ON THE PHOTOCHEMICAL BEHAVIOR OF SOME DIARYLPYRAZOLINES

The excited state behaviour of henyl-3(2,4,6-trimethylphenyl)-4,5-dihy dropyrazole (1) and of the corresponding 1-(4-diethylaminophenyl) (2) and 1-(4-morpholinophenyl) (3) derivatives has been explored. All of t hese compounds fluoresce efficiently and the Stokes shift (11.6 x 10(- 3) cm(-1) with 1) increases by (2-3) x 10(3) cm(-1) with the introduct ion of the electron-donating groups. They are unaffected both by direc t irradiation in deoxygenated solution or by photosensitization with b enzophenone. Compounds 1 and 2, but not 3 are dehydrogenated to the co rresponding pyrazoles by irradiation in deoxygenated acetone. In air-e quilibrated toluene, acetonitrile or ethanol, direct irradiation (lamb da > 300 nm) of these pyrazolines causes only a slow reaction. However , 2 and 3 are dehydrogenated by irradiation at 254 nm in toluene. 1 is dehydrogenated by singlet oxygen in a dye-sensitized reaction, but th is is not the case with 2 and 3, and these compounds act as physical q uencher of singlet oxygen with no measurable reaction. The key factor for rationalizing these reaction is the low oxidation potential of the substrates. (C) 1997 Elsevier Science S.A.