Sun, UV and X-ray triple photochromic properties of three coordination polymers based on 1,1′-bis(3-carboxylatobenzyl)-4,4′-bipyridinium ligand

Abstract

Based on a bipyridinium carboxylate ligand 1,1′-bis(3-carboxylatobenzyl)-(4,4′-bipyridinium)-dichloride (H₂bcbpyCl₂), three new coordination polymers {[Co_0.5(bcbpy)_0.5(H₂O)₂](p-BDC)_0.5·H₂O}_n (1) {[Ni(bcbpy)(H₂O)₄](p-BDC)·2H₂O}_n (2) and [Zn₂(bcbpy)(BTEC)(H₂O)₂·6H₂O]_n (3) are designed and synthesized by varying the benzenecarboxylic acids (1,4-benzenedicarboxylic acid = p-H₂BDC, 1,2,4,5-benzenetetracarboxylic acid = H₄BTEC) to adjust the coordination mode between the viologen ligand and metal ions. Single crystal structure analyses reveal that compounds 1 and 2 both exhibit one-dimensional (1D) chain-like structure, while compound 3 exhibits two-dimensional (2D) layered architecture. Furthermore, due to the formation of viologen radicals, reversible color changes for three compounds can be observed in the air upon sunlight, UV-light and X-ray irradiation. Particularly, compound 3 has high photosensitivity, giving an eye-detectable color change after exposure to sunlight in air within 1 min. In addition, the photoproducts for the three compounds exhibit an ultralonglived charge-separated state.