A dual-functional luminescent Tb(III) metal-organic framework for the selective sensing of acetone and TNP in water

Article abstract of DOI:10.1039/c7ra13494k

Herein, a novel water-stable luminescent terbium metal-organic framework, {[Tb(L₁)(L₂)_0.5(NO₃)(DMF)]·DMF}_n (TPA-MOF), with 1,10-phenanthroline (phen) and 3,3′,5,5′-azobenzene-tetracarboxylic acid (H

Full text of DOI:10.1039/c7ra13494k

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A dual-functional luminescent Tb(III) metal–organic
framework for the selective sensing of acetone and
TNP in water†
Cite this: RSC Adv., 2018, 8, 10746
ab
and Naresh Kumarc
*
Nidhi Goel
‡
Herein, a novel water-stable luminescent terbium metal–organic framework, {[Tb(L1)(L2)0.5(NO3)(DMF)]$
DMF}n (TPA-MOF), with 1,10-phenanthroline (phen) and 3,30,5,50-azobenzene-tetracarboxylic acid
(H4abtc) ligands was solvothermally synthesized and structurally characterized. TPA-MOF possesses
a two-dimensional (2D) extended framework featuring an 8-connected uninodal SP2-periodic net
Received 20th December 2017
Accepted 23rd February 2018
topology with the Schlafli point symbol of {3^12;4^14;5^2}. The p-electron rich luminescent TPA-MOF
¨
exhibits four characteristic emission bands of Tb3+ ion and acts as a selective and sensitive probe for
acetone as well as the electron deficient 2,4,6-trinitrophenol (TNP). Moreover, gas sorption studies
confirm that TPA-MOF displays ultra-micropores and adsorbs moderate amounts of N2 and CO2.
DOI: 10.1039/c7ra13494k
rsc.li/rsc-advances
bladder tumour, which make it dangerous to human beings.4
Therefore, the fast detection of SOMs and NAC explosives is
1. Introduction
quite important for environmental safety, human health, and
national security.5 Fluorescence quenching-based chemo-
sensors have been extensively investigated for the detection of
SOMs and NAC explosives due to their high sensitivity, cost
efficiency, and portability.6 In the past few years, a very small
number of luminescent Ln-MOFs have been discussed for the
detection of acetone and TNP.7 In luminescent Ln-MOFs,
delocalized p-electrons of organic chromophores increase
electrostatic interactions between the MOF and electron-
decient NAC analytes; this results in uorescence quenching
of the initial intensity of the MOF. Moreover, the polymeric
backbone of MOFs facilitates efficient exciton migration to
intensify the sensitivity of MOFs in the detection.8 Hence, an
appropriate choice of organic chromophores and metal ions
plays a signicant role in the fabrication of luminescent
chemical sensors. In this regard, we have focused on the rigid
The advancement of porous lanthanide metal–organic frame-
works (Ln-MOFs) is a progressive area of research and has
attracted signicant attention in recent years not only due to the
alluring topological structures but also the potential of Ln-
MOFs as functional materials for catalysis, gas storage, sepa-
ration, magnetism, drug delivery, luminescence, and nonlinear
optics.1 The outstanding luminescence properties of Ln-MOFs,
originating from ‘f–f’ transitions through the antenna effect of
organic chromophores, make them more relevant candidates
for chemical sensing.2 The selective and sensitive detection of
hazardous materials is an important issue around the world.
Small organic molecules (SOMs), particularly acetone, are air
and water pollutants, and nitro-aromatic (NAC) explosives are
the major components of various unexploded land mines
worldwide.3 Among the numerous NAC explosives, 2,4,6-trini-
trophenol (TNP) is a powerful explosive and toxic pollutant.
Apart from this, its ingestion can cause critical health disorders
such as skin allergies, anaemia, liver or kidney damage, and
1,10-phenanthroline
(phen)
and
3,30,5,50-azobenzene-
tetracarboxylic acid (H4abtc) ligands. Phen is a well-known
light-harvesting organic chromophore, whereas H4abtc
contains a photochromic azo group as well as a conjugated
p-system that are benecial to improve the luminescence
properties.9 In addition, Ln-MOFs with 3,30,5,50-azobenzene-
tetracarboxylic acid are quite less explored.10
aSchool of Physical Sciences, Jawaharlal Nehru University, Delhi 110067, India
bDepartment of Chemistry, Institute of Science, Banaras Hindu University, Varanasi
221005, India
cDiscipline of Biosciences and Biomedical Engineering, Indian Institute of Technology
Herein, we present the synthesis of a new terbium metal–
organic framework {[Tb(L1)(L2)0.5(NO3)(DMF)]$DMF}n (TPA-
MOF) with phen (L1) and H4abtc (L2) organic ligands and its
selective sensing for acetone and TNP through a uorescence
quenching mechanism. To the best of our knowledge, this is the
rst time that a phen and H4abtc ligands-based terbium metal–
organic framework has been explored for the selective sensing
of acetone and TNP in an aqueous solution.
Indore, Simrol, Indore 453552, Madhya Pradesh, India
† Electronic supplementary information (ESI) available: FT-IR, 1H and 13C NMR,
HRMS, crystal structures, TG pattern, UV-vis and uorescence studies,
crystallographic bond lengths, bond angles, and DFT data. CCDC 1533159. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c7ra13494k
‡ Present address: Department of Chemistry, Institute of Science, Banaras Hindu
University, Varanasi 221005, Uttar Pradesh, India. Email: nidhigoel.chem@
bhu.ac.in.
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magnetically stirred for 20 min at room temperature. The
resulting mixture was placed in a Teon-lined stainless-steel
autoclave that was then heated at 120 ꢁC for 72 h under
autogenous pressure and then cooled down to room tempera-
2. Experimental
2.1. Materials and method
To perform all the experiments, reagents and solvents were
procured from commercial sources and used without further
purication. All solvents used were of standard spectroscopic
grade. Tb(NO3)3$6H2O, 1,10-phenanthroline monohydrate (phen),
nitrobenzene (NB), 1,3-dinitrobenzene (1,3-DNB), 4-nitrotoluene
(4-NT), 2,4-dinitrotoluene (2,4-DNT), 2,4,6-trinitrotoluene (TNT), 4-
nitrophenol (4-NP), 2,6-dinitrophenol (2,6-DNP), and 2,4,6-trini-
trophenol (TNP) were purchased from Sigma-Aldrich Chemical
Company, USA. 3,30,5,50-Azobenzene-tetracarboxylic acid (H4abtc)
was synthesized according to a previous report with slight modi-
cations11 and successfully characterized via infrared, NMR, and
HRMS spectroscopy (Fig. S1–S4†). To carry out elemental analysis
using Thermo Finnigan FLASH EA 1112, the puried ligand and
MOF crystals were precisely dried under vacuum for a denite
period. FT-IR spectra were obtained using a Bruker ALPHA FT-IR
ture at the rate of 1 ꢁC hꢀ1. Yellow-orange concave-shaped
crystals were harvested in 61% yield (based on Tb3+
)
and
12
washed with DMF/H2O and dried in air. Anal. calcd. (%) for
C
26H25N6O9Tb: C, 43.11; H, 3.48; N, 11.60. Found: C, 43.05; H,
3.41; N, 11.49. FT-IR (KBr, cmꢀ1): 3426, 2925, 2366, 1612, 1442,
1384, 1321, 1256, 1141, 1103, 929, 779, 723, 670, 503.
2.4. X-ray crystallography
The Bruker D8 Quest single crystal X-ray diffractometer with
˚
graphite monochromated MoKa radiation (l ¼ 0.71070 A) and
the PHOTON 100 CMOS detector were used to obtain the X-ray
data. Lorentz-polarization and empirical absorption corrections
were applied using the SAINT v7.34 and SADABS programs,13,14
respectively. The direct method was used to solve the crystal
structure. Structure solution, renement, data output, and
other calculations were executed using the SHELXL 2014, PLA-
TON 99, and SHELXTL program packages.15,16 Hydrogen atoms
were placed at the geometrically calculated positions through
a riding model, and non-hydrogen atoms were rened aniso-
tropically. The DIAMOND soware was used to create the crystal
images and packing diagrams.17 Channel sizes were also
calculated using the DIAMOND soware,17 and the topology of
the framework was explored using the TOPOS soware.18 The
crystallographic data, structure renement parameters, selected
bond lengths, and bond angles are listed in Table 1 and S1–S2.†
spectrometer in the wavelength range of 4000–400 cmꢀ1. H and
1
13C NMR spectra were obtained using the JNM-ECZ500R/S1 500
MHz spectrometer, and the Bruker-Daltonics micrOTOF-Q II mass
spectrometer was used to obtain HRMS. The Rigaku MiniFlex 600
˚
XRD diffractometer with Cu Ka1 radiation (l ¼ 1.5406 A) was used
to obtain powder X-ray diffraction (PXRD) data at room tempera-
ture in the 2q range of 5–50ꢁ at the scan speed of 5 deg per min.
Thermogravimetry (TG) analysis was performed under N2 at
10 ꢁC minꢀ1 using Mettler Toledo TGA 1 together with Minichiller
MT/230 and STARe soware v13.00. The Shimadzu UV–2450 and
Varian Cary Eclipse uorescence spectrophotometers were used to
obtain the absorption and uorescence spectra, respectively. The
emission spectrum of the solid sample was obtained using the
Edinburgh FLS920 spectrophotometer.
2.5. Computational analysis
2.2. Synthesis of H4abtc (L2)
Geometry optimization of the ligands (L1 and L2) and different
nitroaromatic analytes was performed by applying the density
functional theory (DFT) with the B3LYP/6-311G++ basis set in the
Gaussian 09 program suite.19–21 The coordinates for geometry
optimization are provided in the z-matrix form generated by
GaussView 5.0. ChemCra1.5 and GaussView 5.0 were also used
to visualize the output les, perform theoretical calculations,
and obtain electronic parameters for ligands (L1 and L2) and
different nitro-aromatic analytes.
5-Nitroisophthalic acid (3.16 g, 15 mmol) and NaOH (4.88 g, 122
mmol) were dissolved in 40.0 mL water. Then, to this solution,
dextrose (20 g) in 20 mL hot water was added slowly. The
reaction mixture was reuxed at 85 ꢁC for 24 h, and the resulting
brown solution was cooled at 4 ꢁC for 2–3 days to obtain
a precipitate. The brown precipitate was ltered, dissolved in
50 mL water, and acidied with acetic acid, aer which a yellow
solid was obtained. The solid was ltered, washed with lots of
hot water, and dried in air to afford 68.9% yield of the yellow
powder. Anal. calcd. (%) for C16H10N2O8: C, 53.64; H, 2.81; N,
7.82; found: C, 53.51; H, 2.77; N, 7.69. FT-IR (KBr, cmꢀ1): 3421,
3075, 2989, 2851, 2667, 2583, 1706, 1607, 1463, 1417, 1297,
1219, 1148, 1109, 930, 761, 683, 504. 1H NMR (500 MHz, DMSO-
d6, ppm) d: 8.53 (s, 2H, 4,6-H isoph.), 8.56 (s, 1H, 2-H isoph.),
13.56 (s, br, 2H, –COOH); 13C NMR (125 MHz, DMSO-d6, ppm) d:
166.36, 152.198, 133.31, 133.02, 127.47. HRMS (ESI-TOF): m/z
calculated for C16H10N2O8 381.0329 [M + nNa]; found 381.0322.
2.6. Activation of the TPA-MOF
To conduct the gas adsorption measurements, a volumetric
method using the Micromeritics ASAP 2020 instrument (0–1 bar
pressure) and ultra-pure research-grade gases were used. The
as-synthesized MOF was soaked in DCM for 12 h, and the
supernatant DCM was poured off. Aerwards, fresh DCM was
added to remove the free DMF present in the channel of the
metal–organic framework, and this process was repeated
several times. The resulting framework was dried under
2.3. Synthesis of TPA-MOF
A mixture of Tb(NO3)3$6H2O (0.09 g, 0.2 mmol), phen (0.04 g, dynamic vacuum at 120 ꢁC for 24 h. The observed metal–organic
0.2 mmol), H4abtc (0.07 g, 0.2 mmol), triethylamine (0.08 g, 0.8 framework was free from lattice DMF molecules, as conrmed
mmol), HNO3 (0.05 mL), DMF (5 mL), and H2O (0.5 mL) was by the TG analysis.
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Table 1 Crystallography data and structure refinement for TPA-MOF
3. Results and discussion
TPA-MOF
Concave-shaped yellow crystals (yield 61%) of TPA-MOF were
afforded from the stoichiometric combination of Tb(NO3)3$6H2O,
L1, and L2 under solvothermal conditions (DMF/H2O) at 120 ꢁC for
72 h and successfully characterized.
Formula
C26H25N6O9Tb
Mr
724.45
Crystal system
Space group
Triclinic
P1
ꢀ
˚
A (A)
10.4848(4)
10.6394(4)
13.7737(5)
109.254(2)
100.428(2)
108.162(2)
1307.48(9)
2
1.840
2.772
720
2.145–28.315
6491
3.1. Infrared spectrum
˚
b (A)
˚
c (A)
According to Fig. S5,† the FT-IR spectrum of TPA-MOF displays
a transmittance peak at 1612 cmꢀ1, which is due to the asym-
metric stretching vibration of the carboxylic group (C]O). The
weak absorption at 1442 cmꢀ1 is ascribed to the nas(N]N)
stretching vibration, whereas the aromatic C–H stretching band
appears at 2925 cmꢀ1 for L2 (H4abtc) in TPA-MOF. The C]C
and C]N ring stretching vibrations (1672–1384 cmꢀ1) indicate
the coordination of L1 (phen) to the Tb3+ metal centre. The
bands in the region 1321–1256 cmꢀ1 and 723, 779, and
847 cmꢀ1 are attributed to the strong C–N stretching trans-
mittance and out-of-plane hydrogen deformation vibrations of
L1, respectively. The X-ray crystallographic analysis further
a (ꢁ)
b (ꢁ)
g (ꢁ)
3
˚
V (A )
Z
DCalc (g cmꢀ3
)
m (mmꢀ1
F (000)
)
qmax (ꢁ)
No. of unique reections
No. of observed reections
5654
Data/restraints/parameters
6491/0/383
0.779
Goodness-of t on F2
Final R indices [I > 2s(I)]a,b
conrmed the coordination behaviour of L1 and L2 with Tb3+
.
R1
wR2
0.0320
0.0897
R indices (all data)
3.2. Structure elucidation
aR1
0.0432
0.0897
1533159
P
bwR2
CCDC
ꢀ
The TPA-MOF crystallizes in a triclinic system with the P1 space
group, and the asymmetric unit consists of one Tb3+ ion, one L1,
half of an L2, one nitrate, one coordinated DMF, and one free
DMF molecule in its lattice (Table 1, Fig. 1a and S6a in the ESI†).
Tb3+ ion is nine coordinated and has a slightly distorted tri-
capped trigonal prism geometry with the TbO7N2 chromophore
satised by four O atoms (O1, O2, O6, and O7) from three
different tetra-carboxylate anions (abtc4ꢀ), two N atoms (N1 and
N2) from the phen ligand, and three O atoms (O3, O4, and O5)
from nitrate and coordinated DMF molecules with the bond
P
P
P
a
b
2
R1 ¼ kFo| ꢀ |Fck/ |Fo|. wR2 ¼ { [w (Fo ꢀ Fc2)2]/ w(Fo2)2}1/2
.
2.7. Preparation of the sample for photoluminescence
sensing experiments
2.7.1. Small organic solvent molecules. Typically, 3 mg
nely ground crystalline powder of TPA-MOF was dispersed in
3 mL of H2O and other organic solvents such as acetone, chlo-
roform, DMF, DMSO, benzene, propanol, THF, and o-xylene,
and the sample was ultra-sonicated for 30 min to form
a uniform emulsion. Photoluminescence spectra of the emul-
sions were obtained aer aging for 48 h. For the uorescence
titration experiment, a variable amount of acetone and chloro-
form was added to a standard emulsion (aqueous solution) of
TPA-MOF, and the spectra were obtained.
2.7.2. Nitro-aromatic explosives. Herein, 3 mg nely
ground crystalline powder of TPA-MOF was immersed in 3 mL
of H2O, treated with ultrasonication (30 min) to form
a uniform emulsion, and then aged for 48 h. For uorescence
titration, 3 mL standard aqueous solution of TPA-MOF was
placed in a 10 mm uorescence quartz cuvette and titrated
gradually with a 1 ꢂ 10ꢀ3 M aqueous solution of electron-
decient quenchers including NB, 1,3-DNB, 4-NT, 2,4-DNT,
TNT, 4-NP, 2,6-DNP, and TNP. TPA-MOF was excited at the
respective absorption maximum, and the corresponding
emission spectrum was obtained at room temperature. Both
the excitation and emission slit widths were 3 nm for all the
measurements, and each titration was repeated three times to
obtain concordant values.
˚
˚
distances of Tb1-O1, 2.284(2) A; Tb1-O2, 2.347(3) A; Tb1-O6,
˚
˚
˚
2.486(3) A; Tb1-O7, 2.401(3) A; Tb1-N1, 2.537(3) A; Tb1-N2,
˚
˚
˚
2.599(3) A; Tb1-O3, 2.537(3) A; Tb1-O4, 2.474(3) A; and Tb1-
˚
O5, 2.360(3) A (Fig. 1b and S6b†). Further, the single crystal X-
ray structure analysis suggests that the ligand L2 coordinates
to Tb3+ ion in two different modes: one L2 acts as a bidentate
ligand, whereas the other two are in a chelating-bridging mode
(m2-O), which may be corroborated from the disparity in the Tb–
O bond distances of the coordinated abtc4ꢀ, and one bridged
abtc4ꢀ exhibits a shorter distance than the other (Table S1,
ESI†). Moreover, two proximate Tb3+ ions are conjugated to each
other through two chelated tetra-carboxylate anions (abtc4ꢀ) to
form a wheel-shaped (Tb2(COO)2) dimer with the Tb/Tb
˚
distance of 5.673 A; this dimer is coupled with eight other
identical dimeric units through bridged abtc4ꢀ ions and prop-
agates a 2D extended metal–organic framework along the [100]
˚
plane with a pore size of 4 A (Fig. 1c and S7, ESI†). As shown in
Fig. 1d and S8,† TPA-MOF exhibits micro-channels with the
2
˚
dimensions of ꢃ10.64 ꢂ 6.03 A along the ‘a’ axis that are
incorporated by guest (DMF) molecules. Moreover, the PLATON
analysis manifests that this MOF possesses moderately solvent-
3
3
˚
˚
accessible voids of 17.5% (228.9 A out of 1307.5 A ) of its total
volume. Thus, this entire metal–organic framework may be
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Fig. 1 Molecular structure of TPA-MOF: (a) ball and stick model of the asymmetric unit. Symmetry codes: (i) 1 ꢀ x, 1 ꢀ y, ꢀz; (ii) 1 ꢀ x, ꢀy, ꢀz; and
(iii) 1 ꢀ x, ꢀy, ꢀ1 ꢀ z. (b) Coordination geometry around Tb3+ ion, (c) simplified topological structure depicting the SP2-periodic net having an 8-
connected uninodal net (green lines and maroon balls represent the L2 ligands and Tb3+ metal centres, respectively), (d) the channel, showing
arrangement of solvent (DMF) molecules in TPA-MOF. Color code: Tb; pink, O; red, N; blue, C; teal, and H; green.
simplied as an 8-connected uninodal net having an a moderate amount of N2 with an uptake of 13.6 cm3 gꢀ1, and the
¨
SP2-periodic net topology with the Schlai point symbol of Brunauer–Emmett–Teller (BET) surface area is found to be 179.3
{3^12;4^14;5^2} (Fig. 1c).
m2 gꢀ1. In addition, the sorption of N2 at 273 K is signicantly
less, which shows only a maximum uptake of 4.8 cm3 gꢀ1 at 1
atmospheric pressure. This can be justied due to the small pore
3.3. PXRD and thermogravimetric analysis
3
˚
˚
size (4 A), solvent accessible void (228.9 A ), and comparable
To validate the phase purity of the bulk material, powder X-ray
diffraction was carried out, which conrmed that the experi-
mental PXRD of the synthesized material was consistent with
the corresponding simulated XRD pattern of the TPA-MOF
(Fig. S9a, ESI†). TG analysis was conducted to examine the
thermal stability of the TPA-MOF in the range of 25–800 ꢁC
under a N2 atmosphere at the heating rate of 10 ꢁC minꢀ1
(Fig. S10, ESI†). The thermogram displays the rst weight loss
(calculated 10%; observed 9.2%) corresponding to the release of
free DMF molecules in the temperature range of 45–105 ꢁC. The
˚
kinetic diameter of N2 (3.64 A). Furthermore, the low-pressure
adsorption isotherms for CO2 were measured at 273 and 298 K.
At 273 K, the uptake amount of CO2 is 32.5 cm3 gꢀ1, whereas at
298 K, the uptake amount of CO2 is 17.1 cm3 gꢀ1 (Fig. 2b).
However, the pore sizes are ultra-microporous (0.4 nm) and still
accessible to N2 and CO2; this is in good agreement with the
conformational resilience of the L2 linker and concomitant ex-
ibility of the porous framework. Although N2 (3.64 A) and CO2
(3.3 A) have similar kinetic diameters, the azo group (N]N) in L2
is responsible for the higher uptake of CO2 than N2, which
provides potential coordination sites for CO2 that facilitate
dipole–quadrupole interactions. This narrow pore accessibility
implies extremely slow adsorption kinetics.
˚
˚
ꢁ
TPA-MOF was stable up to 185 C, and the second weight loss
(calculated 18.5%; observed 17.9%) was observed between
185 ꢁC and 350 ꢁC. In this step, the coordinated nitrate and
DMF molecules are released. A sharp decrease is observed in the
ꢁ
plot aer 400 C, which is attributed to the decomposition of
the remaining framework.
3.5. Luminescence studies
To investigate the luminescence properties of the TPA-MOF, the
photo-luminescence spectra of the free ligands (L1 and L2) and
3.4. Gas sorption studies
Gas sorption studies were performed on the activated TPA-MOF TPA-MOF in the aqueous phase were measured at room temper-
at different temperatures. N2 adsorption shows a reversible ature. The emission spectra of L1 and L2 display peaks at 361 (lex
¼
type-I isotherm at 77 K, which is an important feature of micro- 264 nm) and 388 nm (lex ¼ 322 nm), respectively, which may be
porous materials. As shown in Fig. 2a, the TPA-MOF adsorbs attributed to p / p* and n / p* transitions, whereas TPA-MOF
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Fig. 2 Gas adsorption isotherms of TPA-MOF: (a) N2 adsorption and desorption at 77 K and 273 K, (b) CO2 adsorption and desorption at 273 K and
298 K.
exhibits four characteristic transitions of Tb3+ ion in the solid and According to Fig. 4c, the uorescence intensity progressively
solution states (lex ¼ 264 nm) at 488 (5D4 / 7F6), 544 (5D4 / 7F5), decreased with a steady increase in the concentration of TNP in
583 (5D4 / 7F4), and 619 nm (5D4 / 7F3), which can be associated the dispersed solution of TPA-MOF in water, and ultimately, the
with the existence of an efficient ligand-to-metal energy transfer uorescence intensity was almost totally quenched at 400 ppm
process (Fig. S11 and S12, ESI†). Thus, the emissive nature of the of TNP with a quenching efficiency of 93.26%, as calculated
water-stable TPA-MOF in the solid as well as aqueous phase according to h ¼ (F0 ꢀ F)/F0 ꢂ 100%, where F0 and F are the
prompted us to explore its potential for the sensing of SOMs and luminescence intensities of the TPA-MOF before and aer the
NAC explosives. The crystallinity of TPA-MOF in water was also addition of TNP, respectively. In contrast, the other NAC
conrmed via the PXRD analysis of the synthesized material explosives exhibited minimal effects on the uorescence
dispersed for 4 days at 300 K (Fig. S9a†). Fig. 3a–c reveal that the intensity of TPA-MOF (Fig. 4d).
luminescence intensity greatly depends on the solvent molecules
Furthermore, the quenching constant of TPA-MOF with TNP
such as DMF, DMSO, THF, CHCl3, propanol, acetone, benzene, was determined using the Stern–Volmer equation (F0/F ¼ Ksv[Q]
and o-xylene, but exhibits a prominent uorescence quenching + 1, where Ksv is the quenching constant and Q is the concen-
effect by the gradual addition of acetone in a dispersed aqueous tration of TNP), and the SV plot exhibited a nearly linear curve at
solution of TPA-MOF, and its luminescence almost vanishes at an very low concentrations (Fig. 5a). The very slight arc in the SV
acetone content of 18% (v/v). The plot of uorescence intensity plot may be possibly due to mixed static and dynamic photo-
(5D4 / 7F5, 544 nm) versus acetone content (vol%) displays a rst- induced electron transfer.23 From the tted curve, the quench-
order exponential decay (Fig. 3d), which indicates that the acetone ing constant was computed to be 15.97 ppmꢀ1 (5.67 ꢂ 105 Mꢀ1
,
quenching behaviour (98.4% QE; QE ¼ quenching efficiency) of ESI†). These results denitely establish the potential of TPA-
TPA-MOF is controlled by a diffusion process22 (Fig. 3e). We also MOF as a highly selective chemosensor for TNP, which is
performed the uorescence quenching titration with chloroform, crucial for environmental monitoring. To the best of our
but no distinctive quenching effect was observed even at the knowledge, intense selective detection of TNP using a terbium
chloroform content of 80% (v/v), as shown in Fig. S13.† The metal–organic framework in an aqueous solution has not been
mechanism for the impressive quenching effect of acetone may reported to date (Table S4†). Moreover, the sensing ability of 2D
also be elucidated based on the binding interactions of the –N]N– TPA-MOF can be regenerated, and it can be reused for a signif-
group of L2. The small size as well as strong intermolecular icant number of cycles by centrifuging the dispersed solution
interactions (N/O) of acetone with L2 could be responsible for the aer use and washing several times with diethyl ether (Fig. 5b).
notable uorescence quenching of the TPA-MOF.7
The results demonstrated that the initial uorescence intensity
To evaluate the selective and sensing behaviour of TPA-MOF, was approximately recovered over repeated cycles, which sug-
a uorescence quenching titration was performed with the gested a high photostability of TPA-MOF for explosive detection
addition of selected NAC explosives such as NB, 1,3-DNB, 4-NT, applications. PXRD analysis also conrmed the retention of the
2,4-DNT, TNT, 4-NP, 2,6-DNP, and TNP. Remarkable quenching structure even aer immersing the TPA-MOF in an aqueous
in the uorescence intensity was observed in the presence of solution of TNP for 72 h and its reproducibility aer ve cycles
TNP as compared to the case of other molecules (Fig. 4a and b). (Fig. 6 and S9b, ESI†).
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Fig. 3 The photoluminescence spectra of TPA-MOF: (a) intensity of TPA-MOF dispersed in H2O and various organic solvent molecules, (b) 5D4
/ 7F5 transition intensities of TPA-MOF, (c) spectra of TPA-MOF in presence of variable amounts of acetone, (d) emission intensities (5D4 / 7F5,
544 nm transition) as a function of acetone content, (e) quenching efficiency of 5D4
/
7F5 fluorescence intensity for TPA-MOF dispersed in
selected solvents at room temperature.
The encapsulation of TNP into the pores of TPA-MOF is ruled the nely ground TPA-MOF in water may be the reason for the
out due to the existence of very small pores (0.4 nm) within this adsorption of TNP on the surface of the MOF, which promotes
MOF, which has been investigated via crystallographic and gas uorescence quenching and photo-induced electron transfer
adsorption studies (Fig. S7† and 2). The dispersible nature of from the excited TPA-MOF to the electron-decient TNP
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Fig. 4 (a) Intensity of TPA-MOF dispersed in H2O and various NAC analytes, (b) reduction in fluorescence intensity (5D4 / 7F5 transition 544 nm)
after addition of different NAC analytes, (c) fluorescence titration of TPA-MOF by gradual addition of an aqueous solution of TNP (magnified
7
image shows reduction in fluorescence intensity (5D4 / F5 transition, 544 nm)), and (d) QE of fluorescence intensity for TPA-MOF upon the
addition of several NAC analytes.
through interspecies contacts.24 These observations were also theory (ESI†). According to Fig. 7, the lowest unoccupied
investigated using the HOMO and LUMO orbital energies of L1, molecular orbital (LUMO) of L1 (ꢀ1.67052 eV) lies at a higher
L2, and NAC explosives, as calculated by the density functional energy level than those of L2 (ꢀ3.4398 eV) and selected NAC
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Fig. 5 (a) Stern–Volmer plot of F0/F vs. TNP (ppm) concentration in aqueous solution for TPA-MOF, (b) quenching and reproducibility test for
TPA-MOF (initial fluorescence intensity, green; intensity after quenching (400 ppm TNP), blue).
analytes (ꢀ2.42 to ꢀ4.05 eV). However, both ligands (L1 and L2)
However, the electron-withdrawing ability of TNP is very
have higher LUMO energies than TNP; thus, electrons can move poor; this demonstrates that the photo-induced electron
easily from the excited TPA-MOF (electron donor) to the transfer is not the sole mechanism.7 A long-range energy
electron-decient analytes (electron acceptor), and this facili- transfer mechanism also plays a major role in the quenching of
tates uorescence quenching.25
the uorescence intensity that may be attributed to the
Fig. 6 PXRD spectra obtained after immersion in H2O (4 days), acetone, and TNP for 72 h and recycled sample after five cycles.
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Fig. 7 HOMO (red) and LUMO (blue) energy level diagram for the ligands (L1 and L2) and NAC analytes.
electrostatic interaction between TNP and TPA-MOF.26 The
ligand L1 forms an intermolecular hydrogen bonded complex
Acknowledgements
with TNP that is involved in the diffusion of TNP over TPA-MOF;
this leads to excellent quenching through appropriate host–
guest interactions;26 thus, both photo-induced electron and
energy transfer mechanisms occur in the uorescence
quenching of the present TPA-MOF.
N. G gratefully acknowledges the nancial support received
from the Department of Science and Technology, New Delhi,
Government of India (Fast Track Project No. SB/FT/CS-059/
2012). N. G is highly thankful to N. K who carried out the
computational analysis. Author also thanks JNU, New Delhi,
BHU, Varanasi, and IIT Indore for providing instrumental
facilities.
4. Conclusions
References
In summary, a dual-functional, 2D luminescent Tb(III) metal–
organic framework (TPA-MOF) with an SP2-periodic net topology
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ability demonstrates the distinguished application of TPA-MOF
as an efficient uorescent sensor for the detection of small
organic pollutant molecules as well as nitro-aromatic analytes,
and its advantages make it highly feasible for practical use.
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