Specific photochemical behavior of pyrazolines and construction of its electron transfer system

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Radical cations generated from cyclic azoalkanes by photoinduced electron transfer (PET) have recently been investigated and produced a few characteristic compounds by rearrangements and solvent additions. The reaction mechanisms become complex when the generated radical cations have the possibility of rearranging due to their geometric constraints and stabilities, as these species appear to be involved in denitrogenation. In our previous studies using the 1-pyrazolines and 9, 10-dicyanoanthracene (DCA) as an electron accepting sensitizer, the PET reaction was found to cause aryl migration involving the radical cation intermediates. In spite of the fact that the behavior of the nitrogen-retained radical cations are of considerable interest, nitrogen-retained products by the PET reaction have not been obtained, and the products are nitrogen-eliminated compounds. In this project, we report the first example of the use of cyclic azoalkanes for the PET reaction in which nitrogen-retained products are obtained from the radical cation species of 3-alkylidene-1-pyrazolines having an exocyclic double bond, and that the successful detection of the nitrogen-retained radical cations was accomplished by a laser flash photolysis (LFP) technique.