Triple-signaling mechanisms-based three-in-one multi-channel chemosensor for discriminating Cu2+, acetate and ion pair mimicking AND, NOR, INH and IMP logic functions

Article abstract of DOI:10.1039/c5tc00554j

Chemosensor 1 shows three different responses towards Cu²⁺, acetate and Cu(OAc)₂ ion pair at different wavelengths independently in its absorption and fluorescence behaviour following a triple-signaling mechanism, namely, internal ch

Full text of DOI:10.1039/c5tc00554j

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Journal Name
RSCPublishing
ARTICLE
Triple-signaling mechanisms-based three-in-one multi
channel chemosensor for discriminating Cu2+, acetate
and ion pair mimicking AND, NOR, INH and IMP
logic functions
Cite this: DOI: 10.1039/x0xx00000x
Prabhpreet Singha,*, Harminder Singha, Gaurav Bhargavab and Subodh Kumara
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
Chemosensor
1
shows three different responses towards Cu2+, acetate and Cu(OAc)2 ion pair at
www.rsc.org/
different wavelengths independently, in its absorption and fluorescence behaviour following
tripleꢀsignaling mechanisms such as internal charge transfer (ICT), C=N rotation and excited
state intramolecular proton transfer (ESIPT). Chemosensor
1 shows blue shift of the absorption
band from 337 nm to 308 nm in the presence of Cu2+ ions (ICT). It exhibits fluorescence ‘turnꢀ
off’ response at λem 458 nm in the presence of 1 equiv. of Cu2+ ions. Further addition of Cu2+ (6
equiv.) results in ~24 times enhancement in the emission intensity at λem 427 nm mimicking
‘ONꢀOFFꢀON’ molecular switch (ESIPT and C=N rotation). The addition of acetate ions to
solution of
1 results in red shift of the absorption band from 337 to 360 nm (ICT) and ~4 times
enhancement of emission intensity at λem 458 nm (ESIPT). On addition of Cu(OAc)2 ion pair,
1
shows the apparent effect of Cu2+ (blue shift; 337 to 308 nm) and acetate (red shift; 337 to 380
nm) which indicates the synergistic effect of both Cu2+ and AcOꢀ on each other’s binding. The
potential application of chemosensor
1 for construction of INHIBIT (INH), IMPLICATION
(IMP), AND, NOR, YES and NOT logic gates using Cu2+ and acetate as inputs and
absorbance/emission as outputs has been elaborated.
Dual chemosensor for cation and anion includes thioxanthone
based dual channel for Hg2+/Fꢀ;7a Schiff base derivatives of 1ꢀ
1. Introduction
(2ꢀaminophenyl)ꢀ3ꢀphenylthiourea7b and naphthyl based Schiff
base7c for Cu2+/Fꢀ and chemosensor for Cu2+/acetate ion.8
Nitrophenyl urea appended macrocycle shows logic operation
on addition of OHꢀ/Cu2+ and Fꢀ/Hg2+ ions.9 Moreover, in
literature thio(urea) subunits are used in combination with other
hydrogen bonding groups such as –NH, ꢀOH, indole or pyrrole
to execute the task of recognizing the anions.10 The direct
Schiff base conjugates of urea and thiourea with
salicylaldehyde or naphthaldehyde have been reported for the
recognition of metal ions.11ꢀ14 These dual chemosensors utilize
either one or combination of different signaling mechanisms
such as photoinduced electron transfer (PET), internal charge
transfer (ICT), excited state intramolecular proton transfer
(ESIPT), inhibition of C=N rotation etc., with or without
ratiometric8 behavior to signal cation and anion binding.
However, the combination of more than two signaling
mechanisms and separate response for cation, anion and ion
pair at different wavelengths by one chemosensor (3ꢀinꢀ1) to
turn multiple absorption and emission channels independently
has not been reported so far.
For last few years in supramolecular chemistry, the efforts are
being undertaken to develop dual chemosensors which can
perform recognition of both cations and anions, presumably
due to potential applications of cations and anions in the field
of biochemistry, pharmaceuticals, environmental science and
chemical biology.1 Dual chemosensors2ꢀ7 which possess
colorimetric (‘naked eye’ detection) and fluorescent response
for cation and anion at different wavelengths through
modulation of various signaling mechanisms8 are still in great
demand. Although cation complexation (metal coordination)
has roots quite different from anion complexation (involving
hydrogen bonds, electrostatic interactions, deprotonation etc.)9,
yet we have witnessed slow progress in the development of
dual chemosensors to recognize combination of cation and
anion.5ꢀ7
aDepartment of Chemistry, UGC Centre for Advanced Studies, Guru Nanak Dev
University, Amritsar 143 005, India. E-mail:prabhpreet.chem@gndu.ac.in; Tel:
+91-84271-01534
bDepartment of Applied Sciences, Punjab Technical University, Kapurthala-
144601, Punjab, India.
Electronic Supplementary Information (ESI) available: [Experimental details,
additional photophysical studies of chemosensor
See DOI: 10.1039/c000000x/
Among the transition metal ions, copper is considered as
significant pollutant, despite being an essential element in
biological systems15, whereas acetate ions find potential
1ꢀ3 and control compounds 4-6].
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applications in the field of pharmaceuticals, biochemistry and 1H NMR and deutrated solvent in 13C NMR spectra.
environmental science.16 Copper acetate has been known for its Abbreviations used for splitting patterns are s = singlet, bs =
use in textile, dyeing, printing, preservative for cellulose broad singlet, t = triplet, q = quartet, m = multiplet. Fourier
materials, as fungicide/insecticide and as a catalyst for various transform infrared (FTꢀIR) spectra were recorded on Perkin
organic reactions. So, our environment is being continuously Elmer 92035. The fluorescence spectra were recorded by
contaminated by copper and its compounds through industrial excitation at 330 nm unless otherwise stated. The fluorescence
and natural sources. Copper is also considered as notorious titrations were performed on Varian Carey Eclipse fluorescence
quencher of fluorescence and in this context chemosensor and BH Chronos spectrophotometer. The absorption spectra
which shows fluorescence enhancement as a result of copper were recorded on Shimadzuꢀ2450 spectrophotometer from
ion binding are still favored.10,17
Shimadzu. HRMS spectra were recorded on Bruker
MicroToff/QII. H NMR titration of
1
1
against acetate ion, Cu2+
Presently, electronic engineers are facing great challenge to
transform the digital information technology to the nanometer
scale.18 Singleꢀmolecule electronics in which “onꢀoff”, “offꢀon”
and “onꢀoffꢀon” switching behavior at molecular level
modulated by single input (proton, metal ions) has already been
demonstrated when first AND logic gate was mimicked with
optical signals by de Silva et al.19 In subsequent years, several
molecular analogs of AND, OR, NOR, XOR, INHIBIT and
NAND logic gates19ꢀ21 along with half subtractor, half adder,
full adder and full subtractor22ꢀ25 have been described. In this
context, the dual chemosensors which exhibit multiple output
channels are currently of particular interest because they could
be used for construction of simple and complex molecular logic
circuits. In our previous works, we have developed tetrapodal
and dipodal systems based molecular switches and logic
gates.26, 27 Recently, we reported first dual chemosensor based
on perylenediimide for the ratiometric detection of Cu2+/CNꢀ
ions.28 Now, we propose that fluorescent Schiff base and urea
subunits attached to 1,2ꢀdiaminobenzene platform would
provide reasonable preꢀorganization to the dipodal
architectures29ꢀ30 and their individual interactions with cation
and anion would result in different response at different
wavelengths.
ion and Cu(OAc)2 ion pair were performed in DMSO (d6) on
300 and 500 MHz NMR spectrometer. All the data were then
processed in Top Spin software to draw the stacking spectra.
Threotical calculations were carried out using density
functional theory (DFT) at B3LYP/6ꢀ31G(d,p) basis set. The
solutions of chemosensor
1ꢀ3 were prepared in acetonitrile
(HPLC grade). The solutions of metal perchlorates and anions
as tetrabutylammonium salts were prepared in acetonitrile
(HPLC grade) and were added in microliter quantities. All
absorption and fluorescence scans were saved as ACSII files
and further processed in Excel™ to produce all graphs shown.
The spectral data were analyzed through curve fitting
procedures by using nonꢀlinear regression analysis program
SPECFIT 3.0.36 to determine the stability constants and the
distribution of various species. The intermediate compounds
4,531 and 632 were synthesized after modification of the
literature procedures (for synthetic detail of
information).
4ꢀ6 see supporting
In this paper, we report the synthesis of chromoꢀfluorescent
chemosensor
1
(Fig. 1) and its photophysical properties towards
presents
various cations, anions and ion pair. Chemosensor
1
unique and rare example of 3ꢀinꢀ1 molecular system where
multiꢀchannel signals were observed in the presence of Cu2+,
acetate and Cu(AcO)2 ion pair through modulation of tripleꢀ
signaling mechanism (ICT, ESIPT and C=N rotation) at
different wavelengths independently. The potential application
of chemosensor 1 for construction of INH, IMP, AND, NOR,
YES and NOT logic gates has been elaborated. For control
experiments chemosensors 2 and 3 have been synthesized.
Fig. 1 Molecular structure of chemosensors 1ꢀ3.
2.2. Synthesis of Chemosensor 1-3
General procedure: To
a
solution of 2ꢀhydroxyꢀ1ꢀ
naphthaldehyde (127 mg, 0.7350 mmol) in CHCl3ꢀEtOH (1:9)
mixture, 1ꢀ(2ꢀaminophenyl)ꢀ3ꢀ(4ꢀnitrophenyl)urea (4) (200 mg,
2. Experimental
0.7350 mmol) and catalytic amount of pꢀtoluenesulfonic acid
were added and reaction mixture was stirred for 2h at 80oC
(Scheme 1). During this time, yellow solid precipitated out in
the reaction mixture. The yellow solid was filtered and washed
2.1. Materials and Characterization
Chemicals and solvents were of reagent grade and used without
further purification unless otherwise stated. All reactions were
performed under N2 atmosphere. Acetonitrile (HPLC grade)
and ethanol were purchased from Spectrochem India Ltd and
with ethanol to isolate the pure chemosensor 1.
Chemosensor 1: Yellow solid (225 mg, 72 %); m.p 272ꢀ276
1
oC; Rf = 0.35 (50% ethyl acetate:hexane); H NMR (500 MHz,
Changshu
Yangyuan
Chemical
China
respectively.
Chromatographic purification was done with silica gel 60ꢀ120
mesh. TLC was performed on aluminium sheets coated with
silica gel 60 F254 (Merck, Darmstadt). NMR spectra were
recorded on Bruker and JEOL (operating at 500 and 300 for 1H;
125 and 75 MHz for 13C, respectively). The peak values were
obtained as ppm (δ), and referenced to the TMS as reference in
DMSOꢀd6) δ 14.88 (s, 1H), 9.93 (s, 1H), 9.64 (s, 1H), 8.56 (d,
2H,
7.89 (d, 1H,
10 Hz), 7.64 (d, 1H,
1H,
7.13 (d, 1H,
J
= 5 Hz ), 8.19 (d, 2H,
= 5 Hz), 7.86 (d, 1H,
= 5 Hz), 7.58 (t, 1H,
J
= 10 Hz), 8.00 (d, 1H,
J = 10 Hz),
J
J
= 10 Hz), 7.71 (d, 2H, J =
J
J
= 5 Hz), 7.40 (t,
J
= 5 Hz), 7.32 (t, 1H,
J
= 5 Hz), 7.26 (t, 1H, J = 10 Hz),
J
= 10 Hz) ppm; 13C NMR (125 MHz, DMSOꢀd6):
2 | J. Name., 2012, 00, 1-3
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δ 166.8, 159.2, 152.7, 146.8, 141.5, 139.3, 136.5, 133.3, 131.8, emission band centered at 327 nm and 411 nm, respectively. In
129.5, 128.6, 127.5, 127.2, 125.6, 125.3, 124.1, 123.4, 121.2, case of compound , the absorption band between 440ꢀ460 nm
121.1, 120.2, 118.0, 110.1 ppm; νmax (KBr)/cmꢀ1: 3435, 1665, and ESIPT emission band at 458 nm belonging to hydroxyl
1611, 1560, 1533, 1504, 1459, 1342, 1331; HRMS (TOF MS naphthyl units are absent (Fig. 2). Chemosensor contains
2
3
ESI) calculated for C24H18N4O4 m/z 426.1328; found 427.1352. phenyl urea instead of 4ꢀnitrophenyl urea replacing electron
(M+H+).
withdrawing –NO2 group. The absorption spectrum of 3 shows
two absorption bands centered at 315 and 380 nm due to
internal charge transfer (ICT) across –NH fragment to phenyl
and azomethine group transitions, respectively. A low energy
shoulder band at 440ꢀ460 nm was also observed. In the
Chemosensor 2: Yellow solid (235 mg, 78 %); Rf = 0.40 (30%
ethyl acetate:hexane); H NMR (500 MHz, DMSOꢀd6) δ 10.30
1
(s, 1H) , 9.50 (s, 1H), 9.00 (d, 1H,
(d, 1H, = 7 Hz), 8.28 (d, 1H,
Hz), 8.10 (d, 1H, = 8 Hz), 7.75ꢀ7.68 (m, 5H), 7.64 (t, 1H,
7 Hz), 7.51 (d, 1H, = 7 Hz), 7.30 (t, 1H, = 7 Hz), 7.13 (t,
1H,
= 7.5 Hz) ppm; 13C NMR (125 MHz, DMSOꢀd6): δ 159.5,
J
= 8 Hz), 8.67 (s, 1H), 8.49
J
J
= 8 Hz), 8.19 (t, 4H, = 9.5
J
emission spectrum,
3 shows broad ESIPT emission band at 458
J
J =
nm, when excited at 330 nm (Fig. 2).
J
J
J
152.2, 146.9, 141.5, 140.1, 134.1, 133.9, 131.8, 131.7, 129.4,
129.2, 128.0, 127.6, 126.8, 126.0, 125.6, 124.4, 123.3, 119.4,
118.5, 117.9, 117.6 ppm; HRMS (TOF MS ESI) calculated for
C24H18N4O3 m/z 410.1379; found 411.1408 (M+H+).
Chemosensor 3: Yellow solid (280 mg, 83 %); Rf = 0.40 (30%
1
ethyl acetate:hexane); H NMR (500 MHz, DMSOꢀd6) δ 14.89
(s, 1H), 9.64 (s, 1H), 9.22 (s, 1H), 8.56 (d, 1H,
(d, 1H, = 9 Hz), 7.95 (d, 1H, = 7.5 Hz), 7.87 (d, 1H,
8Hz), 7.58 (s, 2H), 7.48 (d, 2H, = 7.5 Hz), 7.40 (t, 1H, = 7.5
Hz), 7.31ꢀ7.27 (m, 3H), 7.20 (t, 1H, = 7 Hz), 7.15 (d, 1H,
9Hz), 6.98 (t, 1H,
= 7 Hz); 13C NMR (125 MHz, DMSOꢀd6):
J = 8.5 Hz), 8.01
J
J
J =
J
J
J
J =
J
δ 166.5, 159.3, 153.1, 140.2, 138.7, 136.6, 133.3, 132.6, 129.5,
129.3, 128.6, 127.5, 127.2, 124.3, 124.1, 122.6, 122.3, 121.2,
121.1, 120.2, 118.7, 110.1 ppm; HRMS (TOF MS ESI)
calculated for C24H19N3O2 m/z 381.1477; found 382.1548.
(M+H+).
Fig. 2 Absorption and emission spectra of chemosensors 1-3 recorded in CH3CN.
We have also recorded the absorption and emission spectra of
the chemosensor
spectroscopic and photophysical properties (Fig. S1, table S2,
ESI). The absorption maxima of shows red shift of about 21
1 in various solvents and calculated the
1
nm on changing the solvent from chloroform to acetonitrile to
DMSO indicating charge transfer character in the ground state.
We also observed that
for deꢀprotonation band (440 ꢀ 460 nm) in the polar protic
solvents. Chemosensor in nonꢀpolar solvents viz, ethyl
1 shows maximum absorption intensity
1
acetate, CH2Cl2, CH3CN, dioxane gives emission at 458 nm due
to ESIPT mechanism but in polar protic and aprotic solvents
(DMF, ethanol, THF) ESIPT band is replaced by normal
emission at 416 nm.
Scheme 1: Synthesis of chemosensors 1ꢀ6.
The response of chemosensor 1 to Cu2+
The absorption spectrum of
1 (10 µM, CH3CN) on addition of
different metal perchlorates of Na+, K+, Sr2+, Ba2+, Pb2+, Ni2+,
Co2+, Fe2+, Zn2+ (100 µM) result in ~20% decrease in
absorbance intensity, however addition of 100 µM Cu2+ ions
caused >50% decrease along with concomitant blue shift of the
absorption maxima. On gradual addition of Cu2+ ions, the
intensity of absorption band at 337 nm was decreased with
concomitant shift of the absorption band from 337 to 308 nm
(blue shift ~29 nm) up to addition of 40 µM of Cu2+ ions and
then attained the plateau (Fig. 3A). The colour of the solution
changed from light yellow to colourless (Fig. 3A, Inset). The
spectral fitting of these data using nonꢀlinear regression
analysis shows the formation of 1:2 (L:M) stoichiometric
complex with log βL(Cu2+) = 7.42 ± 0.02 (L:Cu2+, 1:2). The Job’s
3. Results and Discussion
Spectroscopic comparison of chemosensors 1-3
The absorption spectrum of 1 in CH3CN shows a broad absorption
band centered at 337 nm due to internal charge transfer (ICT)
across –NH fragment to the nitrophenyl group and a low energy
shoulder band between 440ꢀ460 nm attributed to naphthol –OH
moiety. The emission spectrum of
centered at 458 nm, when excited at 330 nm, which is attributed
to ESIPT process (Fig. 2).
1 shows broad emission band
Structurally compound
2 contains naphthyl instead of 2ꢀ
hydroxynaphthyl group and it shows a broad absorption and
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plot shows maxima at the mole fraction of 0.6 and confirms the
formation of 1:2 (
ꢀCu2+) complex (Fig. S2, ESI).
quenching of the emission maxima (Fig. S3, ESI). On the other
3
, on addition of Cu2+ up to 200 µM shows
1
hand chemosensor
decrease in absorption intensity at 380 nm. In the fluorescent
titration of
with Cu2+, the emission band at 458 nm was
3
quenched up to 1 equiv. of Cu2+ ions and on further addition of
Cu2+ ions a new blue shifted emission band at 427 nm was
observed whose intensity increased only 2 times up to 10.0
equiv. of Cu2+ ions concentration (Fig. S4, ESI).
The response of chemosensor 1 to acetate and F- ions
ꢀ
The UVꢀVis spectrum of
CNꢀ remained silent. However
to the solution of the absorption band was red shifted from
337 nm to 360 nm, associated with Hꢀbonding or deꢀ
1
on addition of Clꢀ, Brꢀ, ClO4 and
,
on addition of AcOꢀ and Fꢀ ions
1
protonation of ꢀOH/ꢀNH groups. The UVꢀVis titration of
1 with
AcOꢀ ions shows ratiometric behavior up to 5.0 equiv. (50 µM)
of AcOꢀ ions, with isosbestic point at 349 nm (Fig. 4A).
Fig. 3 (A) UVꢀVis spectra of
absorption intensity vs conc. of Cu2+ ions at 337 nm; Colour changes of
and (10 ꢁM) on addition of
+ Cu2+ ions (40 ꢁM). (B) Fluorescence spectra of
Cu2+ ions; λex 330 nm, Slit width (Ex/Em = 10/20 nm). Inset: Plot of fluorescence
intensity vs conc. of Cu2+ ions at 458 and 427 nm; Fluorescence of
(10 ꢁM)
before and after addition of Cu2+ ions (60 ꢁM) under illumination at 365 nm.
1
(10 ꢁM) on addition of Cu2+ ions. Inset: Plot of
(10 ꢁM)
1
1
1
1
When excited at 330 nm, chemosensor
1 showed fluorescence
emission maxima at 458 nm and addition of 100 µM of Fe2+,
Ba2+, Ni2+, Co2+, Zn2+, Sr2+ and Pb2+ caused 40% quenching of
fluorescence intensity at 458 nm. The fluorescent titration of
1
(10 µM, CH3CN) with Cu2+ showed that on gradual addition of
Cu2+ ions, the emission band at 458 nm was quenched up to
addition of 1.0 equiv. (10 µM) of Cu2+ ions and on further
addition of Cu2+ ions a new blue shifted emission band centered
at 427 nm was observed whose intensity increased ~24 times up
to 6.0 equiv. (60 µM) of Cu2+ and then attained the plateau
Fig. 4 (A) UVꢀVis spectra of
Ratiometric plot of A360 nm/A337 nm vs conc. of AcOꢀ ions; the colour changes of
1
(10 ꢁM) on addition of AcOꢀ ions. Inset:
(Fig. 3B). So, chemosensor
1 behaves as ‘ONꢀOFFꢀON’
1
molecular switch in the presence of Cu2+ ions. The spectral
fitting of these data using nonꢀlinear regression analysis shows
(10 ꢁM);
spectra of
1
1
+ AcOꢀ ions (50 ꢁM) and
1
+ Fꢀ ions (50 µM). (B) Fluorescence
(10 ꢁM) on addition of AcOꢀ ions; λex 370 nm. Slit width (Ex/Em =
the formation of 1:1 and 1:2 (L:M) stoichiometric complexes 10 nm). Inset: Plot of fluorescence intensity vs conc. of AcOꢀ ions; fluorescence
of
1
(10 ꢁM) before and after addition of AcOꢀ ions (20 ꢁM) under illumination at
with log βL(Cu2+) = 3.41 ± 0.2 (L:Cu2+, 1:1) and log βL(Cu2+)
6.78 ± 0.3 (L:Cu2+, 1:2).
=
365 nm.
The plot of ratios of absorbance intensities (A360nm/A337nm) vs.
AcOꢀ ions concentration shows a linear increase between 0ꢀ10
µM of AcOꢀ ions (Fig. 4A, Inset). The spectral fitting of the
In case of chemosensor
2 (10 µM) addition of even 100 µM
concentration of Cu2+ causes negligible blue shift (5 nm) in the
absorption maxima as well as negligible fluorescence
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titration data shows the formation of 1:2 stoichiometric
complex log βL(OAc) = 7.47±0.04 (L:OAc, 1:2). The inflection
point in Job’s plot at the mole fraction of 0.7 further confirms
the formation of 1:2 (1
ꢀAcOꢀ) complex (Fig. S5, ESI).
On the other hand, addition of Fꢀ ions resulted in λmax shift from
337 to 350 nm without ratiometric response along with band at
456 nm due to deꢀprotonation of –OH moiety. The colour of the
solution changed from light green to light yellow on addition of
AcOꢀ ions and dark yellow on addition of Fꢀ ions, (Fig. 4A,
Inset).
The fluorescence titration of
1
in CH3CN against AcOꢀ ions
showed ~4 times increase in fluorescence intensity at 458 nm
up to addition of 6 equiv. (60 µM) of AcOꢀ ions and thus
constitutes fluorescence ‘turn on’ response at 458 nm. The
spectral fitting of the titration data shows the formation of 1:2
stoichiometric complex log βL(OAc) = 7.17 ± 0.02 (L:OAc, 1:2).
The inflection point in Job’s plot at the mole fraction of 0.7
Fig. 5 UVꢀVis spectra of 1 (10 ꢁM) on addition of the Cu(OAc)2 solution showing
potential of 1 for simultaneous estimation of Cu2+ and AcOꢀ ions in the sample. Inset:
further confirms the formation of 1:2 (1
ꢀAcOꢀ) complex.
fluorescence spectra of
1 (10 ꢁM) on addition of Cu(OAc)2 (10 ꢁM); λex 370 nm.
Similar results were also obtained with fluoride due to its
higher basicity.
Slit width (Ex/Em = 10 nm); the colour changes of 1 (10 ꢁM); 1 + Cu(OAc)2
(50 ꢁM).
The control compounds
2 and 3 showed insignificant
To get more insight into our above concept of ion pair
recognition, we also performed UVꢀVis titration of (
1:5) complex in the presence of different concentration of Cu2+
ions. The UVꢀVis spectrum of (
:AcOꢀ, 1:5) complex showed
red shifted absorption band at 360 nm in comparison to 337 nm
for chemosensor alone as discussed earlier. When we
gradually added Cu2+ to the ( :AcOꢀ, 1:5) complex, the
1
:AcOꢀ,
changes in their absorption and emission spectra on addition of
acetate and Fꢀ ions (Fig. S6ꢀ7 ESI). These results clearly signify
the presence of both nitro and ꢀOH moieties essential for having
the selective changes with Cu2+, AcOꢀ and Cu(OAc)2.
1
1
1
absorption peak shifted to 367 nm at 1 equiv. concentration of
The response of chemosensor 1 to Cu(OAc)2 ion pair
Cu2+ ions (10 µM), and on addition of 2 equiv. of Cu2+ ions (20
In the light of above results, we expect that chromoꢀfluorescent
µM) the absorption band of
1 gradually shifted to 380 nm (Fig.
sensor
simultaneous detection of Cu2+ and AcOꢀ ions. The UVꢀVis
titration of (10 µM) on incremental addition of solution of
1 possessing multiple binding sites should permit the
6). We also observed visually detectable solution colour change
from light greenish to dark yellow (50 µM conc. of AcOꢀ ions)
to dark green (up to 20 µM conc. of Cu2+ ions) (Fig. 6, Inset).
However, when we performed the reverse titration i.e. gradual
1
Cu(OAc)2 up to 3 equiv. (30 µM) concentration showed
significant changes and then it achieved the plateau (Fig. 5).
When we gradually added copper acetate, the absorption peak
at 337 nm immediately started decreasing with concomitant
appearance of blue shifted absorption band at 308 nm and red
shifted absorption band at 380 nm. So, we attribute these
absorption changes due to the apparent effect of Cu2+ (blue
addition of acetate ions to the solution of (
no such effect was observed (Fig. S10, ESI).
1
:Cu2+, 1:4) complex,
shift, 337 to 308 nm in line with Cu2+ titration with
(red shift, 337 to 360 nm in line with AcOꢀ titration with
ion pair. The colour of the solution changed from light yellow
to dark green (Fig. 5, Inset). In fluorometric titration of with
1
) and AcOꢀ
1
) as
1
copper acetate, the fluorescence intensity at 458 nm increased
18 times up to addition of 1 equiv. of copper acetate, but blueꢀ
shift of the emission maxima is not observed. To achieve
plateau, fewer quantity of Cu(AcO)2 (3 equiv.) is required than
individual Cu2+ (6 equiv.) and AcOꢀ (5 equiv.) ions.
As a proof of concept, we have also tested the sensing behavior
of other ion pairs such as CuCl2, Cu(NO3)2, Pd(OAc)2 and
Hg(OAc)2 towards chemosensor
1 in CH3CN. In case of these
ion pairs, we observed either only increase in the absorption
intensity at 337 nm without any spectral shift or only individual
effect of Cu2+ or acetate ions associated with deꢀprotonation of
ꢀOH/ꢀNH moiety (Fig. S8ꢀ9, ESI).
Fig. 6 UVꢀVis spectra of (1+AcOꢀ
solution. (i) 10 ꢁM of ; (ii) 10 ꢁM of
(50 ꢁM) + Cu2+ ions (10 µM) and (iv)
µM). Inset: The colour changes of (10 ꢁM);
AcOꢀ ions (50 ꢁM) + Cu2+ ions (20 µM).
)
complex on gradual addition of the Cu2+
and 50 µM of acetate (iii)
+ AcOꢀ ions
+ AcOꢀ ions (50 ꢁM) + Cu2+ ions (20
+ AcOꢀ ions (50 ꢁM) and
1
1
1
1
1
1
1 +
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Similar UVꢀVis titration of (
presence of different concentration of Cu2+ ions was performed. addition of Cu(OAc)2 ion pair.
When we gradually added Cu2+ to the ( :Fꢀ, 1:5) complex, the
1
:Fꢀ, 1:5) complex in the NOT gate at 337 nm and YES gate at 308 and 380 nm on
1
intensity of absorption peak at 356 nm decreases and on
addition of higher concentration of Cu2+ ions (100 µM) the
absorption band of
S11, ESI). Similarly, in the fluorescence titration of (
1
gradually shifts from 356 to 320 nm (Fig.
:Fꢀ, 1:5)
1
complex, the addition of Cu2+ causes only decrease in the
emission intensity (Fig. S12, ESI). So, we conclude that the two
different absorption outcomes could only be produced when
both Cu2+ and AcOꢀ ions are simultaneously present in the
solution.
The UVꢀVis titration of control compounds 2 and 3 on
addition of solution of copper acetate showed insignificant shift
in their absorption bands (Fig. S13, ESI). So, we conclude that
chemosensor
1 presents a unique example where we can
monitor the sensing of Cu(OAc)2 involving two different
wavelengths simultaneously.
The selectivity towards Cu(OAc)2 ion pair was further
ascertained by the competition experiments where 300 µM of
competing ion pairs were added to the
solution. As discussed earlier, the absorption spectrum of
1
ꢀCu(OAc)2 (1:10)
on
1
addition of Cu(OAc)2 shows blue shift from 337 to 308 nm and
red shift from 337 to 380 nm. On addition of 300 µM of various
ion pairs did not cause any further spectral change in the
absorption spectrum except increase in absorbance intensity at
290 nm due to absorbance of Cu2+ itself at high concentration
(Fig. S14, ESI).
Application as Logic Gates
As described above, the absorption and emission spectra of
1
are distinctively affected by the addition of Cu2+ and acetate
ions alone or simultaneously. It shows triple signaling with dual
response for Cu2+ (fluorescence ‘turn on’ response at 427 nm
and fluorescence ‘turn off’ at 458 nm; colorimetric output at
308 nm; ‘ON’ and 337 nm; ‘OFF’) and acetate (fluorescence
‘turn on’ response at 458 nm; colorimetric output at 360 nm;
‘ON’ and 337 nm; ‘OFF’) ions individually as well as
Cu(OAc)2 ion pair (fluorescence ‘turn on’ response at 458 nm;
colorimetric output at 380 nm; ‘ON’). So, using Cu2+ and
acetate as inputs and absorption/fluorescence as output modes,
the following logic gates could be fabricated (Fig. 6).
Molecular switches and logic gates are analogues because both
convert input stimulations into output signals with intrinsic
protocols. In a positive logic convention, a 0 corresponds to the
low output and 1 corresponds to high absorbance/emission
output.
Fig. 7 (A) Truth table for various logic functions (NOR, AND, INH, IMP)
fabricated for chemosensor
using Cu2+ and acetate ions as inputs and
absorption/emission as outputs at different wavelengths; (B) Pictorial
representation of combinatorial circuit from NOR, INH and AND logic gate using
absorption as output mode; (C) IMP logic gate using emission as output mode.
1
The pattern of absorption intensities as a function of cation
and anion input read as a NOR logic function where absorption
band at 337 nm in absence of both ions depicts ‘on’ state but as
either or both of Cu2+ and acetate ions are added, no absorption
band centered at 337 nm exists that could be labelled as ‘off’
state (Fig. 7B). The other demonstration of molecular
information processing was based on a photonic AND logic
gate at A380 nm. Absorbance at 380 nm could be at high value
only when both inputs of Cu2+ and acetate are simultaneously
given to the system. When any one of the above two inputs is
absent, the output will be in low state at A380 nm (Fig. 7B).
The compound
1 acts as an “ONꢀOFFꢀON” switch driven by
An INHIBIT function is basically an AND operation where
one input is reversed, i.e., in our case Cu2+ ion in absorbance
Cu2+ with maximum fluorescence emission at 458 nm when
[Cu2+] = 0 µM. The fluorescence falls off rapidly at 458 nm
with addition of [Cu2+] = 10 µM and then reappears at different
wavelength i.e. at 427 nm with [Cu2+] = 60 µM. So, it behaves
as typical NOT gate at 458 nm and become Cu2+ driven YES
logic at 427 nm. Similarly, we also observe several NOT/YES
mode. The addition of acetate ions to the solution of
1 show
output signal at 360 nm, however the presence of Cu2+ inhibits
the output signal caused due to acetate alone in the absorption
mode and hence, INHIBIT (INH) function at A360nm was
observed (Fig. 7B). Similar INH function was also observed at
F427nm (fluorescence mode) where input of acetate ion is
reversed.
logic gates for chemosensor
1 with different inputs at different
wavelengths in the absorption mode such as NOT gate at 337
nm and YES gate at 308 nm on addition of Cu2+ ions; NOT gate
at 337 nm and YES gate at 360 nm on addition of acetate ions;
The fluorescence output at 458 nm represents rarely reported
IMPLICATION (IMP) logic gate. The IMPLICATION logic
6 | J. Name., 2012, 00, 1-3
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ARTICLE
DOI: 10.1039/C5TC00554J
gate is equivalent to an OR gate except one of its two input 6.53 kcal/mol (Fig. 9). So, there is a dynamic equilibrium
lines contains an inversion operation. At F458 wavelength, the between these two conformations and molecule can attain either
output value is in low state only when Cu2+ is introduced into of two conformations. The optimized structure of
1
shows anti
the system. Otherwise, the output at 458 nm is in high state geometry of the urea group and we could expect cleft –like
(Fig. 7C).
geometry which provides perfect platform for Oꢀ and Nꢀ atoms
of Schiff base chelating group and carbonyl group of urea to
coordinates with cation and –NH of urea to coordinate with
anion.
1H NMR Titration Studies
The interaction of sensor
1 with tetrabutylammonium
acetate, copper perchlorate and copper acetate were also probed
1
through H NMR titration however due to poor solubility of
chemosensor
1 (5 mM) in CD3CN, we prefer DMSOꢀd6 solvent
1
1
for H NMR titrations. The H NMR spectrum of the sensor
1
showed signals at δ 9.93 ppm and δ 9.64 ppm due to –NH
protons of urea moiety. The signals at δ 14.88 ppm and δ 8.56
ppm correspond to –OH group and imine proton (CH=N),
respectively. We observed that the proton signal of the naphthol
hydroxyl group at δ 14.88 ppm and proton signal of the –NH
group attached to pꢀnitrophenyl at δ 9.93 ppm disappeared (due
to fast exchange of Hꢀbond on NMR time scale) on addition of
1.0 equiv of acetate ions along with 0.12 ppm up field shift of
the remaining –NH proton signal. Moreover we also observed
upꢀfield chemical shift of the naphthalene aromatic protons on
addition of acetate ions (Fig. 8).
Fig. 9 The energy minimized structures (DFT 6ꢀ31G*) of chemosensor 1.
The optimized structure of 1
ꢀCu2+ complex shows participation
of three donor atoms i:e., C=O, ꢀOH and C=N towards Cu2+
with bonding distance 1.87, 1.79 and 1.87 Ǻ, respectively. The
optimized structure of
1
ꢀAcOꢀ (1:2) complex showed
participation of AcOꢀ ions in Hꢀbonding with –OH of naphthol
and –NH of urea group (Fig. 10).
The
1
ꢀAcOꢀ complex was found to be more stable by 55.01
AcOꢀ, the
kcal/mol in comparison to 1(A). For complex
1
⊂
optimized distances between acetate and two –NH group were
calculated to be 1.77 and 1.70 Ǻ. The second acetate binds with
1
through –OH of naphthol. The binding of acetate also caused
elongation of –NH bond of urea by 0.039 and 0.031 Ǻ and –OH
bond by 0.30 Ǻ as compared with conformation 1(A)
.
Fig. 8 Partial 1H NMR spectrum of chromoꢀfluorescent sensor
DMSOꢀd6 at different addition (in equivalent) of (A) tetrabutylammonium acetate
1 recorded in
To study the effect of ion pair, similar studies were
conducted using Cu(OAc)2 complex. The optimized
structure of with Cu(OAc)2 ion pair shows simultaneous
1
⊂
1
1
In case of H NMR titration of chemosensor
1 with Cu(ClO4)2,
complexation of three donor atoms (C=O, ꢀOH, C=N) towards
Cu2+ with bonding distance 1.84, 1.81 and 1.87 Ǻ, respectively,
and complexation of –NH group of urea with acetate with
bonding distance of 1.53 and 1.55 Ǻ.
we observed decrease in the intensity for peak corresponding to
ꢀOH group which again indicates Hꢀbonding or deꢀprotonation
mechanism. Moreover Schiff base proton and urea –NH
protons underwent very small shift which indicate weak
complexation of urea –NH with Cu2+ ions. However in case of
The 1⊂Cu(OAc)2 also showed elongation of –C=O bond by
0.027 Ǻ when compared with conformation 1(A). This complex
also showed elongation of –NH bond of urea by 0.080 and
1H NMR titration of chemosensor
1 with Cu(AcO)2 we
observed decrease in intensity for both –OH and –NH protons
indicating complexation of ion pair at their respective binding
site (see Fig. S15, ESI).
0.069 Ǻ as compared with conformation 1(A). The
complex was found to be more stable by 135.28 kcal/mol in
1
ꢀCu2+
comparison to 1(A). So,
1
ꢀCu2+ complex was found to be more
1
ꢀAcOꢀ complex and thus provides
stable in compare to
Structure optimization using DFT
evidence for displacement of AcOꢀ ions on gradual addition of
Geometries of chemosensor
1
and its 1:1 adducts with Cu2+,
Cu2+ ions to the ꢀAcOꢀ solution (discussed above).
1
AcOꢀ and Cu(OAc)2 ion pair, respectively were also fully
optimized with B3LYP functional and 6ꢀ31G* basis set. Firstly,
the two possible conformations of
1 were optimized using
B3LYP (6ꢀ31G*) basis set. The energy of conformation ‘
B
’
was found to be more stable than conformation ‘A’ by only
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Fig. 10 The energy minimized structures (DFT 6ꢀ31G*) of Cu2+ Cu2+), AcOꢀ ( AcOꢀ) and [
(1⊂ 1⊂ 1⊂Cu(OAc)2] complex.
increase in fluorescence intensity at 458 nm (ESIPT)
Proposed mechanism for complexation
phenomena (Fig. 11). The lack of –OH group in chemosensor
2
Mechanistically, we propose that one Cu2+ ion coordinates
with Schiff base and with urea moiety of chemosensor and
thus inhibits the ICT from urea nitrogen to electron deficient
resulted in no fluorescence enhancement on addition of AcOꢀ
ions.
1
p
ꢀ
nitrophenyl causing blueꢀshift of λmax from 337 to 308 nm
along with decrease in absorption intensity due to binding of
C=N of azomethine and –OH of hydroxynaphthyl with Cu2+
When Cu(OAc)2 was added for simultaneous estimation of
both ions, the individual ions coordinated with the respective
binding sites. However, the increased redꢀshift of 337 nm band
to 380 nm on addition of Cu(OAc)2 as compared to 337 to 360
nm in case of AcOꢀ alone is attributed to the synergistic effect
of both Cu2+ and AcOꢀ on each other’s binding. Probably,
simultaneous binding of Cu2+ with carbonyl oxygen as depicted
in figure 11 increases the acidity of urea group and results in
enhanced ICT on binding with acetate. This fact was proved on
ions. In fluorescence spectrum of chemosensor 1, the addition
of 1 equiv. of Cu2+ inhibited ESIPT band at 458 nm due to
binding with –OH. Further addition of Cu2+ ions, resulted in
release of normal emission of naphthalenoxy moiety
supplemented due to restriction in rotation in imine (C=N)
moiety (Fig. 11). Lack of –OH in chemosensor
2, reduced
significantly the copper complexation as observed in UVꢀVis
and fluorescence titrations. Similarly, absence of –NO2 group in
titration of (1
+AcOꢀ) complex on addition of Cu2+ ions where
the λmax first shift from 360 to 380 nm as discussed earlier.
Probably, this mode of Cu2+ binding is also responsible for the
chemosensor 3, resulted in weak ICT mechanism and no blue
shift on addition of Cu2+.
observed blue shift in the absorption spectrum of
Cu(OAc)2 was added.
1 when
Conclusion
In summary, we have synthesized chromoꢀfluorescent
chemosensor that possesses Schiff base and urea moieties as
1
dual binding sites for recognition of Cu2+, AcOꢀ ions and
Cu(OAc)2 ion pair. In coherence participation of internal charge
transfer, ESIPT and restriction in rotation of C=N bond
phenomena have been responsible for the observed
absorbance/fluorescence changes in the presence of Cu2+, AcOꢀ
and both. We have also demonstrated the potential application
of chemosensor
1 for construction of INH, IMP, AND, NOR,
YES and NOT logic gates operated by Cu2+ and acetate as
inputs and absorbance/emission as outputs.
Fig. 11 Proposed binding modes of chromoꢀfluorescent sensor
and Cu(OAc)2 salt showing different modulation of ICT, ESIPT and C=N rotation
channel.
1
with AcOꢀ, Cu2+
Acknowledgements
When acetate ions were added to the solution of
chemosensor
nitrophenylurea, and increase ICT between electron rich anion
bound urea and electron deficient ꢀnitrophenyl moiety with
1
, one AcOꢀ ion coordinated with ꢀNH groups of
This work was supported by UGC grant for newly recruited
faculty, CSIR (Grant No. 02(0084)/12/EMRꢀII) and UPE
programmes. H.S. is thankful to the UGC for the research
fellowship under UPE programme.
p
redꢀshift of λmax from 337 to 360 nm. The second acetate
molecule coordinates with naphthyl –OH group and caused
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DOI: 10.1039/C5TC00554J
Triple-signaling mechanisms-based three-in-one multi-channel
chemosensor for discriminating Cu2+, acetate and ion pair
mimicking AND, NOR, INH and IMP logic functions
Prabhpreet Singha,*, Harminder Singha, Gaurav Bhargavab and Subodh Kumara
aDepartment of Chemistry, UGC Centre for Advanced Studies, Guru Nanak Dev University, Amritsar 143 005, India.
e-mail: prabhpreet.chem@gndu.ac.in; Tel: +91-84271-01534
bDepartment of Applied Sciences, Punjab Technical University, Kapurthala-144601, Punjab, India.
Chemosensor 1 shows three different responses towards Cu2+, acetate and Cu(OAc)2 ion pair at different
wavelengths independently, following triple-signaling mechanisms (ICT, C=N rotation and ESIPT) and
also demonstrate fabrication of INH, IMP, AND, NOR, YES and NOT logic gates.