A diacetylene compound, 1,4-diphenyl-1,3-butadiyne, was photolyzed with 2,3-dimethyl-2-butene, 1,4-cyclohexadiene, dimethyl fumarate, and methyl crotonate, as a model reaction of the phototoxic conjugated polyynes with DNA or RNA and [2 + 2] photocycloadducts were obtained except for 1,4-cyclohexadiene. In the photoreaction of 1,4-diphenyl-1,3-butadiyne with 2,3-dimethyl-2-butene, a [2 + 2 + 2] photoadduct was additionally obtained. The photolysis of 1,4-di-t-buyl-1,3-butadiyne with 2,3-dimethyl-2-butene also yielded a [2 + 2] photoadduct. Fluorescence was observed from all the photoadducts while the reactants did not show any fluorescence.