Site-directed regiocontrolled synthesis of a 'head-to-head' photodimer via a single-crystal-to-single-crystal transformation involving a linear template

Co-crystallisation of 1,8-naphthalenedicarboxylic acid (1,8-nap) with trans-1-(3-pyridyl)-2-(4-pyridyl) ethylene (3,4-bpe) gives a discrete molecular solid-state assembly, 2(3,4-bpe).2(1,8-nap) 1, that is held together by four O-H...N hydrogen bonds wherein the diacid directs a regiocontrolled [2 + 2] photodimerization; the reaction occurs by way of a single-crystal-to-single-crystal transformation.