Modification of Extended Open Frameworks with Fluorescent Tags for Sensing Explosives: Competition between Size Selectivity and Electron Deficiency

Abstract

Three new electron-rich metal-organic frameworks (MOF-1-MOF-3) have been synthesized by employing ligands bearing aromatic tags. The key role of the chosen aromatic tags is to enhance the p-electron density of the luminescent MOFs. Single-crystal X-ray structures have revealed that these MOFs form three-dimensional porous networks with the aromatic tags projecting inwardly into the pores. These highly luminescent electron-rich MOFs have been successfully utilized for the detection of explosive nitroaromatic compounds (NACs) on the basis of fluorescence quenching. Although all of the prepared MOFs can serve as sensors for NACs, MOF-1 and MOF-2 exhibit superior sensitivity towards 4-nitrotoluene (4-NT) and 2,4-dinitrotoluene (DNT) compared to 2,4,6-trinitrotoluene (TNT) and 1,3,5-trinitrobenzene (TNB).MOF-3, on the other hand, shows an order of sensitivity in accordance with the electron deficiencies of the substrates. To understand such anomalous behavior, we have thoroughly analyzed both the steady-state and time-resolved fluorescence quenching associated with these interactions. Determination of static Stern-Volmer constants (K S ) as well as collisional constants (K C ) has revealed that MOF-1 and MOF-2 have higher K S values with 4-NT than with TNT, whereas for MOF-3 the reverse order is observed. This apparently anomalous phenomenon was well corroborated by theoretical calculations. Moreover, recyclability and sensitivity studies have revealed that these MOFs can be reused several times and that their sensitivities towards TNT solution are at the parts per billion (ppb) level.

Photoluminescent Metal–Organic Frameworks for Gas Sensing

“…Similar with common luminescent materials, the luminescence of MOFs can be influenced by many environmental factors,3, 4 including physical stimulus and chemical stimulus (ions,30, 31, 32, 33 solvents,34, 35, 36 vapors,37 pH,38, 39 gases,1 and so on40, 41, 42, 43, 44, 45). However, unlike other luminescent materials, the intrinsic porosity, large surface area, and strong adsorption affinity of MOFs can effectively enrich guest species.…”

The vapour phase detection of explosive markers and derivatives using two fluorescent metal–organic frameworks

“…Pioneering work within this field was performed by Li et al who demonstrated the sensitive detection of DMNB and 2,4-DNT in the vapour phase. 37 This work inspired a number of other researchers who subsequently demonstrated the successful detection of explosives, with some selectivity, using zinc, [38][39][40][41][42][43] cadmium, [44][45][46] lithium, 47 indium, 48 europium, [49][50][51][52] and terbium containing metal-organic frameworks. 53 However, the majority of the published research reports the detection of explosives using MOFs through solution-based titrations.…”

Self‐Assembled Discrete Molecules for Sensing Nitroaromatics

“…In this regards, supramolecular polymers and metal-organic frameworks (MOFs) have emerged as highly promising materials in recent times for sensing NACs explo- TNT high [34] sives. [35][36][37] Our designed discrete systems are quite selective for electron poor nitroaromatics. After achieving such selectivity, the next objective would be to enhance sensitivity so that the presence of even a few molecules of explosive can be detected efficiently.…”