Crystal-to-crystal photodimerization of trans-cinnamamides

In the crystals of trans-4-methylcinnamamide, C₁₀H₁₁NO (I), trans-4-chlorocinnamamide, C₉H₈ClNO (II), trans-3-(2-thienyl)acrylamide, C₇H₇NOS (III), and trans-cinnamamide, C₉H₉NO (IV), the shortest intermolecular C⋯C distances between the C=C double bonds are 3.670 (2), 3.632 (2), 3.762 (3) and 4.120 (2) Å, respectively, for the pair of molecules related by a center of symmetry. The structure analysis was also carried out for trans-2-(p-chlorophenyl)-cis-4-(p-chlorophenyl)-1-trans-3-diamidocyclobutane, C₁₈H₁₆Cl₂-N₂O₂ (V), which is the α-type photodimer of (II). The N -H⋯O hydrogen-bond networks in (I)-(III) are composed of two-dimensional pleated sheets, and those in (IV) and (V) of one-dimensional flat ribbons. The single crystals of (I), (II) and (IV) were photoirradiated with a 250 W ultra-high-pressure mercury lamp through a band-pass filter or a long-pass filter for 2-5 h. The photodimer was produced in each crystal with retention of the single-crystal form. The populations of the dimers were converged to 86.2 (4), 48.4 (6) and 4.5 (2)% in the refinement of the crystals after photoirradiation, (I′), (II′) and (IV′), respectively. Although the intermolecular N-H⋯O hydrogen-bond network remained in (I′) and (II′), the network was partly broken in (IV′) in the process of photoreaction.