Synthesis, structure and characterization of two solvatochromic metal-organic frameworks for chemical-sensing applications

Article abstract of DOI:10.1039/c8ce00204e

Two isostructural Zr and Hf metal-organic frameworks (PU-1 and PU-2) were constructed from a hitherto unreported anthracene-derived dicarboxylate ligand and inorganic [M₆O₄(OH)₄(CO₂)₁₂] (M = Zr⁴⁺ or Hf⁴⁺) secondary building unit under solvothermal conditions. They crystallized in the cubic Fd3m space group with doubly interpenetrated fcu-c topology. PU-1 exhibited an eye-catching color change within different solvents (especially in water) and could be a solvatochromic sensing material for recognition of solvent molecules.

Full text of DOI:10.1039/c8ce00204e

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Synthesis, structure and characterization of two solvatochromic
Metal-organic Frameworks for chemical sensing application
Sheng Gao,a Limin Zhao,*a, b Lin Han,c Zhenjie Zhang,c Hong Zhao*a, b
rReceived 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
differential stabilization of the electronic states or changes in
the coordination geometry of the centre which leads to changes
in the visible dꢀd energy transitions19 as well as metalꢀtoꢀ
Two isostructural Zr and Hf metal-organic frameworks (PU-1
and PU-2) were constructed from
a hitherto unreported
anthracene-derived dicarboxylate ligand and inorganic
[
21
ligandꢀchargeꢀtransfer (MLCT) and π to π* transitions.20,
M6O4(OH)4(CO2)12] (M=Zr4+ or Hf4+) secondary building unit
under solvothermal conditions. They both crystallize in the cubic
Some MOFs constructed with flexible linker that are readily
generate hydrogen bond with solvent molecules also exhibit a
solvatochromic behavior. The use of solvatochromic as a direct
detection method have been reported. For example, Arijit
Mallick et al.22 reported the MgꢀNDI based MOFs to detect
solid state organic amine by visual colour change; G. Mehlana
et al.23 design several solvatochromic MOFs using
pyridylbenzoate linkers and Co(II) salts, solvatochromic MOFs
as a chemosensor show a great potential application.
Fd-3m space group with
a doubly interpenetrated fcu-c
topology. PU-1 exhibits an eye-catching colour change inclusion
of different solvents, especially in water, which serve as a
potential solvatochromic sensing materials for recognition of
solvent molecules.
However, the poor stability of MOFs, performed as
coordination bonds disconnected when soak in solvents, is
commonly regarded as the major drawback for their practical
applications.24ꢀ28 Directly construct MOFs with instinctive
stability in structure and composition become a solution to this
problem.29 The discovery of Zr6(ꢁ3ꢀO)4(ꢁ3ꢀOH)4(BDC)6 (UiOꢀ
66) with 12ꢀcoordinated Zr6(ꢁ3ꢀO)4(ꢁ3ꢀOH)4(CO2)12 clusters by
Cavka et al.30 make a significant step. The structure of Zrꢀbased
Metal organic frameworks (MOFs) are a kind of highly ordered
crystalline porous material constructed from metal ions and
organic linkers,1ꢀ3 exhibit a wide variety of applications such as
gas storage, separation
，drug delivery, heterogeneous catalysis,
sensor applications.4ꢀ7
Fluorescent MOFs have promising potential application as an
excellent chemical sensor in the detection of small molecules,
cations, and anions.8ꢀ12 MOFꢀbased chemosensors can exhibit
pronounced luminescence enhancement or quenching when
guest molecules enter the MOFs pore. The microscopic
interaction between host framework and guest molecules
require special tools to translate into signals that can be
perceived by the outside world. Unlike conventional detection
instruments such as luminescence analysis that are either
expensive or nonꢀportable, chromism MOFs provides a simpler
and often easily visible method to detect organic molecules.13
Chromism show a visible colour changes that can be caused by
photoꢀirradiation,14 heat,15 compression,16 or solvent effects17.
Among four discoloration behaviour mentioned above,
solvatochromic are fundamental properties may arise from
different mechanisms, such as the change in the energy gap
between the ground state and excited state of different
polarities,18 since a change of solvent polarity generates
MOFs exhibits exceptional stability, especially hydrothermal
，
that exceeds most reported MOFs.31ꢀ34 We herein synthetize a
new anthraceneꢀderived dicarboxylate ligand and combine the
inorganic building blocks of M6(M= Zr or Hf) oxo clusters to
construct high stable ZrꢀMOF and HfꢀMOF denoted as PUꢀ1
and PUꢀ2. The coordination interaction between zirconium ions
(or hafnium ions) and carboxylate ligands are strong enough for
the MOFs to be a solvatochromic sensor for detection of
solvent molecules.
To obtain a MOF with solvatochromic properties, we firstly
prepared a strong fluorescence conjugated linker, termed L,
a.
School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical
University，Guangzhou 510000, China. E-mail: zhaolimin@gdpu.edu.cn
b.
c.
Guangdong Cosmetics Engineering & Technology Research Center, Guangzhou
510000, China. E-mail: zhaohong1001@sina.com
College of Chemistry, Nankai University, Tianjin 300071.
† Electronic Supplementary Information (ESI) available: Detailed synthesis proce-
dures, PXRD patterns, and 1H-NMR spectra. CCDC 1819073 and 1819074. For ESI
and crystallographic data in CIF or other electronic format see DOIS ee DOI:
Scheme 1: Synthetic scheme for Ligand L.
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based on Heck cross coupling reaction and saponification
reactions35 (Scheme 1). Details of the synthesis procedure are
given in the ESI. The single crystal of the ligand was also
cultured by the volatilization method. (Figure S4, S5)
PUꢀ1 and PUꢀ2 were obtained through the solvothermal
reaction of L and ZrCl4 (HfCl4) in the presence of modulator
(benzoic acid for the former, trifluoroacetic acid for the latter)
in the tube. Crystallographic data for the structural analysis
have been deposited with the Cambridge Crystallographic Data
(CCDC 1819073 and CCDC 1819074). Detail crystallographic
parameters and representation of selected bond lengths and
bond angles of two MOFs were listed in the ESI (Table S1ꢀS3).
The singleꢀcrystal Xꢀray diffraction analysis reveal that PUꢀ1
and PUꢀ2 both crystallized in the cubic system space group Fdꢀ
3m, a=39.1912 Å and 39.1198 Å respectively. Crystal structure
of PUꢀ1 as a representative example is selected to describe in
detail.
Figure 2: (a) The experimental, simulated and the activated PXRD patterns of PUꢀ1. (b)
The TGA curve of asꢀsynthesized PUꢀ1 and after activated.
[Zr6O4(OH)4(CO2)12] secondary building unit (SBU)36 in PUꢀ
1 consist of six Zr4+ ions with squareꢀantiprismatic coordination
geometry by eight oxygen atoms (Figure 1a). The Zr6 cluster
are bridged by 12 highly conjugated anthraceneꢀderived
dicarboxylate ligands showing the isostructure of PIZOF37
series with doubly interpenetrated fcuꢀc topology (Figure 1b,
1d). Distinct from the structure of PIZOF MOFs, linkers were
replaced by alkenyl group anthracene ligand. Twoꢀfold
interpenetration interactions between anthracene ligands
redistrict the total void space into two distinct cages,38 one large
tetrahedral (19 Å) and smaller octahedral (10 Å) cage (Figure
1c) .
PXRD analyses were carried out at ambient conditions. The
PXRD of asꢀsynthesized PUꢀ1 and PUꢀ2 were almost in accord
with those patterns calculated for their respective singleꢀcrystal
structures (Figure 2a, Figure S6). The asꢀsynthesized PUꢀ1 can
also undergo a solvent exchange process with methanol for 3
days (4 times one day) to afford the activated sample. The
Figure 3: The N2 sorption isotherms of PUꢀ1 at 77 K.
agreement of three curves suggested the high phase purity of
powder, and the activated sample can retain its rigid
framework.
The thermal stability of the asꢀsynthesized PUꢀ1 were
measured by thermal gravimetric analysis (TGA) under N2
atmosphere. As shown in Figure 2b, the curve of the asꢀ
synthesized PUꢀ1 reveals two distinct steps suggests that from
room temperature to 150 °C, a weight loss of 30% can be
observed, which corresponds to the loss of DMF molecules,
then PUꢀ1 can be stable to 470 °C. Above 470 °C, the mass loss
is ascribed to oxidation of the linkers and the residue
component is zirconium dioxide. The thermogravimetric test on
this activated samples were also performed and the curve
showed almost no weight loss from room temperature to 470
°C, which mean almost all of the DMF molecules were
swapped out.
After the exchange process, activation of methanolꢀ
containing samples were placed inside a supercritical CO2
dryer(ScD). And the methanol was exchanged with liquid CO2
over a period of 6 hours, during this time the liquid CO2 were
vented under positive pressure for five minutes every 30
minutes. After that, all the valves were closed and the
temperature of the chamber was raised to 40 ºC to reach the
critical point of CO2. The chamber was held in these conditions
for an hour and vented slowly over 6 hours. The dried sample
was tested for N2 adsorption immediately. N2 sorption
isotherms reveal that PUꢀ1 samples treated by ScD have a
BrunauerꢀEmmettꢀTeller (BET) surface area of 1470 m2/g at
77K, and the calculated pore volume is 0.56 cm3/g (Figure 3).
This BET surface area, similar with isostructural materials,
matches that computed based on the crystal structure.36ꢀ38 The
CO2 adsorptionꢀdesorption isotherms were measured at 298 K,
where it shows CO2 uptakes as high as 43 cm3/ g (Figure S7).
The maximum fluorescence emission of MOFꢀ1 located at 545
nm in the desolvated state under room temperature atmosphere
(Figure S8). The desolvated powders of PUꢀ1 were immersed in
Figure 1: Crystal structure of PUꢀ1 and PUꢀ2: (a) [ M6O4(OH)4(CO2)12] (M=Zr4+ or Hf4+
SBU and linear linker. The SBU is replaced with cuboctahedron shapes, ligand L is
replaced by cyan stick. (b) SBU coordinated by 12 ligands. (c) A view of PUꢀ1 cage
)
packing structure, brown represent the large 19
Å tetrehedral cages, light purple ten kinds of representative common solvents (toluene, diethyl
represents the small 10 Å cages. (d) Doubly interpenetrated fcuꢀc net. Atom labelling
scheme: Zr, O, and C atoms are in green, red and black colours, respectively. H atoms
are omitted for clarity.
ether, dichloromethane, THF, acetone, DMF, acetonitrile,
ethanol, methanol and H2O) for 2 d. Subsequently, the samples
were airꢀdried and the fluorescence spectra were
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between Zr clusters and ligand linkers resulting in the huge
colour change. From the infrared spectrum, carbonyl
characteristic peaks at 1670 cmꢀ1 disappeared when one Zr4+
cation and two carboxylates combined with bidentate
coordination mode (Figure S11). In contrast, a clear carbonyl
characteristic peaks signal both occurred in the infrared
spectrum of ligand L and waterꢀtreatment material. The
evidence may prove the fracture of the coordination bond after
water soaking.
In this study, two anthraceneꢀfunctionalized metalꢀorganic
frameworks PUꢀ1 and PUꢀ2 were successfully constructed and
characterized. Their structures were completely determined by
Figure 4: (a) Solidꢀstate fluorescence emission spectra of PUꢀ1 after various solvent
soaking (ex=425 nm). (b) Wavelengths of emission peak maxima of PUꢀ1 obtained in
10 kinds of representative common solvents.
singleꢀcrystal Xꢀray diffraction analyses with
interpenetrated fcuꢀc net containing
a
dually
fascinating
M6O4(OH)4(CO2)12 clusters (M= Zr and Hf). PUꢀ1 shows an
excellent thermal stability, chemical stability and a high BET
surface area of 1470 m2/g. Extraordinarily, PUꢀ1 exhibits
solvatochromic behavior for sensing small solvent molecules
that display an eyeꢀcatching colour change in different solvents,
a particular chromism was observed after waterꢀtreatment and
the probable mechanism were also discussed. The special
property indicates that an approach to produce a new type of
sensing material was found.
Acknowledgements
This work was supported by the Foundation of the National
Natural Science Foundation of China (No. 21401029).
Figure 5: PXRD diffraction patterns of PUꢀ1 after soaking in solvents.
Conflicts of interest
There are no conflicts to declare.
followed uniformly recorded39 (Figure 4a). The stability of such
MOFs containing solvents were examined by PXRD (Figure 5),
materials remained a good peak after soaking in different
solvents which illustrated that their structural integrity were
conserved. The experimental results confirmed our conjecture,
the obvious differences in different solvents for maximum
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Title:
Synthesis, structure and characterization of two solvatochromic
Metal-organic Frameworks for chemical sensing application
Authors: Sheng Gao, Limin Zhao, * Lin Han, Zhenjie Zhang, Hong Zhao*
Contents graphic and synopsis
Two isostructural Zr and Hf metal-organic frameworks with doubly interpenetrated fcu-c
topology, exhibits obvious solvatochromic behaviour for solvent sensing application.