Synthesis, characterization and metal ion-sensing properties of two Schiff base derivatives

Article abstract of DOI:10.1016/j.saa.2012.01.046

Two new Schiff base derivatives {2,2′-diphenyl-N,N′-bis(2- pyridylmethylene)biphenyl-4,4′-diamine} (1) and {2,2′-diphenyl-N, N′-bis(salicylidene)biphenyl-4,4′-diamine} (2) were synthesized and characterized by means of elemental analysis, ¹H NM

Full text of DOI:10.1016/j.saa.2012.01.046

Spectrochimica Acta Part A 91 (2012) 375–382
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, characterization and metal ion-sensing properties of two Schiff base
derivatives
Huihua Xu, Xian Tao, Yueqin Li, Yingzhong Shen∗, Yanhong Wei
Applied Chemistry Department, School of Material Science & Engineering, Nanjing University of Aeronautics & Astronautics, Nanjing 210016, PR China
a r t i c l e i n f o
a b s t r a c t
Two new Schiff base derivatives {2,2ꢀ-diphenyl-N,Nꢀ-bis(2-pyridylmethylene)biphenyl-4,4ꢀ-diamine} (1)
and {2,2ꢀ-diphenyl-N,Nꢀ-bis(salicylidene)biphenyl-4,4ꢀ-diamine} (2) were synthesized and characterized
by means of elemental analysis, 1H NMR, FT-IR, and standard spectroscopy techniques. The molecular
structure of 2 has been determined by X-ray single crystal analysis. The analyses of fluorescence properties
of the compounds revealed that 1 and 2 are both poorly fluorescent and display sensitive fluorescence
responses to a panel of 24 monovalent, divalent, and trivalent metal ions in CH3CN–DMSO (9:1, v/v).
Article history:
Received 10 November 2011
Received in revised form 6 January 2012
Accepted 18 January 2012
Keywords:
Schiff base
Metal ion sensor
Fluorescence
Results with imine 1 showed that Fe3+, Cu2+, Zn2+, Cd2+, Mn2+, Zr4+, Hg2+, Cr2+, Pb2+, Sn2+, Bi2+, Al3+, Ce3+
,
La3+, Sm3+, Gd3+, Nd3+, Eu3+ and Dy3+ yields red shifts in emission and increases in intensity. And the
greatest spectral changes for imine 2 include enhancements in emission intensity coupled with red shifts
(Zr4+, Sn2+, Al3+ and Zn2+) and strong quenching (Fe3+). The fluorescence enhancement mechanism of
1 and 2 for metal ions is based on: (i) C N isomerization; (ii) chelation-enhanced fluorescence (CHEF)
effect; and (iii) excited-state intra/intermolecular proton transfer (ESPT).
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
attractive electronic and photophysical properties [15]. In addi-
tion, Schiff base derivatives incorporating a fluorescent moiety are
The
design
and
synthesis
of
new
fluorescent
appealing fluorescent probes for metal ions [16]. In this paper, two
Schiff bases 2,2ꢀ-diphenyl-N,Nꢀ-bis(2-pyridylmethylene)biphenyl-
4,4ꢀ-diamine (1) and 2,2ꢀ-diphenyl-N,Nꢀ-bis(salicylidene)biphenyl-
4,4ꢀ-diamine (2) were synthesized and full characterized. Both
compounds are poorly fluorescent and were found to be sensitive
to the presence of 24 different metal ions, displaying significant
changes in their emission spectra. And the mechanism of the
increase of fluorescence intensity is discussed as well.
probes/chemsensors capable of selectively recognizing biologi-
cally and environmentally important ion species have attracted
considerable attention in the realm of chemistry, biochemistry and
environmental chemistry [1–3]. A fluorescent probe/chemsensor
is a molecular system for which the physicochemical properties
change upon interaction with a chemical species in such a way
as to produce a detectable fluorescent signal [4]. Fluorescent
sensors offer significant advantages over other chemsensors
for metal ion detection and measurement because of its gener-
ally nondestructive character, better sensitivity, and real-time
monitoring with fast response time [5–7]. To date, a variety of
signaling mechanisms have been developed and widely applied
for the optical detection of different species, including photo-
induced electron/energy transfer (PET) [8], metal–ligand charge
transfer (MLCT) [9], intramolecular charge transfer (ICT) [10],
excited-state intra/intermolecular proton transfer (ESPT) [11],
2. Experimental
2.1. Materials and methods
2,2ꢀ-Dibromobiphenyl-4,4ꢀ-diamine was synthesized on the
basis of a previously reported method [17]. All other chemi-
cals for the syntheses and the solvents used were of analytical
grade quality from commercial sources and were used with-
out further purification. Inorganic salts including FeCl3·6H2O,
CoCl2·6H2O, NiCl2·6H2O, CuSO4·5H2O, ZnAc2·2H2O, 3CdSO4·8H2O,
MnCl2·4H2O, ZrOCl2·6H2O, HgCl2, CrCl2, PbCl2, SnCl2·6H2O,
Bi2(SO4)3, AlCl3·6H2O, NaNO2, KBr, BaCl2·2H2O, Ce(NO3)3·6H2O,
LaCl3·6H2O, SmCl3·6H2O, GdCl3·6H2O, NdCl3·6H2O, EuCl3·6H2O
and DyCl3·6H2O were purchased from commercial suppliers.
Melting points were measured on a WRS-1B digital melting
point apparatus and are uncorrected. The 1H NMR spectra were
C
N isomerization [12], etc.
As we all know, Schiff bases are good ligands for metal ions
[13]. Schiff bases and their metal complexes are well known
to have antibacterial properties, antitumor activities [14] and
∗
Corresponding author. Tel.: +86 25 52112906 84765; fax: +86 25 52112626.
E-mail address: yz shen@nuaa.edu.cn (Y. Shen).
1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2012.01.046

376
H. Xu et al. / Spectrochimica Acta Part A 91 (2012) 375–382
Table 1
recorded on a Bruker Advance 300 spectrometer in DMSO-d6 solu-
tion with TMS as internal standard. IR spectra were recorded on
a Bruker Vector 22 as KBr pellets in the 400–4000 cm−1 region.
Elemental analyses were carried out on a Perkin-Elmer 240 C
elemental analyzer. Absorption spectra were recorded with a UV-
2550 UV–visible spectrophotometer. Fluorescence spectra were
obtained with an eclipse fluorescence spectrometer.
Crystallographic data and analysis parameters for 2.
Formula
C38H28N2O2
Formula weight
Space group
Crystal size (mm)
Crystal system
Z value
544.62
¯
P 1
0.2 × 0.2 × 0.2
Triclinic
2
a (Å)
7.0827 (14)
b (Å)
11.848 (2)
17.284 (4)
90.01 (3)
96.99 (3)
102.52 (3)
1.287
2.2. Syntheses
c (Å)
˛ (◦)
2.2.1. 2,2ꢀ-Diphenylbipheny-4,4ꢀ-diamine
ˇ (◦)
A
solution of 2,2ꢀ-dibromobiphenyl-4,4ꢀ-diamine (1.710 g,
ꢁ (◦)
Dcalc [g/cm3]
Index ranges
Crystal shape/color
F (000)
5 mmol), phenylboronic acid (1.341 g, 5.5 mmol)), Pd(PPh3)4
(0.087 g, 0.08 mmol) and K2CO3 (2.764 g, 20 mmol) in toluene
(30 mL) and water (18 mL) was heated at 100 ◦C under an nitro-
gen atmosphere for 5 h. After cooling to the room temperature,
the organic layer was separated and the aqueous layer was
extracted twice with 20 mL portions of toluene. The combined
organic layer was washed twice with NaHCO3 solution and water,
respectively, then dried over MgSO4, filtered and the solvent was
evaporated. Recrystallized from ether/ethyl acetate, the pure 2,2ꢀ-
diphenylbipheny-4,4ꢀ-diamine was obtained as a white solid. Yield:
1.401 g (83% based on 2,2ꢀ-dibromobiphenyl-4,4ꢀ-diamine). M.p.:
152–153 ◦C
−7 ≤ h ≤ 8, −14 ≤ k ≤ 14, −20 ≤ l ≤ 20
Prism/colorless
572
0.08
ꢂ (Mo K␣) (mm−1
)
´
˚
ꢀ (Mo-K␣) (A)
0.71073
153(2)
3.1–25.4
5082
0.054
0.126
Temperature (K)
 range (◦)
Independent reflections [(I) > 2ꢃ(I)]
R1 [I > 2ꢃ(Â)]
wR2 [I > 2ꢃ(Â)]
Goodness-of-fit on F2c
1.03
´
˚
ꢄ
ꢄ
max (eA−3
min (eA−3
)
0.19
´
˚
)
−0.19
(w(Fo2 − Fc2)2)/
ꢁ
ꢂ
ꢀ
ꢀ
ꢀ
ꢀ
1/2
R1
=
(||Fo| − |Fc||)/
|Fo|; wR2
=
w(Fo4
)
w = 1/[ꢃ2(Fo2) + (0.050P)2 + 0.3633P], P = (Fo2 + 2Fc2)/3
S =
2.2.2. 2,2ꢀ-Diphenyl-N,Nꢀ-bis(2-pyridylmethylene)biphenyl-4,4ꢀ-
diamine
(1)
ꢁ
ꢂ
ꢀ
2
w(Fo2 − Fc2
)
/(n − p)1/2
,
n = number of reflections, p =
parameters used.
2,2ꢀ-Diphenylbipheny-4,4ꢀ-diamine (0.672 g, 2 mmol) and 2-
pyridylaldehyde (0.428 g, 4 mmol) were dissolved in absolute
ethanol (20 mL). The mixture was refluxed for 5 h under an
N2 atmosphere during which a yellow precipitate appeared.
The resulting precipitate was filtered, washed with ethanol, and
finally recrystallized from MeCN to afford 2,2ꢀ-diphenyl-N,Nꢀ-bis(2-
pyridylmethylene)biphenyl-4,4ꢀ-diamine as yellow crystals. Yield:
0.783 g (76% based on 2,2ꢀ-diphenylbipheny-4,4ꢀ-diamine). M.p.:
224–225 ◦C. Anal. Cal. for C36H26N4 (%): C, 84.09; H, 5.04; N,
10.94. Found (%): C, 84.02; H, 5.09; N, 10.89. 1H NMR (DMSO-d6,
300.13 MHz): ı 8.73 (d, J = 4.6 Hz, 2 H), 8.68 (s, CH N, 2 H), 8.17 (d,
J = 7.8 Hz, 2 H), 7.97 (t, J = 7.5 Hz, 2 H), 7.62–7.49 (m, 2 H), 7.41 (q,
J = 8.0 Hz, 4 H), 7.21–7.03 (m, 8 H), 6.75 (d, J = 7.3 Hz, 4 H). IR (KBr pel-
let, cm−1): 3050 (w), 3027 (w), 2878 (w), 1617 (s), 1593 (s), 1570
(s), 1467 (m), 1277 (s), 1174 (s), 1149 (m), 902 (m), 814 (s), 752
(vs), 696 (s), 630 (s), 544 (m), 492 (m). UV–vis (CH3CN–DMSO (9:1,
v/v), c = 1 × 10−5 mol L−1): ꢀmax (ε) = 266 (26,800), 345 (15,600) nm
(L mol−1 cm−1).
2.3. Crystallographic data collection and refinement
A suitable sample of size 0.20 mm × 0.20 mm × 0.20 mm for 2
was chosen for the crystallographic study and then mounted on
a BRUKER SMART Apex CCD detector equipped with graphite
˚
monochromatic Mo K␣ radiation (ꢀ = 0.71073 A) at 153 K. Date
collection and processing (cell refinement, data reduction, and
empirical absorption correction) were performed using the CRYS-
TALCLEAR program package [18]. The structure was solved using
direct methods and refined by full-matrix least-squares procedures
on F2 (SHELX-97) [19]. All of the non-hydrogen atoms were refined
with anisotropic displacement parameters. Crystallographic data
and details on refinement are presented in Table 1.
3. Result and discussion
3.1. Synthesis and characterization
2.2.3. 2,2ꢀ-Diphenyl-N,Nꢀ-bis(salicylidene)biphenyl-4,4ꢀ-diamine
(2)
An outline of the synthesis of the Schiff base derivatives 2,2ꢀ-
diphenyl-N,Nꢀ-bis(2-pyridylmethylene)biphenyl-4,4ꢀ-diamine (1)
and 2,2ꢀ-diphenyl-N,Nꢀ-bis(salicylidene)biphenyl-4,4ꢀ-diamine (2)
is presented in Scheme 1. 2,2ꢀ-Diphenylbipheny-4,4ꢀ-diamine was
prepared according to a Suzuki cross-coupling reaction starting
from 2,2ꢀ-dibromobiphenyl-4,4ꢀ-diamine and phenylboronic. Com-
pound 1 (or 2) was synthesized by a condensation reaction between
one equivalent of 2,2ꢀ-diphenyl-N,Nꢀ-bis(salicylidene)biphenyl-
4,4ꢀ-diamine and two equivalent of 2-pyridylaldehyde (or salicy-
laldehyde) in refluxing ethanol. The products were obtained in high
yield and characterized by elemental analysis, FT-IR and 1H NMR,
respectively.
2,2ꢀ-Diphenyl-N,Nꢀ-bis(salicylidene)biphenyl-4,4ꢀ-diamine
was prepared via a synthetic method analogous to that for
1
except that salicylaldehyde (0.488 g, 4 mmol) was used
instead of 2-pyridylaldehyde. Yield: 0.885 g (81% based on
2,2ꢀ-diphenylbipheny-4,4ꢀ-diamine). M.p.: 250–251 ◦C. Anal. Cal.
for C38H28N2O2 (%): C, 83.74; H, 5.15; N, 5.19. Found (%): C, 83.80;
H, 5.18; N, 5.14. 1H NMR (DMSO-d6, 300.13 MHz): ı 13.07 (s, O H,
2 H), 9.06 (s, C N, 2 H), 7.72–7.60 (m, 2 H), 7.54–7.35 (m, 6 H),
7.27 (d, J = 1.4 Hz, 2 H), 7.18–7.12 (m, 6 H), 6.98 (t, J = 7.4 Hz, 4
H), 6.78 (d, J = 6.7 Hz, 4 H). IR (KBr pellet, cm−1): 3406 (m), 3053
(m), 3025 (m), 2893 (w), 1623 (m), 1588 (m), 1471 (s), 1439 (m),
1177 (w), 997 (w), 894 (m), 820 (s), 770 (s), 734 (s), 696 (vs),
669 (w), 629 (m), 612 (m), 580 (m). UV–vis (CH3CN–DMSO (9:1,
v/v), c = 1 × 10−5 mol L−1): ꢀmax (ε) = 266 (15,200), 356 (21,800) nm
(L mol−1 cm−1).
The FT-IR spectra (Section 2) of compounds 1 and 2 present
the characteristic absorption of ꢅ (C N) vibration at 1624 and
1617 cm−1, respectively. An O H stretching band for 2 appears as a
broad band centered at 3406 cm−1, an indication of intermolecular
O
H· · ·N hydrogen bonding between the phenol OH and the imine

H. Xu et al. / Spectrochimica Acta Part A 91 (2012) 375–382
377
CHO
N
1
( )
N
N
N
N
EtOH, reflux
H2N
NH2
EtOH, reflux
2
( )
N
N
OH
HO
CHO
OH
Scheme 1. Synthesis of Schiff bases 1 and 2.
N. The 1H NMR spectra of 1 and 2 in DMSO-d6 (Fig. 1) exhibit the
signals corresponding to half of the molecule as expected due to
the internal symmetry, respectively. The spectrum of 1 displays a
broad singlet at ı 8.68 ppm, corresponding to CH N proton. In the
spectrum of 2, the chemical shift of CH N proton shifted downfield
about 9.06 ppm comparing with that of 1, which confirms the for-
mation of an intermolecular hydrogen bonding of the phenol OH
with the imine N as indicated in the FT-IR studies.
3.3. UV–vis absorption and fluorescence properties
The UV–vis absorption and fluorescence emission spectra of
compounds 1 and 2 in CH3CN–DMSO (9:1, v/v) solution at room
temperature are displayed in Fig. 3. Seen from Fig. 3, both com-
pounds exhibit an intense band at 266 nm due to the ꢆ–ꢆ*
transition, and a longer-wavelength absorption band centered at
345 and 356 nm, respectively, which could be assigned to the n–ꢆ*
transition associated with imine chromophore. Emission spectra
were measured with excitation at the long-wavelength bands and
showed emission maxima at 427 and 425 nm for 1 and 2, respec-
tively. Both compounds are poorly fluorescent, in part due to
isomerization of the C N double bond in the excited state [21] and
in part due to the excited-state intra/intermolecular proton transfer
(ESPT) involving the phenolic protons of the salicylic amide moiety
[22].
3.2. Single crystal structure of 2
Single crystals of 2 suitable for X-ray diffraction were obtained
from a MeCN/CHCl3 solution by slow evaporation at room temper-
ature. The molecular structure of compound 2 is depicted in Fig. 2.
Selected bond distances (Å) and bond angles (◦) are given in Table 2.
The compound 2 crystallizes in the triclinic with space group
¯
P 1. Seen from Fig. 2, compound 2 contains two identical arms on
3.4. Metal ion responsive properties
the opposite side. Each arm adopts a unique configuration in which
the phenol ring and the biphenyl part are in a trans relationship. In
the central part of 2, the three serial benzene units, C (C14 C19),
B (C8 C13) and D (C20 C25), constitute almost one cycle of the
helical structure and the terminal E ring (C26 C31) is oriented out
of the helical array, which is similar to that observed in a naph-
thalene tetramer [20]. The dihedral angles between two adjacent
benzene units are as follows: B C = 43.5◦, B D = 58.1◦, D E = 49.2◦.
The dihedral angles formed by the phenol rings A (C1 C6) and F
(C33 C38) with the adjacent benzene rings B and D are 32.1◦ and
42.4◦, respectively. These angles are remarkably smaller than that
formed by the benzene rings B and D (58.1◦), possibly due to the
concomitant effects of the steric hindrance of the phenyl groups
and the presence of two intramolecular O H· · ·N hydrogen-bond
interactions (Table 3).
In order to obtain an insight into the binding properties of 1
and 2 toward metal ions, the changes of the fluorescence proper-
ties of 1 × 10−5 mol L−1 of 1 and 2 were investigated upon addition
of a wide range of 40 equivalent cations (Fe3+, Co2+, Ni2+, Cu2+
,
Zn2+, Cd2+, Mn2+, Zr4+, Hg2+, Cr2+, Pb2+, Sn2+, Bi2+, Al3+, Na+, K+,
Ba2+, Ce3+, La3+, Sm3+, Gd3+, Nd3+, Eu3+, Dy3+) in CH3CN–DMSO (9:1,
v/v) solution at room temperature. Fluorescence emission spectra
of 1 and 2 were measured with excitation wavelength at 345 and
356 nm, respectively. And the chemosensory behavior of 1 and 2
are depicted in Figs. 4 and 5, respectively.
Results with compound 1 showed (Fig. 4, Table 4) that a few
metal ions (Co2+, Ni2+, Na+, K+, Ba2+) had little or no effect on
the fluorescence. The remaining metal ions yielded both red shifts
and marked increases in emission intensity, the order of emis-
sion enhancement being Fe3+ (2-fold increase in intensity with
32 nm red-shift) ≈ Mn2+ (2-fold increase with 15 nm shift) < Zn2+
(4-fold increase with 119 nm shift) ≈ Cu2+ (4-fold increase with
23 nm shift) < Pb2+ (7-fold increase with 17 nm shift) ≈ Zr4+ (7-
fold increase with 26 nm shift) < Al3+ (10-fold increase with 25 nm
shift) < La3+ (11-fold increase with 22 nm shift) ≈ Sn2+ (11-fold
Table 2
Selected bond lengths (Å) and bond angles (◦) for 2.
Bond lengths (Å)
O(1) C(1)
N(1) C(7)
N(1) C(8)
C(6) C(7)
1.351 (2)
1.283 (2)
1.423 (2)
1.450 (2)
O(2) C(34)
N(2) C(23)
N(2) C(32)
C(32) C(33)
1.356 (2)
1.443 (3)
1.285 (3)
1.455 (2)
Table 3
Hydrogen-bond geometry (Å,◦) for 2.
Bond angles (◦)
C(7) N(1) C(8)
O(1) C(1) C(2)
O(1) C(1) C(6)
N(1) C(7) C(6)
120.51 (16)
118.81 (17)
121.29 (17)
121.85 (17)
C(32) N(2) C(23)
N(2) C(32) C(33)
O(2) C(34) C(33)
O(2) C(34) C(35)
119.12 (19)
122.43 (19)
122.92 (15)
117.07 (15)
D
H· · ·A
D
H
H· · ·A
D· · ·A
D
H· · ·A
O(1) H(1)· · ·N(1)
O(2) H(2)· · ·N(2)
0.82
0.82
1.87
1.94
2.598 (2)
2.651 (3)
148
145

378
H. Xu et al. / Spectrochimica Acta Part A 91 (2012) 375–382
15 16
17
18
14
4
2
5
8
3
N
N
N
N
11
12
3, 8, 12, 16
CH=N
2
14, 18
15, 17
5
4
11
8.50
8.00
7.50
7.00
ppm (t1)
15 16
17
18
14
4
5
8
3
2
N
N
OH
HO
11
12
5
8
15
17
12
2
CH=N
OH
4
16
3
14
18
11
13.0
12.0
11.0
10.0
9.0
8.0
7.0
ppm (t1)
Fig. 1. 1H NMR spectrum (300.13 MHz, DMSO-d6) of 1 (top) and 2 (bottom).
increase with 26 nm shift) < Bi2+ (12-fold increase with 22 nm
shift) < Cd2+ (13-fold increase with 22 nm shift) < Ce3+ (16-fold
increase with 22 nm shift) < Nd3+ (17-fold increase with 22 nm
shift) ≈ Hg2+ (17-fold increase with 22 nm shift) < Sm3+ (19-fold
increase with 22 nm shift) < Gd3+ (20-fold increase with 22 nm
shift) ≈ Cr2+ (20-fold increase with 22 nm shift) < Dy3+ (21-fold
increase with 22 nm shift) < Eu3+ (23-fold increase with 22 nm
shift). The effects are summarized in Table 4.
intensity coupled with red shifts (shifts of 17, 20 and 24 nm, respec-
tively). In contrast, Zn2+ gave obvious red shift in emission (shift of
120 nm) with slight increase on intensity.
A summary of the results for both compounds is outlined
in Table 4 and points out the similarities and differences. In
CH3CN–DMSO (9:1, v/v) solution, alkali metal and alkaline earth
metal cations (Na+, K+, Ba2+) were unable to improve any fluores-
cence signal of both compounds as they have feeble bonding affinity
toward the chelating C N group [23]. Most of the transition and
Metal ion effects on compound 2 were also measured (Fig. 5,
post-transition metal cations (Fe3+, Cu2+, Zn2+, Cd2+, Mn2+, Zr4+
,
Table 4). With 2, sixteen metal ions (Co2+, Ni2+, Cd2+, Mn2+, Hg2+
,
,
Hg2+, Cr2+), except Co2+ and Ni2+, induce fluorescence enhancement
with red shifts with 1, which can be explained as follows: upon
chelating with certain metal ions, the C N isomerization is inhib-
ited (Fig. 6); moreover, the reaction of a metal ion with a chelating
Cr2+, Pb2+, Bi2+, Na+, K+, Ba2+, Ce3+, La3+, Sm3+, Gd3+, Nd3+, Eu3+
Dy3+) showed slight quenching of the fluorescence, while Fe3+
showed apparently strong quenching in addition to a 17 nm red-
shift. Addition of Zr4+, Sn2+, Al3+ gave marked increases in emission

H. Xu et al. / Spectrochimica Acta Part A 91 (2012) 375–382
379
Fig. 2. The molecular structure of 2, showing 30% probability displacement ellipsoids and the atom numbering scheme. The O H· · ·N hydrogen-bond interactions are
indicated by dashed lines. H atoms not involved in hydrogen bonding have been omitted for clarity.
(Ce3+, La3+, Sm3+, Gd3+, Nd3+, Eu3+, Dy3+) produced several folds
enhancements of fluorescence with 1 but slight quenching with 2,
which suggested that the singlet (S1) and the triplet (T1) of 1 are
lower than the lowest 4f levels of the rare earth metal cations, but
that of 2 are higher than the lowest 4f levels of the cations [25]. Both
compounds showed similar changes for Sn2+ and Al3+ (fluorescence
agent induces rigidity in the resulting molecule and tends to pro-
duce a large CHEF effect which induces the large enhancement of
fluorescence [24]. Compound 2 showed different responses for the
transition and post-transition metal cations: Fe3+ (strong quench-
ing), Co2+, Ni2+, Cd2+, Mn2+, Hg2+ and Cr2+ (slight quenching), Zr4+
and Zn4+ (red shifts and fluorescence enhancements). The mech-
anism of fluorescence increase is similar to that of 1, additionally,
the coordination of 2 with the metal ion prevents ESPT, leading to
fluorescence enhancement (Fig. 7), and the differences suggested
that the pyridyl unit plays an important role in complexation with
the metal ions in compound 1. Addition of rare earth metal cations
500
(a)
1 only
Fe3+
Co2+
400
Ni2+
Cu2+
(a)
Zn2+
1
2
0.25
300
Cd2+
Mn2+
0.20
200
100
0
Zr4+
Hg2+
Cr2+
Pb2+
Sn2+
0.15
0.10
0.05
0.00
400
500
600
700
Wavelength/nm
300
400
500
500
400
300
200
100
0
Wavelength/nm
(b)
1 only
Bi2+
Al3+
(b)
1
2
25
20
15
10
5
Na+
K+
Ba2+
Ce3+
La3+
Sm3+
Gd3+
Nd3+
Eu3+
Dy3+
0
400
500
600
700
400
500
600
Wavelength/nm
Wavelength/nm
Fig. 4. Fluorescence spectra of compound 1 (1 × 10−5 M) in the presence of 24
metal ions (4 × 10−4 M) in CH3CN–DMSO (9:1, v/v) solution at room temperature,
ꢀex = 345 nm.
Fig. 3. (a) UV–vis absorption and (b) normalized fluorescence spectra (excited at
the long-wavelength absorption maxima) of compounds 1 and 2 in CH3CN–DMSO
(9:1, v/v) solution with the concentration of 1 × 10−5 M at room temperature.

380
H. Xu et al. / Spectrochimica Acta Part A 91 (2012) 375–382
120
100
80
60
40
20
0
(a)
2 only
Fe3+
Co2+
Ni2+
Cu2+
Zn2+
Cd2+
Mn2+
Zr4+
Hg2+
Cr2+
Pb2+
Sn2+
400
450
500
550
600
650
700
Wavelength/nm
120
100
80
60
40
20
0
(b)
1 only
Bi2+
Al3+
Na+
K+
Ba2+
Ce3+
La3+
Sm3+
Gd3+
Nd3+
Eu3+
Dy3+
400
450
500
550
600
650
700
Wavelength/nm
Fig. 5. Fluorescence spectra of compound 2 (1 × 10−5 M) in the presence of 24
metal ions (4 × 10−4 M) in CH3CN–DMSO (9:1, v/v) solution at room temperature,
ꢀex = 356 nm.
enhancements with red shifts) and differences with Pb2+ and Bi2+
(enhancement of fluorescence with 1 versus slight quenching with
2).
As a whole, the fluorescence response of compounds 1 and 2
are remarkably diverse. Compound 1 in CH3CN–DMSO gives emis-
sion maxima over a 119 nm wavelength range and can act as a
useful “turn on” sensor of most of the transition metal and rare
earth metal ions. Compound 2, differing from 1 only by phenol unit,
gives a similar 123 nm emission wavelength range and can also
be used as enhanced-signal reporters of Zr4+, Sn2+, Al3+ and Zn2+
,
and the response to these are different enough to allow separate
identification of these metals in an unknown sample. In addi-
tion to fluorescence response involving an increase or a decrease
in intensity, both compounds change their emission wavelengths
remarkably in the presence of some metal ions. Compounds 1 and
2 show their greatest red shifts in wavelength for Zn2+ (119 and
120 nm, respectively). This is especially useful for two reasons: first,
it allows for ratiometric measurements of concentration, as the
ratio of two wavelengths can easily be measured; second, it allows
for distinguishing between background signal and positive signal
in applications where separating the reporter from the analyte
is undesirable or impossible, such as in real-time solution-phase
assays and intracellular assays [26].

H. Xu et al. / Spectrochimica Acta Part A 91 (2012) 375–382
381
Fig. 6. Proposed C N isomerization (take compound 1 for example).
Fig. 7. Excited-state intra molecular proton transfer (ESPT) to compound 2.
4. Conclusions
Acknowledgments
In conclusion, two new Schiff base fluorescent indicators 1 and 2
have been synthesized and fully characterized. Significant emission
enhancements and red shifts in wavelength of 1 were detected in
This work was supported by National Science and Technology
of Major Project (2009ZX02039-002), Funding of Jiangsu Innova-
tion Program for Graduate Education (CX10B 100Z) and Funding
for Outstanding Doctoral Dissertation in NUAA (BCXJ10-11) were
also acknowledged.
the presence of Fe3+, Cu2+, Zn2+, Cd2+, Mn2+, Zr4+, Hg2+, Cr2+, Pb2+
,
Sn2+, Bi2+, Al3+, Ce3+, La3+, Sm3+, Gd3+, Nd3+, Eu3+, Dy3+ ions, the
highest emission enhancement was caused by Eu3+ and the greatest
red shift in wavelength was caused by Zn2+. With 2, addition of Zr4+
,
Appendix A. Supplementary data
Sn2+, Al3+ and Zn2+ caused marked increases in emission intensity
coupled with red shifts. The fluorescence enhancement mechanism
is as follows: the formation of a rigid framework between Schiff
bases and metal ions inhibits the C N isomerization, ESPT and a
large CHEF effect caused by the chelation between metal ion and
ligand.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2012.01.046.
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