Different Topologies of Hg(II)-Bispidine 1D Coordination Polymers: Dynamic Behavior in Solvent Adsorption and Exchange Processes

One-dimensional (1D) coordination polymers (CPs) featuring three different topologies, comprising zig-zag, ribbon-like and poly-[n]-catenane structures, were obtained by reaction of Hg(II) ions with a novel bispidine ligand L3 , and structurally characterized by SC- and P-XRD methods. The CPs obtained in the form of microcrystalline powders were tested for their ability to undergo solvent adsorption and exchange by P-XRD and ¹H NMR spectroscopy. The extent of their dynamic behavior was then correlated to their structural features, highlighting the role of interchain interactions established among their constituting linear arrays. Zig-zag CPs proved to be resilient to external chemical stimuli, while they differently respond to thermal treatments, depending on the solvent originally included within the CP. In the case of polycatenated structures, we observed transformations where the original topology was maintained upon guest exchange, but also cases where it changed to zig-zag, even under solid/vapor conditions (i. e., no complete dissolution of the CP). Given the presence of linear interconnected 1D channels, 3 ⋅ ClBz-polycatenane^Pwd is also able to trap volatile guests such as n-hexane when exposed to its vapors.     

The Principle of Covariance and the Hamiltonian Formulation of General Relativity

The implications of the general covariance principle for the establishment of a Hamiltonian variational formulation of classical General Relativity are addressed. The analysis is performed in the framework of the Einstein-Hilbert variational theory. Preliminarily, customary Lagrangian variational principles are reviewed, pointing out the existence of a novel variational formulation in which the class of variations remains unconstrained. As a second step, the conditions of validity of the non-manifestly covariant ADM variational theory are questioned. The main result concerns the proof of its intrinsic non-Hamiltonian character and the failure of this approach in providing a symplectic structure of space-time. In contrast, it is demonstrated that a solution reconciling the physical requirements of covariance and manifest covariance of variational theory with the existence of a classical Hamiltonian structure for the gravitational field can be reached in the framework of synchronous variational principles. Both path-integral and volume-integral realizations of the Hamilton variational principle are explicitly determined and the corresponding physical interpretations are pointed out.     

Azobenzene: A Photoactive Building Block for Supramolecular Architectures

The development of nanoscale systems capable to perform specific functions under external control is a challenging task and a fascinating objective in Chemistry. Photochromic compounds undergo radical changes in their physico‐chemical properties upon light excitation, for this reason they are valuable building blocks for the construction of photo‐controllable molecular devices, machines and materials. The E–Z photoisomerization of azobenzene has been known for almost 80 years and – owing to its high efficiency and excellent reversibility – has been widely employed to introduce an element of photo‐control in a large variety of compounds, biomolecules, nanosystems and materials. Here we present some of our research results highlighting how this outstanding photochrome can be utilized to develop systems with light‐induced functionalities.     

Isothermal elongational behavior of liquid-crystalline polymers and LCPs based blends

The rheological behavior of two flexible thermoplastics, Nylon-6 (Ny) and bisphenol-A polysulfone (PSu), and two wholly aromatic liquid crystalline polymers, Vectra-A900 (VA) and Vectra-B950 (VB), as well as that of Ny/VB and PSu/VA blends with 10% LCP, has been investigated by the use of capillary viscometers equipped with cylindrical dies having different length-to-diameter ratios. The elongational viscosity of all materials was calculated, from the results of isothermal measurements carried out at 290°C, by means of the Cogswell's analysis, based on the estimation of the pressure drop due to the converging flow at the die inlet. The behavior in elongational flow was compared with the rheological behavior in shear flow conditions. It was found that the elongational viscosities of VA and VB are very large and account for a fairly marked pressure drop at the die entrance, due to the orientation of the LCP domains taking place in the converging flow zone. For these materials, the ratio of the elongational viscosity to the Newtonian shear viscosity is up to two orders of magnitude higher than the value expected on the basis of the Trouton rule. For the flexible resins, the Trouton ratio is 3 and ca. 3–10, are common values for high molar mass linear polymers. The addition of 10% LCP into the flexible resins strongly increases their elongational viscosity and makes the blends resemble neat LCPs in their extensional flow behavior. In shear flow, on the contrary, the addition of LCP was shown to induce a marked reduction of the melt viscosity, even when, as for the Ny/VB blend, the LCP is more viscous than the matrix.     

Radiant energy extraction from a gasdynamic laser. Static behavior

Summary The radiant energy extraction from a gasdynamic laser has been analyzed. The considered system of energy extraction is the MOPA one, in which the flowing gas medium is used as a Power Amplifier of a Master Oscillator, a primary laser beam introduced in the gas in which an inversion has been achieved. The analysis has been performed in general terms, in order to evaluate in a simple way the effect of the beam inclination, the effect of the losses due to the collisional relaxation and to the turbulent scattering and the effect of the ratio between the primary laser flux and the maximum available laser flux in static conditions.

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Sommario S'è studiata l'estrazione di energia radiante da un laser gasdinamico. Il sistema di estrazione analizzato è quello MOPA, cioè quello in cui il mezzo è sostanzialmente usato come amplificatore, (Power Amplifier), di un fascio primario, (Master Oscillator), immesso nel gas stesso in cui è stata realizzata l'inversione delle popolazioni. Lo studio è stato condotto con criteri di generalità, al fine di valutare il più semplicemente possibile l'effetto dell'inclinazione del fascio laser sull'asse, delle perdite per diseccitazione collisionale e per la diffusione nel mezzo, l'effetto del rapporto tra flusso primario entrante e flusso totale estraibile dal gas in condizioni stazionarie.

     

Line radiation in non local thermodynamic equilibrium

Summary This paper deals with the problem of line radiation in conditions of non thermodynamic equilibrium and applies the results obtained to the study of the imprisonment of resonance radiation. It was found that the excited level, and by consequence the resonance radiation, decays in two ways in competition each other: one is a diffusive decay, due to the fact that the gas is generally very thick for frequencies near the center of the absorption line, and the other is an exponential decay due to the far wings of the absorption coefficient, where the gas is optically thin.

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Sommario E' stata trattata la radiazione di linea in condizioni di non equilibrio termodinamico, e i risultati ottenuti sono stati applicati allo studio del cosiddetto intrappolamento della radiazione risonante. S'è trovato che il livello eccitato, e di conseguenza la radiazione risonante, decade in due modi in competizione fra loro: il primo è un decadimento di tipo diffusivo, ed è dovuto al fatto che il gas è generalmente otticamente opaco al centro della linea di assorbimento, mentre il secondo è di tipo esponenziale, ed è dovuto alle code del coefficiente di assorbimento, dove il gas è otticamente sottile.

     

Giant Shape-Persistent Tetrahedral Porphyrin System: Light-Induced Charge Separation

Tetraphenylmethane appended with four pyridylpyridinium units works as a scaffold to self-assemble four ruthenium porphyrins in a tetrahedral shape-persistent giant architecture. The resulting supramolecular structure has been characterised in the solid state by X-ray single crystal analysis and in solution by various techniques. Multinuclear NMR spectroscopy confirms the 1 : 4 stoichiometry with the formation of a highly symmetric structure. The self-assembly process can be monitored by changes of the redox potentials, as well as by modifications in the visible absorption spectrum of the ruthenium porphyrin and by a complete quenching of both the bright fluorescence of the tetracationic scaffold and the weak phosphorescence of the ruthenium porphyrin. An ultrafast photoinduced electron transfer is responsible for this quenching process. The lifetime of the resulting charge separated state (800 ps) is about four times longer in the giant supramolecular structure compared to the model 1 : 1 complex formed by the ruthenium porphyrin and a single pyridylpyridinium unit. Electron delocalization over the tetrameric pyridinium structure is likely to be responsible for this effect.     

Synthesis and Photochemical Properties of Monolithic TiO2 Nanowires Diode

In this paper, the structural and photochemical properties of a monolithic photochemical diode are discussed. The present structure is composed, from the top to the bottom, of a TiO₂ nanowire layer, a TiO₂ film, a Ti foil, and a porous layer made of Pt nanoparticles. The synthesis of the nanowires was simply carried out by Au-catalysed-assisted process; the effects of the annealing temperature and time were deeply investigated. Morphological and structural characterizations were performed by scanning electron microscopy and Raman spectroscopy. The analyses showed the rutile structure of the TiO₂ nanowires. The photocatalytic properties were studied through the degradation of methylene blue (MB) dye under UV light irradiation. The nanowires induced an enhancement of the photo-degradation rate, compared to TiO₂ in a bulk form, due to an increase in the surface area. Moreover, the presence of a nano-porous Pt layer deposited on the rear side of the samples provided a further increase in the MB degradation rate, related to the scavenging effect of Pt nanoparticles. The overall increment of the photo-activity, due to the nano-structuration of the TiO₂ and to the presence of the Pt layer, resulted a factor 7, compared to the bulk reference. In addition, photovoltage measurements allowed to assess the effects of TiO₂ nano-structuration and Pt nanoparticles on the electron accumulation.     

Lanthanoid/Alkali Metal β‐Triketonate Assemblies: A Robust Platform for Efficient NIR Emitters

The reaction of hydrated lanthanoid chlorides with tribenzoylmethane and an alkali metal hydroxide consistently resulted in the crystallization of neutral tetranuclear assemblies with the general formula [Ln(Ae ? HOEt)( L )₄]₂ (Ln=Eu³⁺, Er³⁺, Yb³⁺; Ae=Na⁺, K⁺, Rb⁺). Analysis of the crystal structures of these species revealed a coordination geometry that varied from a slightly distorted square antiprism to a slightly distorted triangular dodecahedron, with the specific geometrical shape being dependent on the degree of lattice solvation and identity of the alkali metal. The near‐infrared (NIR)‐emitting assemblies of Yb³⁺ and Er³⁺ showed remarkably efficient emission, characterized by significantly longer excited‐state lifetimes (τ_obs≈37–47 μs for Yb³⁺ and τ_obs≈4–6 μs for Er³⁺) when compared with the broader family of lanthanoid β‐diketonate species, even in the case of perfluorination of the ligands. The Eu³⁺ assemblies show bright red emission and a luminescence performance (τ_obs≈0.5 ms, ${{\Phi}{{{\rm L}\hfill \atop {\rm Ln}\hfill}}}$ ≈35–37 %, η_sens≈68–70 %) more akin to the β‐diketonate species. The results highlight that the β‐triketonate ligand offers a tunable and facile system for the preparation of efficient NIR emitters without the need for more complicated perfluorination or deuteration synthetic strategies.     

Modulation of ionization and structural properties of weak polyelectrolytes due to 1D, 2D,and 3D confinement

What is the impact of reducing the space available to molecules onto their properties is a fundamental question for capillary systems, molecular biology and transport, protein and material sciences. Possibly influenced by space restriction, ionization degree has rarely been studied for confined polyelectrolytes; Monte Carlo titrations and coarse‐grained models are thus used to investigate structural and ionization changes induced on a single polyelectrolyte chain by confinement into slit (1D), cylindrical (2D), or spherical (3D) cavities. Four polyelectrolyte models differing in chain stiffness and the possible formation of charged hydrogen bonds (c? H? bonds) are studied. Low pH effective ionization constants (pK_a ) of confined chains are lower than for the free species if c? H? bonds can be formed. This is especially evident for 3D‐confined stiff chains, a finding rationalized by the impact of global compression onto chain conformations. If no c? H? bonds are allowed, chain ionization is largely unaffected by 1D or 2D confinement, while it is depressed by 3D. Chain confinement Helmholtz energy (ΔA _conf) was computed as a function of both pH and confining width (W) to gauge the impact of ionization‐induced stiffening onto ΔA _conf versus W behavior, the partition coefficient governing absorption, and the average number of c? H? bond formed. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 1088–1102