Christian Reber

Sensitization of lanthanoid luminescence by organic and inorganic ligands in lanthanoid-organic-polyoxometalates

The reaction of terbium and europium salts with the lacunary polyxometalate (POM) [As(2)W(19)O(67)(H(2)O)](14-) and 2-picolinic acid (picH) affords the ternary lanthanoid-organic-polyoxometalate (Ln-org-POM) complexes [Tb(2)(pic)(H(2)O)(2)(B-β-AsW(8)O(30))(2)(WO(2)(pic))(3)](10-) (1), [Tb(8)(pic)(6)(H(2)O)(22)(B-β-AsW(8)O(30))(4)(WO(2)(pic))(6)](12-) (2), and [Eu(8)(pic)(6)(H(2)O)(22)(B-β-AsW(8)O(30))(4)(WO(2)(pic))(6)](12-) (3). A detailed synthetic investigation has established the conditions required to isolate pure bulk samples of the three complexes as the mixed salts H(0.5)K(8.5)Na[1]·30H(2)O, K(4)Li(4)H(4)[2]·58H(2)O, and Eu(1.66)K(7)[3]·54H(2)O, each of which has been characterized by single crystal X-ray diffraction. Complexes 2 and 3 are isostructural and can be considered to be composed of two molecules of 1 linked through an inversion center with four additional picolinate-chelated lanthanoid centers. When irradiated with a laboratory UV lamp at room temperature, compounds K(4)Li(4)H(4)[2]·58H(2)O and Eu(1.66)K(7)[3]·54H(2)O visibly luminesce green and red, respectively, while compound H(0.5)K(8.5)Na[1]·30H(2)O is not luminescent. A variable temperature photophysical investigation of the three compounds has revealed that both the organic picolinate ligands and the inorganic POM ligands sensitize the lanthanoid(III) luminescence, following excitation with UV light. However, considerably different temperature dependencies are observed for Tb(III) versus Eu(III) through the two distinct sensitization pathways.

Luminescence spectroscopy of europium(III) and terbium(III) penta-, octa- and nonanuclear clusters with β-diketonate ligands

A series of Eu(III) and Tb(III) clusters as well as their Y(III) analogues with increasing nuclearities of 5, 8 and 9 have been synthesised using beta-diketonate ligands with decreasing steric hindrance. Their molecular structures have been established from X-ray diffraction on single crystals for most clusters and studied by luminescence and Raman spectroscopy. The Raman spectra have distinctive patterns for each nuclearity in accordance with their crystal structure. The luminescence spectra of the Eu(III) and Tb(III) clusters also show distinctive features.

Broadband near‐infrared luminescence of Cr+3 in the elpasolite lattices Cs2NaInCl6, Cs2NaYCl6, and Cs2NaYBr6

The broadband luminescence of isolated Cr+3 ions in the title lattices is investigated. The 4T2g lowest excited state is found to undergo Jahn–Teller distortion in the eg coordinate, in addition to expanding along the a1g. Quantitative determination of these displacements is made by analysis of extremely rich fine structure, the Ham quenched spin‐orbit splitting of the electronic origins, and the Stokes shifts. The Jahn–Teller effect is found to influence the temperature dependence of lifetimes at less than 50 K. In the yttrium containing hosts, nonradiative deactivation of the 4T2g is observed above 200 K. Analysis of lifetime data with four models for nonradiative decay indicates that the active vibrations are again the a1g and eg. The trends observed among the host materials are rationalized on the basis of the freedom of the 4T2g to relax along the a1g and eg directions.

A physical approach to modify the hydraulic reactivity of α-tricalcium phosphate powder

A microsized alpha-tricalcium phosphate (alpha-TCP) powder was calcined at various temperatures (350 degrees C<T<800 degrees C) for various durations (1-24h) and the resulting physico-chemical and reactivity changes were measured. Without calcination, the alpha-TCP powder started reacting within minutes after contacting a 0.2M Na(2)HPO(4) solution as measured by isothermal calorimetry. The overall reaction was finished within a few days. After calcination at 350 degrees C< or =T < or =550 degrees C for 24h, no significant changes in the crystalline composition, crystallite size, particle size or specific surface area were noticed. However, the powder reactivity was progressively changed. More specifically, the hydraulic reaction of the powders calcined at 500 and 550 degrees C only started after 2-3h whereas the overall hydraulic reaction was only slightly postponed, suggesting that physical or chemical changes had occurred at the particle surface. As mainly physical changes were detected at the particle surface during calcination at 500 degrees C, it was speculated that the appearance of this reaction delay (=induction time) was due to the disappearance of surface defects during the calcination step, i.e. to the need to create surface defects to induce dissolution and hence reaction.

Hingeless negative linear compression in the mechanochromic gold complex [(C6F5Au)2(μ-1,4-diisocyanobenzene)]

The compression of a crystalline material under hydrostatic pressure always results in a reduction in volume of the solid, with the observed reduction typically occurring in all crystallographic cell parameters.1 However, there are a number of exceptional materials that have been shown to expand in a specific direction upon hydrostatic compression while maintaining a positive volume compression. The phenomenon of uni-or biaxial expansion under compression is known as negative linear compressibility (NLC).1 Rare examples of NLC have attracted attention recently owing to the potential applications in a range of materials, including body armor, artificial muscle actuators, and pressure sensors.2

Interference dips in molecular absorption spectra calculated for coupled electronic state potential surfaces

Interference effects in electronic absorption spectra caused by coupling between excited states are calculated and interpreted by using numerical integration of the time‐dependent Schrodinger equation and the time‐dependent theory of electronic spectroscopy. The interference between nearby spin‐forbidden doublet and spin‐allowed quartet states of chromium (III) and vanadium (II) metal complexes causes sharp decreases of intensity or dips in the envelopes of absorption bands. The states are coupled by spin–orbit coupling. The origin of the dips is explained in terms of interference between wave packets moving on potential energy surfaces representing the states. The importance of curve crossing and amplitude transfer between the states is analyzed quantitatively. The theory is applied to the absorption spectrum of a chromium (III) complex.

Terbium(III) and Yttrium(III) Complexes with Pyridine-Substituted Nitronyl Nitroxide Radical and Different β-Diketonate Ligands. Crystal Structures and Magnetic and Luminescence Properties

A terbium(III) complex of nitronyl nitroxide free radical 2-(2-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro1H-imidazolyl-1-oxy-3-oxide (NIT2Py), [Tb(acac)3NIT2Py]·0.5H2O (3) (acac = acetylacetonate), was synthesized for comparison with the previously reported [Tb(hfac)3NIT2Py]·0.5C7H16 (1) (hfac = hexafluoroacetylacetonate), together with their yttrium analogues [Y(hfac)3NIT2Py]·0.5C7H16 (2) and [Y(acac)3NIT2Py]·0.5H2O (4). The crystal structures show that in all complexes the nitronyl nitroxide radical acts as a chelating ligand. Magnetic studies show that 3 like 1 exhibits slow relaxation of magnetization at low temperature, suggesting single-molecule magnet behavior. The luminescence spectra show resolved vibronic structure with the main interval decreasing from 1600 cm(-1) to 1400 cm(-1) between 80 and 300 K. This effect is analyzed quantitatively using experimental Raman frequencies.

The cyano nitronyl nitroxide radical: experimental and theoretical evidence for the fourth case of the McConnell-I mechanism.

The nitronyl nitroxide 2-cyano-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (1) crystallises in the tetragonal P42(1)m space group with a=7.4050(7), c=8.649(1) A. In the crystal the molecules form layers parallel to the ab plane in which they are orthogonal to each other. In the layers there are close contacts, 2.953(2) A, between the NO groups and the bridging carbon atoms of the O-N-C-N-O fragment of neighbouring radicals. The calculated spin density shows a positive population mainly and equally localised on the NO groups and small but significant negative spin densities on the bridging carbon atom and the cyano nitrogen. Absorption spectra show temperature-dependent transitions related to the magnetic behaviour. The temperature dependence of the magnetic susceptibility in the range 2-300 K reveals that couplings between the radicals are antiferromagnetic, and is interpreted by considering a two-dimensional square array of spin S=1/2 antiferromagnetically coupled (J=-10 cm(-1) and g=2.01). This is interpreted as an exchange coupling through close contact between positive and negative spin densities in orthogonal orbitals on oxygen and carbon atoms, respectively.

Near-infraded luminescence spectroscopy and relaxation behaviour of V3+ doped in Cs2NaYCl6−mBrm(m =0, 0.3, 3, 6

Abstract The single crystal luminescence spectra of V 3+ doped elpasolites Cs 2 NaYX 6 (X = Cl, Br) are presented. At 2 K the chloride crystal shows sharp near-infrared luminescence with a decay time of 17.2 ms. Broad band emission arises with increasing temperature, dominating the spectra above 100 K. The decay time drops to 300 μs at 176 K. In the bromide elpasolite broad band emission is observed at all temperatures with a decay time of 228 μs at 16 K. Sharp-line luminescence in the bromide can be induced by the application of external pressure at 20 K. Close to room temperature nonradiative relaxation sets in in both lattices. Experiments on mixed crystals containing both chloride and bromide ions show that VCl 3- 6 is the only center with a sharp line luminescence at low temperatures, all the mixed coordinations showing broad-band emission. All the experimental results are quantitatively interpreted resulting in a very detailed picture of the emitting states.

Gold(I)-dithioether supramolecular polymers: synthesis, characterization, and luminescence.

A series of discrete compounds and supramolecular polymers were synthesized by self-assembly of dithioether building blocks and HAuCl4.3H2O. In complexes 1 {[AuL(1-Me)Cl], where L(1-Me) is bis(methylthio)methane} and 2 {[Au2L(2-Ph)Cl2], where L(2-Ph) is 1,2-bis(phenylthio)ethane}, adjacent units are connected via aurophilic interactions. Complex 1, a one-dimensional (1D) supramolecular polymer, and complex 2, a two-dimensional supramolecular network, both feature nearly linear [Au-Au-](infinity) chains. Complexes 4a, 4b, and 4c, all of which contain 1,3-bis(phenylthio)propane (L(3-Ph)), are polymorphs having the composition [Au2L(3-Ph)Cl2]. Complex 3 {[Au2L(1-Ph)Cl2], where L(1-Ph) is bis(phenylthio)methane}and complexes 4a and 4b consist of nearly identical 1D supramolecular polymers formed through Au-Au interactions. The third polymorph, 4c, is a molecular complex, as it does not have metal-metal interactions. Complex 5 {[Au2L(4-Ph)Cl2], where L(4-Ph) is 1,4-bis(phenylthio)butane} is also molecular. UV-vis spectra showed that the absorption bands of these complexes are allowed ligand-centered transitions between 230 and 260 nm. Complexes 1, 2, and 6 {[AuL(3-Me)Cl], where L(3-Me) is 1,3-bis(methylthio)propane} exhibited solid-state luminescence at 5 K with vibronic progressions and band maxima at approximately 570 nm. It is suggested that complex 6 contains [Au-Au-](infinity) chains.