Nanoarchitectonics of two organic cocrystals with zwitterionic ligands: Structures, photochromic and chemochromic properties

Nanoarchitectonics of two organic cocrystals constituted by zwitterionic ligands 1,1′-bis(4-carboxyphenyl)-(4,4′-bipyridinium) dichloride (H₂bcbpCl₂) and 1,1′-bis(3,5-dicarboxyphenyl)-4,4′-bipyridinium dichloride (H₄bdcbpCl₂), has been successfully synthesized and characterized. Hydrogen bonding interactions play an important role in the formation of supramolecular slipped-stacking architecture of both compounds. It is worth noting that these two compounds can quickly change from brown to green upon exposure to ethylamine, indicating that the nucleophilic attack of ethylamine molecules on the viologen moieties in both compounds contributes to the formation of free viologen radicals. In addition, compound 1 also exhibits photochromic property, and, its color rapidly and noticeably changes from brown to green under the irradiation of xenon light due to the generation of free viologen radicals. However, compound 2 lacks photochromic property due to its relatively long distance of electron donor and acceptor.