Solvent-Dependent Zn(II) Supramolecular Isomers and [2 + 2] Photocycloaddition Reaction via an SCSC Transformation

Abstract

Each of the supramolecular isomerism and the solid-state [2 + 2] photocycloaddition has been developed as an independent research topic in crystal engineering. We herein report the solvent-dependent formations of the olefin-based supramolecular isomers (1 and 2) showing different photoreactivity. In the solvothermal reaction of Zn(NO₃)₂·6H₂O, 1,2-bis(4-pyridyl)ethylene (bpe), and 1-adamantanecarboxylic acid (Hadc) in dimethylacetamide(DMA)/water, we obtained a zigzag-shaped coordination polymer of the type [Zn(bpe)(adc)₂] (1). When dimethylformamide (DMF) was used instead of DMA, the railroad-shaped supramolecular isomer 2 was isolated. In 2, the dinuclear repeating units, Zn₂(adc)₄ as railroad ties, are dilinked by bpe ligands. Under ultraviolet (UV) irradiation, isomer 1 was photoinert due to the large separation between the olefin (C═C) bonds in the bpe ligands. In contrast, isomer 2 was photoreactive because the olefin bonds of the parallel bpe linker ligands are well aligned in a face-to-face mode and separated by a distance of 3.79 Å to undergo [2 + 2] photocycloaddition quantitatively via an single-crystal-to-single-crystal (SCSC) manner. All of these products exhibit yellow emission in the 385–388 nm region with different intensities depending on the local structures.