the PMMA[TbEuL1 tppo]1 film (L1 =4,4,4-trifluoro-1-phenyl-1,3-butadionate; tppo=triphenylphosphine oxide), which showed good temperature sensing of Sr =4.21 % K-1 at 313 K, whereas for the PMMA films doped with Tb3+ and Sm3+ complexes the best performing was the PMMA[TbSmL2 tppo]3 film (L2 =4,4,4-trifluoro-1-(4-chlorophenyl)-1,3-butadionate), with Sr =3.64 % K-1 at 313 K. Additionally, SiO2 NPs coated with the best performing films from each of the series of PMMA films (Tb-Eu and Tb-Sm) and their temperature-sensing properties were studied in water, showing excellent performance in the physiological temperature range (PMMA[TbEuL1 tppo]1@SiO2 : Sr =3.84 % °C at 20 °C; PMMA[TbSmL2 tppo]3@SiO2 : Sr =3.27 % °C at 20 °C) and the toxicity of these nanoparticles on human cells was studied, showing that they were nontoxic.

A Visible-Light-Harvesting Covalent Organic Framework Bearing Single Nickel Sites as a Highly Efficient Sulfur-Carbon Cross-Coupling Dual Catalyst.

Covalent Organic Frameworks (COFs) have recently emerged as light-harvesting devices, as well as elegant heterogeneous catalysts. The combination of these two properties into a dual catalyst has not yet been explored. We report a new photosensitive triazine-based COF, decorated with single Ni sites to form a dual catalyst. This crystalline and highly porous catalyst shows excellent catalytic performance in the visible-light-driven catalytic sulfur-carbon cross-coupling reaction. Incorporation of single transition metal sites in a photosensitive COF scaffold with two-component synergistic catalyst in organic transformation is demonstrated for the first time.

Design and visualization of second-generation cyanoisoindole-based fluorescent strigolactone analogs.

Strigolactones (SLs) are a family of terpenoid allelochemicals that were recognized as plant hormones only a decade ago. They influence a myriad of both above- and below-ground developmental processes, and are an important survival strategy for plants in nutrient-deprived soils. A rapidly emerging approach to gain knowledge on hormone signaling is the use of traceable analogs. A unique class of labeled SL analogs was constructed, in which the original tricyclic lactone moiety of natural SLs is replaced by a fluorescent cyanoisoindole ring system. Biological evaluation as parasitic seed germination stimulant and hypocotyl elongation repressor proved the potency of the cyanoisoindole strigolactone analogs (CISAs) to be comparable to the commonly accepted standard GR24. Additionally, via a SMXL6 protein degradation assay, we provided molecular evidence that the compounds elicit SL-like responses through the natural signaling cascade. All CISAs were shown to exhibit fluorescent properties, and the high quantum yield and Stokes shift of the pyrroloindole derivative CISA-7 also enabled in vivo visualization in plants. In contrast to the previously reported fluorescent analogs, CISA-7 displays a large similarity in shape and structure with natural SLs, which renders the analog a promising tracer to investigate the spatiotemporal distribution of SLs in plants and fungi.

DNA Intercalating Near-Infrared Luminescent Lanthanide Complexes Containing Dipyrido[3,2-a:2',3'-c]phenazine (dppz) Ligands: Synthesis, Crystal Structures, Stability, Luminescence Properties and CT-DNA Interaction.

In order to create near-infrared (NIR) luminescent lanthanide complexes suitable for DNA-interaction, novel lanthanide dppz complexes with general formula [Ln(NO3)3(dppz)2] (Ln = Nd3+, Er3+ and Yb3+; dppz = dipyrido[3,2-a:2',3'-c]phenazine) were synthesized, characterized and their luminescence properties were investigated. In addition, analogous compounds with other lanthanide ions (Ln = Ce3+, Pr3+, Sm3+, Eu3+, Tb3+, Dy3+, Ho3+, Tm3+, Lu3+) were prepared. All complexes were characterized by IR spectroscopy and elemental analysis. Single-crystal X-ray diffraction analysis of the complexes (Ln = La3+, Ce3+, Pr3+, Nd3+, Eu3+, Er3+, Yb3+, Lu3+) showed that the lanthanide's first coordination sphere can be described as a bicapped dodecahedron, made up of two bidentate dppz ligands and three bidentate-coordinating nitrate anions. Efficient energy transfer was observed from the dppz ligand to the lanthanide ion (Nd3+, Er3+ and Yb3+), while relatively high luminescence lifetimes were detected for these complexes. In their excitation spectra, the maximum of the strong broad band is located at around 385 nm and this wavelength was further used for excitation of the chosen complexes. In their emission spectra, the following characteristic NIR emission peaks were observed: for a) Nd3+: 4F3/2 → 4I9/2 (870.8 nm), 4F3/2 → 4I11/2 (1052.7 nm) and 4F3/2 → 4I13/2 (1334.5 nm); b) Er3+: 4I13/2 → 4I15/2 (1529.0 nm) c) Yb3+: 2F5/2 → 2F7/2 (977.6 nm). While its low triplet energy level is ideally suited for efficient sensitization of Nd3+ and Er3+, the dppz ligand is considered not favorable as a sensitizer for most of the visible emitting lanthanide ions, due to its low-lying triplet level, which is too low for the accepting levels of most visible emitting lanthanides. Furthermore, the DNA intercalation ability of the [Nd(NO3)3(dppz)2] complex with calf thymus DNA (CT-DNA) was confirmed using fluorescence spectroscopy.

Vibrational Quenching in Near-Infrared Emitting Lanthanide Complexes: A Quantitative Experimental Study and Novel Insights.

Two series of novel NIR-emissive complexes of Nd3+ , Sm3+ , Er3+ and Yb3+ with two different ß-diketonate ligands (L1 =4,4,4-trifluoro-1-phenyl-1,3-butadione and L2 =4,4,4-trifluoro-1-(4-chlorophenyl)-1,3-butadione) are reported. The neutral triphenylphosphine oxide (tppo) ligand was used to replace coordinated water molecules in the first coordination sphere of the as-obtained [Ln(L1(2) )3 (H2 O)2 ] complexes to afford water-free [Ln(L1(2) )3 (tppo)2 ] molecular species. Upon replacement of water molecules by tppo units, the NIR emission lifetimes of the Nd3+ , Er3+ and Sm3+ complexes increase by about one order of magnitude up to values of ≈9, 8 and 113 ms while Yb3+ complexes reach intrinsic quantum yields as high as to ΦYb =6.5 %., which are remarkably high for fully hydrogenated complexes. Vibrational quenching by CH and OH oscillators has been quantitatively assessed by implementing the Förster's model of resonance energy transfer on the basis of experimental data. This study demonstrates that highly efficient NIR-emitting lanthanide complexes can be obtained with facile, cheap and accessible syntheses through a rational design.

Correction: Mutual energy transfer luminescent properties in novel CsGd(MoO4)2:Yb3+,Er3+/Ho3+ phosphors for solid-state lighting and solar cells.

Correction for 'Mutual energy transfer luminescent properties in novel CsGd(MoO4)2:Yb3+,Er3+/Ho3+ phosphors for solid-state lighting and solar cells' by Kai Li et al., Phys. Chem. Chem. Phys., 2019, DOI: 10.1039/c8cp06538a.

Mutual energy transfer luminescent properties in novel CsGd(MoO4)2:Yb3+,Er3+/Ho3+ phosphors for solid-state lighting and solar cells.

In this work, we prepared a novel kind of Yb3+,Er3+/Ho3+ co-doped CsGd(MoO4)2 phosphors with a different structure from the reported ALn(MoO4)2 (A = Li, Na or K; Ln = La, Gd or Y) compounds using a high-temperature solid-state reaction method. X-ray diffraction showed that the as-prepared samples had a pure phase. Based on the efficient energy transfer from Yb3+ to Er3+/Ho3+, the up-conversion (UC) luminescence of the optimal CsGd(MoO4)2:0.30Yb3+,0.02Er3+ sample showed intensely green light with dominant emission peaks at 528 and 550 nm corresponding to Er3+ transitions 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2, respectively, as well as a weak emission peak originating from 4F9/2 → 4I15/2 at 671 nm, under 975 nm laser excitation. The CsGd(MoO4)2:Yb3+,Ho3+ samples mainly displayed two emission bands around 540 and 660 nm together with a negligible one at 755 nm, which corresponded to Ho3+ transitions 4F4,5F2 → 5I8, 5F5 → 5I8 and 4F4,5F2 → 5I7, respectively, under 975 nm laser excitation. With increasing Yb3+ concentration in CsGd(MoO4)2:Yb3+,Ho3+ phosphors, the emission color could be tuned from orange red to light yellow due to the large energy gap between levels 4F4,5F2 and 5F5. In addition, the CsGd(MoO4)2:Yb3+,Er3+ showed green light under 376 nm UV irradiation similar to that upon 975 nm laser excitation. However, the emissions for CsGd(MoO4)2:Yb3+,Ho3+ samples under 358 nm UV or 449 nm blue excitation showed dominant emission peaks at 540 nm and weak 660 nm and 752 nm peaks, which were a bit different from those under 975 nm excitation. Interestingly, we observed efficient energy transfer phenomena (possible quantum cutting) from Er3+/Ho3+ to Yb3+ and a Yb3+-O2- charge transfer (CT) transition in the molybdates, which was deduced from the visible and near-infrared emission spectra and the decrease of the Er3+/Ho3+ luminescent lifetimes with increasing Yb3+ concentration in the CsGd(MoO4)2:Yb3+,Er3+/Ho3+ samples. The luminescence properties of these phosphors suggest their potential possibility for applications in solid-state lighting and displays as well as in c-Si solar energy conversion systems.

New platinum(II) and palladium(II) complexes with substituted terpyridine ligands: synthesis and characterization, cytotoxicity and reactivity towards biomolecules.

A series of palladium(II) (1-3) and platinum(II) chloride complexes (4 and 5) with 2,2':6',2″-terpyridine (terpy) derivatives substituted at the 4' position, was synthesized and fully characterized. Single crystal X-ray diffraction analysis of complexes 2, 3 and 5 showed tridentate coordination of the 4'-substituted terpyridine (terpy) ligands to the metal center. Moreover, in vitro cytotoxic activity of these complexes toward a panel of human cancer cell lines (lung cancer A549, colorectal cancer HCT116, ovarian cancer IGROV-1) and toward normal cell line HDF (dermal fibroblast) was determined by Trypan Blue exclusion assay. Overall, the tested compounds manifested a relevant cytotoxicity for the selected cancer cell lines with complex 4 also showing a modest cytotoxicity on the normal cell lines. To better understand the mode of action of these metal complexes, their reactivity with three model proteins, i.e. hen egg white lysozyme (HEWL), cytochrome c (cyt c) and ribonuclease A (RNase A) were comparatively investigated through ESI-MS analysis. The results highlighted a different behavior between the two series of complexes being platinum compounds more reactive toward RNase and cyt c than palladium compounds. Based on the obtained results, it is proposed that in presence of RNase A and cyt c, the platinum complexes undergo activation through release of labile ligands followed by binding to the protein. In contrast, palladium complexes revealed a far lower reactivity implying the likely occurrence of a different mechanism of action.

Advances in tailoring luminescent rare-earth mixed inorganic materials.

This tutorial review focuses on the recent advances in the topic of mixed inorganic materials, specifically rare-earth mixed inorganic materials, which have similar crystal structures to the parent material. In this review we overview the available synthetic methods employed to prepare rare-earth mixed inorganic materials and the common approaches to characterize the differences in the crystal structures when the composition varies within the mixed material. The luminescence properties are investigated after adjusting the composition of the mixed material and doping with suitable lanthanides. We focus on understanding their luminescence performance, which is a crucial step for designing new materials, and exploring their future applications. Last, the possible applications of such rare-earth mixed inorganic materials are also described and further prospects of the exploration of such materials are presented.

Excitation- and Emission-Wavelength-Based Multiplex Spectroscopy Using Red-Absorbing Near-Infrared-Emitting Lanthanide Complexes.

Multiplex imaging in the red and near-infrared (NIR) should be an enabling tool for the real-time investigation of biological systems. Currently available emitters have short luminescent lifetimes, broad absorption and emission bands, and small Stokes shifts, which limits multiplexing in this region to two colors. NIR-emitting luminescent lanthanide (Ln) complexes carrying hydroporphyrin (chlorin) sensitizing antennae are excitable in the red through the narrow, intense and tunable chlorin absorptions. Both emission- and excitation-based multiplexing are possible, the former by exciting the same antenna appended to different Lns, the latter by attaching different chlorins with nonoverlapping absorptions to the same Ln. The combination of excitation and emission spectroscopies allows for the straightforward differentiation of up to four different complexes.

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 Sa of 0.011 K-1 (210 K) and relative sensitivity Sr 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.

Low-Percentage Ln3+ Doping in a Tetranuclear Lanthanum Polyoxometalate Assembled from [Mo7O24]6- Polyanions Yielding Visible and Near-Infrared Luminescence.

A rare case of low-percentage trivalent lanthanide doping in multinuclear lanthanide polyoxometalates (LnPOMs) was investigated. The [La4(MoO4)(H2O)16(Mo7O24)4]14- polyanion was chosen as the host material for this study. In this polyanion the central [La4(MoO4)]10+ core is coordinated by four heptamolybdate groups as well as 16 water molecules. The tetranuclear lanthanum POM was doped with 5% of Eu3+, Tb3+, Sm3+, Dy3+, Nd3+, Er3+, and Yb3+ (according to synthesis), and the structures and luminescence properties of the x%Ln:LaPOMs were investigated. Additionally a series of tetranuclear lanthanide POMs built from [Mo7O24]6- heptamolybdate polyanions with Eu3+, Tb3+, Sm3+, Dy3+, and Nd3+ instead of La3+ were synthesized, and a detailed analysis revealed that the tetranuclear clusters formed monomers or dimers linked through oxygen bridges. The smaller lanthanide ions, namely, Er3+ and Yb3+, did not form tetranuclear clusters, but instead mononuclear sandwich-type POMs were obtained. The obtained structures were shown to be lanthanide-specific, and not a result of different synthetic/crystallization conditions. The luminescence properties of the x%Ln:LaPOMs were compared with the luminescence properties of the LnPOMs.

Discovery of (S)-3'-hydroxyblebbistatin and (S)-3'-aminoblebbistatin: polar myosin II inhibitors with superior research tool properties.

In search of myosin II inhibitors with superior research tool properties, a chemical optimization campaign of the blebbistatin scaffold was conducted in this paper. (S)-Blebbistatin is the best known small-molecule inhibitor of myosin II ATPase activity. Unfortunately, as a research tool this compound has several deficiencies: it is photolabile and (photo)toxic, has low water solubility, and its (fluorescent) precipitates interfere in (fluorescence) readouts. In view of obtaining tool compounds with improved properties, both enantiomers of a series of D-ring modified polar analogs were prepared. We identified (S)-3'-hydroxyblebbistatin (S)-2 and (S)-3'-aminoblebbistatin (S)-3 as two myosin II inhibitors with a 30-fold higher water solubility than (S)-blebbistatin. These molecules furthermore do not cause interference in (fluorescence) readouts. (S)-2 and (S)-3 thus are superior alternatives to (S)-blebbistatin as research tools to study myosin II.

Controlled fluorescence in a beetle's photonic structure and its sensitivity to environmentally induced changes.

The scales covering the elytra of the male Hoplia coerulea beetle contain fluorophores embedded within a porous photonic structure. The photonic structure controls both insect colour (reflected light) and fluorescence emission. Herein, the effects of water-induced changes on the fluorescence emission from the beetle were investigated. The fluorescence emission peak wavelength was observed to blue-shift on water immersion of the elytra whereas its reflectance peak wavelength was observed to red-shift. Time-resolved fluorescence measurements, together with optical simulations, confirmed that the radiative emission is controlled by a naturally engineered photonic bandgap while the elytra are in the dry state, whereas non-radiative relaxation pathways dominate the emission response of wet elytra.

Nano- and micro-sized rare-earth carbonates and their use as precursors and sacrificial templates for the synthesis of new innovative materials.

This review focuses on rare-earth carbonate materials of nano- and micro-size. It discusses in depth the different types of rare-earth carbonate compounds, diverse synthetic approaches and possibilities for chemical tuning of the size, shape and morphology. The interesting luminescence properties of lanthanide doped rare-earth carbonates and their potential applications for example as efficient white light sources and biolabels are reviewed. Additionally the use of these materials as precursors for the synthesis of nano-/micro-sized oxides, and their application as sacrificial templates for morphology-controlled synthesis of other materials such as YVO4, LaF3, NaYF4 and others is overviewed.

Highly Luminescent, Water-Soluble Lanthanide Fluorobenzoates: Syntheses, Structures and Photophysics, Part I: Lanthanide Pentafluorobenzoates.

Highly luminescent, photostable, and soluble lanthanide pentafluorobenzoates have been synthesized and thoroughly characterized, with a focus on Eu(III) and Tb(III) complexes as visible emitters and Nd(III) , Er(III) , and Yb(III) complexes as infrared emitters. Investigation of the crystal structures of the complexes in powder form and as single crystals by using X-ray diffraction revealed five different structural types, including monomeric, dimeric, and polymeric. The local structure in different solutions was studied by using X-ray absorption spectroscopy. The photoluminescence quantum yields (PLQYs) of terbium and europium complexes were 39 and 15 %, respectively; the latter value was increased almost twice by using the heterometallic complex [Tb0.5 Eu0.5 (pfb)3 (H2 O)] (Hpfb=pentafluorobenzoic acid). Due to the effectively utilized sensitization strategy (pfb)(-) →Tb→Eu, a pure europium luminescence with a PLQY of 29 % was achieved.

Enhanced luminescence in Ln³âº-doped Y2WO6 (Sm, Eu, Dy) 3D microstructures through Gd³âº codoping.

Microstructures of Y2WO6 were prepared by applying a hydrothermal synthesis in the presence of sodium dodecyl sulfate (SDS) surfactant, after which the materials were heat-treated at a temperature of 1100 °C. When prepared at pH 3, the spherical 3D microstructures were built from nanosized particles. Raising the pH gave materials built from differently shaped building blocks, which influenced the final architecture. These materials, similarly to other previously investigated and reported rare-earth tungstate materials, were found to show very interesting luminescence properties. However, quantum yield (QY) values have scarcely been reported for such materials. In this work, a detailed study of the photoluminescence characteristics, decay times, and quantum yields of Y2WO6 doped with Sm(3+), Eu(3+), and Dy(3+) is presented. When doped with different concentrations of Ln(3+) ions, the luminescence properties of the samples changed. The 2.5% Dy:Y2WO6 sample gave white-light emission and showed a QY of 17%. For the optimal lanthanide-ion concentrations, the systems were codoped with 2% and 10% Gd(3+) ions to test the possible enhancement of luminescence through energy transfer from W-O and/or Gd(3+) to Ln(3+). The Eu(3+),Gd(3+)-codoped system showed QYs as high as 79%. The Sm,Gd-codoped system showed the highest enhancement of QY. After incorporation of Gd(3+) ions, the 2.5% Sm_10% Gd:Y2WO6 materials showed a QY approximately 2.4 times larger than that of the 2.5% Sm:Y 2WO6 material.

Rare earth tungstate and molybdate compounds - from 0D to 3D architectures.

This tutorial review focuses on the recent development of diverse synthetic approaches and possibilities for chemical tuning of the size, shape, and morphology, as well as properties such as luminescence, of rare earth tungstate and molybdate materials. The use of rare earth tungstate and molybdate nano- and micromaterials as single materials for the generation of white light is reviewed. Additionally, the use of these materials as red phosphors in phosphor-converted white light emitting diodes (pc-WLEDs) when employing GaN or InGaN chips as the primary light source is explored.

Enantioselective assembly of a ruthenium(II) polypyridyl complex into a double helix.

Evolution can increase the complexity of matter by self-organization into helical architectures, the best example being the DNA double helix. One common aspect, apparently shared by most of these architectures, is the presence of covalent bonds within the helix backbone. Here, we report the unprecedented crystal structures of a metal complex that self-organizes into a continuous double helical structure, assembled by non-covalent building blocks. Built up solely by weak stacking interactions, this alternating tread stairs-like double helical assembly mimics the DNA double helix structure. Starting from a racemic mixture in aqueous solution, the ruthenium(II) polypyridyl complex forms two polymorphic structures of a left-handed double helical assembly of only the Λ-enantiomer. The stacking of the helices is different in both polymorphs: a crossed woodpile structure versus a parallel columnar stacking.