Halogen-Bond Mediated [2+2] Photodimerizations: À la Carte Access to Unsymmetrical Cyclobutanes in the Solid State.

The ditopic halogen-bond (X-bond) donors 1,2-, 1,3-, and 1,4-diiodotetrafluorobenzene (, , and , respectively) form binary cocrystals with the unsymmetrical ditopic X-bond acceptor -1-(2-pyridyl)-2-(4-pyridyl)ethylene (). The components of each cocrystal ()·(), ()·(), and ()·() assemble via N···I X-bonds. For ()·() and ()·(), the X-bond donor supports the C=C bonds of to undergo a topochemical [2+2] photodimerization in the solid state: UV-irradiation of each solid resulted in stereospecific, regiospecific, and quantitative photodimerization of to the corresponding head-to-tail () or head-to-head () cyclobutane photoproduct, respectively.