Probing the Landscape of Photoactive Mixed Cocrystals: Unexpected Photostability Involving a Br/I Halogen Exchange

The formation, structural diversity, and photoreactivity of members of a series of hydrogen-bonded mixed cocrystals were achieved by changing the halogen atoms of trihalophenols; namely, 2,4,6-trichlorophenol, 2,4,6-tribromophenol, and 2,4,6-triiodophenol. The inclusion of two isosteric hydrogen-bond donors led to structural diversity and different photochemical behavior when combined with trans-1,2-bis(4-pyridyl)ethylene. The realization of a three-component mixed cocrystal was achieved when combining neighboring halogen atoms such as Cl/Br and Br/I pairs. The mixed cocrystal based upon 2,4,6-trichlorophenol and 2,4,6-tribromophenol was the only mixed solid to undergo a [2 + 2] photocycloaddition reaction. The overall yield for the equal molar-mixed cocrystal lies at the midpoint between the two binary cocrystals, which demonstrates that the extent of a solid-state photoreaction can be affected using halogen exchange. The mixed cocrystal containing 2,4,6-tribromophenol and 2,4,6-triiodophenol yielded a different face-to-face π-π stacking pattern, which illustrates that the type of halogen atom can influence crystal packing. The mixed cocrystal was determined to be photostable owing to improper positioning of the carbon─carbon double bond. To test structural reliability, an additional isostructural mixed cocrystal for each system was also realized by using 1,2-bis(4-pyridyl)acetylene as a hydrogen-bond acceptor.