A Multifunctional Coordination Polymer Constructed by Viologen Derivatives: Photochromism, Chemochromism, and MnO₄^– Sensing

Abstract

A multifunctional Cd-based coordination polymer based on a viologen derivative and a secondary carboxylic ligand, namely, {[Cd₂(bcbp)(pta)₂ (H₂O)]·2H₂O}_<i>n</i> (<b>1</b>, H₂bcbpCl₂ = 1,1'-bis(4-carboxyphenyl)-(4,4'-bipyridinium) dichloride, H₂pta = <i>p</i>-phthalic acid), has been solvothermally synthesized and structurally characterized. Compound <b>1</b> is a 3D framework built on {Cd₂(pta)₂ (H₂O)}_<i>n</i> net with square channels which intalls bcbp ligands at the diagonal position of the two adjacent square channels. Notably, compound <b>1</b> exhibits a reversible photochromic transformation between yellowish gray and dark green under xenon light irradiation, which should be attributed to the formation of free viologen radicals. Furthermore, compound <b>1</b> also displays a chemochromic phenomenon from yellowish gray to dark green after exposure to ammonia and hydrazine, which could be ascribed to the generation of viologen radicals and charge-transfer complexes triggered by nucleophilic attack of electron-rich ammonia and hydrazine molecules to the electron-deficient viologen moiety in the bcbp ligand. Moreover, compound <b>1</b> is capable of quickly and selectively detecting MnO₄^- with a high quenching coefficient and a low limit of detection via a luminescent quenching process caused by the competitive absorption of excitation energy.