How paramagnetic and diamagnetic LMOCs detect picric acid from surface water and the intracellular environment: A combined experimental and DFT-D3 study

Article abstract of DOI:10.1039/c6cp01620k

Diamagnetic and Paramagnetic Luminescent Metal Organic Complexes (LMOCs) have been reported for Explosive and Pollutant Nitro Aromatic (epNAC) recognition. The diamagnetic complex shows a highly intense AIE induced by NEt₃H⁺, which disappears after picric acid recognition and subsequently RET will quench the emission intensity. Radical stabilized paramagnetic LMOCs seem to be active but show lower sensing efficiency in comparison with diamagnetic LMOCs. Solution and solid state spectroscopy studies along with DFT-D3 have been executed to enlighten the host guest interaction. Limit of PA detection is ～250 ppb with a binding constant of 1.2 × 10⁵ M^-1. Time-stepping, i.e. intervening in the problem of picric acid recognition from surface water collected from several places of West Bengal, India, has been performed. Mutagenic picric acid has been successfully detected in an aqueous medium inside both prokaryotic and eukaryotic cells at a ppm level using fluorescence microscopy.

Full text of DOI:10.1039/c6cp01620k

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How paramagnetic and diamagnetic LMOCs
detect picric acid from surface water and the
intracellular environment: a combined
experimental and DFT-D3 study†‡
Cite this:DOI: 10.1039/c6cp01620k
Pritam Ghosha and Priyabrata Banerjee*ab
Diamagnetic and Paramagnetic Luminescent Metal Organic Complexes (LMOCs) have been reported for
Explosive and Pollutant Nitro Aromatic (epNAC) recognition. The diamagnetic complex shows a highly
intense AIE induced by NEt3H+, which disappears after picric acid recognition and subsequently RET will
quench the emission intensity. Radical stabilized paramagnetic LMOCs seem to be active but show lower
sensing efficiency in comparison with diamagnetic LMOCs. Solution and solid state spectroscopy studies
along with DFT-D3 have been executed to enlighten the host guest interaction. Limit of PA detection is
Received 9th March 2016,
Accepted 19th April 2016
B250 ppb with a binding constant of 1.2 ꢀ 105
M
ꢁ1. Time-stepping, i.e. intervening in the problem of
picric acid recognition from surface water collected from several places of West Bengal, India, has been
performed. Mutagenic picric acid has been successfully detected in an aqueous medium inside both
prokaryotic and eukaryotic cells at a ppm level using fluorescence microscopy.
DOI: 10.1039/c6cp01620k
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coordination polymers whereas MOCs are mostly supramolecular
polymers. In comparison to MOFs, MOCs are easier to prepare, less
Introduction
In recent days, Luminescent Metal Organic Complexes (LMOCs) expensive and posses a comparatively greener way of synthesis.
are attracting much attention due to their vibrant and potential With these added advantages and in the house handling experi-
application in selective catalysis, sensing several sensational ence of LMOFs as an explosive sensor, these researchers are
properties which are also similarly produced by other multi- compelled to focus on the development of LMOCs for rapid
functional materials.1 Incorporation of suitable metal ions along detection of such explosives concerning their wide application in
with the selective luminescent organic skeleton could be a perfect forensic and criminal investigations. Homeland security is one of
choice in preparing LMOCs as a chemosensor. In very recent our major concerns as terrorist groups most commonly use IEDs
times, Prof. Li et al., Prof. Mukherjee et al. and Prof. Ghosh et al. (improvised explosive devices) in mostly crowded places as it is
have reported the use of Luminescent Metal Organic Frameworks their calling card towards exposure of their ideology. Surprisingly,
(LMOFs) as an explosive (such as picric acid or nitro benzoic to date most of the devices and attention for explosive detection
acid etc.) chemosensor.2
have been focused only on 2,4,6-trinitro toluene (TNT). There are
Supramolecular LMOCs can be sub-categorised under LMOFs several other such nitro explosives specially 2,4,6-trinitro phenol
and the difference between MOFs and MOCs is that MOFs are or picric acid (PA) whose explosion power is even more lethal
than that of 2,4,6-trinitro toluene (TNT).2e In between, deadly
a Surface Engineering & Tribology Group, CSIR-Central Mechanical Engineering
Research Institute, Mahatma Gandhi Avenue, Durgapur 713209, West Bengal,
India. E-mail: pr_banerjee@cmeri.res.in, priyabrata_banerjee@yahoo.co.in;
Web: www.cmeri.res.inwww.priyabratabanerjee.in
impacts are occurring one after another in our homeland where
terrorists have used picric acid as one of the explosive materials.3
Apart from being used as an explosive, picric acid is well
known due to its high toxicity when it is metabolized to picramic
acid in the mammalian digestive cycle. Picric acid is under the
category of nitro phenols which are known to be hematotoxic,
hepatotoxic and promote mutagenesis and carcinogenesis inside
human cells and other living organisms. It is an important key
maker of several fungicides, analytical reagents, staining agents,
fireworks, germicides, matches, glasses and leather industries.4a
It can be produced as a by-product of nitrobenzene synthesis.
b Academy of Scientific & Innovative Research (AcSIR) at CSIR-CMERI,
Mahatma Gandhi Avenue, Durgapur 713209, West Bengal, India
† Dedicated to the martyrs of Pathankot, India who lost their lives in a terrorist
attack in January, 2016.
‡ Electronic supplementary information (ESI) available: ESI-MS, FT-IR data of both
the ligand dtHBAP and Co metallated complex PP; X-ray crystallographic data of PP;
UV-Vis spectra; CV; ESI-MS of the PPꢂ ꢂ ꢂTNP adduct; coordinates used in the DFT
and DFT-D3 study; bond distances related to quinoidal distortion and the Stern
Volmer plot; limit of detection calculation. CCDC 1430015 (PP). For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c6cp01620k Therefore there is a huge possibility of incautious release of
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epNACs like picric acid from industry to the environment, which relevant phenoxyl radical metalloligand and its sensing and
may eventually cause serious detrimental impact to soil or the recognition property for picric acid like guest analyte will always
aquatic ecosystem equilibrium. Another possibility of picric acid be of interest. Herein authors briefly discuss the preparation
contamination to ground water is unexploded land mines. These of dia- and paramagnetic LMOCs (PP and PPox) confirmed by
are found to contain unused nitro derivatives.4 With time there is several characteristic studies like UV-Vis, EPR, and room tem-
a large possibility that these nitro explosives might contaminate perature magnetic measurements and finally density functional
the ground water and cause several pernicious effects to aquatic theory (DFT) as supportive evidence in favour of paramagnetic
life. Therefore, detection of picric acid is vital, concerning its LMOC generation. Detection of epNACs with both diamagnetic
major impact on environmental and health-related issues, and PP and paramagnetic PPox has been carried out. Thorough
significantly from the view of homeland security.
characterisation (both in solid and solution state) in support
To date, the detection of epNACs is mainly dependent on of their detection has been performed. Interestingly, a very
canine’s efficacy or other several sophisticated instrumental strong supportive tool [e.g.; modern DFT (DFT-D3)] has also
techniques. These techniques for epNAC sensing are not only been carried out to investigate the sensing of picric acid by the
expensive but also difficult to carry due to poor portability. In this host receptor.7c DFT-D3 concerns about the polarisation factor
regard, development of fluorescent MOCs for epNAC detection is of hydrogen make the outcome more accurate and effective.
nowadays drawing more attention for its unique features like Resonance energy transfer (RET) has been found to be the best
safety, portability, high sensitivity, quick response time, cost mechanism in explaining epNAC sensing.4 Moreover, detection
effectiveness in preparation and detection. Prof. Mukherjee has been performed not only from organic solvents but also from
et al., Prof. Ajayaghosh et al. and Prof. Ghosh et al. have recently several surface water specimens too (collected from various parts
reported several interesting imidazolium, carbazole, self assembled of West Bengal, India) where commonly interfering agents like
cage, pentafluoro unit base organogel molecules or Schiff base type Na+, K+, Ca2+, Mg2+, Clꢁ, HCO3ꢁ, SO42ꢁ and gases O2, N2 and CO2
compounds for detection of picric acid or tri nitrotoluene.5
may be present and interfere with the fluorescence of the host.
In this connection, it will be very interesting to work with Still, the presence of those interfering agents has no way affected
both diamagnetic and paramagnetic LMOCs as there is no such the quenching event even in a subtle amount. It is well known
report of working with LMOCs as a supramolecular host for that picric acid is highly toxic to aquatic life and obviously to
detection of epNACs. Diamagnetic MOCs normally interact with mankind. Therefore in vitro detection is highly necessary in line
possible epNACs through strong hydrogen bonding. Precise with saving aquatic life and for human health-related concerns.
modification in the backbone of MOCs makes them electro- In line with this thought, in vitro detection of picric acid has been
chemically sound for easy oxidation in the ligand centre, which performed from both eukaryotic and prokaryotic cells through
will eventually produce a radical stabilized paramagnetic complex. fluorescence microscopy.7a–d
The main reason for working with paramagnetic MOCs is the
behavior of the loosely bound unpaired electron towards a guest
like epNACs.
Results and discussion
Synthesis and characterization
As a continuation of our ongoing research activity in coordi-
nation6 and supramolecular chemistry,7 in this report emphasis
The ligand dtHBAP and its corresponding dia and paramagnetic
has been given towards preparation of an easy to make LMOCs in
cobalt coordination complexes (PP and PPox) are characterized by
its both diamagnetic (PP) as well as paramagnetic (PPox) state.
Paramagnetic PPox is a ligand centred phenoxyl radical complex,
produced from the diamagnetic complex PP by both a chemical
FT-IR, ESI-MS, UV-Vis, CV, EPR and single crystal X-ray study.
In connection with the experimental evidences, density functional
theory (DFT) has been carried out as a theoretical support for PPox
and electrochemical pathway. The phenoxyl radical stabilised
formation. Modern DFT or DFT-D3 has been carried out in order
ligand has been synthesized due to precise modification of the
to investigate the details of the PA sensing pathway with both
ligand by tertiary butyl group substitution. Tertiary butyl group
PP and its oxidized product PPox. Significantly, the outcome of
functionalisation in the ligand dtHBAP has added advantages
DFT-D3 is in line with fluorescence spectroscopy outcomes.
over the others:
(i) It makes the LMOCs electron rich and eventually highly
emissive by improved charge transfer
Crystallographic description of diamagnetic LMOCs
(ii) Phenoxyl radical stabilized paramagnetic LMOC generation The synthesized Co complex PP (Scheme 1) crystallizes in a
Both diamagnetic and paramagnetic LMOCs can selectively monoclinic space group P21/n (no. 14) (see Table S1, ESI‡) and
detect picric acid by a spectrofluorimetric technique. Though shows distorted octahedral geometry. Two tridentate O,N,O
dtHBAP has been previously explored,8 its diamagnetic and donor Schiff base ligands are ligated with a central metal atom
paramagnetic cobalt complex possibility has not been reported of cobalt and the overall charge of the complex is balanced by a
for sensing epNACs like picric acid. The reason behind choosing counter cation NEt3H+. Geometrically the complex is meridional
cobalt is that cobalt metallated complexes with a redox non- and the relative orientations are cis[O1–O3], trans[N1–N2] and
innocent multidentate Schiff base ligand mimics cobalamin cis[O2–O5] respectively (Table S2, ESI‡). Two ligands are situated
(vitamin B12), a metal complex in our body with a CQN linkage.9 in a 901 fashion towards each other. In between two coordinated
In this perspective generation of a redox active and biologically ligands, one is participating in a weak interaction with NEt3H+
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in their consecutive chelate ring size (Table S3, ESI‡). O2–Co1–N1
or O5–Co1–N2 is forming a six membered chelate ring, on the
contrary for O1–Co1–N2 and O3–Co1–N1, the chelate ring is only
five membered.
Electronic spectra and emission property of diamagnetic
LMOCs
Electronic spectra of dtHBAP and PP have been recorded in
a dichloromethane solvent [1 ꢀ 10ꢁ4 (M) conc.] in the UV/Vis
region (250–800 nm). The ligand (dtHBAP) exhibits one intense
absorption peak at 290 nm and another transition around 385 nm,
which can be assigned as phenolate p - p* and n - p* type
transitions respectively.6m,11 A broad response around 480 nm can
be assigned to intramolecular charge transfer. In the metal
complex (PP) a new broad peak is again appearing at 520 nm
apart from peaks at 275 and 390 nm (sh), which can be assigned as
LMCT transitions (phenolate–CoIII charge transfer).6m,11
Fluorescence experimentation for PP has been carried out in
a 1 ꢀ 10ꢁ5 (M) concentration in acetonitrile (vide supra). It shows
high emission of the receptor PP. The possible reasons of this
high emission are as follows:
Scheme 1 Schematic representation of complex PP preparation.
Fig. 1 Zig-zag view of PP (green arrow shows the direction).
(a) Inhibition of the CQN bond rotation
(b) Extensive intramolecular charge transfer (ICT) inside the
complex PP
while the other doesn’t (Fig. 1). An oxygen atom O(1) from the
amino phenol unit of the participating ligand is weakly interact-
ing with the N(3) atom of the counter cation NEt3H+. Eventually,
a weak interaction of the mononuclear complex makes a 1D
zigzag supramolecular structure. The extensive hydrogen bonding
makes a p–p interaction between tertiary butyl substituted units of
one ligand with the amino phenol unit of the other ligand (bond
distances are 4.2 Å). It shows a unique T shaped p–p interaction
which results in a better fluorescence property of the complex PP
(Fig. 2).10
In the crystal structure of PP no quinoidal distortions
(reflecting two shorter CQC double bonds and four longer C–C
bonds in a six member aromatic ring) have been observed. The
chelate bite angle average of O2–Co1–N1 or O5–Co1–N2 (95.21)
is larger than the average chelate bite angle of O1–Co1–N2 and
O3–Co1–N1 (86.21), which is quite evident due to the difference
(c) NEt3H+ driven closely packed supramolecular entity for T
shaped p–p interaction results in better charge transfer: in
short it is an example of NEt3H+ induced aggregation for better
emission intensity.
Electrochemical study of diamagnetic LMOCs
Cyclic voltammetry study for PP [conc. 5 mM, scan rate 25 mV sꢁ1
,
temperature: 298 K, working electrode: platinum, supporting elec-
trolyte: TBAPF6 (0.10 mol dmꢁ3)] has been executed in dichlor-
omethane. The current vs. voltage curve shows one reversible
and another quasi reversible response in the potential range
ꢃ1.2 V vs. saturated calomel electrode (SCE). In the oxidation
region the reversible response at B0.45 V can be attributed to
ligand based oxidation of one of the schiff base ligands (L)2ꢁ to
its corresponding phenoxyl radical species (Lꢄ)1ꢁ (furthermore
supported by EPR, spectroelectrochemistry and DFT; ESI‡) as
there is hardly any scope for further oxidisation of the CoIII centre.
The oxidative response is mainly reversible but the reductive one
is not. In particular, when the tertiary butyl (–tBu) group having
a high +I effect is present in a molecule, it makes the molecule
electron rich and thus it becomes prone to oxidation and
obviously difficult to reduce (Fig. S5a, ESI‡). The reversibility of
the oxidation peak is verified under the domain of a mathema-
tical perspective. In cyclic voltammetry, the Randles–Sevcik (R–S)
equation is used to understand the effect of the scan rate on the
peak current ip (Fig. S5b, ESI‡).
Diamagnetic to paramagnetic LMOC preparation
Oxidised complexes are active due to the unpaired electron and
several enzymatic pathways are known to proceed through a
radical pathway. To the best of our knowledge, to date there is
no direct evidence of working with a radical stabilised complex
Fig. 2 Segmented polyhedral view of the supramolecular network of PP.
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as a host for detection of picric acid like epNACs. It would be
Expectedly, quinoidal distortions for PPox have been observed.
of enormous interest if a radical complex can be stabilised The ligand on which the electron density is delocalized has
and investigated for epNAC sensing. A cyclic voltammetry study shown quinoidal distortion (i.e.; two shorter CQC double bonds
reveals that the Co-complex PP has a single reversible oxidative and four longer C–C bonds in the six membered aromatic ring
response at 0.45 V. Under an argon blanketing glove box the of paramagnetic PPox in comparison to diamagnetic PP). Other
oxidation process of PP with AgNO3 in a suitable potential ligands have no such effect. This finding supports the existence
range was executed in dry dichloromethane.12 The X-band EPR of the one electron oxidized phenoxyl radical based species PPox
study of the product obtained from the reaction has been (see Table S7, ESI‡).13
executed in a dichloromethane–toluene (1 : 1) solution at liquid
The proposed chemical formula of the complex PP after
nitrogen (77 K) temperature. A single line curve suggests the oxidation (PPox) is as follows:
presence of an organic radical in the chemically generated
[CoIII(L2ꢁ)(L2ꢁ)]1ꢁ (S = 0) - [CoIII(Lꢄ1ꢁ)(L2ꢁ)]0 (S = 1/2)
complex (Fig. S6, ESI‡). The room temperature (298 K) mag-
netic study supports the one electron paramagnetic nature of
the complex (m B 1.78 BM).
(PP, colour: brown) (PPox, colour: reddish brown)
A coulometric study in controlled potential for PP was executed
in dichloromethane at required potentials for 1eꢁ oxidation under
Picric acid recognition: analytical application of LMOCs
an argon blanketing atmosphere in order to produce the oxidized It is always important to explore the functional property of an
species by an electrochemical way. The species has also shown a easy to synthesize and economically viable ligand and its suitable
similar nature as chemically generated species in EPR and the metal complexes in the dia- as well as paramagnetic state. The
room temperature magnetic study. At this stage it is confirmed same group has reported similar type of organic compounds as
that PP is oxidized to an organic radical based complex (PPox) both corrosion inhibitor.14 Herein, both the ligand and its Co metal
chemically and electrochemically, but it is not yet confirmed complex are explored well in an emergent area of chemistry:
whether the radical is azomethine or phenoxyl based. At this picric acid recognition.
juncture spectroelectrochemistry helps regarding its radical fea-
Picric acid is a very well known pollutant and it converts to
ture. It has been noticed that a chemically and electrochemically mutagenic picramic acid during the mammalian metabolic
oxidised complex shows a similar nature in the UV-Visible spec- process. It is also purposefully used as an explosive in several
trum. In UV-Vis the changes at 390 and 680 nm can be treated as a military applications. Several deadly impacts have taken place
marker of generation of phenoxyl radical species during oxidation many times in recent years by terrorists where the use of picric
of diamagnetic PP at a constant potential of 0.8 V (see ESI‡ for acid was observed. Therefore selective detection of picric acid
details). The 390 nm peak gradually diminishes and a low energy as an organic explosive and pollutant has become highly essential.
band at 680 nm has concomitantly been generated.6m Therefore,
Keeping all these factors in mind, we have investigated the
from a chemical and electrochemical perspective one can conclude recognition of several epNACs such as 2,6-DNT (2,6-dinitro
that the 1eꢁ oxidised product is a paramagnetic CoIII–phenoxyl toluene), NB (nitro benzene), 1,3-DNB (1,3-dinitro benzene), 3,4-
radical species (PPox).13
DNT (3,4-dinitro toluene), 3,5-dinitro benzoic acid (DNBA) and PA
Density functional theory (DFT) calculations could be help- in a 5 ꢀ 10ꢁ4 (M) concentration in an acetonitrile : water (2 : 1, v/v)
ful as supportive evidence to further confirm the radical nature. solution for titrating PP and dtHBAP. In case of titration with
Spin unrestricted Kohn–Sham (UKS) calculation with a B3LYP PPox the epNAC solution has been prepared in acetonitrile at
hybrid functional was executed for geometry optimization of the same concentration.
the diamagnetic PP complex. Spin distributions along with the
The 1 ꢀ 10ꢁ5 M acetonitrile solution of the host (ligand
geometry optimization of paramagnetic PPox was also carried dtHBAP and complex PP and PPox) was used for the fluorescence
out with the same UKS and B3LYP functional.6a,d,e,m
titration study. dtHBAP shows emission at 560 nm at room
The diamagnetic complex PP shows no unpaired electron in temperature. Emission of dtHBAP actually remained unaltered
the Loewdin spin density plot (Fig. 3a). In case of PPox, the spin after addition of picric acid. On the other hand, PP shows an
density plot shows the presence of one unpaired electron over intense emission at 430 nm, and upon gradual addition of picric
one dtHBAP ligand (Fig. 3b).
acid (up to 2 equivalents) solution, the emission intensity was
rapidly quenched (Fig. 4a). Other epNACs (upto 2.5 equivalents)
like 2,6-DNT, NB, 1,3-DNB, 3,4-DNT and 3,5-DNBA didn’t show
any detectable influence on the emission of PP (Fig. 4a inset).
In the case of a radical stabilized complex the emission peak is
more or less same in its position, but the nature is a bit different.
Actually fluorescence is related with the electronic distribution of
the complex, so if the distribution is altered then obviously it will
influence the emission property of the complex. That’s why the
difference in the emission peak nature has been observed in
the radical complex PPox. It is generally found that the unpaired
electrons are loosely bounded. There must be chances of better
Fig. 3 Spin density population of (a) PP and (b) PPox
.
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Fig. 4 Fluorescence response of (a) PP [inset: response of PP with other epNACs] (b) PPox with PA.
picric acid sensitivity by that unpaired electron. But, when respectively (Fig. 5b). In every case the PA stretching has been
picric acid [as of PP, i.e.; 2 equivalents] was added gradually moved to a higher wavenumber suggesting lowering of the
it shows a comparatively weaker response in comparison with bond length and enhancement of the bond order, which is
PP. Perhaps electron deficiency of the paramagnetic complex basically the reflection of the withdrawing effect of the electron
PPox is responsible for this anomaly (Fig. 4b). In addition, PP from PP to picric acid.15 This stretching event fully supports the
has been tested with nitrophenol and di nitrophenol (ESI,‡ formation of a supramolecular entity between PP and picric
Fig. S7).
acid in a solid state too.
ESI-MS study
Jobs plot
An appreciably strong hydrogen bonding interaction occurring
between PP and picric acid is obviously reflected from the
desired supramolecular aggregation between p electron rich
PP and electron deficient picric acid. In order to investigate
the stoichiometry of the adduct formed in between PP and
picric acid, ESI-MS has been executed in a methanolic medium.
In ESI-MS it was observed that at the most abundant peak
around B1165 amu reflects a 1 : 2 adduct (PPꢂ ꢂ ꢂpicric acid)
formation in between the formed adduct (ESI,‡ Fig. S8).
In order to measure the stoichiometric ratio of diamagnetic PP
a series of solutions of picric acid and PP (4 ꢀ 10ꢁ5 M) have been
prepared in acetonitrile solution. The fluorescence at 430 nm has
been plotted against the mole fraction of picric acid. The curve
indicates a 1 : 2 binding stoichiometry phenomenon between
host PP and picric acid (Fig. 5a).
FT-IR study
In the solution state study it has been found that PP can
recognise PA. Thus to investigate the recognition capability of
PP in a solid state condition, a sophisticated tool like FT-IR has
been purposefully used. It has been noticed that after inter-
action with PP the stretching frequency of PA has been shifted
to a higher energy. The stretching at 1250 cmꢁ1 responsible for
C–N stretching has now been shifted to 1270 cmꢁ1. C–O bond
stretching at 1145 cmꢁ1 has swung to 1165 cmꢁ1. Symmetric
stretching of –NO2 group at 1340 cmꢁ1 and asymmetric stretching
at 1525 cmꢁ1 have been moved to 1365 cmꢁ1 and 1555 cmꢁ1
DFT-D3 study
The pathway through which the receptor PP is sensing PA needs
to be explored for more vivid studies and for better insight into
this current endeavour. In this connection, density functional
theoretical calculation with dispersion factor can enlighten us
to understand the interaction occurring between the host PP
and guest analytes. The dispersion as well as polarization factor
of the hydrogen atom have to be considered in this sort of
interaction because PA or 3,5-DNBA contains dissociable protons.
Modern DFT or DFT-D3 has been used herein, for correlation of
the experimental and theoretical findings for PP with the guest
analytes and thus to firmly establish a general methodology for
using it which will be easier for predicting the receptor level of
sensitivity and selectivity towards epNAC recognition before
doing experiments.
In the host–guest interaction strategy, to find out whether
the recognition pathway is based on a supramolecular domain
or something else, DFT-D3 has been successfully executed. In the
fluorescence titration study, the quenching is only observed with
PA whereas other epNACs do not have any detectable influence
on PP emission. In view of the above, to understand the host
Fig. 5 (a) Jobs plot of host PP with guest PA, (b) FT-IR study of PP and PA
complexation (blue: PP; cyan: PA; red: PP and PA mixture).
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guest interaction of PP with analytes, a detailed computational phenolic oxygen unit/sidearm in the amino phenol part
analysis using ORCA (version 3.0.3, developed by Prof. Dr Frank of the ligand [O1 (guest)–H1ꢂ ꢂ ꢂO2 (host): 1.13 Å and 1.29 Å/
Neese, Department of Molecular Theory and Spectroscopy, MPI O10 (guest)–H10ꢂ ꢂ ꢂO20 (host): 1.12 Å and 1.30 Å] (Fig. 6b). PA is
fu¨r chemische Energiekonversion for ORCA)16 and version 6.5 for interacting with the host through its phenolic proton and there-
Turbomole has been carried out. The Becke–Johnson damping after facilitating the electron transfer from the host PP to the guest
parameter has also been included during the calculation.7c The PA leading to fluorescence quenching. In the case of another
coordinates and subsequent geometry of PP have been obtained acidic guest molecule, i.e.; 3,5-DNBA there is no such indication of
from single crystal X-ray structural analysis data. After geometry an interaction. Picric acid, which is one of the important mem-
optimization of PP, the converged geometry was taken for further bers of the epNAC family, is highly electron deficient and will
study of the weak interaction between PP and guest analytes. obviously interact in better way with an electron rich receptor like
During calculation, picric acid has been considered in a 1 : 2 ratio PP. In this connection PPox is electron deficient in comparison
with PP. As a consequence, other analyte recognition events have with PP and thus after experimentation it was found that fluores-
been considered in a similar fashion (see Tables S8–S15, ESI‡). cence quenching of PPox with PA is less efficient than PP as
In the case of PPox a similar procedure for calculation is followed. expected. In the DFT-D3 calculation, the distance between phe-
dtHBAP is segmented from CIF of PP for execution of the DFT-D3 nolic hydrogen of the PA unit and the host phenolic oxygen unit
calculation.
of the amino phenol part [O1 (guest)–H1ꢂꢂꢂO2 (host): 1.03 Å and
An energy minimized geometry optimized structure leads to 1.56 Å/O10 (guest)–H10ꢂꢂꢂO20 (host): 1.04 Å and 1.52 Å] supports a
the conclusion that PA is no way interacting with the ligand comparatively weak bonding of PPoxꢂꢂꢂpicric acid (Fig. 6c). From
dtHBAP (Fig. 6a), which is also quite evident from the fluores- the spin density calculation it was found that in one electron
cence titration experiment. It is found that PA is positioned oxidised system (i.e.; PPox) the unpaired electron is predominantly
at around a 3 Å distance from the sensor PP. It is due to steric residing particularly on one of the ligand sides only. Therefore it is
hindrance possessed by a bulky tertiary butyl group and coulombic expected that in between two host oxygen centres (amino phenol
repulsion (d+ꢂ ꢂ ꢂd+) of phenolic protons between the receptor unit) of PPox, the one which is oxidised will interact less in
dtHBAP and guest analyte picric acid. In the case of PP, the comparison with the other centre where electron density remains
phenolic hydrogen of PA is interacting strongly with the host unaltered. Significantly, in the geometry optimized convergent
calculation it was found that PA spatially resided at about 1.56 Å
from the oxidised ligand and to the level of our satisfaction it is
residing at a decreased distance i.e., 1.52 Å from the other ligand
which is not that much affected after 1eꢁ oxidation. Herein PP has
a higher electron density than PPox and thus it forms a stronger
bond with guest electron deficient picric acid (newly formed bond
distance: 1.30 Å).
It is noticeable that DFT-D3 in the present study supports
the experimental findings very nicely and it can be corroborated
that the PA recognition process solely belongs to the realm of the
supramolecular chemistry world. In particular, the main driving
force of sensing is the strong interaction between the guest and
host through phenolic hydrogen sites of the arm of the receptor.
To maintain the resemblance, a DFT-D3 study with other analytes
has also been carried out. In line with experimental outcomes,
herein also, other than picric acid, no other analytes participate
in such a recognition event with the host PP. 3,5-Dinitro benzoic
acid doesn’t even show interaction through its acidic proton with
the host PP (see Fig. S9–S13, ESI‡).
Insight into fluorescence quenching phenomena
The fluorescence quenching process of receptor PP with picric
acid can be explained through resonance energy transfer (RET)
and also in presence of NEt3H+ driven p–p interaction related
phenomena. RET is a well acclaimed methodology for explaining
fluorescence quenching. This phenomenon is mainly concerned
about energy transfer occurring from an electronically excited
donor chromophore to the acceptor (Fig. 7a). Selective sensing
of picric acid in the presence of the complex PP by a fluores-
cence quenching phenomenon lead us to explore the chemistry
behind it.
Fig. 6 PA complexation with (a) dtHBAP, (b) PP and (c) PPox in the DFT-
D3 study.
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the emission. On the other hand, in the case of PPox, it is a neutral
complex and thus has no chance of becoming a supramolecular
host by a NEt3H+ driven pathway as with PP. Thus no chance of a
p–p interaction is there which results in a lower fluorescence
intensity of the complex PPox. RET is the only reason for emission
quenching for paramagnetic PPox with picric acid.
Selectivity of picric acid sensing over other epNACs
It has been already discussed that paramagnetic PPox is less
sensitive and diamagnetic PP shows better selectivity (vide supra).
To establish PP as a universal sensor, i.e. PP can detect picric acid
in the presence of other epNACs, an interference study has been
performed by the following steps:
Fig. 7 Overlap zone of epNAC absorption spectra with fluorescence
spectra of PP (blue: absorption spectra of epNACs and red: emission
spectrum of PP).
(I) The emission of PP in MeCN has shown appreciable
emission intensity (B980 a.u.).
RET can be said to have an effect in fluorescence quenching.
The S–V plot for PP (Fig. 7b) suggests a resonance energy transfer
between PA and PP in this present case, [where (I0/I) = Ksv(A) + 1;
I0 = initial fluorescence intensity of PP and I = fluorescence intensity
of PP in the presence of epNACs, {A} = molar concentration of
epNACs and Ksv = quenching constant (Mꢁ1)].4 The non linear
characteristic in the S–V plot of PP depicts that the quenching
phenomenon basically relies on an energy transfer pathway. The
(II) A concentrated solution of epNACs like 2,6-DNT (10ꢁ2 M)
has been added in the PP solution and the corresponding
emission has been monitored. 2,6-DNT has been added to check
the affinity towards PP, which shows negligible impact on PP
emission. A more or less similar trend was observed with other
epNACs like; NB, 1,3-DNB, 3,4-DNT, and 3,5-DNBA, where
epNACs (other than PA) have shown almost negligible influence
on PP emission. So, it can be concluded that, other than PA, all
epNACs have more or less no effect on PP emission.
(III) Addition of a PA aqueous solution to epNACs (2,6-DNT,
NB, 1,3-DNB, 3,4-DNT and 3,5-DNBA) containing PP, resulted in
appreciable quenching of emission (high degree of fluores-
cence quenching B96%).
calculated Ksv value is 1.2 ꢀ 105
M
ꢁ1. The PA detection limit
calculated for PP is 250 ppb (see ESI‡).
In the resonance energy transfer process, the resonance
energy can be easily transferred from an excited fluorophore
to electron deficient analytes if an overlap persists between the
absorption band of analytes with the emission band of the
fluorophore (PP). It is noteworthy to mention that the level of
energy transfer depends on the extent of spectral overlap between
the analyte absorption band and the emission band of PP.
It could be noticed (Fig. 8) that among the investigated
epNACs, picric acid shows the maximum spectral overlap with
PP while for other epNACs the extent of overlap is significantly
less. Therefore RET is certainly the responsible route for explaining
the fast and rapid fluorescence quenching of PP with picric acid.
In case of PP, it is a supramolecular host and shows high
emission due to a NEt3H+ driven closely packed T shaped p–p
interaction in the complex; AIE (aggregation induced emission).
When picric acid comes in closer vicinity it forms a strong
hydrogen bond with the host PP. Therefore the p–p interaction
is reduced and the supramolecular building block collapsed as,
due to steric hindrance, it is not possible to form further supra-
molecular networks in the presence of picric acid. Eventually, the
emission quenched, i.e. NEt3H+ induced aggregation for emission
(AIE) disappears and RET comes into the scenario for quenching
These results establish the selective sensitivity of picric acid
by PP in the presence of several other epNACs (Fig. 8).
Picric acid detection from an environmental surface water
specimen collected throughout various parts of West Bengal,
India
Picric acid is an explosive and toxic organic pollutant, which can
be converted to mutagenic picramic acid during the mammalian
metabolic process. Selective detection of picric acid as an organic
explosive as well as pollutant is highly essential not only from an
organic medium but also from natural surface water resources
where several interfering agents are present. We have collected
several surface water specimens throughout West Bengal, India
(see ESI‡ for complete details). It was found that PP showed a
better response than PPox. Therefore a picric acid response study
in the surface water has been performed with PP.
Specimen I: rural area, central-western area of West Bengal,
India (Bardhaman development authority supplied water).
Specimen II: north western part of West Bengal, India
(Dubrajpur hand pump water).
Picric acid solution (1 mM) has been prepared with the
collected natural water specimens (Specimen I–VI) and their
effect was monitored on PP emission. An appreciable quench-
ing in fluorescence emission was observed for each and every
case study (Fig. 9).
It is worth mentioning that the blank water samples (without
picric acid) don’t have any detectable effect on PP emission.
Therefore, it can be concluded that, irrespective of water
Fig. 8 Selectivity of PP over picric acid compared to other epNACs.
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solution and after a forty five minute interval those cells were
again incubated in a picric acid solution for thirty minutes. The
control cells are obtained by staining with PP only (see Experi-
mental section for details about the preparation of cells and
procedure of cell imaging). Noticeably, the blue-green emission
of the PP stained cells is quenched with incubation in picric
acid contaminated water.
Fig. 9 Recognition of picric acid from surface water specimens by
fluorescence quenching of PP.
Conclusions
Diamagnetic as well as paramagnetic Luminescent Metal Organic
Complexes (LMOCs) have been synthesized and thoroughly
characterized for Explosive and Pollutant Nitro Aromatic (epNAC)
recognition. A tertiary butyl substituted diamagnetic complex has
been oxidized to a paramagnetic complex by both chemical and
electrochemical pathways. The diamagnetic complex has NEt3H+
induced AIE and shows a highly intense emission. AIE disappears
after picric acid recognition due to the positional change of
NEt3H+ by picric acid. Due to a steric effect of picric acid there
is no chance of a p–p interaction between two adjacent benzene
rings of the host complex. In addition, resonance energy transfer
is also coming into action, which together results in the quench-
ing of emission. Paramagnetic LMOCs seem to be more active
than diamagnetic LMOCs, showing lower emission intensity
and sensing efficiency in comparison with diamagnetic LMOCs.
Insightful solution as well as solid state analytical tools along
with DFT-D3 have been carried out to enlighten the host guest
interaction. The binding constant is 1.2 ꢀ 105 Mꢁ1 and the PA
detection limit is 250 ppb. As a daily life application, detection
of picric acid from surface water collected within West Bengal,
India has been successful. Being toxic and mutagenic, picric
acid is always harmful to the living organism; diamagnetic
LMOCs can detect picric acid inside both eukaryotic (pollen
grains of Techoma stans) and prokaryotic cells (Candida albicans,
a diploid fungus) using fluorescence microscopy in an aqueous
medium. Several research projects with variable LMOCs as
explosive sensors are under active progress in our laboratory.
resources, the PP emission quenching can happen in the presence
of a PA solution. The outcome firmly establishes the ability of PP
as a universal fluorescent sensor of PA like an organic nitroaro-
matic explosive and mutagenic water pollutant.
In vitro detection of picric acid
As picric acid is a highly toxic water pollutant and may affect all
the soft tissues and organs of the human body, in vitro picric acid
detection is very urgent for quick and easy identification of this
contaminant in natural bodies water. To date there are no such
reports found where in vitro picric acid detection is reported. We
have chosen this highly emissive complex PP in order to detect
picric acid inside cells. In West Bengal, Durgapur is an industrial
belt and the surface water in most cases is polluted with several
toxic substances or other elements as well. To establish PP as a
universal sensor of picric acid, we have performed biological
detection of picric acid inside cells grown in the surface water
collected from Durgapur (see ESI‡ for details).
For the in vitro detection study two different types of living
cells viz. Candida albicans (IMTECH no. 3018) (Fig. 10a and b)
and pollen grains of Techoma stans (Fig. 10c and d) have been
used. Candida albicans is prokaryotic and pollen grains are
eukaryotic cells. During the fluorescence titration study, it was
noticed that the emission of PP is quenched with gradual addition
of picric acid. Herein, first of all, the cells were incubated in a PP
Experimental section
Materials
All chemicals used for synthesis are of analytical grade. Precursor
metal salt CoCl2ꢂ6H2O and other chemicals e.g., 3,5-ditertbutyl
salicylaldehyde, 2-amino phenol, and triethyl amine were pur-
chased from Merck India Pvt. Ltd, S.D. Fine Chemicals and
used without any further purification. AgNO3 was purchased
from Merck India. Picric acid was obtained from Lobachemie,
India. Nitro benzene was purchased from Merck India. 2,6-
Dinitro toluene, 1,3-dinitro benzene, and 3,4-dinitro toluene
were procured from Sigma Aldrich. 3,5-Dinitro benzoic acid was
obtained from Nice chemicals, Kerala, India. Solvents such as
dichloromethane (DCM), tetrahydrofuran (THF) and diethyl
ether were purchased from Merck India Pvt. Ltd and dried over
activated molecular sieves and fused calcium chloride then
finally distilled before use. Other solvents e.g., methanol, hexane
Fig. 10 In vitro picric acid detection by PP in both prokaryotic (a and b)
and eukaryotic cells (c and d).
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(HXN), and ethanol were purchased from Merck India Pvt. Ltd 6 h of stirring, excess solvent was removed under vacuum to get
and used as received. complex PP. A microcrystalline brown colored product thus
(Caution: since PA is highly explosive and toxic in nature, obtained was re-dissolved in dichloromethane and layered with
care should be taken during use!!)
hexane to get single crystals of PP suitable for X-ray analysis.
Anal. calc. for C48H66CoN3O4: C, 71.35; H, 8.23; N, 5.20%.
Found: C, 71.29; H, 8.21; N, 5.21%. ESI-MS (m/z, ꢁve mode):
Instrumentation
A Perkin Elmer 2400C elemental analyzer was used to collect the calc. 706.4, found: 705.9 (Fig. S3, ESI‡). FT-IR (n, cmꢁ1, KBr disk):
micro analytical (C, H, N) data. UV-Visible spectroscopy was done 2945(s), 1605(s), 1515(s), 1470(s), 1420(s), 1305(s), 1160(s), 1020(s)
on an ALS-SEC2000 spectrophotometer. A FT-IR spectrometer (Fig. S4, ESI‡).
(spectrum 65, Perkin Elmer) instrument was used for FT-IR
Synthesis of paramagnetic LMOCs
studies (with KBr pellets). A CMS (compact mass spectrometer,
Serial no.: 3013-0140, Advion) was used for ESI-MS. The magnetic
susceptibility experiment was carried out at room temperature
(298 K) on a Sherwood Magnetic Susceptibility Balance. SP-150
Biologic, France made the Potentiostat–galvanostat used for
electrochemical measurements. Cyclic voltammetry (CV) was
performed in dichloromethane solution under an Argon medium
containing tetrabutylammonium hexafluorophosphate (TBAPF6)
(0.10 mol dmꢁ3) as the supporting electrolyte. The CV experi-
mentation was done in a three electrode configured system with a
varied scan rate of B25–170 mV sꢁ1. A platinum electrode (6 mm
diameter) was used as the working electrode. A platinum wire was
used as the counter electrode, and saturated calomel electrode
(SCE) was used as the reference electrode. SEC-C05 thin layer
Quartz glass spectroelectrochemical cell kit (light pass length
of 1 mm) with a SEC-C05 platinum gauze working electrode
(80 mesh) and SEC-C05 platinum counter electrode were used for
spectroelectrochemical measurements. A Ferrocenium/ferrocene
(Fc+/Fc) couple (0.38 eV) was the reference internal standard to all
potentials. Varian, USA made E-112, ESR spectrometer was used
for electron paramagnetic resonance (EPR) spectroscopy. Density
functional theory (DFT) and DFT-D3 calculations were carried out
in a Z-200 workstation with the help of the ORCA program
package (version 3.0.3) and Turbomole software. The argon
blanketing M-Braun glove box was used to perform the oxidation
reaction of the Co complex PP to PPox.
AgNO3 (17 mg, 0.1 mmol) as an oxidizing agent was added to
the stirring solution of PP (B71 mg, 0.1 mmol) in dry dichloro-
methane (5 mL) under an argon blanketing glove box. The
brown color of PP changed to reddish brown after 5 h of the
reaction. The reaction mixture was filtered through celite and
excess solvent was removed under pressure. PPox thus obtained
was taken for further study. The room temperature magnetic
susceptibility measurement shows that PPox is paramagnetic
(m B 1.78 BM). Anal. calc. for C42H50CoN2O4 calc.: C, 71.47;
H, 7.14; N, 3.97%. Found: C, 71.25; H, 7.07; N, 3.89%. FT-IR
(n, cmꢁ1, KBr disk): 2980(s), 1615(s), 1550(s), 1475(s), 1445(s),
1350(s), 1250(s), 1145(s).
DFT study
DFT calculations have been carried out for [CoIII(dtHBAP)2]
[NHEt3] (PP) in its different plausible oxidation states [0 or ꢁ1]
with the ORCA programme package (version 3.0.3).16,17 Geometry
optimizations were performed with the B3LYP functional. All
electron Gaussian basis sets were developed by the Ahlrichs
group.18 In the calculation, TZV(P) triple-z quality basis sets were
used with one set of polarization functions on a central metal and
also on other atoms which are directly coordinated to the central
metal. In case of atoms like carbon and hydrogens, slightly
smaller polarized split-valence SV(P) basis sets were used which
were of double-z quality in the valence region and have a
polarizing set of d functions on the nonhydrogen atoms. SCF
calculations were converged tightly (1 ꢀ 10ꢁ8 Eh in energy,
1 ꢀ 10ꢁ7 Eh in the density change, and 1 ꢀ 10ꢁ7 in maximum
element of the DIIS error vector).19 The geometry searches
for PP were carried out in their redundant internal coordi-
nates without imposing any constraints in their geometry.
Corresponding20 orbital and density plots were performed
using Molekel.21
Synthesis of dtHBAP
Ligand dtHBAP was prepared by simple condensation of 3,5-
ditertbutyl salicylaldehyde and 2-amino phenol in methanolic
medium at a 1 : 1 ratio at room temperature. The yellow colored
product thus obtained was dried under vacuum. Anal. calc.
C
21H27NO2: C, 77.50; H, 8.36; N, 4.30%. Found: C, 77.30; H,
8.16; N, 4.38%. ESI-MS (m/z, ꢁve mode): calc. 325.2, found:
324.2 (Fig. S1, ESI‡). FT-IR (n, cmꢁ1, KBr disk): 3540(s), 2950(s),
1615(s), 1580(s), 1490(s), 1422(s), 1250(s), 1170(s), 970(s) (Fig. S2,
ESI‡).
Acknowledgements
Synthesis of diamagnetic LMOCs
Department of Science and Technology, Govt. of India sponsored
Ligand (dtHBAP) (650 mg, 2 mmol) was taken in 30 mL of Fast Track Project (vide ref. no.: SB/FT/CS-003/2012 and project
methanol. Triethyl amine as a base (560 mL, 4 mmol) was added. no.: GAP-183112) for a young researcher is acknowledged. PG is
A methanolic solution of the metal [CoIICl2ꢂ6H2O (238 mg, grateful to DST (GAP-183112) for his fellowship. Authors are
1 mmol)] was added at a time to the ligand solution. An thankful to Dr Subhra Kanti Mukhopadhyay, Burdwan University
immediate color change from reddish orange to brown was for his kind support and valuable discussions. PG is thankful
noticed (Scheme 1). Aerial oxidation lead to the +3 oxidation to Mr Saibal Jana, IIT-K for his support in the DFT-D3
state of the cobalt center from +2. At room temperature, after calculations.
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