Taking advantage of the radical character of tris(2,4,6-trichlorophenyl)methyl to synthesize new paramagnetic glassy molecular materials

This paper describes the synthesis of the novel bis[4-(N-carbazolyl)-2,6-dichlorophenyl](2,4,6-trichlorophenyl)methyl radical (2*) and tris[4-(N-carbazolyl)-2,6-dichlorophenyl]methyl radical (3*). A Friedel-Crafts reaction on [4-(N-carbazolyl)-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl]methyl radical (1*), 2*, and 3* leads to the introduction of acyl chains in the 3- and 6-positions of the carbazolyl moiety without impairment of the radical character of the molecule to give radicals 5*, 6*, and 7*. All of these novel radical adducts are thermally stable, 5* and 6* being amorphous solids by differential scanning calorimetry. Electron paramagnetic resonance spectra of them show a multiplet at low temperature due to the electron-coupling with six aromatic hydrogens. They show electrochemical amphotericity being reduced and oxidized to their corresponding stable anionic and cationic species, respectively. These radical adducts have luminescent properties covering the red spectral band of the emission with high intensities.     

[4-(N-Carbazolyl)-2,6-dichlorophenyl]bis(2,4,6-trichlorophenyl)methyl radical an efficient red light-emitting paramagnetic molecule

We report the synthesis, luminescent properties, electrochemical behavior and electron paramagnetic resonance of a novel stable radical adduct of the tris(2,4,6-trichlorophenyl)methyl radical and carbazole.     

Direct evidence of a radical mechanism in the addition reaction of iododifluoroesters to olefins by spin trapping

We present EPR analysis of the reaction of ethyl iododifluoroacetate with 1-tetradecene in the presence of Zn + NiCl2 x 6H2O, confirming the mechanistic studies that provide evidence of a single electron transfer process. We have trapped for the first time the ethoxycarbonyldifluoromethyl radical with a variety of spin traps, such as phenyl tert-butyl nitrone (PBN), 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), 2-methyl-2-nitrosopropane (MNP), and 2-nitro-2-nitrosopropane (NNP), and the EPR spectra of the corresponding adducts have been recorded. In a second step the ethoxycarbonyldifluoromethyl radical adds to the olefin to furnish a second radical intermediate, which can be trapped with NNP. Evidence of this second radical was obtained by EPR only with electron-rich olefins, such as alpha-methylstyrene and 2,4,6-trimethylstyrene, and the new adducts were recorded and interpreted. In addition, we also report the EPR spectra of the corresponding adducts when other alkylating reagents are used, such as ethyl iodoacetate, n-perfluorohexyl iodide, methyl omega-iodohexadecanoate, and n-butyl iodide.     

High-spin radical cations of alternating poly(m-p-anilines)

Three alternating poly(m-p-anilines) have been synthesized via palladium-catalyzed amination reactions. Polymers were oxidized to radical cations by the use of chemical and electrochemical methods. The presence of radical cations was manifested by the appearance of two new bands in UV-vis spectra and a strong EPR signal. Moreover, EPR spectra at low temperatures confirmed the formation of a high-spin state. The magnetization measurements of polymers oxidized to radical cations revealed the paramagnetic-type behavior with weak antiferromagnetic interactions. Radical cations underwent the degradation processes in the presence of air, which led to the decrease of spin concentration.     

Radical reactions of a stable N-heterocyclic silylene: EPR study and DFT calculation

Reaction of the stable silylene, 1,3-di-tert-butyl-1,3,2-diazasilol-2-ylidene, with the free radical sources TEMPO, Hg[P(O)(OPri)2]2, (CO)3CpM-MCp(CO)3 (M = W, Mo), (CO)5Re-Re(CO)5, and toluene leads to radical adducts. The EPR spectra of these radicals indicate that the unpaired electron is delocalized over the silicon-containing five-membered ring.     

Deuterated analogues of the free radical trap DEPMPO: synthesis and EPR studies

Three analogues of 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO, 1) labelled with two (1-d2), five (1-d5) or seven (1-d7)2H were synthesized and used to trap the tert-butylperoxyl radical. The EPR spectra of 1-d2-OOBu(t) and 1-d7-OOBu(t) spin adducts exhibited more straightforward patterns and better signal to noise ratio than those obtained with 1 or 1-d5. The use of the easily available 1-d2 as spin trap could help significantly the analysis of the EPR signals when the signal of either superoxide or alkylperoxyl spin adduct is superimposed with the signals of other spin adducts.     

The cycloaddition of the 1,3-butadiene radical cation with acrolein and methyl vinyl ketone

The 1,3-butadiene radical cation reacts with acrolein and methyl vinyl ketone to produce ‘stable’ adducts. The nature of the reaction and the structures of the adducts were investigated by collisional activation decomposition (CAD) combined with tandem mass spectrometry (MS/MS), and also by Fourier transform mass spectrometry. The CAD spectra of gas-phase adducts were compared with those of suitable model compounds. On that basis, it was determined that the 1,3-butadiene radical cation undergoes a cycloaddition with these α,β-unsaturated carbonyl compounds. The butadiene radical cation serves as the ‘ene’, and the acrolein and methyl vinyl ketone react as dienes, forming cycloadducts having 2-ethenyl-2,3-dihydropyran radical cation structures.     

Detection of hydrogen atom adduct of spin-trap DEPMPO. The relevance for studies of biological systems

We proposed EPR spectroscopy using spin-trap DEPMPO as a novel method for the detection of a hydrogen atom (*H) produced by chemical and biological systems. In complex EPR spectra of DEPMPO adducts in biological systems, spectral lines of unknown origin have been observed. We have assumed (Baci?, G.; Mojovi?, M. Ann. N. Y. Acad. Sci. 2005, 1048, 230-243) that those lines represent the spectrum of a hydrogen atom (*H) adduct i.e., DEPMPO/H. An electrochemical system known to produce only *H radicals was used here in order to obtain a separate spectrum of the DEPMPO/H adduct. An acquired spectrum as well as a computer spectral simulation of the DEPMPO/H adduct showed considerable resemblance with additional lines in the EPR spectra of DEPMPO adducts in biological systems-plant plasma membranes and cell walls. This shows that such a radical is produced by plants as well as that DEPMPO is suitable for detection in both electrochemical and biological systems.     

Radical reactions of a stable N-heterocyclic germylene: EPR study and DFT calculation

The first radical adducts of a stable N-heterocyclic germylene were investigated. Novel radical species were produced from a variety of precursors and studied by EPR spectroscopy. DFT (B3LYP) calculations of radical adducts of different (C, Si, Ge) unsaturated N-heterocyclic divalent species with phenoxyl radical show that in the radicals studied the unpaired electron is delocalized over the five-membered ring and the spin density on the central atoms decreases in the following order: C > Si > Ge. These trends can be understood in terms of zwitterionic structure of radical adducts.     

Radical trapping properties of 3-aryl-2H-benzo[1,4]oxazin-4-oxides

A series of 3-aryl-2H-benzo[1,4]oxazin-4-oxides was prepared, and their ability to trap free radicals was investigated by EPR spectroscopy. In organic solvents, these compounds were able to efficiently scavenge all carbon- and oxygen-centered radicals tested, giving very persistent aminoxyls, except with superoxide anion whose spin adducts were unstable. The main feature of these nitrones as spin traps lies in the possibility to recognize the initial radical trapped. In fact, besides a g-factor and aminoxyl nitrogen EPR coupling constant dependence on the species trapped, the EPR spectra also show different patterns due to hyperfine splittings characteristic of the radical scavenged. This last important feature was investigated by means of density functional theory calculations.     

Bis(n-octylamino)perylene-3,4:9,10-bis(dicarboximide)s and their radical cations: synthesis, electrochemistry, and ENDOR spectroscopy

1,6- and 1,7-bis(n-octylamino)perylene-3,4:9,10-bis(dicarboximide) were synthesized by reaction of n-octylamine with the corresponding dibromo compounds. These compounds display intense charge-transfer optical transitions in the visible spectrum (approximately 550-750 nm) and fluoresce weakly (Phi(F) < 0.06). Cyclic voltammetry reveals that each chromophore undergoes facile and reversible oxidation and reduction. Spectroelectrochemical studies show that the radical cations of these chromophores are stable and show no signs of deprotonation of the secondary amines. Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) studies of the chemically generated radical cations of these chromophores corroborate the spectroelectrochemical data by showing that the radical cations persist for days at room temperature in methylene chloride solution. These experiments and complementary density functional theory (DFT) calculations provide a comprehensive picture of the molecular orbitals, spin density distributions, and geometries of the radical cations. The redox properties and stability of these alkylamino-functionalized perylene compounds make them a valuable addition to the family of robust perylene-based chromophores that can be used to develop new photoactive charge transport materials.     

Synthesis and Photophysical Properties of C60‐carbazole Adducts

Three C₆₀‐carbazole adducts have been synthesized by 1, 3‐dipolar cycloaddition reaction. Intramolecular energy/electron transfer from carbazole to C₆₀ was observed by steady‐state absorption and fluorescence spectra. The fluorescence spectra of these adducts were similar to each other and dependent on the excitation wavelength and solvent.     

A contribution to the theory of OD EPR of spin-correlated radical pairs

A composite structure of OD EPR spectra is predicted. The OD EPR spectra consist of a broad component which manifests many features similar to conventional CW EPR spectra and a narrow component which appears as a hole in the broad spectrum. The width of the narrow component is estimated. The contribution of singlet-triplet dephasing to the shape of the OD EPR spectra is discussed comprehensively. It is shown that changes of the OD EPR spectra with an increase in the rate of singlet-triplet dephasing can be interpreted in terms of a redistribution of the populations of mixed (singlet and triplet) states of radical pairs and of exchange of the coherences corresponding to the EPR transitions for radical pairs.     

Carborane Dyads for Photoinduced Electron Transfer: Photophysical Studies on Carbazole and Phenyl‐o‐carborane Molecular Assemblies

o‐Carborane‐based donor–acceptor dyads comprising an o‐carboranyl phenyl unit combined with N‐carbazole ( 1 ) or 4‐phenyl‐N‐carbazole ( 2 ) were prepared, and their dyad characters were confirmed by steady‐state photochemistry and photodynamic experiments as well as electrochemical studies. The absorption and electrochemical properties of the dyads were essentially the sum of those of the carbazole and o‐carboranyl phenyl units; this indicates negligible interaction between the carbazole and o‐carborane units in the ground state. However, the emission spectra of 1 and 2 indicated that carbazole fluorescence was effectively quenched and a new charge‐transfer (CT) emission was observed in solvents, varying from hexane to acetonitrile, which exhibited large Stoke shifts. The CT emission properties of o‐carborane‐based dyads were further analyzed by using Lippert–Mataga plots to show that unit charge separation occurred to form a charge‐separated species in the excited state, namely, 1?2 . This excited‐state species was confirmed by nanosecond transient absorption spectra and spectroelectrochemical measurements; the photoexcitation of carbazole generated the CT state in which a radical cation and anion were formed at the carbazole and o‐carborane units, respectively, within a few nanoseconds. DFT calculations corroborated the presence of this CT species and showed localized populations of the highest singly occupied molecular orbital on 2 in the reduced anionic state. As a result, molecular assemblies formed by linking the carbazole group with the o‐carborane cage through a phenylene or multi‐phenylene spacer revealed that the photoinduced electron‐transfer process occurred intramolecularly.     

Detection and characterization by mass spectrometry of radical adducts produced by linoleic acid oxidation

The formation of linoleic acid radical species under the oxidative conditions of the Fenton reaction (using hydrogen peroxide and Fe (II)) was monitored by FAB-MS and ES-MS using the spin trap 5,5-dimethyl-1-pyrrolidine-N-oxide, DMPO. Both the FAB and ES mass spectra were very similar and showed the presence of ions corresponding to carbon- and oxygen centered spin adducts (DMPO/L*, DMPO/LO*, and DMPO/LOO*). Cyclic structures, formed between the DMPO oxygen and the neighboring carbon of the fatty acid, were also observed. Electrospray tandem mass spectrometry of these ions was performed to confirm the proposed structure of these adducts. All MS/MS spectra showed an ion at m/z 114, correspondent to the [DMPO + H]+, and a fragment ion due to loss of DMPO (loss of 113 Da), confirming that they are DMPO adducts. ES-MS/MS spectra of alkoxyl radical adducts (DMPO/LO*) showed an additional ion at m/z 130 [DMPO - O + H]+, while ES MS/MS of peroxyl radical adducts (DMPO/LOO*) showed a fragment ion at m/z 146 [DMPO - OO + H]+, confirming both structures. Other fragment ions were observed, such as alkyl acylium radical ions, formed by cleavage of the alkyl chain after loss of water and the DMPO molecule. The identification of fragment ions observed in the MS/MS spectra of the different DMPO adducts suggests the occurrence of structural isomers containing the DMPO moiety both at C9 and C13. The use of ES tandem mass spectrometry, associated with spin trapping experiments, has been shown to be a valuable tool for the structural characterization of carbon and oxygen-centered spin adducts of lipid radicals.     

Theoretical study of the spin trapping of hydroxyl radical by cyclic nitrones: a density functional theory approach

The hydroxyl radical (*OH) is an important mediator of biological oxidative stress, and this has stimulated interest in its detection. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) and its alkoxycarbonyl and alkoxyphosphoryl analogues have been employed as spin traps for electron paramagnetic resonance (EPR) spectroscopic radical detection. Energies of optimized geometries of nitrones and their corresponding *OH adducts were calculated using density functional theory (DFT) at the B3LYP/6-31+G//B3LYP/6-31G level. Calculations predict that the trans adduct formation is favored in alkoxycarbonyl nitrones, while cis adducts with intramolecular H-bonding is favored for alkoxyphosphoryl nitrones. Addition of *OH to a phosphoryl-substituted nitrone is more exoergic than the carbonylated nitrones. Charge and spin densities on the nitrone spin traps were correlated with their rates of addition with *OH, and results show that the charge density on the nitronyl C, the site of *OH addition, is more positive in phosphorylated nitrones compared to DMPO and the alkoxycarbonyl nitrones. The dihedral angle between the beta-H and nitroxyl O bonds is smaller in phosphorylated nitrones, and that aspect appears to account for the longer half-lives of the spin adducts compared to those in DMPO and alkoxycarbonyl nitrones. Structures of nitrones with trifluoromethyl-, trifluoromethylcarbonyl-, methylsulfonyl-, trifluoromethylsulfonyl-, amido-, spiropentyl-, and spiroester substituents were optimized and their energies compared. Amido and spiroester nitrones were predicted to be the most suitable nitrones for spin trapping of *OH due to the similarity of their thermodynamic and electronic properties to those of alkoxyphosphoryl nitrones. Moreover, dimethoxyphosphoryl substitution at C-5 was found to be the most efficient substitution site for spin trapping of *OH, and their spin adducts are predicted to be the most stable of all of the isomeric forms.     

Trapping of fatty acid allyl radicals generated in lipoxygenase reactions in biological fluids by nitroxyl radical

Nitroxyl radicals can trap fatty acid allyl radicals on ferric‐lipoxygenases at lower oxygen content, which are an intermediate in the lipoxygenase reaction. In the present study, we examined whether nitroxyl radical‐trapping of fatty acid allyl radicals on the enzyme proceeds in biological fluids with abundant antioxidants. The fatty acid allyl radical–nitroxyl radical adducts were quantified by HPLC with electrochemical detection (HPLC‐ECD); the adducts in eluate degraded into nitroxyl radical by passing through heating coil at 100°C, and then nitroxyl radical was detected by electrochemical detector. Soybean 15‐lipoxygenase and nitroxyl radical (3‐carbamoyl‐2,2,5,5‐tetramethyl‐3‐pyrroline‐N‐oxyl, CmΔP) were mixed with rat serum prepared from fresh venous blood, and the solution was stood at 37°C for 1 h. One volume of the solution was mixed with 5 vols of cold acetonitrile. After centrifugation, the supernatant was subjected to HPLC‐ECD. Arachidonate allyl radical–CmΔP adducts as well as linoleate allyl radical–CmΔP adducts were detected in the solution, and the content of these adducts remarkably increased in the presence of phospholipase A₂. It is proved for the first time that nitroxyl radical traps fatty acid allyl radicals generated in the lipoxygenase reaction in biological fluid without competition from endogenous antioxidants. Copyright © 2010 John Wiley & Sons, Ltd.     

Identification of oxidative processes during simulated mastication of uncooked foods using electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy is used to measure directly the generation of free radicals during a simulation of the mastication process. This involves the gentle grinding of the food product in the presence of a spin trap, a molecule which reacts selectively with unstable free radicals to generate (more) stable radical adducts, which can then be characterised. With mushrooms of the Agaricus family, adducts consistent with a carbon-centred radical are seen with a wide range of spin traps and this radical has been confirmed as 4-(hydroxymethyl)phenyl. In plant tissues that are rich in ascorbic acid, this molecule competes successfully with spin traps for the free radicals and the (monodehydro)ascorbate radical, formed by the 1-electron oxidation of ascorbic acid, is seen in the EPR spectra. However, with >50% of the plant tissue samples studied in the present experiment, free radicals resulting from oxidation of the spin traps were observed. The formation of such molecules, for which oxygen was found to be necessary, requires the existence of powerful oxidation processes as the plant tissue is broken down. Such pro-oxidant behaviour is contrary to the popular assumption that the beneficial effects of uncooked plant tissues are the result of their high levels of anti-oxidant molecules.     

Spectroscopic and computational investigations of stable radical anions of triosmium benzoheterocycle clusters

The variable temperature (1)H and (13)C NMR and EPR spectra of the stable radical anions [Os(3)(CO)(9)(micro(3)-eta(2)-L)(micro-H)] (LH=phenanthridine, 1; 5,6-benzoquinoline, 2), and [Os(3)(CO)(10)(micro(3)-eta(2)-L)(micro-H)] (LH=quinoxaline, 3) are reported. The radical anions 1(-), 2(-), and 3(-) can be prepared by both exhaustive electrolysis and partially by chemical reduction with cobaltocene and with sodium dispersion (only with sodium dispersion in the case of 3(-)). DFT calculations on 1-3 reveal that the LUMO for the electron-deficient compounds 1 and 2 involves significant contributions from both the heterocyclic ligand and the two metal atoms bridged by the ligand and the micro-hydride. The character of this orbital rationalizes the previously observed regioselective reactions of these complexes with nucleophiles. In contrast, the LUMO for the electron precise 3 involves only ligand-based orbitals. Partial chemical reduction of 1 and 2 requires an excess of either cobaltocene or sodium, and their (1)H and (13)C NMR spectra reveal selective line broadening of those proton resonances that are predicted by DFT calculations to bear the greatest amount of free spin density. The variable temperature behavior of the partially chemically reduced species of 1 and 2 indicates that electron transfer between the reduced/unreduced cluster pair and between the cobaltocene/cobaltocenium pair occurs on the NMR timescale. The radical anions of 1 and 2 prepared by exhaustive electrolysis show an EPR signal at room temperature, while the NMR signals are uniformly broadened. Compound 3 appears to be partially reduced by sodium at room temperature and shows uniformly broadened (1)H NMR resonances at room temperature that sharpen significantly at -80 degrees C. The temperature dependence of the spectra are discussed in terms of the effects of relative electron nuclear relaxation processes, chemical exchange, and the results of the DFT calculations.     

Electron paramagnetic resonance of the excited states of the three-spins-1/2 ZnTPP-3-NOPy complex

EPR spectra of the excited quartet and doublet molecular states of (tetraphenylporphinato)zinc(II) covalently bounded to 3-(N-nitronyl-notroxide) pyridine stable radical are modeled in terms of the spin-Hamiltonian given by the sum of the contributions from the radical and triplet moieties, and the interaction between them. The later is represented by anisotropic point dipolar and isotropic exchange electron spin-spin interactions. It is shown that the high field (W-band) EPR spectra depend on energy separation between the electronic doublet (D) and quartet (Q) states. This dependence was utilized to estimate the upper limit of the intensity of exchange interaction between the radical and porphyrin moieties.