Cationic Metal–Organic Frameworks Based on Linear Zwitterionic Ligands for Cr₂O₇^2– and Ammonia Sensing

Abstract

A series of cationic lanthanide-based metal–organic frameworks (MOFs), {[Ln3(bcbp)3(NO3)7]·NO3·ClO4} n (Ln = Tb (1), Gd (2), Eu (3); H2bcbpCl2 = 1,1′-bis­(4-carboxyphenyl)­(4,4′-bipyridinium) dichloride), have been synthesized under solvothermal conditions and structurally characterized. Single-crystal X-ray diffraction analyses reveal that compounds 1–3 are isostructural, in which Ln3-SBUs (SBUs = secondary building units) are connected with six adjacent Ln3-SBUs by six protonated bcbp ligands, leading to cationic lanthanide-based metal–organic frameworks. In addition, the counteranions (nitrate anions and perchlorate anions) reside in the frameworks via weak C–H···O hydrogen bonds. Furthermore, Eu-MOF displays strong red luminescence, which is further proven to be a highly selective and sensitive luminescent sensor toward Cr2O7 2– with a high quenching coefficient and low detection limit. Interestingly, Eu-MOF is also sensitive to ammonia and exhibits a rapid color change from yellow to blue-gray that can be distinguished by the naked eye when it is exposed to ammonia vapor. The UV–vis, IR, Raman, and EPR data reveal that the color change can be ascribed to an electron-transfer process and the radical generation caused by the nucleophilic attack of ammonia molecules.

Metal–organic framework bearing new viologen ligand for ammonia and Cr2O72− sensing

“…It is well known that ammonia molecules with good electron donating ability can interact with the Lewis acid centers of viologen-containing compounds 1, 2 and 3 through acid-base interactions. 11,25,37,38 The results indicate that the color changes of compounds 1, 2 and 3 during ammonia sensing may be the result of electron transfer from electron-rich ammonia molecules to viologen. The response time is basically the same as that of ammonia vapor sensors that have been reported in recent years (Table S8 †).…”

“…Or, by using the synergistic effect between these components, the hybrid crystal network can theoretically endow various response characteristics, and become an excellent material with a variety of stimulus response capabilities. 1,[7][8][9][10][11][12][13] Anionic MOFs with a negatively charged skeleton are a subcategory of MOFs and have unique applications in gas adsorption, sensors, uorescent probes, and so on. [14][15][16] 4,4 0 -Bipyridine and its derivatives (viologen and its derivatives) are a typical electron-decient substance with a strong tendency to carry out electron transfer (ET) and induce energy transfer (EnT).…”

“…And even at low concentrations, there is a certain degree of toxicity. 11,25,37,38 On the other hand, although Cr 2 O 7 2À is widely used in industry, it has carcinogenic effects on the human body and is an environmental pollutant. 11,33,39,40 Therefore, it is particularly important and urgent to design a sensor for detecting ammonia and Cr 2 O 7…”

Mechanistic Insight into Charge and Energy Transfers of Luminescent Metal–Organic Frameworks Based Sensors for Toxic Chemicals

“…The viologen moiety in the linker promoted the interaction between ammonia and the MOF. [208] The presence of electronic charges on the solvents and MOFs should aid recognition. Recently, Reproduced with permission.…”

“…The viologen moiety in the linker promoted the interaction between ammonia and the MOF. [ 208 ] The presence of electronic charges on the solvents and MOFs should aid recognition. Recently, 2‐phenyl‐1 H ‐imidazole‐4,5‐dicarboxylic acid‐linked Tb(III)‐MOF and Eu(III)‐MOF materials exhibited good sensing potential for formic acid.…”