Fluorescence Turn-On Response Amplified by Space Confinement in Metal–Organic Frameworks

Sensitive fluorescence turn-on response to specific substances is highly desired for development of chemical sensors and switches. Here we utilized a “two-in-one” strategy to prepare ionic metal–organic frameworks (MOFs) functionalized with the cationic bipyridinium receptors at the frameworks and anionic fluorescent indicators in the pores. The MOFs are rendered a fluorescence-resting state because the indicator’s fluorescence is efficiently quenched by the ground-state charge-transfer (CT) complexation between the indicator and receptor. Addition of an alkylamine efficiently turns on the fluorescence because the indicator is displaced by the CT complexation between alkylamine with receptor. The turn-on response is highly specific to alkylamines. The MOFs can be used as recyclable sensors for selective and sensitive detection of alkylamines, with ultralow detection limits (0.5 nM). The fluorescence in solid state can be reversibly switched on and off with high contrast. The sensitive and high-contrast response can be attributed to the space confinement effects of the porous frameworks. The confined space can significantly enhance indicator–receptor and analyte–receptor interactions, and thereby both the quenching efficiency in the off state and the displacement efficiency in the on state are amplified

Modulating Excitation Energy of Luminescent Metal–Organic Frameworks for Detection of Cr(VI) in Water

Luminescent metal–organic frameworks (LMOFs) are newly emerged sensory materials, but rational design of LMOFs for specific analytes still faces many challenges for lack of sufficient knowledge about sensory mechanisms and effective approaches to match frameworks to analytes. In this paper, the highly fluorescent thiazolo­[5,4-d]­thiazole-extended viologen chromophore was for the first time incorporated to obtain a Zn­(II) LMOF. The LMOF exhibits excellent stability in common solvents including boiling water and acidic to basic media. It can selectively sense CrO42– and Cr2O72– in water with high sensitivity and good recyclability, the performance exceeding that of the free ligand and most previous LMOFs for sensing of Cr­(VI). By plotting the quenching ratio against excitation energy, we obtained profiles that closely resemble the Cr­(VI) absorption spectra. This illustrates a simple but useful experimental protocol to provide definite evidence for the sensing mechanism of competitive optical absorption. On the basis of density functional theory calculations in comparison with an isoreticular MOF, we demonstrate that the excitation energy can be modulated by varying the core motif of the extended viologen ligand to gain a better match between sensors and analytes

Modulating Excitation Energy of Luminescent Metal–Organic Frameworks for Detection of Cr(VI) in Water

Luminescent metal–organic frameworks (LMOFs) are newly emerged sensory materials, but rational design of LMOFs for specific analytes still faces many challenges for lack of sufficient knowledge about sensory mechanisms and effective approaches to match frameworks to analytes. In this paper, the highly fluorescent thiazolo­[5,4-d]­thiazole-extended viologen chromophore was for the first time incorporated to obtain a Zn­(II) LMOF. The LMOF exhibits excellent stability in common solvents including boiling water and acidic to basic media. It can selectively sense CrO42– and Cr2O72– in water with high sensitivity and good recyclability, the performance exceeding that of the free ligand and most previous LMOFs for sensing of Cr­(VI). By plotting the quenching ratio against excitation energy, we obtained profiles that closely resemble the Cr­(VI) absorption spectra. This illustrates a simple but useful experimental protocol to provide definite evidence for the sensing mechanism of competitive optical absorption. On the basis of density functional theory calculations in comparison with an isoreticular MOF, we demonstrate that the excitation energy can be modulated by varying the core motif of the extended viologen ligand to gain a better match between sensors and analytes