Photochromism and hydrochromism of three two-dimensional coordination polymers constructed from a carboxybenzyl viologen ligand

Stimulus-responsive viologen coordination polymers (CPs) have been very popular in the past few years due to the increasing demand for smart materials. In order to obtain such CPs, in this work, a flexible carboxybenzyl viologen ligand 1,1'-bis(4-carboxybenzyl)-4,4'-bipyridinium dichloride (H(2)BpybcCl(2)), and its self-assembly with transition metal ions in the presence of 1,4-benzenedicarboxylic acid (p-H2BDC) leads to the formation of three novel isostructural CPs {[TM4(Bpybc)(6)(H2O)(12)]center dot(OH)(2)center dot(p-BDC)(3)center dot 22H(2)O}(n) (TM = Co (1), Ni (2), Mn (3)). Each Bpybc ligand bridges two metal ions to create an infinite two-dimensional network. Due to the formation of Bpybc radicals, all three compounds are photochromic under UV irradiation. At the same time, the three compounds exhibit unique hydrochromic properties. The reversible color changes are caused by both electron transfer and the modulation of d-d transitions in the ligand field through dehydration and rehydration accompanied by structure transformations.