Fluorescent recognition of Fe3+ ion with photoinduced electron transfer (PET) sensor

Article abstract of DOI:10.1016/j.cplett.2013.08.029

We synthesized a fluorescence receptor 2,2-(pyridine-2,6-diylbis(azanediyl) )bis(methylene)diphenol (2) and report it as a photoinduced electron transfer (PET) cation sensor that is capable of indicating the presence of Fe ³⁺ ion via a fluoresc

Full text of DOI:10.1016/j.cplett.2013.08.029

Chemical Physics Letters 584 (2013) 165–171
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Fluorescent recognition of Fe3+ ion with photoinduced electron transfer
(PET) sensor
a,
Umesh Fegade a,b, Sanjay Attarde b, , Anil Kuwar
⇑
⇑
a School of Chemical Sciences, North Maharashtra University, Jalgaon 425001, (MS), India
b School of Environmental and Earth Sciences, North Maharashtra University, Jalgaon 425001, (MS), India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 July 2013
In final form 10 August 2013
Available online 21 August 2013
We synthesized a fluorescence receptor 2,2-(pyridine-2,6-diylbis(azanediyl))bis(methylene)diphenol (2)
and report it as a photoinduced electron transfer (PET) cation sensor that is capable of indicating the pres-
ence of Fe3+ ion via a fluorescence signal. It was observed that fluorescence intensity changes and
quenched. The association constant (Ka) of receptor (2) with Fe3+ ions was calculated from Benesi–Hilde-
brand and Scatchard Plot at 1.60 and 1.30 ꢀ 104 Mꢁ1 respectively.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
complexes with Fe3+ ions in semi aqueous solution, resulting in a
significant fluorescence quenching.
In recent years the development of fluorescent receptors for the
detection of cation is the attractive research area in the field of
supramolecular chemistry because of it’s use in material science
and biology [1–3]. The development of molecular systems capable
of signaling different guest molecules or ions have been energetic
areas of modern research [4–6]. One of the most frequently utilized
design processes for fluorescence signaling system is the develop-
ment of three-component system with a fluorophore-spacer-recep-
tor architecture containing components that are selected such that
the communication between the fluorophore and receptor leads to
quenching of the fluorescence of the system. In the presence of a
guest, the communication between the terminal moieties, which is
responsible for fluorescence quenching, gets cut off, thus leading
to the recovery of the fluorescence of the system. This is commonly
referred to as ‘off–on’ fluorescence signaling of a guest. Interestingly,
the off–on fluorescence signaling systems for the transition-metal
ions, which are well-known for their fluorescence quenching abili-
ties, have been developed only very recently [7,8].
N
H
N
N
H
Fe3+
OH
HO
2. Experimental
2.1. Reagents
Among the transition metal ions Fe3+ is having much impor-
tance as it plays a crucial role in the growth and progress of living
systems. For example numerous enzymes use iron as a catalyst for
oxygen metabolism, electron transfer as well as DNA and RNA syn-
thesis. Both its lack and excess in the body can cause serious dis-
eases [9–12].
In the present letter, we prepared a simple and efficient fluoro-
genic receptor 2 containing the two amine and two hydroxyl frag-
ment (Scheme 1). It was found that the compound selectively
All spectroscopy grade chemicals and solvents were used with-
out further purification. The fluorescence and UV–visible spectra
were recorded on Fluoromax-4 Spectrofluorometer and Shimadzu
UV-24500 with 5 nm slit of fluorescence and the nitrate salt of metal
were used in the letter. Ultrapure water with a Millipore Purification
System (Milli-Q water) was used throughout the analytical experi-
ments. 1H NMR spectra were recorded on a Varian NMR mercury
system 300 spectrometer operating at 300 MHz in DMSO-d6.
2.2. Sample preparation
⇑
A stock solution of probe 2 (1.00 ꢀ 10ꢁ3 M) in CH3OH/H2O
(70:30, v/v) solution was prepared (probe 2 is freely soluble in
Corresponding authors. Fax: +91 257 2257403.
E-mail address: kuwaras@gmail.com (A. Kuwar).
0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cplett.2013.08.029

166
U. Fegade et al. / Chemical Physics Letters 584 (2013) 165–171
O
a
2
N
N
N
+
H2N
N
NH2
OH
OH
HO
1
b
N
H
N
H
N
OH
HO
2
Scheme 1. Synthesis of receptor 2 (a) CH3OH, 3 h (b) NaBH4, CH3OH, 0–5 °C and 1.5 h.
MeOH, DMF and DMSO at 25 °C) and the corresponding working
solutions (c = 1.00 ꢀ 10ꢁ5 M) were simply prepared by diluting
with CH3OH/H2O (70:30, v/v). All stock and working solutions were
prepared in ultrapure water and spectroscopic grade CH3OH. Stock
solution of cation (1.00 ꢀ 10ꢁ2 M) was prepared with CH3OH/H2O
(70:30, v/v) solution and the corresponding working solutions
(c = 1.00 ꢀ 10ꢁ4 M) were simply prepared by diluting with CH3-
OH/H2O (70:30, v/v).
114.5, 122.6, 127.0, 129.3, 143.7, 153.0, 159.25. MS (EI): m/z
C19H19N3O2 = 321.37; found 321.00.
2.5. Synthesis of receptor 2ꢂFe3+ complex
2ꢂFe3+ complex was synthesized by reaction of one mole of
receptor 2 (0.64 g, 0.20 mM) with one moles of FeCl3 (0.32 g,
0.20 mM) in MeOH 50 ml reflux with stirring for 3 h. The precipita-
tion was collect at room temperature and dried in vacuum. Further
it was washed with water then ethanol followed by petroleum
ether. Yield-82%, MS (ESI): m/z requires C19H17N3O2Fe: 375.28,
found 375.07.
2.3. Fluorescence analysis
The fluorescence titration experiments were carried out with a
Fluoromax-4 spectrofluorometer in CH3OH/H2O (70:30, v/v) sol-
vent system at room temperature (298 K) with the aim of deter-
mining the association constant (Ka) for receptor 2–cation in this
solvent system. These titration experiments were accomplished
through a stepwise addition of metal salt solutions (0.02 ml,
3. Result and discussion
3.1. Synthesis of receptor 2
1.00 ꢀ 10ꢁ4 M, guest) to
a solution of receptor 2 (2 ml,
Compound 1 was synthesized by reacting one mole of 1,2-
diaminopyridine with two moles of 2-hydroxy benzaldehyde in
methanol with stirring for 3 h. Compound 1 was obtained with
quantitative yield. Receptor 2 was obtained from compound 1 by
reduction under NaBH4 with good yield (Scheme 1). The synthe-
sized receptors were characterized by melting point, IR, 1H NMR,
13C NMR and mass spectroscopic methods. The spectral investiga-
tion gave consistent data of structure of both.
1.00 ꢀ 10ꢁ5 M) in CH3OH/H2O (70:30, v/v) in the cell. The fluores-
cence intensity was recorded at kex/kem = 314/380 nm alongside a
reagent blank. The excitation and emission slits were both set to
5.0 nm.
2.4. Synthesis of receptor 2
Compound 1 was synthesized by reacting one mole of 2,6-
diaminopyridine (1.09 g, 1.00 mM) with two moles of 2-hydroxy
benzaldehyde (2.40 g, 2.00 mM) in methanol, with stirring for 3 h
at an ambient temperature. Compound 1 was obtained with excel-
lent yield and having appearance of orange coloured powder. Sol-
ubility in methanol, 85% yield, mp >250 °C were observed. Further
receptor 2 was obtained from compound 1 by reduction under
NaBH4 in methanol with good yield.
3.2. Fluorescence studies of receptors 2
The solvent ratio CH3OH/H2O (70:30, v/v) respectively for
receptor 2 was kept constant during the titrations. The fluores-
cence properties of receptors 2 was studied upon addition of vari-
ous metal salts (c = 1.00 ꢀ 10ꢁ4 M) Ca2+, Mg2+, Ba2+, Cr3+, Mn2+
,
Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Hg2+, Pb2+, Cd2+, Bi3+ ions. The receptors
2 showed fluorescence emission at 380 nm upon excitation at
314 nm. Upon addition Fe3+ in receptor 2, the intensity of emission
band at 380 nm decreased along with the appearance of a new red-
shifted emission band at 409 nm (Figure 1). The fluorescence was
selectively and significantly red shifted and quenched in the pres-
ence of Fe3+ ions. There were no changes in the fluorescence emis-
sion of receptor 2 in the presence of other metal ions tested. The
Name: 2,2-(pyridine-2,6-diylbis(azanediyl))bis(methylene)
diphenol (2), yield 76% (white powder) solubility in methanol,
mp P250 °C. IR (KBr, cmꢁ1):
v
= 621, 648, 719, 754,746, 871, 970,
3064, 3273, 3560 cmꢁ1
.
1H NMR (300 MHz, DMSO-d6, ppm)
d = 3.33 (s, 4H, Ar-CH2), 4.258 (s, 2H, NH), 5.22 (s, 2H, Ar-OH),
6.12 (d, J = 7.2 Hz, 2H, Py-H), 6.55–7.06 (m, 8H, Ar-H), 7.23 (T,
J = 7.2 Hz, 1H, Py-H). 13C NMR (75 MHz, DMSO-d6) d = 37.8, 98.5,

U. Fegade et al. / Chemical Physics Letters 584 (2013) 165–171
167
Wavelength (nm)
Figure 1. Fluorescent intensity (kex = 314 nm) of receptor 2 upon the addition of a particular metal cation salt in CH3OH/H2O (70:30, v/v). Figure 1 inset corresponding
fluorescent colour change in CH3OH/H2O (70:30, v/v) solution of 2 induced by Fe3+ (from left to right: 2 only and 1+ Fe3+).
Metal Ions
Figure 2a. Fluorescence ratiometric response of chemosensor 2 (c = 1.00 ꢀ 10ꢁ5 M) upon the addition of a particular metal salt (c = 1.00 ꢀ 10ꢁ4 M) in CH3OH/H2O (70:30, v/
v).
Metal Ions
Figure 2b. Interference of metals at the time of Fe3+ ion detection.
fluorescence of receptor 2 is quenched and red shifted upon addi-
tion of Fe3+ due to the thermodynamically favored PET between the
pyridine nitrogen and the excited chromophore. Increasing the oxi-
dation potential of the pyridine nitrogen due to cation binding pre-
vents this PET and regenerates the fluorescence of the
chromophore.

168
U. Fegade et al. / Chemical Physics Letters 584 (2013) 165–171
Wavelength (nm)
Figure 3a. Changes in fluorescence spectrum of probe 2 (c = 1.00 ꢀ 10ꢁ5 M) after the addition of Fe3+ salt of (c = 1.00 ꢀ 10ꢁ4 M) in CH3OH/H2O (70:30, v/v).
Log G
Figure 3b. Normalized response of fluorescence signal to changing Fe3+ concentrations in the CH3OH/H2O (70:30, v/v) solution (receptor 2 c = 1.00 ꢀ 10ꢁ5 M and Fe3+ salt of
c = 1.00 ꢀ 10ꢁ4 M).
Fluorescence ratiometric response of receptor 2 and interfer-
ence toward the surveyed metal ions is shown in Figures 2a and
2b. The results show a highly selective response of receptor 2 to-
wards Fe3+ ions as compared to the other metal ions such as tran-
sition and heavy-metals.
sity and correlation coefficient is high. Normalized response of
fluorescence signal to changing concentration of Fe3+ cation in CH3-
OH/H2O (70:30, v/v) solution is shown in Figure 3b. Receptor 2 can
be used for selective recognition of Fe3+ ions in quite a wide range
of iron concentrations, and it can detect as little as 0.50
ion concentration [14].
l
M of Fe3+
We also determined the fluorescence quantum yields (U) of 2
and 2ꢂFe3+. The quantum yield of receptor 2 is high (0.36) and
2ꢂFe3+ is low (0.02). It is noted that, upon complexation with iron
metal, the corresponding quantum yield dramatically decrease,
which is consistent with the fluorescence spectra feature. These
observations suggest that the receptor 2 is highly selective and
sensitive towards Fe3+ ion [13].
The binding pattern of receptor 2 explores the importance of
pyridine nitrogen and hydroxyl group in cation binding. The pyri-
dine nitrogen and hydroxyl groups are considered as good binding
sites and play an effective role in binding of Fe3+ ions. Therefore, it
appears that the core functionality required for receptor 2 to effi-
ciently bind with Fe3+ upon excitation is a pyridine nitrogen and
hydroxyl linkage. However, due to binding of Fe3+ ion in the gap,
the fluorophore interaction is modulated.
3.3. Calculation of association constant for receptor 2
On the basis of Benesi–Hildebrand Plot [15] Eq. (1) and Scat-
chard Eq. (2) [16] methodologies, association constant (Ka) was cal-
culated by following equation (Ka).
1=ðF ꢁ F0Þ ¼ 1=ðF1 ꢁ F0ÞKa½Gꢃ þ 1=ðF F0Þ
ð1Þ
1
ðF ꢁ F0Þ=½Gꢃ ¼ ðF1 ꢁ F0ÞKa ꢁ ðF ꢁ F0ÞKa
ð2Þ
Figure 3a shows the emission spectra of receptor 2 on addition
Applying the above equations for receptor 2, graph obtained is
illustrated in Figures 4a and 4b. Where F0 represents the fluores-
cence intensity in the absence of guest ion, F represents the fluo-
of Fe3+ on the various concentrations. The fluorescence intensity of
receptor 2 at 380 nm was shifted up to 409 nm (Dk = 29 nm) after
addition of Fe3+ ion. The emission maximum 409 nm was decreases
with increase in concentration of Fe3+. The titration experiment is
plan for showing the linear response between receptor 2 and
changing concentrations of Fe3+ cation. A linear relationship be-
tween concentrations was observed with the fluorescence inten-
rescence intensity in presence of guest ion,
F
1
represents
fluorescence intensity after titration and [G] is the concentration
of guest. The association constant (Ka) of receptor (2) with Fe3+ ions
was calculated from Benesi–Hildebrand and Scatchard Plot at 1.60
and 1.30 ꢀ 104 Mꢁ1 respectively.

U. Fegade et al. / Chemical Physics Letters 584 (2013) 165–171
169
1/G
Figure 4a. Benesi–Hildebrand Plot receptor 2 (adjusted equation: 1=F ꢁ F0 ¼ ꢁ6E ꢁ 13 þ 1E ꢁ 081=½Gꢃ, R = 0.98) at the Ka value 1.60 ꢀ 104 Mꢁ1
.
F-Fo
Figure 4b. Scatchard Plot for receptor 2 (adjusted equation: F ꢁ F0/[G] = ꢁ13530x + 4E + 11, R = 0.97) at the Ka value 1.30 ꢀ 104 Mꢁ1
.
3.4. The Stern–Volmer quenching constant of 2ꢂFe3+
phores, and thus no change in the absorption spectra is predicted.
It was found that the measured spectra were not influenced by the
metal ions in the presence of metal ions.
The quenching can be mathematically expressed by the Stern–
Volmer Eq. (3), which allows for calculating quenching constants
[17].
3.5. Stoichiometry of complexations
ðF0=FÞ ¼ ð1 þ kqs0Þ½Qꢃ ¼ ð1 þ KsvÞ½Qꢃ
ð3Þ
The binding stoichiometry was found by the continuous varia-
tion method (Job’s plot) [18]. Figure 6 shows the Job plot of the
fluorescence intensity of free receptor 2 and the intensity of the
system with the molar fraction of the host {[H]/([H] + [G])} for a
series of solutions, in which the total concentration of host and
guest was constant, with the molar fraction of host continuously
varying. The results showed that the complex 2ꢂFe3+ formed be-
tween 2 and Fe3+ is with 1:1 stoichiometry. This data was further
confirmed by mass spectroscopy analysis. ESI-MS data showed
the formation 1:1 complex between two deprotonated ligand
(receptor 2) and MS (ESI): m/z requires C19H17N3O2Fe: 375.28,
found 375.07.
Where F0 and F are the fluorescence intensities in the absence
and presence of the quencher, kq is the bimolecular quenching con-
stant, s0 is the lifetime of the fluorescence in the absence of the
quencher [Q] is the concentration of the quencher, and Ksv is the
Stern–Volmer quenching constant. In the presence of a quencher
(Lns), the fluorescence intensity is reduced from F0 to F. The ratio
(F0/F) is directly proportional to the quencher concentration [Q].
Evidently:
Ksv ¼ kq ꢂ s0
ð4Þ
ðF0=FÞ ¼ ð1 þ KsvÞ½Qꢃ
ð5Þ
3.6. UV–Vis absorption spectral studies
According to Eq. (5), a plot of F0/F versus [Q] shows a linear
graph (Figure 5) with an intercept of 2 and a slope of Ksv indicating
that fluorescence quenching is dynamic in nature in the linear part.
In order to elucidate the static or dynamic quenching without mea-
surement of fluorescence lifetime, the absorption spectra were
measured carefully to distinguish static and dynamic quenching.
Dynamic quenching only affects the excited states of the fluoro-
The absorption spectrum of probe 2 with Fe3+ ion is shown in
Figure 7. Receptor 2 exhibit maximum peaks at 209, 251, 280
and 314 nm in CH3OH/H2O (70:30, v/v). The longer wavelength
band at 314 nm may be assigned to transitions associated with
phenol ring. The addition of Fe3+ ion into receptor 2, there had
no change in receptor 2. This is fact that the UV–vis spectra of 2

170
U. Fegade et al. / Chemical Physics Letters 584 (2013) 165–171
Q (M)
Figure 5. The Stern–Volmer plot of the titrations of probe 2 with different concentrations of Fe3+ metal salt show normalized graph with regression 0.98.
[H]/[H]+[G]
Figure 6. 1:1 Stoichiometry of the host guest relationship realized from the Job’s plot for receptor 2.
Wavelength (nm)
Figure 7. Changes in absorbance spectrum of probe 2 (c = 1.00 ꢀ 10ꢁ5 M) upon the addition of Fe3+ metal (c = 1.00 ꢀ 10ꢁ4 M) in CH3OH/H2O (70:30, v/v).
Fe3+
Figure 8. Proposed binding mode and mechanism of sensing of probe 2 with Fe3+ ions.

U. Fegade et al. / Chemical Physics Letters 584 (2013) 165–171
171
before and after the addition of 10 equiv of Fe2+ remain relatively
unchanged illustrated that it did not bound with Fe3+ in ground
state and receptor 2 need to be excited for binding [9].
of Fe3+ ions in CH3OH/H2O (70:30, v/v) solvent system. The addi-
tion of Fe3+ gave rise to major fluorescent colour change, which
can be easily seen by the naked eye under UV irradiation. Further-
more, our sensor is not affected by the common interference of
other ions. The 1:1 stoichiometry of the host guest relationship
was realized from the Job’s plot and the association constant (Ka)
of receptor (2) with Fe3+ions was calculated from Benesi–Hilde-
brand and Scatchard Plot at 1.60 and 1.30 ꢀ 104 Mꢁ1 respectively.
3.7. Naked eye detection under UV irradiation
In addition, the selective recognition of receptor 2 with Fe3+ ion
gives a remarkable fluorescent colour quenched which can be visu-
alized by the naked eye under UV irradiation. Visual detection
studies of receptor 2 (c = 1.00 ꢀ 10ꢁ5 M) were conducted with all
cations. The receptor 2 in mixed solutions of CH3OH/H2O (70:30,
v/v) was appeared to be fluorescent at room temperature. With
the addition of these ions (10 equiv) in receptor 2, only Fe3+ ion
cause’s spectacular fluorescent quenched shown in the Figure 1
inset.
References
[1] A.L. Schumacher, J.P. Hill, K. Ariga, F. D’Souza, Electrochem. Commun. 9 (2007)
2751.
[2] D.A. Jose, D.K. Kumar, P. Kar, S. Verma, A. Ghosh, B. Ganguly, H.N. Ghosh, A. Das,
Tetrahedron 63 (2007) 12007.
[3] E.A. Musad, R. Mohamed, B.A. Saeed, B.S. Vishwanath, K.M. LokanathaRai,
Bioorg. Med. Chem. Lett. 21 (2011) 3536.
[4] S. Kubik, RSC 39 (2010) 3648.
3.8. Proposed binding mode and mechanism of sensing
[5] H. Jung, N. Singh, D.O. Jang, Tetrahedron Lett. 49 (2008) 2960.
[6] M. Kumar, J. Nagendra Babu, V. Bhalla, J. Inclusion Phenom. Macrocyclic Chem.
66 (2010) 139.
[7] J.A. Kemlo, T.M. Shepherd, Chem. Phys. Lett. 47 (1977) 158.
[8] P. Ghosh, P.K. Bharadwaj, J. Roy, S. Ghosh, J. Am. Chem. Soc. 119 (1997) 11903.
[9] N. Singh, N. Kaur, R.C. Mulrooney, J.F. Callan, Tetrahedron Lett 49 (2008) 6690.
[10] J.D. Lee, Concise of inorganic chemistry, Wiley, India, 2008.
[11] S.J. Lippard, J.M. Berg, University Science, Mill Valley, 1994.
[12] W. Kaim, B. Schwederski, Bioinorg. Chem. (1995).
[13] W.H. Melhuish, J. Phys. Chem. 65 (1961) 229.
From the fluorescence spectra, a possible binding model of
probe 2 with Fe3+ ion was proposed which is shown in Figure 8.
The pyridine nitrogen and hydroxyl are capable to bind cation.
The Fe3+ could be bound with phenolic OH via deprotonation and
pyridine nitrogen donates lone pair. The proposed binding mecha-
nism was confirmed by mass spectroscopic method.
[14] M. Shortreed, R. Kopelman, M. Kuhn, B. Hoyland, Anal. Chem. 68 (1996) 1414.
[15] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703.
[16] G. Scatchard, Ann. N.Y. Acad. Sci. 51 (1949) 660.
[17] O. Stern, M. Volmer, Phys. J. 20 (1919) 183.
4. Conclusion
[18] P. Job, Ann. Chim. 9 (1928) 113.
In summary, we have synthesized the selective and sensitive
fluorescent chemical sensor that is, receptor 2 for the detection