Article pubs.acs.org/IC
A Flexible Fluorescent Zr Carboxylate Metal−Organic Framework for the Detection of Electron-Rich Molecules in Solution Paul Rouschmeyer,† Nathalie Guillou,† Christian Serre,†,‡ Gilles Clavier,§ Charlotte Martineau,† Pierre Audebert,§ Erik Elkaïm,∥ Clémence Allain,*,§ and Thomas Devic*,†,‡ †
Institut Lavoisier, UMR 8180 CNRSU. Versailles St. Quentin, Université Paris-Saclay, 45 avenue des Etats-Unis, 78035 Versailles, France § PPSM, UMR 8531 CNRSENS Cachan, Université Paris-Saclay, 61 avenue du président Wilson, 94235 Cachan, France ∥ Synchrotron Soleil, beamline Cristal, L’Orme des Merisiers, Saint-Aubin, 91192 Gif-sur Yvette, France S Supporting Information *
ABSTRACT: A novel Zr(IV) dicarboxylate metal organic framework (MOF) built up from an s-tetrazine derived ligand was prepared. This solid, which exhibits a diamond type network, combines a good stability in water, a structural flexibility, and fluorescence properties thanks to the organic ligand. It is noteworthy that this fluorescence is quenched when exposed to electron-rich molecules in solution, such as amines or phenol, this phenomenon being associated with the adsorption of the quencher, as unambiguously proven by X-ray diffraction (XRD) analysis. Finally, the quenching efficiency is shown to be governed not only by electronic and steric factors but also by the relative polarity of the solvent, the MOF, and the quencher. This work thus suggests that it is possible to develop new MOF-based sensors presenting in a given medium (such as water) highly selective responses.
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medium.21−23 The Zr(IV) cation appears suitable for such a purpose, as it usually leads to chemically stable MOFs (especially in humid environment),24−29 and does not involve any absorption in the visible range. Indeed, Zr(IV) carboxylatebased MOFs efficient for optical detection (mostly by fluorescence),21−23,30−37 especially of the pH,38,39 have been reported. Regarding the fluorophore, we focused our attention on a small electron-poor luminophore, the s-tetrazine, whose fluorescence is known to be quenched by electron-rich molecules,40,41 and which has shown useful properties and applications.42−44 Few MOFs built up from s-tetrazine derived ligands have been reported to date, mostly based on M(II) cations.45−52 One of these solids was found to exhibit solvatochromism upon adsorption of solvents.53 Recently, Kaskel et al. reported the doping of the Zr(IV) terephthalate UiO-66 by the hydrotetrazinedicarboxylate (i.e., reduced form of s-tetrazine) for the detection of oxidative gases (nitrous gas and bromine) by redox reactions.54 Nevertheless, to the best of our knowledge, their fluorescence properties have not been exploited for detection purposes so far. We report here the synthesis and characterization of a Zr(IV) MOF built up from a newly designed s-tetrazine ligand, later denoted MIL-161 (MIL stands for Material Institute Lavoisier). This solid is shown to be stable in aqueous medium, structurally flexible (i.e., the pore size and shape adapt to its
INTRODUCTION While porous coordination polymers (PCPs) or metal organic frameworks (MOFs) have been primarily considered as promising candidates mainly for separation and storage purposes, their use for the detection of chemical species by adsorption is also appealing.1,2 Thanks to their porosity and high chemical and structural diversity, MOFs might indeed combine a high level of detection (thanks to the adsorption) and a tunable selectivity, based on the combination of steric (size exclusion) and electronic3 (preferential/specific host− guest interactions such as hydrogen or halogen bonds or donor−acceptor interactions) factors. Fluorescence-based optical detection by MOFs is probably the most studied case.1,2,4−7 While lanthanide-based MOFs have been extensively used, luminescence arising from the organic ligand (either as an intrinsic property or indirectly through ligand to metal or metal to ligand charge transfer) is of high interest, notably because of the likely higher sensitivity of the luminophore to the guest (especially when organic). MOFs presenting a fluorescence sensitive to inorganic anions and cations, protons, gases, and volatile organic compounds (VOCs)8 or nitroaromatic explosives (electron-poor molecules)9 have thus been studied recently. Electron-rich species, such as aromatic amines or phenol derivatives, have been addressed less frequently10−19 although their potential hazard, e.g., in water, is established.20 Our aim here was to build up a robust MOF suitable for the detection of electron-rich molecules, especially in aqueous © 2017 American Chemical Society
Received: May 2, 2017 Published: July 3, 2017 8423
DOI: 10.1021/acs.inorgchem.7b01103 Inorg. Chem. 2017, 56, 8423−8429
Article
Inorganic Chemistry
Rietveld plots) in order to shed some light on the flexible behavior (see below). MIL-161 crystallizes in a tetragonal setting, space group I4̅2d (No. 122), with cell parameters depending on the nature of the absorbed solvents (see Table 1).
content while the solid remains crystalline 55−57), and fluorescent, such fluorescence being quenched by electronrich species. Specifically, the combination of fluorescence and structural flexibility19,58−63 is used to shed some light on the quenching mechanism. Finally, the impact of the hydrophobic/ hydrophilic balance within the MOF, the electron-rich quencher, and the solvent on the quenching efficiency is addressed.
Table 1. Unit Cell Parameters of MIL-161 Dispersed in Various Solvents and in the Presence of Electron-Rich Molecules (Space Group I4̅2d)
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RESULTS AND DISCUSSION Synthesis and Structure Determination. As the simplest dicarboxylic s-tetrazine is rather unstable and poorly fluorescent, a sulfur derivative of tetrazine bearing two carboxylic groups (later labeled H2STz, see Scheme 1) was prepared at the gram scale in one step starting from the 3,6bis(3,5-dimethylpyrazol-1-yl)-s-tetrazine (see Supporting Information for details).
solvent DMF H2O EtOH EtOH EtOH EtOH H2O
Scheme 1. s-Tetrazine Dicarboxylic Acid (H2STz) Used in the Present Study
electron-rich molecule
a (Å)
c (Å)
V (Å3)
DMA TPA phenol phenol
26.1084(3) 27.6288(4) 28.301(1) 28.443(2) 28.336(4) 27.51(2) 27.493(7)
14.3685(3) 13.5007(5) 11.0641(5) 10.7384(2) 10.981(2) 14.52(1) 13.589(3)
9794.2(3) 10305.8(5) 8862.0(8) 8687(1) 8817(2) 10985(7) 10272(6)
The asymmetric unit contains two Zr(IV) ions, three μ3-oxo bridges, one STz ligand, and solvent molecules (DMF and water). This leads to the formula Zr6O8(STz)4(solvent)n, which was confirmed by infrared spectroscopy, thermogravimetric analysis, solid state NMR, and chemical analyses (see Supporting Information for details). Both Zr(IV) ions are 8-fold coordinated and assemble into the well-established Zr6O8 oxocluster (Figure 1a). Each inorganic unit is surrounded by eight carboxylate groups arising from the ligands. While MOFs built up from 12connected Zr6 oxoclusters are the most common,25−27 many recent studies have described new solids built up from carboxylate deficient Zr6 nodes (either 10, 8, or 6 connected). In the case of the 8-fold connection, the carboxylates are usually organized to define a cubic geometry (point group symmetry D4h).65−75 In the present solid, they define a rare triangular gyro-biprism or gyrobifastigium (obtained by fusing together two triangular prisms through a square face after a 90° rotation)76,77 with an ideal D2d symmetry (see Figure S7). The ligands assemble into dimers with their rings roughly parallel to each other, with one carboxylate group in the plane of the aromatic ring and one outside (see Figures 1b and S8). These dimers connect two oxoclusters together, leading on the whole to a distorted diamond (dia) type network (Figure 1c). Regarding the protonation state of the inorganic μ3 ligands (O2−/OH−), the structure solution indicates the presence of terminal DMF molecules and hence likely charge balancing oxo bridges giving rise to the formula Zr6O8(STz)4(DMF)8. Nevertheless, the final protonation state might depend on the activation procedure, and its unambiguous determination in carboxylate deficient Zr6 units is still an issue and a matter of debate.78−80 Indeed, as proposed by Zhou et al. for PCN-222,69 a washing step with an aqueous HCl solution prior to activation seems to qualitatively improve the crystallinity of MIL-161, together with a positive impact on the reversibility of the flexible behavior (see Figure S10). While the mechanism is not clear, it might be associated with acid−base reactions involving bridging and/or terminal ligands.81−84 One of the striking features of some MOFs is their structural flexibility, i.e., the adaptation of the pore size and shape to their content, while the solid remains crystalline.55−57 Adsorption of guests can in this case be directly (and unambiguously) observed by diffraction techniques, as the unit-cell parameters
As expected for such a s-tetrazine derivative, the UV−visible absorption spectrum presents two absorption bands, centered at 409 and 525 nm (ε = 818 and 481 L mol−1 cm−1 respectively) and associated with π−π* and n−π* electronic transitions, respectively.40 After excitation at either 409 or 525 nm, an emission peak centered at 580 nm is observed with a modest quantum yield (0.4(2) %), again in agreement with previous studies of thiol functionalized s-tetrazines (Figure S16). The reactivity of H2STz with Zr salts in N,N-dimethylformamide (DMF) was first investigated using the high-throughput methodology developed by Stock et al.,64 which allowed exploration of a broad range of reaction conditions with a minor amount of reactants. Few key parameters were identified, namely, the nature of the Zr(IV) precursor, the temperature, and the hydrochloric solution (HCl) content. Among the Zr(IV) precursors tested (isopropoxide, acetylacetonate, chloride), only the latter led to crystalline product. At a moderate temperature (60−80 °C) and in the presence of HCl (HCl/Zr = 4), the reaction of a stoichiometric amount of ZrCl4 with H2STz afforded a red polycrystalline powder. Using a higher temperature (100 °C) led to an improvement of the crystallinity together with the appearance of a crystalline impurity. This later was isolated as single crystals and identified as resulting from an intramolecular cyclization reaction of the ligand (see Supporting Information for details). Thus, all characterizations were then carried out on the pure material obtained at low temperature, except the structure determination from X-ray powder diffraction (XRPD), which was performed on a batch synthesized at 100 °C (see Supporting Information for details). Because of the flexible character of MIL-161 leading to a broadening of the Bragg peaks upon drying (see below and Figure S11), high resolution synchrotron data suitable for structure determination were collected while the solid was suspended in solvents. Structures were solved both in DMF and in water (see Figures S4 and S5 for the final 8424
DOI: 10.1021/acs.inorgchem.7b01103 Inorg. Chem. 2017, 56, 8423−8429
Article
Inorganic Chemistry
Figure 2. XRPD patterns of MIL-161 when dispersed in various solvents. The evolution of the 200, 101, 220, and 211 Bragg peaks is highlighted (λ = 1.5406 Å).
from the thioalkyl links in the ligand. Indeed, when comparing the structure of MIL-161 in water and DMF, while the N−C− S−CH2 dihedral angles remain rather unchanged (variation
