Spin trapping is powerful technique to detect oxygen radicals (O2-/⋅O2H and ⋅OH) since these radicals have very short life. The free radical reacts with a spin trap to produce a more stable radical adduct, which can be measured by electron spin resonance (ESR) spectroscopy. Nitrones, such as DMPO (5.5'-dimethyl-1-pyrroline N-oxide), PBN (α-phenyl N-tert-butylnitrone) and POBN (α-pyridyl-l-oxide N-tert-butylnitrone), are used as spin traps because of relatively high solubility in water and stability of the radical adducts, though they are not necessarily ideal reagents. Oxygen radical adducts of nitrones have finite life time, especially their ⋅O2H adducts decay within several minutes in water, while the carbon centered radical adducts are sufficiently stable. Practical suggestions on the advantage and disadvantage of these spin traps are presented. Recent applications of this technique on the action mechanism of antitumor agents and the initiator of lipid peroxidation are also reviewed. It is clearly revealed that quinoid antitumor mediated ⋅OH causes significantly DNA degradation and metal ion plays important role in the ⋅OH generation (Fenton's reaction) . On the other hand, ⋅OH, which is produced during the enzymatic reduction of a mixture of Fe3+-ADP-phosphate complex and adriamycin or of Fe3+-ADP-EDTA complex, induces no lipid peroxidation.
Hyperfine sublevel correlation spectroscopy (HYSCORE) spectra have been measured for the radicals in eight well-characterized Argonne Premium coals and their model compounds. The broad 1H and 13C hyperfine spectra of all coals were detected at room temperature. The hyperfine structure was analyzed by the comparison with model radicals and semiempirical molecular orbital calculations. The main coal radicals are considered as two-dimensionally condensed π radicals with more than seven aromatic rings. The 13C/1H ratio of the hyperfine spectra increases with coal rank, suggesting a new index of coal rank.
The short-lived triplet state of the keto tautomer of 2-(2′-hydroxyphenyl)benzothiazole generated by excited state intramolecular proton transfer has been measured using a time-resolved electron paramagnetic resonance method.
Several di(1-azulenyl)(6-azulenyl)methanes and 1,3-bis[(1-azulenyl)(6-azulenyl)methyl]azulenes were prepared by the condensation reaction of azulenes with diethyl 6-formylazulene-1,3-dicarboxylate under acidic conditions. The products were converted into di(1-azulenyl)(6-azulenyl)methylium hexafluorophosphates and azulene-1,3-diylbis[(1-azulenyl)(6-azulenyl)methylium] bis(hexafluorophosphate)s via hydride abstraction reaction with DDQ following the exchange of counterions. These mono- and dications exhibited high stability with large pK(R)(+) values (5.6-10.1), despite the captodative substitution of azulenes. The electrochemical reduction of the monocations upon cyclic voltammetry (CV) exhibited a reversible two-step, one-electron reduction wave with a small difference between the first reduction potential (E(1)(red)) and the second one (E(2)(red)), which exhibited the generation of highly amphoteric neutral radicals in solution. The electrochemical reduction of dications showed voltammograms, which were characterized by subsequent two single-electron waves and a two-electron transfer upon CV attributable to the formation of a radical cation, a diradical (or twitter ionic structure), and a dianionic species, respectively. Formation of a persistent neutral radical from a monocation was revealed by ESR and UV-vis spectroscopies and theoretical calculations. The ESR spectra of the neutral radical gave two hyperfine coupling constants: a(H) = 0.083 (6H) and 0.166 mT (9H) (g = 2.0024), indicating that an unpaired electron delocalizes over all three of the azulene rings. The stable monoanion, which shows the localization of the charge on the 6-azulenyl substituent, was also successfully generated from the di(1-azulenyl)(6-azulenyl)methane derivative.
A two-laser, two-color time-resolved EPR (TREPR) technique has been applied to the investigation of the electron spin polarization in the excited triplet states of anthracene and its halogen anthracene in rigid glassy matrixes. The one-laser excitation of anthracenes with 355 nm gave the triplet TREPR spectra of the lowest excited triplet (T1) states with EEE/AAA polarization pattern. For 9-bromoanthracene and 9,10-dibromoanthracene, the reexcitation by a dye laser (442 nm) to the upper excited triplet (Tn: n > 3) states brought about the opposite phase polarization in the T1 states. The phenomena were interpreted in terms of the sublevel selective reverse intersystem crossing (RISC) from the Tn (n = 2 or 3) state to the S1 state. The time profiles of the polarization were analyzed taking into account the anisotropic RISC processes. On the other hand, for anthracene and 9,10-dichloroanthracene, little effect was observed on the electron spin polarization by two-step laser irradiation.
