A test for homology: photoactive crystalline assemblies involving linear templates based on a homologous series of phloroglucinols.

[reaction: see text] Cocrystallization of trans-1,2-bis(4-pyridyl)ethylene (4,4'-bpe) with eight members of a homologous series of phloroglucinols 1a-h yields molecular solids 2(1a-h).2(4,4'-bpe) with components held together by four O-H...N hydrogen bonds. In each case, the molecules assemble to form a discrete four-component assembly with olefins preorganized for a [2 + 2] photodimerization. UV irradiation of each member in the series of solids produces rctt-tetrakis(4-pyridyl)cyclobutane (4,4'-tpcb), stereospecifically, in up to 100% yield.