Edge-to-Edge C–H···N Hydrogen Bonds in Two-Component Co-crystals Aide a [2 + 2] Photodimerization

C–H···N hydrogen bonds support an intermolecular [2 + 2] photodimerization in the solid state. The photocycloaddition occurs in a co-crystal of composition ( tbrb )·( 4,4′-bpe ) (where tbrb = 1,2,4,5-tetrabromobenzene; 4,4′-bpe = trans -1,2-bis(4-pyridyl)ethylene) that consists of one-dimensional (1D) chains that self-assemble to form corrugated sheets. Stacking of 4,4′-bpe between sheets conforms to the topochemical postulate of Schmidt. The alkene reacts to form rctt -tetrakis(4-pyridyl)cyclobutane ( 4,4′-tpcb ) stereoselectively and in quantitative yield. C–H···N hydrogen bonds also support the assembly of the components in ( tbrb )·( 4,4′-azo ) (where: 4,4′-azo = 4,4′-azopyridine) into planar sheets. Density-functional theory calculations reveal identical face-to-face stacks of tbrb to aide in the stabilization of the structures of the co-crystals.