A new fluorescent chemosensor for Al3+ ion based on schiff base naphthalene derivatives

Article abstract of DOI:10.1016/j.saa.2014.02.101

A new naphthalene derivative receptor (H₂L) was synthesized. The chemosensor (H₂L) exhibited a strong fluorescence enhancement in the presence of trace amounts of Al³⁺, attributable to chelation-enhanced fluorescence (CHEF) effect, which also displayed high selectivity over a series of other metal cations (Na⁺, K⁺, Cs⁺, Mg²⁺, Ba²⁺, Pb²⁺, Cr ³⁺, Mn²⁺, Fe³⁺, Fe²⁺, Co ²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd ²⁺, Hg²⁺ and Ag⁺) in ethanol.

Full text of DOI:10.1016/j.saa.2014.02.101

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 329–334
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A new fluorescent chemosensor for Al3+ ion based on schiff base
naphthalene derivatives
⇑
Reza Azadbakht , Somaye Rashidi
Department of Chemistry, Payame Noor University, Tehran, Iran
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
A new fluorescent chemosensor was
prepared.
The chemosensor exhibits a
significant fluorescent enhancement
toward Al3+ in ethanol.
ꢀ
In the presence of Al3+, H2L exhibits
significant fluorescence
enhancements with a blue shift
owing to CHEF.
a r t i c l e i n f o
a b s t r a c t
Article history:
A new naphthalene derivative receptor (H2L) was synthesized. The chemosensor (H2L) exhibited a strong
fluorescence enhancement in the presence of trace amounts of Al3+, attributable to chelation-enhanced
fluorescence (CHEF) effect, which also displayed high selectivity over a series of other metal cations
(Na+, K+, Cs+, Mg2+, Ba2+, Pb2+, Cr3+, Mn2+, Fe3+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+ and Ag+) in ethanol.
Ó 2014 Elsevier B.V. All rights reserved.
Received 1 December 2013
Received in revised form 14 February 2014
Accepted 16 February 2014
Available online 4 March 2014
Keywords:
Fluorescence
Chemosensor
Al3+ ion
Schiff base
Naphthalene
Introduction
After absorption, aluminum distributes unequally to a number of
organs and tissues including the brain and spinal cord. The iron
The development of fluorescent chemosensors for metal ions,
based on ion-induced changes in fluorescence, are particularly
attractive because of their simplicity, high sensitivity, and instan-
taneous response [1–3]. Aluminum widely distributed in natural
waters and most biological tissues in its ionic form Al3+. Peoples
are widely exposed to aluminum because aluminum is used in food
additives, aluminum-based pharmaceuticals, and cooking utensils.
binding protein is known to be the main carrier of Al3+ in plasma,
and Al3+ can enter the brain and reach the placenta and fetus. Al3+
may persist for a very long time in various organs and tissues be-
fore it is excreted in the urine.
It has been shown in many studies that aluminum can cause
health hazard like Alzheimer’s disease [4,5], osteomalacia [6] and
even to the risk of cancer of the breast [7]. The upper limit for
the weekly dietary intake of aluminum for humans is estimated
to be 7 mg/kg of body weight. Due to the potential impact of Al3+
⇑
Corresponding author. Tel.: +98 811 2546838; fax: +98 811 2546840.
E-mail address: r_azadbakht@yahoo.com (R. Azadbakht).
http://dx.doi.org/10.1016/j.saa.2014.02.101
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

330
R. Azadbakht, S. Rashidi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 329–334
ions on human health and the environment, selective and sensitive
chemosensors for Al3+ are hence highly demanded.
without further purification. 2-[2-(2-Formylnaphthoxy)eth-
oxy]naphthaldehyde was synthesized according to the literature
method [16]. NMR spectra were obtained using a Bruker A
V300 MHz spectrometer. Infrared spectra were recorded in KBr
Schiff bases are well known for the formation of stable com-
plexes with transition metal ions, and their behavior as ion carriers.
The structure of Schiff bases render geometric and cavity control of
host–guest complexation and produce remarkable selectivity, sen-
sitivity and stability for a specific ion. Consequently, Schiff base
complexes have attracted increasing attention in the area of ionic
binding. Schiff bases with proper placement of additional N or O
as donor atoms are well known to form strong complexes with tran-
sition metal ions and have been used as the ionophore in optical
sensor for determining various cations [8–10]. Several publications
have demonstrated the use of purposefully designed Schiff base li-
gands for optical sensing of metal ions [11–15].
Herein, we describe a new fluorescent probe with mixed nitro-
gen, oxygen donor sites and naphthalene groups as fluorophore
(H2L) (Scheme 1) for Al3+ ions. In the presence of Al3+, H2L exhibits
significant fluorescence enhancements with a blue shift owing to
CHEF (chelation-enhanced fluorescence). CHEF is an attractive de-
sign principle for developing luminescent chemical devices, which
combine the ability to recognize and respond to an external input
mostly with mediation of photoinduced electron transfer (PET).
pellets using
a BIO-RAD FTS-40A spectrophotometer (4000–
400 cmÀ1). Positive ion FAB mass spectra were recorded on a Kra-
tos-MS-50 spectrometer with 3-nitrobenzyl alcohol as the matrix
solvent. UV–Vis absorption spectra were obtained on a Varian Cary
Eclipse 300 spectrophotometer. The fluorescence spectra were re-
corded on a Varian spectrofluorometer. Both excitation and emis-
sion bands were set at 5 nm.
Synthesis of 2-((Z)-(2-(2-(1-((Z)-(2-hydroxyphenylimino)methyl)
naphthalen-2-yloxy)ethoxy)naphthalen-1-yl)methyleneamino)phenol
(H2L)
A solution of 2-Aminophenol (0.217 g, 2 mmol) in absolute
methanol (25 mL) was added dropwise to a hot solution of 2-[2-
(2-Formylnaphthoxy)ethoxy]naphthaldehyde (0.374, 1 mmol) in
absolute methanol (50 mL). The solution was gently refluxed for
6 h. The solution was then concentrated in a rotary evaporator to
a volume of ca. 15 mL. A precipitate was obtained by standing
overnight at 0 °C. Yield (70%), Scheme 1. Anal. Calc. for
Experimental
C36H28N2O4: C, 78.24; H, 5.11; N, 5.07. Found: C, 78.28; H, 5.07;
N, 5.11%. IR (KBr, cmÀ1) 1619 (C@N), 3419 (OH); 1HNMR dH (CDCl3,
ppm) 4.66 (s, 4H), 6.91–7.88 (m, 20 H), 9.26 (s, 2H), 11.41 (b, OH);
13C NMR dC (CDCl3, ppm) 68.86, 114.06, 115.83, 118.18, 120.10,
124.51, 126.14, 126.62, 128.66, 137.68,151.86, 155.63, 168.69.
The mass spectrum show peak at m/z = 552 corresponding to H2L.
Materials and instruments
All solvents were of reagent grade quality and purchased com-
mercially. 2-Aminophenol was obtained from Merck and was used
Scheme 1. The synthetic route of H2L and absence of ESIPT in H2L.

R. Azadbakht, S. Rashidi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 329–334
331
Theoretical calculations
The complex ([AlL]+) gave fine powders and we were not able to
prepare single crystals for structure determination by X-ray spec-
troscopy. Instead, The geometry of [AlL]+ was fully optimized at
DFT [17,18] levels of theory using the GAUSSIAN 03 program
[19], on a Pentium-PC computer with a 4000 MHz processor. A
starting semiempirical structure was optimized using the HYPER-
CHEM 5.02 series of programs [20].
Results and discussion
Synthesis and characterization
Synthesis of 2-((Z)-(2-(2-(1-((Z)-(2-hydroxyphenylimino)
methyl)naphthalen-2-yloxy)ethoxy)-naphthalen-1-yl)methyle-
neamino)phenol (H2L) can be achieved very easily in 80% yield by
Schiff base condensation of 2-[2-(2-Formylnaphthoxy)ethoxy]
naphthaldehyde [16] with 2-Aminophenol in ethanol (Scheme 1).
H2L is air-stable solid, moderately soluble in common organic
solvents. H2L have low solubility in the water or mixed aqueous
medium. The IR spectrum of H2L reveals a strong absorption band
at ca. 1619 cmÀ1 due to stretching vibrations of C@N bands, while
no bands attributable to C@O is detected. The 1H NMR spectrum
for H2L (C2v) in the region of aliphatic shows a singlet signal due
to methylen protons at 4.66 ppm, a single 1H imine resonance at
9.26 ppm, demonstrating the equivalence of the two imine envi-
ronments. The 1H NMR spectrum also shows a broad signal at
11.4 ppm attributed to the hydroxy protons. In the aromatic region
(6.91–7.88 ppm) five signals were found, and since the protons do
not show scalar-coupling we were not able to assign explicitly the
signals to each of the aromatic systems present in the molecule.
The 13C NMR spectrum of H2L in the region of aliphatic shows
one signal at 68.86 ppm attributed to methylene carbons. The 13C
NMR spectrum shows that the imine carbon atoms (168.69 ppm)
are equivalent. In the region corresponding to the signals of aro-
matic ring carbons (114.06–155.63 ppm), 11 peaks are observed.
Fig. 1. B3LYP optimized structure of [AlL]+.
Table 1
The selected bond lengths (Å) and angles (°) for [AlL]+.
Bond length (Å)
Bond angle (°)
Al(1)AN(1)
Al(1)AN(2)
Al(1)AO(1)
Al(1)AO(2)
Al(1)AO(3)
Al(1)AO(4)
1.889
1.886
1.871
1.871
1.836
1.834
N(1)AAl(1)AN(2)
O(2)AAl(1)AO(4)
O(1)AAl(1)AO(3)
O(3)AAl(1)AN(1)
172.94
175.17
178.22
84.63
O(1)AAl(1)AN(1)
O(1)AAl(1)AN(2)
O(3)AAl(1)AN(2)
O(2)AAl(1)AN(1)
O(4)AAl(1)AN(1)
O(4)AAl(1)AN(2)
O(2)AAl(1)AN(2)
94.40
90.57
90.53
92.25
92.47
82.40
92.97
LUMO (-13.088)
HOMO (-13.823)
Fig. 2. Molecular orbital amplitude plots and energies of the main frontier molecular orbitals of H2L computed at B3LYP/6-31G.

332
R. Azadbakht, S. Rashidi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 329–334
The complex of H2L with AlCl3 ([AlL]+) was prepared in ethanol.
resulted in a drastic fluorescence emission change. The fluores-
cence peak at 380 nm blue-shifted to 355 nm with a marked fluo-
rescence intensity enhancement (up to 7.07-fold enhancement
when compared to the fluorescence intensity of free sensor H2L
at 355 nm, see Fig. 3). A mild fluorescence enhancement at
355 nm was also detected for Cr3+, as seen in Fig. 3. The observa-
tion that the fluorescence intensity is completely quenched by
Cu2+, Fe2+ and Fe3+ is consistent with the fact that these transition
metal ions have intrinsic quenching nature [24–26]. The addition
of the other metal ions elicited no noticeable changes to the fluo-
rescence spectra of sensor H2L (Fig. 3). To further investigate the
properties of receptor H2L as a sensor for Al3+, fluorescence titra-
tions were performed by adding of Al3+ to the solutions of sensor
We compared the IR spectra of H2L with [AlL]+, two differences
were observed: (i) the stretch vibration of imine group (C@N) at
1619 cmÀ1, which was a mild and sharp absorption band, changed
to 1641 cmÀ1, a strong and broad band; (ii) The broad and intense
absorption band at 3419 cmÀ1 corresponding to the OAH stretch-
ing vibration disappeared. These differences suggest that the imine
groups and the phenolic oxygen as phenolate took part in the coor-
dination with Al3+
.
Theoretical study
The structure of [AlL]+ was optimized by Density Functional
Theory (DFT) calculations using B3LYP/6-31GÃ basis set. The result-
ing structure for [AlL]+ was used for further calculations using the
effective core potential (ECP) standard basis set LanL2DZ [21–23]
for Al3+ ion and the standard 6-31GÃ basis set for all other atoms.
The resulting structural diagrams are shown in Fig. 1 and selected
calculated bond distances and angles relating to them are shown in
Table 1. Aluminum is bound by the two imine nitrogens, two ethe-
ric oxygens and two oxygen atoms from two phenolate groups in a
distorted octahedral arrangement. The optimized geometry of
these molecules predicts that the free rotation around C@N bond
is restricted by metal ion.
H2L in ethanol solvent (Fig. 4). Upon continuous addition of Al3+
,
fluorescence intensity increased with a blue shift of
Dk = 25 nm.
The sensing mechanism of H2L for Al3+ might be attributed to
the combination of PET and CHEF mechanisms in H2L. ESIPT is
not possible due to the absence of conjugation between the nitro-
gen of the imine and OAH of the phenolic ring as shown in
Scheme 1.
The FEF (fluorescence enhancement factor) values of fluorescent
macrocycles H2L responding to different metal ions are shown in
Fig. 5. The FEF (IX À IL)/IL was calculated using minimal (IL) and
maximal (IX) fluorescence intensities recorded before and after
The spatial distributions and orbital energies of HOMO and
LUMO of [AlL]+ were also determined (Fig. 2). It was clearly shown
that the HOMO distribution of the complex was located essentially
over the phenolate and naphthalen moieties. The energy gap be-
tween HOMO and LUMO was computed to be 0.734 eV (Fig. 2) Cor-
responding to absorption band of aromatic groups in the UV–Vis
spectrum.
200
[M]/[L]
1.2
1.1
180
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
160
140
120
100
80
Spectral properties of fluorescent H2L
Sensor H2L exhibited one typical fluorescence peak at 380 nm.
The sensing properties of compound H2L toward various metal ions
60
were examined by treating sensor H2L (5 lM) with metal ions such
as Na+, K+, Cs+, Mg2+, Ba2+, Al3+, Pb2+, Cr3+, Mn2+, Fe3+, Fe2+, Co2+
,
40
Ni2+, Cu2+, Zn2+, Cd2+, Hg2+ and Ag+ in ethanol solution at ambient
temperature. The addition of Al3+ to the solution of sensor H2L
20
0
300
350
400
450
500
Wavelength (nm)
Fig. 4. Changes in the fluorescence spectra of H2L (5 l
M) as a function of added Al3+
concentration in ethanol. Excitation wavelength was 288 nm. Both the excitation
and emission slit widths were 5.0 nm.
Fig. 3. Fluorescence spectrum of H2L (5
Na+, K+, Cs+, Mg2+, Ba2+, Al3+, Pb2+, Cr3+, Mn2+, Fe3+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+
Hg2+ and Ag+ (1000
M). Both the excitation and emission slit widths were 5.0 nm.
l
M) in the presence of various metal ions
Fig. 5. The fluorescence enhancement factor (FEF) of H2L upon addition of various
metal ions in methanol at 25 °C. Both the excitation and emission slit widths were
5.0 nm.
,
l

R. Azadbakht, S. Rashidi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 329–334
333
Fig. 6. The fluorescence enhancement factor (FEF) of H2L upon addition of Al3+ ions in the presence of various metal ions in methanol at 25 °C. Both the excitation and
emission slit widths were 5.0 nm.
900
[Al(III)]/[H2L]
0.0
1
800
700
0.9
0.8
600
1.2
0.7
0.6
0.5
0.4
0.3
0.2
0.1
500
400
300
200
100
0
0
0.2
0.4
0.6
0.8
1
0
265
[Al3+]/([Al3+]+[H2L])
315
365
415
465
Wavelength (nm)
Fig. 7. Job’s plot for addition for H2L + Al3+
.
Fig. 8. Changes in the absorption spectra of H2L (50
l
M) as a function of added Al3+
concentration in ethanol.
addition of metal ions, respectively. The highest fluorescence
enhancement of for H2L has been observed in the presence of
Al3+ ions.
of Al3+ decreases to 5.0 Â 10À7 M, an observable stripping peak was
observed. When further decreasing the concentration, the stripping
peak almost disappears. So, the limit of detection is evaluated to be
5.0 Â 10À7 M.
To evaluate the potential application and efficiency of H2L as a
fluorescence sensor for Al3+, competition experiments were con-
ducted in ethanol in the presence of other metal cations (Fig. 6).
The results show that the fluorescence of H2L upon complexation
to Al3+ did not significantly change in the presence of Na+, K+,
Mg2+, Ba2+, Pb2+, Mn2+, Zn2+, Cd2+ and Hg2+ with a concentration
up to 1000 mM.
Conclusion
Synthesis of H2L can be achieved very easily in 80% yield by
Schiff base condensation of 2-[2-(2-Formylnaphthoxy)eth-
oxy]naphthaldehyde [16] with 2-Aminophenol. Optimized geo-
metric structures of [AlL]+ were calculated. H2L possesses a high
affinity and selectivity for aluminum ions relative to most other
In order to determine the stoichiometry of the LAAl3+ complex,
the Job’s method was used [27,28]. In the Job’s plot, a maximum
fluorescence change was observed with a 0.5 M fraction of iono-
phore to Al3+ for L, which indicated that only a 1:1 complex was
formed (Fig. 7).
competitive metal ions like Na+, K+, Cs+, Mg2+, Ba2+, Pb2+, Cr3+
,
Mn2+, Fe3+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+ and Ag+ by
enhancement of the fluorescence emission. We expect that the
present design strategy and the remarkable photophysical proper-
ties of this chemosensor will help to extend applications of fluores-
cent chemosensor for metal ions.
The absorption spectrum of H2L exhibits structured absorption
spectrum corresponding to the
p–
pà transitions of naphthalene
unit observed in naphthalene derivatives. However, the change in
the environment of the naphthalene unit in the present systems
H2L causes change in the intensities and positions of the
bands compared to that of pure naphthalene (Fig. 8).
p–
pÃ
Acknowledgments
Limit of detection
The author gratefully acknowledges the Department of Chemis-
try of Payame Noor University and Ministry of Science Research
and Technology.
The limit of detection was estimated utilizing the method of
gradually decreasing concentration of Al3+. When the concentration

334
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[16] R. Azadbakht, M. Parviz, E. Tamari, H. Keypour, R. Golbedaghi, Spectrochim.
References
Acta A82 (2011) 200–204.
[17] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[18] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
[1] (a)M. Dong, Y.-M. Dong, T.-H. Ma, Y.-W. Wang, Y. Peng, Inorg. Chim. Acta. 381
(2012) 137–142.
[19] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R.
Cheeseman,V.G. Zakrazewski, J.A. Montgomery Jr., R.E. Startmann, J.C.
Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O.
Farkas, J. Tomasi, V. Barone,M. Cossi, R. Cammi, B. Mennucci, C. Adamo, S.
Clifford, J. Ochterski, G.A. Petersson,P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malik,
A.D. Rabuck, K. Raghavachar, J.B. Foresman, J. Cioslowski, J.V. Ortiz, B.B.
Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.Komaromi, R. Gomperts, R.L. Martin,
D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanyakkara, C. Gonzalez, M.
Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres, M.
Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian 03, Gaussian, Pittsburgh, PA,
2003.
[20] HYPERCHEM, Release 5. 02, Hypercube Inc., Gainesville, 1997.
[21] K.D. Dobbs, W.J. Hehre, J. Comp. Chem. 8 (1987) 880.
[22] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 299.
[23] P.C. Hariharan, J.A. Pople, Theor. Chim. Acta 28 (1973) 213.
[24] K. Rurack, Spectrochim. Acta A 57 (2001) 2161–2195.
[25] Y. Ma, W. Luo, P.J. Quinn, Z. Liu, R.C. Hider, J. Med. Chem. 47 (2004) 6349–6362.
[26] B. Bodenant, T. Weil, M. Businelli-Pourcel, F. Fages, B. Barbe, I. Pianet, M.
Laguerre, J. Org. Chem. 64 (1999) 7034–7039.
[2] C. Gou, S.-H. Qin, H.-Q. Wu, Y. Wang, J. Luo, X.-Y. Liu, Inorg. Chem. Commun. 14
(2011) 1622–1625.
[3] K. Kaur, V.K. Bhardwaj, N. Kaur, N. Singh, Inorg. Chem. Commun. 26 (2012) 31–
36.
[4] T.P. Flaten, Brain Res. Bull. 55 (2001) 187.
[5] C.N. Martyn, C. Osmond, J.A. Edwardson, D.J.P. Barker, E.C. Harris, R.F. Lacey,
Lancet 333 (1989) 61.
[6] G.C. Woodson, Bone 22 (1998) 695.
[7] P.D. Darbre, J. Inorg. Biochem. 99 (2005) 1912.
[8] D. Maity, T. Govindaraju, Inorg. Chem. 49 (2010) 7229–7231.
[9] D. Maity, T. Govindaraju, Chem. Commun. (2010) 4499–4501.
[10] R. Azadbakht, J. Khanabadi, Tetrahedron 69 (2013) 3206–3211.
[11] N. Aksuner, E. Henden, I. Yilmaz, A. Cukurovali, Sens. Actuators B 134 (2008)
510–515.
[12] N. Aksuner, E. Henden, I. Yilmaz, A. Cukurovali, Dyes Pigments 83 (2009)
211–217.
[13] Z. Yang, M. She, J. Zhang, X. Chen, Y. Huang, H. Zhu, P. Liu, J. Li, Z. Shi, Sens.
Actuators B 176 (2013) 482–487.
[14] L. Yang, W. Zhu, M. Fang, Q. Zhang, C. Li, Spectrochim. Acta. A 109 (2013)
186–192.
[15] M. Kumar, J.N. Babu, V. Bhalla, J. Incl. Phenom. Macrocycl. Chem. 66 (2010)
39–145.
[27] W.C. Vosburgh, G.R. Cooper, J. Am. Chem. Soc. 63 (1941) 437–442.
[28] R.R. Avirah, K. Jyothish, D. Ramaiah, Org. Lett. 9 (2007) 121–124.