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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 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.

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.

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...

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 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.

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.

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.

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.

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.

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.

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...

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.

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.