A highly sensitive and selective novel fluorescent chemosensor for detection of Cr3+ based on a Schiff base

Article abstract of DOI:10.1016/j.ica.2017.03.041

N,N′-bis(salicylidene)-2-(6-(2-aminophenyl)-4-phenylpyridin-2-yl) (3) was synthesized by condensation reaction of 2,6-bis(2-aminophenyl)-4-phenylpyridine and 2-hydroxybenzaldehyde in EtOH and characterized by its melting point, ¹H, ¹³

Full text of DOI:10.1016/j.ica.2017.03.041

Accepted Manuscript
Research paper
A highly sensitive and selective novel fluorescent chemosensor for detection of
3
+
Cr based on a Schiff base
Gholam Babaei Chalmardi, Mahmood Tajbakhsh, Ahmadreza Bekhradnia,
Rahman Hosseinzadeh
PII:
S0020-1693(17)30465-6
http://dx.doi.org/10.1016/j.ica.2017.03.041
ICA 17501
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Accepted Date:
24 March 2017
28 March 2017
Please cite this article as: G.B. Chalmardi, M. Tajbakhsh, A. Bekhradnia, R. Hosseinzadeh, A highly sensitive and
3
+
selective novel fluorescent chemosensor for detection of Cr based on a Schiff base, Inorganica Chimica Acta
2017), doi: http://dx.doi.org/10.1016/j.ica.2017.03.041
(
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A highly sensitive and selective novel fluorescent chemosensor for detection of
3
+
Cr based on a Schiff base
a
a
b*
Gholam Babaei Chalmardi , Mahmood Tajbakhsh *, Ahmadreza Bekhradnia , Rahman
Hosseinzadeh
a
a
Department of Organic Chemistry, Faculty of Chemistry, University of Mazandaran, Babolsar 47416-95447, Iran
b
Pharmaceutical Sciences Research Center, Department of Medicinal Chemistry, Mazandaran University of
Medical Sciences, Sari, Iran
E-mail: Mahmood.Tajbakhsh32@gmail.com, abekhradnia@gmail.com
Graphical Abstract
N
O
CHO
N
Cr3+
N
N
NH4OAC
N
N
Cr3+
CH
HC
CH
NO2
HC
OH HO
OH HO
Fluorescence
OFF
Fluorescence
ON
(
PET ON)
(PET OFF)
Highlights
of efficient Schiff base 3
3
+
Theoretical study together with Job’s plot method showed 1:1 binding ratio of Cr with Schiff
base 3.
4
-1
3+
It has high affinity (K =8.77×10 M ) and selectivity for Cr .
a
1

3
+
-7
The detection limit of sensor (3) towards Cr is 2.2×10 M.
Abstract
N,N’-bis(salicylidene)-2-(6-(2-aminophenyl)-4-phenylpyridin-2-yl) (3) was synthesized by
condensation reaction of 2,6-bis(2-aminophenyl)-4-phenylpyridine and 2-hydroxybenzaldehyde
1
13
in EtOH and characterized by its melting point, H, C-NMR and molecular mass. The addition
3
+
of Cr ion makes the significant increase of its fluorescent intensity at 537 nm in CH CN/H O
3
2
(95/5%), however, other metal ions have almost no influence on the fluorescence. The reason for
3
+
this phenomenon might be attributed to the formation of a 1:1 stoichiometric 3-Cr complex
which inhibits photo-induced electron transfer (PET) process. The 1:1 binding stoichiometry
3
+
between 3 and Cr was established using Job’s plot, and the complex structure was proposed by
4
-1
DFT calculation. The association constant (K ) was determined to be 8.77×10 M and the limit
a
-7
of detection (LOD) was calculated to be 2.2×10 M.
Keywords: Fluorescence, Chemosensor, Chromium ion, DFT, PET, Schiff base
1
. Introduction
Complexation of metal ions with specific ligands causing an induce changed in
fluorescence are particularly attractive for development of fluorescent chemosensors for metal
ions, due to their simplicity, high sensitivity, and rapid response [1]. Selective detection of
chromium ion is very important as it is an essential component of a balanced human and animal
diet, amounting 50-200 mg per day [2]. Excessive chromium reported to cause genotoxic
whereas deficiency of chromium can increase the risk factors associated with diabetes and
cardiovascular diseases [3]. Furthermore, chromium is one of the most notorious environmental
pollutants [4]. Therefore developments of selective chemosensors for chromium ion are
2

interesting in environmental and biological fields. Many analytical methods such as X-ray,
3
+
HPLC, DPP and ICP-AES have been applied for determination of Cr ion, but they are usually
of high cost and inconvenient for routine analysis. Thus, it is attractive to introduce a more
3
+
convenient, faster, and lower cost method for trace Cr detection, for instance, analyzing by ion-
selective sensors [5-11]. Formation of stable complexes of Schiff base with transition metal ions
and their application as ion carriers are well known. The host-guest complexion of Schiff bases
with metal ions can produce remarkable selectivity, sensitivity and stability for a specific ion.
Schiff bases with N, O or S as donor atoms form strong complexes with transition metal ions that
can be used as the ionophore in optical sensors for detection of various cations. Schiff base
complexes have been widely used in the area of ionic binding, as these binding sites can
recognize the metal ions [12-19]. Several publications have reported the use of Schiff base
ligands for optical sensing of cations [20-25]. Different mechanisms have been proposed for
designing fluorescent chemosensors including chelation-enhanced fluorescence (CHEF) effect,
excited state intermolecular proton transfer (ESIPT), photoinduced proton transfer (PPT), metal
to ligand charge transfer (MLCT), internal charge transfer (ICT), C=N isomerization and
3
+
photoinduced electron transfer (PET) [26-31]. As, Cr has been known as one of the most
efficient fluorescence quenchers [32, 33], its detection via chelation enhanced fluorescence
(CHEF) mechanism is an attractive task. Furthermore, due to the lack of selective ionophore,
3
+
only few turn-on fluorescent sensors for Cr have been reported [34].
we report a new
fluorescent probe with mixed nitrogen, oxygen donor sites as fluorophore 3 exhibiting a distinct
3
+
3+
fluorescence enhancement in the presence of Cr which forming a 1:1 3-Cr complex which
can inhibit photo-induced electron transfer (PET) process. Other metal ions do not induce
significant fluorescence changes.
3

2
. Experimental
2
.1. General and reagents
1
13
Melting points were measured on an Electrothermal 9100 apparatus. H and C spectra
were measured with Bruker DRX-400 AVANCE spectrometer at 400.1 and 100.6 MHz,
respectively. Mass spectra were obtained on an Agilent 5975c spectrometer operating at 70 eV.
The UV-Vis spectra were obtained using a Perkin-Elmer lambda-EZ 201 and a Jasco FP-200
spectrafluorometer was used to record fluorescence emission spectra. Data were recorded on-line
and analyzed by Excel software on a PC computer. Fluorescence intensity measurements were
performed at room temperature in solvent CH CN/H O (95/5%).
3
2
2
2
.2. Synthesis
.2.1. Synthesis of 2,6-bis(2-nitrophenyl)-4-phenylpyridine(1)
This compound was synthesized according to reported Procedure [35]. Typically in a
round-bottomed flask (250 mL) equipped with a reflux condenser, a mixture of benzaldehyde
6.4 g, 0.06 mol), 2′-nitroacetophenone (20 g, 0.12 mol), ammonium acetate (60 g), and glacial
(
acetic acid (150 ml) was refluxed for 2 h. Upon cooling, crystals separated, which were filtered
and washed first with acetic acid (50%) and then with cold ethanol. The dark yellow crystals
were recrystallized from absolute ethanol, and dried at 60 ºC under vacuum (Scheme1). Yield:
◦
1
6
6%, mp 188-191 C, H NMR (400 MHz, CDCl ), δ, ppm: 7.94 (2H, d, J =8Hz), 7.72-7.74 (3H,
3
m), 7.70-7.71 (m, 2H), 7.68-7.69 (3H, m), 7.56-7.60(2H, m), 7.51-7.536(2H, m), 7.51-7.53 (2H,
1
3
m). C NMR (100 MHz, CDCl ), δ, ppm: 155.82, 150.43, 149.16, 137.60, 134.99, 132.73,
3
1
31.68, 129.53, 129.46, 129.25, 127.31, 124.42 and 120.16. The mass spectrum showed the
molecular peak at m/z = 397.1 corresponding to compound 1(Fig. S1-S2).
4

2
.2.2. Synthesis of 2,6-bis(2-aminophenyl)-4-phenylpyridine(2)
In a two-necked round-bottomed flask (500 ml) equipped with a reflux condenser and a
dropping funnel, a suspension of 2,6-bis(2-nitrophenyl)-4-phenylpyridine (13.75 g, 0.033 mol),
palladium on carbon 5% (1.4 g), and ethanol (500 mL) was prepared. The mixture was warmed,
and while being stirred magnetically, hydrazine hydrate 85% (35 ml) in ethanol (50 ml) was
added dropwise over 1.5 h period through the dropping funnel while maintaining the temperature
at about 50 ºC. The reaction mixture was then refluxed for 2 h and filtered while hot. On cooling,
the filtrate gave white-cream colored crystals of the title diamine compound, which was
◦
1
recrystallized from ethanol and vacuum dried (Scheme1). Yield: 68%, mp 167-169 C, H NMR
400 MHz, CDCl ), δ, ppm:7.76 (2H, d, J=2.4Hz), 7.73 (2H, d, J=1.2Hz), 7.62 (2H, dd, J=1.6Hz,
(
3
J=7.2Hz), 7.53-7.57 (2H,m), 7.49-7.51 (1H, m), 7.23 (2H, Td, J=1.6Hz, J=7.6Hz), 6.85 (2H, Td,
1
3
J=0.8 Hz, J=7.6 Hz), 6.8(2H, dd, J=0.8, J=8.4Hz), 5.41(4H, s, NH2). C NMR (100 MHz,
CDCl ), δ, ppm: 158.22, 150.67, 146.10, 138.90, 129.98, 129.87, 129.15, 129.06, 127.18,
3
1
3
2
23.18, 118.64, 117.81 and 117.032. The mass spectrum showed the molecular peak at m/z =
36.1 corresponding to compound 2 (Fig. S3-S4).
.2.3. N,N’-bis(salicylidene)-2,6-bis(2-aminophenyl)-4-phenylpyridine(3)
A solution included 2,6-bis(2-aminophenyl)-4-phenylpyridine(1 mmol) in absolute
ethanol was added to an ethanol solution of 2-hydroxybenzaldehyde (2 mmol). The mixture was
refluxed for 8 h and then cooled to room temperature. Then, the solvent was evaporated and the
◦
1
yellow product was recrystallized from ethanol (Scheme1). Yield: 88%, mp 221-224 C, H
NMR (400 MHz, CDCl ), δ, ppm:12.97 (2H, s), 8.69 (2H, s), 7.76 (2H, dd, J=1.2Hz, J=7.6Hz),
3
7
.69 (2H, s), 7.63-7.65 (2H, m), 7.39-7.47 (6H, m), 7.34-7.37 (3H, m), 7.22–7.26 (4H, m), 6.97
13
(
3H, d, J=8Hz), 6.94 (1H, d, J=0.8Hz). C NMR (100 MHz, CDCl ), δ, ppm: 163.72, 160.97,
3
5

1
1
57.21, 148.78, 147.03, 138.66, 135.01, 133.23, 132.39, 131.22, 129.67, 128.86, 128.67, 127.48,
26.90, 121.69, 119.43, 119.16, 119.05 and 117.22. The mass spectrum showed the molecular
peak at m/z = 545.3 corresponding to compound 3 (Fig. S5-S6).
O
CHO
acetic acid
NH OAC
4
NO2
reflax
N
NO2 O N
2
1
Pd/C, NH NH2
2
N
N
EtOH
NO2 O N
NH2 H N
2
2
2
CHO
OH
N
EtOH
r.t
2
N
N
N
CH
HC
NH2 H2N
OH HO
3
Scheme. 1. The synthetic route for compound 3.
2
.3. Spectrometric procedure
The stock solutions of nitrate salts (10 µM) of Na , Ag , K , Ca , Pb , Hg , Mn ,
+
+
+
2+
2+
2+
2+
2
+
2+
2+
2+
3+
3+
3+
Co , Cu , Zn , Cd , Fe ,Cr and Al in CH CN/H O (95/5%) were prepared. Another
3
2
solution comprises compound 3 was dissolved in acetonitrile (10 µM). Then 2 mL of each (2mL
of compound 3+2 mL of nitrate salt solution) were added to the measurement cell. For all
measurements, excitation wavelength was at 273 nm at room temperature.
6

3
. Results and discussion
3
.1. Spectral studies
Fig. 1 and 2 show changes in the absorption and fluorescence spectra in CH CN/H O (95/5%) of
3
2
3
, upon the addition of various cations salts. The absorption spectrum of 3 (10 µM) in
CH CN/H O (95/5%) solution exhibits three bands at 212 nm, 273 nm and 333 nm. The
3
2
3
+
absorption intensity of compound 3 decreased with the addition of various metal ions e.g. Cr ,
+
+
+
2+
2+
2+
2+
2+
2+
2+
2+
3+
3+
Na , Ag , K , Ca , Pb , Hg , Mn , Co , Cu , Zn , Cd , Fe , Al . The cations studied led
to hypochromic effects in absorbance in the UV-Vis spectra of 3 (Fig. 1). By UV-Vis analysis, it
was difficult to identify specific cations over other metal ions because of the ambiguous
3
+
absorption spectra [36]. Therefore, to investigate the selectivity of 3 for Cr in the presence of
other metal ions, the fluorescence response of 3 toward various metal ion solutions (10 µM) was
studied in CH CN/H O (95/5%) (Fig. 2)
3
2
a) Schiff base 3
b)
+
+
+
2+
2+
2+
2+
Na ,K ,Ag ,Pb ,Cd ,Zn ,Hg ,
Mn ,Al ,Ca ,Co ,Fe , Cu
2
+
3+
2+
2+
3+
2+
3+
c) Cr
Fig. 1. Absorption spectra of Schiff base 3 (10 µM) before (a) and after addition of heavy metal mixtures (2 equiv)
3
+
3+
in the absence of Cr (b) and heavy metal mixtures in the presence of Cr in CH CN/H O (95/5%) solution. (c).
3
2
The fluorescence emission intensity of 3 was recorded at 537 nm when it was excited at
3
+
2
73 nm. Upon addition of Cr , 3 shows a large fluorescence enhancement due to the formation
3
+
of a 1:1 3-Cr complex which inhibits photo-induced electron transfer (PET) process [37-39].
7

+
+
+
2+
2+
2+
2+
2+
2+
2+
2+
3+
Other metal ions including Na , Ag , K , Ca , Pb , Hg , Mn , Co , Cu , Zn , Cd , Fe
3
+
and Al have almost no influence on the fluorescence, which indicated the Schiff base 3 that is
3
+
a highly selective fluorescent sensor for Cr .
a)
b)
Fig. 2. (a) Fluorescence responses of compound 3 (10 µM) in CH CN/H O (95/5%) solution upon the addition of
3
2
various metal ions (λex = 273nm, 2 equiv). (b) Bar graph representing the change of the relative emission intensity
of 3 at 537 nm upon treatment with various metal ions (λex = 273 nm).
3
+
The addition of Cr ion to a mixture of 3 and the other potentially competitive metal ions
mentioned above in a CH CN/H O (95/5%) solution, led to an enhancement of fluorescence
3
2
intensity (Fig. 3). These observations demonstrate that Schiff base (3) could be used as an
3
+
efficient and selective fluorescence chemosensor for Cr ion.
8

Fig. 3. Fluorescence intensity of 3 (10 µM) in the absence of metal ions (red curve), and presence of 2 equiv of all
kinds of competitive metal ions (blue curve) in CH CN/H O (95/5%, λex = 273 nm). The black curve represents the
3
2
3
+
addition of Cr to the above mixture. Each spectrum was acquired 1 minute after cations addition at room
temperature.
3
+
To further confirm the properties of receptor 3 as a sensor for Cr ion titrations were
3
+
performed by adding of Cr to the solutions of sensor 3 in CH CN/H O (95/5%) solvent (Fig.
3
2
4
).
3
+
Cr
3
+
Fig.4. Fluorescence titration of 3 (10 µM) with various concentration of Cr in CH CN/H O (95/5%) solution
(
3
2
3+
excitation = 273 nm) at room temperature. inset: changes of fluorescence upon addition of Cr (0-2 equiv) at
emission=537 nm.
9

The association constant (K ) of compound 3 with metal ions was determined using the
a
Benesi–Hildebrand equation [40], as follows:
F and F represent the fluorescent intensity of compound 3 in the presence and absence of
0
metal ions, respectively. Fmax is the saturated fluorescent intensity of compound 3 in the presence
of excess amount of metal ions that [M] is the concentrate ion of added metal ions. According to
3
+
Benesi–Hildebrand equation, the association constant between receptor 3 and Cr was calculated
4
-1
from the fluorescence titration result and was found to be 8.77×10 M . Moreover, the detection
limit was calculated based on the fluorescence titration. To determine the S/N ratio, the emission
3
+
intensity of the complex (3-Cr ) was measured and the standard deviation of blank
measurements was calculated. The detection limit was then calculated according to 3 × δb/m.
where δb was the standard deviation of blank solutions, and m was the slope between intensity
3
+
-7
vs. sample concentration [41]. The detection limit for Cr was calculated to 2.2×10 mol/L.
Fluorescence quantum yield was determined by comparing the emission and absorption
intensities of the probe with those of a fluorescence standard, fluorescein in 0.1 N NaOH [42].
For all fluorescence measurements, the excitation wavelength was 273 nm with excitation and
3
+
emissions slit widths of 3.0 nm. Quantum yield of 15.9% was calculated for the 3-Cr complex.
Nevertheless, low quantum yields were obtained for combined receptor and other cations as well
as free receptor and no worth to report.
3
.2. Binding mode studies
3
+
In order to determine the stoichiometry of the Schiff base 3 with Cr complex, the method of
3
+
continuous variation (Job’s plot) was used. The total concentration of the complex 3-Cr was
3
+
constant (10 µM), with a continuous variable molar fraction of guest ([3]/[ Cr ] + [3]). Fig. 5
1
0

3
+
3+
shows the Job plot of Schiff base 3 with Cr (at 537 nm), the 3-Cr complex concentration
3
+
3+
approaches a maximum when the molar fraction of Cr is 0.5, confirming that 3 and Cr forms
a 1:1 complex.
3
+
Fig. 5. Job’s plot for determining the stoichiometric complexation of 3 with Cr in CH CN/H O (95/5%) at room
temperature. The total concentration of 3 and Cr is 10 µM.
3
2
3+
3
+
1
To understand the complexation between the compound 3 and Cr , the H NMR titration
3
+
experiments were conducted by addition increasing concentrations of Cr upon the 3 solution.
As is seen in (Fig. 6), the OH signal (H ) of 3 appeared at δ 12.96 ppm. But after addition of 2.0
a
3
+
equiv of Cr to the 3 solution this signal was disappeared which means that phenolic OH group
may be deprotonated and this implies that the phenolic OH group is efficient on formation of
3
+
complex between the 3 and Cr . The imine proton (H ) of 3 at around δ 8.69 ppm decreased on
b
3
+
addition of Cr to the 3 solution. It indicates that the nitrogen atom of the Schiff base 3 may
3
+
participate in coordination with Cr ions. In addition, the protons of benzene rings were
decreased. For instance, H and H were affected due to complex formation. This observation
c
d
may imply that phenolic OH group and imine moiety efficient on the formation of complex
3
+
between 3 and Cr [43].
1
1

3
+
3
3
3
+ 2eq Cr
+ 1eq Cr
N
d
N
N
Cr3+
CH
HC
b
OH HO
3
+
a
c
3
+
+0. 5eq Cr
d
b
a
c
N=CH
1
Fig. 6. Partial H NMR spectra of 3 (10 µM) in CD CN at room temperatureand the corresponding changes after the
3
3+
addition of increasing amounts of Chromium nitrate (0-2 equiv). Each spectrum was acquired 1 minute after Cr
addition at room temperature.
3
.3. Theoretical study
The complex (3-Cr ) gave fine powders and we were not able to prepare single crystals
3
+
for structure determination by X-ray spectroscopy [25]. Instead, to get insight into the proposed
binding mode, full geometry optimizations were performed without any symmetry constraints by
means of hybrid functional B LYP and the 6–311G* basis set, employing the Gaussian package.
3
The optimized geometry and the highest occupied molecular orbital (HOMO) and lowest
3
+
unoccupied molecular orbital (LUMO) of 3 and its Cr complex are presented in (Fig. 7 and Fig.
S7-S8).
1
2

a)
LUMO
HOMO-4
b)
LUMO
HOMO-4
3+
Fig. 7. DFT Optimized structure of HOMO and LUMO orbitals of a) 3 and b) 3-Cr complex.
To attain a deeper insight into the UV-Vis analysis, the TDDFT calculations of free
ligand 3, was performed with the comparable studies. The obtained UV-Vis spectra by means of
TDDFT calculation were confirmed by experimental UV-Vis spectroscopy (Fig. S9). From the
3
+
Density Functional Theory (DFT) calculations, λmax of 3 and 3-Cr were obtained by 273 nm,
which are confirmed by the experimental results. The results of TDDFT have interesting
coincidence with the obtained UV-Vis achievements (Fig. S9 and Fig. 8).
1
3

Fig. 8. Molecular Orbitals of 3, HOMO (green) and LUMO (red).
In 3, the contributions of HOMO-2 to LUMO+2, HOMO-3 to LUMO+1, HOMO-4 to
LUMO transitions were 12.96%, 27% and 3.61%, respectively. Due to the importance of the
lone pair electrons of the nitrogen atom, we carefully analysed its contribution to the relevant
orbitals. It was found that the nitrogen lone pair electron belongs to HOMO-4 in 3. Thus,
HOMO-4→LUMO transition (3.61%) is responsible for the fluorescence quenching. On the
3
+
3+
other hand, in 3-Cr , the nitrogen atom has strong interaction with Cr d-orbital to produce
bonding type orbitals that block the PET process from the nitrogen lone pair electron to the
benzene, hence block the fluorescence quenching. The possibility of the restricted chelation-
3
+
enhanced fluorescence (CHEF) process of 3 on coordination to Cr could also contribute to the
fluorescence enhancement along with the PET process [44] (Scheme 2 and Fig. 9).
1
4

Ex=273
N
N
Cr3+
N
HC
N
N
N
Cr3+
CH
CH
HC
OH HO
OH HO
Em=537
Fluorescence
OFF
Fluorescence
ON
(
PET ON)
(PET OFF)
3
+
Scheme. 2. Proposed mechanism for detection of Cr by 3.
Fluorophore orbital
Nitrogen lone-pair orbital
hv
hv
hv'
Cr3+
3-Cr3+
3
Fig. 9. Proposed mechanism of fluorescence enhancement.
In order to show of proposed reliability our suggestion mechanism and to improve the
accuracy in estimating energy levels of occupancy orbitals, we have found all the related
molecular orbitals of ligand 3 using TDDFT theory. Our computational results showed the
+
3
fluorescence enhancement by the Cr could be rationalized in the sense of the HOMO-LUMO
gap. The output diagram of our calculation was depicted in Fig.8 it was seen, that energy gap
1
5

-19
between HOMO-4 and LUMO is 5 eV (8.0×10 j). Noticeably, in our experiments the
-19
excitation wavelength for ligand 3 was found 273 nm which related to 4.6 eV (7.3×10 j) gap.
Those results have showed that our experimental data are consistence with theoretical outcome.
Thus, the results of the DFT calculations further corroborate the proposed sensing mechanism.
3+
As depicted in Fig. 10 and Table 1, sensor 3 forms a 1:1 complex with Cr . Furthermore,
3
+
Cr has favourable interactions with the two oxygen atoms of hydroxyl groups and the two
nitrogen atoms of two amine groups and two oxygen atoms of a nitrate of the sensor [45, 46].
3
+
3+
Then, the structural parameters of 3 changed slightly upon addition of Cr and formed 3-Cr as
3
+
the dihedral angle N -O -N -O was 19.763 in 3 and 24.466 in 3-Cr . Moreover, the distance
2
6
42
25
41
between N and Cr is 2.149 Å which indicates that no bond is formed between two atoms.
1
0
43
(
a)
(b)
Fig. 10. Optimized structure of a) 3 and b) 3-Cr (III) by DFT.
1
6

Table 1
°
The selected bond lengths (Å) and angles ( ) for 3-Cr (III).
0
0
Bond length (Å)
Cr -N
Bond angle ( )
N -Cr -N
Dihedral angle( )
1.998
1.997
1.901
1.902
1.922
1.925
150.237
89.387
70.267
138.580
92.334
89.460
91.413
70.971
139.437
174.925
90.954
83.781
91.784
85.141
69.063
N -O -N -O
24.466
103.064
177.883
178.643
18.462
4
3
25
26
41
42
44
46
25
43
26
41
44
46
42
42
41
46
44
42
44
46
46
44
46
26
42
25
41
Cr -N
N -Cr -O
O -O -O -O
4
3
25
43
42 46 44
41
Cr -O
N -Cr -O
C -O -Cr -N
29 41 43
4
3
25
43
26
25
42
41
26
25
43
43
43
43
44
46
46
44
Cr -O
N -Cr -O
C -O -Cr -N
4
3
25
43
40 42 43
Cr -O
N -Cr -O
C -N -Cr -O
24 25 43
4
3
25
43
Cr -O
N -Cr -O
C -N -Cr -O
19.786
4
3
26
43
18
26
43
N -Cr -O
C -N -Cr -N
119.200
119.984
22.288
21.895
57.832
2
6
43
27
25
43
N -Cr -O
C -N -Cr -N
2
6
43
34 26 43
N -Cr -O
C -C -O -Cr
28 29 41
2
6
43
O -Cr -O
C -C -O -Cr
4
1
43
35
40
42
O -Cr -O
C -C -N -Cr
4
1
43
19
24
25
O -Cr -O
C -C -N -Cr
57.637
4
1
43
13
18
26
O -Cr -O
C -N -Cr -O
112.907
113.816
107.198
108.678
4
2
43
18
26
43
O -Cr -O
C -N -Cr -O
4
2
43
24 25 43
O -Cr -O
C -O -Cr -O
29 41 43
4
4
43
C -O -Cr -O
4
0
42
43
3
.4. Solvent effect
Optical sensing nature of chemosensors was found to be depended on the solvent nature, so the
3
+
response of ligand 3 toward Cr ion was also examined in different solvents such as DMSO,
DMF, methanol, acetonitrile, acetonitrile/H O at maxima of emission intensity. As shown in fig.
2
1
1, the optimum fluorescence enhancement occurred both in acetonitrile and acetonitrile/H O
2
(
95/5%). The aprotic solvents (DMSO, DMF) and protic solvents (methanol, H O) decrease
2
significant the intensity of fluorescent, which might be due to hydrogen bonding of this solvents
with hydroxyl groups, rather than general solvent effects [47]. However, the highest fluorescent
intensity was observed in acetonitrile and acetonitrile/H O (95/5%). So acetonitrile/H O (95/5%)
2
2
was chosen as solvent [48].
1
7

3
+
Fig. 11. Solvent effect on fluorescent intensity of 3 (10 µM) with Cr (2 equiv) in different solvent DMSO, DMF,
methanol, acetonitrile, acetonitrile/H O (50/50%), acetonitrile/H2O (80/20%), acetonitrile/H2O (95/5%).
2
3
+
3
.5. Effect of pH on the binding affinity of Cr to 3
Since pH of the medium plays a crucial role in fluorescence detection, the effect of pH in
the range of (2–13) on the fluorescence response of 3 was investigated in CH CN/H O (95/5%)
3
2
mixture. The changes in the fluorescence intensity at 537 nm of 3 in the absence and presence of
3
+
3+
Cr ion were plotted as a function of pH (Fig. 12). The overall fluorescence intensity of 3–Cr
remained higher than the free receptor 3.
3
+
Cr
3
3
3
+
Cr
3
3
+
Cr
[49].
1
8

3+
Fig. 12. The variation in fluorescence intensity with the pH of the 3 (10 µM) in the presence of Cr (2 equiv) at
emission wavelength 537 nm.
3
.6. Comparison
The N,N’-bis(salicylidene)-2-(6-(2-aminophenyl)-4-phenylpyridin-2-yl)
fluorescence sensing of Cr is very interesting. In addition, we compared the present sensor with
(3)
for
3
+
3
+
those previously reported chemosensors for Cr (Table 2). Chemosensor 3 exhibited better
response relatively.
Table 2: Comparative analysis of chemosensor 3 with previously reported sensors.
Ionophores
mechanism
Methods of
detections
Medium of
detection
Detection
limit
Binding
Sensing
response
pH
range
Constant
-
1
(
M)
M
-
5
4
3+
Ref. 50
Ref. 51
-
fluorescent
CH CN/H O
7.94 × 10
1.6 × 10
Cr (Turn on)
7
3
2
(
8:2)
-
6
4
3+
PET
colorimetric
and
MeOH
1.21 × 10
1.01× 10
Cr (Turn on)
2-7
fluorescent
colorimetric
and
fluorescent
colorimetric
and
fluorescent
fluorescent
fluorescent
-
5
4
3+
Ref. 52
Ref. 53
-
MeOH
1.1 × 10
1.4 × 10
Cr (Turn on)
-
-
-
-
6
4
3+
C=N
isomerization
and ESIPT
-
PET and
CHEF
CH CN
2.75 × 10
9.86 × 10
5.97 × 10
Cr (Turn on)
3
6
3
3+
Ref. 54
CH CN
2.1× 10
8.77×10
Cr (Turn on)
6-8
8
3
-
7
4
3+
3
CH CN/H O
3
2
2.2×10
Cr (Turn on)
(95/5%)
1
9

4
. Conclusions
Synthesis of compound 3 can be achieved by the condensation reaction of 2-(6-(2-
aminophenyl)-4-phenylpyridin-2-yl)benzenamine with 2-hydroxybenzaldehyde. It possesses a
3
+
+
high affinity and selectivity for Cr relative to other different competitive metal ions like Na ,
+
+
2+
2+
2+
2+
2+
2+
3+
2+
3+
3+
Ag , K , Ca , Hg , Mn , Co , Cu , Zn , Cr , Cd , Fe , Al by enhancement of the
fluorescence emission. It is expected that the present design strategy and the remarkable photo
physical properties of this chemosensor will help to extend applications of fluorescent
chemosensors for metal ions. The results of Job’s plot method together with theoretical study
3
+
indicate that 3 and Cr form a 1:1 complex. The association constant K was determined to be
a
4
-1
-7
8
.77×10 M and the limit of detection (LOD) was calculated to be 2.2×10 M exhibiting more
3
+
efficient than other reported Cr sensors [50–54].
References
[
[
1] S. Pawar, U. Fegade,V.K. Bhardwaj, N. Singh, R. Bendre, A. Kuwar, Polyhedron. 87 (2015)79-85.
2] N.R. Council, Recommended dietary allowances. Food and Nutrition Board, Commission on Life Sciences,
National Research Council. Washington: National Academic (1989).
[
[
3] R. McRae, P. Bagchi, S. Sumalekshmy, C.J. Fahrni, Chem. Rev. 109 (2009) 4780-4827.
4] C. Cervantes, J. Campos-García, S. Devars, F. Gutiérrez-Corona, H. Loza-Tavera, J.C. Torres-Guzmán, R.
Moreno-Sánchez, Microbiology Reviews. 25 (2001) 335-347.
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
5] V.K. Gupta, A.K. Jain, P. Kumar, S. Agarwal, G. Maheshwari, Sens. Actuators B. 113 (2006) 182-186.
6] H.A. Zamani, G. Rajabzadeh, M.R. Ganjali, Sens. Actuators B. 119 (2006) 41-46.
7] A.K. Singh, V.K. Gupta, B. Gupta, Anal. Chim. Acta. 585 (2007) 171-178.
8] A.J. Weerasinghe, C. Schmiesing, E. Sinn, Tetrahedron Lett. 50 (2009) 6407-6410.
9] H.W. Wang, Y.Q. Feng, C. Chen, J.Q. Xue, Chin. Chem. Lett. 20 (2009) 1271-1274.
10] Y. Wan, Q. Guo, X. Wang, A. Xia, Anal. Chim. Acta. 665 (2010) 215-220.
11] Y. Lei, Y. Su, J. Huo, Spectrochim. Acta A: Mol. Biomol. Spectrosc. 83 (2011) 149-154.
12] D. Maity, T. Govindaraju, Inorg. Chem. 49 (2010) 7229-7231.
13] D. Maity, T. Govindaraju, Chem. Commun. 46 (2010) 4499-4501.
14] R. Azadbakht, J. Khanabadi, Tetrahedron. 69 (2013) 3206-3211.
15] J.C. Qin, Z.Y. Yang, P. Yang, Inorg. Chim. Acta .432 (2015) 136-141.
16] W. Ruo, J. Guang-Qi, L. Xiao-Hong, Inorg. Chim. Acta. 455 (2017) 247–253.
17] C. Bhaumik, D. Maity, S. Das, S. Baitalik, RSC Advances. 2 (2012) 2581–2594.
18] C. Bhaumik, S. Das, D. Maity, S, Baitalik, Dalton Trans. 40 (2011) 11795-11808.
19] D. Zhou, C. Sun, C. Chen, X. Cui, W. Li, Journal of Molecular Structure. 1079 (2015) 315–320.
20] N. Aksuner, E. Henden, I. Yilmaz, A. Cukurovali, Sens. Actuators B. 134 (2008) 510-515.
21] N. Aksuner, E. Henden, I. Yilmaz, A. Cukurovali, Dyes Pigments. 83 (2009) 211-217.
22] Z. Yang, M. She, J. Zhang, X. Chen, Y. Huang, H. Zhu, P. Liu, J. Li, Z. Shi, Sens. Actuators B. 176 (2013)
4
82-487.
2
0

[
1
23] L. Yang, W. Zhu, M. Fang, Q. Zhang, C. Li, Spectrochim. Acta A: Mol. Biomol. Spectrosc. 109 (2013) 186-
92.
[
[
[
[
[
[
[
(
[
(
[
[
[
(
(
[
[
[
[
[
[
[
(
(
[
(
[
(
[
[
24] M. Kumar, J.N. Babu, V. Bhalla, J. Incl. Phenom. Macrocycl. Chem. 66 (2010) 139-145.
25] R. Azadbakht, S. Rashidi, Spectrochim. Acta A: Mol. Biomol. Spectrosc. . 127 (2014) 329-334.
26] Z. Li, L. Zhang, X. Li, Y. Guo, Z. Ni, J. Chen, L. Wei, M. Yu, Dyes Pigments. 94 (2012) 60-65.
27] M. Li, H.Y. Lu, R.L. Liu, J.D. Chen, C.F. Chen, J. Org. Chem. 77 (2012) 3670-3673.
28] E. Hao, T. Meng, M. Zhang, W. Pang, Y. Zhou, L. Jiao, J. Phys. Chem. A. 115 (2011) 8234-8241.
29] D. Ray, P.K. Bharadwaj, Inorg. Chem. 47 (2008) 2252-2254.
30] J.S. Wu, W.M. Liu, X.Q. Zhuang, F. Wang, P.F. Wang, S.L. Tao, X.H. Zhang, S.K. Wu, S.T. Lee, Org. Lett. 9
2007) 33-36.
31] X.L. Tang, X.H. Peng, W. Dou, J. Mao, J.R. Zheng, W.W. Qin, W.S. Liu, J. Chang, X.J. Yao, Org. Lett. 10
2008) 3653-3656.
32] B. Tang, T. Yue, J. Wu, Y. Dong, Y. Ding, H. Wang, Talanta. 64 (2004) 955-960.
33] J.H. Chang, Y.M. Choi, Y.K. Shin, Bull. Korean Chem. Soc. 22 (2001) 527-530.
34] (a) J. Mao, L. Wang, W. Dou, X. Tang, Y. Yan, W. Liu, Org. Lett. 9 (2007) 4567-4570.
b) K. Huang, H. Yang, Z. Zhou, M. Yu, F. Li, X. Gao, T. Yi, C. Huang, Org. Lett. 10 (2008) 2557-2560.
c) Z. Zhou, M. Yu, H. Yang, K. Huang, F. Li, T. Yi, C. Huang, Chem. Commun. 29 (2008) 3387-3389.
35] B. Tamami, H. Yeganeh, Polymer. 42 (2001) 415-420.
36] M.Z. Kassaee, A.R. Bekhradnia, J. Biosci. Bioeng. 95 (2003) 526-529.
37] N.C. Lim, S.V. Pavlova, C. Bruckner, Inorg. Chem. 48 (2009) 1173-1182.
38] P. Alaei, S. Rouhani, K. Gharanjiga, J. Ghasemi, Spectrochim. Acta, Part A. 90 (2012) 85-89.
39] R.M.F. Batista, S.P.G. Costa, M.M.M. Raposo, J. Photochem. Photobiol. A: Chem. 259(2013) 33-40.
40] K. Rajendran, R. Perumal, J. Lumin. 130 (2010) 1203-1210.
41] (a) X. Chen, T. Pradhan, F. Wang, J.S. Kim, J. Yoon, Chem. Rev. 112 (2011) 1910-1956.
b) D. Liu, T. Pang, K. Ma, W. Jiang, X. Bao, RSC Adv. 4 (2014) 2563-2567.
C) Z. Zhou, M. Yu, H. Yang, K. Huang, F. Li, T. Yi, C. Huang, Chem. Commun. 29 (2008) 3387-3389.
42] (a) C.A. Parker, W.T. Rees, Analyst. 85 (1960) 587-600.
b) Y. Gabe, Y. Urano, K. Kikuchi, H. Kojima, T. Nagano, J. Am. Chem. Soc. 126 (2004) 3357-3367.
43] S. Goswami, A. K. Das, A.K. Maity, A. Manna, K.S. AichMaity, P. Saha, T.K. Mandal, Dalton Trans. 43
2014) 231-239.
44] P. Das, A. Ghosh, H. Bhatt, A. Das, RSC Adv. 2 (2012) 3714-3721.
45] M. Mukherjee, B. Sen, S. Pal, M.S. Hundal, S.K. Mandal, A.R. Khuda-Bukhsh, P. Chattopadhyay, RSC
Advances. 43 (2013) 19978-19984.
[
[
[
[
(
(
(
[
[
[
[
[
46] A. Bekhradnia, E. Domehri, M. Khosravi, Spectrochim. Acta, Part A. 152 (2016) 18-22.
47 J.R. Lakowicz, Principles of fluorescence spectroscopy, springer US. (2006).
48] R.B. OrfãoJr, F. de Carvalho, P. Homem-de-Mello, F.H. Bartoloni, J. Braz. Chem. Soc. (2017) 1-9.
49] (a) S. Goswami, A.K. Das, A.K. Maity, A. Manna, K. Aich, S. Maity, P. Saha, T.K. Mandal, Dalton Trans.43
2014) 231-239.
b) Y. Yang, H. Xue, L. Chen, R. Sheng, X. Li, K. Li, Chin. J. Chem. 31 (2013) 377-380.
c) J. Zhang, L. Zhang, Y. Wei, J. Chao, S. Wang, S. Shuang, Z. Cai, C. Dong, Anal. Methods. 5 (2013) 5549-5554.
50] P. Saluja, H. Sharma, N. Kaur, N. Singh, D.O. Jang, Tetrahedron. 68(2012) 2289–2293.
51] L. K. Kumawat, N. Mergu, M. Asif, V. K. Gupta, Sens. Actuators B. 231 (2016) 847-859.
52] Y.J. Jang, Y.H. Yeon, H.Y. Yang, J.Y. Noh, I.H. Hwang, C. Kim,Inorg. Chem. Commun. 33 (2013) 48–51.
53] W. Zhu, L. Yang, M. Fang, Z. Wu, Q. Zhang, F. Yin, Q. Huang, C. Li, J. Lumin. 158 (2015) 38–43.
54] S. Saha, P. Mahato, E. Suresh, A. Chakrabarty, M. Baidya, S. K. Ghosh, A. Das, Inorg. Chem. 51 (2011) 336-
3
45.
2
1