A charge transfer amplified fluorescent Hg2+ complex for detection of picric acid and construction of logic functions

Article abstract of DOI:10.1021/ol3029753

A chemosensor 3 based on the N,N-dimethylaminocinnamaldehyde has been synthesized which shows fluorescence turn-on response with Hg²⁺ ions, and the in situ prepared 3-Hg²⁺ complex has been used for detection of picric acid via electrostatic interaction and construction of a combinatorial logic circuit with NOR and INHIBIT logic functions.

Full text of DOI:10.1021/ol3029753

ORGANIC
LETTERS
2012
Vol. 14, No. 23
6084–6087
A Charge Transfer Amplified Fluorescent
Hg2þ Complex for Detection of Picric Acid
and Construction of Logic Functions
Manoj Kumar,* Shahi Imam Reja, and Vandana Bhalla
Department of Chemistry, UGC Sponsored Centre for Advanced Studies-1,
Guru Nanak Dev University, Amritsar-143005, Punjab, India
mksharmaa@yahoo.co.in
Received October 29, 2012
ABSTRACT
A chemosensor 3 based on the N,N-dimethylaminocinnamaldehyde has been synthesized which shows fluorescence turn-on response with Hg2þ
ions, and the in situ prepared 3ꢀHg2þ complex has been used for detection of picric acid via electrostatic interaction and construction of a
combinatorial logic circuit with NOR and INHIBIT logic functions.
Recently, development of fluorescent chemosensors for
the selective detection of harmful ions or molecules has
received considerable attention.1 Among the various soft
metal ions, mercury is considered to be highly toxic2aꢀc
owing to its greater affinity toward the SH groups of
biomolecules, and ionic mercury can be converted into
methyl mercury by bacteria in the environment, which enters
the food chain and accumulates in higher organisms.2d
Further, mercury rapidly crosses the bloodꢀbrain barrier,
is stored preferentially in the pituitary gland, thyroid gland,
hypothalamus, and causes disorders such as autism, schizo-
phrenia, dyslexia, learning disabilities, tubular necrosis, etc.3
On the other hand, picric acid with phenolic and nitro
functionalities is also hazardous to the living system.4 The
electron-deficient nature of picric acid generates a highly
xenobiotic character that makes the biodegradation of picric
acid more difficult and is responsible for chronic diseases
such as sycosis and cancer.5 The use of picric acid in organic
synthesis, drug analysis, and manufacture of rocket fuel,5
fireworks, and matches releases a large quantity of it in the
environment. Picramic acid, which is a monoamino trans-
formation product of picric acid, is 10 times more mutagenic
than picric acid. Thus, keeping in mind the adverse effects of
picric acid and mercury, the development of techniques for
mercury and picric acid hazard assessment and pollution
management are in great demand.
Our research work6 involves the design, synthesis, and
evaluation of artificialreceptorsfor selectivesensing of soft
metal ions, anions, nitroaromatics, and construction of
different logic functions. In continuation of this work, we
have now synthesized a dimethylaminocinnamaldehyde-
based fluorescence chemosensor 3 that undergoes selective
fluorescence enhancement in the presence of Hg2þ ions, and
the in situ prepared 3ꢀHg2þ complex acts as a fluorescent
electron transfer (ET) donor to the nonemissive picric acid
(PA) acceptor via electrostatic interaction for the selective
detection of the PA. In addition, the 3ꢀHg2þ complex
(1) (a) Nolan, E. M.; Lippard, S. J. Chem. Rev. 2008, 108, 3443. (b)
Kim, H. N.; Ren, W. X.; Kim, J. S.; Yoon, J. Chem. Soc. Rev. 2012, 41,
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Lett. 2007, 9, 2501. (b) Kim, J. S.; Quang, D. T. Chem. Rev. 2007, 107,
3780. (c) Kim, H. N.; Lee, M. H.; Kim, H. J.; Kim, J. S.; Yoon, J. Chem.
Soc. Rev. 2008, 37, 1465. (d) Harris, H. H.; Pickering, I. J.; George, G. N.
Science 2003, 301, 1203.
(3) Tchounwou, P. B.; Ayensu, W. K.; Ninashvili, N.; Sutton, D.
Environ. Toxicol. 2003, 18, 149.
(4) Yost, S. L.; Pennington, J. C.; Brannon, J. M.; Hayes, C. A. Mar.
Pollut. Bull. 2007, 54, 1262.
(5) Shen, J.; Zhang, J.; Zuo, Y.; Wang, L.; Sun, X.; Li, J.; Han, W.;
He, R. J. Hazard. Mater. 2009, 163, 1199.
(6) (a) Kumar, M.; Kumar, N.; Bhalla, V. Org. Biomol. Chem. 2012, 10,
1769. (b) Kumar, M.; Kumar, N.; Bhalla, V.; Sharma, P. R.; Qurishi, Y.
Chem. Commun. 2012, 48, 4719.
r
10.1021/ol3029753
Published on Web 11/28/2012
2012 American Chemical Society

behaves as a combinatorial logic circuit with two outputs
having NOR and INHIBIT logic functions with PA/Hþ as
chemical inputs. To the best of our knowledge, this is the first
report where fluorogenic sensor for Hg2þ ions has been used
for detection of picric acid and construction of logic circuit.
Condensation of 2-aminothiophenol (1) with N,N-di-
methylaminocinnamaldehyde (2) in ethanol gave the de-
sired compound 3 in 86% yield (Scheme 1 and Supporting
Information, SI). The structure of compound 3 was con-
firmed from its spectroscopic and analytical data (SI). The
binding behavior of receptor 3 was studied toward differ-
ent cations by UVꢀvis and fluorescence spectroscopy. All
the titration experiments were carried out in THF/H2O
(9:1, v/v) by aliquots of different metal ions. The absorption
Figure 1. UVꢀvis spectra of (A) receptor 3 (5.0 μM) with Hg2þ
ions (0ꢀ130 μM) (C) 3ꢀHg2þ complex (5.0 μM) in the presence
of PA (0ꢀ200 μM) in THF/H2O (9:1, v/v) buffered with
HEPES, pH = 7.0; (B) and (D) are the SEM images of “A”
and “C”, respectively.
Scheme 1. Synthesis of Compound 3
band at 458 nm along with the appearance of a blue
fluorescenceemission(inset ofFigure 2A). The appearance
of an emission band at 458 nm is attributed to the inter-
action of Hg2þ ions with sulfur and imino nitrogen
atoms toform the 3ꢀHg2þ complex asa result of which the
possibility of PET from imino nitrogen atom to dimethy-
laminovinylbenzene moiety is suppressed responsible for
the intramolecular charge transfer (ICT) from the nitrogen
atom of the dimethylamino group6a to the imino moiety.
The binding mode of receptor 3 with Hg2þ ions is proven
by 1H NMR and IR spectroscopy. The imino protons of
receptor 3 undergoa downfield shift ofΔδ =1.37ppm(SI)
on addition of Hg2þ ions, proving interaction of receptor 3
with Hg2þ ions through imino nitrogen atoms. On the
spectrum of receptor 3 (5 μM) is characterized by typical
absorption band at 405 nm corresponding to πꢀπ* transi-
tions (Figure 1A). Among the various metal ions tested
(Pb2þ, Ba2þ, Hg2þ, Cd2þ, Agþ, Zn2þ, Cu2þ, Ni2þ, Co2þ
,
Fe3þ, Fe2þ, Kþ, Mg2þ, Naþ, and Liþ), only the addition of
Hg2þ ions changes the absorption spectrum of receptor 3.
The addition of Hg2þ ions (130 μM) to the solution of 3
results in the enhancement of absorption band at 405 nm
attributed to the interaction of Hg2þ ions with imino
nitrogen and sulfur atoms (Figure 1A). Further, the appear-
ance of level-off long wavelength tails in the absorbance
spectra of 3 with Hg2þ ions indicates the formation of
nanoaggregates attributed to the Mie scattering which is
confirmed by scanning electron microscopy (SEM). The
SEM image as shown in Figure 1B clearly indicates
the nanoaggregate formation upon addition of Hg2þ to
the solution of 3. However, the addition of picric acid to the
3ꢀHg2þ complex led to the appearance of a new absorbance
band at 365 nm (Figure 1C) with simultaneous disappear-
ance of level-off long wavelength tails. The disappearance of
level-off long wavelength tails suggests that the nanoaggre-
gate form of 3ꢀHg2þ complex no longer exists on addition
of picric acid which is well supported by the SEM image
(Figure 1D).
other hand, in IR spectra the absoption band at νmax
=
1602 cmꢀ1 corresponding to the imino moiety of 3 shifted
to 1591 cmꢀ1 (SI) on addition of Hg2þ ions to the solution
of 3, which clearly indicates that the Hg2þ interact with
imino nitrogen atoms. Further, the emission spectrum of
receptor 3 did not change by the addition of other metal
ions (SI). In order to check the practical ability of receptor
3 as Hg2þ selective fluorescent chemosensor, we carried
out competitive experiments in the presence of Hg2þ mixed
with different metal ions. The fluorescence response of
3ꢀHg2þ complex system remained same by comparison
with or without the other metal ions (SI). It was found that
the receptor 3 has a detection limit of 80 ꢁ 10ꢀ9 M (SI) for
Hg2þ ions which is sufficiently low for the detection of
nanomolar concentration range of Hg2þ ions found in
many chemical system. Fitting the changes in the fluores-
cence spectra of receptor 3 with Hg2þ ions using the non-
linear regression analysis program SPECFIT7 gave a good
fit and demonstrated that 1:2 stoichiometry (host/guest)
The fluorescence spectrum of receptor 3 in THF/H2O
(9:1, v/v) does not exhibit any fluorescence emission when
excited at 380 nm (Figure 2A). The quenched fluorescence
emission of receptor 3 is ascribed to the photoinduced
electron transfer (PET) from imino nitrogen atom to the
photoexcited dimethylaminovinylbenzene moiety. How-
ever, the addition of Hg2þ ions (0ꢀ130 μM) to the solution
of receptor 3 results in the formation of a new emission
(7) Gampp, H.; Maeder, M.; Meyer, C. J.; Zhuberbulher, A. D.
Talanta 1985, 32, 95.
Org. Lett., Vol. 14, No. 23, 2012
6085

Scheme 2. Schematic Representation of the Electron Transfer
from 3ꢀHg2þ Complex to PA via Electrostatic Interaction
Figure 2. Fluorescence emission spectra of (A) receptor 3 (1.0 μM)
with Hg2þ ions (0ꢀ130 μM). (B) 3ꢀHg2þ complex (1.0 μM) in the
presence of PA (0ꢀ200 μM) in THF/H2O (9:1, v/v) buffered with
HEPES, pH = 7.0; λex = 380 nm.
was the most stable species in the solution. The binding
constant (log β) of 3ꢀHg2þ complex was found to be 9.16 (
0.03. The method of continuous variation (Job’s plot) was
also used to prove the 1:2 stoichiometry (SI).8a The forma-
tion of the complex is confirmed by the mass spectroscopy,
which shows a peak at m/z 406 [M þ 2Hg2þ þ 2ClO4ꢀ þ
Kþ þ H2O] (SI) corresponding to the 3ꢀHg2þ, complex
which also proves the 1:2 stoichiometry of the host and guest
species. The fluorescence quantum yield (QY)8b of the
3ꢀHg2þ system is found to be 0.29 as compared to that of
free receptor 3 (0.001) at 458 nm, which shows good
agreement with fluorescence spectra obtained for receptor
3 in the presence of Hg2þ ions. We also carried out a
reversibility experiment, which proved that binding of
Hg2þ ions to the receptor 3 is reversible. In the presence of
KI, the iodide ions because of its strong affinity for Hg2þ
ions, forms a complex with it, which results in the decom-
plexation of the receptor Hg2þ complex. On further addi-
tion of Hg2þ ions, the fluorescence intensity was revived
again indicating the reversible behavior of the receptor 3 for
the Hg2þ ions (SI).
Since the in situ prepared 3ꢀHg2þ complex is highly
fluorescent in solution, we studied the fluorescence behav-
ior of the 3ꢀHg2þ complex toward different nitro deriva-
tives such as 2,4,6-trinitrotoluene (TNT), picric acid (PA),
2,4-dinitrotoluene (DNT), 1,4-dinitrobenzene (DNB), nitro-
methane (NM), nitrotoluene (NT), nitrobenzene (NB), and
2,3-dimethyl-2,3-dinitrobutane (DMDNB). Among the dif-
ferent nitro derivatives tested, the fluorescence intensity of
3ꢀHg2þ complex decreases with increase in the concentra-
tion of PA (Figure 2B). The addition of 45 μM of PA results
in the 50% fluorescence quenching of 3ꢀHg2þ complex. As
PA has a lower π* energy of the LUMO (ꢀ3.89 eV) than the
energy of the LUMO for 3ꢀHg2þ complex (ꢀ3.2 eV) (SI) the
fluorescence of 3ꢀHg2þ complex can be effectively quenched
via the electron transfer from the 3ꢀHg2þ complex to picric
acid. The addition of PA to the solution of 3ꢀHg2þ complex
initially results in the protonation of the dimethylamino
groups which (i) trammels the intramolecular charge transfer
from the dimethylamino group to the imino units and (ii)
leads to strong electrostatic interaction between the proto-
nated form of 3ꢀHg2þ complex and picrate anion. Both of
these processes are responsible for highest fluorescence
quenching observed with picric acid among the different
nitro derivatives tested9 (Scheme 2). To confirm the proto-
nation of the dimethylamino moiety, we studied the fluores-
cence behavior of probe 3ꢀHg2þ at lower pH value and 1H
NMR titation with picric acid (PA). At pH 6.0, an emission
band at 404 nm corresponding to protonated form of
3ꢀHg2þ complex (QY = 0.14), along with weak ICT
emission band at 458 nm (SI) was observed, which clearly
indicates the hampering of ICT6b process from dimethyla-
mino moiety to imino binding site as dimethylamino moiety
is no longer available for ICT owing to its protonation. Thus,
the protonation of dimethylamino moiety is required to
hamper the ICT emission. However, the appearance of
emission band at 404 nm reveals in the absence of quencher.
But in the presence of PA the quenched fluorescence ob-
served via protonation (SI) followed by the electrostatic
interaction through electron transfer from the electron-rich
donor (3ꢀHg2þ) to electron-deficient acceptor (PA). In the
1H NMR spectra of the 3ꢀHg2þ complex, the N,N-dimethy-
lamino protons undergo downfield shift of Δδ = 0.17 ppm
(SI) on addition of picric acid, which indicates the protona-
tion of the dimethylamino moiety. However, no change in
chemical shift was observed for the imino protons, which
indicates that (i) imino nitrogen atom is not protonated in the
presence of picric acid and (ii) picrate anion is not coordinat-
ing with the metal center, which in the case of coordination
would have weakened the interaction between Hg2þ and
imino nitrogen atom leading to upfield shift of the imino
protons. From these 1H NMR observation we conclude that
picrate anion interact with protonated form of N,N-dimethy-
lamino moiety of 3ꢀHg2þ complex via electrostatic interac-
tion (Scheme 2). The detection limit of 3ꢀHg2þ complex for
the detection of PA was found to be 170 ꢁ 10ꢀ9 M (SI). The
quenching response of picric acid was studied by using the
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Org. Lett., Vol. 14, No. 23, 2012

Table 1. Truth Table
input-1
input-2
Hþ
output-1
458 nm
output-2
404 nm
entry
PA
1
2
3
4
0
0
1
1
0
1
0
1
1
0
0
0
0
1
0
0
Figure 3. (a) Fluorescence emission spectra of 3ꢀHg2þ complex
at different input conditions: (1) no inputs; (2) Hþ; (3) PA; (4)
PA þ Hþ. Fluorescence intensities higher than the threshold
value specified at 458 nm (50), 404 nm (130) are assigned as “1”
and intensities lower than the value are assigned as “0”. (b) The
combinatorial logic circuit diagram.
458 nm with the formation of a new emission band at 404 nm
(Figure 3a). Thus, from the above fluorescent behavior of
3ꢀHg2þ complex and by using sequential addition of chemi-
cal inputs, we costruct a truth table (Table 1) for a combina-
torial logic circuit having NOR and INHIBIT logic functions
(Figure 3b). In the presence of any one of the chemical input,
3ꢀHg2þ shows no characteristic band at 458 nm, i.e., output 1
becomes “0”. By the operation of Hþ to 3ꢀHg2þ complex,
output 1 becomes “0”, and a new blue-shifted emission band
appears at 404 nm which is considered as output 2; hence, the
fluorescence becomes ON and the output 2 is “1”. In the
presence of both chemical inputs no emission bands were
observed; i.e., output 1 and 2 become “0”. In the absence of
both chemical inputs the characteristic band at 458 nm is
observed; i.e., output 1 becomes “1”. The truth table value for
output 1 forms a NOR logic gate, and the truth table values
for output 2 form a INHIBIT logic gate. The combination of
these intrinsic properties with selectivity of actions by different
chemical inputs allows its implementation to design a complex
molecular switch as shown in Figure 3.
In conclusion, we designed and synthesized a new ICT
base receptor 3 that selectively binds Hg2þ ions and shows
turn-on fluorescence response. Further, we used in situ
prepared 3ꢀHg2þ complex as a fluorescent electron trans-
fer (ET) donor to the nonemissivepicricacid(PA) acceptor
via electrostatic interaction for the selective detection of
the PA. In addition, the 3ꢀHg2þ complex behaves as a
combinatorial logic circuit with two outputs having NOR
and INHIBIT logic functions.
SternꢀVolmer equation (SI). At lower concentration of PA,
a linear SternꢀVolmer relationship with SternꢀVolmer
constant10 (KSV) = 9 ꢁ 104 Mꢀ1 was observed which is
found to be higher than other fluorescent chemosensors for
PA and suggests a static quenching through the excited state
interaction. This type of fluorescence behavior is not ob-
served by the addition of other nitro derivatives (SI). Thus,
3ꢀHg2þ complex is an efficient system for the selective
detection of picric acid via an electrostatic interaction
induced fluorescence quenching mechanism.
Further, with the above fluorescence outputs at 458 and
404 nm, we used our system for the construction combi-
natoriallogic gatewith NOR andINHIBITlogicfunctions
in the presence of PA and Hþ (pH 6.0) as chemical inputs.
The design and development of supramolecular systems
which function as a molecular logic gate, is an area of
intense research activity.11aꢀc However, most of the molec-
ular logic gates comprise of only one output mode (i.e.,
single fluorescence) and thus the recent interest is focused
on an integrated system involving multiple fluorescent
output modes.11d Thus, we utilized the 3ꢀHg2þ system
as a combinatorial logic circuit with two inputs and two
outputs.
The addition of PA to the solution of 3ꢀHg2þ complex
results in quenching of the fluorescence intensity at 458 nm
(Figure3a). Onthe otherhand, the additionof Hþ (pH6.0)
to the 3ꢀHg2þ complex quenched the emission band at
Acknowledgment. We are thankful to CSIR, UGC, and
DST (New Delhi) for financial support. S.I.R. is thankful
to UGC for a junior research fellowship.
(10) Venkatramaiah, N.; Kumar, S.; Patil, S. Chem. Commun. 2012,
48, 5007.
Supporting Information Available. Experimental pro-
cedure; characterization data including mp, 1H, 13C
NMR, IR, and mass spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
(11) (a) de Silva, A. P.; McClenaghan, N. D.; McCoy, C. P. Molec-
ular Level Electronics, Imaging and Iinformation, Energy and Environ-
ment, in Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH:
Weinheim, 2001. (b) Raymo, F. M. Adv. Mater. 2002, 14, 401. (c) Balzini,
V.; Credi, A.; Venturi, M. ChemPhysChem 2003, 4, 49. (d) Maligaspe, E.;
D’Souza, F. Org. Lett. 2010, 12, 624.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 23, 2012
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