Photochemical dimerization of a fluorinated dibenzylideneacetone in chloroform solution.

(1E,4E)-1,5-Bis(2,6-difluorophenyl)penta-1,4-dien-3-one, C17H10F4O, (I), dimerizes under sunlight in chloroform solution to form the corresponding cyclobutane derivative, (2E,2'E)-1,1'-[2,4-bis(2,6-difluorophenyl)cyclobutane-1,3-diyl]bis[3-(2,6-difluorophenyl)prop-2-en-1-one], C34H20F8O2, (II). The crystal structure of (I) explains why no topochemical dimerization can occur in the solid state. In the solid, molecules of dimer (II) show the `truxillic acid'-type arrangement of crystallographic centres of inversion, with half a molecule per asymmetric unit and cell dimensions closely related to those of the monomer. Intermolecular interactions in both solids are dominated by C-H···O and C-H···F contacts and also comprise interactions with aromatic systems (C-H···π and π-π).