Fosca Conti

The sign of the exchange interaction between triplet excited fullerene and nitroxide free radicals

The sign of the exchange interactionJ in a series of radical triplet pairs (RTPs), formed by a nitroxide free radical and a triplet excited fullerene, has been determined from the spin polarization of time-resolved electron paramagnetic resonance spectra. Radical and fullerene are linked together by covalent bonds in different geometries. It is shown that the sign ofJ depends on the overlap between the orbital of nitroxide unpaired electron and the LUMO of fullerene, which is singly occupied in the excited triplet state. When the overlap does not vanish, a negative contribution toJ arises from the admixing of a charge transfer structure in the wave function of the excited doublet state D* of the RTP, which does not take place in the excited quartet state Q*. The mixing of D* and Q* states lowers the energy of the former spin state and gives antiferromagnetic coupling.

Effect of high pressure CO2 on the structure of PMMA: a FT-IR study.

Conformational changes in polymer films exposed to high-pressure CO(2) have been investigated with Fourier transform infrared (FT-IR) spectroscopy. The experimental setup, based on a custom-made stainless steel optical cell with CaF(2) windows, allows measurements in a CO(2) environment for pressures up to 6 MPa, in a temperature range from 293 to 353 K and in the mid-infrared (1000-4000 cm(-1)). Poly(methyl methacrylate) (PMMA), a polymer with a side group (C-type), was studied to monitor the spectral changes as a function of CO(2) pressure and was compared to poly(D,L-lactic-co-glycolic acid) (PLGA), a polymer without a side group (B-type). By monitoring the characteristic carbonyl bands, conformational changes that occur due to molecular interactions between the high-pressure CO(2) and the polymers were explored at a constant pressurization rate (0.02 MPa/min) and temperature. Spectral changes are observed only for PMMA, where the vibrational band at 1680 cm(-1) disappears with increasing pressure. The spectra of PLGA do not show any significant change in the presence of high pressure CO(2) in the investigated range. The behavior of the absorbance peak as a function of pressure and temperature highlights the presence of dynamic cross-links (DCs) between the side groups of PMMA films obtained by solvent casting below the glass transition temperature of the polymer. The spectral features are correlated using a model that accounts for CO(2) diffusion and the relaxation kinetics of the polymer chains in the thin film. The disappearance of the vibrational band attributed to the DCs for PMMA is related to the glass transition temperature, and a retrograde vitrification phenomenon is observed. This approach can be considered a useful alternative to magnetic suspended balance for the study of polymer-gas systems.

Carbon NMR investigation of the polybenzimidazole–dimethylacetamide interactions in membranes for fuel cells

Polybenzimidazole dissolved in dimethylacetamide has been studied by 1H and 13C one and two dimensional NMR methods to investigate the polymer–solvent interactions. The resonance signals have been fully assigned, giving a complete picture of the present species and allowing a study of the involved dynamic processes. The results have been correlated with the formation of H-bonds, and with the presence of tautomeric processes in the polymer.

Spin polarization and photoinduced electron transfer between ferrocene and fullerene derivatives containing a nitroxide group

Fulleropyrroliddine derivatives containing a TEMPO group (FPNOs) are examined in toluene solution, between 200 and 270 K, by TR-EPR spectroscopy after LASER excitation. Signals of the ground and excited states are observed and their time profiles are recorded. In presence of ferrocene the time pattern of the polarization is modified, showing the occurrence of intermolecular electron transfer from the donor ferrocene to the excited FPNOs. The rate constantkE. is obtained from the simulation of the EPR signal time evolution of the ground and excited states.

EPR and ENDOR of perfluoroalkyl radical intermediates in the reaction between perfluorohypofluorites and perfluoroalkenes

EPR and ENDOR spectra of the reaction products of sterically hindered perfluoroalkenes with perfluoromethyl hypofluorite and with the perfluoroether hypofluorite X(CF2O)n(CF2CF2O)mCF2OF, where X is either OCF3 or a second hypofluorite group OCF2-OF, have been recorded. Tertiary and secondary free radicals are produced by the addition to the alkene double bond of a fluorine atom or of the partner radical formed by the homolysis of the hypofluorite O—F bond. Analysis of the spectra reveals that hyperfine splitting by 19F nuclei in the γ and δ positions, and even further from the radical centre, contribute spectra. The α- and β-19F hyperfine splitting are discussed in relation to the radical structure and conformation. The β-19F splittings are accounted for by the well known equation aβF=B cos2(θ); different values of the constant B have to be used according to the number of β-fluorine substituents bonded to the same α-carbon.

Time-resolved EPR observation of synthetic eumelanin–superoxide radical pairs

TR-EPR spectra of UV irradiated synthetic eumelanin in equilibrium with air oxygen, reveal the formation of melanin-superoxide radical pairs and allow to determine both the electron dipolar interaction D and the exchange interaction J between the two radicals.

A fulleropyrrolidine binitroxide: Synthesis, EPR and electrochemical features

The synthesis, electrochemical characterization and EPR study of a fulleropyrrolidine monoadduct, bearing two TEMPO groups differently linked at positions 2 and 5 of the pyrrolidine ring, are presented. The exchange interaction J between the two unpaired nitroxide electrons is of the same order of magnitude of the 14N hyperfine coupling (aN). EPR investigation in three different solvents showed that J decreases as the temperature increases. Chemical or electrochemical reduction produces a stable triradical anion that, in the case of chemical reduction, was characterized ia EPR spectroscopy. Further chemical reduction produces an unstable species probably corresponding to a protonated compound. The electrochemical study highlights an interesting effect of the fullerene moiety on the kinetics of TEMPO-centered electrochemical reduction, which was analyzed in the frame of Marcus theory of heterogeneous electron transfer.

Time-resolved EPR investigation of intramolecular photoinduced electron transfer in spin-labeled fullerene/ferrocene dyads

Intramolecular photoinduced electron transfer from ferrocene-linked TEMPO-labeled fulleropyrrolidines has been studied by time-resolved EPR using laser excitation. The EPR signal of the quartet excited state of 2 and 3, which corresponds to triplet fullerene/doublet nitroxide, was substantially quenched relative to model compound 1 without ferrocene. Kinetic parameters of the electron transfer process have been obtained by simulating the time evolution of the EPR signal of both the ground electronic state and the excited quartet state that become spin polarized during the excitation and decay paths. The distance between donor and acceptor, when the photoinduced electron transfer occurs, has been estimated for both dyads on the basis of the Marcus electron transfer theory.

Phosphoric Acid and its Interactions with Polybenzimidazole-Type Polymers

In high-temperature polymer electrolyte membranes, phosphoric acid is used as dopant for polybenzimidazole-type membranes to provide the protonic conductivity. In addition, phosphoric acid also serves as proton conductor in the porous electrodes in order to establish the three-phase boundary. In the first part of this chapter a short overview is given on the physico-chemical properties of (aqueous) phosphoric acid. In the second part the focus is on the adsorption of phosphoric acid as a protic electrolyte on polybenzimidazole-type polymers. Although polybenzimidazole-type membranes are routinely doped with phosphoric acid, few studies on the exact nature of the acid inside the membrane have been published. Experimental data from our institute and data compiled from literature indicate that the polymer chain is protonated by the acid and that the anions are bound by coulomb interactions. Additional electrolyte molecules can interact with the polymer chain by formation of H bonds or via H bonds with other H3PO4 molecules. It is demonstrated that the uptake of phosphoric acid can be described by a modified BET isotherm, assuming multilayer-like adsorption. The assumption of free phosphoric acid in the membrane at high doping levels is supported by Raman spectroscopy.

Broadband electric spectroscopy at high CO2 pressure: dipole moment of CO2 and relaxation phenomena of the CO2-poly(vinyl chloride) system.

Broadband electric spectroscopy (BES) is a technique that shows promise in studying the interactions of dense or supercritical gases with polymers, particularly with respect to chain mobility. Polymers that are treated with dense gases show a reduction in the viscosity, glass transition, and melting temperature. A high pressure cell for BES has been constructed that can be used from ambient temperature and pressure to 353 K and 15 MPa and over a frequency range from 20 Hz to 1 MHz. In the past, the dielectric constant of CO(2) was determined by measurements at only one or two frequency values. New instrumentation and technology allow this experiment to be expanded to cover a wider frequency range. BES measurements of CO(2) do not show any relaxation peaks in the permittivity from 20 Hz to 1 MHz and 1 to 6 MPa. By these measurements, the CO(2) dielectric constant was evaluated between 0.1 and 6 MPa. Cell testing with poly(vinyl chloride) (PVC) at 323 K and CO(2) pressures from 0.1 to 13 MPa indicate an increase in the chain segmental motion at high pressures resulting from a reduction in the glass transition temperature of the PVC-CO(2) system due to plasticization by CO(2).