Masatoshi Igarashi

Effects of high magnetic fields on the dynamic behavior of radical pairs generated in micellar solutions

Abstract 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.

Effects of large magnetic fields on the dynamic behavior of radical ion pairs in a non-viscous solution at room temperature

Abstract 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 on the photochemical hydrogen abstraction of 2′,3′,4′,5′,6′-pentafluoroacetophenone in a Brij 35 micellar solution

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.

Photo-induced electron transfer reaction between hexamethyldisilane and quinones as studied by a CIDNP technique

Abstract 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.

Formation of α-keto alkyl radical in the photoreduction of chromone as studied by the laser photolysis—ESR technique

Abstract 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.

Reversion of magnetic field effects under large magnetic fields observed in the photochemical hydrogen abstraction reactions of fluorinated acetophenone and benzophenone 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...

Photo-generated stable complex with efficient power of photo-induced charge separation between porphyrin and quinone

Abstract A stable charge-transfer complex was formed between 5,10,15,20-tetraphenyl-21 H ,23 H -porphine and a quinone in a neutral or acidic chloroform solution on irradiation with blue light. The existence of this photo-generated complex was confirmed by the change of visible absorption spectra, fluorescence spectra and NMR spectra. It had a high power of photo-induced charge separation even when irradiated with red light, as was proven from the intensity dependence of chemically induced dynamic nuclear polarization on the wavelength of excited light and on the irradiation time.

Catalytic function of ss-carotene on the disproportionation reaction of semiquinone radicals based on CIDNP spectra and its excitation profile.

成初期過程の光励起電荷分離反応のモデルと見なされるポルフィリン色素・キノン均一溶液系において,光CIDNPおよびその励起プロフィルの測定が行われた。β-カロチンの共存により,ペンゾキノンおよびそのメチル置換キノンにヒドロキノン-Hの強いCIDNP信号が見いだされた。これは同時に観測されたキノンーHのCIDNP信号の経時変化およびβ-力ロチン濃度依存性を考慮して,β-チンがセミキノンラジルの不均化反応を促進した結果だ雛定さ泌。不均化反応の直接カのロ先駆ゆげ物質は,CIDドPの純分極効果および増大因子の検討から,セミキノンカチオンラジカルとセミキノソスニオンラジカルであり,おそらくβ-カロチンのプゴトン伝達機能により形状を異にしたラジカルィオン対と考えられる。