LAUSR

We describe hydrogen bonds (i.e., N+–H⋯N) in combination with cation⋯π interactions to enable a cascade-like [2 + 2] photodimerization of 4-stilbazole in a salt cocrystal. A four-component crystal assembly is composed of photoactive pyridinium ion pair 4-stilbzH+ (4-stilbz = trans-1-(4-pyridyl)-2-(phenyl)ethylene) and photostable molecule pair 4-stilbz. UV irradiation gives rctt-1,2-bis(4-pyridyl)-3,4-bis(phenyl)cyclobutane (4-pyr-ph-cb) in quantitative yield. An intermediate structure 2(4-stilbz)·(4-pyr-ph-cb)2H+ is isolated that undergoes a partial single-crystal-to-single-crystal transformation. The single-crystal X-ray data provides a snapshot of movements in the salt cocrystal in the course of the photodimerization.