Photochromic and photomodulated luminescence properties of two metal–viologen complexes constructed by a tetracarboxylate-anchored bipyridinium-based ligand

In this study, two metal-viologen complexes, formulated as [Cd-1.5(H2L)(0.5)(Cl)(3)(CH3OH)](n) (1) and [Eu-2(L)(NO3)(4)(HOCH2CH2OH)(2)](n) (2), were constructed by a tetracarboxylate anchored bipyridinium-based ligand 1,1-bis(3,5-dicarboxybenzyl)-4,4-bipyridinium dichloride (H4LCl2). Single-crystal X-ray analyses revealed that complex 1 featured a 2D-layered structure, while complex 2 showed a 1D ladder-like chain structure. The difference in the central metal ions, anions, solvent molecules, deprotonation degree of the tetracarboxylate ligand, coordination modes and conformations of the ligand have a great influence on the final structure. As expected, incorporation of the viologen moiety to the frameworks led to the predefined photochromism: when samples 1 and 2 were exposed to UV-visible light, a reversible color change from light yellow to blue/dark blue occurred. Moreover, complex 2 exhibited the characteristic emission of Eu(iii) ion in the visible region, and it also displayed an interesting reversible tunable photo-induced luminescence switching behavior: under the irradiation of UV-vis light, the luminescence emission intensity of complex 2 gradually decreased and reached 15% of the starting value after 8 min. These results may not only help to further understand the structure-photoresponse relationships of viologen-based MOFs, but also help to guide the design and synthesis of luminescent materials with photo-modulated switching ability.