Biomedical subjects

Haim Levanon

Publications and source records attributed to Haim Levanon.

8 recordsLinked to original sources

PubMed2006

Triplet topology of self-assembled zinc porphyrin-pyridylfullerene complex.

This work extends a recent EPR study on light-driven electron and energy transfer in a self-assembled zinc porphyrin-pyridylfullerene (ZnP-PyrF) complex. We report on a triplet line shape analysis of the photoexcited PyrF monomer and the ZnP-PyrF complex dissolved in isotropic and anisotropic matrixes of different polarity, namely, toluene, tetrahydrofuran (THF), and the nematic liquid crystals (LCs), E-7 and ZLI-4389. The line shape of the unbound *(3)PyrF obtained in both isotropic matrixes exhibits triplet parameters similar to those obtained for other monoadducts of C(60) under similar experimental conditions. On the other hand, 8(3)PyrF oriented in the LCs shows a complicated line shape, which is attributed to two conformers: (a) an axial dominant (85%) configuration characterized by triplet parameters, similar to those obtained in the isotropic matrixes and (b) a bent configuration associated with spin density localized about the poles accompanied by sign reversal of the ZFS parameter D of the *(3)C(60) moiety. Further, since in both LCs the ZnP-PyrF complex mainly exhibits a conformation with axial symmetry, the differences between the electron and the energy transfer routes in each LC are attributed to their different polarity. This study reflects the strength of LC matrixes to serve as a topological tool, enabling us to determine the conformers' distribution and to differentiate between electron and energy transfer routes.

Journal Article↗

PubMed2005

Electron spin polarization of functionalized fullerenes. Reversed quartet mechanism.

Time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used to study two functionalized fullerenes consisting of a C60 moiety covalently linked to TEMPO radical via spacers of different length. Photoinduced electron spin polarization (ESP) reflecting a non-Boltzmann population within the energy levels of the spin system was observed in the electronic ground and excited states. Both fullerenes are characterized by a sign inversion of their TREPR spectra. A new mechanism of ESP generation was suggested to explain the experimental results. This mechanism, termed as the reversed quartet mechanism (RQM), includes the intersystem crossing process, which generates ESP in the excited trip-doublet and trip-quartet (2T1 and 4T1) states. This ISC is accompanied by ESP transfer to the ground state (2S0) by either electron-transfer reaction (in our case via charge transfer state, 2CT, i.e., 2T1--> 2CT --> 2S0 or internal conversion, 2T1--> 2S0.

Journal Article↗

PubMed2005

Intramolecular electron and energy transfer in an axial ZnP-pyridylfullerene complex as studied by X- and W-band time-resolved EPR spectroscopy.

Light-driven electron transfer (ET) and energy transfer (EnT) in a self-assembled via axial coordination Zn-porphyrin-pyridylfullerene (ZnP-PyrF) complex were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy at 9.5 GHz (X-band) and 95 GHz (W-band). The studies over a wide temperature range were carried out in media of different polarity, including isotropic toluene and tetrahydrofuran (THF), and anisotropic nematic liquid crystals (LCs), E-7 and ZLI-4389. At low temperatures (frozen matrices), photoexcitation of the ZnP donor results mainly in singlet-singlet EnT to the pyridine-appended fullerene acceptor. In fluid phases ET is the dominant process. Specifically, in isotropic solvents the generated radical pairs (RPs) are long-lived, with lifetimes exceeding that observed for covalently linked donor-acceptor systems. It is concluded that in liquid phases of both polar and nonpolar solvents the separation of the tightly bound complex into the more loosely bound structure slows down the back ET (BET) process. Photoexcitation of the donor in fluid phases of LCs does not result in the creation of the long-lived RPs, since the ordered LC matrix hinders the separation of the complex constituents. As a result, fast intramolecular BET takes place in the tightly bound complex. Contrarily to the behavior of covalently linked donor-acceptor systems in different LCs, the polarity of the LC matrix affects the ET process. Moreover, in contrast to covalently linked D-s-A systems, utilization of LCs for the coordinatively linked D-s-A complexes does not reduce the ET rates significantly.

Anisotropy↗

PubMed2005

Magnetic field dependence of electron spin polarization generated through radical-triplet interactions.

The magnetic field dependence of electron spin polarization (ESP), generated in free radicals when they encounter photoexcited triplets, was measured experimentally and analyzed theoretically. The time-resolved electron paramagnetic resonance measurements were performed with a microwave setup consisting of low-loss dielectric ring resonators with tunable microwave frequencies and the corresponding magnetic fields. The ESP of the radical was found in the magnetic field range of 170-370 mT, and the results of the calculation based on the numerical solution of the stochastic Liouville equation were found to be in line with the experimental data showing that ESP decreases when the magnetic field increases.

Journal Article↗

PubMed2004

Electron spin dynamics in photoexcited diamagnetic and paramagnetic corroles.

Three corroles, which differ by their cavity's core, namely, diamagnetic free-base tris(pentafluorophenyl)corrole and its gallium(III) complex and the paramagnetic oxo-chromium(V) complex, were studied by steady-state and time-resolved electron paramagnetic resonance (EPR) spectroscopy. The magnetic and orientational parameters of the corroles, oriented in a nematic liquid crystal, were determined and interpreted in terms of their structure, geometry, and excited states spin dynamics. It was shown that both diamagnetic corroles, photoexcited to their triplet states, exhibit similar EPR line shapes, which is characterized by a negative zero-field splitting parameter, D, whose origin is due to molecular "stretching". Photoexcited Cr(V)O-corrole exhibits polarized ground-state EPR spectrum in emission mode. This polarization stems from the sequence of photophysical and photochemical reactions, involving the formation of the trip-quartet/trip-doublet composite states and their selective quenching via a charge transfer state.

Journal Article↗

PubMed2003

On the interactions of free radicals with gold nanoparticles.

Electron paramagnetic resonance (EPR) spectroscopy was used to study the interactions between stable free radicals and gold nanoparticles. The nitroxyl free radicals used were TEMPO, TEMPAMINE, and TEMPONE. Two sizes of Au particles, 15 and 2.5 nm in diameter, were synthesized to investigate the interactions with the metallic particles. We find that the EPR signal is reduced upon adsorption of the radicals onto the 15 nm Au particle surface. Despite the strong adsorption of TEMPAMINE on the particles, the signal intensity recovers upon the introduction of a high concentration of ethanolamine to the solution. The signal reduction was proportional to the concentration of Au particles, and the signal totally disappeared at high concentrations of Au particles. Possible explanations of the signal reduction are discussed in this Article. We propose that the reduction in signal intensity arises from exchange interactions between the unpaired electrons of the adsorbed radicals and conduction-band electrons of the metallic particles. In addition, in the presence of oxygen, the adsorbed TEMPAMINE radicals are catalytically oxidized to the carbonyl derivative, TEMPONE. A mechanism for this unexpected catalytic reaction is proposed.

Journal Article↗

PubMed2003

Using spin dynamics of covalently linked radical ion pairs to probe the impact of structural and energetic changes on charge recombination.

We have synthesized a series of structurally related, covalently linked electron donor-acceptor triads having highly restricted conformations to study the effects of radical ion pair (RP) structure, energetics, and solvation on charge recombination. The chromophoric electron acceptor in these triads is a 4-aminonaphthalene-1,8-dicarboximide (6ANI), in which the 4-amine nitrogen atom is part of a piperazine ring. The second nitrogen atom of the piperazine ring is part of a para-substituted aniline donor, where the para substituents are X = H, OMe, and NMe(2). The imide group of 6ANI is linked to a naphthalene-1,8:4,5-bis(dicarboximide) (NI) electron acceptor across a phenyl spacer in a meta relationship. The triads undergo two-step photoinduced electron transfer to yield their respective XAn(*)(+)-6ANI-Ph-NI(*)(-) RP states, which undergo radical pair intersystem crossing followed by charge recombination to yield (3)NI. Time-resolved electron paramagnetic resonance experiments on the spin-polarized RPs and triplet states carried out in toluene and in E-7, a mixture of nematic liquid crystals (LCs), show that for all three triads, the XAn(*)(+)-6ANI-Ph-NI(*)(-) RPs are correlated radical pairs and directly yield values of the spin-spin exchange interaction, J, and the dipolar interaction, D. The values of J are all about -1 mT and show that the LC environment most likely enforces the chair conformation at the piperazine ring, for which the RP distance is larger than that for the corresponding boat conformation. The values of D yield effective RP distances that agree well with those calculated earlier from the spin distributions of the radical ions. Within the LC, changing the temperature shows that the CR mechanism can be changed significantly as the energy levels of the RPs change relative to that of the recombination triplet.

Journal Article↗

PubMed2002

Filling factor in a pulsed electron paramagnetic resonance experiment.

A definition and mathematical treatment to calculate the filling factor in a pulsed electron paramagnetic resonance (EPR) experiment are presented. The differences between filling factors in traditional, continuous wave (CW)-EPR experiments (eta), and in pulsed-EPR experiments (eta(p)), are discussed. We present some examples to demonstrate how eta(p) depends upon the particular pulse sequence and sample characteristics.

Biophysical Phenomena↗