Anders Døssing

Preparation of a ZnS shell on CdSe quantum dots using a single-molecular ZnS precursor.

A simple synthetic route to the preparation of a thin ZnS shell on CdSe quantum dot cores from the air-stable, single-molecular precursor zinc diethyldithiocarbamate, Zn(S(2)CNEt(2))(2), in the three-component solvent system octadec-1-ene/oleylamine/tri-n-octylphosphine (ODE/OLA/TOP) is presented. The one-pot synthesis proceeds through heating of the solution of CdSe cores and the amount of crystalline Zn(S(2)CNEt(2))(2) corresponding to a shell thickness of two monolayers of ZnS to 110-120 °C for 1-2 h. The role of the surfactants OLA and TOP and the significance of the temperature and the amount of Zn(S(2)CNEt(2))(2) have been investigated with optical absorption and luminescence spectroscopy. We show that the presence of both OLA and TOP is crucial for the low-temperature growth and that the amount of precursor corresponding to two monolayers of ZnS results in the highest quality of core/shell CdSe/ZnS quantum dots.

Dynamics of the excited states of the europium podant, [Eu C36H44N8O5Cl2]Cl·5H2O, in solid state and solutions

Heterocyclic ligands are common sensitizers for lanthanide ion emission under UV irradiation. Due to their specific emission properties, lanthanide podants are promising materials for medical applications. In this report we present spectroscopic and decay-time studies in the 360–77 K temperature range for an Eu(III) podand in the solid state, in solutions, and incorporated into a gel. The compound was synthesized and the luminescence and emission excitation spectra measured for the solid complex and dissolved in H2O, MeOH and MeCN. Two peaks in the spectral region of the 5D0→7F0 transition are interpreted in terms of the existence of two forms of the complex in aqueous and MeOH solutions, but only one in MeCN. The role played by the radiative and nonradiative paths in emission intensity are discussed. Dynamic processes in solutions have been considered and are correlated with earlier spectroscopic, and X-ray studies. The pressure and temperature dependence of the 5D0→7F0 transition of the Eu(III) complex in water has been measured and thermodynamic parameters (ΔV, ΔH) have been determined.

Flash and continuous photolysis studies of the thionitrosyl complex Cr(CH3CN)5(NS)2+ and the nitric oxide analogs: reactions of nitrogen monosulfide in solution.

Photolysis of the thionitrosyl complex Cr(CH3CN)5(NS)2+ (1) in acetonitrile solution leads to the dissociation of nitrogen monosulfide (NS). In deaerated solution, this reaction is reversible, and flash photolysis studies demonstrate that NS reacts with Cr(CH3CN)62+ according to the rate law d[1]/dt = kon[Cr(CH3CN)62+][NS] (kon = 2.3 x 108 M-1 s-1 at 298 K). The photolysis of 1 in deaerated acetonitrile with added Fe(S2CNEt2)2 leads to the transfer of NS and the formation of a species concluded to be Fe(S2CNEt2)2(NS) based on its electron paramagnetic resonance spectrum. Analogous photolysis of 1 in the presence of added NO leads to clean formation of the nitrosyl complex Cr(CH3CN)5(NO)2+ (2) presumably by NO capture of the photoproduct Cr(CH3CN)6(2+) (3). When 1 was photolyzed in aerated acetonitrile solution, the reactive species 3 was trapped, thus leading to net photochemical transformations with excitation-wavelength-dependent quantum yields of 0.3-1.0 mol/Einstein. Mass spectroscopic studies of the product solutions demonstrate the formation of S8, presumably from the decomposition of NS. The quantitative photochemical behaviors of 1 and the nitrosyl analog 2 are compared.

SYNTHESES AND STRUCTURE OF DINUCLEAR COBALT(II) COMPLEXES OF THE POLYPYRIDINE LIGAND 6,6'-BIS BIS(2-PYRIDYLMETHYL)AMINOMETHYL-2,2'-BIPYRIDINE

Abstract A series of dinuclear cobalt(II) complexes of the octadentate polypyridine ligand 6,6′-bis[bis(2-pyridylmethyl)aminomethyl]-2,2′-bipyridine (L) has been prepared. The crystal structure of the complex [{Co(NCS)} 2 L] · 0.4H 2 O has been determined by X-ray diffraction methods. The cobalt atoms, bridged by L ( CoCo = 4.901(4) A ) , are trigonal bipyramidally coordinated by one nitrogen atom of the thiocyanato group and four nitrogen atoms of L. Magnetic measurements show that no magnetic interaction between the metal centers occurs.

Crystal structure, luminescence and other properties of some lanthanidecomplexes of the polypyridine ligand6,6′-bis[bis(2-pyridylmethyl)aminomethyl]-2,2′-bipyridine

Europium(III) and gadolinium(III) form 1 : 1 complexes with the octadentate polypyridine ligand 6,6′-bis[bis(2-pyridylmethyl)aminomethyl]-2,2′-bipyridine L 2 . The crystal structure of the complex [GdL 2 Cl 2 ]Cl·2H 2 O·0.5EtOH has been determined. The gadolinium atom is co-ordinated to seven nitrogen atoms of the L 2 ligand and two chloride ligands. One of the pyridine groups of L 2 is dangling unco-ordinated. Irradiation of water or MeOH solutions of the [EuL 2 ]Cl 3 complex with UV light led to a metal-centred luminescence. The luminescence lifetimes at 296 K in water, D 2 O, MeOH and CD 3 OD were 0.78, 1.28, 0.98 and 1.25 ms, respectively. From these data the average number of co-ordinated solvent molecules (water or MeOH) was calculated to be 0.5 according to earlier empirical correlations. The luminescence consists of a series of narrow lines corresponding to the envelopes of 5 D 0 → 7 F J transitions, the most intense occurring at 613 nm (→ 7 F 2 ). The integrated intensities of these emission lines give the respective quantum yields of 0.046 and 0.089 in water and D 2 O. The equilibrium constant for formation of the europium complex in MeOH, I = 0.100 mol dm -3 NBu 4 ClO 4 , was determined to be 10 7.09 ± 0.09 dm 3 mol -1 .

Crystal structures and nitrosation reactions of triazido complexes of chromium(III)

Two triazido chromium(III) complexes [Cr(tame)(N3)3] · H2O (1) and [Cr(tacn)(N3)3] (2) (tame = 1,1,1-tris(aminomethyl)ethane and tacn = 1,4,7-triazacyclononane) have been prepared and characterized by single-crystal X-ray crystallography at 122 K. The crystal structure of 1 is monoclinic, space group P21/c, with a = 9.2460(9), b = 11.0500(11), c = 13.1980(9) Å, β = 97.244(7)° with four formula units in the cell. The crystal structure of 2 is triclinic, space group , with a = 7.698(4), b = 8.5800(6), c = 10.236(3) Å, α = 89.497(15), β = 83.70(2), γ = 72.83(2)° with two formula units in the cell. The reactions between NOBF4 and 1 or 2 in acetonitrile give [Cr(tame)(NCCH3)3](BF4)3 and [Cr(tacn)(NCCH3)3](BF4)3, respectively.

Recent advances in the chemistry of nitrosyl complexes of chromium

Abstract A review of the chemistry of nitrosyl complexes of chromium published since 2002 is given including coordination complexes as well as organometallic complexes. The syntheses, electronic structures, optical absorption spectra, electron paramagnetic resonance parameters, photochemistry, reactivity, and magnetic properties of the complexes are presented.

(2-pyridylmethyl)ammonium pyridine-2-carboxylate: the product of oxidative degradation of bis(2-pyridylmethyl)amine.

The title compound, C6H9N2+*C6H4NO2-, has been formed by oxidative degradation of neat bis(2-pyridylmethyl)amine. Hydrogen bonds link the ions into infinite chains along the a axis.