Recent Developments in the Preparation and Chemistry of Metallacycles and Metallacages via Coordination

The detection of explosives is one of the current pressing concerns in global security. In the past few decades, a large number of emissive sensing materials have been developed for the detection of explosives in vapor, solution, and solid states through fluorescence methods. In recent years, great efforts have been devoted to develop new fluorescent materials with various sensing mechanisms for detecting explosives in order to achieve super-sensitivity, ultra-selectivity, as well as fast response time. This review article starts with a brief introduction on various sensing mechanisms for fluorescence based explosive detection, and then summarizes in an exhaustive and systematic way the state-of-the-art of fluorescent materials for explosive detection with a focus on the research in the recent 5 years. A wide range of fluorescent materials, such as conjugated polymers, small fluorophores, supramolecular systems, bio-inspired materials and aggregation induced emission-active materials, and their sensing performance and sensing mechanism are the centerpiece of this review. Finally, conclusions and future outlook are presented and discussed.

Fluorescence based explosive detection: from mechanisms to sensory materials.

There is great need for stand-alone luminescence-based chemosensors that exemplify selectivity, sensitivity, and applicability and that overcome the challenges that arise from complex, real-world media. Discussed herein are recent developments toward these goals in the field of supramolecular luminescent chemosensors, including macrocycles, polymers, and nanomaterials. Specific focus is placed on the development of new macrocycle hosts since 2010, coupled with considerations of the underlying principles of supramolecular chemistry as well as analytes of interest and common luminophores. State-of-the-art developments in the fields of polymer and nanomaterial sensors are also examined, and some remaining unsolved challenges in the area of chemosensors are discussed.

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Supramolecular Luminescent Sensors

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 -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.

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

π-Electron rich small molecule sensors for the recognition of nitroaromatics

[2 + 4] self-assembly of a pyrene-functionalized PtII8 tetragonal prism (2) is achieved using a newly designed star-shaped organometallic acceptor (1) in combination with an amide-based “clip” donor (L). The propensity of this prism (2) as a selective sensor for nitroaromatics (2,4-dinitrotoluene, 1,3,5-trinitrotoluene, and picric acid), which are the chemical constituents of many commercial explosives, has been examined.

Self-Assembly of an Octanuclear Platinum(II) Tetragonal Prism from a New PtII4 Organometallic Star-Shaped Acceptor and Its Nitroaromatic Sensing Study

Self-assembly of a chloro-bridged half-sandwich p-cymene ruthenium(II) complex [Ru2(μ-Cl2)(η6-p-cymene)2Cl2] 1 with linear ditopic donor [L; trans-1,2-bis(4-pyridyl)ethylene] in presence of 2 eq. AgNO3 in CH3CN yielded a chloro-bridged molecular rectangle 2. The rectangle 2 was isolated as nitrate salt in high yield (90 %) and characterized by infra-red, 1H NMR spectroscopy including ESI–MS analyses. Molecular structure of 2 was determined by single crystal X-ray diffraction study The diffraction analysis shows that 2 adopts a tetranuclear rectangular geometry with the dimensions of 5.51 A × 13.29 A and forming an infinite supramolecular chain with large internal porosity arising through multiple π–π and CH–π interactions between the adjacent rectangles. Furthermore, rectangle 2 is used as selective receptor for phenolic-nitroaromatic compounds such as picric acid, dinitrophenol and nitrophenol. A Chloro-bridged molecular rectangle is synthesized via two-component self-assembly and its molecular structure is confirmed by various spectroscopic methods including X-ray diffraction analysis. In addition, tendency of this rectangle as receptor for phenolic nitroaromatics has been investigated.

/pdf/self-assembly-of-chloro-bridged-arene-ruthenium-based-4jks0dicp6.pdf

Self-assembly of Chloro-Bridged Arene–Ruthenium Based Rectangle: Synthesis, Structural Characterization and Sensing Study

Coordination self-assembly of a series of tetranuclear Pt(II) macrocycles containing an organometallic backbone incorporating ethynyl functionality is presented. The 1 : 1 combination of a linear acceptor 1,4-bis[trans-Pt(PEt3)2(NO3)(ethynyl)]benzene (1) with three different dipyridyl donor ‘clips’ (Laa–Lcc) afforded three [2 + 2] self-assembled PtII4 macrocycles (2a–2c) in quantitative yields, respectively [Laa = 1,3-bis(3-pyridyl)isothalamide; Lbb = 1,3-bis(3-pyridyl)ethynylbenzene; Lcc = 1,8-bis(4-pyridyl)ethynylanthracene]. These macrocycles were characterized by multinuclear NMR (1H and 31P); ESI-MS spectroscopy and the molecular structures of 2a and 2b were established by single crystal X-ray diffraction analysis. These macrocycles (2a–2c) are fluorescent in nature. The amide functionalized macrocycle 2a is used as a receptor to check the binding affinity of aliphatic acyclic dicarboxylic acids. Such binding affinity is examined using fluorescence and UV-Vis spectroscopic methods. A solution state fluorescence study showed that macrocycle 2a selectively binds (KSV = 1.4 × 104 M−1) maleic acid by subsequent enhancement in emission intensity. Other aliphatic dicarboxylic acids such as fumaric, succinic, adipic, mesaconic and itaconic acids caused no change in the emission spectra; thereby demonstrating its potential use as a macrocyclic receptor in distinction of maleic acid from other aliphatic dicarboxylic acids.

Coordination self-assembly of tetranuclear Pt(II) macrocycles with an organometallic backbone for sensing of acyclic dicarboxylic acids

Three π-electron rich fluorescent supramolecular polymers (1–3) have been synthesized incorporating 2-methyl-3-butyn-2-ol groups in reasonable yield by employing Sonagashira coupling. They were characterized by multinuclear NMR (1H, 13C), ESI-MS and single crystal X-ray diffraction analyses [1 = 1-(2-methyl-3-butyn-2-ol)pyrene; 2 = 9,10-bis(2-methyl-3-butyn-2-ol)anthracene; 3 = 1,3,6,8-tetrakis(2-methyl-3-butyn-2-ol)pyrene]. Single crystal structures of 1–3 indicated that the incorporation of hydroxy (–OH) groups on the peripheral of the fluorophores helps them to self-associate into an infinite supramolecular polymeric network via intermolecular hydrogen bonding interactions between the adjacent discrete fluorophore units. All these compounds showed fluorescence characteristics in chloroform solution due to the extended π-conjugation and were used as selective fluorescent sensors for the detection of electron deficient nitroaromatics. The changes in photophysical properties of fluorophores (1–3) upon complex formation with electron deficient nitroaromatic explosives were studied in chloroform solution by using fluorescence spectroscopy. All these fluorophores showed the largest quenching response with moderate selectivity for nitroaromatics over various other electron deficient/rich aromatic compounds tested (Chart 1). Analysis of the fluorescence titration profile of 9,10-bis(2-methyl-3-butyn-2-ol)anthracene fluorophore (2) with 1,3,5-trinitrotoluene/2,4-dinitrotoluene provided evidence that this particular fluorophore detects nitroaromatics in the nanomolar range [2.0 ppb for TNT, 13.7 ppb for DNT]. Moreover, sharp visual color change was observed upon mixing nitroaromatic (DNT) with fluorophores (1–3) both in solution as well as in solid phase. Furthermore, the vapor-phase sensing study of thin film of fluorophores (1–3) showed efficient quenching responses for DNT and this sensing process is reproducible. Selective fluorescence quenching response including a sharp visual color change for nitroaromatics make these tested fluorophores (1–3) as potential sensors for nitroaromatic compounds with a detection limit of ppb level.

Electron rich supramolecular polymers as fluorescent sensors for nitroaromatics

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Self-Assembly of an Octanuclear Platinum(II) Tetragonal Prism from a New PtII4 Organometallic Star-Shaped Acceptor and Its Nitroaromatic Sensing Study

Self-assembly of Chloro-Bridged Arene–Ruthenium Based Rectangle: Synthesis, Structural Characterization and Sensing Study

Coordination self-assembly of tetranuclear Pt(II) macrocycles with an organometallic backbone for sensing of acyclic dicarboxylic acids

Electron rich supramolecular polymers as fluorescent sensors for nitroaromatics