Supramolecular Sandwiches: Halogen-Bonded Coformers Direct [2+2] Photoreactivity in Two-Component Cocrystals.

The halogen-bond (X-bond) donors 1,3- and 1,4-diiodotetrafluorobenzene ( and , respectively) form cocrystals with -1,2-bis(2-pyridyl)ethylene () assembled by N···I X-bonds. In each cocrystal, 2()·2() and ()·(), the donor molecules support the C=C bonds of to undergo an intermolecular [2+2] photodimerization. UV irradiation of each cocrystal resulted in stereospecific and quantitative conversion of to -tetrakis(2-pyridyl)cyclobutane (). In each case, the reactivity occurs via face-to-face -stacked columns wherein nearest-neighbor pairs of molecules lie sandwiched between X-bond donor molecules. Nearest-neighbor C=C bonds are stacked criss-crossed in both cocrystals. The reactivity was ascribed to the olefins undergoing pedal-like motion in the solid state. The stereochemistry of is confirmed in cocrystals 2()·() and ()·().