Multistimuli-induced structural variations of a Zn(II)−viologen coordination polymer and the two-step photochromism.

Organic molecules and some coordination polymers that are responsive to external stimuli (such as light, electrical potential, heat, guest inclusion or pressure) often generate structural isomerization, such as ring-opening/closing, tautomerism and cis-trans isomerization. While for viologen derivatives, generally, electron transfer occurs upon the photoirradiation without a significant structural change. Herein we present a new viologen-based Zn-complex (1) which undergoes single-crystal-to-single-crystal (SCSC) transformation to 1G upon crushing. Both 1 and 1G could change to 1Y when stored under air in the dark at ambient for one week. The colorless crystal 1 undergoes a two-step color change to pink (1P) and brown (1Z) upon irradiation with UV for 40 s and 15 min, respectively. More importantly, the complex 1G also has a two-step coloring phenomenon under UV light. The colorless crystals of 1G change into blue (1Q) after irradiation for 15 s and finally become brown (1Z′) with the increase of irradiation time. While 1Y only undergoes one coloration from colorless to cyan 1Y′ after UV irradiation. The possible pathways for electron transfer were confirmed to be through intermolecular hydrogen bonding according to the results of crystal structure and the photochromic behavior of the ground samples. 1Z and 1Z′ could change color to yellow 1H and 1H′ in the dark, respectively. Only the changes between 1 and 1P , 1Z and 1H , 1G and 1Q , 1Z′ and 1H′ , 1Y and 1Y′ are reversible, which implies that the crystal structures varied upon photoirradiation because 1Z and 1Z' can revert to neither the original crystals 1 and 1G , nor the intermediate 1P and 1Q. These findings help to understand multiple photochromism and variations of crystal structures under different external stimuli via SCSC transformations. In addition, complex 1 shows high selectivity for primary aromatic amine aniline among various amines. [Display omitted] • Complex 1 undergoes a single-crystal to single-crystal transformation to 1G upon crush with structure change. • Both 1 and 1G could undergo a two-step coloring phenomenon under UV light. • 1 and 1G change to 1Y when stored in the dark for one week. • Complex 1 shows high sensitivity to aniline. • The pathways for electron transfer of the two-step were discussed in detail. [ABSTRACT FROM AUTHOR]