Multinuclear copper(I) guanidinate complexes

A series of multinuclear Copper(I) guanidinate complexes have been synthesized in a succession of reactions between CuCl and the lithium guanidinate systems Li{L} (L = Me(2)NC((i)PrN)(2) (1a), Me(2)NC(CyN)(2) (1b), Me(2)NC((t)BuN)(2)(1c), and Me(2)NC(DipN)(2) (2d) ((i)Pr = iso-propyl, Cy = cyclohexyl, (t)Bu = tert-butyl, and Dip = 2,6-disopropylphenyl) made in situ, and structurally characterized. The di-copper guanidinates systems with the general formula [Cu(2){L}(2)] (L = {Me(2)NC((i)PrN)(2)} (2a), {Me(2)NC(CyN)(2)} (2b), and {Me(2)NC(DipN)(2)} (2d) differed significantly from related amidinate complexes because of a large torsion of the dimer ring, which in turn is a result of transannular repulsion between adjacent guanidinate substituents. Attempts to synthesis the tert-butyl derivative [Cu(2){Me(2)NC((t)BuN)(2)}(2)] result in the separate formation and isolation of the tri-copper complexes [Cu(3){Me(2)NC((t)BuN)(2)}(2)(μ-NMe(2))] (3c) and [Cu(3){Me(2)NC((t)BuN)(2)}(2)(μ-Cl)] (4c), both of which have been unambiguously characterized by single crystal X-ray diffraction. Closer inspection of the solution state behavior of the lithium salt 1c reveals a previously unobserved equilibrium between 1c and its starting materials, LiNMe(2) and N,N'-di-tert-butyl-carbodiimide, for which activation enthalpy and entropy values of ΔH(?) = 48.2 ± 18 kJ mol(-1) and ΔS(?) = 70.6 ± 6 J/K mol have been calculated using 1D-EXSY NMR spectroscopy to establish temperature dependent rates of exchange between the species in solution. The molecular structures of the lithium complexes 1c and 1d have also been determined and shown to form tetrameric and dimeric complexes respectively held together by Li-N and agostic Li···H-C interactions. The thermal chemistry of the copper complexes have also been assessed by thermogravimetric analysis.     

Dielectric relaxation and ultrafast transient absorption spectroscopy of [C6mim]+[Tf2N]−/acetonitrile mixtures

Mixtures of the ionic liquid (IL) [C(6)mim](+)[Tf(2)N](-) and acetonitrile have been investigated by a combination of dielectric relaxation spectroscopy (DRS) and ultrafast transient absorption techniques using the molecular probe 12'-apo-β-carotenoic-12'-acid (12'CA). Steady-state absorption spectra of the 12'CA molecule have also been recorded. The position of the probe's S(0)→ S(2) absorption maximum correlates linearly with the polarizability of the mixture, suggesting that the bulk composition is a good approximation to the local composition. The lifetime τ(1) of the S(1)/ICT state of 12'CA varies rather smoothly with composition between the value for pure acetonitrile (42 ps) and neat [C(6)mim](+)[Tf(2)N](-) (94 ps). At low IL contents there appears to be an influence of discrete ion pairs. Employing static dielectric constants from the DRS experiments, one finds that the lifetime of the probe in the IL mixtures is shorter than that in pure organic solvents with the same polarity parameter. This suggests an increased stabilization of the S(1)/ICT state in IL-containing mixtures, most likely due to IL-specific Coulombic interactions between the cation and the negative end of the probe's dipole. An ultrafast solvation component is observed which is ca. 0.5 ps in pure acetonitrile, and approaches the value for the pure IL (2.0 ps) already around x(IL) = 0.3. This is interpreted in terms of an efficient perturbation of the cooperative solvation response of acetonitrile by the presence of small amounts of IL and possibly also the viscosity increase when adding IL. This view is also supported by the increase of the average longitudinal relaxation time of acetonitrile upon addition of small IL amounts extracted from the DRS experiments.     

On the applicability of elastic network normal modes in small-molecule docking

Incorporating backbone flexibility into protein-ligand docking is still a challenging problem. In protein-protein docking, normal mode analysis (NMA) has become increasingly popular as it can be used to describe the collective motions of a biological system, but the question of whether NMA can also be useful in predicting the conformational changes observed upon small-molecule binding has only been addressed in a few case studies. Here, we describe a large-scale study on the applicability of NMA for protein-ligand docking using 433 apo/holo pairs of the Astex data sets. On the basis of sets of the first normal modes from the apo structure, we first generated for each paired holo structure a set of conformations that optimally reproduce its C(α) trace with respect to the underlying normal mode subspace. Using AutoDock, GOLD, and FlexX we then docked the original ligands into these conformations to assess how the docking performance depends on the number of modes used to reproduce the holo structure. The results of our study indicate that, even for such a best-case scenario, the use of normal mode analysis in small-molecule docking is restricted and that a general rule on how many modes to use does not seem to exist or at least is not easy to find.     

Interfacial free energy driven nanophase separation in poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester thin films

The nanostructure of thermally annealed thin films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends on hydrophobic and hydrophilic substrates was studied to unravel the relationship between the substrate properties and the phase structure of polymer blends in confined geometry. Indeed, the nature of the employed substrates was found to affect the extent of phase separation, the PCBM aggregation state and the texture of the whole system. In particular, annealing below the melting temperature of the polymer yielded the formation of PCBM nanometric crystallites on the hydrophobic substrates, while mostly amorphous microscopic aggregates were formed on the hydrophilic ones. Moreover, while an enhanced in-plane orientation of P3HT lamellae was promoted on hydrophobic substrates, a markedly tilted geometry was produced on the hydrophilic ones. The observed effects were interpreted in terms of a simple model connecting the interface free energy for the blend films to the different polymer chain mobility and diffusion velocity of PCBM molecules on the different substrates.     

Bio serves nano: biological light-harvesting complex as energy donor for semiconductor quantum dots

Light-harvesting complex (LHCII) of the photosynthetic apparatus in plants is attached to type-II core-shell CdTe/CdSe/ZnS nanocrystals (quantum dots, QD) exhibiting an absorption band at 710 nm and carrying a dihydrolipoic acid coating for water solubility. LHCII stays functional upon binding to the QD surface and enhances the light utilization of the QDs significantly, similar to its light-harvesting function in photosynthesis. Electronic excitation energy transfer of about 50% efficiency is shown by donor (LHCII) fluorescence quenching as well as sensitized acceptor (QD) emission and corroborated by time-resolved fluorescence measurements. The energy transfer efficiency is commensurable with the expected efficiency calculated according to F?rster theory on the basis of the estimated donor-acceptor separation. Light harvesting is particularly efficient in the red spectral domain where QD absorption is relatively low. Excitation over the entire visible spectrum is further improved by complementing the biological pigments in LHCII with a dye attached to the apoprotein; the dye has been chosen to absorb in the "green gap" of the LHCII absorption spectrum and transfers its excitation energy ultimately to QD. This is the first report of a biological light-harvesting complex serving an inorganic semiconductor nanocrystal. Due to the charge separation between the core and the shell in type-II QDs the presented LHCII-QD hybrid complexes are potentially interesting for sensitized charge-transfer and photovoltaic applications.     

N-heterocyclic phosphenium ligands as sterically and electronically-tunable isolobal analogues of nitrosyls

The coordination chemistry of an N-heterocyclic phosphenium (NHP)-containing bis(phosphine) pincer ligand has been explored with Pt(0) and Pd(0) precursors. Unlike previous compounds featuring monodentate NHP ligands, the resulting NHP Pt and Pd complexes feature pyramidal geometries about the central phosphorus atom, indicative of a stereochemically active lone pair. Structural, spectroscopic, and computational data suggest that the unusual pyramidal NHP geometry results from two-electron reduction of the phosphenium ligand to generate transition metal complexes in which the Pt or Pd centers have been formally oxidized by two electrons. Interconversion between planar and pyramidal NHP geometries can be affected by either coordination/dissociation of a two-electron donor ligand or two-electron redox processes, strongly supporting an isolobal analogy with the linear (NO(+)) and bent (NO(-)) variations of nitrosyl ligands. In contrast to nitrosyls, however, these new main group noninnocent ligands are sterically and electronically tunable and are amenable to incorporation into chelating ligands, perhaps representing a new strategy for promoting redox transformations at transition metal complexes.     

Chiral uranium phosphonates constructed from achiral units with three-dimensional frameworks

Two chiral, porous uranium methylenediphosphonates, [C(2)H(10)N(2)]{UO(2)[CH(2)(PO(3))(2)]}·H(2)O (UC1P2N-1) and [N(C(2)H(5))(4)]K{(UO(2))(3)[CH(2)(PO(3))(2)](2)(H(2)O)(2)}·1.5H(2)O (KUC1P2-1), have been synthesized without chiral starting materials. Both compounds display channels ~1 × 1 nm that are large enough for these materials to conduct ion-exchange with coordination complexes such as [Co(en)(3)](3+).     

Pyrene-fluorene hybrids containing acetylene linkage as color-tunable emitting materials for organic light-emitting diodes

New blue- to yellow-emitting materials have been developed by incorporating fluorene-based chromophores on pyrene core with acetylene linkage and using multifold palladium-catalyzed cross-coupling reactions. Both mono- and tetrasubstituted derivatives have been synthesized and characterized. The tetrasubstituted derivatives displayed red-shifted emission when compared to the monosubstituted derivative indicative of an extended conjugation in the former. End-capping with a diphenylamine unit further red-shifted the absorption and emission profiles and imparted a weak dipolar character to the molecules. Amine-containing derivatives displayed positive solvatochromism in the fluorescence spectra indicating a more polar excited state due to an efficient charge migration from the diphenylamine donor to the pyrene π-acceptor. All of the derivatives were tested as emitting dopants with host material 4,4'-bis(9H-carbazol-9-yl)biphenyl (CBP) in a multilayered OLED and found to exhibit bright blue or yellow electroluminescence. The device utilizing 1,3,6,8-tetrasubstituted pyrene derivative as a dopant emitter displayed highest maximum luminescence 4630 cd/m(2) with power efficiency 3.8 lm/W and current efficiency 7.1 cd/A at 100 cd/m(2) attributable to the proper alignment of energy levels that led to the efficient harvesting of excitons. All of the devices exhibited color purity over a wide range of operating voltages.     

The effect of extra framework species on the intrinsic negative thermal expansion property of zeolites with the LTA topology

X-Ray powder diffraction studies have been used to measure the thermal expansion properties of three zeolites with the LTA topology and provide details of the underlying structural changes. Siliceous ITQ-29 and dehydrated and hydrated silver zeolite A have large negative, moderate negative and positive thermal expansion coefficients, respectively.     

4,4,9,9-Tetraphenyl pyrroloindolizine: a structural analogue of calix[2]pyrrole

Synthesis, spectral and structural characterization of a pyrroloindolizine derivative having structural similarity with calix[2]pyrrole is described. Here, two pyrrole rings are connected with two meso-carbon atoms having an N,α-linkage and an α,β-linkage to afford the smallest analogue in the calixpyrrole family. Detailed NMR spectroscopic studies along with single crystal X-ray analysis confirm the assigned structure of the molecule.