Regioselective photoreactions within a series of mixed co-crystals containing isosteric dihalogenated resorcinols with 4-stilbazole.

The formation and photoreactivity of a series of mixed co-crystals containing 4-stilbazole along with both 4,6-dichlororesorcinol and 4,6-dibromoresorcinol at various ratios is reported. In all cases, the quantitative [2 + 2] cycloaddition reaction yields an identical head-to-tail photoproduct, namely  rctt -1,3-bis(4-pyridyl)-2,4-bis(phenyl)cyclobutane. The selectivity in the cycloaddition reaction occurred due to a shorter distance observed between two carbon–carbon double bonds that were found between and not within the hydrogen-bonded assemblies.