Thioether side chains improve the stability, fluorescence, and metal uptake of a metal-organic framework

Article abstract of DOI:10.1021/cm200557e

This work builds on the recently developed hard-soft approach, as is embodied in the carboxyl-thioether combination, for functionalizing metal-organic frameworks (MOFs), and it aims to further demonstrate its efficacy and generality in connection with the

Full text of DOI:10.1021/cm200557e

ARTICLE
pubs.acs.org/cm
Thioether Side Chains Improve the Stability, Fluorescence,
and Metal Uptake of a MetalꢀOrganic Framework
Jun He,† Ka-Kit Yee,† Zhengtao Xu,*,† Matthias Zeller,‡ Allen D. Hunter,‡ Stephen Sin-Yin Chui,§ and
Chi-Ming Che§
†Department of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China
‡Department of Chemistry, Youngstown State University, One University Plaza, Youngstown, Ohio 44555, United States
§Department of Chemistry and HKU-CAS Joint Laboratory on New Materials, The University of Hong Kong, Pokfulam Road,
Hong Kong, China
S Supporting Information
b
ABSTRACT: This work builds on the recently developed hardꢀsoft approach, as is embodied
in the carboxylꢀthioether combination, for functionalizing metalꢀorganic frameworks
(MOFs), and it aims to further demonstrate its efficacy and generality in connection with
the prototypic MOF-5 system [i.e., Zn4O(bdc)3, where bdc is 1,4-benzene dicarboxylate].
Specifically, the thioether side chain CH3SCH2CH2Sꢀ (methylthioethylenethio, or MSES) is
placed at the 2,5- positions of bdc, and the resultant molecule (L) was crystallized with Zn(II)
ions into a porous, cubic network [Zn4O(L)3] topologically equivalent to MOF-5. Compared
with the previously used methylthio (CH3Sꢀ) group, the MSES side chain is more flexible, has
more S atoms as the binding sites (per chain), and extends further into the channel region;
therefore, this side chain is predisposed for more-efficient binding to soft metal species when
installed in a porous MOF matrix. Here, we report the significantly improved properties, with
regard to stability to moisture, fluorescence intensity, and capability of metal uptake. For
example, activated solid samples of 1 feature long-term stability (more than 3 weeks) in air, have a notable sensing response to
nitrobenzene (in the form of fluorescence quenching), and are capable of taking up HgCl2 from an ethanol solution at a
concentration as low as 84 mg/L.
KEYWORDS: sulfurated frameworks, metalꢀorganic frameworks, fluorescent sensor, heavy metal removal, thioether donors
’ INTRODUCTION
We have recently reported a porous coordination network
based on the Pb(II) ion and the bifunctional molecule 2,3,5,6-
tetrakis(methylthio)benzenedicarboxylic acid (TMBD), in
which the reversible uptake of HgCl2 into the channels was
enabled by the methylthio (CH3Sꢀ) functions.9c That particular
system, however, has several limitations. First, the Pb species
poses an environmental concern in practical applications. Sec-
ond, the effective uptake of the HgCl2 species entails a rather
stringent condition of heating in benzene. To further illustrate
the usefulness of thioether groups for functionalizing MOF
systems, here, we report a coordination network based on the
molecule 2,5-bis(2-(methylthio)ethylthio)terephthalic acid (L)
In the fast growing field of porous coordination networks (aka
metalꢀorganic frameworks, MOFs),1 it is of great interest to
achieve systematic functionalization of the pores while maintain-
ing the underlying topology of the main framework. Bifunctional
molecules with primary groups for network construction and
secondary groups for functionalization seem to be potentially
versatile building blocks. Besides earlier systems of bifunctional
building blocks from the groups of Lee (phenylacetylene-based
nitriles),2 Wuest (hydrogen-bonding nets),3 and Lin,4 notable
recent efforts include 2-amino-1,4-benzene dicarboxylic acid,5 an
azide-decorated MOF for click functionalization,6 zeolitic imida-
zolate framework (ZIF) post-synthetic modification,7 and ether/
thioether-tagged MOFs.8 In this regard, the combination of
carboxylic and sulfur functions (see structure L in Chart 1, for
example) provides appealing opportunities for functionalizing
MOFs.8a,9 For example, the ionic, chemically hard carboxylate
groups are predisposed, as the primary group, to interact with
metal ions for network formation, while leaving the neutral, and
generally weaker-binding, thioether groups as free-standing,
secondary donors. In addition, thioethers are generally stable
in air and can be attached to molecular building blocks with
efficient organic synthetic procedures.10
and Zn(II) ions [network composition: Zn4O (L)3, henceforth
3
referred to as 1] that effectively overcomes these limitations.
Besides the environmentally benign Zn species, the methyl-
thioethylenethio side chains of L apparently provide stronger
binding for the metal species (in comparison with the methylthio
groups in TMBD)—for example, HgCl2 can be effectively taken
up from an ethanol solution at a concentration as low as 84 mg/L.
Received: February 22, 2011
Revised:
April 24, 2011
Published: May 11, 2011
r
2011 American Chemical Society
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Chart 1
Moreover, the 2,5-sulfur-substituted L brings about significantly
enhanced photoluminescence in both the free ligand L and the
network compound 1, enabling effective sensing for environ-
mentally relevant guest species, such as HgCl2 and nitrobenzene.
product C10H10O4S2 yielded the following: Calcd [C (46.50%), H
(3.90%)]; Found [C (46.30%), H (4.05%)].
Dimethyl 2,5-bis(2-bromoethylthio)terephthalate (S2). S1
(0.52 g, 2.01 mmol) and anhydrous acetone (40 mL) were loaded into a
100-mL two-neck flask that was charged with a magnetic stirring bar.
The mixture was bubbled with N2 for 10 min. Under N2 protection, 1,2-
dibromoethane (10 mL, 116 mmol), K2CO3 (1.10 g, 7.96 mmol), and
NaI (0.05 g, 0.334 mmol) were added, and N2 was bubbled through the
resulting reaction mixture for another 10 min. The reaction mixture was
then stirred and refluxed (by means of an oil bath; bath temperature:
75 °C) under nitrogen protection for 24 h. After being cooled to room
temperature, the solvents were removed with a rotary evaporator and
the yellowish residue was purified via a silica gel plug (silica gel,
with 1:3 hexane/dichloromethane as the eluent) to provide a bright
’ EXPERIMENTAL SECTION
General Procedure. Starting materials, reagents, and solvents were
purchased from commercial sources (Aldrich, Merck, and Acros) and
used without further purification. Elemental analysis was performed with
a Vario Micro CUBE CHN elemental analyzer. Fourier transform
infrared (FT-IR) spectra were obtained using a Nicolet Avatar 360
FT-IR spectrophotometer. The ratios of the metal ions were determined
using a PerkinꢀElmer Model Optima 2100 DV ICP optical emission
1
1
spectrometer. Solution H and 13C NMR spectra were recorded on a
yellow crystalline solid (0.51 g, 53% based on S1). H NMR (400
MHz, CDCl3): δ 7.844 (s, 2H, CHAr), 3.974 (s, 6H, CH3), 3.4ꢀ3.6
400 MHz Bruker superconducting magnetic high-field NMR spectro-
meter at room temperature, with tetramethylsilane (TMS) as the
internal standard. Thermogravimetric (TG) analyses were carried out
in a nitrogen stream using PerkinꢀElmer Thermal analysis equipment
(Model STA 6000) with a heating rate of 10 °C/min. Single-crystal XRD
data collection was conducted on a Bruker AXS SMART APEX CCD
system, using Mo KR (λ = 0.71073 Å) radiation at 100(2) K. Data were
collected, the unit cell determined and refined, and data were integrated
and corrected for absorption using Apex2 (v2008.2ꢀ4, Bruker AXS Inc.,
Madison, WI, 2008). All absorption corrections were performed using
the SADABS program. The structure was solved and refined by full-
matrix least-squares on F2o, using SHELXL 6.14 (Bruker AXS, Inc.,
Madison, WI, 2003). X-ray diffraction (XRD) patterns for the bulk
samples in Figures 3 and 5 (presented later in this work) were collected
at room temperature on a Bruker Model D8 Advance diffractometer,
using Cu KR radiation (λ = 1.5418 Å) with a power of 40 kV and 40 mA.
Powder XRD patterns for the other samples were collected at room
temperature on a Siemens D500 powder diffractometer (Cu KR radia-
tion, λ = 1.5418 Å). The power of the sealed X-ray tube was 40 kV and
30 mA. All powder samples were spread onto glass slides for data collec-
tion. The program Mercury (from the Cambridge Crystallographic
Data Center) was used to calculate powder patterns from single-crystal
structures.
(m, 8H, CH2). 13C NMR (100 MHz, CDCl3): δ 165.76, 135.33,
132.46, 129.41, 52.84, 34.53, 28.48. FT-IR (KBr pellet, ν/cmꢀ1):
h
3415(w), 3036(w), 2987(w), 2943(w), 2843(w), 1713(s), 1473(m),
1435(m), 1298(m), 1232(m), 1196(m), 1132(w), 1086(s), 966(w),
899(w), 825(w), 775(s). Chemical analysis of the product C14H16-
Br2O4S2 yielded the following: Calcd [C (35.61%), H (3.42%)];
Found [C (35.80%), H (3.25%)].
2,5-Bis(2-(methylthio)ethylthio)terephthalic Acid (L). In a
nitrogen-filled glovebox, sodium thiomethoxide (0.44 g, 6.28 mmol) was
loaded into a 50-mL two-neck flask equipped with a septum and a
magnetic stirring bar. After the flask was taken out of the glovebox,
DMEU (1,3-dimethyl-2-imidazolidinone, 10 mL) was injected via cannula
under N2. When the sodium thiomethoxide was completely dissolved in
DMEU, S2 (0.60 g, 1.27 mmol) was added. The reaction mixture was
then stirred and heated at 90 °C under nitrogen protection. After 2 days,
the reaction mixture was poured into 100 mL of water, and HCl (10%)
was added slowly with vigorous stirring. After the pH was reduced to
below 2, the resultant yellowish solid product was isolated by suction
filtration [0.42 g, 87% based on S2]. The solid product thus obtained was
pure, as indicated by NMR, and was used for crystal growth without
1
further purification. H NMR (400 MHz, DMSO-d6): δ 7.80 (s, 2H,
CHAr), 3.19 (m, 4H, CH2), 2.71 (m, 4H, CH2), 2.14 (s, 3H, CH3). 13
C
NMR (100 MHz, DMSO-d6): δ 167.24, 135.25, 132.76, 128.49, 32.19,
Dimethyl 2,5-dimercaptoterephthalate (S1). A round-bot-
tomed flask (100 mL) was loaded with a magnetic stirring bar, powder
of 2,5-dimercapto-1,4-benzenedicarboxylic acid (DMBD, 2.16 g,
9.39 mmol), methanol (30 mL), and concentrated H2SO4 (2.0 mL).
The flask was then connected to a condenser and the mixture was refluxed
at 80 °C for 48 h. After the reaction mixture was cooled to room temp-
erature, water (200 mL) was added to the reaction mixture; the resulted
yellowish precipitate was filtered off by suction, washed extensively with
31.44, 15.06. FT-IR (KBr pellet, ν/cmꢀ1): 3430(w), 2920(m), 2852(m),
h
2645(w), 2558(w), 1659(s), 1474(m), 1412(m), 1301(m), 1243(s),
1091(m), 895(w), 779(w), 633(w). Chemical analysis of the product
C14H18O4S4 yielded the following: Calcd [C (44.42%), H (4.79%)];
Found [C (44.80%), H (4.52%)].
X-ray Quality Single-Crystals of Zn4O (L)3 (DMF)4 (H2O)4
(1). A mixture of Zn(NO3)2 6H2O (6.0 mg), L (3.8 mg) and N,N-
3
3
3
3
1
water, and dried in air (2.19 g, 90% based on the DMBD). H NMR
dimethylformamide (DMF) (0.8 mL) was sealed in a Pyrex glass tube
(outer diameter, OD = 10 mm; inner diameter, ID = 7.8 mm) and heated
in an oven at 100 °C for 24 h and cooled to room temperature at a rate of
10 °C/h. Bright yellow block-like crystals were obtained (4.3 mg, 80%,
based on L). Larger-scale preparations [e.g., with 36.0 mg of L, 60.0 mg
(400 MHz, CDCl3): δ 7.97 (s, 2H, CHAr), 4.66 (s, 2H, SHAr), 3.96
(s, 6H, CH3). 13C NMR (100 MHz, CDCl3): δ 165.92, 133.57, 133.48,
129.12, 52.72. FT-IR (KBr pellet, ν/cmꢀ1): 3410(w), 3002(w), 2953(w),
h
2535(w), 1713(s), 1560(w), 1434(m), 1303(s), 1247(s), 1136(w),
1090(s), 954(m), 823(m), 777(m), 632(w). Chemical analysis of the
of Zn(NO3)2 6H2O, and 8 mL of DMF in a 12-mL vial at 110 °Cfor 24h],
3
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using a glass vial, provided the same product in similar yields and single-
phase purity (as checked via powder XRD). Chemical analysis of the
product C54H84O21N4S12Zn4, corresponding to Zn4O (L)3 (DMF)4-
(H2O)4, yielded the following: Calcd [C (36.61%), H (4.78%), N
(3.16%)]; Found [C (36.30%), H (4.41%), N (3.31%)].
10 mL of ethanol to give a final product (11.6 mg). This solid product
was then examined via powder XRD to reveal that the main framework
remained intact (see below). Part of this solid product was also dissolved
in concentrated HNO3 for inductively coupled plasma (ICP) elemental
analysis, which indicated a Zn/Hg molar ratio of 5.56:1. Chemical
analysis of the product C45H57Cl1.44Hg0.72O14.5S12Zn4, corresponding
to Zn4O (L)3 (HgCl2)0.72 (EtOH)1.5, yielded the following: Calcd [C
(32.33%), H (3.44%)]; Found [C (32.58%), H (3.06%)].
In another test (to probe the ability of the activated sample of 1 to
take up HgCl2 from dilute solutions), activated crystals of 1 (10.0 mg)
were added into a 5-mL clear vial containing a (weaker) ethanol solution
of HgCl2 (2.0 mL, 84 mg/L). The mixture was then left to stand
undisturbed for six days at room temperature, and ICP was used to detect
how much HgCl2 was left in the solution (see Results and Discussion
section below).
3
3
Activation of Crystals of 1 and MOF-5. A bulk sample of bright-
yellow, cube-shaped crystals of 1 was prepared in a vial as described
3
3
3
above [i.e., with 36.0 mg of L and 60.0 mg of Zn(NO3)2 6H2O]. After
3
the mother liquid (of DMF) was decanted, the crystals were washed
twice with anhydrous DMF (i.e., 2 ꢁ 8 mL)—each time, the crystals
were allowed to soak for 6 h before the DMF was decanted. The crystals
were then similarly washed three times with acetone (3 ꢁ 10 mL)—
again, the crystals were soaked for 8 h before the acetone was decanted,
after which the solid was washed three times with chloroform (3 ꢁ 8 mL;
the soaking time for each wash was 8 h). After the final chloroform wash,
the solvent was decanted and the resultant crystals were evacuated with
an oil pump at room temperature for 4 h to give 33 mg (75%, based on
L) of Zn4O L3 (CHCl3)0.5 as yellowish cube-shaped crystals. Chemical
The HgCl2 was depleted from the host framework via the following
procedure: a mixture of Zn4O (L)3 (HgCl2)0.72 (EtOH)1.5 (5.0 mg)
3
3
3
(the ratio of Zn/Hg was predetermined by ICP) and freshly distilled
anhydrous acetonitrile (2.5 mL) in a sealed Pyrex glass tube was heated
at 100 °C for 2 days and cooled to room temperature. The product was
filtered and washed with freshly distilled acetonitrile and then dried in
vacuo overnight (yield = 4.5 mg). The sample then was dissolved in
concentrated HNO3, and the ensuing ICP elemental analysis yielded a
Zn/Hg ratio of 36.4:1, which indicates that ∼85% of the HgCl2
component had been removed from the solid sample. Chemical analysis
of the product C44.4H51.6Cl0.22Hg0.11N1.2O13S12Zn4, corresponding
to Zn4O (L)3 (HgCl2)0.11 (CH3CN)1.2, yielded the following: Calcd
3
3
analysis for C85H97Cl3O26S24Zn8 yielded the following: Calcd [C
(34.80%), H (3.33%)]; Found [C (34.50%), H (3.62%)]. The powder
XRD pattern of the bulk sample thus activated indicated a single
crystalline phase, which was consistent with the single-crystal structure
of as-made 1.
The preparation of single-crystal samples of MOF-5 and the sub-
sequent activation were carried out according to the literature.11 Zn-
(NO3)2 6H2O (180 mg, 0.610 mmol) and 1,4-benzenedicarboxylic acid
3
(bdc, 33 mg, 0.198 mmol) were dissolved in diethylformamide (DEF,
5.0 mL) in a 12-mL glass vial with a septum. The reaction vessel was
heated in an oven at 80 °C for 10 h and cooled to room temperature at a
rate of 10 °C/h. After the mother liquid of DEF was decanted, the
crystals were washed thrice with anhydrous DMF (i.e., 3 ꢁ 8 mL)—each
time, the crystals were allowed to soak for 8 h before the DMF was
decanted. The crystals were then similarly washed six times with
chloroform (6 ꢁ 8 mL; the soaking time for each wash was 8 h). After
the final chloroform wash, the solvent was decanted and the included
CHCl3 was removed in vacuo to give 34 mg (67%, based on bdc) of
activated MOF-5 product as colorless cube-shaped crystals. Chemical
3
3
3
[C (35.88%), H (3.50%), N (1.13%)]; Found [C (35.62%), H (3.15%),
N (1.48%)].
’ RESULTS AND DISCUSSION
Syntheses and Stability Tests. As shown in Chart 1, mole-
cule L was conveniently synthesized from the methyl ester of 2,5-
dimercaptobenzenedicarboxylic acid (DMBD, which we recently
introduced as a bifunctional building block for coordination
networks9b). In this connection, it is also our intention to further
highlight DMBD as an especially versatile entry point to a large class
of functionalized building blocks for the field of coordination
networks, because a wider array of organic groups can be conve-
niently anchored to the highly nucleophilic sulfur atoms in DMBD
to structurally and functionally impact the prospective MOFs.
Compound 1 was obtained by reacting L with Zn(NO3)2 in N,
N-dimethylformamide (DMF) under solvothermal conditions
(see Experimental Section). Table 1 gives X-ray data regarding 1.
X-ray single-crystal analysis reveals that the network of 1 (see
Figure 1) features the R-Po topology (i.e., a simple cubic net) and
is isoreticular with the reported MOF-5 that builds on 1,4-
benzenedicarboxylate (bdc) linear struts and the Zn4O cluster
octahedral nodes.12 Notice that, in 1, the 2-methylthioethylthio
side chains (ꢀSCH2CH2SCH3) of L are not bonded to the
Zn(II) centers and thus extensively decorate the channels as free-
standing donors for metal guests. These floppy side chains are
highly disordered, and only the S atom on the benzene core was
located, using XRD data (see the Supporting Information for
details). The presence of the ꢀSCH2CH2SCH3 side chains in
the crystals of 1 was verified by solution 1H NMR and IR measu-
rements (i.e., by dissolving crystals of 1 in 1:8 DCl (38% in
D2O)/DMSO-d6, see also Figure S1 in the Supporting Informa-
tion for the NMR spectrum and Figure S2 in the Supporting
Information for the IR spectrum).
analysis of MOF-5 C24H15O14.5Zn4, corresponding to Zn4O (1,4-
3
benzenedicarboxylate)3 (H2O)1.5, yielded the following: Calcd [C
3
(36.17%); H (1.90%)]; Found [C (36.04%); H (2.15%)]. The powder
XRD pattern of the bulk sample thus activated indicated a single
crystalline phase, which is consistent with the single-crystal structure
of as-made MOF-5.
Sorption of Nitrobenzene (NB) for 1 and MOF-5. A small vial
containing ∼10 mg of the activated crystals of 1 (not ground) was placed
into a larger vial containing ∼10 mL of nitrobenzene (NB), with care
being taken to avoid direct contact between the solid sample and the
liquid. The larger vial was then sealed for the vapor sorption experiment
for a period of 3 days. To remove solvent that was fortuitously located on
the exterior of the solid sample, n-hexane was then used to wash the
crystals repeatedly (e.g., 3 ꢁ 2 mL), and the washed crystals were left in
the air, to allow the hexane to evaporate before characterizing the
compound for the presence of NB molecules. Chemical analysis of the
NB-treated crystals thus obtained, C48H59NO18S12Zn4, corresponding
to Zn4O (L)3 (NB)(H2O)3, yielded the following: Calcd [C (36.39%),
3
3
H (3.75%), N (0.88%)]; Found [C (36.18%), H (3.91%), N (0.97%)].
The NB-depleted sample was prepared by evacuating (by means of an oil
pump) the NB-treated sample at 120 °C for 5 h (see Figure S6 in the
Supporting Information for the IR spectrum).
HgCl2 Sorption of 1. Inside a capped 5-mL vial, activated crystals
of 1 (10 mg, equivalent to 0.0071 mmol of Zn4O(L)3) were allowed to
soak undisturbed in an ethanol (ABS, 2.0 mL) solution of HgCl2 (10.0 mg,
0.036 mmol) at room temperature for 2 weeks. The mixture was then
suction-filtered, and the solid that was isolated was then washed with
The crystal samples thus prepared show remarkably enhanced
stability toward moisture, in comparison to MOF-5. For example,
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Table 1. X-ray Crystallographic Data for 1a
parameter
value
chemical formula
formula weight, fw
temperature
C
42H48O13S12Zn4
1407.00
100(2) K
0.71073 Å
cubic
wavelength
crystal system
space group
Fm3m
lattice parameter, a
unit-cell volume, V
Z
25.559(6) Å
16696(4) Å3
8
density, Fcalcd
absorption coefficient
F(000)
1.119 g/cm3
1.474
5728
goodness-of-fit on F2
R1b [I > 2σ(I)]
wR2c [I > 2σ(I)]
0.834
0.0948
0.2878
a Partial model; disordered ꢀCH2CH2SCH3 side chains were unre-
solved; see Supporting Information for more details. b R1 = ∑ Fo| ꢀ |
Fc /∑(|Fo|). c wR2 = {∑ [w(Fo2 ꢀ Fc2)2]/∑[w(Fo2)2]}1/2
.
the activated solid samples (see the Experimental Section for the
preparation of the activated sample) of 1 are stable in air for more
than 3 weeks, as is shown by the powder XRD patterns in Figure S3
in the Supporting Information. However, the prototypical MOF-5
is well-known to degrade steadily within a couple of hours when
placed in air.11a,13 In another experiment, the activated crystals of 1
were immersed in water at room temperature to further test their
stability. As seen in Figure S4 in the Supporting Information,
although the major diffraction peaks of the solid sample of 1 were
substantially retained after 3 h of immersion in water, longer
exposure (e.g., 6 h) led to significant degradation of the crystalline
samples of 1 (apparently due to the inherent water-sensitive nature
of the Zn4O cluster). Overall, the enhanced stability of 1 toward
water can be attributed to the hydrophobic nature of the thioether
side chains of L, as well as to the proximity of such hydrophobic
groups to the Zn4O clusters.
Fluorescence Properties and Sorption of Nitrobenzene.
Photoluminescent data on benzene 1,4-dicarboxylate ions and its
network compound MOF-5 were reported.14 It was found that
the emission maximum occurs at ∼410 nm for the free ligand in
the anionic form, while the network compound MOF-5 emits at
525 nm, which was attributed to ligandꢀmetal charge transfer
(LMCT).15 Notice, however, that the documented data of MOF-
5 were measured on nanosized samples that were obtained as a
precipitate from a “direct mixing” procedure.16 When using the
larger single crystals of the MOF-5 samples (∼0.2 ꢁ 0.2 ꢁ 0.2 mm3,
see the Experimental Section for details) for photoluminescence
measurements (e.g., at the various excitation wavelengths of 280,
320, and 350 nm), we obtained spectra that differ from the data
reported on the nanosized samples,14 with a blue-shifted, broad
emission that peaks at ∼420 nm (curve (b) of Figure S5 in the
Supporting Information). Because of its similarity to the domi-
nant features among the emission spectra of free bdc (1,4-
benzene dicarboxylic acid)—as in spectrum a of Figure S5 in
the Supporting Information, the reported data of its disodium
salts14b—the emission of the MOF-5 single-crystal samples
observed herein seems to be largely arising from ligand-centered
transitions, with the LMCT mechanism reported elsewhere on
Figure 1. Crystal structure of 1: (a) Zn4O(COO)6 cluster shown as a
ball-and-stick model (Zn1ꢀO1, 1.934 Å; Zn1ꢀO2, 1.885 Å; Zn Zn
3 3 3
3.158 Å); (b) the same cluster with ZnO4 shown as tetrahedra; (c) one
of the cavities. The side chain atoms beyond the first S atom (e.g.,
ꢀCH2CH2SCH3) and guest molecules (e.g., DMF and H2O) are
disordered and omitted. The two S atoms on the phenylene core of L
are shown here as disordered over all four (e.g., the 2, 3, 5, and 6)
positions. The large central yellow sphere highlights the void feature.
nanoparticles of MOF being less significant. Further studies are
apparently needed to explain the difference between the lumi-
nescence data observed herein of the MOF-5 single crystals and
those of the nanoparticle samples.17
Compared to the emission maximum (λmax) of ∼390 nm for
the powder sample of the dbc molecule (curve (a) of Figure S5 in
the Supporting Information), the sulfur functions in molecule L
significantly red-shift the emission maximum of its solid sample
to a longer wavelength of 500 nm (with a weaker side peak at
410 nm; see Figure 2), which is consistent with the nꢀπ
conjugation between the sulfur groups and the aromatic core
in molecule L. Similarly, for the framework compound 1, a broad
and intense emission was observed with λmax = 475 nm, together
with a weaker sideband with λmax at 410 nm (e.g., compare curve
(b) of Figure S5 in the Supporting Information and spectrum a of
Figure 2). As seen in Figure 2 (inset), the powder of 1 emits
bright cyan (blue-green) light under UV radiation (365 nm).
Overall, the major features of the emission spectra for L and 1 are
similar, thus indicating a largely ligand-based luminescent pro-
cess. Ligand-based photoluminescent phenomena were also
observed in the isoreticular systems of IRMOF-11 (Zn4O(4,5,9,10-
tetrahydropyrene-2,7-dicarboxylate)3), IRMOF-13 (Zn4O(2,7-
pyrene dicarboxylate)3), and Zn4O(stilbene dicarboxylate)3), in
which the greater degree of π-conjugation in the organic linkers
was presented as being responsible for the ligand-based lumines-
cence.15 In this line of argument, the ligand-based luminescence
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Figure 3. Powder X-ray diffraction (XRD) patterns (Cu KR, λ =
1.5418 Å) from (a) a simulation from the single-crystal structure of 1,
Figure 2. Room-temperature solid-state emission spectra from (a) a
desolvated sample of 1, (b) an NB-loaded sample of 1, (c) an NB-
depleted sample, and (d) a powder sample of molecule L (excitation
wavelength λex = 360 nm). The inset shows photographs of the samples
that generated spectra a, b, and c.
(b) an activated sample of 1 [Zn4O (L)3], (c) a nitrobenzene (NB)-
3
loaded sample [Zn4O L3 (NB)(H2O)3] prepared from treating the
3
3
sample that generated spectrum b with NB vapor, and (d) an NB-
depleted sample prepared from evacuating the sample that generated
spectrum c at 120 °C for 5 h.
of 1 is in accord with the conjugation between the terephthalate
π-system with the electron-donating thioether sulfur substitu-
ents in molecule L.
The strong fluorescence, as well as enhanced air stability of the
network of 1, facilitate the testing of its potential use as a
fluorescent sensor. In our preliminary tests (conducted in air,
with no measures to exclude moisture taken), we discovered that
immersing an activated sample of 1 in nitrobenzene (NB) vapor
for 2ꢀ3 days results in significant inclusion of NB molecules
in the pores of 1 (see Figure S6 in the Supporting Information
for the IR evidence for the uptake of NB), and effectively
suppresses the luminescence, whereas molecules such as benzene
and toluene, under similar conditions, do not significantly alter
the fluorescence intensity or appearance of the solid sample of 1.
Moreover, as shown in Figure 2, the fluorescence intensity of the
host net in 1 can be substantially recovered upon removal of the
NB guests (e.g., by evacuating at 120 °C for 5 h; removal of NB
molecules checked by IR; see Figure S6 in the Supporting Infor-
mation). The structural integrity of the host net was maintained
in this guest exchange process, as seen in the powder XRD
patterns of Figure 3—the positions of the peaks for the NB-
loaded sample and NB-free samples are slightly shifted, while the
relative intensity changes (e.g., in the 111 and 400 peaks) are
apparently due to the electron densities from the NB molecules.
The fluorescent quenching of nitrobenzene on aromatic fluoro-
phores is well-documented, and it is generally considered to
involve a charge transfer from the fluorophore excited state to the
low-lying molecular orbital associated with the strongly electron-
withdrawing nitro group.18
In a comparative study, we also tested the fluorescent
responses of the prototypical MOF-5 system (in an N2-filled
glovebox) and found that the MOF-5 samples in their activated
form also feature a similar response to NB (see Figure S5 in the
Supporting Information for the emission spectra, Figure S7 in
the Supporting Information for the IR spectra regarding the
uptake of NB, and Figure S8 in the Supporting Information for
the powder XRD patterns), although better calibrated tests are
still needed to compare 1 and MOF-5, with regard to the
sensitivity to NB. The use of MOF materials as fluorescent
sensors for organic nitro compounds19 and other analytes20 is
starting to attract interest and further studies on the properties of
Figure 4. Conformations of the TMBD molecule in the crystal
structure of PbTMBD (A) before and (B) after the uptake of HgCl2.
The HgꢀS distance in panel B is 2.692 Å.
1 and related systems might shed more light on its suitability for
practical applications in this direction.
Sorption of HgCl2. As demonstrated in the previously re-
ported network PbTMBD,9c the presence of the methylthio
groups (from the molecule TMBD) in the pores of the network
solid allowed for the uptake of HgCl2. However, that particular
test entailed a stringent condition of heating at 100 °C for 48 h in
a sealed tube with HgCl2 powder, crystals of PbTMBD, and
benzene, while placing the PbTMBD crystals in an HgCl2
solution (e.g., in THF or ethanol) at room temperature did
not lead to significant uptake of HgCl2. We suspect that the
sluggish binding of HgCl2 therein might be due to the steric
factors around the sulfur functions, e.g., the methylthio groups
are directly attached to the rigid and crowded hexasubstituted
benzene cores, and they are installed in a compact microporous
milieu.
Specifically, the pristine crystal of PbTMBD features the space
group C2/c, with the 1,2-bis(methylthio) groups all adopting the
ab (or up/down) configuration—i.e., one methyl group being
above and the other below the benzene plane (see Figure 4).
After taking up HgCl2, the space group changed to C2/m, with
the reflection symmetry imposed on one of the TMBD mol-
ecules (i.e., one out of three) to enforce an aa conformation
(with both methyl groups on the same side of the benzene plane;
see Figure 4). With both methyl groups on the same side of the
benzene plane, the Hg(II) atom can approach the S atoms from
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Figure 6. Room-temperature solid-state emission spectra from (a) a
desolvated sample of 1 [Zn4O (L)3]; (b) a HgCl2-loaded sample of 1
3
[Zn4O (L)3 (HgCl2)0.72]; and (c) a HgCl2-depleted sample from
3
3
Figure 5. Powder XRD patterns (Cu KR, λ = 1.5418 Å) of (a)
calculated pattern from single crystal structure; (b) a desolvated sample
of 1 [Zn4O (L)3]; (c) a HgCl2-loaded sample of 1 [Zn4O (L)3
washing the sample that gave spectrum b with acetonitrile (excitation
wavelength λex = 360 nm). The inset shows photographs of the samples
that yielded spectra a, b, and c.
3
3
3
(HgCl2)0.72]; and (d) a HgCl2-depleted sample obtained from washing
the sample that generated spectrum c with acetonitrile.
100 °C for 2 days and then cooled to room temperature. The
product was filtered and washed with freshly distilled acetonitrile
and then dried in vacuo overnight (yield = 4.5 mg). ICP
elemental analysis yields a Zn/Hg ratio of 36.4:1, indicating that
∼85% of the HgCl2 component has been removed from the
HgCl2-loaded sample (see the Experimental Section for the
details). Powder XRD studies on the HgCl2-depleted sample
thus recovered revealed that the relative intensity of the peaks
was largely restored to resemble that of the pristine samples of 1,
consistent with the recovering of the porous features of the host
net (pattern d in Figure 5).
the other side, without being hampered by the methyl groups.
Since such an open and accessible geometry is more difficult with
the ab conformation in the pristine structure (C2/c), the con-
version into the aa conformation becomes especially helpful for
effecting the HgCl2 uptake. Such a conformational change is
apparently difficult with the hexasubstituted configuration, as
well as its being part of a solid state matrix; therefore, prolonged
heating (or other activating measures) is necessary.
By comparison, molecule L is equipped with flexible and
extended sulfur functions, in order to enhance the accessibility
to the incoming HgCl2 species. Therefore, we expected that the
resultant network 1, with its larger porosity and 3D-connected
channels (in comparison to the above PbTMBD), would provide
for better efficiency of HgCl2 uptake. Indeed, even with ethanol
(absolute) as the solvent for HgCl2 and at a concentration of
0.5% (w:w), significant HgCl2 uptake can be achieved by simply
mixing in the activated crystals of 1 at room temperature (see the
Experimental Section for details). A new composite with the
formula Zn4O L3 (HgCl2)0.72 (EtOH)1.5 (i.e., a molar ratio of
To probe the lower limit of the HgCl2 concentration achiev-
able by 1 as an absorbent, a relatively large amount of activated
sample of 1 (e.g., 10.0 mg) was added to an ethanol solution
(2.0 mL) of HgCl2 with the initial concentration of 84 mg/L.
After standing undisturbed at room temperature for six days, the
concentration of HgCl2 in the solution was decreased to 5.0 mg/L
(equivalent to 5.0 ppm), as determined by the ICP method,
indicating that >94% of the HgCl2 was thus removed in this test.
The uptake of HgCl2 also impacts significantly the photo-
luminescence of the host net of 1. As seen in Figure 6, ∼90% of
the original emission of the activated sample of 1 (spectrum a)
was suppressed after the HgCl2 uptake (to form Zn4O L3
3
3
3
Zn/Hg = 5.56:1, determined from ICP elemental analyses; the
formula also fits with results of CHN elemental analyses, see the
Experimental Section), for example, can be achieved after immer-
sing the crystals in the HgCl2 ethanol solution for 14 days.
The uptake of HgCl2 into the host net of 1 was also monitored
by powder XRD studies. As shown in Figure 5, the lower angle
peaks of 111 and 200 greatly diminished for the HgCl2-loaded
sample (pattern c), whereas the higher-angle peaks (e.g., 220,
400) become relatively strong. Such a drastic change in diffrac-
tion peak intensities reflects the significant increase of electron
density in the channel region (i.e., the system becomes less
porous), as a result of the uptake of HgCl2, while indicating that
the original crystal lattice of 1 remains intact in the process.
Moreover, the majority of the HgCl2 component in the
HgCl2-loaded sample of 1 can be removed by a polar solvent
such as acetonitrile, while maintaining the structural integrity of
the host net of 1. For example, a mixture of the above Zn4O L3
3
3
(HgCl2)0.72, spectrum b). Correspondingly, the powder of the
loaded sample features a dim emission that is barely visible. The
fluorescence substantially recovers upon removing the HgCl2
component (by a polar solvent such as acetonitrile, same as that
previously mentioned). Fluorescence quenching by heavy-metal
ions is a well-documented phenomenon and occurs via many
intramolecular/intermolecular pathways, which include spinꢀ
orbit coupling, energy transfer, and electron transfer.21 In parallel
to the solid-state behaviors observed herein, fluorophors
equipped with thioether side chains also exhibit effective quench-
ing by Hg(II) ions in organic solution.22 Further studies on MOF
systems with the combined features of photoluminescence,
porosity, and thioether donor function should lead to a better
understanding of the potential applicability in the uptake and
monitoring of Hg(II) ions and other metal species.
3
3
(HgCl2)0.72 (EtOH)1.5 (5.0 mg) and freshly distilled anhydrous
Thermogravimetric Analyses. As shown in curve (a) of
Figure S9 in the Supporting Information, no significant weight
3
acetonitrile (2.5 mL) in a sealed Pyrex glass tube was heated at
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loss below 240 °C was observed for the solid sample of free ligand
(L). Subsequently, a sharp weight loss (67.6%) between 250 °C
and 354 °C occurred, which possibly involves the departure of
the ethylenethioether side chain and the carboxylic groups (c.f.,
the CH2CH2SCH3 and COO components together account for
63.5% of the total weight of L). As shown in the TGA plot (curve
(b)) of the as-made sample of 1, the weight loss proceeds rather
smoothly (gradual and continuous weight loss) from room
temperature up to 470 °C. However, two stages of weight losses
can be roughly identified. The first stage (from room tempera-
ture up to 170 °C) registers a weight loss of 21.8%, closely
corresponding to the weight fraction of the DMF and water
guests calculated from the formula determined from the crystal-
lography and elemental analysis [i.e., DMF and water weight
proportion in Zn4O L3 (DMF)4(H2O)4: 20.6%], whereas the
second-stage weight loss (31.2%, from 170 °C to 470 °C) likely
involves the decomposition of the host framework. Notice that
the onset decomposition temperature for the host net in the as-
made sample of 1 (i.e., 170 °C) is substantially lower than that of
the free ligand L (ca. 240 °C), which could be due to the
especially strong collision between the floppy side chains of L and
the nearby Zn4O nodes (e.g., the S atoms on the side chains
could even attack the Zn2þ centers at higher temperatures).
For the activated sample (i.e., prepared from subjecting the as-
made sample to solvent exchange with CHCl3), only a minimal
weight loss (i.e., <2.0%; see curve (c) in Figure S9 in the Suppor-
ting Information) observed below 200 °C, which is probably due
to residual CHCl3 molecules. From 200 °C up to 470 °C, a
sharper decrease of 39.5% involving the decomposition of the
host net occurred, which, notably, largely parallels that of the
corresponding stage of plot 2 (for the as-made sample).
to be especially effective in suppressing the moisture
sensitivity that is symptomatic of the prototypal network
of MOF-5.
(2) Thioether groups are generally stable in air and, in the
hands of a trained synthetic chemist, can be readily modi-
fied in variable lengths and arrangements—for example,
thioether groups have even been used as mainstays for
the assembly of dendritic scaffolds.23
(3) Thioether groups are generally compatible with the
Sonogashira and Suzuki CꢀC coupling reactions and
other well-established synthetic protocols, thus allowing
for the thioether groups to be efficiently integrated into
diverse arrays of organic molecular scaffolds.
The functional advantages of the thioether groups become
especially attractive in light of the recent development of solid
state network materials: ever larger pores (e.g., over 20 Å) are
being achieved in increasing numbers of MOFs,24 and in the
related boroxine or boronate ester-based covalent organic
frameworks25 (COFs). In fact, one does not have to be limited
to crystalline systems: microporous polymer systems, for exam-
ple, are also have been the subjects of intense investigation.26
Conjoining the rich and rapid developments in framework mate-
rials with the functional/synthetic advantages of the thioether
groups, we foresee a wide growth area in which pore features and
functions can be systematically tuned by thioether side chains, to
optimize the characteristic solid-state properties such as guest
absorption/separation, metal uptake, and the consequential
developments with regard to catalysis and advanced electronic
properties within the matrices of framework materials.
3
3
’ ASSOCIATED CONTENT
Unlike curves (b) and (c) in Figure S9 in the Supporting
Information, the TGA plot (curve (d) in Figure S9 in the
Supporting Information) for the nitrobenzene-filled sample of
1 features a more monotonic weight loss below 450 °C, with no
distinct stages of weight losses observed. The monotonic process
of weight loss is presumably related to the less-volatile nature of
the nitrobenzene molecules (compared with the guests of DMF,
water, and CHCl3 for the samples that yielded curves (b) and (c)
in Figure S9 in the Supporting Information). The weight loss (up
to 180 °C) is 11.7%, corresponding to the removal of one
nitrobenzene molecule and three water molecules from the
formula Zn4O L3 (NB)(H2O)3 (calculated: 11.2%). The ensu-
S
Supporting Information. Additional experimental pro-
b
cedures, full crystallographic data in CIF format for 1, powder
diffraction patterns for bulk samples of 1, and solution NMR
spectra, IR spectra, and TGA plots of 1 and related compounds.
This material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: zhengtao@cityu.edu.hk.
3
3
ing weight loss beyond 180 °C very likely involves the collapse of
the host net and decomposition of the organic components.
’ ACKNOWLEDGMENT
This work is supported by City University of Hong Kong
(Project No. 7002321) and the Research Grants Council of
HKSAR [Project 9041322 (CityU 103009)]. The single crystal
diffractometer was funded by NSF (Grant No. 0087210), the
Ohio Board of Regents (Grant No. CAP-491), and by YSU.
’ CONCLUSION
This work serves to further highlight the multiple advantages
of the thioether units for functionalizing metalꢀorganic frame-
works (MOFs) and serves to stir our imagination in several
directions. The advantages include the following:
(1) In general, thioethers are moderate donors (e.g., com-
pared with the stronger-binding phosphines), with a
distinct chemical softness that contrasts with the hard
and ionic carboxylate units. As a result, the MSES (CH3-
SCH2CH2Sꢀ) side chain, despite the chelating disposi-
tion of the two S atoms, refrains from bonding to the
chemically hard Zn(II) centers, and thereby minimizes
interference on the formation of the Zn4O-carboxylate
host net. The chemical softness also implies significant
hydrophobicity of the thioether side chains, which proves
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were told that the MOF-5 single crystals in her hands emit at 525 nm,
like the nanosized samples. We have repeated our experiments a few
more times, and yet we continued to observe the same 420-nm emission.
2947
dx.doi.org/10.1021/cm200557e |Chem. Mater. 2011, 23, 2940–2947