The photophysics of a luminescent ruthenium polypyridyl complex with pendant β-cylodextrin; pH modulation of lifetime and photoinduced electron transfer

At room temperature, [Ru(bpy)₂(phen-CD)][PF₆]₂, (phen-CD is 6 ^A -(5-amino-1, 10-phenanthroline)-6 ^A -deoxy- β-Cyclodextrin and bpy is 2,2'-bipyridine) exhibits an intense metal ligand charge transfer (MLCT) transition at 452 nm and a long lived luminescence, centred at 618 nm. We demonstrate, for the first time, that the luminescence quantum yield and lifetime of the Ru (II) polypyridyl centre depends markedly on the solution pH. The pH sensitive range extends from pH 3.9 to pH 13.2 and the luminescence quantum yield changes by more than 60% over this range. This pH sensitivity is attributed to protonation/deprotonation of the secondary amine group bridge between the phenanthroline unit and CD. The complex exhibits strong host-guest binding to anthraquinone and anthraquinone-2-carboxylic acid, with concomitant quenching of the [Ru(bpy)₂(phen-CD)]²⁺ excited state. This quenching arises from efficient intramolecular electron transfer. The sensitivity of this photoinduced process to the protonation state of the bridge is discussed.     

Magnetic properties of mixed uranium and 3d element chalcogenides of the MU8X17 type

Magnetic susceptibility measurements in the temperature range 4.2–300°K for the MU₈X₁₇ type chalcogenides, where M is a 3d transition metal or Mg, and X = S or Se, were carried out. For comparison, the magnetic properties of UTe₂ are also given. Almost all the mixed uranium and 3d-transition element chalcogenides are induced-antiferromagnets, as indicated from the occurrence of the susceptibility maxima on the χ vs. T curves and large negative values of the paramagnetic Curie temperatures. The influence of the 3d-transition metals on setting up the magnetic order in the MU₈X₁₇-type compounds is discussed.     

Use of factor analysis and liquid chromatography to determine the number of species in reactions of potassium ions with selected polyethers

Abstract factor analysis is used to determine the number of species in chemical systems from data obtained by high-performance liquid chromatography. Retention data for the systems involving the reaction between potassium ions and three polyethers, benzo-5-crown-5, dibenzo-18-crown-6, and dibenzo-24-crown-8, were obtained in methanol at room temperature. The experimental data were factor-analyzed in order to yield the number of species in the reacting system. In addition to the usual criteria, it is proposed that tree-dimensional graphs of calculated data be compared to three-dimensional graphs of raw data in order to evaluate the factor space. It is also suggested that the three-dimensional graphs showing the residual error matrix be examined as a tool in the evaluation of the factor space. Two species were found from the reaction between benzo-15-crown-5 and potassium, one from dibenzo-18-crown-6-, and there from dibenzo-24-crown-8 in methanol.     

Mean field theory-based calculation of FLC polarization

Mean field theory and Monte Carlo sampling are applied to the calculation of the spontaneous polarization density of ferroelectric liquid crystals by the ensemble averaging of single molecules confined in mean field potentials reflecting the SmC environment. Molecules are modelled with atomistic detail, using intramolecular interaction potentials derived from ab initio quantum mechanical calculations. This technique is applied to thirteen members of a family of novel fluoro ether-based compounds. Comparison with experiment shows that the observed variation of polarization density with chemical structure is well reproduced in most cases, but that the observed temperature dependence of polarization density is not captured by our model. The features of molecular organization responsible for the discrepancies between theory and experiment are discussed.     

Assessment of quantum‐chemical methods for electronic properties and geometry of signaling biomolecules

A reasonable balance between accuracy and feasibility of quantum‐chemical methods depends on the complexity of the molecular system and the scientific goals. Six series of indole‐, naphthalene‐, phenol‐, benzoic‐, phenoxy‐, other auxin‐derivatives, and a test set of similar organic molecules have been chosen for an assessment of 13 density functional and semi‐empirical molecular orbital methods with respect to electronic and structural properties. The accuracy and precision of HOMO/LUMO calculations are determined by comparison with experimental ionization potentials and electron affinities. Further comparison was performed at atomic level by covariance analysis. The methods KMLYP, MSINDO, and PM3 are precise and accurate for the whole set of molecules. The method AM1 offers comparable accuracy with the exception of electron affinities of indole derivatives, where significant deviations from experiment were observed. Geometrical properties were best reproduced with the semi‐empirical method MSINDO. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

A Proof of a Conjecture of Ohba

We prove a conjecture of Ohba that says that every graph G on at most vertices satisfies .     

Mixing Homomorphisms,Recolorings, and Extending Circular Precolorings

This work brings together ideas of mixing graph colorings, discrete homotopy, and precoloring extension. A particular focus is circular colorings. We prove that all the ‐colorings of a graph G can be obtained by successively recoloring a single vertex provided along the lines of Cereceda, van den Heuvel, and Johnson's result for k‐colorings. We give various bounds for such mixing results and discuss their sharpness, including cases where the bounds for circular and classical colorings coincide. As a corollary, we obtain an Albertson‐type extension theorem for ‐precolorings of circular cliques. Such a result was first conjectured by Albertson and West. General results on homomorphism mixing are presented, including a characterization of graphs G for which the endomorphism monoid can be generated through the mixing process. As in similar work of Brightwell and Winkler, the concept of dismantlability plays a key role.