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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

With the aid of the time resolved ESR and transient optical absorption spectroscopy, the effect of transition metal ions on the photochemical reaction of naphthoquinone in micelles was investigated in the absence and presence of external magnetic fields below 1 T. The paramagnetic metal ions accelerated the relaxation of the component radicals in the generated radical pair. All quenching rate constants of the paramagnetic ions were around 7 × 108 mol−1 dm−3 s−1 and did not show any dependence on their magnetic properties. This result shows that the relaxation of the electron spins by the paramagnetic transition metal ions is induced by the strong exchange interaction between the radical pair in a micelle and the ions at the water interface of the micelle.

The photoreactions of 3-methoxycarbonyl-N-methylpyridinium perchlorate (1a) and 3-carbamoyl-N-methylpyridinium perchlorate (1b) under N2 give the products methylated at the 6- and 4-positions in methanol. Under O2, the photoreaction of 1a is drastically changed to give a product methoxylated at the 2-position and hydroxylated at the 5-position simultaneously. This compound is unstable and changes to 5-hydroxy-3-methoxycarbonyl-N-methyl-2-pyridone (5a). Only one atom of O2 is incorporated in 5a. The photoreaction of 1b is completely inhibited by O2.

With aid of the time-resolved electron spin resonance technique, the photochemical hydrogen abstraction reactions were investigated for the aromatic carbonyl compounds having ππ* characters in their lowest triplet states. When the hydrogen donors having negatively charged hydrogen atoms, such as NaBH 4 , Et 3 GeH (EtC 2 H 5 ) and n -Bu 3 SnH (BuC 4 H 9 ), are used as the hydrogen donors, these ketones reacted readily with them to form non-ketyl radicals, such as cyclohexadienyl-, benzyl- and α-keto alkyl-type radicals instead of their ketyl radicals. The instrinsic reactivity of the ππ* triplet states is inhibited by the positively charged hydrogen atoms in the usual hydrogen donors since they have a strong affinity for the reaction around the negatively charged carbonyl oxygen.

The reaction mechanism of photo-induced electron transfer reactions from hexamethyldisilane to quinones in chloroform solution was studied at room temperature by means of a CIDNP technique. With irradiation, the formation of the corresponding mono- and di-trimethylsilyl hydroquinones and trimethylsilyl chloride was confirmed by GC, GC-MS, and 1 H NMR spectra. The absorptive CIDNP phase of the mono-trimethylsilyl hydroquinone indicates that this reaction precursor is the triplet radical ion pair of the hexamethyldisilane cation radical and quinone anion radical, and that the former radical reacts with the latter prior to the fission of its SiSi bond.

Transient absorption spectra and triplet decay rates k d of thioxanthone (TX) and 2,4-diisopentylthioxanthone (DITX) in solid polymer matrices were measured using an Nd:YAG laser flash photolysis technique. In a polystyrene matrix, the triplet decay rates of TX and DITX are not dependent on their concentration, indicating that the self-quenching of TX and DITX is not effective in a polymer matrix. In a poly(styrene-co-vinylbenzyl azide) (AZMS) matrix, the triplet decay rates of TX and DITX increase as the number of azido groups in the AZMS polymer increases. The energy transfer rate k et to the azido group in the AZMS polymer matrix is 1.5×10 6 M −1 s −1 for TX and 1.4×10 6 M −1 s −1 for DITX. The k d / k et values of TX and DITX in the AZMS polymer matrix are 20–30 times smaller than the values previously obtained from the measurement of the amount of nitrogen evolved under steady state conditions. The disagreement between the two results strongly indicates that the autocatalytic chain decomposition reaction of the azido group takes place in the AZMS polymer matrix.

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.

In the photochemical hydrogen abstraction reactions of chromone in 2-propanol at room temperature, beside its ketyl radical, the α-keto alkyl radical from chromone was observed, with the aid of the time-resolved electron spin resonance technique. When tri-n-butylin hydride was used as a hydrogen donor, the α-keto alkyl radical was produced predominantly from the lowest ππ* triplet state. The formation of such a non-ketyl radical in the hydrogen abstraction reaction with both of the hydrogen donors indicates the popularity of the hydrogen abstraction reactions from the ππ* triplet state of aromatic carbonyl compounds.

The energy transfer processes from thioxanthone (TX) and 2,4-diisopentylthioxanthone (DITX) to organic azides, such as ethyl- p -azidobenzoate (EAZB) and vinylbenzyl azide (VBAZ), were studied by measuring the transient absorption spectra and triplet decay rates of TX and DITX using an Nd:YAG laser flash photolysis technique. The energy transfer rates from both TX and DITX to the azides are close to the diffusion-controlled rate (10 9 M −1 s −1 ). In ethanol, the rate of energy transfer from TX to the azides is approximately twice as fast as from DITX to the azides. The rate of energy transfer to VBAZ from both TX and DITX is 4–5 times faster than to EAZB. Energy transfer rates to EAZB in ethanol were compared with the values previously obtained from the measurement of the volume of nitrogen evolved under steady state conditions. The values previously obtained are approximately 50 times smaller than the present results, and can be attributed to the autocatalytic chain decomposition reaction of the azido group.

Photochemical primary process and magnetic field effect of methyltriphenyl germane in a micelle have been investigated with the aid of a nanosecond laser-flash-photolysis technique. The germyl radical generated through triplet state was directly observed. The lifetime of the generated radical pair and the yield of the escaping germyl radical were increased with increasing magnetic field strength from 0 to 1.35 T. The yield at 1.35 T became 1.16 ± 0.02 times that at 0 T.

We have found that ordinary magnetic fields below 1.5 T have great influence on dynamic processes of excited molecules such as fluorescence in the gas phase and chemical reactions in solution. We have directly measured the conversion rates between the singlet and triplet states in such processes with the aid of laser photolysis techniques and studied the mechanisms of these magnetic field effects in detail.

The effect of magnetic field on the intensity of electroluminescence from devices made of a poly-$p$-phenylenevinylene (PPV) copolymer was investigated. The emission intensity was enhanced by the application of magnetic field, and the magnitude of the increase depended on operational voltages. When the device was operated under application of low voltages, the intensity increased with magnetic field and reached an 8.5% increase at about $100\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$. With the increase of the operational voltage, the effect of magnetic field was lessened. In addition, when measured at high voltages with increasing magnetic field, the emission intensity started to decrease after passing a maximum, then leveled off. This saturation value was slightly higher than that observed in the absence of magnetic field. These findings suggest that two processes sensitive to magnetic field are included in the emission processes. They are assigned to the charge recombination (CR) of anion and cation radicals and triplet-triplet annihilation (TTA) processes. From the analysis of the effects of magnetic field on the emission intensity based on a kinetic model, we quantitatively determined the fractions of singlet and triplet excitons generated through the CR process to be 0.17 and 0.83, respectively. With the increase of the concentration of triplet excitons in the organic layer, production of singlet excitons through the TTA process was enhanced, and the total yield of the singlet excitons exceeded 0.5 under normal device operational conditions. We conclude that this high yield is responsible for the high emission efficiency observed in the light-emitting devices based on PPVs.

The mechanism and intermediates of photoinduced electron-transfer reaction from hexamethyldigermane to 2, 5-dichloro-1, 4-benzoquinone were studied at room temperature by means of a CIDNP technique. After irradiation, only trimethylgermyl chloride was confirmed by GC, GC-MS, 1H-NMR spectra. However, the absorptive CIDNP signals for ring and hydroxyl-H of 4- (trimethylgermyloxy) phenol were observed during irradiation. The observed CIDNP phase indicates that the reaction intermediate is hexamethyldigermane cation radical rather than trimethylgermyl radical. These results suggest the instability of the cation radical.

Upon laser flash photolysis of p-aminophenyl disulfide at room temperature, the radical pair of two p-aminophenylthiyl radicals was found to be generated by direct excitation and triplet sensitization in the magnetic field range 0-1.35 T. Only in the case of triplet sensitization did the yield of the escaped radical increase with increasing magnetic field strength. The yield at 1.35 T was 1.14±0.2 times as large as that at 0 T. This is the first report of a magnetic field effect on a reaction of two sulfur-centered radicals

Etude par spectrometrie RPE du processus primaire photochimique de la methyl aqua cobaloxime (CH 3 Co(CH 3 C(NOH)C(NO)CH 3 ) 2 (H 2 O)) dans l'eau et le propanol-2

Magnetic field effects on the triplet sensitization reaction of p-aminophenyl disulfide were studied in a sodium dodecylsulfate micellar solution by a nanosecond laser flash photolysis technique un...

The photoreduction of benzophenone with thiophenol was studied by a laser flash photolysis under the magnetic fields of 0–10 T. The escape yields of the benzophenone ketyl and phenylthiyl radicals decreased with increasing magnetic field strength. The observed magnetic field effect can be interpreted by the Δg mechanism in the case of a triplet radical pair.

The hydrogen abstraction reaction of benzophenones in micellar interiors was studied by the effect of a short, resonant microwave pulse on the recombination kinetics under magnetic fields of approx...

The observed triplet—triplet absorption of 3,3-dimethyl-1-(2-naphthyl)-1-butene is assigned to the planar transoid geometry ( 3 t*) from its spectral similarity to the 3,4-dihydrophenanthrene triplet, a rigid planar model, as well as from the measured rate constants for its quenching by azulene and oxygen. The results are in contrast the 1,2-diarylethylene triplets which favor the perpendicularly twisted geometry ( 3 p*).

Photochemical primary processes of benzoyltriphenylgermane and benzoylphenyldimethylgermane were studied at room temperature in cyclohexane and chloroform. The germyl radicals generated were observed directly with the aid of a laser-photolysis technique.

The external magnetic field effects upon product yields in the singlet sensitized photolysis of dibenzoyl peroxide in toluene were studied. The observed field dependencies were classified into four types, and were interpreted in terms of the electronic Zeeman and hyperfine interactions in the intermediate radical pair. Especially large magnetic field effects were observed for the yields of dimethylbiphenyls, new products in this reaction.

2,2′-, 2,3′-, and 3,3′-Dimethylbiphenyls were found in the photodecomposition reaction of dibenzoyl peroxide using toluene as solvent. We carried out the experiments to study the external magnetic field, oxygen scavenging, and excitation light intensity effects on their yields. It is concluded that they are produced through the dimerization of (benzoyloxy)methylcyclohexadienyl radicals. This is a new case of the assisted reaction by benzoyloxyl radical. The yields of dimethylbiphenyls were found to be affected remarkably by the small variation of the concentration of benzoyloxyl radical through the three experiments.

The photochemical introduction of trifluoromethyl group with CF3Br in aromatic and heteroaromatic rings was investigated for 9 compounds. Naphthalene, anthracene, anisole, N,N-dimethylaniline, ferrocene, benzo[b]thiophene, isoquinoline, and N-methylpyrrole gave trifluoromethylated products in 6.5–100% yields. In one step from uracil, a pharmacologically important 5-trifluoromethyluracil can be synthesized by this method in 11% yield. Based on the mechanistic study carried out for the naphthalene–CF3Br–CH3CN system, the reaction is found to proceed via the electron transfer from an excited singlet state of naphthalene to CF3Br.

For the title ketone in solution, strong fluorescence from S 1 (nπ*) was observed at room temperature with a lifetime of 460 ns. Very weak and short-lived fluorescence from S 2 (ππ*) was also observed. Triphenylsilyl and triphenylsilyl acyl radicals were found to be produced from S 2 (ππ*) and T 1 (nπ*).

Phenyl-substituted group 4B element-centered radicals generated by the direct photoejection from the group 4B element-centered anions were observed by laser photolysis at room temperature. In order to clarify the mechanism of this process, CIDEP study of the photolysis of the group 4B element-centered anions was carried out at 77 and 4.2 K. From the emissive patterns observed for the germyl and stannyl anions after the excitation by a pulse laser, the photoejection reactions were most probably considered to occur from triplet anions.

The photochemical hydrogen abstraction reactions of flavone and its derivatives with some hydrogen donors in 2-propanol solutions were investigated at room temperature with the aid of a chemically induced dynamic electron polarization technique. When xanthene and N,N-diethylaniline were used as hydrogen donors, the ketyl radical of flavone was obtained through the n..pi..* character and charge-transfer interaction of the triplet state, respectively. When tri-n-butyltin hydride and sodium borodeuteride were used as hydrogen donors, the benzyl-type radicals of flavones were obtained through the ..pi pi..* character of the triplet states.

The magnetic field effects of the photochemical reactions of benzophenone derivatives and dibenzylketone were studied. The decay rate of the 4.4′-difluorobenzophenone ketyl radical in a sodium dodecylsulfate micelle continued to decrease from 2.9 × 106 s−1 at 0 T to 3.0 × 105 s −1 at 1.34 T.

Photochemical primary processes of carbon–tin and tin–tin bonds were studied at room temperature. The stannyl radicals generated were observed directly by laser-photolysis.

The yield of escaping benzenesulfonyl radical was found to be increased by a magnetic field. The yield at 1.2 T was 1.53 ± 0.05 times that at 0 T. From the observed magnetic field effects, the present reaction was proved to occur from triplet radical pairs.

The photodecomposition reaction of 1,3-diphenyl-2-propanone in a micelle was studied with the aid of ns laser-photolysis. The transient absorption due to the benzyl radical was observed and the amount of escaping benzyl radicals from radical pairs was found to be enhanced by 30 % in the presence of a magnetic field of 70 mT.

The dynamics of naphtosemiquinone and SDS alkyl radicals were studied by CIDEP in a micellar solution. The total emission pattern of the spectrum detected at an early stage after pulsed excitation revealed a predominant contribution of the triplet mechanism. The radical pair mechanism became observable at a later stage. The effect of an external magnetic field discussed.

From CIDEP measurements a new reaction of triplet xanthone, hydrogen abstraction by a benzene ring, was found to occur in the reactions with triethylgermane and sodium borohydride. An anomalous E/A phase pattern within each hyperfine line was also observed for the generated cyclohexadienyl-type radical in a micelle.

The effect of trivalent lanthanoid ions on the photochemical reaction of 1,4-naphthoquinone in a sodium dodecyl sulfate micelle is investigated in the

Photochemical primary processes of aryl-substituted silyl and stannyl anions were studied at room temperature. The silyl and stannyl radicals generated were observed directly by laser-photolysis.

The yields of the escaping ketyl radicals from the radical pairs formed through the reactions of triethylgermane with the triplet states of acetophenone and xanthone were found to be increased by a magnetic field. The observed increases at 1 T were 3 and 27% for the ketyl radicals of acetophenone and xanthone, respectively.

On etudie par une technique de CIDEP le mecanisme de formation du radical de type cyclohexadienyle par mesure de l'effet du solvant sur la reaction d'enlevement d'hydrogene de la xanthone a l'etat triplet par Sn(C 4 H 9 ) 3 H

The photochemistry of organosilicon compounds having a /sigma/(Si-Si)-/pi/(C-C) conjugated system has been the subject of considerable interest in recent years. However, there have been few reports on photochemical studies of the germanium analogue. The authors describe herein the first laser photolysis studies on the aryl-substituted digermanes (Ph/sub n/Me/sub 3-n/Ge)/sub 2/ (1-3, n = 1-3, respectively). Photolysis of the phenylated digermanes causes germanium-germanium bond homolysis which leads to the formation of a germyl radical and probably a germylene.

The photochemical introduction of trifluoromethyl group with CF3Br in aromatic and heteroaromatic rings was investigated for 9 compounds. Naphthalene, anthracene, anisole, N,N-dimethylaniline, ferrocene, benzo[b]thiophene, isoquinoline, and N-methylpyrrole gave trifluoromethylated products in 6.5–100% yields. In one step from uracil, a pharmacologically important 5-trifluoromethyluracil can be synthesized by this method in 11% yield. Based on the mechanistic study carried out for the naphthalene–CF3Br–CH3CN system, the reaction is found to proceed via the electron transfer from an excited singlet state of naphthalene to CF3Br.