Particle bridging in dispersions by small charged molecules: chain length and rigidity, architecture and functional groups spatial position

The particle bridging behaviour of dicarboxylic acid bolaform compounds such as fumaric, oxalic,trans-beta-hydromuconic, trans,trans- and cis,cis-muconic acids were evaluated in terms of their effects on the yield stress of alpha-Al(2)O(3) dispersions. The adsorption behaviour of these additives and their effects on the particle zeta potential were also determined. This study aims to understand and identify molecular factors essential for particle bridging. Very rigid compounds like trans,trans- and cis,cis-muconic and fumaric acids were identified as excellent bridging compounds from the large increase in the maximum gel strength. This strength enhancement increases with chain length and is due to more bridging molecules located in the larger spherical cap bridging area and participating in bridging. Cis,cis-muconic acid with the same chain length as fumaric acid displayed a greater bridging capability as its bolaform carboxylate groups possessed a greater lateral reach. Trans-beta-hydromuconic acid with a more flexible backbone displayed a much diminished particle bridging capability. This study has revealed a number of new molecular structural factors essential for particle bridging attribute. These are (a) the degree of backbone rigidity, (b) chain length, (c) spatial position and (d) lateral displacement of the bolaform charged group. For fumaric, trans,trans- and cis,cis-muconic acids, the maximum gel strength was not located at the pH of zero zeta potential. A particle bridging model taking into account of electrostatic repulsive interactions between the interacting particles was proposed to explain the maximum gel strength enhancement by the bolaform compounds.     

Dynamics and mechanism of bridge-dependent charge separation in pyrenylurea-nitrobenzene pi-stacked protophanes

Herein are reported the synthesis, structure, and electronic properties of a series of tertiary di- and polyarylureas possessing pyrene and nitrobenzene end groups separated by a variable number of internal phenylenediamine bridging groups. These molecules adopt folded "protophane" structures in which the adjacent arenes are loosely pi-stacked. The behavior of both the pyrene and nitrobenzene singlet states has been investigated by means of femtosecond broadband pump-probe spectroscopy, and the transients have been assigned on the basis of comparison to reference molecules. Femtosecond time resolution permits direct observation of the fast internal conversion process for both the pyrene and nitrobenzene upper singlet states, as well as the intersystem crossing of nitrobenzene. The ultrafast (ca. 100 fs) charge separation of the donor-acceptor urea having no bridging group is attributed to an internal conversion process. The slower charge separation and charge recombination of the donor-acceptor urea having a single bridging group occur via a bridge-mediated superexchange process. Addition of a second bridging unit results in a role reversal for the pyrene singlet state, which now serves as an excited-state acceptor with the bridging units serving as the electron donors. The change in the directionality of electron transfer upon addition of a second bridging phenylenediamine is a consequence of a decrease in the bridge oxidation potential as well as a decrease in the rate constant for single-step superexchange electron transfer.     

The equilibrium fraction of bridging chains and the swelling behavior of ABA triblock copolymer mesophases

The physical cross-linked network due to B blocks bridging different A domains of a microphase separated melt formed by an ABA copolymer in the strong-segregation limit is examined. The system is considered to consist of swollen elements of the same size, each containing an A domain and a B layer anchored to the A domain as loops or bridges. A lattice model and a generator-matrix method are employed to calculate the equilibrium fraction of the bridging B chains of the ABA mesophase and the equilibrium swelling concentration of the B layer in a selective solvent for the planar, cylindrical and spherical A domain structures. The effects of chain length and of two-dimensional surface density of the AB joints are discussed. The equilibrium fraction of the bridging chains as a function of chain stiffness and the equilibrium swelling concentration as a function of the fraction of bridging chains and of the interaction parameter are calculated. In addition, the segment density distributions of both loop and bridging chains for both the pure ABA and the swollen ABA mesophases are provided.     

Particle bridging between oil and water interfaces

Particle bridging between a water drop and a flat oil-water interface has been observed when the drop is brought into contact with the interface, leading to the formation of a dense particle monolayer of disc shape (namely, particle disc) that prevents the drop from coalescing into the bulk water phase. Unlike previous observations where particles from opposite interfaces appear to register with each other before bridging, the present experiment demonstrates that the particle registry is not a necessity for bridging. In many cases, the particles from one of the interfaces were repelled away from the contact region, leaving behind the particles from the other interface to bridge the two interfaces. This is confirmed by particle bridging experiments between two interfaces covered with different sized particles, and between a particle-covered interface and a clean interface. The dynamics associated with the growth of the particle disc due to particle bridging follows a power law relationship between the radius of the disc and time: r proportional, variant t0.32+/-0.03. A scaling analysis assuming capillary attraction as the driving force and a hydrodynamic resistance leads to the power law r proportional, variant t1/3, in good agreement with the experiment. In addition, we found that binary mixtures of two different sized particles can undergo phase segregation driven by the particle bridging process.     

Intervalence involvement of bridging ligand vibrations in hexaruthenium mixed-valence clusters probed by resonance Raman spectroscopy

Resonance Raman spectroscopy, performed using spectroelectrochemistry and with excitation in the intervalence bands of three pyrazine-bridged, mixed-valence dimers of trinuclear ruthenium clusters, shows resonant enhancement of symmetric bridging ligand modes. The resonant enhancements and frequency shifts of these bridging ligand modes are observed as a function of varying electronic communication between charge sites, and they show that a three-state vibronic model which explicitly includes the participation of the bridging ligand is needed to explain the spectroscopic behavior of these near-delocalized complexes.     

Molecular polarizability of organic molecules and their complexes: LIV. Molar volumes of polyaryl organoelement compounds in solutions,extrapolated to infinite dilution,and steric structure of the molecules

Molar volumes in various solvents were determined for organic derivatives of silicon, phosphorus, arsenic, sulfur, and tellurium, containing aryl nuclei capable to internal rotation about single bonds between them and bridging groups. Additive analysis of the molar volumes of these compounds showed that the aryl nuclei are acoplanar with respect to the bridging groups. Most probable is a conrotatory mutual orientation of the aromatic rings. Molar volumes were also determined for a series of compounds with two bridging groups, which can serve as models of an extreme case of mutual proximity of aryl ring planes in diaryl systems with one bridging group. A possibility for considerably simplifying the methods for determination of dipole moments and Kerr constants for compounds whose molar volumes can be calculated by our developed additive scheme is demonstrated.     

Structure and reactivity of a novel parallel thiosulfito (SSO2-S:S') rhodium dinuclear complex

The parallel thiosulfite ligand (SSO2) in a rhodium complex, which is formed by oxygenation of a bridging disulfide, is converted to a bridging hydrocarbyl thiolate ligand and sulfur dioxide gas by the reaction with hydrocarbyl halides.     

Bridging vinylalkylidene transition metal complexes

Vinylalkylidene transition metal complexes have been extensively used as ‘multitalent tools’ in organic synthesis, covering a broad field of applications. The vinylalkylidene ligands can be monodentate; alternatively they can adopt a bridging coordination mode in complexes with two adjacent metal atoms. As for other unsaturated organic ligands which can bond in both mono- and di-nuclear modes, the bridging coordination can give rise to new and different chemical properties from those found when the ligand is bound to a single metal centre. Likewise, the synthetic routes to bridging vinylalkylidene complexes offer a broader range of possibilities compared to those used to make mononuclear vinylalkylidenes. In spite of the fact that bridging vinylalkylidene complexes have been known for about 40 years, their synthetic potential as C₃ activated fragments has so far been under-exploited. Comparison with other C₃ bridged ligands (allenyls and allyls) indicates that vinylalkylidene ligands are reactive and versatile species. This review article gives an overview of the chemistry of bridging vinylalkylidene complexes to focus attention on their potential as synthetic tools.     

High-nuclearity iridium carbonyl clusters containing phenylgermyl ligands: synthesis, structures, and reactivity

The reaction of Ir4(CO)12 with Ph3GeH at 97 degrees C has yielded the new tetrairidium cluster complexes Ir4(CO)7(GePh3)(mu-GePh2)2[mu3-eta3-GePh(C6H4)](mu-H)2 (10) and Ir4(CO)8(GePh3)2(mu-GePh2)4 (11). The structure of 10 consists of a tetrahedral Ir4 cluster with seven terminal CO groups, two bridging GePh2) ligands, an ortho-metallated bridging mu3-eta3-GePh(C6H4) group, a terminal GePh3 ligand, and two bridging hydrido ligands. Compound 11 consists of a planar butterfly arrangement of four iridium atoms with four bridging GePh2 and two terminal GePh3 ligands. The same reaction at 125 degrees C yielded the two new triiridium clusters Ir3(CO)5(GePh3)(mu-GePh2)3(mu3-GePh)(mu-H) (12) and Ir3(CO)6(GePh3)3(mu-GePh2)3 (13). Compound 12 contains a triangular Ir3 cluster with three bridging GePh2), one triply bridging GePh, and one terminal GePh3 ligand. The compound also contains a hydrido ligand that bridges one of the Ir-Ge bonds. Compound 13 contains a triangular Ir3 cluster with three bridging GePh2 and three terminal GePh3 ligands. At 151 degrees C, an additional complex, Ir4H4(CO)4(mu-GePh2)4(mu4-GePh)2 (14), was isolated. Compound 14 consists of an Ir4 square with four bridging GePh2, two quadruply bridging GePh groups, and four terminal hydrido ligands. Compound 12 reacts with CO at 125 degrees C to give the compound Ir3(CO)6(mu-GePh2)3(mu3-GePh) (15). Compound 15 is formed via the loss of the hydrido ligand and the terminal GePh3 ligand and the addition of one carbonyl ligand to 12. All compounds were fully characterized by IR, NMR, single-crystal X-ray diffraction analysis, and elemental analysis.     

Magnetic study on a two-dimensional coordination polymer with mixed bridging ligands

A two-dimensional coordination polymer, [Co(mu(1,3)-SCN)(2)(mu(1,6)-dmpzdo)](n)() (where dmpzdo = 2,5-dimethylpyrazine-1,4-dioxide), has been synthesized and its crystal structure determined by X-ray crystallography. In the complex, the adjacent Co(II) ions are coordinated by mu(1,3)-SCN(-) bridging ligands which forms a one-dimensional chain along the a axis; the one-dimensional chains are further connected by mu(1,6)-dmpzdo bridging ligands which leads to the formation of a two-dimensional layer on the ac plane. The theoretical calculations reveal that a ferromagnetic coupling exists between the mu(1,3)-SCN(-) bridging Co(II) ions and an anti-ferromagnetic interaction between the mu(1,6)-dmpzdo bridging Co(II) ions, and the anti-ferromagnetic interaction is stronger than the ferromagnetic interaction. The fitting of the variable-temperature (34-300 K) magnetic susceptibilities reveals that there is an anti-ferromagnetic coupling between the bridging Co(II) ions with the magnetic coupling constant J = -3.52 cm(-1).     

Study on photochromism of diarylethenes with a 2,5-dihydropyrrole bridging unit: a convenient preparation of 3,4-diarylpyrroles from 3,4-diaryl-2,5-dihydropyrroles

Symmetric and nonsymmetric diarylethenes with a 2,5-dihydropyrrole bridging unit have been prepared, and the photochromic properties are investigated. Both symmetric and nonsymmetric diarylethenes with 2,5-dihydropyrrole bridging units undergo reversible ring-opening and ring-closing photoisomerization reactions in nonpolar solvents with UV/vis light, and some of them exhibit good fatigue resistance and no marked degradation detected after 10 cycles via an on/off switch. In polar solvents, however, photochromic diarylethenes with 2,5-dihydropyrrole bridging units produce 3,4-diarylpyrrole derivatives instead of the ring-closing isomer of diarylethenes with UV light irradiation. A class of N-substituted 3,4-diphenylethenes with 2,5-dihydropyrrole bridging units were prepared and used as templates to investigate the conversion reactions. The mechanism of photoconversion of 3,4-diaryl-2,5-dihydropyrroles to 3,4-diarylpyrroles was explored as well.     

Isomerism and solubility of benzene mono- and dicarboxylic acid: its effect on alumina dispersions

Low molecular weight benzenedicarboxylic acid has a very well-defined molecular structure because of its rigid and planar backbone. Therefore, it is hypothesized to have high potential for highly directed bridging between surfaces. However, phthalic acid cannot participate in particle bridging because the two carboxylic acid groups on the benzene ring are located adjacent to each other which prevent the molecule from bridging between two surfaces. Yield stress measurements showed that isophthalic and terephthalic acid failed to cause significant rheological changes to alumina slurries within their solubility limit. However, upon increasing the concentration beyond the solubility limit, terephthalic acid increased the yield stress by a factor of 7 and isophthalic acid by a factor of 2 when compared to the same colloidal alumina system without additive. Benzoic acid, which has low solubility at low pH, also showed an increase in yield stress by a factor of 2 even though it lacks the second carboxylic group to link neighboring surfaces. These observations suggest that highly directed bridging is unlikely to operate when these acids are present in high concentration. Instead, the dominant mechanism is most likely attraction between the negatively charged precipitates and the positively charged alumina particles and/or capillary bridging.     

Thermoelectrics in an array of molecular junctions

The room temperature thermoelectric properties of a three-dimensional array of molecular junctions are calculated. The array is composed of n-doped silicon nanoparticles where the surfaces are partially covered with polar molecules and the nanoparticles are bridged by trans-polyacetylene molecules. The role of the polar molecules is to reduce the band bending in the n-doped silicon nanoparticles and to shift the electronic resonances of the bridging molecules to the nanoparticle conduction band edges where the molecular resonances act as electron energy filters. The transmission coefficients of the bridging molecules that appear in the formulas for the Seebeck coefficient, the electrical conductance, and the electronic thermal conductance, are calculated using the nonequilibrium Green's function technique. A simple tight-binding Hamiltonian is used to describe the bridging molecules, and the self-energy term is calculated using the parabolic conduction band approximation. The dependencies of the thermoelectric properties of the molecular junctions on the silicon doping concentration and on the molecule-nanoparticle coupling are discussed. The maximal achievable thermoelectric figure of merit ZT of the array is estimated as a function of the phononic thermal conductance of the bridging molecules and the doping of the nanoparticles. The power factor of the array is also calculated. For sufficiently small phononic thermal conductances of the bridging molecules, very high ZT values are predicted.     

Photoswitching tetranuclear rhenium(I) tricarbonyl diimine complexes with a stilbene-like bridging ligand

Highly efficient photoswitching tetranuclear rhenium(I) tricarbonyl diimine complexes with a stilbene-like bridging ligand are reported. The ability to directly populate excited states localized on the bridging ligand is the key for the observed efficient photoisomerization.     

Homochiral crystallization of helical coordination chains bridged by achiral ligands: can it be controlled by the ligand structure?

Homochiral/heterochiral crystallizations of helical chiral polymer chains bridged by achiral poly-pyridyl ligands dependent on the structures of the bridging ligands and independent on the solvent are described, implying a possible strategy to design achiral crystals of helical chains using chiral bridging ligands.     

Photoluminescence as Complementary Evidence for Short-Range Order in Ionic Silica Nanoparticle Networks

Recently we published the synthesis of new hybrid materials, ionic silica nanoparticles networks (ISNN), made of silica nanoparticles covalently connected by organic bridging ligands containing imidazolium units owing to a "click-chemistry-like" reaction. Among other techniques small-angle X-ray scattering (SAXS) experiments were carried out to get a better picture of the network extension. It turned out that the short-range order in ISNN materials was strongly influenced by the rigidity of the bridging ligand, while the position of the short-range order peaks confirmed the successful linking of the bridging ligands. The photoluminescence experiments reported in this communication revealed strongly enhanced emission in the hybrid material in comparison with neat imidazolium salts. Moreover the shift of the emission maximum toward longer wavelengths, obtained when varying the aromatic ring content of the bridging ligand, suggested the existence of strong π-π stacking in the hybrid material. Experiments revealed a stronger luminescence in those samples exhibiting the higher extent of short-range order in SAXS.     

Redox-switchable conjugated bimetallic ruthenium(II) complexes

The conjugated bimetallic ruthenium(II) complex composed of 1,4-phenylenediamine as a bridging ligand was synthesized by photo-irradiation to show redox-switching of the emission properties of the terminal Ru(II) units depending on the redox state of the π-conjugated bridging spacer.     

Intervalence charge transfer transition in mixed valence complexes synthesised from RuIII(edta)- and FeII(CN)5-cores

Intervalence charge transfer properties were studied for a set of mixed valence complexes incorporating Ru(III) and Fe(II)-centres linked by various saturated and unsaturated bridging ligands (BL). Studies reveal that degree of ground state electronic interaction and coupling between Ru(III) and Fe(II)-centrescanbe attenuated by changing the nature of the bridging ligand. Further, inclusion of the bridging ligand with interrupted π-electron system in a β-CD cavity initiate an optical electron transfer from Fe(II) to Ru(III) which is otherwise not observed.     

Synthesis, crystal structure, magnetic properties and theoretical studies on a one-dimensional polynuclear copper(II) complex [Cu2(mu1,3-SCN)2(mu'1,3-SCN)2(MPyO)2]n

A new one-dimensional polynuclear copper(II) complex [Cu(2)(mu(1,3)-SCN)(2)(mu'(1,3)-SCN)(2)(MPyO)(2)](n)(where MPyO = 4-methylpyridine N-oxide) has been synthesized and its crystal structure determined by X-ray crystallography. In the complex there exist two kinds of bridging coordination modes, namely, mu(1,3)-SCN(-) equatorial-equatorial (EE) bridging ligand and micro'(1,3)-SCN(-) equatorial-axial (EA) bridging ligand. Two micro(1,3)-SCN(-) EE bridging ligands coordinate two copper(II) ions in a binuclear unit, and the S atoms from the micro'(1,3)-SCN(-) EA bridging ligands as axial coordinated atoms link the binuclear units into one-dimensional chains. The ESR spectra have been investigated, and variable temperature (4-300 K) magnetic measurements were analyzed using a binuclear Cu(ii) magnetic interaction formula and indicate the existence of strong antiferromagnetic coupling with 2J=- 216.00 cm(-1) between bridged copper(II) ions. Density functional calculations have been carried out on this binuclear unit, yielding a similar singlet-triplet splitting. The mechanism of strong antiferromangetic interaction is revealed according to the calculations.     

Bridging Ligand Length Controls AT Selectivity and Enantioselectivity of Binuclear Ruthenium Threading Intercalators

The slow dissociation of DNA threading intercalators makes them interesting as model compounds in the search for new DNA targeting drugs, as there appears to be a correlation between slow dissociation and biological activity. Thus, it would be of great value to understand the mechanisms controlling threading intercalation, and for this purpose we have investigated how the length of the bridging ligand of binuclear ruthenium threading intercalators affects their DNA binding properties. We have synthesised a new binuclear ruthenium threading intercalator with slower dissociation kinetics from ct‐DNA than has ever been observed for any ruthenium complex with any type of DNA, a property that we attribute to the increased distance between the ruthenium centres of the new complex. By comparison with previously studied ruthenium complexes, we further conclude that elongation of the bridging ligand reduces the sensitivity of the threading interaction to DNA flexibility, resulting in a decreased AT selectivity for the new complex. We also find that the length of the bridging ligand affects the enantioselectivity with increasing preference for the ΔΔ enantiomer as the bridging ligand becomes longer.