Fluorine in crystal engineering: photodimerization of (1E,3E)-1-phenyl-4-pentafluorophenylbuta-1,3-dienes in the crystalline state.

(1E,3E)-1,4-Diphenylbuta-1,3-diene is photostable in the crystalline state, while fluoro-substitution induces reactivity. Crystals of (1E,3E)-1-pentafluorophenyl-4-(4-methoxyphenyl)buta-1,3-diene 1, (1E,3E)-1-pentafluorophenyl-4-(4-methylphenyl)buta-1,3-diene 2 and (1E,3E)-l-pentafluorophenyl-4-phenylbuta-1,3-diene 3 undergo double [2+2] photodimerization topochemically to yield anti head-to-tail photodimers in the crystalline state. Remarkably fluoro-substitution brings the reactant molecules into an anti head-to-tail arrangement in the crystal lattice with weak intermolecular interactions: C-H ... F, F ... F, C-H ... pi, pi ... pi.