N-Functionality actuated improved CO2 adsorption and turn-on detection of organo-toxins with guest-induced fluorescence modulation in isostructural diamondoid MOFs

Abstract

Functional-group-assisted pore environment modulation in metal–organic frameworks (MOFs) can support the substantial development of their applications, and structure–property synergy can be best described via comparisons between isoskeletal MOFs that differ in functionality. Herein, we elaborate the effects of functional disparity on selective carbon dioxide (CO₂) adsorption and the luminescence detection of lethal pollutants using a pair of structurally similar and 4-fold interpenetrated dia-networks, CSMCRI-7 and CSMCRI-8 (CSMCRI = Central Salt & Marine Chemicals Research Institute), that vary by a spare nitrogen atom in the constituent bifunctional struts. Pore-wall decoration with accessible triazole ring N-atoms nearly doubles the CO₂ uptake capacity in desolvated CSMCRI-8 (8a) with a 10.9 kJ mol⁻¹ rise in adsorption enthalpy compared to 7a. Interestingly, CO₂ selectivity over N₂ (322.2) displays a two-fold enhancement, validating the strong effects of channel functionalization; this is further supported via microscopic adsorption mechanism studies based on grand canonical Monte Carlo simulation. Benefiting from a strong emission signature and nitrogen-rich pore surface, 8a represents the first-ever turn-on luminescent MOF (LMOF) for highly specific, recyclable and fast responsive detection of toxic 3-(diethylamino)phenol (DEP) and the lethal pesticide isoproturon (IPT), with nanomolar limit of detection (LOD) values (DEP: 79.8 nM; IPT: 226 nM). Importantly, guest-responsive fluorescence modulation is evidenced through the remarkable turn-off sensing of the pesticide dichloran with admirable selectivity, a quick response time, and a very low LOD (124 nM), with sharp colorimetric changes under UV light. The accessibility of the nitrogen sites plays a crucial role in the sensing performance, which is supported by periodic-density functional theory (DFT) calculations, with the unique shuffling of the molecular orbital energy levels of the diamondoid MOFs in the presence of each organo-aromatic, further demonstrating superior framework-analyte non-covalent interactions in the pore-engineered structure.