Multivalency in supramolecular chemistry and nanofabrication

Multivalency is a powerful and versatile self-assembly pathway that confers unique thermodynamic and kinetic behavior onto supramolecular complexes. The diversity of the examples of supramolecular multivalent systems discussed in this perspective shows that the concept of multivalency is a general phenomenon, and that any supramolecular interaction can be employed in multivalent displays to attain the attractive aspects characteristic of multivalent interactions. After a general introduction reviewing the general aspects of multivalency, a number of different supramolecular multivalent complexes are discussed that highlight the different features of multivalent interactions. In contrast to the many biochemical multivalent interactions, supramolecular multivalent interactions are ideal to attain a quantitative and fundamental understanding of multivalency. Several examples in which multivalency has been utilized in supramolecular nanofabrication schemes are described in detail.     

Experimental study of the interplay between long-range electron transfer and redox probe permeation at self-assembled monolayers: evidence for potential-induced ion gating

Evidence for the competition between long-range electron transfer across self-assembled monolayers (SAMs) and incorporation of the redox probe into the film is reported for the electroreduction of Ru(NH(3)) at hydroxyl- and carboxylic-acid-terminated SAMs on a mercury electrode, by using electrochemical techniques that operate at distinct time scales. Two limiting voltammetric behaviors are observed, consistent with a diffusion control of the redox process at mercaptophenol-coated electrodes and a kinetically controlled electron transfer reaction in the presence of neutral HS-(CH(2))(10)-COOH and HS-(CH(2))(n)()-CH(2)OH (n = 3, 5, and 10) SAMs. The monolayer thickness dependence of the standard heterogeneous electron transfer rate constant shows that the electron transfer plane for the reduction of Ru(NH(3)) at hydroxyl-terminated SAMs is located outside the film | solution interface at short times. However, long time scale experiments provide evidence for the occurrence of potential-induced gating of the adsorbed structure in some of the monolayers studied, which takes the form of a chronoamperometric spike. Redox probe permeation is shown to be a kinetically slow process, whose activation strongly depends on redox probe concentration, applied potential, and chemical composition of the intervening medium. The obtained results reveal that self-assembled monolayers made of mercaptobutanol and mercaptophenol preserve their electronic barrier properties up to the reductive desorption potential of a fully grown SAM, whereas those of mercaptohexanol, mercaptoundecanol, and mercaptoundecanoic acid undergo an order/disorder transition below a critical potential, which facilitates the approach of the redox probe toward the electrode surface.     

Reactivity of hindered 1,1-dimethylbutadienes towards α,β-unsaturated enones

The reactivity of several hindered 1,1-dimethylbutadienes towards α,β-unsaturated enones were studied and compared. Computational and experimental results suggest that substituents at C-2 and C-3 of the dienes significantly effect the reactivities. A synthetic application of this reaction is also described.     

Pervaporation of alcohol-toluene mixtures through polymer blend membranes of poly(acrylic acid) and poly(vinyl alcohol)

Homogeneous membranes were prepared by blending poly(acrylic acid) with poly(vinyl alcohol). These blend membranes were evaluated for the selective separation of alcohols from toluene by pervaporation. The flux and selectivity of the membranes were determined both as a function of the blend composition and of the feed mixture composition. The results showed that a polymer blending method could be very useful to develop new membranes with improved permselectivity. The pervaporation properties could be optimized by adjusting the blend composition. All the blend membranes tested showed a decrease in flux with increasing poly(vinyl alcohol) content for both methanol—toluene and ethanol—toluene liquid mixtures. The alcohols permeated preferentially through all tested blend membranes, and the selectivity values increased with increasing poly(vinyl alcohol) content. The pervaporation characteristics of the blend membranes were also strongly influenced by the feed mixture composition. The fluxes increased exponentially with increasing alcohol concentration in the feed mixtures, whereas the selectivities decreased for both liquid mixtures.     

Bis(phenylthienyl)ethene-tethered beta-cyclodextrin dimers as photoswitchable hosts

Two beta-cyclodextrin dimers tethered by photoswitchable bis(phenylthienyl)ethene moieties were synthesized as potentially tunable receptor molecules. The cyclodextrin cavities of these dimers were linked via their secondary sides, with the photochromic bis(phenylthienyl)ethene unit either directly connected to the secondary rim (7) or via propyl spacers (10). By irradiation with light the dimers were reversibly switched between a relatively flexible (open) form and a rigid (closed) form. The photostationary states for both dimers consisted of 92% of the open and 8% of the closed form, enabling the nearly complete conversion between the two forms. The binding properties of the open and closed forms of dimers 7 and 10 were assessed by complexation studies with meso-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) using isothermal titration calorimetry. For the rigidly tethered dimer 7, a factor 8 difference in binding affinity between the open and closed form of the dimer was found. This difference in binding affinity reflects the difference in enthalpy of binding for the two dimers, indicating that the beta-cyclodextrin cavities of the closed dimer 7b are spaced too far apart from each other by the rigid closed bis(phenylthienyl)ethene tether to cooperatively bind TSPP. The difference in binding affinity was sufficient to enable the phototriggered release of TSPP from dimer. The thermodynamic parameters obtained for dimer 10 suggested that the closed tether substantially contributes to the binding of TSPP. The open and closed form of dimer 10 bound TSPP with similar association constants, although the enthalpy of binding for the complexation of TSPP by the closed form of dimer 10 was more favorable than that found for the open form of the dimer.     

Two phase morphology limits lithium diffusion in TiO(2)(anatase): a (7)Li MAS NMR study.

7Li magic angle spinning solid-state nuclear magnetic resonance is applied to investigate the lithium local environment and lithium ion mobility in tetragonal anatase TiO(2) and orthorhombic lithium titanate Li(0.6)TiO(2). Upon lithium insertion, an increasing fraction of the material changes its crystallographic structure from anatase TiO(2) to lithium titanate Li(0.6)TiO(2). Phase separation occurs, and as a result, the Li-rich lithium titanate phase is coexisting with the Li-poor TiO(2) phase containing only small Li amounts approximately equal to 0.01. In both the anatase and the lithium titanate lattice, Li is found to be hopping over the available sites with activation energies of 0.2 and 0.09 eV, respectively. This leads to rapid microscopic diffusion rates at room temperature (D(micr) = 4.7 x 10(-12) cm(2)s(-1) in anatase and D(micr) = 1.3 x 10(-11) cm(2)s(-1) in lithium titanate). However, macroscopic intercalation data show activation energies of approximately 0.5 eV and smaller diffusion coefficients. We suggest that the diffusion through the phase boundary is determining the activation energy of the overall diffusion and the overall diffusion rate itself. The chemical shift of lithium in anatase is independent of temperature up to approximately 250 K but decreases at higher temperatures, reflecting a change in the 3d conduction electron densities. The Li mobility becomes prominent from this same temperature showing that such electronic effects possibly facilitate the mobility.     

Zur Bestimmung der Halogene in organischen Verbindungen

Ohne Zusammenfassung     

Density functional calculations of potential energy surface and charge transfer integrals in molecular triphenylene derivative HAT6

We investigate the effect of structural fluctuations on charge transfer integrals, overlap integrals, and site energies in a system of two stacked molecular 2,3,6,7,10,11-hexakishexyloxytriphenylene (HAT₆), which is a model system for conducting devices in organic photocell applications. A density functional based computational study is reported. Accurate potential energy surface calculations are carried out using an improved meta-hybrid density functional to determine the most stable configuration of the two weakly bound HAT₆ molecules. The equilibrium parameters in terms of the twist angle and co-facial separation are calculated. Adopting the fragment approach within the Kohn–Sham density functional framework, these parameters are combined to a lateral slide, to mimic structural/conformational fluctuations and variations in the columnar phase. The charge transfer and spatial overlap integrals, and site energies, which form the matrix element of the Kohn–Sham Hamiltonian are derived. It is found that these quantities are strongly affected by the conformational variations. The spatial overlap between stacked molecules is found to be of considerable importance since charge transfer integrals obtained using the fragment approach differ significantly from those using the dimer approach.     

Conformational stability studies of a stapled hexa-β3-peptide library

A library of 14-helical hexa β(3)-peptides was synthesized in order to determine the influence of sequence variation as well as staple size and location on conformational stability. From this study we show that appropriately stapled hexa-β(3)-peptides can allow for a number of variations without significant perturbation of the 14-helix.     

Engineering of lipid-coated PLGA nanoparticles with a tunable payload of diagnostically active nanocrystals for medical imaging

Polylactic-co-glycolic acid (PLGA) based nanoparticles are biocompatible and biodegradable and therefore have been extensively investigated as therapeutic carriers. Here, we engineered diagnostically active PLGA nanoparticles that incorporate high payloads of nanocrystals into their core for tunable bioimaging features. We accomplished this through esterification reactions of PLGA to generate polymers modified with nanocrystals. The PLGA nanoparticles formed from modified PLGA polymers that were functionalized with either gold nanocrystals or quantum dots exhibited favorable features for computed tomography and optical imaging, respectively.