A novel photochemical reaction of 4‐bromo‐3‐methyl‐1‐phenyl‐4,5‐dihydro‐pyrazol‐5‐one

UV‐Irradiation of 4‐bromo‐3‐methyl‐l‐phenyl‐4,5‐dihydro‐pyrazol‐5‐one (1) in the presence of various aromatic hydrocarbons gave different types of photoproducts depending on the nature of the hydrocarbons via an electron‐transfer mechanism. In the presence of naphthalene or phenanthrene a photochemical homocoupling reaction of 1 occurred to form 2 or 2 and 3, respectively.