Studies of Highly Reactive Biradical and Cation Radicals

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We have studied the relationship between biradical (BR) and cation radical(CR)intermediates in the photoinduced electron-tranfer(PET)skeletal rearrangement of organic compounds. In general, a nondistonic CR of a donor substrate generated by PET from the excited state of sensitizer (A)can undergo the bond formation or bond cleavage to give a more stable distonic CR intermediates (I^<・+>). Although free distonic CRs have been often observed spectroscopically and/or captured by a trapping reagent such as molecular oxygen or alcohol, reactions of a distonic CR in an ion pair [I^<・+>/A^<・->] have not been well understood yet. There are two possibilities for the decay of a distonic CR intermediate. One is back electron transfer (BET) from A^<・-> to give a singlet or triplet BR intermediate, which give final product (s) by the bond cleavage or formation. The other one is an alternative possible sequence, i.e., bond cleavage or formation in a CR stage followed by BET.In order to know which pro … More cess is operative, we use two complementary laser techniques, time-resolved absorption spectroscopy for the direct observation of CR and BR intermediates and time-resolved photoacoustic calorimetry for the determination of energy of [I^<・+>/A^<・->]. As result, we found three PET reactions that proceed through both CR and BR intermediates. The first is the PET degenerate Cope rearrangement of 2,5-diaryl-1,5-hexadiene, which proceeds in a CR cyclization-BR cleavage mechanism through 1,4-diarylcyclohexa-1,4-diyl CR and 1,4-diarylcyclohexa-1,4-diyl BR.The second example is the degenerate methylenecyclopropane rearrangement of 2,2-diarylmethylenecyclopropane induced by PET,which involves a trimethlenemethane CR (TMM^<・+>) and corresponding BR (TMM). The last case is the generation of singlet and triplet tetramethyleneethane BRs by cyclization of 5,5-dimethy1-2,3-bis (1-phenylvinyl) cyclopentadiene CR.Those three PET reactions provided an intriguing conclusion that BET in an ion pair is an important key process which connects CR chemistry with BR chemistry. This conclusion is quite noteworthy since, up to the present, BET was usually considered as an energy wasting process. Less