Two azo-functionalized luminescent 3D Cd(ii) MOFs for highly selective detection of Fe3+ and Al3+

Article abstract of DOI:10.1039/c8nj02338g

Two cadmium-based 3D luminescent MOFs {[Cd₂(SA)₂(L)₂]·H₂O}_n (Cd-MOF-1) and [Cd(CDC)(L)]_n (Cd-MOF-2) (H₂SA = succinic acid, H₂CDC = 1,4-cyclohexanedicarboxylic acid, L = [3,3′-azobis(pyridine)]) have been assembled by employing organic dicarboxylic acid linkers with an unexploited azo-functionalized N,N′ spacer via a room temperature slow evaporation process, and they are characterized by single crystal X-ray analysis, TGA, FT-IR, PXRD and elemental analysis. The topological analysis reveals that Cd-MOF-1 features a 6c-uninodal rare 'rob' topology with the point symbol {4⁸·6⁶·8}, whereas Cd-MOF-2 shows a 'pcu' alpha-Po primitive cubic topology with the point symbol {4¹²·6³}. These MOFs are highly emissive at 382 nm and 398 nm when excited at 305 nm and 312 nm, respectively. The exposed azo groups are presumed to act as functional sites for the recognition of metal ions through quenching of fluorescence intensity. The fluorescence measurements show that these MOFs can selectively and sensitively detect Fe³⁺ as well as Al³⁺ and thus, they demonstrate potential as dual-responsive luminescent probes for metal-ion sensing. EDS elemental mapping, PXRD of loaded MOF materials, and a plausible quenching mechanism have been discussed. More importantly, both the MOFs exhibit rapid response times toward sensing of Fe³⁺ and Al³⁺.

Full text of DOI:10.1039/c8nj02338g

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Revised manuscript
(NJꢀARTꢀ05ꢀ2018ꢀ002338)
Two Azoꢀfunctionalized Luminescent 3D Cd(II)ꢀMOFs for Highly Selective
Detection of Fe3+ and Al3+
Santanu Chand,a Manas Mandal,a Shyam Chand Pal,a Arun Pal,a Sinchan Maji,b Debaprasad
Mandalb and Madhab C. Dasa*
aDepartment of Chemistry, Indian Institute of Technology Kharagpur, WB, 721302, India
bDepartment of Chemistry, Indian Institute of Technology Ropar, Punjab, 140001, India
E-mail: mcdas@chem.iitkgp.ac.in
________________________________________________________________________
Abstract: Two cadmium based 3D luminescent MOFs {[Cd2(SA)2(L)2]·H2O}n (CdꢀMOFꢀ1)
and [Cd(CDC)(L)]n (CdꢀMOFꢀ2) (H2SA
=
succinic acid, H2CDC
=
1,4ꢀ
Cyclohexanedicarboxylic acid, L = [3,3'ꢀazobis(pyridine)], S = Guest molecules) have been
assembled by employing organic dicarboxylic acid linkers with an unexploited azo
functionalized N,N' spacer via room temperature slow evaporation process and characterized
by single crystal xꢀray analysis, TGA, FTꢀIR, PXRD and elemental analysis. The topological
analysis reveals that CdꢀMOFꢀ1 features a 6cꢀuninodal rare ‘rob’ topology with the point
symbol {48.66.8} while CdꢀMOFꢀ2 shows a ‘pcu’ alphaꢀPo primitive cubic topology with
the point symbol {412.63}. These MOFs are highly emissive at 382 nm and 398 nm while
excited at 305 nm and 312 nm respectively. The exposed azo groups are presumed to act as
functional sites for the recognition of metal ions through quenching of fluorescence intensity.
The fluorescence measurements show that these MOFs can selectively and sensitively detect
Fe3+ as well as Al3+ and thus demonstrating the potential for dualꢀresponsive luminescent
probe for metal ion sensing. EDS elemental mapping, PXRD of loaded MOF materials, and
plausible quenching mechanism have been discussed. More importantly, both the MOFs
exhibit rapid response time toward sensing of Fe3+ and Al3+ as well.

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Introduction
Fe3+ is one of the most important and essential element in the human body and biological
tissues.1 However, both excess and deficiency from the normal permissible limit can induce
serious disorders.2 On the other hand, Al is the third most widespread element and the most
abundant metal in the biosphere.3 Excess Al is acquired by the use of deodorants, cookware,
cans, antacids and drinking water supplies. Al is not as toxic as heavy metals, but there is
evidence of some toxicity if it is consumed in excess.4 Hence, in virtue of the crucial role of
metal ions in the environmental and biological system, the design and synthesis of metal ion
sensing probes have attracted much attention of the researchers. Thus it is very important to
develop novel candidate as well as novel method that can be easy to apply and ability to
detect exclusively Fe3+ and Al3+ as well.
Microporous luminescent metalꢀorganic frameworks (MOFs) are of interest for their
diverse functions, predominantly for the sensing of various metal ions and nitroaromatics.5ꢀ10
Various ways to detect the metal ions have been introduced via diverse detection techniques
such as inductively coupled plasma atomic emission, atomic absorption spectroscopies etc.
Among the various detection methods, fluorometric methods have acquired appreciable
attention because of their facilitated detection with rapid response time and easy manipulation
with high sensitivity.7−8 Particularly, the high porosities of MOFs prompt the
preconcentration of the analytes (metal ions) in their pores, therefore leading to lower
detection limits and higher sensing sensitivities.9 In recent years, the research on MOF
fluorescent chemosensor highlighted the significance of the role of Lewis basic sites within
the porous MOFs, such as unsaturated (open) pyridyl group, free −NH2 group, an anionic
sulfonate group, and −OH group to enhance unique recognition towards sensing of various
metal ions in various solvents.9ꢀ10 For the construction of such kind of MOF materials,
researchers are mostly using lanthanides salts due to their sharp and intense luminescence
intensity. But, because of the high cost and exiguousness of lanthanides, the MOF sensors
based on transition metals are highly desirable.11 In order to harvest plausible binding sites of
selective metal ion sensing, we are interested to immobilize azo functionality of an
unexploited spacer 3,3'ꢀazobis(pyridine) with uncoordinated Nꢀrich sites on the MOF
backbone. Besides, this spacer possesses a highly conjugated πꢀelectronic system to provide
powerful luminescence property. Although a few discrete complexes and hydrogen bonding
networks are reported,12aꢀb to the best of our knowledge this spacer has rarely been used for

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the construction of MOF materials.12cꢀd Based on the above considerations, we have designed
and constructed two new bifunctional 3D Cd(II)ꢀMOFs ({[Cd2(SA)2(L)2]·H2O}n (CdꢀMOFꢀ
1) and {[Cd(CDC)(L)]}n (CdꢀMOFꢀ2)) selfꢀassembled by two different dicarboxylate
organic linkers and 3,3'ꢀazobis(pyridine) spacer with exposed azo binding sites for selective
sensing of Fe3+ as well as Al3+ in DMF/water solution. Further investigation on sensing
mechanism, detection selectivity in presence of other metal ions and response time have been
carried out.
Experimental Section
Materials and reagents.
All the chemicals were of reagent grade quality, and they were purchased from the
commercial sources and used as received. The spacer L was prepared following a method
described in the literature.12c
Synthesis of {[Cd2(SA)2(L)2]·H2O}n, CdꢀMOFꢀ1. Cd(NO3)2·4H2O (0.1 mmol, 0.031 g),
H2SA (0.1 mmol, 0.012 g) and L (0.05 mmol, 0.010 g) were dissolved in 3 mL DMF in a
glass vial. The resulting solution was stirred at room temperature for 1ꢀ2 h, filtered and the
filtrate was kept for crystallization at ambient temperature. Blockꢀshaped orange crystals
were obtained after about 15ꢀ18 days. Yield: 59%. Elemental analysis for C28 H24 Cd2 N8 O8,
calcd: C, 39.87%; H, 3.10%; N, 13.28%. Found: C, 39.84%; H, 3.11%; N, 13.25%. FTꢀIR
(KBr pellet, cm−1): 1659.5(s), 1616.2(w), 1571.7(s), 1495.1(s), 1407.1(s), 1318.4(m),
1288.3(s), 1211.9(s), 1164.5(s), 1133.8(s), 1106.8(s), 870.1(s), 821.2(m), 790.4(s), 748.2(s),
731.6(s), 659.3(s), 624.4(s), 507.1(s), 456.6(s).
Synthesis of {[Cd(CDC)(L)]}n, CdꢀMOFꢀ2. It has been prepared as described for the
synthesis of 1, except that H2SA is replaced by H2CDC (0.1 mmol, 0.017 g). Needle shaped
orange crystals were obtained after about 25 days. Yield: 71%. Elemental analysis for C18 H18
Cd N4 O4, calcd: C, 46.31%; H, 3.88%; N, 12.00%. Found: C, 46.28%; H, 4.13.85%; N,
12.09%. FTꢀIR (KBr pellet, cm−1): 2872.9(w), 1654.5(s), 1607.2(w), 1569.7(s), 1485.1(s),
1387(s), 1318.4(m), 1288.3(s), 1060.2(s), 1013.8(s), 947.3(w), 870.1(s), 821.2(m), 790.4(s),
748.2(s), 731.6(s).
Results and Discussion
The singleꢀcrystal Xꢀray determination reveals 3D coordination frameworks
constituted by SA2ꢀ or CDC2ꢀ and an N,N'ꢀdonor spacer L. CdꢀMOFꢀ1 crystallizes in

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monoclinic crystal system with P21/n space group formulated as {[Cd2(SA)2(L)2]·H2O}n. The
asymmetric unit of CdꢀMOFꢀ1 consists of two Cd(II) metal ion, two deprotonated succinic
acids and two L spacer (Fig. S1, ESI). Each Cd(II) atom is sevenꢀcoordinated with two
Nitrogen atoms ligated from two different L spacer in axial position and five oxygen atoms
from three SA2ꢀ and thus giving rise a distorted pentagonal bipyramidal configuration
{CdO5N2} (Fig. 1a). The Cd–O bond distances range from 2.330(4) to 2.519(4) Å whereas
the Cd–N bond distances range from 2.350(5) to 2.369(5) Å (selected bond distances and
angles of CdꢀMOFꢀ1 are listed in Table S2, ESI). Two different coordination modes have
been adopted by two carboxylate groups of SA2ꢀ ligands, one is bidentate chelating mode and
the other is monodentate chelating fashion. Two Cd(II) centers are bridged by two
carboxylate groups of SA2ꢀ ligands to generate a dimeric Cd2(COO)2 core unit (Fig. S2, ESI)
resulting in the formation of a 1D chain structure along the
Fig. 1 (a) Coordination environment around Cd(II) in CdꢀMOFꢀ1. Color code: Cd, yellow;
N, blue; O, red; C, grey; H atoms are omitted for clarity. (b) Five coordinated SA2ꢀ units with
Cd(II) unit to form a 2D layered network. (c) A representation of overall 3D structure along
crystallographic ‘a’ axis. (d) The topological representation of CdꢀMOFꢀ1 showing overall
framework with the rare ‘rob’ topology. The pink dots, blue and black sticks represent the
Cd2(COO)2 SBUs, SA2ꢀ units and L spacers respectively.

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crystallographic ‘c’ axis (Fig. 1b). The distance between two Cd(II) in the dimeric unit is
3.792 (2) Å. The pyridyl N atoms of the spacer L connect the Cd2(COO)2 unit in a crisscross
fashion and extended in both the directions to build an overall 3D network (Fig. 1c). The
resultant 3D framework contains the solvent molecules capped in the 1D channel oriented
through ‘a’ axis (Fig. 1c). Moreover, the framework of CdꢀMOFꢀ1 can be simplified to a 3D
uninodal, 6ꢀconnected a rare ‘rob’ topology with the point symbol {48.66.8} as depicted in
Fig. 1d. Weak π····π stacking interaction (centroid to centroid distance of 3.564(2) Å) among
the adjacent pyridine moieties of spacer L and several nonꢀcovalent intermolecular
interactions among the ligands further stabilized the framework (Table S3, ESI).
On the other hand, the asymmetric unit of CdꢀMOFꢀ2 consists of one Cd(II) metal
ion, one spacer L, and one deprotonated CDC2ꢀ forming the neutral network with molecular
formula {[Cd(CDC)(L)]}n as depicted in Fig. S3 (ESI). Cd(II) ion is coordinated by four
oxygen atoms from three different CDC2− linkers and two nitrogen atoms from two different
L spacers, showing a distorted octahedral geometry {CdO4N2} (Fig. 2a). The Cd–O bond
distances range from 2.2664(15) to 2.4560(14) Å whereas the Cd–N bond distance ranges
from 2.3143(16) to 2.3193(15) Å (selected bond distances and angles of CdꢀMOFꢀ2 are
listed in Table S4, ESI). Each CDC2ꢀ connects three Cd(II) metal ions through chelating and
bridging bidentate fashion forming Cd2(CO2)2 (Fig. S4, ESI) units with a separation distance
of 3.952(5) Å. Again, these binuclear units are connected with CDC2ꢀ ligands forming two
dimensional (2D) [Cd2(CDC)2]n sheets lying in the crystallographic ‘bc’ plane (Fig. 2b)
which are further connected by the spacer L to construct a 3D pillared layer dense framework
(Fig. 2c). The framework of CdꢀMOFꢀ2 can be simplified to a 3D uninodal, nonꢀ
interpenetrating ‘pcu’ alphaꢀPo primitive cubic topology network with the point symbol of
{412.63} (Fig. 2d). Weak π····π stacking interaction (centroid to centroid distance of 3.638(2)
Å) among the adjacent pyridyl moieties of spacer L and several nonꢀcovalent intermolecular
interactions among the ligands further stabilized the framework (Table S5, ESI).
To investigate the purity of the crystals in bulk phase PXRD experiment was carried
out for both the samples. The asꢀsynthesized and the simulated patterns of PXRD are in good
agreement with each other as depicted in Fig. S5 and S6, ESI. To recheck the phase purity in
DMFꢀH2O mixture (solvent system for sensing measurement studies) we have collected the
PXRD upon immerging both CdꢀMOFꢀ1 and CdꢀMOFꢀ2 in 4:1 DMFꢀH2O mixture for 12

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hours, which shows the identical peak positions with the asꢀsynthesized sample (Fig. S5 and
S6, ESI). To study the thermal behavior, thermogravimetric analysis (TGA) has been carried
out from 30 °C to 800 °C in the nitrogen atmosphere with a heating rate 5 °C per min as
depicted in Fig. S7ꢀS8 (ESI). TGA of asꢀsynthesized CdꢀMOFꢀ1 shows that a weight loss of
2.2% (calculated 2.1%) occurs below 100 °C which can be attributed to the loss of one lattice
water molecule. The framework is stable until 270 °C and after that, the decomposition
started gradually. TGA on the asꢀsynthesized CdꢀMOFꢀ2 indicates that the material is stable
and no weight loss is observed until 300 °C. The expulsion of the organic ligand and spacer
occurs from 350 °C and further heating up to 800 °C reveals continuous weight losses.
Fig. 2 (a) Coordination environment around Cd(II) in CdꢀMOFꢀ2. (b) Four coordinated
CDC2ꢀ units with Cd(II) unit to form a 2D network. (c) A representation of overall 3D
structure along crystallographic ‘a’ axis. (d) The topological representation of CdꢀMOFꢀ2
showing overall framework with the ‘pcu’ alphaꢀPo primitive cubic topology. The pink dots,
black and yellow sticks represent the Cd2(COO)2 SBUs, CDC2ꢀ units and L spacers
respectively.

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The emission spectra of both CdꢀMOFꢀ1, as well as CdꢀMOFꢀ2 and the
corresponding spacer L, have been investigated in solid state at room temperature as depicted
in Fig. S11, ESI. When CdꢀMOFꢀ1 is excited in 305 nm it displays an emission peak at 382
nm that can be attributed to π* → π and π*→ n transitions of the spacer (i.e. intra ligand
charge transfer transition as well as the ligand to ligand charge transfer transition (LLCT)).13
On the other hand when CdꢀMOFꢀ2 is excited in 312 nm it displays an emission peak at 398
nm that can be attributed to π* → π and π*→ n transitions of the conjugated ligands.
Considering this emission property and presence of uncoordinated Lewis basic azo
sites for both the MOFs, we have investigated the fluorescence sensing property towards
metal ions. As previously mentioned, the framework has retained its crystallinity in the DMF
and water (4:1 mL) mixture of solvents used in this sensing experiment. The standard
solution of both the materials have been prepared by adding a finely ground sample of Cdꢀ
MOFꢀ1 and CdꢀMOFꢀ2 (30 mg) in 30 mL DMF solutions and treated by the ultraꢀsonication
for about half an hour. The metal salts (Mn+ = Na+, K+, Ca2+, Mg2+, Ba2+, Fe2+, Ni2+, Cu2+,
Cr2+, Hg2+, Cd2+, Pb2+, Al3+, Cr3+, Fe3+) have been prepared in 1 x 10ꢀ2 (M) solution in water.
For all the experiments the excitation wavelength was 305 nm (λex = 305 nm). The metal ion
detection experiments on both the samples were performed by adding a total 100 µL aqueous
solutions with the sequential addition of 5 µL of the different metal ions to a 2 mL of Cdꢀ
MOFꢀ1 or CdꢀMOFꢀ2 in DMF and subsequently measuring the luminescence spectra
respectively.
After the addition of metal ion (Fig. 3 and 4) the luminescence spectra of CdꢀMOFꢀ1
display that most metal ions, such as Na+, K+, Ba2+, Mg2+, Ca2+, Co2+, Cd2+, Pb2+, Hg2+, and
Cu2+ make negligible difference to the luminescence intensity (Fig. S12, ESI). On the
contrary, Fe2+, Al3+, Cr3+ and Fe3+ ions display different levels of luminescence quenching.
Among the four metal ions, Fe3+ and Al3+ ions lead to almost complete quenching of intensity
of CdꢀMOFꢀ1 indicating its highly selective sensitivity towards both the metal ions. As can
be seen, the emission intensity of CdꢀMOFꢀ1 decreases with gradually increasing the
concentration of Fe3+ and Al3+ (5 µL to 100 µL). Quantitatively, the quenching efficiency
effect can be explained by using the SternꢀVolmer equation: I0/I = 1+ Ksv[M], where I0 and I
are the luminescence intensity of CdꢀMOFꢀ1 before the addition of the metal ion and after
the addition of the metal ion respectively, Ksv is quenching coefficient and [M] is the molar

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concentration of the metal ion. Hence, it is expected that the decrease in the luminescence
intensity will be proportionate to the metal ions concentration. As shown in Fig. S13 (ESI), it
is clear that the Stern−Volmer curves for Fe(III) ion are nearly linear at low concentrations.
However, with increasing the concentrations, the curves deviate from the linearity which can
be explained by an energyꢀtransfer process or selfꢀabsorption.14 Estimated from the plot of
fluorescence intensity and the concentration of the metal ions, the experimental result shows
the value of Ksv = 2.1 ×104 M−1, and 5.4 ×103 M−1 for Fe3+ and Al3+ respectively, which is
comparable and higher than some
Fig. 3 (a) Photoluminescence spectra with a gradual addition of 10ꢀ2 (M) aqueous Fe3+
solution in the DMF dispersed solution of CdꢀMOFꢀ1. [Insets: colour changes before (left)
and after (right) the addition of 100 µL Fe3+ solution (exposed to 365 nm UV light)] (b)
Photoluminescence spectra with a gradual addition of 10ꢀ2 (M) aqueous Al3+ solution in the
DMF dispersed solution of CdꢀMOFꢀ1. [Insets: colour changes before (left) and after (right)
the addition of 100 µLAl3+ solution (exposed to 365 nm UV light)] (c) Relative luminescence
intensities after introducing various metal cations. (d) Comparison of the fluorescence
intensity of CdꢀMOFꢀ1 exposed to mixed metal ions.

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reported results (Table S6 and S7, ESI), indicating the potential of CdꢀMOFꢀ1 to act as a
sensor towards highly sensitive and selective detection of Fe3+ ions. The limit of detection
(LOD) value was determined by employing the equation: LOD = 3σ/K, where σ denotes the
standard deviation of the initial intensity of CdꢀMOFꢀ1 without analytes and K represents the
slope of the linear curve. The estimated LOD value of CdꢀMOFꢀ1 for Fe3+ is 2.4 × 10ꢀ6 M
and 9.3 × 10ꢀ5 M for Al3+.
Generally speaking, many metal ions coexist in practical biological and
environmental system. Thus the interferences of the other metal ions for Fe3+/Al3+ sensing
also needs to be examined. 2 mL of MOF suspension containing mixed Fe3+ (50 µL 1 ×10ꢀ2
M) and each of rest of the cations (50 µL 1 ×10ꢀ2 M) were prepared and fluorescence intensity
was measured. As shown in Fig. 4d the quenching emission effect was retained in the
presence of the other cations, indicating that the quenching effects for the mixture of metal
ions are not mutually interfered.
On the other hand, the luminescence property recorded for CdꢀMOFꢀ2 is shown in
Fig. S15 (ESI). Most of the metal ions have a different degree of quenching effect on the
luminescence intensity of CdꢀMOFꢀ2. For example, the intensity at 312 nm is about the same
(almost no quenching effect) with the original intensity when K+, and Ba2+ are involved. On
contrary, Fe3+ and Al3+ ions lead to almost complete quenching of intensity of CdꢀMOFꢀ2.
These results indicate that CdꢀMOFꢀ2 can selectively sense Fe3+ as well as Al3+ ions through
luminescence quenching. As depicted in Fig. 4, the emission intensity of CdꢀMOFꢀ2
decreases with gradually increasing the concentration of Fe3+ and Al3+ (5 µL to 100 µL) as
could be observed for CdꢀMOFꢀ1. Hence, estimated from the plot of luminescence intensity
and the concentration of the metal ions, the experimental result shows the value of Ksv = 4.9
×103 M−1, and 2.6 ×103 M−1 for Fe3+ and Al3+ respectively, which is higher and comparable as
well with some literature reports (Table S6 and S7, ESI), indicating the potential of Cdꢀ
MOFꢀ2 to act as a sensor towards highly sensitive and selective detection of both of these
ions. The quenching coefficient value Ksv for Fe3+ of CdꢀMOFꢀ1 is roughly one order higher
magnitude than that of CdꢀMOFꢀ2, which is probably because of the presence of azo
functional groups lined open channels running along a axis in CdꢀMOFꢀ1 compared to the
dense framework of CdꢀMOFꢀ2. A similar trend is also observed for the case of Al3+. The
estimated LOD value of CdꢀMOFꢀ2 for Fe3+ is 7.4 × 10ꢀ5 M and 6.1 × 10ꢀ5 M for Al3+.

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As shown via the PXRD analysis, the skeleton of both the materials CdꢀMOFꢀ1 and
CdꢀMOFꢀ2 remains unflawed, which contradicts the collapse of the network during
quenching experiments. We speculate that the strong interaction between the Fe3+/Al3+ ions
and free Lewis basic azo sites of the frameworks is the possible reason for such luminescence
quenching behavior. Moreover, the fluorescence response with respect to time was also
recorded. As shown in Fig. S18 and S19 the fluorescence intensity of the MOF suspension
with 10 µL of Al3+/Fe3+ were continuously monitored after a certain period of time. There are
no degree of enhancement or quenching effect with increasing the time. Hence the rapid
response time may be attributed that both the metal ions Al3+ and Fe3+ can easily scatter into
the channel of Cdꢀ
Fig. 4 (a) Photoluminescence spectra with a gradual addition of 10ꢀ2 (M) aqueous Fe3+
solution in the DMF dispersed solution of CdꢀMOFꢀ2. [Insets: colour changes before (left)
and after (right) the addition of 100 µL Fe3+ solution (exposed to 365 nm UV light)] (b)
Photoluminescence spectra with a gradual addition of 10ꢀ2 (M) aqueous Al3+ solution in the
DMF dispersed solution of CdꢀMOFꢀ2. [Insets: colour changes before (left) and after (right)
the addition of 100 µLAl3+ solution (exposed to 365 nm UV light)] (c) Relative luminescence

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intensities after introducing various metal cations. (d) Comparison of the fluorescence
intensity of CdꢀMOFꢀ2 exposed to mixed metal ions.
MOFꢀ1 or may interact with the azo sites of the MOFs surfaces. The rapid response time is
also higher than that of some previously reported materials.15 It should be noted that the
response time is rarely explored for the reported MOFꢀbased ion sensors.
The possible mechanism for the sensing of Fe3+ ion would be explained by the
followings: firstly, PXRD patterns of both the MOFs after the detection of Fe3+ ion were
measured, which remained identical with their asꢀsynthesized patterns suggesting that the
skeleton of the MOFs remained intact under the experimental conditions (Fig. S5ꢀS6, ESI).
Hence, the quenching effect does not result from the destruction of the frameworks. To get a
better insight into the phenomenon, the energy dispersive spectroscopy (EDS) elemental
mapping of Fe3+ and Al3+ꢀloaded samples were performed. As shown in Fig. S24 to S27
(ESI), Fe and Al in both the samples are homogeneously distributed. Secondly, the resonance
energy transfer is another possible reason for the quenching appearance of the framework due
to sensing of the metal ions. The resonance energy transfer can be distinguished from the
fluorophore (donor site) to the analyte (acceptor site), if the emission spectrum of the donor
has a certain degree of overlap with the absorption spectrum of the acceptor, provided the
appropriate distance between the donor and acceptor.16 Hence, the UVꢀVis spectra of the
various metal ions in water has been investigated. It is observed from Fig. S28 and S29 (ESI)
that there exists a large overlap between the absorption spectra of the Fe3+ ions and the
emission spectra of both the CdꢀMOFs whereas the other metal salts have no apparent
overlap with the emission spectrum of the materials. For the Al3+ sensing, the quenching
mechanism is not like Fe3+ as there is no existence of overlap between the absorption spectra
of the Al3+ ions and the emission spectra of both the CdꢀMOFs. The PXRD patterns of both
the MOFs after the detection of Al3+ ion were measured, which shows the identical peak
position with that of their asꢀsynthesized patterns suggesting that the skeleton of the MOFs
remained intact under the experimental conditions. The introduction of Al3+ metal ion may
consume the energy through the collision interaction and further decrease the energy transfer
from the spacer L to Cd2+ center may raise to the luminescence quenching.17
Conclusions

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In summary, two new 3D CdꢀMOF based on an azo linked N,N′ donor spacer with
two different dicarboxylates have been synthesized and structurally characterized. The
luminescence investigation on both the materials manifested that both MOFs could act as
dual luminescence sensors which can selectively and sensitively detect Al3+ as well as Fe3+
with rapid response time. The current result demonstrates the strategy of constructing
luminescent MOFs through incorporation of functional bare azo sites onto the framework
backbone for selective sensing of metal ions. Given that d10 transition metals are highly
abundant and rich library of conjugated organic linkers and spacers for functional MOFs
construction, this work may contribute to extend the potential application of transition metal
basedꢀMOFs toward dualꢀresponsive luminescent probe for metal ion sensing.
Electronic Supplementary Information (ESI)
Crystallographic data, PXRD, TGA patterns, additional Figures related to sensing
experiments, FESEM images, and EDS analysis study (PDF).
CCDC No: 1835623 for CdꢀMOFꢀ1 and 1835624 for CdꢀMOFꢀ2
ORCIDiD
Madhab C. Das: 0000ꢀ0002ꢀ6571ꢀ8705
Acknowledgments
S.C. thanks IIT KGP for SRF fellowship. M.C.D. gratefully acknowledges the financial
support received from SERB, New Delhi as Early Carrier Research Award
(ECR/2015/000041).
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Table of Content
Two Azo-functionalized Luminescent 3D Cd(II)-MOFs for Highly Selective
Detection of Fe3+ and Al3+
Two 3D luminescent Cd(II)-MOFs (Cd-MOF-1 and Cd-MOF-2) exposing azo functional
sites displayed selective detection of Fe3+ and Al3+ metal ions with high sensitivity.