Two new Zinc (II)-viologen coordination polymers: Syntheses, structures, and photochromic behaviors

The self-assembly of Zn (II) ions with monosubstituted viologen ligand 1-(3-carboxybenzyl)-4,4’-bipyridinium chloride (m-Hbcbpy·Cl) and 1,3,5-benzenetricarboxylic acid (H3BTC) under solvothermal conditions results in two coordination polymers, namely, {[Zn(bcbpy)Cl2]·H2O}n (1) and {[Zn(bcbpy)(HBTC)H2O]·2H2O}n (2). Due to the formation of viologen radicals, the two compounds exhibit a triple photochromic behavior to sunlight, UV light and blue light through electron transfer, and the sequence of light response time is 1 > 2. Photochromic mechanisms and the relationships between structures and properties have been discussed adequately. And using time-dependent density functional theory (TDDFT) to study the discoloration mechanism of compounds 1 and 2. More importantly, compound 1 can be deposited on filter paper and displays potential applications as an inkless printing material. In addition, the capabilities of compound 2 to act as anti-counterfeiting materials for QR codes have also been investigated.