Giuseppe Di Silvestro

Highly Emissive Nanostructured Thin Films of Organic Host–Guests for Energy Conversion

All-organic nanostructured host-guest systems, based on dyes inserted in the nanochannels of perhydrotriphenylene (PHTP) and deoxycholic acid (DCA), show enhanced fluorescence properties with quantum yields even higher than those of the dyes in solution, thanks to the high concentration of emissive molecules with controlled spatial and geometrical organization that prevents aggregation quenching. Both host molecules crystallize, growing with the long axis oriented along the direction of the nanochannels where the linear-chain dyes are inserted, to yield crystals emitting well-polarized light. For the DCA-based host-guests, homogeneous thin films suitable for several applications are obtained. Colour emission in such films can be tuned by co-inclusion of two or three dyes due to resonant energy-transfer processes. We show that films obtained by low-cost techniques, such as solution casting and spin-coating, convert UV light into visible light with an efficiency much higher than that of the standard polymeric blends.

Encapsulation of volatiles in nanofibrous polysaccharide membranes for humidity-triggered release

A single-step electrospinning process will be applied to a blend of edible carbohydrate polymers (pullulan and β-cyclodextrin) to encapsulate bioactive aroma compounds and allow a humidity-triggered release. The encapsulation is rapid and efficient and the final product is an active nanofibrous membrane that can be directly used for food or active packaging applications. The membrane hosts small and homogeneously dispersed crystals of cyclodextrin-aroma complexes which are formed during the electrospinning. With this type of structure, the release of aroma compound is negligible at ambient conditions (23 °C and 55% UR) even at high temperature (up to 230 °C), and it occurs beyond a given relative humidity threshold (90%), useful for food packaging applications. The mass fraction of free aroma released is directly related to the water activity of the system, namely, φ=aW(n)/(aW(n)+Kapp) explaining the observed key role played by the relative humidity on the release of the aroma compounds.

Advantages of Surface-Initiated ATRP (SI-ATRP) for the Functionalization of Electrospun Materials

Surface-initiated atom transfer radical polymerization (SI-ATRP) is successfully applied to electrospun constructs of poly(L-lactide). ATRP macroinitiators are adsorbed through polyelectrolyte complexation following the introduction of negative charges on the polyester surface through its blending with a six-armed carboxy-terminated oligolactide. SI-ATRP of glycerol monomethacrylate (GMMA) or 2-(N,N-diethylamino)ethyl methacrylate (DEAEMA) allows then to grow surface films with controllable thickness, and in this way also to control the wetting and interactions of the construct.

A Unifying Approach to the Thermal Behavior of Inclusion Compounds

Abstract A rational approach to the understanding of the thermal stability of inclusion compounds requires an examination of all the involved phases, in particular the solid and the liquid phase. Equations for phase equilibria were derived within the framework of the regular solution theory: they contain the interaction parameter W which measures the tendency of solvent and solute to segregate. The shape of P-T and T-x diagrams changes dramatically with W, especially in cases where a miscibility gap in the liquid phase occur. On this basis it is possible to explain the behavior of the different host-guest systems such as urea- n alkanes and perhydrotriphenylene- n alkanes.

Phase equilibria of binary systems containing crystalline inclusion compounds. Part 1. The perhydrotriphenylene–n-heptane system

The vapour pressure of saturated solutions containing the perhydrotriphenylene (PHTP)–n-heptane inclusion compound as a solid phase shows a maximum at ca. 10 °C below the m.p. of the crystalline adduct. A second pressure maximum coincident with a eutectic point, is observed in the case of mixtures very rich in PHTP. Moreover, a sudden increase in pressure is observed in the case of pure or nearly pure inclusion compound in the close vicinity of the m.p. Such a behaviour is consistent with that predicted on the basis of the phase rule. The experimental values agree with those calculated by assuming the complete dissociation of the adduct in the liquid phase.

Polymerization in Clathrates

Two or more substances can form adducts showing a single crystalline structure, brought together by different kinds of forces. These forces are, in some cases, relatively strong—such as hydrogen bonds or ion–dipole interactions—or, in other cases, only weak interactions exist, which are essentially due to the shape and dimensions of the molecules involved. This latter characteristic is distinctive of ‘clathrates’—or inclusion compounds (IC)—in which the most abundant component (host) packs the molecules of a second component (guest) into its lattice, generating a definite crystalline structure.1

Relationships between solid-state structures of enantiomers and the corresponding racemic compounds in small ring derivatives. Comparison of crystal structures and solid-state properties of (R)-(–)- and racemic 1-chloro-2,2-diphenylaziridine. Solvent effect on the racemization of (R)-(–)-1-chloro-2,2-diphenylaziridine

The synthesis of highly optically pure N-chloro-2,2-diphenylaziridine, in a crystalline form and sufficiently stable at room temperature, has enabled the study and the comparison to be carried out of the solid-state properties (e.g., the m.p. phase diagram, the crystal and molecular structures, and the i.r. spectra) of the enantiomer and racemic species of this small ring derivative. The solvent effect on the racemization of the optically active N-chloro-2,2-diphenylaziridine is relatively small, and this is attributed to the low basicity of the nitrogen atom of the N-chloroaziridine ring.

Control of Macromolecular Architecture of Polyamides by Poly‐Functional Agents, 1. Theoretic and Experimental Approaches to Star‐Branched Polyamides

In this first paper of the series, a statistical model for star-branched polycondensation of AB type monomers in the presence of a polyfunctional agent RA f was completely developed. The analytical expression obtained for the number-average (DP n ) and weight-average (DP w ) degree of polymerization, and the dispersion index (D) for whole polymer species, linear and star macromolecular chains, were derived as a function of the conversion of the functional group of RA f . An important molecular parameter, mole fraction of star-branched polymer, was introduced. Numerical examples are reported on the relationship between molecular parameters and conversion of the functional group of RA f . It is illustrated that the molecular weight properties of the linear and star-branched polymers in the mixture of the products are very important factors for the application of this kind of polymeric materials. Polymerization of 6-aminocaproic acid was carried out in the presence of terephthalic (T2) and trimesic (T3) acids as tri- and bifunctional agents. The molecular weights calculated are in good agreement with those obtained by size exclusion chromatography (SEC).

Solid-Liquid-Vapor Equilibria of Chiral Compounds

Abstract We report the thermodynamic equations that control the phase relationships of chiral compounds and discuss the corresponding binary phase diagrams with special concern to the vapor phase. Some unusual features are observed in these diagrams when the vapor-liquid equilibrium intersects the field of existence of the solid phases. Comparison with available data shows a good agreement between theory and experiment.

Ring reversal of cis-cyclohexane-1,2,3,4,5,6-hexacarboxylic acid and its hexamethyl ester

cis-Cyclohexane-1,2,3,4,5,6-hexacarboxylic acid and its hexamethyl ester have been synthesized and shown, by n.m.r., to exist in solution at room temperature as an equilibrium of slowly exchanging chair conformations. From complete line shape analysis of the 13C n.m.r. spectra measured at different temperatures the activation parameters have been determined. A relatively high value (ca. 17 kcal mol–1) of free energy of activation has been found for both molecules, by analogy with the hexamethylcyclohexane case. The energy barrier of the acid has been calculated with the method of molecular mechanics and the computer program MOLBD3: the value obtained (16 kcal mol–1) is a slight overestimate by comparison with the observed value of 13–14.5 kcal mol–1.