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A water-cooled pulsed magnet having a room temperature bore of diameter 20 mm was constructed. Magnetic field effects on a triplet sensitization reaction of p-aminophenyl disulfide (APDS) with xanthone (Xn) were studied in a sodium dodecylsulfate (SDS) micellar solution by a nanosecond laser flash photolysis technique under ultrahigh magnetic fields of up to 28 T. Upon irradiation of the SDS micellar solution containing APDS and Xn, triplet radical pairs of p-aminophenylthiyl radicals were generated. The yield of the escaped p-aminophenylthiyl radical was found to decrease by 6 ± 2% at 28 T compared with that at 0 T.

A study is reported of the dynamics of radical pairs formed by the photochemical reaction of benzophenone and its heavy atom substitutions, dichlorobenzophenone and bromobenzophe-none, in a sodium dodecylsulphate micellar solution. Changes were observed in the time profiles of the transient absorptions and in the yields of the escaping radicals induced by the irradiation of an × (9.15 GHz) or Ku band (17.44 GHz) microwave short pulse. From these experiments, the hydrogen abstraction reaction rates, the recombination rates of the singlet radical pairs, the relaxation rates, the escape rates, the decay rates of the radical pairs without the external magnetic field, and the intersystem recombination rates due to the spin-orbit coupling were determined.

A study has ben made of magnetic field effects (MFEs) on the reaction of the photoexcited triplet of 2-methyl-1,4-naphthoquinone (MNQ) in SDS micellar solution containing the 4-lauroylamino-TEMPO radical (L-R•) under magnetic fields below 1.75 T by a nanosecond laser flash photolysis technique. The triplet MNQ mainly underwent the hydrogen abstraction from an SDS molecule to give a radical pair. The lifetime of the radical pair increased with increasing magnetic field from 0 to 0.62 T. The escaped radical yield also increased from 0 to 1.75 T. The qualitative features of these MFEs were similar to those observed for the photo-reduction of MNQ in SDS micellar solution without L-R•, and these MFEs can be explained by the relaxation mechanism. However, it was found that L-R• affected the MFEs for this reaction in two ways: first, L-R• reacted with the triplet MNQ through H abstraction and/or electron transfer, and second, the spin relaxation of the radical pair was enhanced through the spin-spin interactions...

The photochemical reaction of 2-methyl-1,4-naphthoquinone in sodium dodecyl-sulphate micellar solution was investigated with an optical detection ESR apparatus working at 17.44 GHz (Ku-band). The Ku-band RYDMR spectra are obtained from the transient optical absorption and the stationary absorption of the reaction product, and the shift of the spectral peak compared with the spectra at 331 mT is reproduced well by the difference in g of the component radicals. The microwave pulse length dependence shows the quantum beat originated from the conversion between triplet ±1 states and the mixed state of singlet and triplet 0 states by the microwave field. The decay rate of the radical pair in triplet ±1 at 622 mT was determined to be 7.1 ± 1.1 × 105 s−1 by changing the irradiation time of a short (20 ns) microwave pulse with reference to the laser excitation. This value is smaller than that at 331 mT, as expected by the relaxation mechanism.

Benzophenone and its derivatives in dimethylsulphoxide (DMSO) exhibit biphotonic photochemistry under 355 nm laser photolysis. Flash photolysis electron paramagnetic resonance experiments demonstrate that a single laser pulse is capable of producing and exciting benzophenone triplets, which can sensitize dimethylsulphoxide and subsequently lead to photodecomposition. In decafluorobenzophenone, electron transfer is the dominant process of the highly excited triplet state. Despite the rapid radiationless decay of 2-hydroxybenzophenone (2OHbenzophenone) in non-polar solvents, radical signals are observed from the photoexcitation of 2OHbenzophenone in DMSO. This is attributed to the sufficiently rapid excitation of the triplet state, which competes with the radiationless decay process, aided by the unique solvent properties of DMSO. It is concluded, in contrast to literature data, that the excited triplet state of DMSO is reactive, and can produce methyl radicals that show triplet mechanism polarization via t...

The photo-induced hydrogen abstraction reactions of benzophenone in aqueous micellar SDS and Brij 35 solutions, decafluorobenzophenone in an aqueous micellar Brij 35 solution and naphthoquinone in an aqueous micellar SDS solution were investigated at room temperature under pulsed magnetic fields of 0–29.6T. In each of these reactions, the radical pair decay rate is found to decrease with increasing field from 0 to about 3 T and to increase with increasing field above 3 T. The escape radical yield is found to increase with increasing field from 0 to about 3 T and to decrease with increasing field above 3 T. These magnetic field effects are explained in terms of the relaxation mechanism. In each of the Brij 35 micellar solution systems, the anisotropy in the g tensor of the ketyl radical and its correlation time are evaluated from the observed effects.

Chemically induced dynamic electron polarization (CIDEP) spectra of transient radicals were measured upon photolysis of benzaldehydes (BAs) after 266 nm laser excitation. The substituent effects on CIDEP were studied for a series of substituted derivatives (CH3-, F-, Cl-, Br- and CN-) of BA in 2-propanol at room temperature. For F-, Cl-, and CN-substituted BAs, CIDEP spectra were observed that were due to combinations of, firstly, the triplet mechanism (TM) with emissive (E) phase patterns and, secondly, the S-T,0 mechanism (ST0M) of radical pairs with emissiveoabsorptive (E/A) phase patterns. In contrast, for CH3-substituted BAs, CIDEP spectra due only to the ST0M with E/A phase patterns were observed. The contribution of the TM-induced CIDEP shows an increasing trend for BAs with electron-withdrawing substituents (F-, C1-, and CN-) and a decreasing trend for BAs with an electron-donating substituent (CH3-). These substituent effects on the TM-induced CIDEP were found to be parallel to the effects on the...

Magnetic field effects (MFEs) on the dynamic behaviour of radical pairs generated by the photochemical hydrogen abstractions of 2′, 3′, 4′, 5′, 6′-pentafluoroacetophenone and decafluorobenzophenone in micellar solutions were investigated at large magnetic fields up to 10 T using a superconducting magnet. These MFEs were compared with the MFEs observed in the reactions of acetophenone and benzophenone. The decay rate of a radical pair generated in the reaction of acetophenone in a Brij 35 micellar solution decreased with increasing magnetic flux density (B) from 0 T, and the decrease was saturated at 0·34 T. On the other hand, the decay rate of the pair in the reaction of pentafluoroacetophenone decreased steeply with increasing B from 0 to 2 T, but increased gradually from 2 to 10 T. Such reversions of MFEs were observed for benzophenone and decafluorobenzophenone in Brij 35 micellar solutions, and the degree of the reversion for decafluorobenzophenone was larger than that of benzophenone. These new rever...

The inversion observed at 0·1 T for the magnetic field effects (MFEs) on the photoreduction of xanthone with p-aminothiophenol was investigated in the absence and the presence of a paramagnetic Gd3+ ion with the aid of a laser flash photolysis technique. The MFEs owing to the Δg and relaxation mechanisms were experimentally separated by the addition of Gd3+. Thus, we have proved for the present reaction that the MFEs owing to the hyperfine coupling mechanism and relaxation mechanism appear in a low field region (below 0·1 T) and that the MFEs owing to the Δg mechanism gradually appear in a high field region (above 0·1 T).

Magnetic field effects (MFEs) on the photochemical hydrogen abstraction of 2′,3′,4′,5′,6′-pentafluoroacetophenone in a Brij 35 micellar solution were investigated by a nanosecond laser flash photolysis technique and compared with acetophenone. The decay rate of a radical pair generated in the reaction of acetophenone decreased with increasing magnetic field strength B from 0 T to 0·34 T, but was saturated by 0·34 T. On the other hand, the decay rate of the pair in the reaction of pentafluoroacetophenone decreased with increasing B from 0 T to 1·34 T. The difference in these MFEs is explained in terms of the electron spin relaxation of the radical pairs and the escape rate of the ketyl radicals from these pairs. The magnetic field dependence of these rate constants was obtained by an analysis of the time profiles of the transient absorbance of the ketyl radicals generated.