Temperature-Dependent Emission and Turn-Off Fluorescence Sensing of Hazardous "quat" Herbicides in Water by a Zn-MOF Based on a Semi-Rigid Dibenzochrysene Tetraacetic Acid Linker

Article abstract of DOI:10.1021/acs.inorgchem.0c00307

A zinc metal-organic framework, i.e., Zn-MOF (Zn-DBC), with ca. 27% solvent-accessible void volume was synthesized from a rationally designed tetraacid based on sterically insulated dibenzo[g,p]chrysene core; the latter inherently features concave shapes.

Full text of DOI:10.1021/acs.inorgchem.0c00307

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Article
Temperature-Dependent Emission and Turn-Off Fluorescence
Sensing of Hazardous “Quat” Herbicides in Water by a Zn-MOF
Based on a Semi-Rigid Dibenzochrysene Tetraacetic Acid Linker
Arindam Mukhopadhyay, Swati Jindal, Govardhan Savitha, and Jarugu Narasimha Moorthy*
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ABSTRACT: A zinc metal−organic framework, i.e., Zn-MOF (Zn-DBC), with ca. 27% solvent-accessible void volume was
synthesized from a rationally designed tetraacid based on sterically insulated dibenzo[g,p]chrysene core; the latter inherently features
concave shapes. Due to rigidification of the fluorophore in the MOF, Zn-DBC exhibits a respectable fluorescence quantum yield of
ca. 30% in the solid state. The fluorescent and water-stable Zn-DBC MOF was found to display intriguing temperature-dependent
emission behavior with an activation barrier of 1.06 kcal/mol for radiationless deactivation from the singlet-excited state. It is shown
that the Zn-MOF can be employed as an efficient sensory material for detection of hazardous “quat” dicationic herbicides in water by
diffusion-limited “turn-off” fluorescence. Due to confinement of the cationic guest analytes within the pores of the MOF, the
fluorescence quenching via excited-state charge transfer mechanism is shown to depend on the molecular size of the analyte in
addition to the redox potentials. Remarkably, Zn-DBC permits sensing of DQ, a well-known toxic “quat” herbicide, with a detection
limit as low as 2.8 ppm in water. The unique structural attributes of the Zn-MOF for highly efficient fluorescence sensing of toxic
herbicides in water are thus exemplified for the first time.
dihydrodipyrido[1,2-a:2′,1′-c]pyrazinediium) dibromide (cf.
Chart 1), have gained tremendous attention as nonselective
and contact as well as quick-acting weed killers.6,7 The
mechanism of their action has been established. Accordingly,
the bipyridinium motifs of the dicationic species can accept
electrons available from the plant photosynthetic system I
(PSI), leading to generation of radicals, which in turn react
with molecular oxygen.7 Although they are employed as
aquatic or nonaquatic weed killers, the “quat” herbicides are
regarded as water pollutants. In fact, the lethal dose (LD50) of
PQ is reported to be ca. 20−40 mg/kg of body weight in
INTRODUCTION
■
One of the serious problems plaguing humankind today is the
ever-increasing pollution of our ecosystem.1 The toxic
chemicals emanating from industrial processes, traffic emission,
plant/animal protection, chemotherapy, etc. range from
radioactive wastes to hazardous gases, chemical side products,
herbicides, pesticides, drugs, etc.1 There is thus a dire need to
develop (i) sensitive analytical methods to monitor a large
variety of chemicals that are detrimental to the ecosystem and
(ii) efficient technologies to get rid of hazardous chemicals
distributed in high dilution from the environment. In
particular, there has been a relentless quest to develop
methods to monitor concentrations of herbicides in the
environment; herbicides, also termed weed killers, are widely
used in agriculture and horticulture.2−6 Among the most
commonly used herbicides, “quat” herbicides containing a
diquaternary bipyridyl motif, e.g., paraquat (PQ, 1,1′-dimethyl-
4,4′-bipyridinium) dichloride and diquat (DQ, 6,7-
Received: January 31, 2020
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variations, framework stability, permanent porosity, high
surface area, high thermal and chemical stability, functionality,
etc.28−40 The photoluminescence in LMOFs is known to result
from one or more of the following: organic linkers, metal ions
(e.g., lanthanide ions), both organic linkers and metal ions at
the same time, guest species bound in the porous MOFs, and
scintillations.24 Regardless of the origin of luminescence,
LMOFs have been shown to be highly efficient fluorescence
sensors for detection of small molecules, volatile organic
compounds, explosive nitroaromatics, odorants, transition
metal ions, temperature, pH, bioactive molecules, etc.24−40
In continuation of our endeavors to develop functional
MOFs in a “bottom-up” approach involving de novo design of
organic fluorescent systems such as pyrene, bianthryl,
triptycene, etc.,41−51 we designed a tetracarboxylic acid linker
based on a twisted dibenzo[g,p]chrysene core, namely,
2,7,10,15-tetrakis[2,6-dimethyl-4-(α-carboxy)methoxyphenyl]-
dibenzo[g,p]chrysene (H4DBC), Figure 1. Our rationale for
Chart 1. Molecular Structures of Bipyridinium Dicationic
Quats
a
a
−
The counter anions are 2PF6 .
humans.6 Further, it is known that direct contact and exposure
to PQ via inhalation, ingestion, ocular, skin routes, etc. results
in dangerous health consequences, which may include
pulmonary fibrosis, pulmonary edema, erythema, dermatitis,
mouth ulceration, brain damage, etc.6 Although the toxicity of
DQ is relatively lower than that of PQ (the oral values of DQ
and PQ in rats are 231 and 150 mg/kg, respectively6), the
former is known to (i) instigate corrosion of tissues of skin and
gastrointestinal tract upon exposure and (ii) cause kidney
failure and central nervous system toxicity upon ingestion.6
Thus, efficient methods that detect toxic “quat” herbicides are
of immediate relevance.
Molecular recognition of “quat” type dicationic systems by
crown ethers involving supramolecular host−guest chemistry
has long been known.8−15 The standard and traditional
method for their detection, however, is spectrophotometry of
their radical cations. This method necessitates meticulous
experimental execution due to rapid oxidation of the latter by
the dissolved molecular oxygen.7 Therefore, a number of
laborious and time-consuming analytical methods, e.g.,
chromatography, capillary electrophoresis, immunoassay, etc.,
have been developed.7 However, the requirement of extremely
sophisticated and expensive instruments in these techniques
limits their practical applications. In this context, fluorescence-
based sensing should provide the best possible analytical
method in terms of sensitivity, speed, and portability. Only a
few fluorescent chemosensors have been reported for detection
of PQ and DQ;7,16−19 some of them, for instance, are
porphyrin-based receptors,16,17 coumarin-450-functionalized
metallodendrimers,18 pyrene-appended bis(m-phenylene)-32-
crown-10 host,19 pyrene derivatives,7 etc. Two important
drawbacks with these systems include (i) fluorescence sensing
in organic solvents rather than in an aqueous medium, which
completely precludes practical on-field detection of herbicides,
and (ii) the inability of most of systems to distinguish between
PQ and DQ.
Stimuli-responsive luminescent materials have gained
enormous attention due to their emerging security applica-
tions, e.g., sensing, information storage, encryption, anti-
counterfeiting, etc.20−23 Although a variety of smart
luminescent materials have been reported so far,21−23 there
has been a surge of interest over the past decade to explore
functional applications of luminescent metal−organic frame-
works, termed popularly as LMOFs.24−27 The latter have been
widely exploited for chemosensing applications due to several
advantages such as tunabibility of structures based on ligand
Figure 1. Structure of the semirigid tetracarboxylic acid linker
H4DBC. Note that the dibenzo[g,p]chrysene (DBC) core is shown in
blue.
the design of this linker was based on the following
considerations. First, the linker H4DBC is characterized by a
rigid dibenzo[g,p]chrysene core and orthogonally oriented flat
aromatic rings; the methyl groups installed strategically ensure
inflexibility of the system. Such a structure features concave
shapes, which have been demonstrated to facilitate guest
inclusion in the solid state to permit access to multicomponent
crystals.52−54 Second, the 4-connecting linker also features
carboxymethyl groups at the periphery of the four orthogonal
aromatic rings. The peripheral flexibility in the otherwise rigid
linker may permit metal-assisted self-assembly successfully to
afford MOFs without any structurally imposed constraints.
Third, given the well-known excited-state properties of
dibenzo[g,p]chrysene systems and their extensive applications
in the development of luminescent materials,55−57 the LMOFs
with d10 metal ions should be compelling materials for
fluorescence sensing applications. Herein, we report that the
self-assembly of H4DBC in the presence of Zn2+ leads to a
porous, water-stable, and fluorescent Zn-MOF, namely, Zn-
DBC. The latter is shown to exhibit intriguing temperature-
dependent emission behavior. The respectable fluorescence
emission of Zn-DBC has been exploited for sensing of the
dicationic “quat” herbicides. It is shown that the Zn-MOF is
selective in that a distinction can be made between dicationic
bipyridinium salts, solely based on size and redox properties of
the analytes.
B
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a
Scheme 1. Synthesis of H4DBC
a
Note that the dibenzo[g,p]chrysene (DBC) core is shown in blue.
Figure 2. (a) The coordination modes of the carboxylate groups of DBC and (b) the tetrahedral coordination environment of Zn2+ in Zn-DBC.
Notice the twisted structure of the dibenzo[g,p]chrysene core in (a). (c) Crystal packing diagram of Zn-DBC down the c axis; notice that Zn, O,
and N atoms are represented in cyan, red, and blue, respectively. The hydrogen atoms in (a−c) and the solvent molecules (water and DMF) in (c)
have been omitted for clarity. (d) Simplified topology of the 3-nodal net of Zn-DBC represented down 001 direction.
(1).58 The latter was converted to 2,7,10,15-
tetrabromodibenzo[g,p]chrysene (2) by treatment with
FeCl3. Suzuki coupling reaction of 2 with 2,6-dimethyl-4-
methoxyphenylboronic acid under Pd(0)-catalyzed conditions
furnished 2,7,10,15-tetrakis(2,6-dimethyl-4-methoxyphenyl)-
RESULTS AND DISCUSSION
■
Synthesis of H4DBC. The target linker H4DBC was
synthesized according to the route shown in Scheme 1. To
begin with, 1,1,2,2-tetraphenylethene was brominated with
neat Br2 to access 1,1,2,2-tetrakis(4-bromophenyl)ethene
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Figure 3. Location of the DMA cations (represented as blue spheres) in the voids of Zn-DBC. Notice the strong N−H···O hydrogen bonding
(dN···O = 2.76 Å) of the DMA cation with the oxygen atom of one of the carboxylate groups of DBC.
Figure 4. (a) Steady-state fluorescence spectra (λex = 350 nm) of Zn-DBC and H4DBC in the solid state at rt. Notice that the emission spectra (λem
= 420 nm) are identical for both cases. The insets show the photographs of the bulk crystals of pristine Zn-DBC under regular light (top) and blue
fluorescence of the latter when exposed to 350 nm UV light (bottom). (b) Time-resolved fluorescence decay trace (λex = 308 nm) of Zn-DBC
(dispersed in water) and H4DBC (0.5 μM in DMSO) at rt, cf. SI.
dibenzo[g,p]chrysene (3) in 82% yield. The resultant
tetraanisyl derivative 3 was demethylated with BBr3 to afford
2,7,10,15-tetrakis(2,6-dimethyl-4-hydroxyphenyl)dibenzo[g,p]-
chrysene (4) in almost quantitative yield. The tetraphenol 4
was subsequently subjected to alkylation with ethyl bromoa-
cetate under basic conditions to get hold of 2,7,10,15-
tetrakis[2,6-dimethyl-4-(α-carboethoxy)methoxyphenyl]-
dibenzo[g,p]chrysene (5) in 74% yield. Base hydrolysis of the
tetraester 5 led to the target tetraacetic acid H4DBC in a near-
quantitative yield.
Synthesis and X-ray Single Crystal Structure Analysis
of Zn-DBC. Treatment of the solution of H4DBC with
Zn(NO3)2·6H2O in DMF−DMSO−H2O (3:1:1, v/v) solvent
mixture at 90 °C in a tightly capped glass vial under
autogenous solvothermal conditions led to flaky crystals of
Zn-DBC after 2 days, cf. SI. Single crystal X-ray diffraction data
collection and subsequent structure determination revealed
that the crystals belong to the orthorhombic system with the
Pbcn space group, cf. Table S1 in the SI. The asymmetric unit
was found to contain 1.5 Zn2+ metal ions, one molecule of
DBC linker, and three guests, namely, one uncoordinated
water molecule, one uncoordinated DMF molecule, and one
dimethylammonium (DMA) cation, cf. Figure S1. Thus, the
chemical formula of the asymmetric repeating unit in Zn-DBC
is [Zn1.5(DBC)·(H2O)·(Me2NCHO)·Me2NH2]. The crystal
structure (cf. Figure 2) analysis shows that one molecule of
DBC linker accounts for four units of negative charge, which is
compensated by 1.5 units of Zn2+ ions and one DMA cation,
such that the overall charge neutrality is maintained. Indeed,
the DMA cation is found to be stabilized inside the voids of the
MOF via strong N−H···O hydrogen bonding (dN···O = 2.76 Å)
with the oxygen atom of one of the carboxylate groups of DBC,
cf. Figure 3. The structural analysis also shows that the Zn2+
metal ions in the framework exist in a tetrahedral geometry,
whereby they are coordinated to four oxygen atoms of four
carboxylate groups of DBC, cf. Figure 2. Thus, each Zn2+ metal
ion behaves as a 4-connecting node. Figure 2 also shows the
coordination motif of the tetracarboxylate DBC, which behaves
as a 4-connecting linker. Thus, the assembly of Zn2+ ions and
DBC leads a 3D porous anionic framework structure with the
channels propagating down the c axis, Figure 2. Insofar as the
central chrysene core is concerned, it is found to be
significantly twisted.
The crystal lattice of Zn-DBC is found to be significantly
porous with the dimensions of the pores ranging from ca. 8.4
to 17.9 Å, cf. Figure S3. PLATON59 analysis shows that the
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Figure 5. (a) Temperature-dependent fluorescence spectra of Zn-BDC in water. (b) Contour plot of the fluorescence intensity (Irel) at 420 nm as a
function of temperature. (c) Monoexponential diminution of quantum yields of fluorescence with increasing temperature. (d) Arrhenius-type plot
for determination of activation energy for the activation-controlled nonradiative pathway.
solvent-accessible volume in the crystal lattice after removal of
the two guest solvent molecules, i.e., water and DMF, is ca.
27%. The void volumes in the crystal lattice are generated in
close proximity of the dibenzo[g,p]chrysene core between the
concave shapes, cf. Figure S4. Topological analysis using the
TOPOS60 program reveals that the 3D framework of Zn-DBC
corresponds to a new topology with a point symbol of
{62.84}{62.8}2{63.87}2. The framework corresponds to a three-
centered nodal net, such that the stoichiometry is (3-c)2(4-
c)(5-c)2. Figure 2 shows a simplified topological network of
the three-centered nodal net of Zn-DBC down the 001
direction in a standard representation.
The synthesis of Zn-DBC could be scaled up quite readily.
The material synthesized on a bulk scale was established to be
identical to that of the X-ray determined single crystal by
comparison of its PXRD profile of the pristine Zn-DBC with
that simulated for the structure obtained by single crystal X-ray
analysis, vide infra. The thermal stability of the MOF was
established from TGA, which reveals solvent loss up to 350 °C
corresponding to ca. 30% followed by decomposition, cf.
Figure S7. Notably, the crystals of Zn-DBC were found to be
remarkably stable both when immersed in water for 2 days at
room temperature (rt) and when heated at 85 °C in water for
1 day, as revealed by PXRD analysis, vide infra. The stability of
the MOF was further corroborated by a leaching experiment,
which reveals that the linker does not leach out from the MOF
upon immersion of the crystals in water for 2 days at rt, cf. SI.
Temperature-Dependent Fluorescence Properties of
Zn-DBC. The crystals of Zn-DBC were found to display
brilliant blue emission upon exposure to UV light in the solid
state. In Figure 4 is shown the solid-state fluorescence
spectrum of Zn-DBC with an emission maximum at 420 nm
(λexc at 350 nm); the full width at half-maximum (fwhm) of the
emission was found to be 46 nm. The solid-state fluorescence
emission spectrum of the precursor organic linker H4DBC was
found to be identical to that of Zn-DBC, cf. Figure 4. This is
not surprising given that the orthogonally oriented aryl rings at
the four corners of the dibenzo[g,p]chrysene moiety sterically
insulate the fluorescent dibenzo[g,p]chrysene core, as has been
found for an analogous system based on tetraarylpyrene;45
fluorescence quantum yields for Zn-DBC and H4DBC were
determined in the solid state by an integrating sphere setup at
298 K to be ca. 30% and 16%, respectively. Significant
enhancement of the fluorescence quantum yield of Zn-DBC
when compared to that of the precursor linker H4DBC should
be reconciled from differences in their conformational rigidities
and highly ordered structure of the dibenzo[g,p]chrysene
fluorophore within the framework; structural rigidity is well-
known to attenuate nonradiative pathways that depopulate the
singlet-excited state.45,61 This was further corroborated by
time-resolved fluorescence studies using time-correlated single
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Figure 6. (a) Changes in the UV−vis absorption spectra of the supernatant solution of MB in DMF with time in the presence of added crystals of
Zn-DBC. Insets show the color of the supernatant solution of MB before and after 24 h of dye adsorption. (b) A plot showing the gradual decrease
in absorbance at 664 nm of the MB solution with time. Notice the blue coloration of MOF crystals after 24 h of MB absorption in the inset; the
photograph of the blue crystals was taken after thoroughly washing the crystals with DMF to remove any dye adhering to the surface.
o
l
o
o
|
o
o
photon counting (TCSPC), which reveal that the singlet
lifetime of Zn-DBC (1τ = 7.8 ns) is comparatively longer than
that observed in the case of H4DBC (1τ = 5.0 ns), cf. Figure 4
and SI. This is a characteristic feature of LMOFs in which the
fluorophoric linkers lose some degree of torsional freedom.45,61
To further probe the nonradiative pathways that depopulate
the singlet-excited state of Zn-DBC, the influence of
temperature on the steady-state fluorescence properties was
investigated for the crystals of Zn-DBC suspended as a
dispersion in water, cf. Table S3. Interestingly, the fluorescence
intensity of Zn-DBC was found to depend remarkably on
temperature; of course, the emission maximum (420 nm) was
found to remain affected with temperature. As shown in Figure
5, one observes a drastic decrease in the fluorescence intensity
(λmax = 420 nm) with increasing temperature. Buoyed by this
interesting observation, quantum yields of fluorescence (Φf)
were determined for the aqueous suspension of the MOF at
different temperatures in the range of 278−358 K; the stability
of the MOF crystals in water at room temperature as well as at
elevated temperatures was ensured to be unaffected by PXRD
analysis, vide infra. The quantum yield was found to decrease
rapidly from 35.5% at 278 K to 30% at 298 K and 25.4% at 358
K, cf. Table S3. The diminution in the quantum yield of
fluorescence with absolute temperature (T in K) of the
medium could be easily fitted to a monoexponential function
with an excellent goodness-of-fit (R2 = 0.99), cf. Figure 5.
On the basis of the definition of Φf and the general
assumption that the radiative decay constant (kr) is
independent of temperature,62−64 it could be reasoned that
the influence of temperature on Φf values must arise from
some competitive activation-controlled nonradiative pathways
(knr). In fact, the mechanistic insights of the temperature
dependence of Φf due to operation of such activation-
controlled radiationless processes have been comprehensively
established by several groups.63,64 It is needless to mention that
the relation of Φf and T (K) can be expressed in the form of an
Arrhenius-type equation (eq 1 and eq 2) as shown below:
i
j
y
z
i
j
y
z
k
1
ΔE
nr
j
z
j
z
lnmj z − 1} = lnj z −
j
j
z
z
j
j
z
z
o
o
o
o
Φ
kr
RT
f
(2)
k
{
k
{
n
~
where knr0 is the pre-exponential factor and ΔE is the activation
energy.
Following eq 2, the plot of ln{(1/Φf) − 1} versus 1/T is
indeed found to be linear with a respectable correlation of R2 =
0.97, cf. Figure 5. Accordingly, the activation energy (ΔE)
calculated from the slope of this linear fit turned out to be 1.06
kcal/mol for the radiationless deactivation of the singlet-
excited state of Zn-DBC. This clearly suggests that some
activation-controlled radiationless relaxation mechanism is
operative in the singlet-excited state of the latter. In this
regard, the fact that the diminution in fluorescence intensity is
not a consequence of the loss of crystallinity of the material
was unambiguously established based on (i) similarity of the
PXRD profiles of the material after heating/cooling cycles, vide
supra, and (ii) the observation that the emission spectrum of
the MOF suspension at 298 K is similar to the one recorded
for the suspension heated at 358 K and cooled to 298 K, cf.
Figure S8.
The observed temperature-dependent process should be
reconciled from the structural features of Zn-DBC. As
mentioned earlier, the carboxymethyl groups at the periphery
of the four orthogonal aromatic rings offer structural flexibility
to the DBC linker such that the latter behaves as a semirigid
linker. Thus, one of the probable reasons behind the
activation-controlled radiationless process could be torsional
rotations of the conformationally flexible ether moieties (−O−
CH2−), which are substituted at the para positions of the four
orthogonally oriented aromatic rings in DBC linker. A similar
instance of temperature-dependent torsional rotation and
hence an activation-controlled fluorescence property has
been reported for coumarin-151 dye; for the latter, the flip-
flop motion of the 7-amino group has been reported to serve as
a nonradiative deactivation channel for the singlet-excited state
in a nonpolar solvent.63 Although the excited-state properties
of LMOFs have been a subject of extensive investiga-
tion,24−40,43,45−47,50 the temperature-dependent fluorescence
emission of LMOF due to an activation-controlled radiation-
less process is hitherto unknown.
i
j
j
y
z
z
z
i
j
j
j
y
z
z
z
knr
kr
1
1
kr
i−ΔE y
o
nr
j
j
j
z
z
z
= 1 +
= 1 +
k
exp
j
j
j
z
z
j
j
z
z
Φ
RT
k
{
(1)
f
k
{
k
{
F
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“Turn-Off” Fluorescence Sensing of “Quat” Herbi-
cides by Zn-DBC in Water. We were encouraged by
respectable fluorescence quantum yield, appreciable void
volume of ca. 27% in the crystals, and remarkable water
stability of Zn-DBC to explore guest binding by fluorescence
signaling. It was surmised that the electron-rich aromatic
dibenzo[g,p]chrysene fluorophore that is part of Zn-DBC
should relay information about its interaction with any
electron-deficient (and π-acceptor) aromatic guest species
through changes in fluorescence. However, the fluorescence of
Zn-DBC was found to be unaffected whatsoever in the
presence of various nitroaromatic compounds, e.g., nitro-
benzene, 1,3-dinitrobenzene, 2,4,6-trinitrotoluene, etc.; the
latter are well-known strong π-acceptor analytes, which have
been employed routinely for “turn-off” fluorescence quenching
of LMOFs.26,27,43 We reasoned that the lack of fluorescence
quenching is presumably due to the anionic nature of the Zn-
DBC framework, which warrants that the counter cations be
located in the lattice for overall charge neutrality. Indeed, the
DMA counter cations, as mentioned earlier, are found to be
stabilized within the voids. The counter cations in the crystals
of Zn-DBC may in principle permit transport of guests that are
cationic in nature and capable of replacing the DMA cations in
the voids. In other words, we surmised that the exchange of
neutral and anionic guests in the anionic framework of Zn-
DBC should be precluded.
To unambiguously establish the ability of the Zn-DBC
crystals to undergo postsynthetic exchange (PSE)44,45 of the
DMA cations, the MOF crystals were dispersed in the solution
of a cationic dye, namely, methylene blue (MB). Indeed, the
blue color of MB was found to fade with time in the presence
of the MOF, cf. Figure 6. The gradual dye absorption process
was also monitored by recording UV−vis absorption spectra of
the supernatant solution of MB; as shown in Figure 6, the
absorbance at the λmax of MB (i.e., 664 nm) was found to
undergo regular diminution over a period of 24 h. The dye
exchange process was also corroborated by blue coloration of
the colorless MOF crystals, which could be readily made out
by the naked eye, cf. Figure 6. However, such a coloration was
not observed whatsoever, when the MOF crystals were
dispersed in the solutions of an anionic dye, namely,
bromophenol blue (BB), and a neutral dye, namely, nile red
(NR), even after 4 days, cf. Figure S15. It thus emerges that the
anionic framework of Zn-DBC permits selective transportation
of the cationic guests by exchanging DMA counter cations
while precluding anionic and neutral guests. Additionally, the
adsorption of cationic dye, e.g., MB, could be deadsorbed in a
reversible manner by immersing the colored MOF crsytals in
DMF containing ammonium chloride; the colorless MOF
crystals reappeared slowly with the DMF turning intensely
colored.
with, the fluorescence of the crystals of Zn-DBC suspended as
a dispersion in water was examined in the presence of each of
these analytes; of course, the structural integrity of the Zn-
DBC crystals upon dispersion in water with and without the
dicationic analytes was established to be intact by PXRD
analysis, cf. Figure 7.
Figure 7. PXRD profiles: (a) simulated for the X-ray determined
single crystal structure, (b) pristine MOF crystals synthesized in bulk,
(c) crystals immersed in distilled water for 2 days, which were
subsequently dried, (d) crystals immersed in distilled water and
heated at 85 °C in water for 1 day, which were subsequently cooled to
rt and dried, and (e) crystals immersed in a 5 mM aqueous solution of
DQ(PF6)2, which were subsequently washed with water and dried.
With the addition of an increasing concentration of each of
the dications, a gradual decrease in the fluorescence intensity
of the aqueous suspension of Zn-DBC was observed, cf. Figure
S9. A representative plot of fluorescence quenching titration of
Zn-DBC with DQ(PF6)2 at rt is shown in Figure 8.
Remarkably, the sensing of the dicationic analytes by the
aqueous suspension of the MOF by fluorescence quenching
can be readily made out with the naked eye, cf. Figure 8. The
steady-state fluorescence quenching data for all four analytes
could be readily subjected to a linear regression analysis
following the Stern−Volmer plot. As established from time-
resolved fluorescence titration experiments, the nature of
quenching of fluorescence of Zn-DBC by the cationic analytes
turns out to be dynamic, cf. Figure S10. Given that the
dicationic analytes themselves do not absorb at the λex = 350
nm, the observed fluorescence quenching of MOF with
dicationic analytes is not contributed by competing light
absorption due to the latter, cf. SI.
The Stern−Volmer quenching constants (KSV) thus derived
for all the analytes follow the order DQ (500 M−1) > PQ (110
M−1) > OQ (36 M−1) > TBPQ (4 M−1), cf. Table 1. The
singlet lifetime of Zn-DBC in water (1τ = 7.8 ns) determined
from TCSPC (vide supra) was employed to evaluate absolute
bimolecular quenching rate constants (kq = KSV/1τ) for each of
the analytes, cf. Table 1. Thus, the order of quenching rate
constants (kq’s, × 1010 M−1 s−1) turns out to be DQ (6.41) >
PQ (1.41) > OQ (0.46) > TBPQ (0.051). How can this
observed order of fluorescence quenching data for the cationic
quenchers be rationalized?
With the postsynthetic cation exchange in Zn-DBC
established, we immediately sought to investigate the sensing
of bipyridinium dicationic species and, hence, the “quat”
herbicides. Thus, we comprehensively examined the binding/
sensing of four bipyridinium dicationic species as shown in
Chart 1, namely, 1,1′-dimethyl-4,4′-bipyridinium (PQ), 6,7-
dihydrodipyrido[1,2-a:2′,1′-c]pyrazinediium (DQ), 1,1′-di-
methyl-2,2′-bipyridinium (OQ), and 1,1′-bis(3,5-di-tert-butyl-
benzyl)-4,4′-bipyridinium (TBPQ); the analytes were used in
the form of their hexafluorophosphate salts (cf. SI) with the
The fact that aromatic cations (acceptor) quench the
fluorescence of e-rich fluorophores (donors) via excited-state
charge transfer process by diffusion limited rates is well
−
advantage that the PF6 counteranion is non-coordinating in
nature to interfere with the studies carried out.7,16−19 To begin
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Figure 8. (a) Quenching of the fluorescence intensity of Zn-DBC with increasing concentration of DQ(PF6)2 in water (λex = 350 nm) at rt. Notice
the changes in the fluorescence images (insets) of the aqueous dispersion of the MOF before and after quenching titration. (b) Corresponding
Stern−Volmer quenching plot with increasing concentration of DQ.
donor to the LUMO of the e-deficient analyte, leading to
gradual diminution of fluorescence intensity due to increasing
nonradiative decay rate.43,66−68 We have demonstrated this in
our recent studies on LMOFs based on pyrene-tetracarboxylic
acids.43,45 The observed order of kq for fluorescence quenching
of Zn-DBC by the bipyridinium dications in Chart 1 should, in
principle, be traced to a bimolecular quenching process
occurring as a consequence of charge transfer from the excited
state of DBC fluorophore (donor) to the bipyridinium
dications (acceptor); the latter becomes thermodynamically
more favorable with increasing π-acceptor nature, and hence
electron-deficiency of the analytes. Notably, the first one-
electron reduction potentials (Ered° vs SCE) of the
bipyridinium salts measured from differential pulse voltamme-
try (DPV) analysis are found to follow the trend OQ (−0.60
V) < PQ (−0.45 V) < TBPQ (−0.37 V) < DQ (−0.35 V), cf.
Table 1, Figure S16 and Table S5. Interestingly, the LUMO
energies of these dicationic salts, determined from their onset
potentials of reduction, follow the trend OQ (−3.935 eV) >
PQ (−4.085 eV) > TBPQ (−4.175 eV) > DQ (−4.205 eV), cf.
Table S6. This trend suggests that in this series, OQ has the
lowest reduction potential and hence highest LUMO energy
such that this dication is the most difficult species to undergo
one-electron reduction to the corresponding radical cation.
Table 1. Fluorescence Quenching Data of Zn-DBC for
Various Dicationic Analytes at rt
Ered° (V vs
KSV
kq (1010
η (%) at 12 mM conc. of
a
b
quencher
SCE)
(M−1
)
M−1 s−1
)
quencher
DQ
PQ
OQ
TBPQ
−0.35
−0.45
−0.60
−0.37
500
110
36
6.41
1.41
0.46
0.051
95.2
55.2
40.6
1.7
4
a
The analytes were used in the form of their hexafluorophosphate
b
salts. The reduction potential in each case is reported for the first
one-electron reduction (e.g., EDQ ) in deaerated acetonitrile
2+/DQ+•
using 0.1 M n-Bu4NPF6 as the supporting electrolyte. The first one-
electron reduction potentials determined from differential pulse
voltammetry (DPV) using Ag/AgCl as the reference electrode and
platinum as the working electrode (scan rate = 20 mVs−1) are
eventually referenced to SCE by subtracting 45 mV from the values
obtained versus the Ag/AgCl electrode, cf. Figure S16 and Table S5.
established.65 In general, the quenching of fluorescence
becomes progressively facile for more and more electron-
deficient analytes for which the lowest-unoccupied molecular
orbitals (LUMOs) are located between the valence band (VB)
and conduction band (CB) of electron-rich MOFs.43,66−68
Photoexcitation leads to electron transfer from CB of the
Figure 9. (a) A plot of quenching efficiency (η%) versus concentration of each of the dicationic quenchers. (b) A plot showing a similar trend in kq
and η% (at 12 mM concentration) for the dicationic quenchers; the reduction potentials versus SCE are also shown in green for the analytes.
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Similarly, DQ with the highest reduction potential and hence
lowest LUMO energy should be the easiest to reduce to the
corresponding radical cation. The observed trend in the kq
(1010 M−1 s−1) values, as mentioned before, is DQ (6.41) >
PQ (1.41) > OQ (0.46) > TBPQ (0.051). Thus, a ready
correlation is observed for kq with reduction potentials of the
dications with the exception of TBPQ. In other words, for the
three dications DQ, PQ, and OQ, the fluorescence quenching
rates are diffusion limited and are guided by their reduction
potentials, i.e., the higher the electron deficiency of the analyte,
the greater is the value of kq, cf. Figure 9. For example, the
highest fluorescence quenching rate (6.41 × 1010 M−1 s−1) is
observed for DQ, which has the highest Ered° (vs SCE) of
−0.35 V, while the quenching rates are comparatively lower for
PQ (1.41 × 1010 M−1 s−1) and OQ (0.46 × 1010 M−1 s−1) with
Ered° values of −0.45 V and −0.60 V, respectively. In stark
contrast, one observes that TBPQ with almost a similar
reduction potential (−0.37 V) as that of DQ shows the lowest
fluorescence quenching rate of 5.1 × 108 M−1 s−1. The reason
as to why TBPQ with a respectably high reduction potential
exhibits such an unusually low fluorescence quenching rate
should be reconciled from structural attributes. This is
evidently due to the fact that TBPQ with two 3,5-di-tert-
butylbenzyl groups is enormously large structurally such that it
is not suitable to transport into the relatively smaller voids of
the MOF to exchange the DMA cations. This clearly points to
the fact that the fluorescent Zn-DBC MOF is size-selective in
terms of cationic guest exchange.
To further corroborate the relative abilities of the dications
to quench the fluorescence of Zn-DBC, change in the
fluorescence quenching efficiency (η%) at rt was monitored
with increasing concentration of each of the dications, cf. SI. As
shown in Figure 9, η increases progressively with increasing
concentrations of the quenchers in that the highest η value is
observed for DQ. For instance, at a concentration of ca. 12
mM, one observes a quenching efficiency of ca. 95.2% for DQ,
while the same turns out being ca. 55.2% for PQ, 40.6% for
OQ, and only 1.7% for TBPQ, cf. Table 1. Thus, the trend of η
values is the same as that observed for kq, cf. Figure 9. In other
words, for dications of almost similar size, i.e., DQ, PQ, and
OQ, the fluorescence quenching rate/efficiency is dictated by
the electron deficiency, i.e., reduction potential of the analyte.
However, for TBPQ, a quencher with almost similar reduction
potential as that of DQ but bulky in size, the lowest
fluorescence quenching rate/efficiency is observed, under-
standably due to difficulty in its transportation to the
fluorophoric dibenzochrysene sites in the crystals due to
relatively smaller voids.
concentration as low as 1 μM in DMSO, due possibly to
hydrogen-bonding interactions involving the carboxylic acid
functionalities in the presence of water; complicated/abrupt
fluorescence intensity changes were observed while carrying
out the fluorescence titrations with aqueous solutions of the
dications. Steady-state fluorescence quenching experiments
were thus carried out with the solution (1 μM) of 3 in DMSO
(λex = 350 nm; λem = 420 nm) and aqueous solutions of the
dicationic analytes, cf. Figures S12 and S13 and Table S4. On
the basis of the Stern−Volmer quenching constants thus
derived and the singlet lifetime of 3 (1τ = 7.0 ns) determined
from a time-resolved study (Figure S11), bimolecular
quenching rate constants (kq) were calculated for the dications.
The trend of kq (109 M−1 s−1) for fluorescence quenching of 3
thus turned out to be DQ (10.6) > TBPQ (8.0) > PQ (5.4) >
OQ (3.9), which is in accordance with their redox potentials.
This suggests that for the model system, the fluorescence
quenching is dictated solely by the e-deficiency of the
dicationic analytes and that the model system can not
distinguish the latter by their size; in other words, a sterically
bulky analyte (TBPQ) is sensed with a much faster rate than
analytes of smaller size (PQ and OQ).
Overall, Zn-DBC MOF offers the following advantages over
the pure organic system 3 in terms of fluorescence sensing of
dicationic analytes: (i) The unique stability of the MOF in
water permits sensing of the analytes in aqueous medium
without the need of an organic solvent. (ii) Bimolecular
quenching rates are much faster in case of the MOF (ca. 1010
M
−1 s−1) as compared to the model system (kq on the order of
109 M−1 s−1), which points to better sensitivity of detection.
(iii) Fluorescence sensing of the analytes with the MOF is
controlled by both size and reduction potential, while for the
model system, the same is dictated only by the reduction
potential, and (iv) the MOF can distinguish between the two
herbicides DQ and PQ better than the model system in terms
of the ratio of fluorescence quenching rate constants, i.e.,
kq(DQ)/kq(PQ); the latter are ca. 4.5 and 2.0 for the MOF
and model system, respectively. Two plausible reasons24−27 for
the observed contrasting differences between MOF and the
organic model system are (i) rigidification of the fluorophore
when present as part of the MOF and (ii) confinement of the
cationic guests within the pores of certain size/shape.
To assess the sensitivity limits of Zn-DBC for detection of
the dicationic herbicides, fluorescence quenching experiments
were carried out at very low concentrations of DQ, a
representative case, in water, cf. Figure S14. Accordingly, a
plot of quenching efficiency (η%) versus the quencher
concentration reveals that the minimum concentration for
detection of DQ is ca. 15 μM, cf. Figure S14. In other words,
the Zn-DBC MOF permits fluorescence sensing of DQ with a
detection limit of ca. 2.8 ppm. As mentioned at the outset,
although LMOFs with tunable luminescence properties have
been employed for diverse sensing applications, they have
heretofore been not explored quite inexplicably for fluores-
cence detection of toxic herbicides. The results unveiled herein
offer intriguing mechanistic insights of temperature-dependent
emission behavior of an LMOF based on the dibenzo[g,p]-
chrysene fluorophore and application of the latter for “turn-off”
fluorescence sensing of toxic herbicides in water with a
detection limit of ca. 2.8 ppm. The results thus constitute an
invaluable addition to the gamut of ever-increasing applications
being uncovered for MOFs in general.
It is noteworthy that the Zn-DBC MOF offers a unique
advantage over the precursor organic linker H4DBC in terms of
fluorescence sensing of the bipyridinium dications. For
instance, the latter could be sensed by the MOF suspension
in water without employing any organic solvent, whereas the
same was not possible in the case of the organic linker due to
its complete insolubility and obvious aggregation in aqueous
media. For further comparison of the advantageous attributes
of the MOF versus organic system, fluorescence quenching
experiments were carried out with a model system, namely,
tetramethoxy ether of dibenzo[g,p]chrysene, i.e., 3 in Scheme
1, which is devoid of carboxylic acid functionalities at the
periphery. Notably, the reason for employing the tetraether 3
instead of the tetra-acid H4DBC itself for quenching studies is
that the latter was found to undergo aggregation even at a
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CONCLUSIONS
AUTHOR INFORMATION
■
■
Corresponding Author
A fluorescent dibenzo[g,p]chrysene-based organic tetracarbox-
ylic acid linker H4DBC with inherent concave features was
rationally designed and synthesized to access porous LMOFs
via metal-assisted self-assembly. Treatment of H4DBC with
Zn(NO3)2 yielded a water-stable Zn-MOF, namely, Zn-DBC,
with a solvent-accessible void volume of ca. 27% in the crystal
lattice. The crystals of Zn-DBC exhibit fluorescence emission
with a quantum yield of 30% and were found to exhibit
intriguing temperature-dependent emission behavior with an
activation barrier of 1.06 kcal/mol for radiationless deactiva-
tion of the singlet-excited state; the temperature-dependent
deactivation should be reconciled from torsional rotations of
the conformationally flexible ether moieties of DBC. It is
shown that the fluorescent Zn-DBC MOF can be applied for
“turn-off” sensing of toxic dicationic “quat” herbicides in water
with diffusion-controlled rates. The quenching of fluorescence
of the Zn-MOF has been rationalized based on exchange of the
cationic DMA guests in the voids of the MOF crystals by the
“quat” dicationic species. The latter, being extremely electron-
poor aromatic systems, are proposed to be involved in charge-
transfer interactions with the protected and e-rich singlet-
excited state of dibenzo[g,p]chrysene fluorophore of DBC to
account for the observed quenching. It is further shown that
the “turn-off” fluorescence quenching of Zn-DBC is dictated
both by size and redox potential of the cationic guests. Indeed,
Zn-DBC MOF is shown to efficiently distinguish between two
common herbicides, DQ and PQ. The limit for sensing of DQ
by Zn-DBC is remarkably low (2.8 ppm). The latter has been
attributed to rigidification of the fluorophore in the MOF and
confinement of the cationic analytes within the pores.
Although LMOFs have been immensely explored for diverse
sensing applications, the presently uncovered results constitute
a first-ever demonstration of LMOF for “turn-off” sensing of
toxic “quat” herbicides in water.
Jarugu Narasimha Moorthy − Department of Chemistry,
Indian Institute of Technology, Kanpur 208016, India; School
of Chemistry, Indian Institute of Science Education and Research
Thiruvananthapuram (IISER TVM), Vithura 695551, Kerala,
India; orcid.org/0000-0001-9477-5015;
Email: moorthy@iitk.ac.in
Authors
Arindam Mukhopadhyay − Department of Chemistry, Indian
Institute of Technology, Kanpur 208016, India
Swati Jindal − Department of Chemistry, Indian Institute of
Technology, Kanpur 208016, India
Govardhan Savitha − Department of Chemistry, Indian Institute
of Technology, Kanpur 208016, India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00307
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.N.M. is thankful to the Science and Engineering Research
Board (grant number SB/S2/JCB-52/2014) in New Delhi for
generous funding through a J. C. Bose fellowship. A.M. and S.J.
gratefully acknowledge the Council of Scientific and Industrial
Research (CSIR, SRF) and Ministry of Human Resource
Development (MHRD, GATE), New Delhi, respectively, for
their research fellowships. We thank Mr. M. S. Krishna for his
contributions in the initial stages during the synthesis of the
linker.
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00307.
Synthetic procedures for linker and MOF, details of X-
ray crystallography, table of crystal data and refinement,
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