Integrated Photoresponsive Alkaline Earth Metal Coordination Networks: Synthesis, Topology, Photochromism and Photoluminescence Investigation

Design and Construction of the Uranyl Coordination Polymer with Multifunction Stimulus Response: Fluorescent Sensors for Halide Ions and Photochromism

It can provide ideas for the use of uranium elements in the treatment of spent fuel from nuclear wastewater to explore the application potential of uranium element. Thus, it is necessary to research the structure and properties of a novel uranyl coordination polymer (CP) for uranium recovery and reuse. Herein, we designed and prepared a new uranyl CP U-CMNDI based on UO22+ and H2CMNDI (H2CMNDI = N, N'-bis(carboxymethyl)-1,4,5,8-naphthalenediimide). Structural analysis shows that two uranyl ions are connected by two parallel deprotonated CMNDI ligands to form a discrete uranyl dimer structure. U-CMNDI can act as a potential stimulus-responsive halide ion sensor by a fluorescence "turn on" response in water. The limit of detection for fluoride (F-), bromide (Br-), iodide (I-), and chloride (Cl-) is 5.00, 5.32, 5.49, and 5.73 μM, respectively. The fluorescence "turn on" behavior is based on the photoinduced electron transfer (PET) mechanism between halide ions and electron-deficient NDI cores. In addition, U-CMNDI demonstrates a color response to ultraviolet light, exhibiting reversible photochromic behavior with a notable color change. The color change mechanism can contribute to the PET process and the radical process.

Dual Photo-/Electrochromic Pyromellitic Diimide-Based Coordination Polymer

By the combination of N,N'-bis(carboxymethyl)-pyromellitic diimide (H2CMPMD, 1) and zinc ions, a novel PMD-based coordination polymer (CP), [Zn(CMPMD)(DMF)1.5]·0.5DMF (2) (DMF = N,N'-dimethylformamide), has been prepared and characterized. 1 and 2 exhibit completely different photochromic properties, which are mainly reflected in the photoresponsive rate (5 s for 1 vs 1 s for 2) and coloration contrast (from colorless to light green for 1 vs green for 2). This phenomenon should be attributed to the introduction of zinc ions and the consequent formation of the distinct interfacial contacts of electron donors (EDs) and electron acceptors (EAs) (dn-π = 3.404 and 3.448 Å for 1 vs dn-π = 3.343, 3.359, 3.398, and 3.495 Å for 2), suggesting a subtle modulating effect of metal ions on interfacial contacts, photoinduced intermolecular electron transfer (PIET) and photochromic behaviors. Interestingly, the photochromic performance of 2 can be enhanced after the removal of coordinated DMF, which might be ascribed to the decrease of the distance of EDs/EAs caused by lattice shrinkage, which further improves the efficiency of PIET. Meanwhile, 2 displays rapid electrochromic behavior with an obvious reversible color change from colorless to green, which can be used in an electrochromic device. This work develops a new type of EA for the construction of stimuli-responsive functional materials with excellent dual photo-/electrochromic properties.

Stimulus-Responsive Lanthanide MOF Materials Encapsulated with Viologen Derivatives: Characterization, Photophysical Properties and Sensing on Nitrophenols

Viologens (1,1'-disubstituted 4,4'-bipyridyls) possessing electron-deficient properties and redox activity are a class of suitable chromophores to assemble metal-organic hybrid photochromic materials. Thus, viologen-functionalized metal-organic frameworks (MOFs) have attracted much attention for their photochromic properties; however, the syntheses of lanthanide-viologen hybrid crystalline photochromic materials still face many challenges. For example, the structures and properties of the final products are difficult to predict and are limited by molecular configurations. In this work, host-guest composite-material Ln-NH₂ BDC-pbpy MOFs were constructed by encapsulating viologen derivative pbpyCl₂ . The pbpy²⁺ moieties are uniformly embed by their π-π conjugation in the pores of the 3D structure by electrostatic interactions. Due to the encapsulation of the chromophore pbpy²⁺ moieties, Ln-NH₂ BDC-pbpy MOFs have reversible photochromic properties: they can change color after irradiation and can return to the original color after being protected from light or heating. Interestingly, the fluorescence intensity decreases with illumination time and recovers in the dark. As a result, Ln-NH₂ BDC-pbpy MOFs show both photochromic and photomodulated fluorescence. Based on the outstanding fluorescence performance of the Ln-NH₂ BDC-pbpy MOFs, they also show a wonderful effect for detecting nitrophenols, especially TNP.

Synthesis of a Heterometallic [Zn2Ca] Pinwheel Array Stabilized by Amide-Amide Synthons

The rational design of heterometallic compounds bearing s-block metal ions have been a difficult task for chemists owing to their lack of preferential geometries. However, some strategies, such as the design of coordinating pockets with different sizes and/or donor atoms, have offered great results. In this work, this strategy has been tested using Ca(II) as an s-block metal ion and a compound previously obtained by our group with the formula [Zn3(μ-ACA)6(4-phpy)2], which contains tetrahedral N,O- and octahedral O-coordinating pockets as a model structure. From this work, the corresponding heterometallic compound with the formula [Zn2Ca(μ-ACA)6(4-phpy)2]·EtOH (1) has been successfully synthesized, and fully characterized, and its crystal structure has been elucidated. Furthermore, we have compiled all the crystal structures containing [Zn2M] pinwheel secondary building units (SBUs), where M stands for an s-block metal ion, and the observed tendencies, as well as the promising applications as template SBUs for the preparation of 1D–3D coordination polymers, have been discussed. Finally, solid-state UV-Vis and photoluminescence have been recorded and compared with the homometallic [Zn3(μ-ACA)6(4-phpy)2] compound.

Stimuli-Responsive Naphthalenediimide Cd-MOFs Tuned by Different Aliphatic Dicarboxylic Acids with Extended Spacers

A series of novel Cd metal-organic frameworks (MOFs) (1-9) with different extended spacers with seven kinds of the aliphatic dicarboxylic acids as secondary building linkers based on N,N'-di(4-pyridylacylamino)-1,4,5,8-naphthalenediimide (NDI-A) have been designed and synthesized by changing the volume ratio of solvents under solvothermal conditions. In addition, the secondary building linkers of aliphatic dicarboxylic acids have different spacer lengths, resulting in different structures of complexes 1-9. So, their packing structures are affected by the degree of distortion of the NDI-A ligand, the different aliphatic dicarboxylic acids ligands, and the hydrogen-bonding patterns. Complexes 1-9 showed stimuli-responsive emission tuned by different aliphatic dicarboxylic acids with extended length spacers under UV light irradiation, accompanied by the color change from light orange to dark brown, and achieved reversible photochromic under heating, which indicates that they could serve as secret erasable inks. Moreover, complexes 1-9 exhibited selective vaporchromic behavior to methylamine (MA), and the vaporchromic sample could be recovered after washing with MeOH. It is worth noting that the preparation of poly(vinyl alcohol) (PVA)-NDI-MOF films enables the photochromic and vaporchromic properties of complexes 1-9 to apply in practice. In addition, complexes 1-9 exhibited good fluorescence properties as sensing probes toward 2,4,6-trinitrophenol (TNP) with lower limits of detection. In short, this work provides a broad field to explore the creative NDI-MOF materials with photoactive and luminescent properties.

Photochromic Metal-Organic Framework for High-Resolution Inkless and Erasable Printing

Inkless and erasable printing as a new technology has received intense attention in reducing paper waste and environmental hazards caused by the use of large amounts of ink. However, achieving high-resolution printing by inkless and erasable printing for practical applications remains a huge challenge. Herein, a new metal-organic framework (MOF) has been synthesized, which exhibits a reversible photochromic behavior. None of the unpaired electrons of metal ions and a unique three-dimensional network hinder electron transfer between the ligands and metal nodes, as well as between the ligands themselves, which are conducive to prolonging the photo-generated color lifetime and suitable for inkless and erasable printing. By virtue of the proper photo-generated color lifetime, strong contrast color before and after light irradiation, and reversible color transformation, a high-resolution printing content for inkless and erasable printing can be achieved by light irradiation. Notably, the paper coated with this MOF can be used for printing not only simple patterns such as pictures but also even texts for practical applications, surpassing other photochromic MOF materials for inkless and erasable printing, and almost comparable to ink and laser printing in terms of practicality and resolution. In addition, the MOF-coated paper can be reused for multiple cycles without significant deterioration.

Reversible Photochromic Coordination Polymer by Phototriggered Subtle Molecular Conformation Variations

Photochromic materials are constructed with molecules accompanied by structural change after triggering by light, which are of great importance and necessity for various applications. However, because of space-confinement effects, molecule stacking of these photoresponsive chromophores within coordination polymers (CPs) always results in an efficiency decrement and a response delay, and this phenomenon will lead to a poor photochromic property. Herein, a CP (named CIT-E) with a 3-fold-interpenetrating network structure, which was prepared with (Z)-1,2-diphenyl-1,2-bis[4-(pyridin-3-ylmethoxy)phenyl]ethene (1Z) and a CuI cluster, showed fast reversible photochromic behavior. Under UV-light illumination, the color of CIT-Z changed from pale yellow to reddish brown. With the illumination of green light, the polymer could return to its initial color within 10 s. To reveal the mechanism of reversible photochromic behavior of CIT-Z, single-crystal structures of each color state were fully studied, and other scientific study methods were also used, such as time-dependent density functional theory calculation and control experiments. It was found that, with light illumination, this behavior of CIT-Z was the result of a ligand-to-metal charge-transfer process, and this process was triggered by subtle molecular conformation variation of tetraphenylethylene. It should be noted that CIT-Z has high thermal and chemical stability, which are excellent advantages as smart photoresponsive materials. As a proof of concept, a uniform thin film with such a fascinating photochromic property allows applications in invisible anticounterfeiting and dynamic optical data storage. Overall, the present study opens up a new avenue toward reversible photochromic materials.