Stacking of double bonds for photochemical [2+2] cycloaddition reactions in the solid state.

trans-1,2-Bis(4-pyridyl)ethylene (bpe), containing a C=C bond, has been extensively studied for solid-state photochemical [2+2] cycloaddition reactions, in discrete molecular and metal complexes, hydrogen bonded and coordination polymeric structures. The challenges in orienting a pair or more of bpe molecules in the solid-state using crystal engineering principles, and their photochemical behaviour and implications, based on Schmidt's postulates, are discussed.