Teng Gong

Pillared-Layer Metal–Organic Frameworks for Improved Lithium-Ion Storage Performance

Recently, more and more metal-organic frameworks (MOFs) have been directly used as anodic materials in lithium-ion batteries, but judicious design or choice of MOFs is still challenging for lack of structural-property knowledge. In this article we propose a pillared-layer strategy to achieve improved Li-storage performance. Four Mn(II) and Co(II) MOFs with mixed azide and carboxylate ligands were studied to illustrate the strategy. In these 3D MOFs, layers (1, 3, and 4) or chains (2) with short bridges are linked by long organic spacers. All the MOFs show very high lithiation capacity (1170-1400 mA h g-1 at 100 mA g-1) in the first cycle owing to the rich insertion sites arising from the azide ion and the aromatic ligands. After the formation cycles, the reversible capacities of the anodes from 1, 3, and 4 are kept at a high level (580-595 mA h g-1) with good rate and cycling performance, while the anode from 2 undergoes a dramatic drop in capacity. All the MOFs lose the crystallinity after the first cycle. While the amorphization of the chain-based framework of 2 leads to major irreversible deposit of Li ions, the amorphous phases derived from the pillared-layer frameworks of 1, 3, and 4 still retain rich accessible space for reversible insertion and diffusion of active Li ions. Consistent with the analysis, electrochemical impedance spectra revealed that the pillared-layer MOFs led to significantly smaller charge-transfer resistances than 2. Soft X-ray absorption spectroscopy suggested that no metal conversion is involved in the lithiation process, consistent with the fact that the isomorphous Co(II) (3) and Mn(II) (4) MOFs are quite similar in anodic performance.

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.