A Flexible Fluorescent Zr Carboxylate Metal-Organic Framework for the Detection of Electron-Rich Molecules in Solution

Article abstract of DOI:10.1021/acs.inorgchem.7b01103

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.

Full text of DOI:10.1021/acs.inorgchem.7b01103

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,∥ Clemence Allain,*,§ and Thomas Devic*,†,‡
́
†Institut Lavoisier, UMR 8180 CNRSU. Versailles St. Quentin, Universite
́
Paris-Saclay, 45 avenue des Etats-Unis, 78035 Versailles,
France
§PPSM, UMR 8531 CNRSENS Cachan, Universite
Paris-Saclay, 61 avenue du president 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.
medium.21−23 The Zr(IV) cation appears suitable for such a
INTRODUCTION
■
purpose, as it usually leads to chemically stable MOFs
While porous coordination polymers (PCPs) or metal organic
(especially in humid environment),24−29 and does not involve
frameworks (MOFs) have been primarily considered as
any absorption in the visible range. Indeed, Zr(IV) carboxylate-
promising candidates mainly for separation and storage
based MOFs efficient for optical detection (mostly by
purposes, their use for the detection of chemical species by
fluorescence),21−23,30−37 especially of the pH,38,39 have been
adsorption is also appealing.1,2 Thanks to their porosity and
reported. Regarding the fluorophore, we focused our attention
on a small electron-poor luminophore, the s-tetrazine, whose
high chemical and structural diversity, MOFs might indeed
combine a high level of detection (thanks to the adsorption)
fluorescence is known to be quenched by electron-rich
and a tunable selectivity, based on the combination of steric
molecules,40,41 and which has shown useful properties and
(size exclusion) and electronic3 (preferential/specific host−
applications.42−44 Few MOFs built up from s-tetrazine derived
guest interactions such as hydrogen or halogen bonds or
ligands have been reported to date, mostly based on M(II)
donor−acceptor interactions) factors. Fluorescence-based
cations.45−52 One of these solids was found to exhibit
optical detection by MOFs is probably the most studied
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.
case.1,2,4−7 While lanthanide-based MOFs have been exten-
sively 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 mole-
cules)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
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
Our aim here was to build up a robust MOF suitable for the
detection of electron-rich molecules, especially in aqueous
Received: May 2, 2017
© XXXX American Chemical Society
A
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content while the solid remains crystalline55−57), and
fluorescent, such fluorescence being quenched by electron-
rich 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.
Rietveld plots) in order to shed some light on the flexible
behavior (see below). MIL-161 crystallizes in a tetragonal
setting, space group I42d (No. 122), with cell parameters
̅
depending on the nature of the absorbed solvents (see Table
1).
Table 1. Unit Cell Parameters of MIL-161 Dispersed in
Various Solvents and in the Presence of Electron-Rich
Molecules (Space Group I42d)
̅
RESULTS AND DISCUSSION
■
electron-rich
molecule
solvent
a (Å)
c (Å)
V (Å3)
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 3,6-
bis(3,5-dimethylpyrazol-1-yl)-s-tetrazine (see Supporting In-
formation for details).
DMF
H2O
26.1084(3)
27.6288(4)
28.301(1)
28.443(2)
28.336(4)
27.51(2)
14.3685(3)
13.5007(5)
11.0641(5)
10.7384(2)
10.981(2)
14.52(1)
9794.2(3)
10305.8(5)
8862.0(8)
8687(1)
EtOH
EtOH
EtOH
EtOH
H2O
DMA
TPA
8817(2)
phenol
phenol
10985(7)
10272(6)
27.493(7)
13.589(3)
Scheme 1. s-Tetrazine Dicarboxylic Acid (H2STz) Used in
the Present Study
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 12-
connected 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
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-dimethylforma-
mide (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 determi-
nation 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
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
B
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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
<1°), the variation of the C−S−CH2−CO2 angles is more
pronounced (∼6 and 11°, see Figure S8 for details).
Photophysical Properties. The optical properties of MIL-
161 were evaluated in suspension in solvents (either water or
ethanol, see below), and compared with the one of the free
H2STz ligand in solution, while keeping the concentration of
the s-tetrazine core constant. As expected, both H2STz and
MIL-161 are luminescent, with an emission band centered at
580 nm (excitation at either 409 or 525 nm) in both cases
(Figure S17), but with a faster decay for the solid, with an
average lifetime of 0.95 and 0.44 ns in ethanol for H2STz and
MIL-161 respectively (see Figure S15 and Table S2).
The quenching ability of various electron-rich molecules
dissolved in either ethanol or water was further evaluated. In a
first step, two aromatic amines, namely, N,N-dimethylaniline
(DMA) and triphenylamine (TPA), were considered. Both
H2STz and MIL-161 were exposed to increasing amount of
both electron-rich molecules. As expected, both DMA and TPA
induce a quenching of the intensity of luminescence I of free
H2STz (see Figures S19 and S21). The luminescence decays
can be fitted with the Stern−Volmer equation,
Figure 1. Crystal structure of MIL-161 (here DMF@MIL-161). ZrO8
polyhedra are shown in blue: (a) the Zr6O8(CO2)8 inorganic unit
(carboxylate groups and DMF molecules are shown in yellow and gray
respectively); (b) pair of ligands (carbon, white; sulfur, yellow;
nitrogen, blue); (c) the whole structure (ligands either in red or in
yellow).
could be characteristic of the pore content.85 As mentioned
earlier, desolvation of MIL-161 is accompanied by a broadening
of Bragg peaks on the XRPD pattern. This phenomenon is
reversible, as the redispersion of the dry solid in solvents leads
to the recovery of the crystalline state (see Figure S11),
suggesting a flexible behavior. The dried state adsorbs a very
minor adsorption of N2 at 77 K, and hence does not present
any permanent porosity. As the broadening of the Bragg peaks
is not associated with a significant shift, such a result probably
arises from local distortions of the intrinsically flexible ligands
leading to too small pore windows rather that to pore closure,
as commonly found in flexible MOFs built up from rigid ligands
(such as the MIL-53(Fe) and MIL-88 series).86−88 In contrast,
adsorption in the liquid phase is feasible, as confirmed by the
examination of the XRPD patterns of MIL-161 suspended in
various solvents (Figure 2). In all cases (ethanol, DMF, and
water), the solids remain highly crystalline. This first confirms
the robustness of MIL-161, especially in aqueous medium, as
the XRPD diagram remains unaltered after weeks. Moreover,
the position of the Bragg peaks strongly depends on the
solvent. These shifts are associated with noticeable changes of
the unit-cell parameters (see Table 1), with variation of a, c, and
V up to 8, 29, and 15% respectively, hence confirming the
adsorption of the solvent. Here, the structural flexibility arises
I0/I − 1 = KSV[quencher]
where I0 is the initial fluorescence intensity, and KSV is the
Stern−Volmer constant (see Figure 3 and Supporting
Information for details), that quantifies the sensitivity toward
a given quencher. This equation is indeed suitable for
intermolecular quenching reactions and properly models the
behavior of s-tetrazines in solution, the value of KSV being
slightly higher for DMA than TPA (see Table S3).
The behavior of MIL-161 strongly differs. While, for the
smaller amine (DMA, see Figure S18), a clear quenching is
observed, the effect of the bulkier TPA is far less marked
(Figure S20). In the first case, the variation of I0/I with the
concentration of quencher remains linear (Figure 3, top), with
a KSV value of the same order of magnitude as H2STz (see
Table S3). On the contrary, for TPA, only a slight decrease
followed by a plateau is observed (Figure 3, bottom), which
prevents the calculation of KSV.
This suggests that, in the case of MIL-161, the quenching of
the fluorescence arises not only from electronic factors (as
found in solution), but also from steric effects: the smaller
DMA molecules are likely to enter the pores, while the bulkier
TPA cannot, hence leading to a limited quenching ability
related to its interaction only at the outer surface of the MOF
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and free H2STz was evaluated both in ethanol and in water.
The corresponding Stern−Volmer plots are shown Figure 5
Figure 5. Stern−Volmer plots of a suspension of MIL-161 (blue or
green) and free H2STz (red or black) as a function of the
concentration of phenol both in ethanol and in water (respectively).
(see Figures S22−S25 and S28 for the corresponding
fluorescence spectra and XRPD patterns respectively). In all
cases, the exposure to phenol leads to a quenching of the
fluorescence of the tetrazine core. Nevertheless, in ethanol, the
efficiency is rather poor, for both the solid and the free ligand
(see Table S3), while significantly higher in water. More
interestingly, in this solvent, the quenching is more pronounced
for MIL-161 than for H2STz, with a sensitivity three times
higher (see Figure 5 and Table S3).
As mentioned above, the quenching of the fluorescence
requests here an interaction between the tetrazine core and the
electron-rich molecules. In water, both organic species (H2STz
and phenol) are prone to interact together, whereas ethanol
solvates more easily these organic species and prevents their
close contact. Related features should happen for MIL-161. As
shown above, the optical detection relies in this case on the
adsorption of the quencher within the pores. As already
highlighted, e.g., in the case of the encapsulation of drugs within
MOFs,89,90 the adsorption of molecules from solution results
from the competition between various intermolecular inter-
actions, namely, MOF−solvent, MOF−guest, and guest−
solvent. Because of its organic constituent, MIL-161 is expected
to interact more strongly with polar organic species such as
phenol and ethanol than with water. MOF−solvent and
phenol−solvent interactions are thus expected to be lower in
water than in ethanol, hence favoring the adsorption of phenol
and thus the quenching of the fluorescence in water.
Figure 3. Stern−Volmer plots of a suspension of MIL-161 (black) and
free H2STz (red) as a function of the concentration of either DMA
(top) or TPA (bottom).
particles. In order to support such a mechanism, we took
advantage of the flexible behavior of MIL-161. XRPD patterns
of the title solid suspended in ethanol, without any quencher,
and in the presence of either TPA or DMA were collected (see
Figure 4). Whereas, in the presence of TPA, the XRPD pattern
Figure 4. XRPD patterns of MIL-161 dispersed in pure ethanol
(bottom), a 20 mM solution of TPA in ethanol (middle), and a 100
mM solution of DMA in ethanol (top) (λ = 1.5406 Å).
CONCLUSIONS
■
remains almost unchanged, the one in the presence of DMA is
different. This was confirmed by the change of the unit-cell
parameters (see Table 1), which indicated a shrinkage of the
structure in the presence of DMA. This unambiguously proves
that DMA is adsorbed in the pores, hence leading to a pore
opening distinct from the one in pure ethanol. This means that
the quenching of the fluorescence is here truly associated with
adsorption of DMA within the pores. This experiment still does
not entirely rule out any adsorption of TPA within the pores,
although this seems unlikely considering the pore size together
with the shape of the quenching curve (which is in favor of a
TPA−STz interaction at the surface of the crystallites, as only a
small fraction of STz (<15%) is active).
We reported here a new porous MOF built up from an s-
tetrazine dicarboxylate derivative and Zr(IV), which was found
to be flexible as well as stable in aqueous medium. Because of
the nature of the organic ligand, this solid is able to detect
electron-rich molecules thanks to the quenching of its
fluorescence, such phenomenon being associated with
adsorption, as unambiguously proven by X-ray diffraction
analyses. As evidenced with aromatic amines, such adsorption-
driven detection is size-selective thanks to the molecular sieving
effect associated with the microporosity. It is noteworthy that,
thanks to the amphiphilic character of the MOF, the sensibility
to phenol was moreover found to be significantly higher in
water than in ethanol, likely because of its higher adsorption in
the aqueous medium resulting from a more favorable MOF−
solvent, MOF−phenol, and phenol−solvent set of interactions.
Phenol is another class of electron-rich chemicals which
deserves to be detected. Its quenching ability toward MIL-161
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Hence, when compared to s-tetrazine derivatives dissolved in
solution, the two specific features associated with the use of
solid porous fluorophores (steric constraints and polarity of the
walls) allow the achievement of good sorption selectivities
leading to an enhanced detection selectivity. Although the
fluorescence of the title solid is too low for any practical use,
this study suggests that fluorescent MOFs built up from more
emissive tetrazine derivatives40 might be interesting for the
development of new solid state sensors for electron-rich
molecules.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01103.
Synthetic procedures, details about the structural and
optical characterizations, and the quenching experiments
(PDF)
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Accession Codes
CCDC 1433758−1433759 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: clemence.allain@ppsm.ens-cachan.fr.
*E-mail: thomas.devic@cnrs-imn.fr.
ORCID
Clemence Allain: 0000-0002-2908-264X
́
Thomas Devic: 0000-0001-7074-6498
Present Address
‡C.S.: Institut des Mater
́
iaux Poreux de Paris, FRE2000 CNRS,
ieure, Ecole Superieure de Physique et de
Chimie Industrielles de Paris, PSL Research University, 75005
Paris, France. T.D.: Institut des Materiaux Jean Rouxel (IMN),
Ecole Normale Super
́
́
́
Universite
Houssinier
́
de Nantes, CNRS UMR 6502, 2 rue de la
e, BP 32229, 44322 Nantes cedex 3, France.
̀
Author Contributions
P.R. conducted all the synthesis and spectroscopic experiments,
N.G. and E.E. performed the XRD analysis, C.M. performed
the solid state NMR experiments, C.S., G.C., P.A., C.A., and
T.D. coordinated the project, and C.A. and T.D. wrote the
article.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge the DGA and the CNRS for the
Ph.D fellowship of P.R., as well as the synchrotron Soleil for
providing access to the beamline Crystal and labex
CHARMMMAT (ANR-11-LABEX-0039). Arnaud Brosseau is
acknowledged for his help with the fluorescence lifetime
measurements, and Georges Mouchaham is thanked for the
Connolly surface analysis.
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