Slippage of a Porphyrin Macrocycle over Threads of Varying Bulkiness: Implications for the Mechanism of Threading Polymers through a Macrocyclic Ring

Threading of a polymer through a macrocyclic ring may occur directly, that is, by finding the end of the polymer chain, or by a process in which the polymer chain first folds and then threads through the macrocyclic ring in a hairpin‐like conformation. We present kinetic and thermodynamic studies on the threading of a macrocyclic porphyrin receptor ( H₂1 ) onto molecular threads that are blocked on one side and are open on the other side. The open side is modified by groups that vary in ease of folding and in bulkiness. Additionally, the threads contain a viologen binding site for the macrocyclic receptor, which is located close to the blocking group. The rates of threading of H₂1 were measured under various conditions, by recording as a function of time the quenching of the fluorescence of the porphyrin, which occurs when receptor H₂1 reaches the viologen binding site. The kinetic data suggest that threading is impossible if the receptor encounters an open side that is sterically encumbered in a similar way as a folded polymer chain. This indicates that threading of polymers through macrocyclic compounds through a folded chain mechanism is unlikely.     

Strong optical nonlinearities of self-assembled polymorphic microstructures of phenylethynyl functionalized fluorenones

The polymorphs of a phenylethynyl functionalized fluorenone derivative, and their controlled self-assembly for microstructures with different morphologies have been studied. These polymorphic microcrystals exhibit very distinctive NLO properties, which are highly correlated to their electronic and supramolecular structures.     

Materials Space-Tectonics: Atomic-level Compositional and Spatial Control Methodologies for Synthesis of Future Materials

Reactions occurring at surfaces and interfaces necessitate the creation of well-designed surface and interfacial structures. To achieve a combination of bulk material (i.e., framework) and void spaces, a meticulous process of “nano-architecting” of the available space is necessary. Conventional porous materials such as mesoporous silica, zeolites, and metal–organic frameworks lack advanced cooperative functionalities owing to their largely monotonous pore geometries and limited conductivities. To overcome these limitations and develop functional structures with surface-specific functions, the novel materials space-tectonics methodology has been proposed for future materials synthesis. This review summarizes recent examples of materials synthesis based on designing building blocks (i.e., tectons) and their hybridization, along with practical guidelines for implementing materials syntheses and state-of-the-art examples of practical applications. Lastly, the potential integration of materials space-tectonics with emerging technologies, such as materials informatics, is discussed.     

Processive rotaxane systems. Studies on the mechanism and control of the threading process

The threading behavior of a zinc analogue of a previously reported processive manganese porphyrin catalyst onto a series of polymers of different lengths is reported. It is demonstrated that the speed of the threading process is determined by the opening of the cavity of the toroidal porphyrin host, which can be tuned with the help of axial ligands that coordinate to the metal center in the porphyrin.     

“Helter‐Skelter‐Like” Perylene Polyisocyanopeptides

Exciton migration! Spectroscopic analyses and extensive molecular dynamics studies revealed a well‐defined 4₁ helix in which the perylene molecules (see figure) form four “helter‐skelter‐like” overlapping pathways along which excitons and electrons can rapidly migrate.

     

Upper limits on the mechanical loss of silicate bonds in a silicon tuning fork oscillator

Silicon optics suspended by silicon ribbons or silicon fibers and kept at low temperature are being considered, because of promising optical and mechanical properties, for the test masses of third generation interferometric gravitational-wave detectors. To interconnect the suspension elements the technique of hydroxide catalysis (or silicate) bonding can be used. In order to estimate the bond loss a tuning fork was fabricated from silicon ribbons, which were then silicate bonded. The temperature dependence of mechanical loss of this tuning fork was measured in the temperature range of 95–295 K. The ratio of the energy stored in the bond layer to the total energy of the tuning fork was calculated by numerical simulation. This provided an upper limit of the bond loss as a function of the temperature. It is $(5\pm 2)\times {10}^{?3}$ at 123 K, a proposed operating temperature of cryogenically operating ground based interferometric gravitational-wave detectors.     

Nanoparticle morphology and aspect ratio effects in Ag/PVDF nanocomposites

Optical response in silver/polyvinylidene fluoride nanocomposite materials with nonspherical inclusions was examined using direct dipolar interband transitions, from density functional theory. We discuss here the dependence of the optical response of the material on the geometry, crystallographic makeup and end-cap morphology of the metallic inclusions, as well as on their orientation relative to the polarization direction of the applied electromagnetic field. Each periodic unit cell contained a single inclusion and a polymer matrix; thus, the composite behaved as a monodisperse, perfectly oriented material. Overall, the spectral location of the composite excitation spectrum was tied to that of the metallic inclusions and correlated well to quantum confinement models for the direction of polarization: As linear size of the inclusion increased in a given direction, the excitation spectrum of light polarized in that direction was red-shifted. The effect of the polymer matrix was also examined. Coulomb repulsion from matrix energy states led to splitting of nanoparticle-based energy levels, and the matrix conduction band became involved in high-energy transitions. These effects led to extensions of the spectra of nanocomposites with less stable {100}–basal plane inclusions to very low excitation energies. Attenuation or redshifting of nanoparticle peaks with high photon energies was also observed for materials with small linear sizes along the excitation direction. Comparisons with experimental and time-dependent density functional theory results suggest that estimating the complex dielectric constant from interband transition dipole moments, in a time-independent fashion, provides reliable qualitative spectra for these systems.     

Monolayer and aggregate formation of a modified phthalocyanine on mica determined by a delicate balance of surface interactions

An ordered layer of a phthalocyanine modified with ether tails can be formed on muscovite mica if removed from solution and dried. This ordered layer forms on potassium terminated as well as on sodium terminated mica, but not on a hydronium terminated surface. The molecules lie flat on the surface, as shown by AFM and X-ray diffraction, giving a layer thickness of approximately 1 nm. In solution, however, no in-plane ordering exists. The material is attracted to the substrate surface but instead of ordering, it aggregates in a liquid-like mobile fashion. This is likely caused by the fact that the water present in solution has a stronger interaction with the potassium ions on the mica surface than with the ether tails of the phthalocyanine.     

Eigenfunction Statistics in the Localized Anderson Model

We consider the localized region of the Anderson model and study the distribution of eigenfunctions simultaneously in space and energy. In a natural scaling limit, we prove convergence to a Poisson process. This provides a counterpoint to recent work, [9], which proves repulsion of the localization centres in a subtly different regime. Submitted: December 12, 2005; Revised: April 22, 2006; Accepted: May 3, 2006