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Piersandro Pallavicini

Publications and source records attributed to Piersandro Pallavicini.

12 recordsLinked to original sources
PubMed2007

Cathodic electrografting of versatile ligands on Si(100) as a low-impact approach for establishing a Si--C bond: a surface-coordination study of substituted 2,2'-bipyridines with CuI ions.

Three distinct wet chemistry recipes were applied to hydrogen-terminated n- and p-Si(100) surfaces in a comparative study of the covalent grafting of two differently substituted 2,2'-bipyridines. The applied reactions require the use of heat, or visible light under a controlled atmosphere, or a suitable potential in an electrochemical cell. In this last case, hydrogen-terminated silicon is the working electrode in a cathodic electrografting (CEG) reaction, in which it is kept under reduction conditions. The resulting Si--C bound hybrids were characterized by a combination of AFM, dynamic contact-angle, and XPS analysis, with the help of theoretical calculations. The three distinct approaches were found to be suitable for obtaining ligand-functionalized Si surfaces. CEG resulted in the most satisfactory anchoring procedure, because of its better correlation between high coverage and preservation of the Si surface from both oxidation and contamination. The corresponding Si-bipyridine hybrid was reacted in a solution of CH3CN containing CuI ions coordinatively bound to the anchored ligands, as evidenced from the XPS binding-energy shift of the N atom donor functions. The reaction gave a 1:2 Cu-bipyridine surface complex, in which two ligands couple to a single CuI ion. The surface complex was characterized by the Cu Auger parameter and Cu/N XPS atomic-ratio values coincident with those for pure, unsupported CuI complex with the same 2,2'-bipyridine. Further support for such a specific metal-ligand interaction at the functionalized Si surface came from the distinct values of Cu2p binding energy and the Cu Auger parameter, which were obtained for the species resulting from CuI ion uptake on hydrogen-terminated Si(100).

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PubMed2007

Fluorescent sensors for Hg(2+) in micelles: a new approach that transforms an ON-OFF into an OFF-ON response as a function of the lipophilicity of the receptor.

A new approach to the use of micelles in the fluorescent sensing of metal cations is proposed and applied to the case of Hg(2+). We demonstrate how it is possible to transform a system from an ON-OFF to an OFF-ON sensor by changing the length of the chain used to lipophilise a ligand that resides inside TritonX-100 micelles together with pyrene as the fluorophore. Three tetrathia-monoaza macrocyclic ligands have been synthesised with the same ring but functionalised on the nitrogen atom with a methyl (C1-NS4), an n-butyl (C4-NS4) or an n-dodecyl (C12-NS4) chain. The three ligands have been fully characterised in water containing TritonX-100 micelles by means of potentiometric titrations and their apparent protonation and complexation constants with Hg(2+) were determined. On the basis of the distribution diagrams obtained, the more lipophilic C12-NS4 has been developed as an ON-OFF fluorescent sensor for mercury: working at pH<4, in the absence of Hg(2+) the ligand is inside the micelles, protonated and non-quenching, while on addition of mercury the [C12-NS4Hg](2+) complex forms which remains inside the micelles and is quenching. On the other hand, the ligand of intermediate chain length, C4-NS4, can be used to obtain an OFF-ON sensor at 7.0 7.0 the ligand is unprotonated, it stays inside the micelles and is quenching, while addition of Hg(2+) in the 7.0-9.5 pH range results in the formation of [C4-NS4Hg](2+), which is hydrophilic enough to leave the micelles and to be released into the bulk solution where it is no longer capable of quenching pyrene fluorescence. Additional studies on C1-NS4, C3-NS4 and C8-NS4 indicate that the optimal chain length to observe this OFF-ON behaviour is C(3)-C(4).

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PubMed2006

Single and double pH-driven Cu2+ translocation with molecular rearrangement in alkyne-functionalized polyamino polyamido ligands.

A new series of ligands, containing one (L1H(2)-L4H(2)) or two (L5H(4)-L6H(4)) 1,4,8,11-tetraaza-5,7-dione units and functionalized with a propargyl group on the C atom between the C=O moieties, has been synthesized. Protonation constants for the ligands and formation constants of their Cu(2+) complexes have been determined in water, and the coordination geometry of the complexes existing at various pH values has been investigated by coupled pH-metric and spectrophotometric titrations. Ligands capable of simple uptake of Cu(2+) with the formation of neutral, square-planar complexes containing the -2-charged diamino-diimido donor sets and ligands containing further coordinating groups (quinoline or pyridine) capable of single and double cation translocation have been investigated. The role of the substituents on the amino groups and the structural role played by the propargyl group have been examined as regards Cu(2+) complexation and translocation. In the double-translocating ligand L6H(4), when the two Cu(2+) ions move inside the diamino-diamido donor set, the slim propargyl group allows an unprecedented folding of the whole ligand with apical coordination of one pyridine to form a five-coordinate, square-pyramidal Cu(2+) ion. The crystal and molecular structures of this unusual [L6Cu(2)] complex have been determined by X-ray diffraction. Finally, oxidation of Cu(2+) to Cu(3+) has been studied by cyclic voltammetry in water, which revealed that the redox reaction occurs only when the copper cation is within the diamino-diimido compartment. Moreover, both functionalization of the primary amines with bulky substituents and apical coordination of Cu(2+) make access to the 3+ oxidation state more difficult and disrupt the reversibility of the electrochemical process.

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PubMed2006

Micelles for the self-assembly of "off-on-off" fluorescent sensors for pH windows.

A micellar approach is proposed to build a series of systems featuring an "off-on-off" fluorescent window response with changes in pH. The solubilizing properties of micelles are used to self-assemble, in water, plain pyrene with lipophilized pyridine and tertiary amine moieties. Since these components are contained in the small volume of the same micelle, pyrene fluorescence is influenced by the basic moieties: protonated pyridines and free tertiary amines behave as quenchers. Accordingly, fluorescence transitions from the "off" to the "on" state, and viceversa, take place when the pH crosses the pK(a) values of the amine and pyridine fragments. To obtain an "off-on-off" fluorescent response in this investigation we use either a set of dibasic lipophilic molecules (containing covalently linked pyridine and tertiary amine groups) or combinations of separate, lipophilic pyridines and tertiary amines. The use of combinations of dibasic and monobasic lipophilic molecules also gives a window-shaped fluorescence response with changes in pH: it is the highest pyridine pK(a) and the lowest tertiary amine pK(a) that determine the window limits. The pK(a) values of all the examined lipophilic molecules were determined in micelles, and compared with the values found for the same molecules in solvent mixtures in which they are molecularly dispersed. The effect of micellization is to significantly lower the observed protonation constants of the lipophilized species. Moreover, the more lipophilic a molecule is, the lower the observed logK value is. Accordingly, changing the substituents on the basic moieties or modifying their structure, tuning the lipophilicity of the mono- or dibases, and choosing among a large set of possible combination of lipophilized mono- and dibases have allowed us to tune, almost at will, both the width and the position along the pH axis of the obtained fluorescent window.

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PubMed2004

Three-component systems for conventional and window-shaped response fluorescent pH indicators.

Three-component systems made of a tetradentate bis-amino bis-quinoline ligand, a transition metal cation (Ni2+ or Cu2+) and a fluorescent indicator (Coumarin 343) have been studied in a water-dioxane (1 : 4 v/v) mixture, through potentiometric, pH-spectrophotometric and pH-fluorimetric titrations. For the Cu2+ containing systems, an "on-off-on" variation of fluorescence intensity vs. pH has been observed, whereas in the presence of Ni2+ a simple "on-off" profile of the fluorescence intensity vs. pH was obtained. These ternary systems thus behave as window-shaped or conventional pH-indicators, depending whether Cu2+ or Ni2+ is used as the cation.

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PubMed2003

Bistable copper complexes of bis-thia-bis-quinoline ligands.

The new ligands R,R-trans-S,S'-bis[methyl(2'-quinolyl)]-1,2-dithiacyclohexane, cis-S,S'-bis[methyl(2'-quinolyl)]-1,2-dithiacyclohexane, and 1,6-bis(2'-quinolyl)-2,5-dithiahexane have been synthesized and their complexes with Cu(I) and Cu(II) prepared. The ligand/metal systems are bistable, as the complexes with copper in both its oxidation states are stable under the same conditions as solids and in solution. The crystal and molecular structure of [Cu(I)(1,6-bis(2'-quinolyl)-2,5-dithiahexane)]ClO(4) has been determined by X-ray diffraction and reveals that the complex is monomeric, with the ligand folding around the Cu(+) cation, imparting to it a tetrahedral coordination. UV-vis, MS-ESI, and NMR data indicate that the same is found for the Cu(I) complexes of all three ligands. Also, the Cu(II) complexes are monomeric, but with a square arrangement of the ligands around Cu(2+). On changing the oxidation state, the change in the geometrical arrangement is fast and complete in less than 80 ms, as demonstrated by cyclic voltammetry experiments. In the CV profiles, the oxidation and reduction events take place at separated E(ox) and E(red) values, with no return wave even at the fastest scan rates. In the E(ox)-E(red) interval (which ranges from 450 to 650 mV, depending on the ligand), the ligand/copper system can thus exist in one of its two states, depending on its history, and thus display electrochemical hysteretical behavior. The electrochemical cycle leading from the tetrahedral [Cu(I)(ligand)](+) to the square [Cu(II)(ligand)](2+) complex (and vice versa) is reversible and repeatable without degradation, as checked by coupled UV-vis-controlled potential coulometry experiments.

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PubMed2003

Monitoring the redox-driven assembly/disassembly of a dicopper(I) helicate with an auxiliary fluorescent probe.

The assembly/disassembly of a dicopper(I) helicate with a bis-bidentate imine-quinoline ligand is driven by the Cu(II)/Cu(I) redox change and is signaled by a fluorescent probe bearing a -COO(-) group (coumarine 343). The probe coordinates the Cu(II) center of the monomeric complex, which quenches its emission (fluorescence off), and is released upon reduction and formation of the Cu(I) helicate (fluorescence on).

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PubMed2002

'On-off-on' fluorescent indicators of pH windows based on three separated components.

'On-off-on' fluorescent indicators for pH windows are obtained with ternary systems in which the three separated components are Cu2+, a tetraaza ligand and the fluorophore Coumarin 343: protonation of Coumarin 343, its coordination to Cu2+ and its displacement from Cu2+ by OH- give an inverse bell-shaped variation of emission with pH.

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