Experimental and theoretical exploration of sensing and magnetic properties of a triply bridged dicopper(II) complex: The first discrete metal complex to sense picric acid in pure water

Article abstract of DOI:10.1016/j.jphotochem.2019.111987

This article reports the syntheses, crystal structures, fluorescence spectra, picric acid (PA) sensing and magnetic properties of a μ-phenoxo-bis(μ-chloro) dicopper(II) compound of composition [Cu₂L(μ-Cl)₂]Cl·4.5H₂O (1), w

Full text of DOI:10.1016/j.jphotochem.2019.111987

Accepted Manuscript
Title: Experimental and theoretical exploration of sensing and
magnetic properties of a triply bridged dicopper(II) complex:
The first discrete metal complex to sense picric acid in pure
water
Authors: Samit Majumder, Abhishek Pramanik, Shuvankar
Mandal, Sasankasekhar Mohanta
PII:
S1010-6030(19)30974-8
DOI:
Article Number:
https://doi.org/10.1016/j.jphotochem.2019.111987
111987
Reference:
JPC 111987
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
7 June 2019
17 July 2019
22 July 2019
Pleasecitethisarticleas:MajumderS, PramanikA, MandalS, MohantaS, Experimental
and theoretical exploration of sensing and magnetic properties of a triply bridged
dicopper(II) complex: The first discrete metal complex to sense picric acid in pure
water, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019),
https://doi.org/10.1016/j.jphotochem.2019.111987
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

1
Experimental and theoretical exploration of sensing and magnetic properties
of a triply bridged dicopper(II) complex: The first discrete metal complex to
sense picric acid in pure water
,a,b
a
a
Samit Majumder,* Abhishek Pramanik, Shuvankar Mandal, and
Sasankasekhar Mohanta*,a
aDepartment of Chemistry, University of Calcutta, 92 A.P.C Road, Kolkata 700009, India.
bDepartment of Chemistry, Bhairab Ganguly College, 2 Feeder Road, Belghoria, Kolkata
7
00056, West Bengal, India.
*
Corresponding author. E-mail address: sm_cu_chem@yahoo.co.in (S. Mohanta) and
samitmaj@gmail.com (S. Majumder).
Graphical abstract
Syntheses, crystal structures, magnetic and fluorescence properties and turn-off fluorescent
sensing behaviour in pure water, for PA, of a triply bridged µ-phenoxo-bis(µ-chloro) dinuclear
copper(II) complex, derived from a dinucleating acyclic Schiff-base ligand, for selective
detection of PA in aqueous medium are performed in this investigation. DFT modelling of the
sensing and magnetic properties are also discussed.

2
Triple Bridged Dicopper(II)
PA
PA
Magnetic Property
Sensing Property
PA = Picric Acid
Highlights of the Work:
This article reports the syntheses, crystal structures, fluorescence spectra, picric acid (PA) sensing and
magnetic property of a µ-phenoxo-bis(µ-chloro) dicopper(II) compound of composition [Cu L(µ-
Cl) ]Cl·4.5H O (1), where HL is the [1+2] condensation product of 2,6-diformyl-4-ethylphenol and 2-(2-
2
2
2
aminoethyl)pyridine. Compound 1 is water soluble and exhibits fluorescence behaviour in water.
The major outcomes of the present investigation may be summarized as follows:




The title compound (1) is the first example of a discrete metallo-organic system to sense
picric acid in pure water.
The title compound is the first example of a structurally characterized copper(II) compound
to sense picric acid;
The values of quenching constant and association constant of the title compound is the
highest among the known PA sensors in pure water;
Experimental and theoretical magnetic properties of 1 have been studied and this is the
first time that J value (experimental or theoretical) of a copper(II) system having µ-O-bis(µ-
chloro) bridging moiety (O = any of phenoxo, hydroxo, alkoxo, oxo) has been determined.
Abstract
This article reports the syntheses, crystal structures, fluorescence spectra, picric acid (PA)
sensing and magnetic properties of a µ-phenoxo-bis(µ-chloro) dicopper(II) compound of
composition [Cu
2
L(µ-Cl)
2
]Cl·4.5H O (1), where HL is the [1+2] condensation product of 2,6-
2

3
diformyl-4-ethylphenol and 2-(2-aminoethyl)pyridine. Compound 1 is water soluble and exhibits
fluorescence behaviour in pure water. Interestingly among the various aromatic, non aromatic
nitro compounds as well as non nitro aromatic compounds screened, PA specifically quenches
the fluorescence intencity of 1 significantly in water, revealing that 1 is a turn-off fluorosensor
for PA. The sensing pathways have been investigated by spectral titration, time resolved
fluorescence decay and DFT studies. Spectral and theoretical (DFT) studies suggest that the
observed fluorescence quenching is associated with ground state (GS) charge transfer as well as
electrostatic interactions between 1 and picrate ion. Unaltered fluorescence life time of 1 in the
absence and presence of PA strongly suggests that quenching follows a static mechanism. Nature
of binding modes or interactions between the complex 1 and PA was disclosed by the single
crystal structure analysis of the corresponding association complex of composition [CuII
Cl)(H O) ]2Picrate·DMF (2). The quenching constant (Ksv), association constant (K ) and
detection limit (LOD) of the complex 1 for picric acid was found to be 1.1×10 M , 4.64×10
L(µ-
2
2
2
a
5
‒1
10
‒
2
M and 4.8 µM respectively. Variable-temperature (2.5‒300 K) magnetic susceptibility
measurement of the complex 1 reveals antiferromagnetic exchange interaction with J = ‒ 132.40
‒
1
cm . The magnetic property has been nicely rationalized in terms of spin density and magnetic
orbitals within broken-symmetry (BS) framework. Compound 1 is the first example of a discrete
metal compound to sense picric acid in pure water. Some other interesting aspects have been
discussed.
Keywords: Triple bridged dicopper(II); Picric acid sensing in water; Magnetic property; Crystal
structure; DFT

4
1
.
Introduction
Development of cheap and efficient fluorescent chemosensors having high selectivity and
sensitivity for detection of hazardous or explosives molecules/ions has attracted much attention
in recent times for wide applications in both national security and environmental protection
[
1‒4]. Trinitrophenol or picric acid (PA), being a powerful explosive among different
nitroexplosives and a highly water-soluble hazardous pollutant, has received considerable
attention recently in the field of chemosensors. In this regard, a wide variety of fluorosensors
based on small organic molecules [5,6], organic polymers including conjugated polymers [7],
discrete metallo-organic compounds [8], including macrocycles/cages [8i,8j], coordination
polymers including MOFs [9‒11], nanoaggregates [12], carbon or quantum dots [13], have been
reported for detection of PA. The detection mechanism for PA is mostly composed of proton
transfer from the hydroxy group of PA to the chemosensor, leading to a hydrogen bonding
network followed by electron transfer from the donor to the acceptor. However, most of them
show one or more of the following drawbacks: (i) Real applicability is limited as most sensors
function in non-aqueous media; (ii) There is synthetic difficulty because of the requirements of
time consuming multi-step synthesis procedure (which usually ends in low yield of the final
product), high temperature and expensive materials in the synthesis of most of the chemosensors.
Moreover, structural evidence of the association complex formed between PA and fluorescent
chemosensors, which is very important to gain deep insight into the exact interaction, is scarce.
In this regards, Schiff base sensors deserve special attention due to their simplicity, sensitivity
and low cost features. Their structures and solubility can be chemically tuned to increase the
sensing selectivity. Therefore, it is highly desirable to explore new Schiff bases or their metal

5
complexes as inexpensive and efficient fluorescent chemosensors for the detection of PA in
aqueous medium.
Copper is the third most abundant transition metal found in living system, and it plays an
essential role in biological systems. Over the past few decades extensive studies have been made
on the chemistry of copper(II) complexes derived from N,O donor type ligands to explore the
areas such as magnetism [14,15], optical properties [16], gas adsorption [17], homogeneous and
heterogeneous catalysis [18], DNA and antibacterial activities [14i‒14k,19], etc. In contrast,
II
sensing properties of such Cu compounds have been seldom reported [20]. It is worth
mentioning also that there is no structurally characterised copper(II) system that senses PA.
Clearly, exploration of copper(II) complexes as PA sensor, particularly in pure water, deserve
attention.
Although magnetic properties of copper(II) compounds have been well investigated
[
14,15], there are definitely types of systems for which magnetic studies have not been yet well
explored. The copper(II) compounds having µ-O-bis(µ-chloro) bridging moiety belong to such
unexplored area [15f]. Therefore, magnetic studies of such systems deserve attention.
DFT method has been established as a powerful tool to model observed properties. There
are several reports in which magnetic properties and sensing behaviours have been understood
in-depth by DFT calculations [5a‒5c,5f,6b,6c,6g,6f,8a,8b,9b,10c,14e‒14k,21].
Over the last few years, we have been working on the structural, biomimetic,
spectroscopic, magnetic and sensing aspects of metal compounds derived from
iminophenol/aminophenol types of N,O-donor ligand having 2,6-diformyl-4-ethylphenol as a
fragment in the synthesis of the final ligands [15a‒15d,22]. Some interesting structures/
properties have been observed in the complexes derived from some acyclic or macrocyclic

6
ligands having 2,6-diformyl-4-ethylphenol as a fragment. In continuation of our efforts, we
present herein the syntheses, crystal structures, magnetic and fluorescence properties and turn-off
fluorescent sensing behaviour in pure water, for PA, of a triply bridged µ-phenoxo-bis(µ-chloro)
dinuclear copper(II) complex [Cu
2
L(µ-Cl)
2
]Cl·4.5H O (1), derived from a dinucleating acyclic
2
Schiff-base ligand (HL; Chart 1), for selective detection of PA in aqueous medium. DFT
modelling of the sensing and magnetic properties are also discussed.
2
. Experimental
2
.1. Materials and Physical Methods
All reagents and solvents were purchased from commercial sources and used as received.
,6-diformyl-4-ethylphenol was synthesized by a known procedure [23]. (Caution: PA and 2,4-
2
dinitrophenol (DNP) are highly explosives and should be handled carefully and in small
amounts). HL was synthesized by the previous reported method [15a]. Elemental (C, H, and N)
analyses were performed on a Perkin-Elmer 2400 II analyzer. IR spectra were recorded in the
‒
1
region 400‒4000 cm on a Bruker-Optics Alpha‒T spectrophotometer with samples as KBr
disks. Absorbance spectra were recorded using a Shimadzu UV-3600 spectrophotometer, and a
Perkin-Elmer LS-50B spectrofluorometer was utilized to study emission spectra. In all
measurements, the sample concentration 50 µM was maintained. Only freshly prepared solutions
were used for spectroscopic study, and all experiments have been carried out at room
temperature (298 K). Time resolved fluorescence decay was recorded using a TCSPC lifetime
system of Horiba Jobin Yvon IBH Ltd. having a picoseconds diode laser (Model: Fluorocube-01-
NL). Fluorescence quantum yield (φ) of the compound 1 was estimated at room temperature for
its water solution by a relative method using quinine sulphate (φ = 0.546 in 0.5 (M) H
2
SO ) [24]
4

7
as the secondary standard using the following equation Where 퐼 is the area under the emission
spectral curve, OD
2
 S
 R
I S
I R
(OD ) R  S



2
(OD ) S  R
denote absorbance at the wavelength of exciting light,  is the refractive index of the medium
and φ is the fluorescence quantum yield and subscripts S and R stand in the recognition of the
respective parameters of studied sample and reference, respectively. Fluorescence lifetimes were
determined from time-resolved intensity decay by the method of time correlated single-photon
counting using a diode laser at 512 nm and 375 nm as light source. The excellence of the fits was
2
judged by the χ criterion. Average fluorescence life time (‹τ
f
›) for the multiexponential decay
curve was obtained from the decay time constants (τ) and pre-exponential factors (α) using the
th
following equation in which 훼 is the pre-exponential factor corresponding to the i decay time
푖
2
 i i

i

 f  
 i i

i
constant 휏 . Magnetic susceptibility measurements at 0.1 T magnetic field were performed with a
푖
SQUID-VSM (Quantum Design) instrument. Diamagnetic correction was done using Pascal
constants.
2
.2.1. Synthesis of the Ligand (HL)
The ligand, HL, 2,6-bis[N-(2-pyridylethyl)formidoyl]-4-ethylphenol, was prepared [15a] by
condensation of 2,6-diformyl-4-ethylphenol (0.890 g, 5 mmol) with 2-(2-aminoethyl)pyridine
(1.22 g, 10 mmol) in methanol. The mixture was refluxed for 2 h. After cooling and filtration to
remove any suspended particles, the volume of the solution was adjusted to 50 mL. This

8
"solution of HL" was considered to contain 5 mmol of the ligand HL and was used for
subsequent reaction without further purification.
2
.2.2. Synthesis of [CuII
2
L(µ-Cl)
2
]Cl·4.5H O (1)
2
To a mixture of a 10 mL "solution of HL", containing 1 mmol HL, a methanol (10 mL) solution
of copper(II) chloride (0.268 g, 2 mmol) was added under reflux. Immediately a brown
compound started to deposit. Reflux was continued for 3 h, and then the resulting deep brown
solid was collected by filtration and washed with methanol. Recrystallization from water
produced crystalline compound containing diffraction quality single crystals. Yield: ~0.59 g
(~85%). Anal. Calc. for C24
H
34
N
4
O5.5Cl
3
Cu
2
: C, 41.19; H, 4.89; N, 8.00%. Found: C, 41.46; H,
‒
1
4
.81; N, 8.15. IR (KBr pellet, cm ): ν(H
2
O), 3443 (b); ν (C=N), 1645 (s).
2
.2.3. [CuII
2
L(µ-Cl)(H
2
O) ]2Picrate·DMF (2)
2
An aqueous (5 mL) solution of 1 (0.345 g, 0.5 mmol) was added to an aqueous solution (5 mL)
of picric acid (0.229 g, 1 mmol) at room temperature. The solution was stirred for 10 min and a
green precipitate appeared. This was dissolved by adding the requisite amount of
dimethylformamide (DMF). After one hour stirring, the solution was filtered and the filtrate was
kept for slow evaporation. The green crystalline compound, containing diffraction quality green
crystals, that deposited over a period of a few days was collected by filtration and washed with
water. Yield: 0.39 g (70%). Anal. Calc. for C39
Found: C, 42.32; H, 3.70; N, 13.68%. IR (KBr pellet, cm ): ν(H
s); νas (NO ), 1561,; ν (NO ), 1362 (m), 1333 (m).
H
40
N
11
O
18ClCu
2
: C, 42.07; H, 3.62; N, 13.84%.
‒
1
2
O), 3417 (b); ν (C=N), 1641
(
2
s
2

9
2
.3. Crystal structure determination of 1 and 2. The crystallographic data of the compounds 1
and 2 are summarized in Table S1 (Supplementary material). Intensity data of 1 and 2 were
collected at 296 K and 292 K respectively, on a Bruker-APEX II SMART CCD diffractometer
using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The packages SAINT [25a]
and SADABS [25b] were used for data processing and absorption correction. All data were
corrected for Lorentz-polarization effects. The structures were solved by direct and Fourier
2
methods and refined by full-matrix least-squares based on F using SHELXL-97 [25c] and
SHELXL-2014/7 [25d] packages.
During the development of the structure of 1, the counter chloride anion (Cl3) was found
as disordered over two sites. The disorder was modelled by refining these two positions freely
for each case and setting sum of their occupancies to be equal to one and the final occupancy
parameters were set as 0.60/0.40 for Cl3A/Cl3B. It was not possible to assign properly the
solvent water molecules in the structure of 1 and therefore these solvent molecules were
eliminated by using the SQUEEZE facility of PLATON to improve the refinement [26]. The
number of solvent of crystallization in formula has been taken as 4.5 H O, as the electron count
2
per unit cell for the eliminated solvent molecules is 88 and the Z value for all of 1 is 2. All other
hydrogen atoms in 1 were inserted at geometrically calculated positions with isotropic thermal
parameters and refined. In 2, four hydrogen atoms of two coordinated water molecules were
located from a difference Fourier map and refined with constraints. All other hydrogen atoms in
2
were inserted at geometrically calculated positions with isotropic thermal parameters and
refined. All the hydrogen atoms were refined isotropically, while all the non hydrogen atoms
were refined anisotropically. The final least-squares refinements (R
1
) based on I > 2σ (I)
converged to 0.0586 and 0.0618 for 1 and 2, respectively.

1
0
2
.4. Calculation of Stern-Volmer constant (KSV), association constant (K
a
) and limit of
detection (LOD): The KSV, K
a
and LOD values were calculated by comparing the fluorescence
intensity obtained for 1 in the presence and absence of PA as reported earlier in
literature.5b,5c,6e,8c,9e,24,27 The fluorescence quenching titrations were carried out in pure water
placing 3 mL (50 µM solution) of 1 in a quartz cuvette of 1cm path length. Then, an aqueous
solution of PA (0‒1.5 equiv.) was added in an incremental fashion. The sensitivity of 1 towards
PA was determined by calculating the Stern-Volmer constant (KSV).5c,6e,9e,24 The relative
fluorescence intensity was plotted against the PA concentration to estimate KSV using the well
known Stern-Volmer equation (I
0
/I) = KSV [A] + 1. Binding stoichiometry and association
constant (K ) of 1 with PA in water solution have been determined by using Benesi-Hildebrand
a
2
7
(
B-H) plot of fluorescence intensity as a function of PA concentration. The limit of detection
(
LOD) was also calculated on the basis of the fluorescence titration. The detection limit was
calculated using the following equation; Detection limit = 3σ/k where σ is the standard deviation
of blank measurement, and k is the slope of the calibration curve obtained from linear dynamic
plot of fluorescence intensity vs PA concentration.5b,8c
2
.5. Theoretical Calculations. All DFT calculations (exchange coupling constant and full
geometry optimization) were done using ORCA program package (version 4.0.1.2) [28]. At the
DFT level, the magnetic exchange coupling constant was predicted within the broken symmetry
(BS) framework [29] by considering the experimental X-ray structure without further
geometrical optimization. Geometry optimizations were performed for the singlet state of picric
acid and picrate anion and the molecular fragments based on the experimental X-ray structures of
1
. The hybrid B3LYP functional [30] and polarized triple-ζ quality basis set def2-TZVP [31]
have been used in all calculations as implemented in the ORCA program package (version

1
1
4
.0.1.2). All the calculations utilized the chain-of-spheres (RIJCOSX) [32] approximation.
Increased integration grids (Grid4 in ORCA convention) and tight SCF convergence criteria
were used in all calculations. In the theoretical calculation of magnetic coupling, relativistic
effects have been incorporated by zeroth-order regular approximation (ZORA) within the BS
[
33]. AutoAux procedure is incorporated using the keyword “AutoAux” for BS calculation as
described in the program manual [34a]. Auxiliary Def2/J coulomb fitting basis set was used in
geometry optimizations [34b]. The spin densities, magnetic orbitals and highest occupied
molecular orbitals (HOMO) as well as lowest unoccupied molecular orbitals (LUMO) were
visualized with the programs VESTA 3 and Avogadro (version 1.2.0) [35].
3
. Results and discussion
3
.1. Syntheses and characterisation.
The compound of composition [CuII
L(µ-Cl)
]Cl·4.5H
O (1) was readily obtained in high yield
2
2
2
on reacting copper(II) chloride and HL in 1:2 ratio; methanol was used as the medium of reaction
and water as the solvent of recrystallization. On the other hand, treatment of 1 with picric acid in
water-dmf mixture results in the formation of compound of composition [CuII
Cl)(H O) ]2Picrate·DMF (2).
The characteristic C=N stretching in 1 and 2 appears as a strong band in almost the same position
L(µ-
2
2
2
‒
1
(
1641‒1645 cm ). The presence of water molecules in both complexes is evidenced by the
‒
1
‒1
appearance of a broad band centred at 3443 cm for 1 and 3417cm for 2. Absence of the O‒H
‒1
stretching band at 3106 cm , as observed in case of free picric acid [36], clearly indicates the
presence of picrate ion instead of picric acid in 2. The stretching frequencies due to asymmetric

1
2
and symmetric vibrational modes of ‒NO
group of the picrate moiety in 2 arises at 1561 cm‒1
2
‒
1
and 1333‒1362 cm , respectively.
3
.2.
Cl)
Table S2 (Supplementary material). The structure reveals that 1 is a dicopper(II) system. The
Description of crystal structures of 1 and 2. The crystal structure of [CuII
2
L(µ-
2
]Cl·4.5H O (1) is shown in Fig. 1, while the selected bond lengths and angles are listed in
2
dicopper(II) moiety is monocationic and the charge is balanced by one chloride anion. Each of
‒
the two N(pyridine)N(imine)O(phenoxide) compartments of L is occupied by a copper(II) ion.
The metal centers are also coordinated to two bridging chloride ligands. This way, the two metal
centres are triply bridged by the phenoxide oxygen atom and two chloride atoms. Clearly, both
metal centres are pentacoordinated by two nitrogen, one oxygen and two chlorine atoms. The
values of distortion parameter (τ) for the pentacoordinated CuN
2
OCl environment of Cu1 and
2
Cu2 are 0.276 and 0.352, respectively, which indicate significantly distorted square pyramidal
geometry; Cl1 occupies the apical position for Cu1 and Cl2 occupies the apical position for Cu2.
Obviously, each of the two bridging chloride ligands is equatorial to one copper(II) centre and
axial to the second copper(II) centre, i. e., both chlorides bridge the metal ions in equatorial-axial
(eq-ax) fashion.
The deviation of the donor centres and the displacement of the metal ion from the respective
least-squares basal plane are 0.350 and 0.194 Å for Cu1 and 0.359 and 0.226 Å for Cu2. In
the basal plane, the Cu‒N/O bond distances are close and lie in the ranges 1.959‒1.996 Å for
Cu1 and 1.963‒1.997 Å for Cu2. Both Cu–Cl bond distances are significantly longer than the
Cu–N/O bond distances. However, Cu–Cl(ax) bond distances (Cu1–Cl1 = 2.811 Å, Cu2–Cl2
=
2.761 Å) are much longer than the Cu–Cl(eq) bond distances (Cu1–Cl2 = 2.339 Å, Cu2–

1
3
Cl1 = 2.342 Å), which is caused by Jahn-Teller distortion of the copper(II) ion. The two
transoid angles in the basal plane are 151.21 and 167.78° for Cu1 and 169.46 and 148.34° for
Cu2. The ranges of cisoid angles for Cu1 and Cu2 are 72.97‒116.01° and 74.03‒116.88°,
respectively. In brief, the bond angles are deviated much from the ideal values and the
overall ranges of bond distances (1.959‒2.811 Å and 1.963‒2.761 Å) indicate appreciable
distortion of coordination geometry. The Cu–O(phenoxide)–Cu and Cu–Cl–Cu bridge angles
are, respectively, 99.94, 70.63 and 71.60°, and the two metal ions in the dinuclear core are
separated by a distance of 3.003 Å.
The crystal structure of [CuII
L(µ-Cl)(H
O) ]2Picrate·DMF (2) is shown in Fig. 2 and the
2
2
2
selected bond lengths and angles are listed in Table S3 (Supplementary material). The
structure reveals that it is a heterobridged µ-phenoxo-µ-chloro dicopper(II) compound in
‒
which the metal centers are boubly bridged by the phenoxo oxygen atom of L and one
chloride ligand. The heterobridged µ-phenoxo-µ-chloro dicopper(II) moiety is dipositive
cationic in 2, where the charge is balanced by two picrate anions. There is one
dimethylformamide as solvent of crystallization. The basic difference of the dicopper(II) core
in 2 from that in 1 is that one of the two chloride bridges in the latter are replaced by two
monodentate water ligands, each of which is coordinated to one copper(II) centre. Here also,
both metal centers are pentacoordinated and square pyramidal. However, the CuN
2
O
2
Cl
environment for Cu1 and Cu2 in 2 are less distorted than the CuN OCl environment in 1, as
2
2
evidenced by much smaller values of discrimination parameter (τ = 0.156 for Cu1 and 0.005
for Cu2 in 2). Notably, the bridging chloride ligand in 2 occupies an equatorial position for
both copper(II) centers, i. e., here it is eq-eq.

1
4
The deviation of the donor centres and the displacement of the metal ion from respective
least-squares basal plane are 0.135 and 0.213Ǻ for Cu1 and 0.039 and 0.142 Ǻ for Cu2.
Interestingly, the Cu–Cl bond distances (Cu1–Cl1 = 1.907 Å, Cu2–Cl1 = 1.919 Å) in 2 are
clearly shorter than the Cu–N/O bond distances (1.957‒1.995 Å for Cu1, 1.963‒2.001 Å for
Cu2) in the basal plane. As usual, the apical bond distances (Cu–O(water) = 2.366 Å for Cu1
and 2.360 Å for Cu2) are appreciably longer than the bond lengths in the basal plane. The
two transoid angles in the basal plane are 159.79 and 169.15° for Cu1 and 167.77 and
1
7
68.06° for Cu2. The ranges of cisoid angles for Cu1 and Cu2 are 78.14‒107.15° and
8.17‒95.33°, respectively. All these values indicate that the geometry of both Cu1 and Cu2
is distorted square pyramidal, where the amount of distortion is much smaller than that in 1.
As shown in Fig. 2, one hydrogen atom of the coordinated water molecule (O3) in Cu2 forms
strong hydrogen bond with phenoxide (O4) oxygen atom of picrate anion, where the
donor...acceptor distance is 2.839 Å. Another interesting structural feature of compound 2 is
the presence of a π–π interaction. The aromatic ring, C(1)C(2)C(3)C(4)C(5)C(6), of the
ligand interacts with the aromatic ring, C(31)C(32)C(33)C(34)C(35)C(36), of a picrate anion;
the distance between the two centroids is 3.668 Å and the dihedral angle between the two
least-squares planes is 2.95°. The combined effect of hydrogen bonding and π–π interactions
is likely to favour and stabilize the dicopper(II) moiety···picrate adduct [CuII
L(µ-
2
Cl)(H O) ]2Picrate·DMF (2).
2
2
3
.3. Magnetic Properties of 1. Variable-temperature magnetic susceptibility data in the
temperature range 2.5‒300 K of complex 1 is shown in Fig. 3 in the form of a χ T vs. T plot.
T value at 300 K for 1 is 0.64 cm K mol , which is smaller than the expected value of
M
3
‒1
The χ
M

1
5
3
‒1
0
.75 cm K mol for two isolated copper(II) ions with g = 2.0 and S
1
= S
2
= ½. On lowering of
3
‒1
temperature, χ
M
T decreases gradually to reach 0.012 cm K mol at 2.5 K. The profile clearly
indicates that the metal centers in 1 are coupled by antiferromagnetic interaction. Taking Ĥ =
2JS ·S as the model Hamiltonian, the simulation (Fig. 3) was performed using PHI software
37] and the converging parameters are J = ‒ 132.40 cm , g = 2.20, temperature independent
‒
1
2
‒
1
[
‒
6
3
‒1
paramagnetism (TIP) = 259 × 10 cm mol and impurity (ρ ) = 1.3%. To the best of our
knowledge, this is the first example of a triply bridged µ-phenoxo-bis(µ-chloro) dicopper(II)
complex for which the magnetic property is explored. As already discussed, while the bridging
phenoxide oxygen atom occupies basal positions (eq-eq), each of the two bridging chloride
atoms are eq-ax. So, the magnetic interactions mediated by chloride bridges should be
insignificant in comparison to that propagated through the phenoxide bridge. Crystal structure
II
analysis reveals that both Cu centers in 1 have square-pyramidal geometry. Thus, the unpaired
II
electron in each Cu center will be in dx2-y2 magnetic orbital. Consequently, there exists an
appreciable overlap between the magnetic-orbitals through the bridging phenoxide oxygen atom.
The Cu–O(phenoxide)–Cu bridge angle is 99.94(14)°, greater than the cross-over angle (ca.
9
7.5). As a result, moderate antiferromagnetic interaction takes place.
3
.4. Theoretical rationalization of magnetic-exchange in 1. In order to get better insight into
magnetic exchange phenomena in the dinuclear complex 1, “broken-symmetry” density
functional theory (DFT) calculations have been carried out using hybrid B3LYP functional in the
ORCA program. The calculations were performed on molecular cation [Cu
2
L(-Cl)
]+ of 1. The
2
J-values were evaluated from the energy difference (Δ), between the high spin (HS) and broken-
symmetry (BS) states (Δ = EBS  EHS) by Ginsberg’s approach [38]

1
6
J = (EBSEHS)/S2max
where the following spin Hamiltonian for the dinuclear system was used
Ĥ = 2J(S
1
∙S
2
)
The BS-state (M
s
= 0) is found to be more stable compared to the high-spin state (HS-state) (M
s
=
1) and thus antiferromagnetic interaction is normal. Following the above equation, the J value
‒
1
is calculated to be ‒59.77 cm which is correct in sign but somewhat smaller than the
‒
1
experimental value of ‒132.40 cm .
To understand the exchange-pathway, we have investigated the Mülliken spin-density of
HS state and the BS state. The spin density values on the Cu centers and the donor atoms of the
ligands are listed in Table 1 and corresponding plots of HS and BS states of compound 1 is
presented in Fig.4. For the high-spin state (HS), the Mülliken spin density data show that
significant spin (0.78 e) is delocalized through the ligands, and the rest (1.22 e) is present in Cu
ions. The spin density in the high-spin state is slightly higher in the phenoxide O atom (0.135 e)
compared to the two μ-chloride atoms (0.108 e and 0.116 e for Cl1 and Cl2, respectively), which
indicates that the largest overlap was observed through bridging phenoxide O atom. In Table 1 it
is shown that the spin densities on the two Cu(II) ions in 1 have similar values but opposite in
sign in the broken symmetry state which clearly indicates the existence of antiferromagnetic
exchange interaction [14e]. Importantly, the bridging phenoxide oxygen atom has a spin-density
of 0.135 in the HS-state, which is found to be reduced to a value of ‒0.002 in the BS-state. This
II
indicates that phenoxide bridge is involved in the overlapping of two Cu centers. It is evident
from the values in the Table 1 that the sign of spin density in broken-symmetry state on one of
the two bridging chloride atom (Cl2) is positive and is the same to the sign of the Cu1 center. For
the other bridging chloride (Cl1) atom, the sign of the spin density of broken-symmetry state is

1
7
the same (negative) as that of Cu2. This shows that the bridging chloride atom (Cl2) which is
equatorial to Cu1, is mainly affected by the delocalization from Cu1, while for the other bridging
chloride atom (Cl1) which is equatorial to Cu2, the delocalization from Cu2 is predominant. The
spin population computed at the imine (N1 and N4) and pyridine (N2 and N3) nitrogen atoms in
the BS-state have the same signs as that of the Cu centers to which they are bonded.
Obviously, in this dinuclear structure, the effect of spin delocalization (~39% of the spin is
delocalized to the ligand framework) results in the occurrence of a strong antiferromagnetic
interaction. Analysis of magnetic orbitals (Fig. 5) shows that each of the two HOMOS, 147α
(alpha-HOMO) and 147β (beta-HOMO),contains one unpaired electron where one HOMO is
located on one copper(II) while the second is located on the second copper(II) centre. A close
scrutiny reveals that magnetic orbital on each copper(II) interacts with p orbitals of the
phenoxide oxygen atom and one chloride chlorine atom in antibonding sense. Notably, one
chloride is involved in bonding with one copper(II) while the second chloride is involved with
II
the second copper(II) and therefore exchange interactions between the two Cu ions are
efficiently blocked through the chloride bridges. As a result, the magnetic interaction in 1 is
solely mediated through the phenoxide oxygen atom. The non-orthogonal magnetic orbitals of
the broken symmetry state are significantly coupled (Sαβ = 0.107) through the phenoxide bridge
resulting in an antiferromagnetic exchange.
3
.5. Electronic absorption spectra, Luminescent Properties and Detection of PA. The
electronic spectra of complex 1 was recorded in pure water in the range of 200‒800 nm. The
electronic spectrum of the complex 1, as shown in Fig. S1 (Supplementary material), exhibits
‒
1
‒1
one highly intense band at 253 nm (ε = 57400 M cm ) due to π→π* transition and one ligand-

1
8
‒
1
‒1
II
to-metal charge–transfer band at 368 nm (ε = 8580 M cm ). The d‒d transition for the Cu
‒
1
‒1
centre(s) for 1 appears at 625 nm (ε = 119 M cm ) which is shown in the inset of Fig. S1
(Supplementary material). Fluorescence spectra of the complex 1 has been investigated in pure
water at room temperature. As evident from Fig. S2 (Supplementary material), the fluorescence
spectrum (excited at 375 nm) of 1 in aqueous medium exhibits a fluorescence maximum at 525
‒
3
nm with a quantum yield of 3.1× 10 in reference to quinine sulphate as the secondary standard
(
φ = 0.546 in 0.5 (M) H SO ). This emission band for the complex 1 can be tentatively
2
4
attributed to the intra-ligand π→π* fluorescence.
The fluorescence of 1 in aqueous medium prompted us to investigate its potential application in
the selective sensing of explosive aromatic/non-aromatic nitro analytes (NACs). To check the
nitro explosive sensing ability of 1 in water, fluorescence intensities of 1 in water were
monitored upon addition of aqueous or methanol solutions (50 mM; 3µL each) of various
analytes such as picric acid (PA), p-nitrophenol (PNP), 2,4-dinitrophenol (DNP), m-
dinitrobenzene (DNB), p-nitrotoluene (NT), 4-nitrobenzaldehyde (NBZ), nitrobenzene (NB),
nitromethane (NM), p-nitrobenzoic acid (NBC), phenol (PH) and p-cresol (PC). To our delight,
as evident from Fig. 6, an efficient fluorescence quenching of ~77% of 1 occurs after adding just
one equivalent of PA, which indicates its effectiveness to detect PA over other analytes. In case
of nitrophenols, the order of quenching proficiency follows the order, PA > 2,4-DNP > NP,
which is in agreement with the order of the acidity of these nitrophenols. The fluorescence
intensity of compound 1 decreases like an on-off sensing response. To understand whether acidic
property of PA is responsible for fluorescence change, fluorescence spectrum of 1 was recorded
in presence of an protic acid, HCl. It can also be seen from Fig. 6 that addition of HCl leads to no
noteworthy effect on fluorescence quenching which clearly suggests that there is no significant

1
9
structural change of the fluorophore moieties present in the complex 1 due to the acidic nature of
PA and clearly acidic property of PA is not the reason of the observed fluorescence quenching.
This way, the capability of complex 1 towards sensing of picric acid in water is established.
We have performed fluorescence titration of complex 1 in water by incremental addition
of PA to revive more information about the interaction between complex 1 and PA. Fig. 7 shows
the changes in the fluorescence behaviour upon titrating complex 1 with PA in water. As can be
seen in Fig. 7, addition of PA is accompanied by band broading, which is indicative of charge
transfer characteristics. The fluorescence quenching data were analysed using Stern-Volmer (S-
V) equation: (I
0
/I) = KSV [A] + 1, where I
0
is the initial fluorescence intensity of 1 before the
addition of analyte (A), I is the fluorescence intensity of 1 in the presence of analyte, [A] is the
‒
1
molar concentration of analyte, and KSV is the quenching constant (unit: M ). It is evident from
Fig. 8 that at low concentration range of PA a linear increase in the S-V plot was observed,
whereas at higher concentration range the curve deviates from linearity and shows an upward
curvature. From this it is clear that quenching efficiency increases with increasing concentration
of PA and may be termed as super amplified quenching effect [5b,12c].
The nonlinear nature of the S-V plot may be attributed to many reasons such as existence
of both static and dynamic quenching processes or an energy transfer (fluorescence resonance
energy transfer) or self-absorption processes [8d,10a,11j]. At lower concentration of PA, a
linear Stern-Volmer plot was obtained (inset of Fig. 8) and it furnishes a quenching constant
5
‒1
(KSV) of 1.1×10 M .
Next, we constructed the Benesi-Hildebrand (B-H) plot to investigate the binding stoichiometry
and association constant. The association constant and binding stoichiometry were determined by
the fluorescence spectral change using B-H plot analysis, as reported in literature [27]. As

2
0
2
shown in Fig. 9, the B-H plot of 1/(F
0
-F) vs 1/[PA] from the fluorescence titration of 1 with PA
provides a straight line through out the entire range of PA concentration indicating 1:2 complex
1
0
‒2
formation with association constant (K
a
) 4.64 ×10 M . To confirm our assumption, we also
constructed the B-H plot for 1:1 complexation as shown in inset of Fig. 9. The nonlinear plot in
this case again supports that complexation occurs only through 1:2 stoichiometry. The high value
of association constant indicates that strong complex formation takes place between PA and 1.
The limit of detection (LOD) of PA is 4.8 µM (Fig. S3, Supplementary material). Time-resolved
fluorescence quenching experiments were carried out in order to explore the quenching
mechanism (static versus dynamic). In these experiments, the fluorescence decays of 1 were
monitored with and without the addition of PA. It is evident from Fig. 10 and Table S4
(Supplementary material) that the average life time (<τav>) of 1 (4.62 ns) does not display any
significant change after adding PA (4.48 ns and 4.47 ns on addition of 0.2 and 1.0 equivalent
respectively). Since the life time remains almost constant in presence of PA, this confirms that
quenching occured primarily via a static mechanism and ruled out the possibility of fluorescence
resonance energy transfer (FRET) in the quenching process [7c]. The static quenching can be
attributed due to the ground-state charge-transfer complex formation, which is confirmed by Uv-
vis and single-crystal X-ray diffraction experiments. We recorded the Uv-vis spectrum of 1 in
the presence of PA in water. The UV-visible titration of 1 with the incremental addition of PA is
shown in Fig. 11. Remarkable increase of absorbance with a shift in peak of 1, as shown in Fig.
1
1, suggests the formation of ground state complexation between electron deficient picric acid
and 1. All the absorption curves in this spectrophotometric titration pass through an isobestic
point at about 272 nm which indicates that ground state complex formation takes place more or
less smoothly. The electronic spectra of the compound 2 (50 µM) in methanol, as shown in fig.

2
1
1
1, is found to be identical with the spectra of 1:2 mixture of 1 and PA. Little difference in the
electronic spectra definitely may be due to the solvent effect. In order to understand the
interactions in the solid state, we isolated and determined the single crystal X-ray structure of the
association complex, [CuII
2
L(µ-Cl)(H
2
O) ]2Picrate·DMF (2), formed between PA and complex
2
1
. As discussed earlier, the association complex 2 contains a weak interaction directed aggregate
of a dicopper(II) moiety, [CuII ]2+, and two picrate anions; in the course of the
L(µ-Cl)(H
O)
2
2
2
formation of the aggregate 2, one of the two bridging chlorides is replaced by two monodentate
water ligands. Obviously, the formation of the aggregate 2 not only confirms that static
quenching but also depicts the reason of picric acid sensing behaviour of complex 1.
DFT based theoretical calculations were performed to support the static mechanism and
ground state charge transfer between 1 and PA or picrate. The HOMO-LUMO energy calculation
of the sensor 1 along with PA and picrate by gas-phase DFT calculations reveal (as shown in the
Fig. 12) that the LUMO (‒5.66 eV) of 1 resides above the HOMO (‒8.37 eV) of PA and the
HOMO (‒8.71 eV) of 1 resides below the LUMO (‒4.04 eV) of PA. Therefore, ground-state
charge transfer from sensor to PA as well as from PA to sensor are unfavourable processes. On
the other hand, the energy of the LUMO of 1 is less than the energy of the HOMO (‒3.17 eV) of
picrate and so the ground state electron charge transfer from the HOMO of picrate to the LUMO
of 1 can take place and it is the reason of the efficient fluorescence quenching.
3
.6. Comparison of 1 with the reported related systems. Several fluorescent materials have
been reported previously as sensors of picric acid. Most of these are fluorescent turn-off sensors
and only a few are fluorescent turn-on sensors [5g,5h,6d,6g]. It is worth mentioning that most of
the PA sensors exhibit the sensing property in non-aqueous media

2
2
[
5c‒5e,5g‒5i,6a‒6d,6f,7c,7f,8c‒8k,9a‒9c,10,12a‒12c]. Although some picric acid sensors in
aqueous medium are known, majority of them exhibit the properties in buffer solutions of
various pH values [8a,8b,8k,13]. There are only a few examples of picric acid sensors in pure
water but those are coordination polymers including MOFs (Table S5; Supplementary material)
7a‒7c,9d,9e,11]. Interestingly, the title compound 1 is the only example of a discrete metal
[
complex that senses PA in pure water. Notably, sensing of PA in aqueous medium is vital for
practical utility. Moreover, it is more desirable for practical application that the sensing takes
place in pure water as the electrolytes used in the buffer solutions may bring secondary pollution.
From that perspective, being the first discrete metal complex to sense PA in pure water, the title
compound is an important addition.
The previously reported metallo-organic PA sensors (aqueous or non-aqueous) contain various
IV
IV
III
III
III
III
II
II
II
II
II
metal ions such as Zr , Sn , Eu , Tb , Ir , Al , Pt , Pd , Hg , Cd , Zn [8c‒8j,9,10,11].
II
There are rare examples of Cu complexes as PA sensors but those examples were not
structurally characterized [8a,8b]. Hence, the title compound is the first example of a structurally
characterized copper(II) complex to show PA sensing behaviour.
The isolation and structural characterization of the sensor···analyte aggregate is crucial to
understand the origin of sensing behaviour. Notably, as already discussed, its has been possible
to characterize the sensor···analyte aggregate by determination of single crystal X-ray structure.
At this context, it is worth mentioning that the number of structurally characterized
sensor···analyte aggregates in the family of PA sensors is much less than the number of reported
sensors [5d,5e,6a,6b,6d,8c,8g,11i,11k].
The Stern-Volmer quenching contant (KSV), association constant (K
a
) and limit of
detection (LOD) of 1 and the most of the known metallo-organic PA sensors in pure water, are

2
3
listed in Table S5 (Supplementary material). Although there are sensors having better LOD
values (12.9 nM, 0.06 µM, 0.11 µM, 0.12 µM and 0.27 µM), this value of 1 is comparable to
those of the most of the reported sensors. However, KSV value of 1 is the best among the known
PA sensors in pure water. The K values of the metallo-organic PA sensors in pure water were
not reported. However, it may be mentioned that the K
a
1
0
‒2
a
value (4.64 ×10 M ) of 1 is better than
9
‒1
the K
a
values ( 1.67× 10 M ) the best organic PA sensors in pure water [5f]. All in all, the title
compound 1 is a good sensor for PA.
As per CSD search (CSD version 5.38) [39], there is only four copper(II) system in which the
there are µ-O-bis(µ-chloro) bridging moiety (O is any type of oxygen atom such as phenoxide,
hydroxide, alkoxide, oxide, N-oxide) [15f,18e,19]. Out of these, two are dinuclear and other two
are trinuclear. One dinuclear and two trinuclear compounds have µ-O(phenoxo)-bis(µ-chloro)
bridging moiety while the second dinuclear compound is µ-O(N-oxide)-bis(µ-chloro) bridging
moiety. Among these four, magnetic properties have been studied only for the µ-O(N-oxide)-
bis(µ-chloro) dicopper(II) compound [15f]. However, magnetic data were collected only at two
temperatures (305 K and 89 K), from which it was only concluded that the nature of magnetic
exchange interaction is not significant. On the other hand, variable-temperature (2.5-300 K)
magnetic susceptibility measuments, determination of experimental J value and DFT modelling
of magnetic behaviour have been done for compound 1 in this investigation. Clearly, the present
investigation may be taken as the first report of magnetic properties of a copper(II) compound
having µ-O-bis(µ-chloro) bridging moiety.
4
. Conclusions

2
4
In this study, [Cu
2
L(µ-Cl)
2
]Cl·4.5H O (1) has been established as a turn-off fluorescent sensor
2
for picric acid (PA). The sensing property has been explored by spectral titration, crystal
structure determination of the sensor···analyte aggregate and DFT calculations. Magnetic
property of 1 has been studied and the moderate antiferromagnetic interaction has been well
‒
1
‒1
modelled by DFT calculations (Jexp = ‒ 132.40 cm ; JDFT = ‒59.77 cm ). The major outcomes
of the present investigation may be summarized as follows: (i) The title compound is the first
example of a discrete metallo-organic system to sense picric acid in pure water; (ii) The title
compound is the first example of a structurally characterized copper(II) compound to sense
picric acid; (iii) The values of quenching constant and association constant of the title compound
is the highest among the known PA sensors in pure water; (iv) Experimental and theoretical
magnetic properties of 1 have been studied and this is the first time that J value (experimental or
theoretical) of a copper(II) system having µ-O-bis(µ-chloro) bridging moiety (O = any of
phenoxo, hydroxo, alkoxo, oxo) has been determined.
Appendix A. Supplementary data
CCDC 1915604 and 1892390 contains the supplementary crystallographic data for 1 and 2
respectively. Supplementary material related to this article can be found online.
Acknowledgements
Financial support for this work has been received from Government of India through
Department of Science and Technology (INSPIRE Fellowship to S. Mandal), Council for
Scientific and Industrial Research (Fellowship to A. Pramanik) and University Grants
Commission (contingency from CAS-V project to S. Mohanta). S. Majumder thanks

2
5
Department of Chemistry, Bhairab Ganguly College for providing chemical facilities.
Crystallography was performed at the DST-FIST-funded Single Crystal Diffractometer
Facility at the Department of Chemistry, University of Calcutta. Centre for Research in
Nanoscience and Nanotechnology (CRNN), University of Calcutta, is acknowledged for
magnetic studies.

2
6
References
[
1]
Z. Hu, B. J. Deibert, J. Li, Luminescent metal–organic frameworks for chemical sensing
and explosive detection, Chem. Soc. Rev. 43 (2014) 5815‒5840.
[
[
2]
3]
J. Yinon, Detection of explosives by electronic noses, Anal. Chem. 75 (2003) 99‒105.
Y. Salinas, R. Martinez-Manez, M. D. Marcos, F. Sancenon, A. M. Costero, M. Parra, S.
Gil, Optical chemosensors and reagents to detect explosives, Chem. Soc. Rev. 41 (2012)
1
261‒1296.
[
[
4]
5]
S. Mardanya, S. Karmakar, M. Bar, S. Baitalik, Pyrene-biimidazole based Ru(II) and
Os(II) complexes as highly efficient probes for the visible and nearinfrared detection of
cyanide in aqueous media, Dalton Trans. 44 (2015) 21053‒21072.
(a) S. Joshi, S. Kumari, E. Chamorro, D. D. Pant, R. Sakhuja, Fluorescence quenching of
a benzimidazolium-based probe for selective detection of picric acid in aqueous medium,
ChemistrySelect 1 (2016) 1756‒1762; (b) D. Das, A. Maity, M. Shyamal, S. Maity, N.
Mudi, A. Misra, Aggregation induced emission of 9-Anthraldehyde microstructures and
its selective sensing behavior towards picric acid, J. Mol. Liq. 261 (2018) 446‒455; (c) B.
Roy, A. K. Bar, B. Gole, P. S. Mukherjee, Fluorescent tris-imidazolium sensors for picric
acid explosive, J. Org. Chem. 78 (2013) 1306‒1310; (d) P. Vishnoi, S. Sen, G. N.
Patwari, R. Murugavel, Charge transfer aided selective sensing and capture of picric acid
by triphenylbenzenes. New. J. Chem. 39 (2015) 886‒892; (e) A. Ding, L. Yang, Y.
Zhang, G.
Zhang, L. Kong, X. Zhang, Y. Tian, X. Tao, J. Yang, Complex-formation-
enhanced fluorescence quenching effect for efficient detection of picric acid, Chem. Eur.
J. 20 (2014) 12215‒12222; (f) K. Duraimurugan, R. Balasaravanan, A. Siva, Electron
rich triphenylamine derivatives (D-π-D) for selective sensing of picric acid in aqueous

2
7
media, Sens. Actuators. B: Chem. 231 (2016) 302‒312; (g) Y. Erande, S. Chemate, A.
More, N. Sekar, PET governed fluorescence “Turn ON” BODIPY probe for selective
detection of picric acid, RSC Adv. 5 (2015) 89482‒89487; (h) S. Chemate, Y. Erande, D.
Mohbiya, N. Sekar, Acridine derivative as a “turn on” probe for selective detection of
picric acid via PET deterrence, RSC Adv. 6 (2016) 84319‒84325; (i) S. Paul, M. Karar,
P. Paul, A. Mallick, T. Majumdar, Dual mode nitro explosive detection under crowded
condition: Conceptual development of a sensing device, J. Photochem. Photobiol. A:
Chem. 379 (2019) 123‒129; (j) S. Kandel, V. Sathish, L. Mathivathanan, A. N. Morozov,
A. M. Mebel, R. G. Raptis, Aggregation induced emission enhancement (AIEE) of
tripodal pyrazole derivatives for sensing of nitroaromatics and vapour phase detection of
picric acid, New J. Chem. 43 (2019) 7251‒7258.
[
6]
(a)Y. Peng, A.-J. Zhang, M. Dong, Y.-W. Wang, A colorimetric and fluorescent
chemosensor for the detection of
an explosive—2,4,6-trinitrophenol (TNP), Chem.
Commun. 47 (2011) 4505‒4507; (b) M. Dong, Y.-W. Wang, A.-J. Zhang, Y. Peng,
Colorimetric and fluorescent chemosensors for the detection of 2,4,6-trinitrophenol and
investigation of their co-crystal structures, Chem. Asian J. 8 (2013) 1321‒1330; (c) Z.-H.
Fu, Y.-W. Wang, Y. Peng, Two fluorescein-based chemosensors for the fast detection of
2
,4,6-trinitrophenol (TNP) in water, Chem. Commun. 53 (2017) 10524‒10527; (d) J.
Pitchaimani, A. Kundu, S. Karthikeyan, S. P. Anthony, D. Moon, V. Madhu, A crab claw
shaped molecular receptor for selective recognition of picric acid: supramolecular self-
assembly mediated aggregation induced emission and color change, CrystEngComm 19
(2017) 3557‒3561; (e) S. Mukherjee, A. V. Desai, A. I. Inamdar, B. Manna, S. K. Ghosh,
Selective detection of 2,4,6-trinitrophenol (TNP) by a π-stacked organic crystalline solid

2
8
in water, Cryst. Growth Des. 15 (2015) 3493‒3497; (f) P. Ghosh, S. Paul, P. Banerjee,
How explosive TNP interacts with a small tritopic receptor: a combined crystallographic
and thermodynamic approach, CrystEngComm 19 (2017) 6703‒6710; (g) G. Sivaraman,
B. Vidya, D. Chellappa, Rhodamine based selective turn-on sensing of picric acid, RSC
Adv. 4 (2014) 30828‒30831.
[
7]
(a) T. Wang, N. Zhang, R. Bai, Y. Bao, Aggregation-enhanced FRET-active conjugated
polymer nanoparticles for picric acid sensing in aqueous solution, J. Mater. Chem. C 6
(2018) 266‒270; (b) S. Hussain, A. H. Malik, M. A. Afroz, P. K. Lyer, Ultrasensitive
detection of nitroexplosive–picric acid via a conjugated polyelectrolyte in aqueous media
and solid support, Chem. Commun. 51 (2015) 7207‒7210; (c) A. S. Tanwar, S. Hussain,
A. H. Malik, M. A. Afroz, P. K. Lyer, Inner filter effect based selective detection of
nitroexplosive-picric acid in aqueous solution and solid support using conjugated
polymer, ACS Sens. 1 (2016) 1070‒1077; (d) B. Xu, X. Wu, H. Li, H. Tong, L. Wang,
Selective detection of TNT and picric acid by conjugated polymer film sensors with
donor‒acceptor architecture, Macromolecules 44 (2011) 5089‒5092; (e) B. Gole, S.
Shanmugaraju, A. K. Bar, P. S. Mukherjee, Supramolecular polymer for explosives
sensing: role of H-bonding in enhancement of sensitivity in the solid state, Chem.
Commun. 47 (2011) 10046‒10048; (f) S. Krishnan, C. V. Suneesh, Fluorene‒Triazine
conjugated porous organic polymer framework for superamplified sensing of
nitroaromatic explosives, J. Photochem. Photobiol. A: Chem. 371 (2019) 414‒422; (g) H.
Du, G. He, T. Liu, L. Ding, Y. Fang, Preparation of pyrene-functionalized fluorescent
film with a benzene ring in spacer and sensitive detection to picric acid in aqueous phase,
J. Photochem. Photobiol. A: Chem. 217 (2011) 356‒362.

2
9
[
8]
(a) K. L. Reddy, A. M. Kumar, A. Dhir, V. Krishnan, Selective and sensitive fluorescent
detection of picric acid by new pyrene and anthracene based copper complexes, J.
Fluoresc. 26 (2016) 2041‒2046; (b) R. Pandey, L. Reddy, S. Ishihara, A. Dhir, V.
Krishnan, Conformation induced discrimination between picric acid and nitro
derivatives/anions with Cu-pyrene array: the first decision making photonic device, RSC
Adv. 3 (2013) 21365‒21368; (c) V. Béreau, C. Duhayon, J.-P. Sutter, Supramolecular
control over recognition and efficient detection of picric acid, Chem. Commun. 50 (2014)
2061‒12064; (d) A. Kumar, A. Kumar, D. S. Pandey, N,N-diethylamine appended
1
binuclear Zn(II) complexes: highly selective and sensitive fluorescent chemosensors for
picric acid, Dalton Trans. 45 (2016) 8475‒8484; (e) M. Kumar, S. I. Reja, V. Bhalla, A
2
+
charge transfer amplified fluorescent Hg complex for detection of picric acid and
construction of logic functions, Org. Lett. 14 (2012) 6084‒6087; (f) P. Alam, G. Kaur, V.
Kachwal, A. Gupta, A. R. Choudhury, I. R. Laskar, Highly sensitive explosive sensing by
‘‘aggregation induced phosphorescence’’ active cyclometalated iridium(III) complexes,
J. Mater. Chem. C 3 (2015) 5450‒5456; (g) R. Soman, S. Sujatha, C. Arunkumar,
Protonation and axial ligation intervened fluorescent turn-off sensing of picric acid in
freebase and tin(IV) porphyrins, RSC Adv. 5 (2015) 93243‒93247; (h) N. Goel, N.
Kumar, A stable nonanuclear Tb(III) cluster for selective sensing of picric acid, Inorg.
Chim. Acta 463 (2017) 14‒19; (i) D. Samanta, P. S. Mukherjee, PtII
with an organometallic backbone as sensors for picric acid, Dalton Trans. 42 (2013)
6784‒16795; (j) S. Kumar, R. Kishan, P. Kumar, S. Pachisia, R. Gupta, Size-Selective
Detection of picric acid by fluorescent palladium macrocycles, Inorg. Chem. 57 (2018)
693‒1697; (k) J. Sahoo, S. B. Waghmode, P. S. Subramanian, M. Albrecht, Specific
nanoscopic cages
6
1
1