Photoinduced Electron Transfer Reactions of 3H-Pyrazole Derivatives. Formation of Solvent Adduct by Specific Sensitizer

3H-pyrazoles were photolyzed in the presence of 2,4,6-triphenylpyrylium tetrafluoroborate (TPP^+) or 9,10-dicyanoanthracene (DCA) as a sensitizer. The obtained products were different depending on the sensitizers employed. The cyclopropene derivatives were produced in the cases of the DCA and TPP^+ sensitizations and a solvent adduct was obtained for the TPP^+ sensitization reaction. The difference in products is explained by the rates of the back electron transfer from the electron accepted sensitizers to the radical cation intermediates. In the DCA sensitization, the absorption band of the diazoalkene was observed but in the TPP^+ sensitization it was not observed. From these results, it is apparent that the back electron transfer rates of the DCA sensitization reactions are faster than those of the TPP^+ sensitization reactions.