A cyanide-selective colorimetric "naked-eye" and fluorescent chemosensor based on a diketopyrrolopyrrole-hydrazone conjugate and its use for the design of a molecular-scale logic device

Article abstract of DOI:10.1039/c6ra21669b

A novel diketopyrrolopyrrole (DPP)-hydrazone based receptor 1 was designed and synthesized as a selective fluorescent and colorimetric chemosensor for cyanide without interference from other common anions including fluoride and acetate. The spectral respo

Full text of DOI:10.1039/c6ra21669b

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A cyanide-selective colorimetric “naked-eye” and fluorescent
chemosensor based on diketopyrrolopyrrole-hydrazone conjugate
and its use for the design of molecular-scale logic device
Lingyun Wang *, Xianggen Chen, Derong Cao
Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of
Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, China,
510641
*Corresponding author: Tel. +86 20 87110245; fax: +86 20 87110245. Eꢀmail:
lingyun@scut.edu.cn
1

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Abstract
A novel diketopyrrolopyrrole (DPP)ꢀhydrazone based receptor 1 was designed and
synthesized as a selective fluorescent and colorimetric chemosensor for cyanide
without interference from other common anions including fluoride and acetate. The
spectral responses of 1 to cyanide were studied: an approximately 130 nm red shift in
absorption spectra (from λmaxꢀab = 516 nm to λmaxꢀab = 646 nm and the color change
from pink to indigo) and “turnꢀoff” emission response was observed. The
experimental results revealed that the formation of anionic species from deprotonation
of the hydrazone NꢀH moiety by cyanide was responsible for the spectral changes.
Density function theory calculations were conducted to rationalize the optical
response of the receptor 1. In the presence of cyanide, receptor 1 could act as an
“OffꢀOn” fluorescent probe for trifluoroacetic acid (TFA) based on the reversible
protonation process, which may be represented by
a
complementary
“IMPLICATION/INHIBIT” logic gate at molecular level employing CN−ꢀTFA as the
chemical inputs. The detection limit was 0.23 ꢁM using the fluorescence spectra
changes, which was far lower than the WHO guideline of 1.9 ꢁM.
Keywords: Colorimetric; Chemosensor; Diketopyrrolopyrrole (DPP); Anions sensing;
Fluorescence; Logic gates
2

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1 Introduction
The recognition and detection of anions is extremely necessary because anions play
so many positive and negative effects in the lives. Cyanide is well known as one of
the most toxic species and is extremely harmful to mammals. Any accidental release
of cyanide to the environment causes serious problems.1 Therefore, the maximum
permissive level of cyanide in drinking water is set at 1.9 ꢁM by the World Health
Organization (WHO). Nevertheless, cyanide salts are still widely used as industrial
materials in gold mining, electroplating, plastics production and other fields.
Widespread applications of cyanide raises a number of environmental concerns and
these issues necessitate the development of sensor molecules that can adequately
detect this anion in a variety of settings. So, it is highly desirable to develop sensitive,
selective and quick detection methods for toxic cyanide anions.
On the other hand, in biological and environmental systems, anionꢀreceptor
interactions commonly occur in aqueous media. Therefore, much attention has been
paid to develop anion sensors that work in the aqueous phase. However, examples of
chromogenic sensor molecules that are capable of recognizing cyanide ions in
aqueous environments are much fewer compared to those that display sensing features
in nonaqueous solutions.2 High solvation energy of the cyanide ion in aqueous
environments is known to affect adversely the hydrogenꢀbonded adduct formation
between the receptor unit and the CN−.3 The recognition and separation of the CN−
anion from an aqueous media is a challenging task. An alternate approach, adopted by
many researchers, for recognition of CN− in aqueous solution is based on
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chemodosimetric methodology. This involves coordination of free cyanide ion to a
transition metal ion or unsaturated boron center,4 or to an electrophilic carbonyl
functionality.5 However, such examples have certain limitations such as delayed
response, lack of reversibility or use of elevated reaction temperature. The few reports
that describe the recognition of CN− in aqueous solution in the presence of F−, AcO−,
−
H2PO4 , and SO42−, utilizing the hydrogen bond motif, suffer from lack of selectivity
due to its poor ability to form hydrogen bonds with receptors.6ꢀ18 Furthermore, most
of these sensors are normally irreversible and one time use for sensing cyanide ion. A
hydrazone (ꢀCH=NꢀNHꢀ) contained hydrogen donor moiety of NꢀH, which had the
tendency for binding of anions was used as binding site with cyanide.19ꢀ23 Recently, a
Schiffꢀbase
based
on
5ꢀ(4ꢀnitrophenyl)ꢀ2ꢀfuran
and
naphtho[2,1ꢀb]furanꢀ2ꢀcarbohydrazide was reported for the selective detection of
cyanide in aqueous medium via deprotonating between the hydrazide moiety of probe
and cyanide.24 However, this probe showed low detection limit due to low activity of
hydrazide.
1,4ꢀDiketoꢀ3,6ꢀdiphenylpyrrolo[3,4ꢀc]pyrrole (DPP) and its derivatives25 represent
a class of brilliant red and strongly fluorescent highꢀperformance pigments and have
exceptional light, weather and heat stability and high light fastness. In view of this
and as a longstanding interested in molecular recognition of our group based on
DPP,26 herein, we report a simple, easy to prepare yet highly selective cyanide
receptor 1 (Scheme 1) for the rapid detection and colorimetric sensing of cyanide
from aqueous media. The NꢀH protons on hydrazone become appreciably activiated
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due to presence of the strong electronꢀwithdrawing effect of 2,4ꢀdinitrophenyl, C=N
and DPP moieties. There was a deprotonation process between the probe molecule
and cyanide, which increased the ability of intramolecular charge transfer which
enlarge the conjugate system of the probe and then gave rise to large changes in the
absorption spectra (from λmaxꢀab = 516 nm to λmaxꢀab = 646 nm and the color change
from pink to indigo) and “turnꢀoff” emission response at 601 nm with 92%
fluorescence quenching. To the best of our knowledge, it was the first sensor for
cyanide ever reported with a 130 nm red shift response, which could gave a
colorimetric and “onꢀoff” fluorescence response instantly. Notably, this probe serves
as a recyclable component in sensing materials by adding of trifluoroacetic acid
(TFA). After the addition of TFA, 1ꢀCN shows an immediate visible change in color
from indigo to pink within a short time of 5 s, and the fluorescence recovered at the
same time. A complementary “IMPLICATION/INHIBIT” logic gate at molecular
level employing CN−ꢀTFA as the chemical inputs can be achieved.
2. Experimental
2.1. Chemicals and instruments
Nuclear magnetic resonance spectra were recorded on Bruker Avance III 400 MHz
and chemical shifts are expressed in ppm using TMS as an internal standard. The
UVꢀvis absorption spectra were recorded using a Helios Alpha UVꢀvis scanning
spectrophotometer. Fluorescence spectra were obtained with a Hitachi Fꢀ4500 FL
spectrophotometer
with
quartz
cuvette
(path
length
=
1cm).
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2,5ꢀDioctylꢀ3,6ꢀbis(4’ꢀformylphenyl)pyrrolo[3,4ꢀc]pyrroleꢀ1,4ꢀdione (compound 5)
was synthesized according to our published literature.27
The aqueous solutions of anions (F−, CN−, Cl−, Br−, I−, AcO−, HPO42−, H2PO4 ,
−
−
2ꢀ
3ꢀ
NO3 , SCN−, CO3 , PO4 , S2ꢀ) were prepared from their tetrabutylammonium or
sodium salts with distilled water. 1 was dissolved in THF at room temperature to
afford the stock solution (10ꢀ3 M). Test solutions were prepared by placing 1 mL of 1
stock solution into a 100 mL volumetric flask, adding an appropriate aliquot of each
anion stock and 7 mL distilled water, and diluting the solution to 100 mL with THF.
The resulting solution was shaken well before recording the absorption and emission
spectra.
2.2. Computational methods
The groundꢀstate geometry optimization of 1 and its receptorꢀadditive form
(1ꢀ2CN−) were performed on density functional theory (DFT) with the
B3LYP/6ꢀ31G(d) level of the Gaussian 03 program. The UVꢀvis absorptions of
receptor 1 and its deprotonated species were calculated with the B3LYP/6ꢀ31G(d)
level of the Gaussian 03 program based on the optimized groundꢀstate geometry.
2.3. Synthesis of receptor 1
113.6 mg (0.2 mmol) coumpond 5 and 198.2 mg (1 mmol) 2,4ꢀdinitrophenylhydrazine
were dissolved in 50 mL ethanol and six drops acetic acid was added to start the
reaction, then the resulting solution was heated and refluxed for 4 h. Dark red
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precipitate was filtered and washed twice with hot ethanol (2 × 10 mL). Finally, 166.3
mg dark red solid in 87.4% yield was obtained. m.p. 281 °Cꢀ282 °C. 1H NMR (CDCl3,
400 MHz, δ, ppm):11.40 (s, 2H), 9.15 (s, 2H), 8.38 (d, 2H, J=8 Hz), 8.17 (s, 2H), 8.11
(d, 2H, J=8 Hz), 7.93 (s, 8H), 3.79 (t, 4H，J= 8 Hz), 1.57ꢀ0.77 (m, 30H). 13C NMR
(100 MHz, δ, ppm): 160.05, 143.91, 138.35, 134.65, 131.05, 128.67, 123.84, 122.11,
115.59, 108.47, 43.02, 32.81, 30.59, 28.92, 24.93, 14.53. HRMSꢀESI: m/z calcd (%)
for C48H52N10O10: 928.3868 [M+Na]+; Found: 951.3760.
3. Results and discussions
3.1. Design and Synthesis of receptor 1
Although several literatures utilized hydrogen bonding approach based on DPP for
detection of fluoride ion in organic solvents,28ꢀ30 however, a cyanide receptor based on
DPP that operates in aqueous solvent mixture has never been reported. The problems
are mainly related to poor interaction between the receptor and cyanide anion due to
its higher Gibbs energy of hydration. With this in mind, we decided to introduce
electron withdrawing 2,4ꢀdinitrophenyl and Schiff base groups, as well as
electronꢀdeficient DPP moiety to enhance activity of NꢀH based on receptor 1. Thus,
it is anticipated that receptor 1 should be highly reactive and selective to CN− from
aqueous media.
A convenient synthetic route to receptor 1 was developed as shown in Scheme 1.
2,5ꢀDioctylꢀ3,6ꢀbis(4’ꢀformylphenyl)pyrrolo[3,4ꢀc]pyrroleꢀ1,4ꢀdione (compound 5)
was prepared from pꢀcyanobenzaldehyde as a starting material in four steps. Receptor
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1 was obtained by a straightforward condensation reaction of compound 5 with
2,4ꢀdinitrophenylhydrazine in 87.4% yield as a dark red solid. The 1H NMR spectrum
of receptor 1 showed the distinctive signals at 11.4 ppm corresponding to the
resonance of hydrazone NꢀH adjacent to C=N bond (ꢀCH=NꢀNHꢀ). The 1H NMR, 13C
NMR and HRMS spectra of receptor 1 are shown in Figs. S1ꢀS3.
3.2 Spectroscopic responses of 1 to anions
A hydrazone (ꢀCH=NꢀNH) had the tendency for binding of anions, and DPP moiety
was an efficient fluorophore. Therefore, the combination of the hydrazone and DPP in
a single molecular framework was very useful to create a probe by following the
changes in the spectral properties.
The anion binding abilities of the receptor 1 with F−, CN−, Cl−, Br−, I−, AcO−,
HPO42−, H2PO4 , NO3 , SCN−, CO3 , PO4 , S2ꢀ in aqueous media were studied
through UVꢀvis, fluorescence spectra and naked eye color changes. The anions tested
were used as their tetrabutylammonium or sodium salts in distilled water while
receptor 1 was taken as its 1.0×10−5 M THF/H2O (93/7, v/v) solution.
−
−
2ꢀ
3ꢀ
Receptor 1 showed two major absorption peaks at 396 nm and 516 nm. Upon
addition of 6 equiv of various anions (F−, CN−, Cl−, Br−, I−, AcO−, HPO42−, H2PO4 ,
−
−
2ꢀ
2ꢀ
NO3 , SCN−, CO3 , PO4 , S2ꢀ) to 1, there were no significant UVꢀvis spectral change
in the nature of the spectra except for CN−, as shown in Fig. 1a. It was interesting to
observe that upon interaction of 1 with CN−, two new absorbance peaks emerged at
462 and 646 nm. Meanwhile, the absorbance peaks at 396 and 516 nm disappeared.
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However, addition of AcO− and F− only gave minor response to 1.
The difference in 1 response to anions could thus be used to develop a colorimetric
assay. The color change from pink to indigo occurred very rapidly (within 10 seconds),
which was sufficiently distinct to be discriminated from other anions through naked
eye itself. The corresponding naked eye changes were present in Fig. 1b.
A titration of 1 with CN− indicated that there was a gradual increase in the
absorbance at 462 and 646 nm upon increment of the CN− concentration, as shown in
Fig. 2. At the same time, the absorbance peaks at 396 and 516 nm disappeared. Two
clear isosbestic points at 315 and 439 nm were observed during the titration process,
indicating formation of intermediates species with the receptor 1. The absorption at
646 nm began to increase significantly and reached the limit value after 6 equiv of
CN− were added, indicating that a CN−ꢀinduced deprotonation of hydrazone NꢀH had
occurred. This was confirmed by adding OH− (as Bu4NOH) to the solution of receptor
1, which gave similar UVꢀvis spectral changes (Fig. S4) to those observed with CN−
ion.
Receptor 1 was strongly fluorescent with maximum emission band at 601 nm upon
excitation at 516 nm. Upon addition of 5 equiv. CN− to the solution, the fluorescence
quenching of 92% was observed and the emission peak underwent a minor blue shift
from 601 nm to 593 nm, suggesting that the new species is much less luminescent
than the original one (Fig. 3). This may be ascribed to the NꢀH···CN hydrogen
bonding, which enhanced the electronꢀdonating ability of N atom and facilitated the
intramolecular photoinduced electron transfer, resulting in fluorescence turnꢀoff. But
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addition of Cl−, Br−, I−, HPO42−, H2PO4 , NO3 , SCN−, CO3 , PO4 , S2ꢀresulted in
insignificant changes in the fluorescent spectra (Fig. 4a). In the case of AcO− and F−
anions, fluorescence quenching rates of 15% and 18% were obtained, respectively.
However, these changes were smaller in magnitude when compared to CN−. Thus, the
significant change of 1 with a clear emission color change from red fluorescent
solution to nonfluorescent solution in presence of CN− was present, an agreement
with the appearance of fluorescence quenching of 92% in the corresponding
fluorescence spectra. In the presence of the other anions, no changes in fluorescence
were observed. The corresponding naked eye fluorescence changes were present in
Fig. 4b. All these findings suggested that 1 allowed selective and sensitive CN−
detection over other anions by both colorimetric and fluorometric analyses.
−
−
2ꢀ
2ꢀ
It is well known that cyanide ions have high solvation energy in water (ꢂGh = −295
kJ/mol). However, 1 had no interaction with other anions that have a comparable
hydration energy (ꢂGh = −365, −340, −315, and −300 kJ/mol for OAc−, Cl−, Br−, and
−
NO3 , respectively) to cyanide anion. Here comes a question: Why does 1 only
respond to CN− in water? The much stronger basicity of CN− in water may be the
main reason. The pKa value of HCN in water (9.30) is much higher than that of other
acid (pKa = 4.75, 3.17 for HOAc and HF in water, respectively). When the strong
basic CN− anion gets close to the acidic NꢀH fragment of 1, hydrogen bonding
interaction occurs, leading to a partial proton transfer from 1 to CN−, an increase of
electron density on DPP center, and redꢀshifts of the absorption and decrease of
emission intensity. The excellent selectivity of 1 toward CN− can be attributed to the
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fitness in the activity of its NH groups in 1, which is tuned to be able to distinguish
the subtle difference in the affinity of CN− to proton in water.
To test whether 1 can detect CN− selectively even in the presence of other anions,
competitive anion titrations were carried out. 1 was treated with 6 equiv of CN− in the
presence of 6 equiv of all other anions. The results indicated miscellaneous
competitive anion did not lead to any significant spectral change and CN− ions still
resulted in the similar fluorescence quenching in the presence of competitive anions,
as shown in Fig. S5. Thus, the selectivity of 1 toward CN− was not affected by the
presence of other anions. All the above results clearly indicated that 1 could be used
as a new chemosensor for CN− by the colorimetric and fluorescence dual mode.
For determination of stoichiometry between 1 and CN−, Job’s plot analyses were
used. The method is that keeping total concentration of 1 and CN− at 10.0 ꢁM, and
changing the molar ratio of CN− (XM; XM = [CN−]/{[1] + [CN−]}) from 0.1 to 0.9.
From Fig. S6, when molar fraction of CN− was 0.3, the I0ꢀI value got to maximum,
indicating that forming a 1:2 complex between 1 and CN−.
Moreover, the binding stoichiometry and the binding constant to CN− were
determined by BenesiꢀHildebrand double reciprocal method following Eq. (1). Here I0
and Imin are the fluorescence intensities at zero and the maximum concentrations of
CN−, [CN−] is the total CN− concentration, Kb is the binding constants for 1:2 binding
mode. For receptor 1, a good linear fit could be obtained by Eq. (1), indicative of the
binding stoichiometry of 1:2, which conformed to the presence of two NꢀH groups in
receptor 1 (Fig. S7). The binding constant was found to be 2.02×1010 Mꢀ2. In addition,
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limits of detection (LOD) were determined from the equation LOD = K× Sb1/S, where
K=3, Sb1 is the standard deviation of the blank solution and S is the slope of the
calibration curve (Fig. S8). The obtained result was 0.23 ꢁM for CN− with 1 using
emission measurements.
1
1
1
=
+
K(I0 − Imin )[CN ꢀ ]2
I0 − I
I0 − Imin
(1)
Active materialsꢀbased test strips represent a group of convenient probing substrate
for practical utilization. ꢀbased test strip was thus fabricated by immersing filter
paper into the THF solution of
(1.0 × 10−3 M) and drying in air, which was energyꢀ
1
1
and costꢀeffective. The corresponding probing experiments were carried out
subsequently. The results indicated that this protocol really took effect. The obvious
color change from pink to indigo was observed by immersing this test strips in
solution of CN− , exhibiting colorimetric changes differentiable to naked eyes (Fig.
S9).
3.3 1H NMR studies
1
The cyanide ion binding to receptor
1
was also monitored by H NMR titration
1
studies in CDCl3. The changes in H NMR signals of
1 upon addition of increasing
amounts of cyanide ion were shown in Fig. 5. The signal of proton on the hydrazone
NꢀH at 11.4 ppm disappeared entirely upon addition of 2 equiv of cyanide owing to
the hydrogen bonding between this proton and cyanide. The deprotonation of the
hydrazone NꢀH group was supported by the fact that aromatic protons showed distinct
upfield shifts when 2 equiv of cyanide was introduced compared with the free
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receptor arising from an overall change of the electron distribution in the
chromophore when the hydrazone NꢀH group was deprotonated.
3.4 DFT calculations
To gain insights into spectroscopic responses of
1 cyanide anions, receptor 1 and
the corresponding deprotonated species were also examined by DFT calculations at
the B3LYP/6ꢀ31 G(d) level of the Gaussian 03 program. The optimized structures of
receptor
ring was tilted by about 74.3
hydrazone (ꢀCH NꢀNH) and phenyl spacer was 41.3
phenyl ring was tilted by about 82.8 toward the DPP core, and the hydrazone
moiety was almost coplanar with the phenyl spacer moiety with twisted angle 3.4
The bathochromic shift of maximum absorption of receptor when binding with
cyanide could further be understood in terms of decrease in the potential energy of its
HOMO and LUMO. The HOMO and LUMO orbitals of receptor and deprotonated
species were shown in Fig. 7. For receptor , the HOMO was distributed on the DPP
1
and deprotonated species were shown in Fig. 6. For receptor
toward the DPP core and the twisted angle between
. For deprotonated species, the
1, the phenyl
°
=
°
°
°
.
1
1
1
core, hydrazone moiety and 2,4ꢀdinitrophenyl moiety, while the LUMO was more
distributed on the 2,4ꢀdinitrophenyl and hydrazone moiety. For the deprotonated
species, the HOMO was mostly located on both the hydrazone moiety and
2,4ꢀdinitrophenyl moiety, while the LUMO was more distributed on both the DPP
core, 2,4ꢀdinitrophenyl and the hydrazone moiety. As described above that the LUMO
was the electronꢀdeficient, and cyanide would prefer it more rather than the HOMO.
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Hence, on binding with cyanide, the potential energy of the LUMO was raised to
comparatively lesser extent than that of the HOMO, which was less preferred by
cyanide. This led to narrowing of the energy gap between HOMO and LUMO (3.37
eV and 2.76 eV for
ultimately responsible for the bathochromic shift of maximum absorption of receptor
on binding with cyanide. This indicated that the binding of cyanide with receptor
1 and its deprotonated species, respectively), which was
1
1
made a polar due to the homogenization of electron density through extended
πꢀconjugation. In other words, the binding of cyanide introduced a high extent of
planarity into the receptor 1, which led to the homogeneous distribution of electron
density throughout. This phenomenon had been well documented in the literature as
planar intramolecular charge transfer. The 1H NMR studies also indicated similar type
of electron density changes crossing the receptor
signals and merging of a few of them.
1 in form of upfield shifting of the
3.5 Reversibility and Reusability of the receptor
1
Most of the CN− ion sensors available in the literature are irreversible and can be
used only one time. It is very beneficial if a probe can be reversible in nature and be
reusable for sensing selective anion. To test the reversibility and reusability of
carried out systematic titration studies of
ꢀCN− complex upon addition of increasing
amounts of trifluoroacetic acid (TFA) by absorption and fluorescence techniques. The
absorption spectral titration of
ꢀCN− complex with increasing addition of TFA (0ꢀ2
1, we
1
1
equiv) in aqueous solution is shown in Fig. 8. Addition of increasing equivalents of
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TFA to a solution of
1
ꢀCN− complex resulted in the significant increase in the
absorption bands at 396 and 516 nm, whereas two absorbance peaks at 462 and 646
nm were greatly decreased. It was clearly shown that only addition of 2 equiv of TFA
to
findings supported protonation of the NꢀH negative ion. Similarly, we also monitored
the systematic titration studies of
ꢀCN− complex upon addition of increasing
equivalents of TFA by fluorescence studies. Upon addition of increasing amounts of
TFA to
ꢀCN− complex, the fluorescence emission gradually increased, and emission
1 1. These
ꢀCN− complex was needed to go back to the initial spectrum of original
1
1
peak underwent a minor red shift from 593 nm to 601 nm. Finally, 34ꢀfold
enhancement was observed on addition of 2 equiv of TFA (Fig. 9). Thus, upon CN−
ion addition, the NꢀH moiety was electronꢀrich via deprotonation and engaged in
photoinduced electron transfer quenching of the DPP excited state. But, upon titration
with TFA, the photoinduced electron transfer quenching was prevented because of
protonation of NꢀH negative ion. So, the absorption and fluorescence properties were
reverted (Scheme 2). The reversible and reusable response of
1 was demonstrated by
carrying out four alternate cycles of titration of
1
with CN− ion followed by addition
of TFA by absorption measurement (Fig. 10). The repeated demonstration of visual
color changes from pink to indigo and then back to pink was shown in Fig. 11,
demonstrating the reversibility and reusability of probe 1. Herein, in the presence of 2
equiv TFA, the color change from indigo to pink and fluorescence recovery occurred
very rapidly (within 10 seconds), indicating high efficiency was the unique feature of
1
.
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3.6 The reversible colorimetric switch with complementary“IMP/INH” logic
functions
As discussed above, concomitant with a color change from pink to indigo, a drastic
redꢀshift of 130 nm was observed in the UVꢀvis spectra of receptor
1 upon cyanide
binding. More interestingly, we found that the cyanideꢀinduced chromogenic process
is totally reversed with TFA (see Figs. 8 and 9). The eyeꢀdetected change was clearly
reflected in the UVꢀvis spectra, in which the addition of 2 equiv of TFA results in
vanishing of the band at 646 nm and reappearance of the absorption at 516 nm (Fig.
4). The reversible pinkꢀindigoꢀpink cycle could be repeated for several times by
alternating addition of 6 equiv of cyanide and 2 equiv of TFA. However, this
reversible chromogenic switching process did not occur when TFA, instead of cyanide,
is previously added to the solution. A set of comparison experiments demonstrated
that the successive addition of 2 equiv of TFA and 6 equiv of cyanide does not
produce
that the presence of TFA would inhibit the interaction of cyanide with the sensor
since TFA is a more acidic species compared to . As elucidated in Scheme 2, the
interaction of TFA with the complex ; meanwhile
ꢀ2CN− reproduces the receptor
1 any change in both its color and absorption spectrum. The results indicate
1,
1
1
1
the released cyanide ions could be converted into other species. This also explains
why the colorimetric switch can work in a reproducible manner, despite the fact that
the added cyanide and TFA are not of equal molar amount in one single cycle. The
reversible and reproducible colorimetric switching process may be represented by a
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molecular “INHIBIT” logic gate, employing CN− (InC) and TFA (InF) as the inputs
and the absorbance at 646 nm as the output. When using the absorbance at 516 nm as
another output, an “IMPLICATION” logic gate is fabricated. In this way, a
complementary IMP/INH logic function can be realized based on the receptor
1, as
shown in Fig. 12. Thus, the color changes of in THF (optical output) are controlled
1
by the input of two anions: CN− “switches” ON the optical output, while TFA
“switches” OFF the optical output. By alternately adding CN− (6 equiv) and TFA (2
equiv.) into the receptor solution, a reversible colorimetric switch could be created in
a reproducible manner (Fig. 12). To the best of our knowledge, this kind of reversible
and reproducible switch is of great interest for molecularꢀlevel information processing.
In the field of information technology, the switching process must be based on a
reversible chemical process to perform any useful calculation.31 Therefore, the present
logic device has a great advantage over early reported relative systems at least in
terms of the reversible and reproducible characteristics.
4. Conclusions
In summary, a new colorimetric and onꢀoff fluorescence receptor
diketopyrrolopyrrole (DPP)ꢀhydrazone conjugate for cyanide anion detection in
aqueous media was designed and synthesized. Receptor displayed a rapid response,
1 based
1
high selectivity and sensitivity toward cyanide over other anions, with a significant
130 nm bathochromic shift of the absorption maximum. Meanwhile, it could be easily
observed that the receptor
1
for cyanide ion changed from pink to indigo by the naked
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eye, and from fluorescence red to nonꢀfluorescence dark under UV lamp immediately
after the cyanide ion was added. The experimental results revealed that the formation
of anionic species from deprotonation of the hydrazone moiety by cyanide ion was
responsible for the spectral changes. Density function theory calculations were
conducted to rationalize the optical response of the receptor
1. In addition, receptor
1
has been used to construct the molecularꢀlevel
complementary
“IMPLICATION/INHIBIT” logic gate employing CN− and TFA as the inputs. The
present work provides deep insights for the future design of anions sensors as
multifunctional logic devices.
Acknowledgements
The supports by National Natural Science Foundation of China (No. 21274045),
the Fundamental Research Funds for the Central Universities (2015ZZ037) and the
Natural Science Foundation of Guangdong Province (2015A030313209,
2016A030311034) are gratefully acknowledged.
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Scheme 1. Synthesis of receptor 1.
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Scheme 2. Proposed interaction mode of receptor
1
with CN− and TFA.
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1.0
0.8
0.6
0.4
0.2
0.0
Receptor+Cyanide
Receptor+other ions
-
-
Receptor+F /AcO
300
400
500
600
700
800
Wavelength (nm)
(b)
Figure 1. (a) The absorption spectra and (b) color changes of receptor
THF/H2O (93/7, v/v) upon addition of 6 equiv of various ions.
1 (10 ꢁM) in
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1.0
0.8
0.6
0.4
0.2
0.0
646 nm
6 eq
0 eq
396 nm
516 nm
462 nm
315 nm
439 nm
300
400
500
600
700
800
Wavelength (nm)
Figure 2. The absorption spectra of receptor
1 (10 ꢁM) in THF/H2O (93/7, v/v) with
the addition of different equivalents of cyanide ions.
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1400
1200
1000
800
600
400
200
0
601 nm
0 eq
5 eq
600
Wavelength (nm)
650
700
Figure 3. The emission spectra of receptor
1 (10 ꢁM) in THF/H2O (93/7, v/v) with the
addition of different equivalents of cyanide ions (λ ex=516 nm).
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1400
1200
1000
800
600
400
200
0
receptor
receptor+other ions
receptor+cyanide
600
700
Wavelength(nm)
(b)
Figure 4. The fluorescence spectra of receptor
1 (10 ꢁM) in THF/H2O (93/7, v/v)
solution upon addition of 5 equiv of various ions (λ ex= 516 nm).
29