Trifluoromethyl Groups in Crystal Design of 1,4-Diphenyl-1,3-butadienes for Topochemical [2 + 2] Photodimerization

UV irradiation of the powdered crystalline sample of each of three (E,E)-1,4-di(trifluoromethyl-substituted)phenyl-1,3-butadienes (1−3) was
found to yield a single [2 + 2] cycloaddition product in the solid state. Moreover, upon irradiation, the crystalline samples of two (E,E)-1,4-di(trifluoromethyl- and fluorine-substituted)phenyl-1,3-butadienes (4, 5) undergo a similar conversion to afford a [2 + 2] cycloaddition product,
respectively. Our observations suggest that trifluoromethyl groups can be used to direct 1,4-diphenyl-1,3-butadiene molecules to form a
parallel, offset-stacked orientation suitable for topochemical [2 + 2] cycloaddition.