Loredana Luvidi

Supramolecular chirality in solvent-promoted aggregation of amphiphilic porphyrin derivatives: kinetic studies and comparison between solution behavior and solid-state morphology by AFM topography.

The solvent-promoted aggregation behavior of some amphiphilic porphyrin derivatives bearing chiral functionality in the form of a charged L-proline group has been investigated by UV/Vis, resonance light scattering, fluorescence and circular dichroism spectroscopy. The investigated macrocycles give rise to aggregates featuring supramolecular chirality with high ellipticity. Kinetic studies reveal peculiar differences in the fashion of aggregation, depending on the intimate nature of the chiral functionality, namely, cationic (nitrogen-quaternized L-proline, 3H(2)) or anionic (carboxylate residue, 6H(2)) group. Formation of anionic 6H(2) aggregates shows a diffusion-limited kinetic behavior. AFM topography studies show formation of tighter globular structures. On the other hand, the corresponding 3H(2) aggregates are formed by a cooperative, fractal-type decay, and appear as long-fibrous, looser structures. In the templated aggregation of 3H(2) over preformed 6H(2) aggregates, AFM images show formation of globular structures with reduced sizes, as a likely consequence of shorter interchromophore distances, due to favorable Coulombic interactions. The results obtained show an interesting parallelism between the solution behavior and the solid-state aggregate structures, corroborating the sergeant-soldier effect observed in the templated aggregation. The results presented give important insights for understanding the complex mechanisms involved in these issues, which are of key importance for the development of chiral supramolecular materials and stereoselective sensors and devices.

Spectroscopic, Morphological, and Mechanistic Investigation of the Solvent‐Promoted Aggregation of Porphyrins Modified in meso‐Positions by Glucosylated Steroids

Solvent-driven aggregation of a series of porphyrin derivatives was studied by UV/Vis and circular dichroism spectroscopy. The porphyrins are characterised by the presence in the meso positions of steroidal moieties further conjugated with glucosyl groups. The presence of these groups makes the investigated macrocycles amphiphilic and soluble in aqueous solvent, namely, dimethyl acetamide/water. Aggregation of the macrocycles is triggered by a change in bulk solvent composition leading to formation of large architectures that express supramolecular chirality, steered by the presence of the stereogenic centres on the periphery of the macrocycles. The aggregation behaviour and chiroptical features of the aggregates are strongly dependent on the number of moieties decorating the periphery of the porphyrin framework. In particular, experimental evidence indicates that the structure of the steroid linker dictates the overall chirality of the supramolecular architectures. Moreover, the porphyrin concentration strongly affects the aggregation mechanism and the CD intensities of the spectra. Notably, AFM investigations reveal strong differences in aggregate morphology that are dependent on the nature of the appended functional groups, and closely in line with the changes in aggregation mechanism. The suprastructures formed at lower concentration show a network of long fibrous structures spanning over tens of micrometres, whereas the aggregates formed at higher concentration have smaller rod-shaped structures that can be recognised as the result of coalescence of smaller globular structures. The fully steroid substituted derivative forms globular structures over the whole concentration range explored. Finally, a rationale for the aggregation phenomena was given by semiempirical calculations at the PM6 level.

Atmospheric corrosion of B6 bronze evaluated by the thin layer activation technique

Abstract The applicability of thin layer activation (TLA) technique to outdoor degradation monitoring of a bronze alloy (B6 bronze), which is of interest in the field of cultural heritage, was evaluated by thickness loss measurements of specimens exposed to different climatic conditions. Activated samples were exposed in Rome and in Venice and withdrawn for measurements after different exposure periods. The morphology and the nature of the corrosion products were respectively investigated by OM observations and XRD analyses. The sensitivity of TLA was also verified by thickness loss measurements of activated specimens treated with the 5-octyl-1,2,3-benzotriazole corrosion inhibitor and outdoor exposed for a period of six months.

Thin layer activation technique application in bronze corrosion monitoring

Abstract Proton activation of bronze is of interest for the potential application of the Thin Layer Activation (TLA) technique in the corrosion monitoring of bronze artefacts. The data, which are here reported, are the first results concerning the application of TLA to bronze specimens which have been submitted to artificial weathering.

Solid state deposition of chiral amphiphilic porphyrin derivatives on glass surface

Herein, we present a straightforward method to achieve optically active films based on porphyrin derivatives. The introduction of an aminoacid functionality on the porphyrin platform confers to the macrocycle both the amphiphilic and chiral character exploited for its solvent-promoted self-aggregation leading to the formation of chiral supramolecular architectures. These ordered suprastructures have the propensity to spontaneously layer as solid films on glass surfaces. The deposited material has been characterized by means of UV-visible, fluorescence emission, circular dichroism spectroscopy and AFM. The reported studies show once more how the stereochemical information stored on a single porphyrin framework can induce the formation of supramolecular chiral architectures, in solution, as well as in solid state. Furthermore, slight modifications on the porphyrin skeleton can influence the aggregation process and the structural features of the final assemblies, leading to solid surfaces featuring different morphologies. These combined aspects can be of great importance for the achievement of solid state chemical sensors with stereoselective properties.

Effect of alkali cations on the methoxide ion addition to corands incorporating a thiopyrylium subunit

Abstract The incorporation of a thiopyrylium ring in a corand structure forms a system with a weak binding ability in methanol towards the alkali cations K + , Rb + and Cs + . The binding substantially increases in basic solution because of the reversible addition of methoxide ion to the heteroaromatic ring to form a neutral 4 H adduct. The effect of the length of the polyoxyethylene chain and of the presence of a bulky group ( tert -butyl) in the γ position of the heteroaromatic ring has been quantitatively investigated. Results obtained from molecular mechanics calculations were used to rationalise the depressing effect of the γ tert -butyl group on the ligand ability of thiopyrans.