Qi Sui

Coordination-modulated piezochromism in metal–viologen materials

While stimuli-responsive chromic phenomena are well known for various viologen-containing organic and metal–organic materials, viologen-based piezochromism is a very recent discovery in organic compounds. Here we present the first piezochromic metal–viologen material and the modulation of the pressure-responsive behavior through coordination structures. By means of ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance, in situ/ex situ X-ray diffraction and DFT calculations, we demonstrated that a zigzag-chain CdII coordination polymer (1) with a viologen-dicarboxylate zwitterionic ligand shows reversible piezochromism, with modulations in threshold pressure and visible-light absorption (color) compared with the free ligand. We also illustrated that piezochromism can be suppressed upon coordination of the same ligand in a rigid 3D framework with the same metal ion. Two basic requirements were proposed from viologen-based piezochromism: appropriate donor–acceptor contacts providing electron transfer pathways, and structural flexibility allowing pressure to further reduce the contacts. We expect great prospects in tuning piezochromism and designing new pressure-responsive materials through diverse metal–viologen combinations. The very fast photochromic response of compound 1 at ambient pressure was also studied, which was attributed to the rather short donor–acceptor contacts in the structure.

A stable electron-deficient metal–organic framework for colorimetric and luminescence sensing of phenols and anilines

A 3D metal–organic framework (LVMOF-1) with unique electron-deficient channels was synthesized and its sensing properties for electron-rich benzene derivatives were demonstrated. The MOF is built of robust [Eu(OH)(COO)2]n columns and tetratopic viologen-based crosslinkers and shows excellent chemical stability. The structure integrates Eu(III) centers to luminesce and viologen moieties to accept electrons, and most notably, the electron-deficient viologen moieties, like those in box-like diviologen cyclophanes, are ideally spaced for sandwiching electron-rich aromatic rings. The MOF shows a bimodal response (color and luminescence) to phenols, anilines, benzenediols and aminophenols, with excellent selectivity against a wide range of other organic molecules. The chromogenic phenomena allow facile, quick and naked-eye test-paper detection of these priority contaminants in water, while the luminescence response affords very fast and sensitive quantitative detection. In particular, the detection limits for anilines and benzenediols are as low as 1–9 ppb. The charge transfer and energy transfer mechanisms for the sensing properties were elucidated on the basis of X-ray crystallography after single-crystal-to-single-crystal adsorption and orbital energy analyses according to electrochemical and spectroscopic data and also DFT calculations. The MOF bridges the gap between discrete cyclophanes functioning in solution and extended porous lattices in the solid state and can provide a blueprint for further development of sensory MOFs.

Switchable Ferro-, Ferri-, and Antiferromagnetic States in a Piezo- and Hydrochromic Metal–Organic Framework

The Mn(II) metal-organic framework with a viologen-based tetracarboxylate ligand exhibits reversible optical (color) and magnetic changes concomitant with stimuli-induced electron transfer from carboxylate to viologen. Compression causes a magnetic transformation from ferro- to ferrimagnetic, while water release/reuptake switches the magnetic behavior between ferro- and antiferromagnetic.