Guest-triggered assembly of zinc(ii) supramolecular isomers with accompanying dimensional change and reversible single-crystal-to-single-crystal transformation

Abstract

Solvent-directed assembly of two-dimensional (2D) and 3D supramolecular isomers as well as guest-exchange induced transformation of the 2D isomer accompanied by photoluminescence switching are reported. Solvothermal reaction of Zn(NO₃)₂·4H₂O, 1,4-bis(4-pyridyl)piperazine (bpp), and benzene-1,3-dicarboxylic acid (H₂bdc) in dimethylformamide (DMF) afforded a self-penetrated 3D coordination polymer, {[Zn₂(bpp)₂(bdc)₂]·4DMF·4H₂O}_n (1), adopting a firewood-like stacking structure. When the above synthetic procedure was repeated employing dimethylacetamide (DMA) as a solvent, a 2D coordination polymer, {[Zn₂(bpp)₂(bdc)₂]·4DMA·6H₂O}_n (2), was obtained, whose structure shows a crinkled square (4,4) grid type, as a supramolecular isomer of 1. The dimensional difference between isomers 1 and 2 stems mainly from the directional arrangement and the conformation of the bpp ligand. In the guest-exchange experiment, isomer 2 with the crinkled square grid structure not only represents a reversible spreading–folding conversion due to the conformational change of the bpp ligand but also displays a photoluminescence switching behaviour.