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