Temperature-induced switching of the mechanism for intramolecular energy transfer in a 2,2':6',2' '-Terpyridine-based Ru(II)-Os(II) trinuclear array

The synthesis and photophysical properties of a linear 2,2':6',2' '-terpyridine-based trinuclear Ru(II)-Os(II) nanometer-sized array are described. This array comprises two bis(2,2':6',2' '-terpyridine) ruthenium(II) terminals connected via alkoxy-strapped 4,4'-diethynylated biphenylene units to a central bis(2,2':6',2' '-terpyridine) osmium(II) core. The mixed-metal linear array was prepared using the "synthesis at metal" approach, and the Ru(II)-Ru(II) separation is ca. 50 A. Energy transfer occurs with high efficiency from the Ru(II) units to the Os(II) center at all temperatures. Forster-type energy transfer prevails in a glassy matrix at very low temperature, but this is augmented by Dexter-type electron exchange at higher temperatures. This latter process, which is weakly activated, involves long-range superexchange interactions between the metal centers. In fluid solution, a strongly activated process provides for fast energy transfer. Here, a charge-transfer (CT) state localized on the bridge is populated as an intermediate species. The CT triplet does not undergo direct charge recombination to form the ground state but transfers energy, possibly via a second CT state, to the Os(II)-based acceptor. The short tethering strap constrains the geometry of the linker, especially in a glassy matrix, such that low-temperature electron exchange occurs across a particular torsion angle of 37 degrees . The probability of triplet energy transfer depends on temperature but always exceeds 75%.     

Building molecular-scale bridges having restricted rotation

The synthesis is reported of a binuclear ruthenium(II) bis(2,2′:6′,2″-terpyridine) complex for which the polytopic ligand incorporates a central, torsionally-constrained biphenylene group. In principle, the dihedral angle between the two phenylene rings can be controlled by the length of the constraining strap.     

Perfluoro-n-propylbromine(V) tetrafluoride

Perfluoro-$\text{n}$-propylbromine(V) tetrafluoride has been prepared by fluorination of perfluoro-$\text{n}$-propyl bromide at 0°C with elemental fluorine. It has been characterized by ¹⁹FNMR, mass spectroscopy and elemental analysis.     

Digoxin and metabolites in urine and feces: a fluorescence derivatization--high-performance liquid chromatographic technique

A high-performance liquid chromatography method is described for the determination of digoxin and its metabolites digoxigenin, digoxigenin monodigitoxoside, digoxigenin bis-digitoxoside and dihydrodigoxin (20S and 20R) excreted in urine and feces. The urine sample or fecal supernatant is extracted with methylene chloride in the presence of digitoxigenin or digitoxin as internal standard. Pre-column derivatization is achieved using 1-naphthoyl chloride with subsequent separation of the derivatized compounds on either a normal- or reversed-phase system with fluorescence detection. Recoveries for digoxin and all metabolites from fecal samples were in the range 60-74%, which is comparable to that previously determined for urine samples. Standard curve data revealed linearity over a wide range of concentrations. Coefficients of variation for the analysis were less than 10% for all compounds over a range of 5-125 ng per ml urine and 10-250 ng per 200 mg feces. Peaks for digoxin and metabolites in urine and feces were obtained when human excreta were analyzed.     

Radiofrequency fluorination of bromobenzene using elemental fluorine

Passage of undiluted elemental fluorine gas through a radiofrequency discharge region is known to produce atomic fluorine. Reaction of atomic fluorine with bromobenzene vapor gave the corresponding bromofluorobenzene isomers and small amounts of fluorobenzene and difluorobenzene isomers. From the isomer distributions the substitution of the atomic fluorine in observed to be more selective than previously thought.     

Antimony-121 mössbauer spectra of some triphenyl-antimonate(V) schiff base complexes

The 121Sb Mössbauer spectra at 9 and 78 K of R₃Sb-Schiff base complexes which do not have trigonal bipyramidal structures are discussed, and a mer-octahedral structure is proposed.     

Hyperfine interactions at 57Fe in octaethylhaemin

Mössbauer measurements on octaethylhaemin have been made between 4.2 and 300 K, and (at the lower temperature) in applied magnetic fields of up to 50 kG. From a spin hamiltonian treatment the saturation value of the hyperfine field at the ⁵⁷Fe nucleus is found to be 495 kG, and the splitting parameter for the ferric ion levels by the axial ligand field is D = 8.0 cm^?1. The electric field gradient 1/2e²qQ = +0.93 mm s^?1.     

From second to first order transitions in a disordered quantum magnet

The cross sections and transverse spin asymmetries in the hard exclusive electroproduction of decuplet baryons are calculated in the large N(c) limit and found to be comparable to that of octet baryons. Large N(c) selection rules for the production amplitudes are derived, leading to new sensitive tests of the spin aspects of the QCD chiral dynamics both in the nonstrange and strange sectors. Importance of such studies for the reliable extraction of the pion form factor from pion electroproduction is explained.     

Electron exchange in conformationally restricted donor-spacer-acceptor dyads: angle dependence and involvement of upper-lying excited States

The rate constant for triplet energy transfer (k(TET)) has been measured in fluid solution for a series of mixed-metal Ru-Os bis(2,2':6',2'-terpyridine) complexes built around a tethered biphenyl-based spacer group. The length of the tether controls the central torsion angle for the spacer, which can be varied systematically from 37 to 130 degrees . At low temperature, but still in fluid solution, the spacer adopts the lowest-energy conformation and k(TET) shows a clear correlation with the torsion angle. A similar relationship holds for the inverse quantum yield for emission from the Ru-terpy donor. Triplet energy transfer is more strongly activated at higher temperature and the kinetic data require analysis in terms of two separate processes. The more weakly activated step involves electron exchange from the first-excited triplet state on the Ru-terpy donor and the size of the activation barrier matches well with that calculated from spectroscopic properties. The pre-exponential factor derived for this process correlates remarkably well with the torsion angle and there is a large disparity in electronic coupling through pi and sigma orbitals on the spacer. The more strongly activated step is attributed to electron exchange from an upper-lying triplet state localized on the Ru-terpy donor. Here, the pre-exponential factor is larger but shows the same dependence on the geometry of the spacer. Strangely, the difference in coupling through pi and sigma orbitals is much less pronounced. Despite internal flexibility around the spacer, k(TET) shows a marked dependence on the torsion angle computed for the lowest-energy conformation.