Di- and Tri-nuclear VIII and CrIII Complexes of Dipyridyltriazoles: Ligand Rearrangements, Mixed Valency and Ferromagnetic Coupling.

The first dinuclear and trinuclear chromium(III) and dinuclear vanadium(III) complexes of N 4-R-substituted-3,5-di(2-pyridyl)-1,2,4-triazole (Rdpt) ligands have been prepared by solvothermal complexations under inert atmospheres, and characterized. The reactions of CrIII and VIII with adpt (R = amino) resulted in deamination of the ligand and yielded the dinuclear doubly-triazolate bridged complexes [ V 2 III (dpt -)2Cl4] (1) and [ Cr 2 III (dpt -)2Cl4] (2). In the case of the CrIII complex 2 this bridging results in a rare example of ferromagnetic coupling for a dinuclear CrIII compound. DFT studies confirm that in 2 the ferromagnetic coupling pathways dominate over the antiferromagnetic pathways, whereas in 1 the reverse occurs, consistent with the observed overall antiferromagnetic coupling in that case. It was also found that the use of different additives in the reaction allows the nuclearity of the CrIII product to be manipulated, giving either the dinuclear system, or the first example of a trinuclear circular helicate for a Rdpt complex, [ Cr 3 III (dpt)3Cl6]·1¾MeCN·»DCM (3). Reaction of N 4 -pydpt (R = 4-pyridyl) with VIII led to an unusual shift of the pyridyl substituent from N 4 to N 1 of the triazole, forming the ligand isomer N 1 -pydpt, and giving a dinuclear doubly-triazole bridged complex, [ V 2 III ( N 1 -pydpt)2Cl6]·2MeCN (4). Reaction with CrIII results in loss of the 4-pyridyl ring and a mixture of the di- and trinuclear complexes, 2 and 3. Interestingly, partial oxidation of the VIII in dinuclear complex 4 to vanadyl VIV=O was identified by crystallographic analysis of partially oxidized single crystals, [(VIVO)0.84(VIII)1.16( N 1 -pydpt)2Cl5.16]·0.84H2O·1.16MeCN (5).

Single Crystal Investigations Unravel the Magnetic Anisotropy of the "Square-In Square" Cr4Dy4 SMM Coordination Cluster.

In the search for new single molecule magnets (SMM), i.e., molecular systems that can retain their magnetization without the need to apply an external magnetic field, a successful strategy is to associate 3d and 4f ions to form molecular coordination clusters. In order to efficiently design such systems, it is necessary to chemically project both the magnetic building blocks and the resultant interaction before the synthesis. Lanthanide ions can provide the required easy axis magnetic anisotropy that hampers magnetization reversal. In the rare examples of 3d/4f SMMs containing CrIII ions, the latter turn out to act as quasi-isotropic anchors which can also interact via 3d-4f coupling to neighbouring Ln centres. This has been demonstrated in cases where the intramolecular exchange interactions mediated by CrIII ions effectively reduce the efficiency of tunnelling without applied magnetic field. However, describing such high nuclearity systems remains challenging, from both experimental and theoretical perspectives, because the overall behaviour of the molecular cluster is heavily affected by the orientation of the individual anisotropy axes. These are in general non-collinear to each other. In this article, we combine single crystal SQUID and torque magnetometry studies of the octanuclear [Cr4Dy4(µ3-OH)4(µ-N3)4(mdea)4(piv)8]·3CH2Cl2 single molecule magnet (piv=pivalate and mdea=N-methyldiethanol amine). These experiments allowed us to probe the magnetic anisotropy of this complex which displays slow magnetization dynamics due to the peculiar arrangement of the easy-axis anisotropy on the Dy sites. New ab initio calculations considering the entire cluster are in agreement with our experimental results.

Electrically Conductive, Monolithic Metal-Organic Framework-Graphene (MOF@G) Composite Coatings.

We present a novel approach to produce a composite of the HKUST-1 metal-organic framework (MOF) and graphene, which is suited for the fabrication of monolithic coatings of solid substrates. In order to avoid the degradation of graphene electrical properties resulting from chemical functionalization (e.g., oxidation yielding graphene oxide, GO), commercial, nonmodified graphene was utilized. The one-pot synthesis of the moldable composite material allows for a controllable loading of graphene and the tuning of porosity. Potentially, this facile synthesis can be transferred to other MOF systems. The monolithic coatings reported here exhibit high surface areas (1156-1078 m2/g). The electrical conductivity was high (a range of 7.6 × 10-6 S m-1to 6.4 × 10-1 S m-1) and was found to be proportional to the graphene content. The ability to readily attain different forms and shapes of the conductive, microporous composites indicates that the MOF@G system can provide a compelling approach to access various applications of MOFs, specifically in electrochemical catalysis, supercapacitors, and sensors.

Isolation of a wide range of minerals from a thermally treated plant: Equisetum arvense, a Mare's tale.

Silica is the second most abundant biomineral being exceeded in nature only by biogenic CaCO3. Many land plants (such as rice, cereals, cucumber, etc.) deposit silica in significant amounts to reinforce their tissues and as a systematic response to pathogen attack. One of the most ancient species of living vascular plants, Equisetum arvense is also able to take up and accumulate silica in all parts of the plant. Numerous methods have been developed for elimination of the organic material and/or metal ions present in plant material to isolate biogenic silica. However, depending on the chemical and/or physical treatment applied to branch or stem from Equisetum arvense; other mineral forms such glass-type materials (i.e. CaSiO3), salts (i.e. KCl) or luminescent materials can also be isolated from the plant material. In the current contribution, we show the chemical and/or thermal routes that lead to the formation of a number of different mineral types in addition to biogenic silica.

Coordination cluster nuclearity decreases with decreasing rare earth ionic radius in 1:1 Cr/Ln N-butyldiethanolamine compounds: a journey across the lanthanide series from Cr4(III)La4­Cr4(III)Tb4 via Cr3(III)Dy3 and Cr3(III)Ho3 to Cr2(III)Er2­Cr2(III)Lu2.

Reactions of the N-substituted diethanolamine ligand N-n-butyldiethanolamine with chromium(II) and lanthanide(III)/rare earth salts (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y) in the presence of coligands give access to three series of isostructural 1:1 3d(Cr(III))/4f(Ln(III)) coordination cluster compounds that can be designated in terms of octanuclear "square-in-square" (Ln = La-Tb), hexanuclear "triangle-in-triangle" (Ln = Dy, Ho, Y) and tetranuclear "butterfly" or defect dicubane core (Ln = Er-Lu) topologies as revealed by single-crystal X-ray crystallographic analysis. The bulk magnetic properties were also measured. The influences of the various components in the reaction system on the final topology and the role of the ionic radius are discussed.

Size-dependent oxidation of monodisperse silicon nanocrystals with allylphenylsulfide surfaces.

The synthesis and characterization of size-separated silicon nanocrystals functionalized with a heteroatom-substituted organic capping group, allylphenylsulfide, via photochemical hydrosilylation are described for the first time. These silicon nanocrystals form colloidally stable and highly photoluminescent dispersions in non-polar organic solvents with an absolute quantum yield as high as 52% which is 20% above that of the allylbenzene analogue. Solutions of the size-separated fractions are characterized over time to monitor the effect of aging in air by following the change of their photoluminescence and absolute quantum yields, supplemented by transmission electron microscopy.

looking inside a working SiLED.

In this study, we investigate for the first time morphological and compositional changes of silicon quantum dot (SiQD) light-emitting diodes (SiLEDs) upon device operation. By means of advanced transmission electron microscopy (TEM) analysis including energy filtered TEM (EFTEM) and energy dispersive X-ray (EDX) spectroscopy, we observe drastic morphological changes and degradation for SiLEDs operated under high applied voltage ultimately leading to device failure. However, SiLEDs built from size-separated SiQDs operating under normal conditions show no morphological and compositional changes and the biexponential loss in electroluminescence seems to be correlated to chemical and physical degradation of the SiQDs. By contrast, we found that, for SiLEDs fabricated from polydisperse SiQDs, device degradation is more pronounced with three main modes of failure contributing to the reduced overall lifetime compared to those prepared from size-separated SiQDs. With this newfound knowledge, it is possible to devise ways to increase the lifetimes of SiLEDs.

Multicolor silicon light-emitting diodes (SiLEDs).

We present highly efficient electroluminescent devices using size-separated silicon nanocrystals (ncSi) as light emitting material. The emission color can be tuned from the deep red down to the yellow-orange spectral region by using very monodisperse size-separated nanoparticles. High external quantum efficiencies up to 1.1% as well as low turn-on voltages are obtained for red emitters. In addition, we demonstrate that size-separation of ncSi leads to drastically improved lifetimes of the devices and much less sensitivity of the emission wavelength to the applied drive voltage.

Assembling photoluminescent silicon nanocrystals into periodic mesoporous organosilica.

A contemporary question in the intensely active field of periodic mesoporous organosilica (PMO) materials is how large a silsesquioxane precursor can be self-assembled under template direction into the pore walls of an ordered mesostructure. An answer to this question is beginning to emerge with the ability to synthesize dendrimer, buckyball, and polyhedral oligomeric silsesquioxane PMOs. In this paper, we further expand the library of large-scale silsesquioxane precursors by demonstrating that photoluminescent nanocrystalline silicon that has been surface-capped with oligo(triethoxysilylethylene), denoted as ncSi:(CH(2)CH(2)Si(OEt)(3))(n)H, can be self-assembled into a photoluminescent nanocrystalline silicon periodic mesoporous organosilica (ncSi-PMO). A comprehensive multianalytical characterization of the structural and optical properties of ncSi-PMO demonstrates that the material gainfully combines the photoluminescent properties of nanocrystalline silicon with the porous structure of the PMO. This integration of two functional components makes ncSi-PMO a promising multifunctional material for optoelectronic and biomedical applications.