Studies on Conservation of Spin-Polarization in Hydrogen Atom Transfer Reactions.

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The time-resolved ESR technique was applied to obtain molecular constants of triplet states and to elucidate mechanisms of intersystem crossing, triplet-triplet energy transfer, hydrogen atom transfer reaction and etc. Main results obtained are summarized as follows:(1) The <^3n(pi)^*> state of biacetyl: From the spin-polarized ESR signals of the <^3n(pi)^*> state of biacetyl produced directly by the <T_1> <- <S_0> absorption, the sign of the D value is determined to be positive. The marked matrix effect on spin polarization in the <^3n(pi)^*> state is interpreted by the switching of intersystem crossing channel in ethanol and nonpolar solvents.(2) Triplet-triplet energy transfer: Transient spin-polarized ESR spectra of triplet state resulting from triplet-triplet energy transfer were studied in organic matrices at 77 K. The spin-polarized ESR spectra of the acceptor triplet produced by the photosensitization were completely different from those of direct photoexcitation. It is conclud … More ed that the spin polarization of the donor is transferred to the acceptor by triplet-triplet energy transfer and the Zeeman quantum number is conserved, which is interpreted by the exchange mechanism.(3) Spin-polarization transfer in the triplet-doublet system: The interaction between triplet molecules and nitroxide radicals was studied in solution. Spin polarization induced in the radical reflects that of the triplet molecule which is an encounter partner. The spin-polarized ESR signals observed in nitroxide radicals are interpreted in terms of the electron and/or spin exchange mechanisms.(4) Reversal of spin-polarization in the triplet benzyl: The CIDEP signals of produced radicals were observed by the photoirradiation of the benzyl-hydrogen atom donor (aniline, phenoles) systems in benzen solusion. The polarities of the CIDEP signals were reversed by the change of donor and benzyl concentrations and donor reactivities. The revers of spin-polrization was interpreted in terms of the triplet energy transfer between benzyl molecules followed by the structure change of benzyl from nonplanar to planar. Less