Carlo Corvaja

Spin polarization in fullerene derivatives containing a nitroxide group. Observation of the intermediate photoexcited quartet state in radical triplet pair interaction

A series of C60 fullerene derivatives containing a nitroxide group has been photoexcited by short LASER pulses in the microwave cavity of a cw-EPR spectrometer. Strongly spin polarized signals have been observed, in glassy matrix as well as in liquid solution, for both the ground electronic state and the excited quartet state. In the quartet state the excitation resides in the fullerene part and the molecule constitutes a triplet-radical pair with the partner covalently linked. The absorptive or emissive character of the transitions is explained in terms of the mechanism of radicaltriplet interaction producing spin polarization. Opposite initial sign and polarization patterns are observed for molecules with different spacer between nitroxide and fullerene. The time evolution of the relevant sublevel populations is fitted by a kinetic model taking into account quartet decay constants, quartet and doublet spin-lattice relaxation rates and branching ratios.

EPR studies of the excited triplet states of C60O and C60C2H4N(CH3) fullerene derivatives and C70 in toluene and polymethylmethacrylate glasses and as films

Abstract The EPR spectra of excited triplet states of two fullerene derivatives, C60O and C60C2H4N(CH3), in the glassy matrices of toluene and of polymethylmethacrylate (PMMA) and as films were examined at low temperature and the magnetic parameters were measured. In films, species with zfs parameters smaller than those in toluene were found and they were assigned to triplet excitations visiting more than one molecule in the crystallites that form the film. When C60C2H4N(CH3) was excited in PMMA grown by in sity thermal polymerization a new triplet species was originated characterized by a dipolar splitting constant D about three times larger than that measured in toluene. Evidence was gained that such species participates to cross-linking in the polymer. The effect of PMMA matrix on the molecular dynamics of 3C70 was also investigated. It was found that a dynamical model holds based on the pseudorotation of the molecule around the axis of larger dipolar splitting. Motion is activated with activation energy of 210 ± 50 cm−1 a value comparable with that obtained for pseudorotation of 3C70 in cyclohexane.

E.P.R. and ENDOR investigation of tempone nitroxide radical in a single crystal of tetramethyl-1,3-cyclobutanedione

The E.P.R. and ENDOR spectra of the free nitroxide radical tempone diluted in a single crystal of 2,2,4,4-tetramethylcyclobutanedione (TMCB) have been analysed and the g tensor, nitrogen hyperfine and quadrupole interaction tensors, and methyl protons tensors have been determined. It is found that the principal axes of the g tensor and of the tensors A N and Q N do not fully coincide. The negative sign of the quadrupole interaction tensor element Qzz where Z is along the direction of the nitrogen p orbital agrees with previous measurements on similar systems.

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.

Solar cells based on a fullerene–azothiophene dyad

A power conversion efficiency of 0.37%, under white light of 80 mW cm-2 intensity, was obtained when a fullerene-azothiophene dyad was used as the active layer of a photovoltaic cell.

Thermal decomposition of branched-chain perfluoroalkanes

Abstract The pyrolysis of some branched perfluoroalkanes has been studied. Homolytic cleavageof the most hindered carbon-carbon bond occurs, followed by coupling and rearrangementof the radicals so formed. This mechanism accounts for all the reaction products. Somekinetic and thermodynamic data are presented.

CIDEP Effects of Intramolecular Quenching of Singlet and Triplet Excited States by Nitroxide Radicals in Oligopeptides: A Potentially Useful New Method for Investigating Peptide Secondary Structures in Solution

Two hexapeptides, each bearing one photoactive alpha-amino acid (Bin or Bpa) and one nitroxide-containing TOAC residue, have been synthesized and fully characterized. FT-IR absorption measurements indicate that a 3(10)-helical conformation is adopted by these peptides in solution. As two amino acid units separate the photoactive residue from TOAC in the peptide sequences, the two moieties face each other at a distance of about 6 A after one complete turn of the ternary helix. Irradiation by a light pulse from an excimer laser populates the excited states localized on the chromophores. An intramolecular interaction between the singlet (Bin) or triplet (Bin and Bpa) excited states and the doublet state of the TOAC nitroxide makes a spin-selective decay pathway possible, that produces transient spin polarization. In addition, in order to determine whether the intramolecular exchange interaction occurs through-bond or through-space, we have prepared linear and cyclic TOAC-Bin dipeptide units. A CIDEP study revealed that a through-space intramolecular interaction is operative. The observation of spin polarization makes the two helical hexapeptides suitable models to test the possibility of application of this novel technique to conformational studies of peptides in solution.

Carotenoid triplet detection by time-resolved EPR spectroscopy in carotenopyropheophorbide dyads

Carotenoid triplets play a photoprotective role in natural photosynthesis. The main process of carotenoid triplet formation is known to be triplet-triplet energy transfer from chlorophyll triplets. The structural requirements for high transfer yields are still a matter of discussion and the presence of competitive triplet formation pathways has not been excluded. Transient EPR measurements of triplet states formed by photoexcitation allow detection of the initial spin polarization. This pattern derives from the mechanism of triplet formation. In the case of triplet-triplet energy transfer, if the condition of spin angular momentum conservation is fulfilled, simulation of the EPR spectra gives information about the donor-acceptor mutual orientation. We describe transient EPR experiments on two artificial photosynthetic dyads, consisting of a carotenoid covalently-linked to a free-base or zinc substituted pyropheophorbide moiety and we discuss the results in terms of possible dyad conformations.

Aluminum (III) induces alterations on the physical state of the erythrocytic membrane: an ESR evaluation.

The action of aluminum [Al(III)] as Al(acac)3 on erythrocytes causes biophysical effects such as osmotic fragility and echino-acanthocytes formation. In this paper, we present these effects in terms of variation of membrane fluidity, together with findings regarding conformational modifications of membrane proteins consequent to Al(III) exposure, as well as the effects on the mobility of the membrane protein bound sialic acid. To this end, we utilized ESR measurements of rabbits and humans erythrocytic ghosts after probing or labeling with suitable stable radicals used as spin probes or labels. Our results show that the lipophilic, hydrolytically stable toxicant Al(acac)3 causes a remarkable reduction of membrane fluidity in rabbit erythrocytes, an appreciable structural compacting effect on cytoskeletal and transmembrane proteins, as well as a reduction of rotational mobility of cell-surface sialic acid of human erythrocytes.

Electron spin polarisation effects of ISC into a superposition of spin states: Triplet excitons in biphenyl-tetracyanobenzene 1:1 complex

Abstract The behaviour in a magnetic field of the spin polarisation of triplet excitons in biphenyl-tetracyanobenzene is accounted for by the generation of the triplet state into a superposition of spin states. The molecular conformation in the excited singlet state is discussed.