On the photochemical behaviour of some diarylpyrazolines

Abstract

The excited state behaviour of 1-phenyl-3(2,4,6-trimethylphenyl)-4,5-dihydropyrazole ( 1) and of the corresponding 1-(4-diethylaminophenyl) ( 2) and 1-(4-morpholinophenyl) ( 3) derivatives has been explored. All of these compounds fluoresce efficiently and the Stokes shift (11.6 × 10 −3 cm −1 with 1) increases by (2–3) × 10 3 cm −1 with the introduction of the electron-donating groups. They are unaffected both by direct irradiation in deoxygenated solution or by photosensitization with benzophenone. Compounds 1 and 2, but not 3 are dehydrogenated to the corresponding pyrazoles by irradiation in deoxygenated acetone. In air-equilibrated toluene, acetonitrile or ethanol, direct irradiation (λ > 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 this is not the case with 2 and 3, and these compounds act as physical quencher of singlet oxygen with no measurable reaction. The key factor for rationalizing these reaction is the low oxidation potential of the substrates.