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A five-membered metallacycloallene complex, 1-zirconacyclopenta-2,3-diene, was prepared from low-valent zirconocene and 2,4-bis(trimethylsilyl)but-1-en-3-yne. The complex reacted with carbonyl compounds to afford alkynyl alcohols after hydrolysis via C=O insertion into Zr–C bond. Similarly, nitriles inserted into the Zr–C bond to give the corresponding ketones after hydrolysis. The insertion of C=O and CN selectively took place at zirconium-sp3 carbon of the metallacycle.

The E isomer of a [5]cumulene derivative, 2,2,9,9-tetramethyl-3,8-diphenyldeca-3,4,5,6,7-pentaene (1), which was previously believed to be unisolable owing to very fast E/Z isomerization, was isola...

The magnetic field effects (MFEs) on the photo-induced electron transfer (PET) reaction of zinc(II) tetraphenylporphyrin (ZnTPP) with 2-methyl-1,4-naphthoquinone (2MNQ) in a mixed solvent of cyclohexanol and 2-propanol were investigated at 293 K by a nanosecond laser flash photolysis technique. Upon irradiation of ZnTPP, the electron transfer from ZnTPP to 2MNQ occurred and the corresponding cation and anion radicals were generated. The relative yield of the escaped 2MNQ anion radical, R(B), showed appreciable MFEs. The R(B) values increased between 0 and 0.1 T, then decreased again between 0.1 and 1.65 T. The observed MFEs can be explained in terms of the relaxation mechanism.

We investigated the dynamics of the radical ion pairs formed by photoinduced electron-transfer reaction from zinc tetraphenyl porphyrin to 2-methyl-1,4-naphthoquinone in mixtures of 2-propanol and cyclohexanol. By the irradiation of a resonant X- (9.16 GHz) or Ku-band (17.41 GHz) microwave pulse, the time profiles of the transient absorptions was modified and the yields of escaping radical ions decreased. From these experiments, we determined the kinetic parameters of the radical ion pairs at various solvent viscosities. The recombination rates of the singlet pairs were (4 +/- 4), (8 +/- 3), and (16 +/- 3) x 10(6) s(-1) at 5, 10, and 15 cP, respectively. The escape rates were (1.7 +/- 0.2), (1.4 +/- 0.1), and (0.9 +/- 0.2) x 10(6) s(-1) at 5, 10, and 15 cP, respectively. The viscosity dependence of the kinetic parameters was followed by the simple continuum diffusion model.

We investigated the quantum beats, the oscillation between singlet and triplet states of radical pairs induced by the microwave field resonant to one of the component radicals. They were observed as the alternation of the yields of the component radicals by a nanosecond time-resolved optical absorption with the X-band (9.15 GHz) resonant microwave pulse. This technique was applied to the photochemical reaction of benzophenone, benzophenone-d(10), and benzophenone-carbonyl-(13)C in a sodium dodecylsulfate micellar solution with a step-by-step increase of the resonant microwave pulse width. The yields of the component radicals showed alternation with an increase of the microwave pulse width. This indicates that the radical pair retains spin coherence in the micellar solution. The magnetic isotope effect on the amplitude of the quantum beat was observed. The MW effect on the quantum beat of BP-(13)C decreases from 80% to 60% of that of BP by irradiation of the pi-pulse MW due to spin-locking. The kinetic parameters were also determined using the X- or Ku-band (17.44 GHz) region. They are almost similar to each other except for the intersystem recombination rate in the system of BP-(13)C, which may be slightly higher than those in other systems.

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.

Silylborates (Li[PhnMe3-nSiBPh3], n = 1−3) and germylborates (Li[PhnMe3-nGeBPh3], n = 1−3; M[Et3GeBPh3], M = Li, Na, K) were prepared by the reaction of the corresponding silyl- and germylalkali me...

In this review, we present an overview of magnetic field effects (MFEs) and chemically induced electron polarization (CIDEP) due to the d-type triplet mechanism (TM) in intra-molecular reactions of photo-excited triplet molecules. Because the conditions for such MFEs and CIDEP are rather strict, they had been observed only for a few types of radicals generated through triplet exciplexes of some excited dye molecules with heavy-atom substituted electron donors in polar solvents, until quite recently. Under such circumstances, Paul's group and that of the present authors have recently found MFEs and CIDEP due to the d-type TM for intra-molecular reactions from photo-excited triplet states of azoalkanes and triarylphosphines, respectively, irrespective of solvent polarity. These studies have extended the research field of the d-type TM from very limited reactions through triplet exciplexes to more general intra-molecular ones of triplet molecules. In the present review, we would like to summarize the various mechanisms for MFEs and CIDEP, to explain a theory for MFEs and CIDEP due to the d-type TM, and to present the new advances obtained experimentally by the two groups.

The magnetic field effects (MFEs) on the photodecomposition reactions of triphenylphosphine and its halogen and methyl derivatives are investigated in fluid solutions. The yield of diarylphosphinyl radicals decreased withincreasing magnetic field from 0.1 to 5 T but was stationary below 0.1 T and above 5 T. The MFE becomes larger by the substitution of halogen atoms and the 3- or 4-methyl group. In cyclohexane, the yields of the escaped diarylphosphinyl radicals at 1 T are reduced to 0.69, 0.55, 0.59, and 0.56 of those at 0 T for triphenylphosphine and its tris(4-chloro), tris(3-methyl), and tris(4-methyl) derivatives, respectively. This magnetic field dependence was ascribed to originate from the deactivation process of the excited triplet state, which is a variant of the d-type triplet mechanism originally proposed by Steiner. The interaction between the closely lying nπ* and ππ* states makes their solvent dependence complicated.

We investigated the magnetic field effect on the yields of the escaping radicals formed by the hydrogen abstraction reaction of excited triplet 4-methoxybenzophenone with 4-chlorothiophenol in mixtures of 2-propanol and cyclohexanol. The yields of the escaping radicals decreased with increasing external magnetic field owing to the Δg mechanism. The relative yield of the escaping radicals to that in the absence of the external magnetic field in 2-propanol was 0.9 at 1.4 T. The magnetic field effects in a nonviscous solution such as 2-propanol were proportional to the square root of the external magnetic field and the solvent viscosity, (Bη)1/2. However, the deviation from this relationship, the decrease in the exponent, was observed by increasing the solvent viscosity, up to 60 cP. The relative yield in cyclohexanol was 0.88 at 0.8 T and roughly constant up to 1.4 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.

Photoexcitation of zinc porphyrin−bridge−naphthalenediimide (ZP−B−NI) dyads, 1 and 2, generated the short- and long-distance charge-separated (CS) states, [ZP•+−B−NI•-], through the intramolecular electron-transfer from excited ZP to NI in solvents of various polarity. The energy level of [ZP•+−B−NI•-] was either higher (in benzene and 1,4-dioxane) or lower (in solvents of higher polarity) than that of 3ZP*−B−NI. When generated in the singlet spin state, the short-distance CS state derived from 1 rapidly (109−1010 s-1) decayed through the charge recombination (CR) leading to the ground state. On the other hand, when generated from the triplet excited state of 1, the decay of the CS state was much slower and showed magnetic field effects attributable to the level-crossing mechanism. For the long-distance CS state derived from 2, the decay dynamics and its magnetic field dependence exhibited quite different features. To examine the effects of a neighboring additional radical on the decay dynamics of these C...

A photodiode-array (PDA) UV–VIS detector for liquid chromatography is applied to time-resolved reaction yield detected magnetic resonance (RYDMR) measurements. The results derived from the yields of cage and escape products in the photoreaction of 2-methyl-1, 4-naphtnoquinone in a sodium dodecylsulfate micelle are found to be identical with those derived from the yield of escaping semiquinone radical detected by transient optical absorption. This implies practical linearity between the yields of escaping radicals and escape products. High sensitivity of the PDA detector enables application of escape product yields for kinetic analysis by reducing microwave-induced perturbation.

A pulsed magnet which is useful for the research of chemical reactions has recently been constructed by our group. Here, the world record of the maximum magnetic field (30 T) has been attained in this research area. An optical detected X-band ESR apparatus has also been developed by our group. The world’s best sensitivity and resolution have been realized in this apparatus. In this account, we review our recent results on new magnetic field effects and spin dynamics obtained with such novel methods as described above for radical reactions at room temperature in solution. In Section 1, typical mechanisms of MFEs on radical pairs such as the Δg mechanism and the relaxation mechanism are explained as the introduction of this review. In Section 2, the saturation of MFEs due to the Δg mechanism observed for radical pairs generated from the photoreduction of 4-methoxybenzophenone with benzenethiol in fluid solutions under ultrahigh fields are described. In Section 3, the saturation and reversion of MFEs due to ...

Magnetic field effects (MFEs) on reactions of biradical radical ion pairs (BRIPs, S = 3/2 and 1/2) were investigated for the electron transfer (ET) reactions of the triplet state of 10-methylphenothiazine (3MPTZ*) (S = 1) and electron acceptors linked with 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) (An−R·) (S = 1/2) in various solvents by means of a nanosecond laser photolysis technique. In 2-propanol, the yield of free ions dramatically increased with increasing magnetic field (B) from 0 to 2 T, but slightly decreased from 2 to 10 T. The magnitude of the MFEs was much larger than those observed in ET reactions of 3MPTZ* with acceptors without TEMPO (S = 0). To interpret the MFEs observed in the reactions with An−R·, we considered the following two factors. (i) In the quenching of 3MPTZ* by An−R·, not only the ET reaction but also triplet−doublet (T−D) quenching takes place. The T−D quenching efficiency decreases with increasing B, resulting in a higher yield of BRIPs ([MPTZ•+ An•-−R·]). However, this fa...

Magnetic-field effects on the decay kinetics of chain-linked triradicals have been investigated in acetonitrile at 293 K. The decay rate of the triradical linked by an 8-methylene chain was 1×10 7 s −1 under zero field and decreased with increasing field to attain a constant value of 2×10 6 s −1 at 2–10 T. These values were one order larger than those of the corresponding ketyl-diphenylmethyl biradical.

Magnetic field effects on both the quenching of the triplet excited state of 10-methylphenothiazine ( S =1) with 4-(4-cyanobenzoyloxy)TEMPO ( S =1/2) and the dynamic behavior of generated radical ion pairs ( S =1/2 and 3/2) were directly observed by a nanosecond laser photolysis technique. In 2-propanol, with increasing field from 0 T to 2 T, the quenching rate was decreased by (11.8±1.2)% and the yield of the escaped cation radical was increased by (90±8)%.

The optically detected ESR technique is used to record the spectra of a radical pair generated in the photolysis of 2,4,6-trimethylbenzoyl diphenylphosphine oxide in a micellar sodium dodecylsulfate solution. Along with the signals of the radicals of the pair, additional resonances have been detected in the spectrum, whose signal is inverted with respect to the fundamental ones. It is shown that the inverted lines consist of narrow and wide components, which arise from the effects of crossing and anticrossing of pair spin energy levels, respectively.

The radical pair dynamics in a photochemical hydrogen abstraction reaction of 2-methyl-1,4-naphthoquinone in a sodium dodecylsulfate micelle was modulated by a microwave pulse. After a short resonant 180° microwave pulse, the recombination of the radical pair was enhanced, its rate constant being determined to be (8.3±0.8)×106 s−1. Other kinetic parameters were determined by the scanning of the microwave pulse position as follows: the formation of the radical pair (3.3±0.3)×107 s−1, the relaxation rate from the triplet (T±1) levels to the singlet–triplet (T0) mixed one (3.3±0.3)×105 s−1 at 331 mT, and the radical escape rate (5.8±0.6)×105 s−1.

The effects of the geminate recombination induced by the pulsed microwave irradiation on the electron paramagnetic resonance spectra of radicals generated in the photoinitiated hydrogen abstraction reaction of 2-methyl-1,4-naphthoquinone in a micellar sodium dodecylsulphate solution have been studied using an inversion recovery method of pulsed electron paramagnetic resonance. It was found that the stimulated nuclear polarization resulting from the microwave-induced recombination of the geminate radical pairs at one (or a limited group) of nuclear spin configurations affects the polarization and intensity of the electron paramagnetic resonance spectra of escaped radicals via the electron-nuclear cross-relaxation mechanism. The long lifetimes of the radical pairs in a micellar system facilitate the observation of the effects. Similar effects, though of much smaller scale, were found to be detectable in certain conditions also in homogeneous solutions.

The photoreduction of benzophenone in micellar SDS and Brij 35 solutions was investigated at room temperature under magnetic fields of 0–29.6T. In the SDS solution, the radical-pair lifetime increased with increasing field from 0T (0.4 μs) to 3.36T (1.6 μs), but the lifetime attained a constant value with increasing field from 3.36 and 29.6T. In the Brij 35 solution, the lifetime increased with increasing field from 0T (0.7 μs) to 3.36–6.30T (about 3 μs) and decreased with increasing field from 6.30 to 29.6T (2.1 μs), approaching a saturated value at 29.6T. These magnetic field dependences could be explained by the previous predictions derived from the relaxation mechanism.

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 on the photochemical electron-transfer reactions of 10-methylphenothiazine with 1,4-dicyanobenzene and tetrafluoro-1,4-dicyanobenzene are investigated by a nanosecond laser photolysis technique. For these reactions, the recombination of the geminate radical ion pairs as well as the yields and decay rates of the escaped radical ions showed clear magnetic field dependence in nonviscous homogeneous solutions. The mechanisms of the magnetic field effects are ascribed to the hyperfine coupling and Δg mechanisms at low and high magnetic fields, respectively. The smaller magnetic field effects on the reaction of the fluorinated derivative than those of 1,4-dicyanobenzene are ascribed to its smaller back-electron-transfer rate because its free-energy change is smaller than that of the nonsubstituted one.

Magnetic field effects on the photodissociation reaction of triphenylphosphine at 293 K were investigated in several non-viscous homogeneous solvents by means of a nanosecond laser photolysis technique under magnetic fields of 0–1.75 T. The generated neutral radical 3air of the diphenylphosphinyl and phenyl radicals showed clear magnetic field effects. The yield of the escaped diphenylphosphinyl radical at 1.5 T reduced to 74% ± 3% of that at 0 T in 2-propanol ( η = 2.379 at 293 K. Similar magnetic field effects were observed at 293 K even in more fluid solvents such as n -hexane (84% ± 5% at 1.5 T), cyclohexane (65% ± 2% at 1.5 T), and p -dioxane (64% ± 3% at 1.5 T).

Photochemical reactions of triplet xanthone and 2,5-dimethyl- p -benzoquinone with 2-propanol, xanthene, and N,N-diethylaniline in 2-propanol were investigated at 293 K by a nanosecond laser photolysis technique under magnetic fields of 0–10 T. In the reactions with 2-propanol and xanthene hydrogen transfer was found to occur and generated neutral radical pairs showed no or a very small magnetic field effect. On the other hand, electron transfer was found to occur in the reactions with N,N-diethylaniline and the generated radical ion pairs showed fairly large magnetic field effects. Here, reverted magnetic field effects were also observed in 2-propanol like those observed in micellar solutions.

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...

We present our recent studies on magnetic field effects (MFEs) observed in reactions of heavy atom-centered radicals such as Si-,P-, Ge-, and Sn-radicals with a ns-laser photolysis technique under magnetic fields of 0–10 T. Although the MFES of heavy atom-centered radicals are much smaller than those of C-radicals due to the spin-orbit interaction of heavy atoms, we have found appreciable MFEs in many reactions of such heavy atom-centered radicals. Comparing the MFES of C-radicals with those of heavy atom-centered ones, we have explained the MFEs of heavy atom-centered radicals in terms of the Δg and relaxation mechanisms. We have found that the separation between the MFES due to the Δg mechanism and those due to the relaxation one is possible with the enhancement of spin relaxation by the addition of a paramagnetic ion. We have also tried to enrich magnetic isotopes of heavy atoms with the magnetic isotope effect (MIE), using the reactions which show fairly large MFES. Recently, we have succeeded in enriching 73Ge. This is the heaviest isotope which has so far been enriched with the MIE from samples of natural abundance.

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).

The inversion of magnetic field effects (MFEs) observed in the photoreduction of decafluorobenzophenone was studied in the absence and presence of a paramagnetic Gd3+ ion. The MFEs appearing at both low (B ≤ 2T) and high (2T < B ≤ 10T) fields were simultaneously quenched by the addition of Gd3+. For the present reaction, it is concluded that the inversion of the MFEs is due to the relaxation mechanism.

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.

The recombination of pairs of para-substituted phenylthiyl radicals inside micellar supercages was studied by means of a nanosecond laser flash photolysis technique. Singlet and triplet pairs were formed by direct excitation and by triplet-sensitized photolysis of the corresponding disulfides, respectively. Magnetic field effects (MFEs) on the yield of escaped thiyl radicals were found for the triplet-sensitized photolysis of the p-amino compound. For the p-hydroxy-, p-methoxy-, and p-butoxy compounds, short-lived MFEs were observed but it is unlikely that they arise from the recombination of the thiyl radicals. No MFE was found on pairs of p-chloro-substituted and unsubstituted phenylthiyl radicals within experimental error

Magnetic field effects on the dynamic behavior of radical pairs generated in micellar solutions of benzophenone, naphthoquinone, and 2′,3′,4′,5′,6′-pentafluoroacetophenone were studied under high magnetic fields (⩽10 T). For the reaction of pentafluoroacetophenone, the lifetime of a radical pair and the yield of the escaped ketyl radical were found to increase steeply with increasing field strength from 0 to 2 T, but to decrease gradually from 2 to 10 T. For the other reactions, such a phenomenon was not observed. These new magnetic field effects were explained in terms of the relaxation mechanism and the molecular dynamics in micellar solutions.

Hydrogen abstraction reactions of thiobenzophenone with thiophenol in solutions of varying viscosities (η=0.29-42.0 cP) were studied by a nanosecond laser flash photolysis under magnetic fields of 0-15.5 T. In alcoholic solutions, the escaped radical yield (Y) of thiobenzophenone ketyl radical showed appreciable magnetic field effects (MFEs). The observed MFEs can be interpreted with the Δg mechanism through the triplet radical pair. The relative escaped radical yield (R(1.7T)=Y(1.7T)/Y(0T)) decreased with increasing η at 0η≤3.33 cP, but then the yield increased with increasing η at 3.33 cPη≤22.2 cP. At much higher viscosity 22.2 cPη≤42 cP, R(1.7T) values become 1.0 within experimental errors. Such quenching of MFE was explained by the spin-orbit coupling recombination of close radical pairs associated with high viscosity. The MFEs on the present reaction is extremely sensitive to the solvent viscosity in the vicinity of the radical pairs. Using this probe reaction, microviscosities of sodium dodecyl sulfate (SDS) and Brij35 micellar solutions were estimated.

The photoreduction of xanthone with p-aminothiophenol in a sodium dodecylsulfate solution was investigated with the aid of a nanosecond laser flash photolysis technique at magnetic fields of 0–10 T. The lifetime of the radical pair generated by hydrogen abstraction of triplet xanthone and the yields of the escaped xanthone ketyl and p-aminophenylthiyl radicals increased steeply with increasing magnetic field strength (B) in a low-field region (B⩽0.05 T) and decreased more slightly with increasing B in a high-field region (0.05 T

The photochemical reaction of benzophenone in a micellar sodium dodecylsulfate solution was investigated under magnetic fields of 0–10 T. The decrease of decay rate of the ketyl radical and the increase of the yield of the escaped radical were saturated around 2 T. The contribution of the Δg mechanism was not observed below 10 T since the Δg value is very small for this system.

External magnetic field effects on photochemical reactions of triplet xanthone with tert-butyl-substituted phenols in an SDS micelle were studied by a transient optical absorption technique. These systems, which have been know to give CIDEP spectra due to quasi-stable radical pairs, also showed large magnetic field effects. These effects were rationalized by hyperfine and relaxation mechanisms. The effect of the substituent on spin dynamics is discussed.

The photochemical reactions of aryl-substituted digermanes were investigated by trapping experiments and a laser flash photolysis technique. The photolysis of phenylated digermanes resulted in germanium–germanium bond homolysis to give a pair of two germyl radicals. The germyl radicals abstracted a chlorine atom from carbon tetrachloride to give chlorogermanes. The pair of germyl radicals also underwent ipso-substitution, which was a precursor of the germylenes. The mechanism for the photochemistry of phenylated digermanes is discussed.

The recombination of pairs of para-substituted phenylthiyl radicals inside micellar supercages was studied by means of a nanosecond laser flash photolysis technique. Singlet and triplet pairs were formed by direct excitation and by triplet-sensitized photolysis of the corresponding disulfides, respectively. Magnetic field effects (MFEs) on the yield of escaped thiyl radicals were found for the triplet-sensitized photolysis of the p-amino compound. For the p-hydroxy-, p-methoxy-, and p-butoxy compounds, short-lived MFEs were observed but it is unlikely that they arise from the recombination of the thiyl radicals. No MFE was found on pairs of p-chloro-substituted and unsubstituted phenylthiyl radicals within experimental error

UV irradiation of 1-methyl-4-carbomethoxypyridinium perchlorate (1) in methanol solutions in the presence of O2 brings about selective methoxylation at the 3-position in a good yield, while under N2 atmosphere photohydroxymethylation at the 2-position occurs. Mechanistic studies indicate that the methoxylation under O2 proceeds via an excited singlet state and that singlet oxygen does not participate in the reaction. Hydrogen peroxide was produced during the irradiation; consumption of 1 and methoxylation at the 3-position occurred simultaneously. Kinetic studies postulate that the photomethoxylation under O2 proceeds through two processes: (1) via a terplex which consists of two molecules of 1 and one molecule of oxygen or an excimer which interacts with oxygen, and (2) via an exciplex which consists of one molecule each of 1 and oxygen.

Transient absorptions due to the anion radicals of several aromatic nitriles were measured in their electron-transfer reactions from hexamethyldisilane by means of a nanosecond laser flash photolysis technique. With direct excitation, the yields of the escaped anion radicals of p- and o-dicyanobenzenes increased with increasing magnetic field strength from 0 to 1.35 T. At 1.35 T, the yield of the p-dicyanobenzene anion radical became 1.23±0.05 times as large as that at 0 T

Laser flash photolysis was performed on micellar solutions and oil emulsions of organosilicon and organogermanium compounds ((aryl) n Me n-4 E: E=Si or Ge) in the absence and presence of magneticfields at room temperature. The dynamic behavior of the silyl and germyl radicals formed upon decomposition of the triplet states was directly observed. The lifetime of the radical pair of group 14 element centered and aryl radicals and the yields of the escaped silyl and germyl radicals were observed to increase with magnetic field strength increasing from 0 to 1.35 T