Photochromism of heterocyclic systems

The preparations of photochromic dimers of a 2-aryl-4,5-di-phenylimidazyl radical and of a 2,5-diphenyl-3,4-isobenzopyrrole with different substituents in the homocyclic ring are reported. Of the dienes investigated, only 1-4-diphenyl-1,3-butadiene and 2,3-dimethyl-1,3-butadiene gave isolable amounts of the Diels-Adler adducts.