Tailoring the Solid-State Fluorescence of BODIPY by Supramolecular Assembly with Polyoxometalates.

A cationic boron dipyrromethene (BODIPY) derivative (1+) has been successfully combined with two polyoxometalates (POMs), the Lindqvist-type [W6O19]2- and the ß-[Mo8O26]4- units, into three new supramolecular fluorescent materials (1)2[W6O19]·2CH3CN, (1)2[W6O19], and (1)4[Mo8O26]·DMF·H2O. The resulting hybrid compounds have been fully characterized by a combination of single-crystal X-ray diffraction, IR and UV-vis spectroscopies, and photoluminescence analyses. This self-assembly approach prevents any π-π stacking interactions not only between the BODIPY units, responsible for aggregation-caused quenching (ACQ) effects, but also between the BODIPY and the POMs, avoiding intermolecular charge-transfer effects. Noticeably, the POM units do not only act as bulky spacers, but their negative charge density drives the molecular arrangement of the 1+ luminophore, strongly modifying its fluorescence in the solid state. As a consequence, the 1+ cations are organized into dimers in (1)2[W6O19]·2CH3CN and (1)2[W6O19], which are weakly emissive at room temperature, and in a more compact layered assembly in (1)4[Mo8O26]·DMF·H2O, which exhibits a red-shifted and intense emission upon similar photoexcitation.

New Robust Luminescent Supramolecular Assemblies Based on [Ln(Mo8O26)2]5- (Ln = Eu, Sm) Polyoxometalates.

This work highlights for the first time the photoluminescence (PL) properties of two new [Ln(Mo8O26)2]5- (Ln = Eu, Sm) lanthanide-containing polyoxometalates. Stable crystals of their tetrabutylammonium salts were synthesized, and their structures were confirmed by single-crystal X-ray diffraction. The robustness of the [Ln(Mo8O26)2]5- complexes in an acetonitrile solution has been evidenced by Fourier transform Raman and PL spectroscopies. The tetraphenylphosphonium derivatives were obtained by a salt metathesis reaction. The two series exhibit high thermal stability in air and are efficient phosphors at room temperature.

Strong Solid-state Luminescence Enhancement in Supramolecular Assemblies of Polyoxometalate and "Aggregation-Induced Emission"-active Phospholium.

Two new supramolecular fluorescent hybrid materials, combining for the first time [M6 O19 ]2- (M=Mo, W) polyoxometalates (POMs) and aggregation-induced emission (AIE)-active 1-methyl-1,2,3,4,5-pentaphenyl-phospholium (1+ ), were successfully synthesized. This novel molecular self-assembling strategy allows designing efficient solid-state emitters, such as (1)2 [W6 O19 ], by directing favorably the balance between the AIE and aggregation-caused quenching (ACQ) effects using both anion-π+ and H-bonding interactions in the solid state. Combined single-crystal X-ray diffraction, Raman, UV-vis and photoluminescence analyses highlighted that the nucleophilic oxygen-enriched POM surfaces strengthened the rigidity of the phospholium via strong C-H⋅⋅⋅O contacts, thereby exalting its solid-state luminescence. Besides, the bulky POM anions prevented π-π stacking interactions between the luminophores, blocking detrimental self-quenching effects.

A Multifunctional Dual-Luminescent Polyoxometalate@Metal-Organic Framework EuW10@UiO-67 Composite as Chemical Probe and Temperature Sensor.

The luminescent [EuW10O36]9- polyoxometalate has been introduced into the cavities of the highly porous zirconium luminescent metal-organic framework UiO-67 via a direct synthesis approach, affording the EuW10@UiO-67 hybrid. Using a combination of techniques (TGA, BET, elemental analysis, EDX mapping,…) this new material has been fully characterized, evidencing that it contains only 0.25% in europium and that the polyoxometalate units are located inside the octahedral cavities and not at the surface of the UiO-67 crystallites. Despite the low amount of europium, it is shown that EuW10@UiO-67 acts as a solid-state luminescent sensor for the detection of amino-acids, the growth of the emission intensity globally following the growth of the amino-acid pKa. In addition, EuW10@UiO-67 acts as a sensor for the detection of metallic cations, with a high sensitivity for Fe3+. Noticeably, the recyclability of the reported material has been established. Finally, it is shown that the dual-luminescent EuW10@UiO-67 material behave as a self-calibrated-ratiometric thermometer in the physiological range.

Synthesis and Photoluminescence Properties of Ca2Ga2SiO7:Eu(3+) Red Phosphors with an Intense (5)D0 → (7)F4 Transition.

Novel melilite-type Ca2Ga2SiO7:Eu(3+) red-emitting phosphors with different Eu(3+) contents were synthesized via high-temperature solid-state reaction. The crystal structure, optical absorption, and photoluminescence properties were investigated, while density functional theory calculations were performed on the host lattice. The excitation spectra indicate that phosphors can be effectively excited by near-UV light for a potential application in white-light-emitting diodes. Because of the abnormally high intensity emission at about 700 nm arising from the (5)D0 → (7)F4 transition of Eu(3+), the phosphors Ca2Ga2SiO7:Eu(3+) show a deep-red emission with chromaticity coordinates (0.639, 0.358).

A high fatigue resistant, photoswitchable fluorescent spiropyran-polyoxometalate-BODIPY single-molecule.

The tuning of the fluorescence through the activation of the photochromic part in an unprecedented covalent spiropyran-polyoxometalate-BODIPY tricomponent points out the high photofatigue resistance of such molecular switches.

A Chemical Route Towards Single-Phase Materials with Controllable Photoluminescence.

A promising general approach is proposed that enables the controlled reduction of dopants to tune the photoluminescence properties of single-phase materials. The change of oxidation state of dopants in phosphor phases leads to different emission colors that can be finely tuned. This approach was illustrated with the progressive reduction of the red phosphor SrAl2O4:Eu(3+) to green phosphor SrAl2O4:Eu(2+) to target yellow luminescence.

Polyoxomolybdate Bisphosphonate Heterometallic Complexes: Synthesis, Structure, and Activity on a Breast Cancer Cell Line.

Six polyoxometalates containing Mn(II) , Mn(III) , or Fe(III) as the heteroelement were synthesized in water by treating Mo(VI) precursors with biologically active bisphosphonates (alendronate (Ale), zoledronate (Zol), an n-alkyl bisphosphonate (BPC9 ), an aminoalkyl bisphosphonate (BPC8 NH2 )) in the presence of additional metal ions. The Pt complex was synthesized from a polyoxomolybdate bisphosphonate precursor with Mo(VI) ions linked by the 2-pyridyl analogue of alendronate (AlePy). The complexes Mo4 Ale2 Mn, Mo4 Zol2 Mn, Mo4 Ale2 Fe, Mo4 Zol2 Fe, Mo4 (BPC8 NH2 )2 Fe, and Mo4 (BPC9 )2 Fe contain two dinuclear Mo(VI) cores bound to a central heterometallic ion. The oxidation state of manganese was determined by magnetic measurements. Complexes Mo12 (AlePy)4 and Mo12 (AlePy)4 Pt4 were studied by solid-state NMR spectroscopy and the photochromic properties were investigated in the solid state; both methods confirmed the complexation of Pt. Activity against the human breast adenocarcinoma cell line MCF-7 was determined and the most potent compound was Mn(III) -containing Mo4 Zol2 Mn (IC50 ≈1.3 µM). Unlike results obtained with vanadium-containing polyoxometalate bisphosphonates, cell growth inhibition was rescued by the addition of geranylgeraniol, which reverses the effects of bisphosphonates on isoprenoid biosynthesis/protein prenylation. The results indicate an important role for both the heterometallic element and the bisphosphonate ligand in the mechanism of action of the most active compounds.

Thermochromism in yttrium iron garnet compounds.

Polycrystalline yttrium iron garnet (Y3Fe5O12, hereafter labeled YIG) has been synthesized by solid-state reaction, characterized by X-ray diffraction, Mössbauer spectroscopy, and UV-vis-NIR diffuse reflectance spectroscopy, and its optical properties from room temperature (RT) to 300 °C are discussed. Namely, its greenish color at RT is assigned to an O(2-) → Fe(3+) ligand-to-metal charge transfer at 2.57 eV coupled with d-d transitions peaking at 1.35 and 2.04 eV. When the temperature is raised, YIG displays a marked thermochromic effect; i.e., the color changes continuously from greenish to brownish, which offers opportunities for potential application as a temperature indicator for everyday uses. The origin of the observed thermochromism is assigned to a gradual red shift of the ligand-to-metal charge transfer with temperature while the positioning in energy of the d-d transitions is almost unaltered. Attempts to achieve more saturated colors via doping (e.g., Al(3+), Ga(3+), Mn(3+), ...) remained unsuccessful except for chromium. Indeed, Y3Fe5O12:Cr samples exhibit at RT the same color than the undoped garnet at 200 °C. The introduction of Cr(3+) ions strongly impacts the color of the Y3Fe5O12 parent either by an inductive effect or, more probably, by a direct effect on the electronic structure of the undoped material with formation of a midgap state.