Ultrafast visible-light photochromic properties of naphthalenediimide-based coordination polymers for the visual detection/filtration of blue light†
Two novel naphthalenediimide-based (NDI-based) coordination polymers (CPs), namely [Cd(3-PMNDI)(HNDC)2(DMF)] (1) and [Cd(4-PMNDI)(NDC)]·DMF (2) (H2NDC = 1,4-naphthalenedicarboxylic acid, 3-/4-PMNDI = N,N′-bis(3-/4-pyridylmethyl)-1,4,5,8-naphthalenediimide, DMF = N,N′-dimethylformamide), have been designed and synthesized here from positional isomeric electron-deficient PMNDI (electron acceptors, EAs) and electron-rich H2NDC (electron donors, EDs) in the presence of cadmium ions. The usage of the positional isomeric PMNDI ligands leads to the generation of a one-dimensional (1D) chain in 1 and a two-fold interpenetrated three-dimensional (3D) framework in 2. Interestingly, both compounds exhibit outstanding ultrafast visible-light photochromic properties based on photoinduced intermolecular electron transfer (PIET). The higher sensitivity of 1 and 2 to visible light relative to UV light could be ascribed to the typical arrangements of EDs/EAs in the interfacial contacts (ED–EA–ED in 1vs. ED–EA–EA–ED in 2). Furthermore, the difference in the number and strength of weak interactions (lone pair–π and π–π interactions) between EDs and EAs might be responsible for the discriminative photoresponsive rates (5 s for 1vs. 10 s for 2). Meanwhile, the different coloration states might be contributed by the distinct concentrations of the [PMNDI]˙− anionic radicals and their delicately different matrices or surrounding environments. Notably, both 1 and 2 are highly photosensitive to blue-light wavelengths, which can be viewed as typical of photochromic materials for the detection and filtration of blue light.