Crafting Zn(II) Supramolecular Isomers Using a Styrylpyridyl Ligand: Structure-Controlled [2 + 2] Photocycloaddition in the Solid State

Supramolecular isomerism in metal–organic frameworks (MOFs) and coordination polymers has garnered significant interest in the field of hybrid materials. This interest stems from the fact that these materials share identical linkers and compositions, yet they exhibit diverse properties and applications. In this study, two supramolecular isomers were synthesized using zinc(II) nitrate, 4-styrylpyridine (spy), and 1,3-benzenedicarboxylic acid (m-H2bdc). The first isomer, [Zn(spy)2(m-bdc)] (1), exhibits a one-dimensional (1D) chain structure with octahedral Zn(II) centers that are bridged by m-bdc. Upon UV irradiation, this compound undergoes a quantitative photoreaction of the spy linkers in a head-to-tail manner, resulting in the formation of a 2D layered structure, [Zn(rctt-ppcb)(m-bdc)] (3, ppcb = 1,3-bis(4-pyridyl)-2,4-bis(phenyl)cyclobutane). The second isomer, [Zn(spy)2(m-bdc)]·DMF (4), also exhibits a 1D chain structure with tetrahedral Zn(II) centers bridged by m-bdc. Due to the partial head-to-tail alignment of the spy in 4, this compound showed only 50% photoreaction. Both isomers showed transformation of 1D chains to 2D layers. These isomers not only differ in their structures but also in their photoreactivity and optical properties. Overall, this study elucidates the synthesis, structural characterization, photoreactive behavior, and optical properties of zinc(II)-based supramolecular MOFs, thereby revealing their potential for controlled structural transformations and photochemical applications.