Neutral Luminescent Metal-Organic Frameworks: Structural Diversification, Photophysical Properties, and Sensing Applications

Abstract

Utilizing flexible bis(tridentate)polypyridyl ligands, the two new luminescent 2D metal organic frameworks {Zn(tpbn)(2,6-NDC)} (1) and {[Zn(tphn)(2,6-NDC)]·4HO} (2), where tpbn = N,N',N″,N‴-tetrakis(2-pyridylmethyl)-1,4-diaminobutane, tphn = N,N',N″,N‴-tetrakis(2-pyridylmethyl)-1,6-diaminohexane, and 2,6-HNDC = 2,6-naphthalenedicarboxylic acid, have been isolated in good yields under solvothermal conditions. Their solid-state molecular structures have been determined by single-crystal X-ray diffractometry. Both 1 and 2 have pentacoordinated Zn(II) centers with an NO environment from three nitrogen atoms of the tpbn or tphn ligand and two carboxylate oxygen atoms from two different 2,6-NDC linkers. However, the binding modes of the tridentate part of polypyridyl ligands to the Zn(II) center are different in 1 and 2-meridional (tpbn) vs facial (tphn) due to an increase (1.5 times) in the methylene chain length. Thus, the binding mode of 2,6-NDC to the Zn(II) center differs: bis(monodentate) syn-anti in 1 and bis(monodentate) syn-syn in 2. This difference in binding modes of the components has a profound effect on the conformation of the six-membered ring (metal centers are considered as the vertices in it) within the 2D framework: honeycomb vs chair form for 1 and 2, respectively. In addition to further characterization by elemental analysis and UV-vis and FT-IR spectroscopy, their framework stabilities in water and thermal properties have been studied by powder X-ray diffraction and thermogravimetric analysis, respectively. On the basis of thermodiffractometry, 1 and 2 retain their crystallinity and overall structure up to 350 and 325 °C, respectively. Their luminescent properties have been utilized to demonstrate sensing of various solvents as well as nitro-aromatic compounds in water, which correlate well with their structural differences. Through the spectral overlap, lifetime measurements, and nature of the Stern-Volmer plots, the fluorescence quenching pathway for the nitro-analytes, particularly 2,4,6-trinitrophenol (TNP), is established for 1 and 2. Their recyclability and stability after sensing experiments are found to be excellent.

Structural Diversity in Luminescent MOFs Containing a Bent Electron‐rich Dicarboxylate Linker and a Flexible Capping Ligand: Selective Detection of 4‐Nitroaniline in Water

“…Since both the values are close to 0, it confirms that both have more towards square pyramidal geometry . It is noteworthy to observe that 1 is dinuclear compound instead of a polymeric framework as obtained in case of {Zn 2 (tpbn)(2,6‐ndc) 2 } n (where, 2,6‐ndc=2,6‐naphthalene dicarboxylate, a linear linker), a dinuclear complex is obtained which is due to the presence of a free carboxylate obstructing the growth of the polymer. However, very strong inter‐ and intramolecular H‐bonding interactions are present that leads to the formation of a 1D supramolecular assembly as shown in Figures b,c.…”

“…On the basis of systematic studies, we have recently demonstrated the role of the flexible chain length in the bis(tridentate)polypyridyl capping ligands that connect the two metal ions in the formation of diverse MOFs . Using d 10 metal ions (Zn 2+ and Cd 2+ ) as well as fluorescent linkers containing π‐conjugated moieties (naphthalene and anthracene groups), we continue to develop LMOF sensors for the detection of NACs . In these three‐component systems, we explore our efforts to showcase the effect of shape (linear vs. bent) and an increase in the number of π‐conjugated moieties in the dicarboxylate linkers on their sensing abilities.…”

Construction of Zn(II) Linear Trinuclear Secondary Building Units from A Coordination Polymer Based on α-Acetamidocinnamic Acid and 4-Phenylpyridine

“…In addition, the Zn-N bond lengths are slightly longer than the Zn-O bonds, with values of 2.054(4) and 2.072(4) Å. These values are similar to other Zn(II) compounds with Zn-O coordination through carbonyl groups [54][55][56] or carboxylate ligands which display monodentate coordination modes in combination with pyridine derivative ligands [57,58].…”

A 4′-(4-carboxyphenyl)-3,2′:6′,3′′-terpyridine-based luminescent cadmium(II) coordination polymer for the detection of 2,4,6-trinitrophenol

“…CPs based on 3-Hcptpy should have electron-rich architectures and usually exhibit strong luminescence peoperties, and can thus be developed as sensors for the detection of electron-deficient analytes, such as heavy-metal cations, nitro aromatic compounds (NACs) etc. (Chakraborty & Mandal, 2017). NACs, such as 2,4,6-trinitrophenol (TNP), 2,4-dinitrophenol (2,4-DNP), dinitrotoluene (DNT), 1,3-dinitrobenzene (1,3-DNB) and nitrobenzene (NB), have been widely used in ISSN 2053ISSN -2296 # 2019 International Union of Crystallography the military, dye, agriculture and pharmaceutical industries (Chaudhari et al, 2013;Sathish et al, 2017).…”