Novel Tb-MOF Embedded with Viologen Species for Multi-Photofunctionality: Photochromism, Photomodulated Fluorescence, and Luminescent pH Sensing

Tunable inorganic nodes and modifiable organic linkers enable designable functionality to come to fruition in new-type porous hybrid materials, namely metal–organic frameworks (MOFs). By using viologen-functionalized m-benzenedicarboxylate as an organic linker, a terbium-organic framework embedded with asymmetric viologen species has been constructed. The reversible photochromism from bright yellow to dark green is implemented due to the favorable spatial stack of the electron donor and acceptor. The photochromic component of the organic linkers can modulate the luminescence of the tetranuclear terbium cluster under irradiation of UV light, which is defined as photoluminescence switching behavior. The different protonation levels of uncoordinated N atoms with varying pH enable the MOF as a potential fluorescent pH sensor. Thus, multiphotofunctionality, viz. photoluminescence, photochromism as well as the derivate fluorescent response to irradiation and pH have been combined in the Tb-MOF, which is the first example in the viologen-based photochromic hybrid materials.