A Luminescent Terbium MOF Containing Hydroxyl Groups Exhibits Selective Sensing of Nitroaromatic Compounds and Fe(III) Ions

Article abstract of DOI:10.1021/acs.cgd.6b00930

A new 3D porous Tb(III)-ion based PMOF, {[Tb(L)(DMA)]·(DMA)·(0.5H₂O)} (1) (where L = L^3-, H₃L = 3′-hydroxybiphenyl-3,4′,5-tricarboxylic acid), with uncoordinated hydroxyl groups decorating the pores has been solvothermally synthesized in moderate yield. The hydroxyl groups act as potential interaction sites to sensitize the luminescence output of the Tb³⁺ ion. Luminescence studies reveal that the PMOF can selectively and reversibly sense nitroaromatic compounds and Fe(III) ions in DMF suspension through fluorescence quenching observable under UV light. The probable mechanisms for such sensing processes are, for the nitroaromatic compounds, the host-guest interactions with the PMOF and, for Fe³⁺ ion, the competitive absorption of excitation energy.

Full text of DOI:10.1021/acs.cgd.6b00930

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Article
A Luminescent Terbium MOF Containing Hydroxyl Groups Exhibits
Selective Sensing of Nitroaromatic Compounds and Fe(III) Ions
Sanchari Pal, and Parimal K. Bharadwaj
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00930 • Publication Date (Web): 29 Aug 2016
Downloaded from http://pubs.acs.org on August 31, 2016
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A Luminescent Terbium MOF Containing Hydroxyl Groups
Exhibits Selective Sensing of Nitroaromatic Compounds and Fe(III)
Ions
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Sanchari Pal and Parimal K. Bharadwaj*
Department of Chemistry, Indian Institute of technology Kanpur, 208016, India
Email: pkb@iitk.ac.in
ABSTRACT:
A
new
3D
porous
Tb(III)ꢀion
based
PMOF,
{[Tb(L)(DMA)].(DMA).(0.5H2O)} (1) (where L= L3ꢀ, H3L = 3'ꢀhydroxybiphenylꢀ3,4',5ꢀ
tricarboxylic acid) with uncoordinated hydroxyl groups decorating the pores have been
solvothermally synthesized in moderate yield. The hydroxyl groups act as potential interaction
sites to sensitize the luminescence output of the Tb3+ ion. Luminescence studies reveal that the
PMOF can selectively and reversibly sense nitroaromatic compounds and Fe(III) ions in DMF
suspension through fluorescence quenching observable under UV light. For the nitroaromatic
compounds, it is the hostꢀguest interactions with the PMOF while for Fe3+ ion, it is the
competitive absorption of excitation energy that are the probable mechanisms for such sensing
processes.
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INTRODUCTION
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Design, synthesis and characterization of porous metal organic frameworks (PMOFs)
continue to be a subject of intense research owing to their various practical applications.1ꢀ7
Particularly, chemical sensing through change in the luminescence properties of PMOFs is of
great practical importance.8ꢀ11 The capability of hostꢀguest interactions that modulate the
luminescence output of PMOFs make them promising candidates as chemical sensors.
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Sensing of nitroaromatic compounds (NACs) as well as cations especially Fe3+ ions are
of practical significance. Nitroaromatic compounds pose serious environmental hazards due to
their explosive nature.12 For example, compounds such as nitrobenzene (NB), 4ꢀnitrotoluene
(NT), 2,4ꢀdinitrotoluene (DNT), trinitrotoluene (TNT) and so on, are on the list of antiꢀterrorism
or environment security in different countries. Trinitrophenol (TNP) is very toxic to human
health and its usage in the chemical industry may lead to fatal environmental consequences.
Metabolism of TNP generates picramic acid (2ꢀaminoꢀ4,6ꢀdinitrophenol) as a byꢀproduct that
shows ten times more mutagenic activity than TNP.13 Due to moderate vapor pressure and
limited chemical reactivity, detection of NACs is not easy. Although different analytical
techniques are known to detect NACs, they are either very expensive and timeꢀconsuming, or are
difficult to operate.14 However, NACs being electronꢀdeficient compounds can possibly enter
into the voids of a porous luminescent PMOF that provides electronꢀrich environment. The
efficient hostꢀguest interactions can modulate the luminescence characteristics of the PMOF thus
providing a rapid, inexpensive technique for the detection of trace quantities of NACs. On the
other hand, detection of Fe3+ is also very important since it is a vital element in the environment
as well as in biological systems. It is best known for its role in hemoglobin formation, electron
transfer, oxygen metabolism processes, in the synthesis of DNA and RNA, etc. Deviation from
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normal permissible limit of iron concentration causes serious health disorders. Accumulated data
on iron metabolism suggest that alterations in the intracellular pool of 'redoxꢀactive' iron (also
called 'chelatable iron') can contribute substantially to a variety of injurious processes such as
ischemiaꢀreperfusion injury, ethanol toxicity and oxidative injury after nutrient deprivation and
so on.15
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Lanthanide containing PMOFs have drawn immense attention for their sensing
applications.16 The emission from a lanthanide ion is generally weak because it involves spinꢀ
and/or parityꢀforbidden f–f transitions. Integration of these ions with πꢀconjugated organic
linkers enables electron transfer upon excitation from the linker to the lanthanide ion that greatly
enhances its characteristic emission. This phenomenon is known as the “antenna effect” (Figure
1).17 In lanthanide containing PMOFs, inclusion of guest molecules can alter the emission
intensity by interrupting the electron transfer due to favorable hostꢀguest interactions. Since the
emission profiles of lanthanide ions are highly characteristic and can be easily observed under
standard UVꢀlamp, it offers advantages over d10 metal ion based PMOF sensors though there are
several reports of the d10 metal ion based PMOF showing high selectivity and sensitivity for the
detection of NACs.18ꢀ23 In recent years, a few Eu3+/Tb3+ based luminescent PMOFs have been
reported for the detection of nitroaromatics24ꢀ27 and different metal ions.28ꢀ32
Figure 1. “Antenna effect” induced luminescence in 1.
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In the present work, a new rigid linear linker has been designed to provide uncoordinated
hydroxyl groups in the pores. It forms a 3D PMOF, {[Tb(L)(DMA)].(DMA).(0.5H2O)} (1)
(where L=L3ꢀ, H3L=3'ꢀhydroxybiphenylꢀ3,4',5ꢀtricarboxylic acid) with uncoordinated hydroxyl
groups decorating the channels and hosting highly disordered solvent molecules in the pore. It
emits bright visible characteristic green light under UVꢀlamp. The luminescence is selectively
quenched by nitroaromatic compounds and Fe3+ ions in DMF suspension.
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EXPERIMENTAL SECTION
Materials and measurements
Reagent grade methylꢀ4ꢀiodosalicylate, benzyl bromide, PdCl2, Tb(NO3)3.6H2O, were
procured from SigmaꢀAldrich and used as received. All solvents were obtained from S. D. Fine
Chemicals, India and purified following established procedures prior to use. Characterizations of
the linker have been carried out by a number of spectroscopic techniques as detailed
previously.33 Powder XꢀRay diffraction (PXRD) patterns of the compounds were recorded with a
Bruker D8 Advance diffractometer equipped with nickel filtered CuKα radiation.
Thermogravimetric analyses (TGA) (5 ˚C/min heating rate under nitrogen atmosphere) were
performed with a Mettler Toledo Star System. Solidꢀstate as well as dispersed phase absorption
and emission spectra were recorded using UV–VIS–NIR spectrophotometer (Varian Model Cary
5000) and Jobin Yvon Horiba Fluorologꢀ3 spectrofluorimeter at room temperature. EDS analysis
was done using TungstenꢀElectronꢀMicroscope (JSMꢀ6010LA; JEOL). Gas adsorption
measurements were performed using automatic volumetric BELSORPꢀMINIꢀII adsorption
equipment.
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Single crystal Xꢀray studies
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Single crystal Xꢀray data of 1 was collected at 100 K on a Bruker SMART APEX CCD
diffractometer using graphite monochromated MoKα radiation (λ = 0.71073 Å) and the structure
solution was completed as described earlier.33 Several DFIX commands were used for fixing
some bond distances due to inherent disorder. The disordered guest solvent molecules could not
be located in the successive difference Fourier maps and hence PLATON34 squeeze refinement
was performed. The crystal and refinement data are collected in Table S1 (Supporting
Information). Selective bond distances and angles are given in Table S2 (Supporting
Information).
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Luminescence measurements
All the measurements were done at room temperature. For solid state measurements,
activated crystals (1′) were ground to powder. To investigate luminescence response in different
solvents, 1 mg of the sample (1′) was dispersed in 5 mL of the corresponding solvent, sonicated
and further diluted to equal volume (20 mL) in each case and emission intensity was measured.
For all quantitative measurements, 5 mg of the powered sample (1′) was dispersed in 5 mL DMF
and sonicated until homogeneous suspension was obtained. This solution was then diluted to 100
mL to afford the DMFꢀsuspended 1′ and used as the stock solution for all the measurements.
Synthesis of the Ligand
Synthesis of the linker H3L could be achieved in several steps as illustrated in Scheme 1.
Details of the synthetic procedure are given in the Supporting Information.
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Scheme 1. Synthetic scheme for the linker H3L.
Synthesis of the Complex
{[Tb(L)(DMA)].(DMA).(0.5H2O)}n (1). This was synthesized by mixing H3L (0.030 g, 0.1
mmol), Tb(NO3)3·6H2O (0.09 g, 0.2 mmol), DMA (2 mL) and H2O (1 mL) in a Teflonꢀlined
autoclave that was heated under autogenous pressure at 100 oC for 72 h followed by slow cooling
to room temperature. The resulting colorless needleꢀshaped crystals were isolated by filtration in
~45% yield based on H3L. Anal. Calcd. for C23H26N2O9.5Tb: C, 43.07; H, 4.09; N, 4.37%.
Found: C, 43.55; H, 4.27; N, 4.17%. IR (cmꢀ1): 3428(br), 2927(s), 1618(m), 1518(s), 1454(s),
1375(s), 1296(s), 1211(s), 1010(s), 774(s), 731(s) (Figure S12, Supporting Information).
RESULTS AND DISCUSSION
The PMOF 1 could be easily synthesized in single crystal form under solvothermal
condition in moderate yields. The single crystal Xꢀray diffraction study reveals that it crystallizes
in the tetragonal space group I41/a. The asymmetric unit contains one Tb(III) ion, one linker (L3ꢀ,
denoted as L) and one coordinated DMA molecule (Figure 2a).
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(a)
(b)
(c)
Figure 2. (a) Asymmetric unit of 1, (b) coordination environment around Tb(III), (c) binding
mode of the linker.
Each Tb3+ ion is coordinated to eight O atoms: one bidentate carboxylate, five monoꢀ
dentate carboxylate from five different L and one O atom from a DMA molecule (Figure 2b).
The TbꢀO bond distances lie in the range, 2.716(8) — 2.268(1) Å that are comparable to
reported TbꢀO bond distances.26 The binding mode of the carboxylate towards Tb3+ ion is µ6: η1:
η1: η1: η2: η1: η1 (Figure 2c). The linker is completely deꢀprotonated during the reaction and
binds Tb3+ ions to form a oneꢀdimensional chain along the crystallographic cꢀaxis with the
nearest TbꢀTb distance being 4.330(9) Å (Figure S15a, Supporting Information). These chains
are interlinked with the linker L along the crystallographic a and b axes (Figure S15b, c,
Supporting Information) forming a threeꢀdimensional boxꢀlike structure (Figure 3). Topological
simplification shows that the framework is 6,6ꢀc net forming a new network topology with
Schläfli point symbol {413.62}{48.67} (Figure S15e, Supporting Information). The solvent
molecules present in the cavity are highly disordered; hence, the exact solvent composition was
evaluated by the combination of TGA and elemental analysis and agreement with PLATON
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calculated solvent accessible void volume (5276 Å3; 45%) of the unit cell. The channel
dimensions along the crystallographic c direction is 15.223(3)×15.223(3) Å2 and the free
hydroxyl moieties of the linker are exposed in the channels along a and bꢀaxis. Metalꢀbound
DMA molecules are also directed towards the centre of the channels as viewed along cꢀaxis.
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Figure 3. A perspective view of 1 along crystallographic c-axis (guest free, H atoms are omitted
for clarity).
Thermogravimetric analysis reveals that the PMOF loses 15% of its weight (calculated
o
14.98%) up to 120 C that corresponds to the loss of lattice water and DMA molecules. The
coordinated DMA molecules are removed at higher temperature (up to 270 oC) showing a weight
loss of 14% (calculated 13.57%). Complete decomposition of the compound is achieved beyond
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400 C (Figure S13, Supporting Information). The powder Xꢀray diffraction pattern of the
synthesized crystals also agrees well with the simulated pattern and confirmed its phase purity.
Furthermore, the VTꢀPXRD data indicate that the overall framework integrity of 1 is retained at
temperatures at least up to 150 °C (Figure S14, Supporting Information). Single crystals of 1
were kept in EtOH for a week and then heated to 120 oC under 1 mm pressure for 10 h to obtain
the activated compound 1′. The powder pattern of 1′ matched well with the original complex 1
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(Figure S14, Supporting Information). Due to the porous character of the framework and
availability of surface functionalized channels of 1, gas adsorption studies were conducted up to
a relative pressure (p/po) of 1.0 bar. Gas adsorption characteristics of 1′ were not significant and
hence not pursued further (Figure S29, Supporting Information).
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The solidꢀstate photoluminescence spectra of the free linker (H3L) and 1′ were measured
under ambient conditions by exciting at 345 and 350 nm respectively (Figure 4). It is wellꢀknown
that Tb3+ ion exhibits luminescence properties due to fꢀf transitions that are both spin and parityꢀ
forbidden, thereby having low intensity. However, its emission intensity can be enhanced by
integrating Tb3+ ions with conjugated systems to cause efficient energy transfer from ligand to
the Tb3+ꢀcenter. Thus, 1′ exhibits the characteristic enhanced luminescent emissions at λmax
values of 490, 544, 584, 622 nm, upon excitation at 350 nm, which can be assigned16 to 5D4 →
7F6, 5D4 → 7F5, 5D4 → 7F4 and 5D4 → 7F3 transitions, respectively. The emission arising from the
free ligand (λmax ~ 460 nm for H3L) is not observed here. The absence of ligandꢀbased emission
in 1′ indicates efficient energy transfer from the ligand to the Tb3+ center (“antenna effect”)
greatly enhancing the optical performance of the metal ion. The most prominent characteristic
emission peak of 1′ at 544 nm makes it bright green under standard UVꢀlamp visible in naked
eye which is an added advantage for its sensory application. Due to such luminescence property
of 1′, its emission characteristics with different electronꢀdeficient NACs and metal ions were
probed. The 3D MOF can act as a host for small guest molecules and energy transfer between
them can produce a strong synergistic effect for the recognition of specific guest molecules. The
complex 1′ was finely powdered and then dispersed in different selected analyteꢀsolutions to
investigate its luminescence properties.
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Figure 4. Solid state luminescence spectra of H3L and that of 1′.
Compound 1′ was first dispersed in different commonly used organic solvents like N,N′ꢀ
dimethylacetamide (DMA), N,N′ꢀdimethylformamide (DMF), ethanol (EtOH), acetonitrile
(MeCN), dioxane, tetrahydrofuran (THF), benzene and toluene as well as nitrobenzene (NB).
Interestingly, it was observed that only in nitrobenzene (NB) solvent, the emission was
completely quenched while with the other solvents, the variation of emission intensity was not
discernible (Figure 5).
(a)
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(b)
(c)
Figure 5. (a) Luminescence spectra of 1′ in different solvents, (b) bar diagram representation of
the D4 → F5 transition at 544 nm in different solvents, (c) visual response of 1′ in various
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solvents monitored under UV light.
This observation provided the scope to explore the sensing ability of 1′ towards other
nitroaromatic compounds. Hence, DMF solutions (1 mM) of different nitroaromatic compounds,
viz. nitrobenzene (NB), 1,4ꢀdinitrobenzene (1,4ꢀDNB), 2,4ꢀdinitrochlorobenzene (DNCB), pꢀ
nitrophenol (NP) and 2,4,6ꢀtrinitrophenol (TNP) were prepared. Each of these solutions (1 mL)
was added to a fixed amount of DMFꢀsuspended 1′ (0.1 mL of stock solution). Significant
quenching was observed in each case (Figure 6a). The extent of quenching was calculated by
using the equation (I0ꢀI)/I0×100%, where I0 and I are the luminescence intensities of the stock
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solution of 1′ before and after the addition of the NACꢀanalyte, respectively. Due to high
concentration of the NACꢀanalytes with respect to stock solution, the emission intensity almost
completely quenches in each case. To evaluate the quenching constant quantitative
measurements were performed (Figure S17ꢀS21, Supporting Information) with higher dilution of
the NACꢀanalytes and by using the SternꢀVolmer (SV) equation (I0/I)=KSV[A]+1, where I0
represents the initial intensity of 1′ in absence of analyte, I is the emission intensity in presence
of analyte with molar concentration [A] and KSV is the quenching constant (Mꢀ1). The quenching
efficiency by the analytes are in the following order: TNP > 4NP > DNCB > 1,4ꢀDNB > NB
(Figure 6b).
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(a)
(b)
Figure 6. (a) Quenching efficiency of 1′ in presence of different nitroarmatic compounds
(1mM), (b) corresponding SternꢀVolmer plots of NACꢀanalytes.
To examine the potential of 1′ for sensing of the metal ions, DMF solution containing
different metal ions (0.1 M solution of nitrate salts of Li+, Na+, K+, Mg2+, Al3+, Cr3+, Mn2+, Fe3+,
Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+ and perchlorate salt of Fe2+) were added separately into a fixed
amount of the stock dispersion of 1′. The luminescence intensity quenched in presence of Fe3+
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ions while a moderate intensity reduction was observed upon the addition of Fe2+ or Cu2+ ions.
Other metal ions do not alter the intensity to any noticeable extent as shown in Figure 7.
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(a)
(b)
Figure 7. (a) Luminescence intensity of 1′ in presence of different metal ions (0.1 M) in DMF
solution, (b) bar diagram representation of relative intensity of the same.
Interference by other metal ions with the detection of Fe3+ ion was investigated by
competitive experiments. The results indicated that the presence of other metal ions did not make
any significant change in the sensing of Fe3+ ions (Figure S23, Supporting Information).
Quantitative emission titration experiments were also performed with Fe3+ ions. With the
increasing concentration of Fe(III), the emission intensity of DMFꢀsuspended stock solution of 1′
decreased gradually (in the concentration range of 10ꢀ6 to 10ꢀ2 M) as shown in the figures 8a and
S24 (Supporting Information). In the concentration range of 10ꢀ4 to 10ꢀ3 M, a good linear
correlation (R2 = 0.99243) between the quenching efficiency and the amount of Fe3+ was
observed (Figure 8b).
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(a)
(b)
Figure 8. (a) Variation of luminescence intensity of 1′ suspension with different concentration of
Fe(NO3)3, (b) corresponding SternꢀVolmer plot.
For probing reversibility of the luminescence property of 1′, it was dispersed in a DMF
solution of ferric nitrate/trinitrophenol and centrifuged after one day. After washing several times
with fresh DMF, the luminescence intensity of 1′ was measured. Variation of the luminescence
intensity was not significant at least up to three sensingꢀrecovery cycles in either case indicating
reversibility of sensing (Figure S25, Supporting Information).
Probable mechanism of sensing
The probable mechanism for quenching of emission of 1′ in presence of the experimental
analytes might be due to alternation in crystal structure, metal ion exchange, hostꢀguest
interactions or competition of absorption of excitation energy between the MOF and the analytes.
The PXRD patterns of 1, NACꢀimmersed 1′ and 1′ immersed in a metal salt solution (ferric
nitrate, cupric nitrate) showed good agreement suggesting that the basic framework remained
unaltered (Figure S22, S26, Supporting Information). An examination of the IR spectra of 1 and
nitrobenzene immersed 1 showed that the peaks in 1 to be broadened and slightly redꢀshifted
with respect to those of the pure crystals of 1 (Figure S27, Supporting Information) suggesting
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interactions of nitrobenzene with the host framework.26,35 It is, therefore, presumed that the
NACs, being electronꢀdeficient in nature, can interact with the electronꢀrich cavity of the
framework leading to quenching of the emission of 1′.26ꢀ28
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For probing the Fe3+ induced quenching, the possibility of transmetalation was
investigated. However, no transmetalation reaction in 1 was observed upon keeping crystals of 1
in a 0.5 M DMF solution of ferric nitrate even after 7 d at RT as proven by the EDS experiments
(Table S3, Supporting Information). It is presumed, therefore, that there might exist a
competitive absorption of excitation energy between Fe(III) ions and 1′ which diminish the
essential energy requirement for the antenna effect. The overlapping of absorption band of DMF
solution of Fe(III) ion (strong absorption in the range of 250ꢀ500 nm) and the excitation spectra
of 1′ (~350 nm) also indicates this possibility (Figure. S28, Supporting Information). Similar
observations were found in earlier cases as well.28ꢀ31
CONCLUSION
A new luminescent 3D porous TbꢀMOF which contains free hydroxyl group in the
channels was synthesized and characterized. This framework has established another example as
a potential probe for sensing nitroaromatic compounds as well as cation sensing. The dispersed
solution of activated 1 in DMF medium reveals selective and remarkable quenching effect in
presence of Fe(III) ions and moderately towards Fe(II)/Cu(II) ions. Such sensing of NACs and
Fe(III) is reversible in nature and probably occurred through hostꢀguest interaction and
competitive absorption of excitation energy respectively. Other lanthanideꢀMOFs with this
ligand have been synthesized and further studies are currently underway to examine their
potential applications.
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ASSOCIATED CONTENT
Supporting Information.
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The Supporting Information is available free of charge on the ACS Publications website at DOI:
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Additional experiments and structural figures, Xꢀray crystallographic data, TGA, and PXRD
patterns of 1 (PDF).
Accession Codes
CCDC 1485393 contains the supplementary crystallographic data for this paper. These data can
be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
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 Author
*Eꢀmail: pkb@iitk.ac.in.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
We gratefully acknowledge the financial support received from the Department of Science and
Technology, New Delhi, India (to PKB) and SRF from the CSIR to S.P. We also acknowledge
Dr. Subhadip Neogi for gas adsorption measurements.
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For Table of Contents Use Only
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A Luminescent Terbium MOF Containing Hydroxyl Groups
Exhibits Selective Sensing of Nitroaromatic Compounds and Fe(III)
Ions
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Sanchari Pal and Parimal K. Bharadwaj*
A new 3D porous Tb(III)ꢀion based PMOF has been synthesized which selectively sense
nitroaromatic compounds and Fe(III) ions in DMF suspension by fluorescence quenching
mechanism.
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A new 3D porous Tb(III)-ion based PMOF has been synthesized which selectively sense nitroaromatic
compounds and Fe(III) ions in DMF suspension by fluorescence quenching mechanism.
169x75mm (96 x 96 DPI)
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