A donor-acceptor π-stacked multifunctional Eu-MOF based on a novel viologen-like photosensitive ligand: Degradation of tetracycline, photochromism, and photoluminescence

The photosensitive building blocks of photochromic MOFs are mostly 4,4′-bipyridine. However, the linear structure and kind of 4,4′-bipyridine derivatives limit the development of its properties and applications. Therefore, a “Y"-shaped 2,4,6-tri (4-pyridyl)-1,3,5-triazine derivative ([H₃p-Batpt]Cl₃, TPT) with high conjugation, bare nitrogen and oxygen sites that was selected as a novel photosensitive ligand to replace the traditional 4,4′-bipyridine ligand for photochromic MOFs. Then, [H₃p-Batpt]Cl₃ was combined with Eu^III and terephthalic acid (H₂TPA) to construct a multifunctional Eu-MOF [Eu₂ (p-Batpt) (TPA)₃(H₂O)]n (1). 1 possess degradation of tetracycline (TC), photochromic and photoluminescence properties. The colored 1 (1P) after irradiation with UV–vis light (300 W, Xe-lamp) is difficult to fade due to donor-acceptor (D-A) π-π and radical-π interactions. In addition, 1 is used in photocatalysis because it is an excellent N-type semiconductor. The TC degradation rate of 1 can reach 67.42 % under visible light and be recycled six times. Interestingly, the TC degradation rate of 1P was 6.78 % higher than that of 1. Electrochemical tests show that 1P has higher charge separation efficiency and a smaller band gap. The free radicals capture and ESR experiments of 1 revealed that holes (h⁺) played a leading role in the photocatalytic degradation of TC. This research expands the application of photochromic MOFs and provides a reference for removing organic pollutants.