The impact of metal ions on photoinduced electron-transfer properties: four photochromic metal-organic frameworks based on a naphthalenediimide chromophore

Four novel photochromic metal-organic frameworks (MOFs) self-assembled from a naphthalenediimide-based ligand and different metal cations, [Zn-2(BINDI)(DMA)(2)]center dot 2DMA (1), [Cd(H4BINDI)(H2BINDI)]center dot 4DMF (2), [Ca-2(HBINDI)Cl(DMA)(3)]center dot DMA (3), [Ba-4(BINDI)(2)(DMF)(7)]center dot 7DMF (4) (H-4 BINDI = N, N'-bis(5-isophthalic acid)naphthalenediimide), have been synthesized and characterized by single-crystal X-ray diffraction, powder X-ray diffraction, elemental analyses, IR spectroscopy and TG analyses. Compound 1 exhibits a 3D porous framework with an lvt topology based on paddle-wheel Zn-2(COO)(4) clusters and BINDI as four-connected nodes. Compound 2 features a porous threefold interpenetrating diamondoid framework. Compound 3 shows a microporous 2D double-layer architecture and compound 4 displays a porous twofold interpenetrated 3D network. Interestingly, these four MOFs exhibit reversible photochromic behaviors with a concomitant eye-detectable color change, but different coloration degrees. The comparison of these MOFs indicates that the electron-withdrawing capabilities of metal cations play a significant role in tuning the photosensitive properties of photochromic MOFs; this result may provide a feasible route for the design and synthesis of photochromic MOFs with controllable photoinduced electron-transfer properties. Moreover, the photomodulated photoluminescence properties of compounds 1-4, as well as the solvatochromic behaviors of 3 and 4 were also investigated. Additionally, compounds 1-3 are found to be useful as indicators for the qualitative detection of nitrite by naked eye recognition of color change.