Photochromic properties of three 2D MOFs based on 1-carboxyethyl-4,4'-bipyridinine

Viologen units have been widely used to impart metal-organic frameworks (MOFs) with photochromic properties. However, construction of a stable photochromic system in viologen MOFs has not been fully explored. Herein, we report three examples of MOFs, namely, {[Cd(CEbpy)(m-BDC)(DMF)]·2H₂O} (1), {[Cd(CEbpy)(p-BDC)(H₂O)]·H₂O} (2), and {[Zn(CEbpy)(p-HBDC)(p-BDC)_0.5]·H₂O} (3) based on benzenedicarboxylic acids (m-H₂BDC = 1,3-benzenedicarboxylic acid, p-H₂BDC = 1,4-benzenedicarboxylic acid) and a viologen-derived ligand 1-carboxyethyl-4,4'-bipyridine (L = CEbpy). As expected, the incorporation of the viologen unit into the frameworks results in the predefined photochromism upon both sunlight and UV-light. Compounds 1-3 feature a two-dimensional (2D) layered structure and are all photochromic due to the formation of CEbpy radicals by photoinduced electron transfer (PET). The aggregates build an interesting stable crystalline framework that exhibits long-lived color constancy in the solid state.