Dynamics of a Nitroxide Layer Grafted onto Porous Silicon

This paper reports a rapid electron spin resonance method to assess the dynamical properties of spin-labeled alkyl carboxylate layers, covalently linked onto a porous silicon substrate. IR and X-ray photoelectron spectroscopy analyses complemented the ESR study by proving the successful grafting of the organic radical and coverage of the surface

Stochastic Modeling of CW-ESR Spectroscopy of [60]Fulleropyrrolidine Bisadducts with Nitroxide Probes

In this work, we address the interpretation of continuous wave electron spin resonance (CW-ESR) spectra of fulleropyrrolidine bisadducts with nitroxide addends. Our approach is based on a definition of the spin Hamiltonian which includes exchange and dipolar interactions and on a complete numerical solution of the resulting stochastic Liouville equation, with inclusion of diffusive rotational dynamics. CW-ESR spectra are simulated for a series of C60 bisadducts made up of four trans isomers and the equatorial isomer. A nonlinear least-squares fitting procedure allows extraction directly from the available experimental spectra of a wide range of parameters, namely interprobe relative distances, diffusion tensors, and values of the exchange parameter J. Results are in good agreement with previous, more phenomenological estimates, proving that the combination of sensitive ESR spectroscopy based on multiple spin labeling with nitroxide radicals and sophisticated modeling can be highly helpful in providing structural and dynamic information on molecular systems

Wetting Properties of Flat and Porous Silicon Surfaces Coated with a Spiropyran

We report the immobilization and characterization of a spiropyran (SP) derivative (1) on smooth Si(100) and porous H-terminated silicon surfaces through a thermal hydrosilylation protocol. Under visible light exposure the SP is in a closed, hydrophobic form, whereas under UV irradiation it converts to a polar, hydrophilic open form named merocyanine (MC). The SP−MC photoinduced isomerization gives a small contact angle (CA) change of 9° for smooth Si(100) samples under sequential irradiation cycles with white and UV light. Irradiation of porous silicon (PS) surfaces, under the same conditions, gave a CA change of 11°. Treatment of PS surfaces, bearing the MC form of chromophore 1, with cobalt(II) ions enhances the wettability switching of the PS surface to a much larger extent, giving rise to a CA variation as high as 32°

Easy Access to Water-Soluble Fullerene Derivatives via 1,3-Dipolar Cycloadditions of Azomethine Ylides to C₆₀

Easy Access to Water-Soluble Fullerene Derivatives via 1,3-Dipolar Cycloadditions of Azomethine Ylides to C60</sub

Unexpected Formation of Dienes in the Diels−Alder Reaction of Exocyclic 1-Bromobutadienes of Polycyclic Hydrocarbons

Polycyclic dienes having an exocyclic 1-bromobutadiene moiety react with dienophiles and fullerene-C60 to afford exclusively dienes via a cycloaddition−elimination mechanism. Neither the primary adducts nor the double addition products derived from a second cycloaddition of the dienophile to the diene could be detected. In one case only, i.e. with 4-phenyl-1,2,4-triazoline-3,5-dione, was the double addition product formed. Contrary to expectations, X-ray diffractometric analysis shows that this adduct is formed following a contrasteric approach

Wetting Behavior of Porous Silicon Surfaces Functionalized with a Fulleropyrrolidine

We report the immobilization of a fulleropyrrolidine, bearing a dec-9-ynyl functionality, on silicon surfaces through a thermal hydrosilylation protocol. Contact angle measurements on porous silicon (PS) surfaces reveal an unusual dependence of the angle with the PS roughness that apparently contradicts Wenzel's formula. This result has been explained by an extension of Wenzel's model in which the critical angle, which discriminates between the hydrophilic/hydrophobic character of a solid material, is substantially reduced below 90° by surface roughness

Unexpected Formation of Dienes in the Diels−Alder Reaction of Exocyclic 1-Bromobutadienes of Polycyclic Hydrocarbons

Polycyclic dienes having an exocyclic 1-bromobutadiene moiety react with dienophiles and fullerene-C60 to afford exclusively dienes via a cycloaddition−elimination mechanism. Neither the primary adducts nor the double addition products derived from a second cycloaddition of the dienophile to the diene could be detected. In one case only, i.e. with 4-phenyl-1,2,4-triazoline-3,5-dione, was the double addition product formed. Contrary to expectations, X-ray diffractometric analysis shows that this adduct is formed following a contrasteric approach

Zinc-Induced Switching of the Nonlinear Optical Properties of a Functionalized Bis(styryl)benzene

A large variation of the nonlinear transmission properties of a cyclen-based bis(styryl)benzene can be induced by a small change of the linear absorption spectrum upon Zn2+ binding. This result has been interpreted in the frame of a sequential two-photon process in which one photon is absorbed from the ground state and one photon is absorbed from an excited state

Structural Modifications of the Active Site in Teicoplanin and Related Glycopeptides. 2. Deglucoteicoplanin-Derived Tetrapeptide

The deglucoteicoplanin-derived tetrapeptide (TDTP), a key synthon suitable for the synthesis of modified glycopeptide antibiotics differing in the structure of the active site, was prepared from the product (RH-TD) of reductive hydrolysis of the 2,3-peptide bond of deglucoteicoplanin (TD) upon selective oxidation of the newly formed hydroxymethyl group and following simultaneous removal of amino acids 1 and 3 by double Edman degradation. The oxidation of the alcohol function of residue 2 in RH-TD was accomplished (Jones reagent) after protection of the two free amino groups as tert-butyl BOC carbamates and of most of phenolic hydroxy groups as benzyl CBZ carbonates. Esterification of the C-terminal carboxy group of intermediate di-BOC-RH-TD allowed the formation at the end of the process of the tetrapeptide (TDTP-Me) protected at one carboxy group as methyl ester. Selective protection of the primary N4- and N2-amino groups of TDTP-Me as BOC and CBZ carbamates, respectively, followed by removal of the BOC function, afforded a more suitable intermediate (N2-CBZ-TDTP-Me) for the synthesis of new glycopeptides

Study of the Aggregation Properties of a Novel Amphiphilic C₆₀ Fullerene Derivative

An amphiphilic C60-derivative, AFE, characterized by the presence of the chiral fragment of l-acetyl carnitine in its hydrophilic appendage has been synthesized. In binary (THF/H2O) and ternary (THF/MeOH/H2O) solutions, AFE exhibits a strong tendency to self-aggregation, provided that the Hildebrand polarity index, δ, of the solvent is higher than about 15. A stable aqueous solution of aggregated AFE was obtained. Partition experiments between n-octanol and water show that AFE cannot be spontaneously transferred from water into the organic solvent (and vice versa), although it is effectively “salted-out” by common electrolytes. Light scattering and reversed-phase liquid chromatography experiments carried out on the aqueous solution of AFE suggest for the aggregates an average diameter of 120 nm