Construction of Pillar-Layer Metal–Organic Frameworks for CO2 Adsorption under Humid Climate: High Selectivity and Sensitive Detection of Picric Acid in Water

Adsorption of CO2 under humid conditions is important, as flue gases contain some degree of moisture, while aqueous-phase sensing of nitro-aromatic compounds is critical for environmental protection and anti-terrorism activities. However, implementing both of these aspects in metal–organic frameworks (MOFs) is rare and challenging due to their moisture instability. To this end, we prepared three isostructural, pillar-layer Zn­(II) MOFs where criss-cross pillaring by the linkers tunes the pore opening and pore electronic environment that in turn modulate thermal and/or moisture stabilities. While activated 2 (2′), incorporating an azo group in the linker, exhibits excellent CO2/N2 selectivity (>200), 1′, containing a 4,4′-bipyridine linker displays superior hydrolytic stability with minimum loss in CO2 adsorption–desorption cycles up to 10 days of water vapor exposure. However, framework 3, with a bis­(4-pyridyl)­ethylene linker, is unstable. Importantly, aqueous-phase sensitive detection of picric acid (PA) has been achieved through fluorescence quenching, where the quenching constant for 2′ (3.11 × 104 M–1) is found to be almost double that for 1′ (1.53 × 104 M–1). A combination of experimental and mechanistic studies reveals that the concurrent presence of dynamic and static quenching as well as resonance energy transfer are responsible for such a high fluorescence quenching in 2′. Moreover, strong non-covalent interactions, as observed in the co-crystal of PA and 4-azopyridine linker, provide direct evidence. Together, CO2 adsorption under humid conditions, high selectivity, and very low limit of PA detection in the aqueous phase manifest the present MOFs as potential materials for sustainability.