A new optical sensor for Al3+/Fe3+ based on PET and chelation-enhanced fluorescence

Article abstract of DOI:10.1007/s11164-015-1956-y

A simple probe (compound 1) sensing Al³⁺ or Fe³⁺ by suppression of photoinduced electron/energy transfer (PET) process and chelation-enhanced fluorescence is described. Compound 1 was prepared from 2-(bromomethyl)benzo[ d]thiazole and pyridine-2,6-diamine. Compound 1 showed a much stronger fluorescence response to Al(III) than to Fe(III). The best fluorescence response of 1 to Al³⁺ was found to be at pH near 7. The results indicated that 1 could be a promising fluorescent turn-on chemosensor for Al³⁺ or Fe³⁺. Ratiometric sensing of 1 to Al³⁺ or Fe³⁺ was accomplished by plotting the absorbance ratio at 338 to 310 nm versus Al³⁺ or Fe³⁺ concentration. The fluorescent turn-on response of 1 to Al³⁺ or Fe³⁺ can be attributed to suppression of PET process and chelation-enhanced fluorescence upon binding with Al³⁺ or Fe³⁺. The binding mode of 1-Al³⁺ or 1-Fe³⁺ was found to be a 1:1 complex based on Job's plot, electrospray ionization (ESI) mass spectroscopy (MS) data, and density functional theory (DFT) calculation results.

Full text of DOI:10.1007/s11164-015-1956-y

Res Chem Intermed
DOI 10.1007/s11164-015-1956-y
A new optical sensor for Al3+/Fe3+ based on PET
and chelation-enhanced fluorescence
Meiling Gao1 Puhui Xie2 Lingyu Wang1
•
•
•
Xiaohui Miao1 Fengqi Guo1
•
Received: 25 November 2014 / Accepted: 5 February 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract A simple probe (compound 1) sensing Al3? or Fe3? by suppression of
photoinduced electron/energy transfer (PET) process and chelation-enhanced
fluorescence is described. Compound 1 was prepared from 2-(bromomethyl)ben-
zo[d]thiazole and pyridine-2,6-diamine. Compound 1 showed a much stronger
fluorescence response to Al(III) than to Fe(III). The best fluorescence response of 1
to Al3? was found to be at pH near 7. The results indicated that 1 could be a
promising fluorescent turn-on chemosensor for Al3? or Fe3?. Ratiometric sensing of
1 to Al3? or Fe3? was accomplished by plotting the absorbance ratio at 338 to
310 nm versus Al3? or Fe3? concentration. The fluorescent turn-on response of 1 to
Al3? or Fe3? can be attributed to suppression of PET process and chelation-en-
hanced fluorescence upon binding with Al3? or Fe3?. The binding mode of 1-Al3?
or 1-Fe3? was found to be a 1:1 complex based on Job’s plot, electrospray ion-
ization (ESI) mass spectroscopy (MS) data, and density functional theory (DFT)
calculation results.
Keywords Fluorescent probe ꢀ Ratiometric ꢀ PET ꢀ Chelation-enhanced
fluorescence ꢀ Al(III)
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-1956-y)
contains supplementary material, which is available to authorized users.
&
&
1
Puhui Xie
phxie2013@163.com
Fengqi Guo
fqguo@zzu.edu.cn
College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001,
People’s Republic of China
2
College of Sciences, Henan Agricultural University, Zhengzhou 450002,
People’s Republic of China
123

M. Gao et al.
Introduction
Aluminum, the third most abundant element after oxygen and silicon in the Earth’s
crust [1], is distributed ubiquitously in our daily life. Aluminum-containing
derivatives are widely used in food additives, water treatment, and medicine [2].
However, overloading with aluminum will lead to malfunction of the human central
nervous system [3]. Ferric ion is the most familiar metal ion, being found as an
essential element in myoglobin, hemoglobin, and cytochromes, and playing an
indispensable role in many biological processes such as cellular metabolism, gene
regulation, electron transfer, and regulation of cell growth and differentiation [4–6].
Nonetheless, high concentration of ferric ion in organisms or the environment can
induce biological disorders such as anemia, liver and kidney damage, heart failure,
and diabetes, and cause serious environmental pollution [7, 8]. Therefore, it is
important to develop efficient methods for detection and quantification of Al3? or
Fe3? for food safety and quality control, clinical, and environmental applications.
Among the various methods, fluorescence spectroscopy has become a powerful tool
for sensing trace amounts of samples because of its simplicity, sensitivity, and fast
response time [9–14]. Therefore, it is crucial to develop suitable fluorescent probes
for Al3? or Fe3?
.
Many kinds of signaling mechanisms have been proposed and applied in
fluorescent detection of Al3? or Fe3?, including photoinduced electron/energy
transfer (PET) [15–19], intramolecular charge transfer (ICT) [20–22], fluorescence
resonance energy transfer (FRET) [23–25], excited-state intramolecular proton
transfer [26], spirolactam to ring-open equilibrium of rhodamine derivatives [27–
29], chelation-enhanced fluorescence [30–32], inhibiting nonradiative process by
complexation of metal cations to C=N group [19, 32], etc. It is generally recognized
that one probe with a fluorescent turn-on signal for specific species is more efficient
[33, 34] than one with fluorescent quenching due to high signal-to-noise ratio. As
the paramagnetic nature of Fe3? always leads to a quenching ability, fluorescence
enhancement through chelation of Fe3? with a probe is a challenging research topic
in vitro as well as in vivo. Although several ‘‘turn-on’’ probes that work with
selective response to Al3? or Fe3? have been reported [35–38], their poor solubility
in aqueous solution and complicated synthetic processes inhibit practical applica-
tions. New fluorescent sensors with simple synthetic processes and improved
solubility in water have been developed in our group [39, 40]. In this work, a simple
probe for Al3? or Fe3? with fluorescent enhancement was designed and synthesized.
The nitrogen and sulfur atoms in 1 are well-known acceptor sites for metal ions. If
probe 1 is coordinated with a certain metal ion, its molecular skeleton will be fixed
with rigidity. Chelation of Al3? or Fe3? with probe 1 will result in fluorescent
enhancement and shifts in absorption bands.
123

Optical sensor for Al3?/Fe3? based on PET
Experimental
All chemicals of analytical grade for syntheses were purchased from commercial
suppliers and used without further purification. Tetrahydrofuran (THF) of spectro-
scopic grade and deionized water were used throughout the spectroscopic
experiments as solvents. Nuclear magnetic resonance (NMR) spectra were recorded
with a 300-MHz Varian spectrometer. Electrospray ionization mass spectra (ESI–
MS) were measured on an LC-MSD-Trap-SL instrument. Fluorescence spectra were
measured with a Cary Eclipse fluorescence spectrometer. UV–Vis absorption
spectra were obtained on a UV-5200PC UV/vis spectrophotometer.
Stock solution of 1 (30 lM) in THF/H2O (9/1, v/v) was prepared for
spectroscopic investigations. Stock solutions of metal ions (1 mM) were prepared
in deionized water from nitrates or chlorides of Na? (alkaline metal ion), Mg2?
Ca2? (alkaline earth metal ions), Al3? (group IIIA metal ion), Mn2?, Fe3?, Co2?
,
,
Ni2?, Cu2?, Zn2? (first-row transition-metal ions), and Cd2?, Hg2?, Pb2? (toxic
heavy-metal ions) of analytical grade. In titration experiments, 3 mL stock solution
of 1 was put into a quartz optical cell with optical path of 1 cm. Stock solution of
each metal ion was added into the quartz optical cell step by step via a microsyringe,
and the solution was stirred for 5 min before recording spectra. For fluorescence
measurements, the excitation wavelength was provided at 310 nm, and emission
was collected from 320 to 550 nm. The excitation slit and emission slit were both
set at 5 nm.
The synthetic process of 1 is shown in Scheme 1. The intermediate 2 was
synthesized according to literature [41].
Synthesis of 1
To a mixture of 2,6-diaminopyridine (109 mg, 1 mmol) and anhydrous K2CO3
(303 mg, 2.2 mmol) in CH3CN (15 mL), a solution of 2-(bromomethyl)benzoth-
iazole 2 (510 mg, 2.23 mmol) in 15 mL THF/CH3CN (4/1, v/v) was added
dropwise within 20 min. The mixture was stirred at room temperature for 72 h.
Then, the solvents were removed under reduced pressure. The residue was dissolved
in dichloromethane (40 mL) and washed three times with water. The organic phase
was dried over anhydrous sodium sulfate. After removing the solvents, the crude
product was subjected to column chromatography on silica gel with petroleum
ether/ethyl acetate (1/1, v/v) to afford an off-white solid (110 mg, 43.2 %). 1H NMR
(CDCl3) d (ppm): 4.24 (s, 2H), 4.90 (d, J = 6.0 Hz, 2H), 5.16 (t, J = 6.0 Hz, 1H),
4'
3'
5'
S
N
Br
S
AIBN
NBS
1
6'
2'
N
2
S
H2N
N
NH2
H2N
N
N
H
1'
N
3
2
1
Scheme 1 Synthetic process of 1
123

M. Gao et al.
5.85–5.90 (m, 2H), 7.19–7.25 (m, 1H), 7.31–7.37 (m, 1H), 7.42–7.48 (m, 1H), 7.83
(t, J = 8.0 Hz, 1H), 7.96 (d, J = 8.0 Hz, 1H) (Fig. S1, Electronic Supplementary
Material, ESM). 13C NMR (CDCl3), d (ppm): 44.76, 96.54, 97.90, 121.72, 122.67,
124.83, 125.91, 135.14, 139.55, 153.29, 157.00, 157.54, 172.70 (Fig. S2, ESM). MS
(ESI–MS): m/z calculated for C13H12N4S, 256.33 [M]?. Found: 256.2, 257.2
[M ? H]? (Fig. S3, ESM).
Results and discussion
The response of fluorescence spectra to Al31 or Fe31
The response of 1 to various metal ions, such as Na? (alkaline metal ion), Mg2?
Ca2? (alkaline earth metal ions), Al3? (group IIIA metal ion), Mn2?, Fe3?, Co2?
,
,
Ni2?, Cu2?, Zn2? (first-row transition-metal ions), and Cd2?, Hg2?, Pb2? (toxic
heavy-metal ions) was investigated firstly by fluorescence spectra (Fig. 1). It could
be seen that 1 (30 lmol/L) exhibited no fluorescence in THF/H2O (9/1, v/v)
solution. The intramolecular photoinduced electron transfer (PET) effect from the
lone pair of electrons of the N atom on –NH– of pyridine group to the adjacent
benzo[d]thiazole would result in no fluorescence of 1. However, addition of Al3?
(30 lmol/L) caused an outstanding fluorescence enhancement centered at 393 nm.
Addition of Fe3? (30 lmol/L) caused a moderate fluorescence enhancement
centered at 393 nm. Addition of Cu2? (30 lmol/L) increased the fluorescence
intensity of 1 very slightly. The fluorescence intensity of 1 at 393 nm was not
increased after addition of 30 lmol/L Na?, Ca2?, Mg2?, Co2?, Mn2?, Zn2?, Cd2?
,
Ni2?, Pb2? or Hg2?, respectively. This result indicates that compound 1 shows
superior selectivity towards Al3? over Fe3? and that compound 1 could be used as a
turn-on fluorescence sensor for Al3? or Fe3?. The time dependence of the response
of 1 to Al3? or Fe3? was investigated by recording the fluorescence intensity at
393 nm over time (Fig. S4, ESM). The results revealed that the reaction of
compound 1 (30 lM) and Al3? (100 lM) or Fe3? (100 lM) was completed within
600
500
400
300
200
100
0
Fig. 1 Fluorescence spectra of
1 (30 lmol/L) in the presence of
different metal ions (30 lmol/L)
in THF/H2O (9:1, v/v)
Al3+
Fe3+
1, Na+, Ca2+, Mg2+, Co2+,
Mn2+, Zn2+, Cd2+, Ni2+, Pb2+, Hg2+
Cu2+
350
400
450
500
550
Wavelength (nm)
123

Optical sensor for Al3?/Fe3? based on PET
5 min. Therefore, all measurements were performed after mixing and stirring for
5 min.
The fluorescence spectra of 1 upon titration of Al3? or Fe3? are shown in Fig. 2a
and b, respectively. A new emission band centered at 393 nm appeared, and the
fluorescence was gradually enhanced upon gradual addition of Al3? or Fe3?. The
maximum fluorescent enhancement (67-fold and 28-fold for Al3? and Fe3?
,
respectively) was observed at 393 nm when the amounts of metal ions and 1 were
equal. The fluorescence intensity at 393 nm and the concentration of Al3? or Fe3?
showed good linear relationship in the range from 2 to 40 lM (inset in Fig. 2a, b,
respectively). The detection limit of 1 for Al3? and Fe3? was determined to be 29
and 46 nM (3r/slope), respectively. The fluorescence enhancement for 1 in the
presence of Al3? or Fe3? could be ascribed to the fixing in the molecular skeleton
1000
1000
a
900
800
800
600
700
400
600
200
500
0
0
2
4
6
8
10
12
14
[Al3+ ]/10-5 (mol/L)
400
300
200
100
0
350
400
450
500
550
Wavelength (nm)
400
350
300
250
200
150
100
50
400
350
300
250
200
150
100
50
b
0
0
2
4
6
8
10
12
14
[Fe3+ ]/10-5 (mol/L)
0
350
400
450
500
550
Wavelength (nm)
Fig. 2 a Fluorescence spectra of 1 (30 lmol/L) upon addition of Al3? (inset fluorescence intensity at
393 nm of 1 as a function of Al3? concentration). b Fluorescence spectra of 1 (30 lmol/L) upon addition
of Fe3? (inset fluorescence intensity at 393 nm of 1 as a function of Fe3? concentration)
123

M. Gao et al.
increasing the rigidity of 1 and decreasing the PET effect of the N atom on –NH– of
pyridine group, after it was coordinated with Al3? or Fe3?
.
Job’s plots were used to determine the stoichiometries between 1 and Al3? as
well as 1 and Fe3?. It can be seen in Fig. 3 that the fluorescent maxima of 1 are both
at molar fraction of 0.5 for Al3? or Fe3?, indicating that the stoichiometries between
1 and Al3? as well as 1 and Fe3? are both 1:1.
As several kinds of metal ions often coexist in one system, coexisting metal ions
may interfere with the detection of target ions. To determine the influence of other
cations on the fluorescent detection of Al3? or Fe3? in THF/H2O (9/1, v/v) solution,
300 lmol/L of metal ions such as Na?, Hg2?, Mg2?, Mn2?, Co2?, Ca2?, Cd2?
Cu2?, Pb2?, Zn2?, and Ni2? were added to the solution of 1-Al3? or 1-Fe3?
,
,
respectively. As shown in Fig. 4a, b, the fluorescence intensity of 1-Al3? or 1-Fe3?
was almost unaffected by the presence of the other metal ions. Although Cd2?
caused a slight decrease in the fluorescence intensity of 1-Al3? or 1-Fe3?, this was
negligible compared with the enhancement of the fluorescence upon addition of
Al3? or Fe3?. Based on these results, it was inferred that 1 could be a potential turn-
on fluorescent probe for Al3? or Fe3? with high selectivity and high sensitivity in
THF/H2O (9/1, v/v) solution.
700
600
500
400
300
200
100
Fig. 3 Job’s plot evaluated
from the fluorescence spectra of
1 and Al3? a or Fe3? b at
a
393 nm in THF/H2O (9:1, v/v)
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
[Al3+]/[Al3+]+[1]
900
800
700
600
500
400
300
200
100
b
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
[Fe3+]/[Fe3+]+[1]
123

Optical sensor for Al3?/Fe3? based on PET
a 1.0
0.8
0.6
0.4
0.2
0.0
Al3+
+
Al3+ Al3+ Al3+
Al3+ Al3+
Al3+ Al3+ Al3+ Al3+
Al3+
Al3+
+
Cd
+
+
+
+
+
+
+
+
+
2+ Mn2+
+
2+
2+
2+
2+
2+
2+
Na Hg Mg
Co Ca
2+ Cu Pb Zn Ni2+
b
1.0
0.8
0.6
0.4
0.2
0.0
3+
3+
3+
3+
3+
3+
3+
3+
3+
3+
3+
3+
Fe Fe Fe
Fe
Fe Fe
Fe
Fe Fe Fe
Fe
Fe
+
+
+
+
+
+
+
+
+
+
+
Na+ Mg2+ Hg
2+ Ca
2+ Ni2+ Cd2+
Pb
2+
2+
2+
2+
2+
Mn
Co Cu Zn
Fig. 4 Fluorescence responses of 1 (30 lmol/L) to Al3? (30 lmol/L) a and Fe3? (30 lmol/L) b in
presence of other metal ions. F0: fluorescence intensity of 1 containing Al3? or Fe3? without addition of
competing ions, F: fluorescence intensity of 1 containing Al3? or Fe3? after addition of competing ions
(300 lmol/L)
UV–Vis absorption responses of 1 to Al31 or Fe31
The absorption spectral properties of 1 upon titration of Al3? or Fe3? were studied
in THF/H2O (9/1, v/v). As shown in Fig. 5a, the absorption spectrum of 1 displayed
two absorption bands at 225 and 310 nm. Upon addition of Al3?, the absorption
intensity at 310 nm decreased, and at the same time a new absorption peak at
338 nm appeared and the absorbance increased with increasing concentration of
Al3?. The 28-nm redshift could be explained by the fact that coordination between 1
and Al3? inhibited rotation of the r-bond between the pyridine and benzothiazole
123

M. Gao et al.
Fig. 5 a Absorption spectra of
1 (30 lmol/L) upon addition of
Al3? in THF/H2O (9:1, v/v) and
b Benesi–Hildebrand plot
a
1.0
0.8
0.6
0.4
0.2
0.0
0*10-6
M
6.67*10-6
M
M
-6
13.33*10
20.00*10
26.67*10
-6
M
M
M
M
-6
(absorbance at 338 nm) of 1
between 1/(A - A0) and
-6
53.33*10
80.00*10-6
1/[Al3?
]
-6
106.67*10
133.33*10
160.00*10
M
M
M
-6
-6
200
250
300
350
400
450
500
Wavelength (nm)
b 5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
0
10000 20000 30000 40000 50000
1/[Al3+] M-1
groups and increased the rigidity of the molecular skeleton. In addition, charge
transfer between the electron-rich ligand and electron-deficient metal ions also
increased the conjugation property of the molecule. The increase in the rigidity and
the conjugation property of 1-Al3? turned on its fluorescence. A concomitant
isosbestic absorption point at 315 nm indicated that only one intermediate complex
was formed during the titration process. In view of the fluorescent Job’s plot results,
it could be concluded that only one 1:1 complex was formed in the 1-Al3? system.
The association constant of the complex 1-Al3? was calculated by the Benesi–
Hildebrand expression (Eq. 1) [42].
1
1
1
¼
þ
;
ð1Þ
KaðAmax ꢁ A0Þ½Al3þꢂ
A ꢁ A0
Amax ꢁ A0
where A0 is the absorbance of 1 at 338 nm in the absence of Al3?, A is the ab-
sorbance of 1 at 338 nm at a certain concentration of Al3?, Amax is the absorbance
of 1 at a complete-interaction concentration of Al3?, Ka is the association constant,
and [Al3?] is the concentration of Al3?. On the basis of the plot of 1/(A - A0)
versus 1/[Al3?], the association constant was determined from the slope to be
2.99 9 104 M-1 (Fig. 5b).
123

Optical sensor for Al3?/Fe3? based on PET
The ratio between the absorbance at 338 nm and that at 310 nm of 1 increased
linearly with increasing Al3? concentration in the range of 6.67 9 10-6–1.60 9 10-4
mol/L (Fig. 6). This result indicated that 1 showed high colorimetric selectivity to
Al3? and might be a promising colorimetric sensor for Al3?
.
A similar experimental result was obtained for the titration of Fe3? (Fig. S5,
ESM). Upon titration of Fe3?, the new absorption peak and the concomitant
isosbestic absorption point also appeared at 338 and 315 nm, respectively,
indicating that only one intermediate complex was formed during the titration
process. In view of the results of Job’s plot, it could be concluded that only one 1:1
complex was formed during the titration of Fe3? into the solution of 1. The
association constant of the complex between 1 and Fe3? was also calculated by the
Benesi–Hildebrand expression (Eq. 1) (as shown in Fig. S6, ESM) to be 6.86 9 103
M-1. The ratio between the absorbance at 338 and 310 nm of 1 increased linearly
with increasing Fe3? concentration in the range of 0–1.60 9 10-4 mol/L (Fig. S7,
ESM). This result indicated that 1 also showed high colorimetric selectivity to Fe3?
and might be a promising colorimetric sensor for Fe3?
.
The positions of the response wavelengths were the same for 1 toward Al3? and
Fe3?. Under 365-nm lamp irradiation, the solutions of 1-Al3? and 1-Fe3? turned
blue, while the solutions of 1 in the presence of other ions did not change at all
(Fig. 7). It could still be found that the solution of 1 displayed a light-yellow color
upon addition of Fe3? because of the intrinsic color of Fe3? (Fig. S8, ESM), and the
solution of 1 exhibited no color upon addition of Al3? as well as the other ions.
Therefore, it is easy to distinguish the solution of Fe3? from Al3? in combination
with the fluorescent spectra measurement. The paramagnetic nature of Fe3? might
quench the fluorescence of 1-Fe3? to some degree. The lower association constant
of 1-Fe3? than that of 1-Al3? could also contribute to the superior Al3? compared
with Fe3? sensitivity.
2.0
1.6
1.2
0.8
0.4
0.0
Fig. 6 Ratio between
absorbance at 310 and at
338 nm of 1 upon titration of
Al3?
0
20 40 60 80 100 120 140 160 180
[Al3+] /µM
123

M. Gao et al.
Fig. 7 Photograph of 1 (50 lmol/L) in THF/H2O (v/v, 9:1) solution with 10 equiv metal ions under
irradiation by 365-nm lamp
pH effect on fluorescence of 1 and 1-Al31 complex
To determine a suitable pH range for 1 as a chemosensor for Al3?, the fluorescence
spectra of 1 with and without Al3? in different pH solutions were investigated. As
shown in Fig. 8, 1 revealed no fluorescence at 393 nm between pH 2.6 and 11.1.
After addition of Al3?, the fluorescence of 1-Al3? solution was far stronger than that
of 1 with pH value less than 9. It is well known that Al3? will precipitate as Al(OH)3
in weak basic solution and change to AlO-2 in strong basic solution. These results
indicate that 1 can be applied for monitoring Al3? fluorimetrically over a wide pH
range.
Sensing mechanism
According to the absorption and emission properties of 1 with and without Al3? or
Fe3?, the binding mechanism of 1 with Al3? or Fe3? was assumed. The fluorescence
of the benzo[d]thiazole moiety was quenched by the intramolecular PET effect from
the lone pair of electrons of the N atom on 20 position of pyridine group to the
adjacent benzo[d]thiazole. This PET effect would result in no fluorescence of 1.
After it was bound with Al3? or Fe3?, the lone pair of electrons on the nitrogen of 1
was stabilized, and intramolecular PET to the adjacent benzo[d]thiazole group upon
900
800
700
600
500
400
300
200
100
0
Fig. 8 Fluorescence intensities
(393 nm) of 1 in THF/H2O
(30 lM) (9:1, v/v) with and
without Al3? (2 equiv) as a
function of pH. 1-Al3? (circle)
and 1 (square)
1
1+Al3+
2
4
6
8
10
12
pH
123

Optical sensor for Al3?/Fe3? based on PET
photoexcitation of 1 was thermodynamically unfavorable, resulting in the significant
fluorescence enhancement (Scheme 2). Evidence was obtained by carrying out ESI
mass analysis of 1-Al3?. 1 exhibited a peak at m/z = 256.2 (calcd. 256.3 for
C13H12N4S), which corresponded to [1]? as shown in Fig. S3 (ESM). Several new
peaks were clearly observed after AlCl3 (60 lmol) was added to the solution of 1
(30 lmol), as shown in Fig. S9 (ESM), in which m/z = 318.8 corresponds to
[1 ? Al ? Cl]? with chemical formula C13H12AlClN4S (calcd. 318.7) and
m/z = 301.2 corresponds to [1 ? Al ? H2O] with formula C13H14AlN4OS (calcd.
301.1).
To further understand the recognition process of 1 toward Al3?, highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
distributions of 1 and 1 ? Al3? were determined by density functional theory
(DFT) calculations, in which 6–31 g(d) basic set was used for C, H, N, S, and Cl,
and lanl2dz was used for Al [43]. As shown in Fig. 9, the HOMO distribution of 1
was concentrated exclusively on the amino pyridine moieties without contribution
from benzothiazole group. The LUMO distribution of 1 was concentrated on the
benzothiazole group without contribution from pyridine group. The dihedral angle
between pyridine and benzothiazole groups was about 88.2°. Although the HOMO
distribution of 1 ? Al3? was still concentrated mainly on the amino pyridine
moieties, the contribution of benzothiazole group and Al3? was very obvious. The
LUMO distribution of 1 ? Al3? was also mainly concentrated on the benzothiazole
group, whereas the contribution of pyridine group and Al3? was also very obvious.
The dihedral angle between pyridine and benzothiazole groups changed to about
70.9°, indicating that the structural rigidity was promoted after 1 was bound with
Al3?. This effect made the excited state of 1 ? Al3? radiating. The calculation
results were coincident with our experimental results.
Conclusions
A simple, sensitive ‘‘turn–on’’ chemosensor 1 for detection of Al3? or Fe3? was
synthesized and characterized. The new sensor displayed ratiometric and colori-
metric response toward Al3? or Fe3?. It exhibited fluorescence enhancement with
good sensitivity in the presence of Al3? or Fe3? without significant interference
from other metal ions such as Na?, Hg2?, Mg2?, Mn2?, Co2?, Ca2?, Cd2?, Cu2?
,
no electron transfer
electron transfer
4'
3'
2'
5'
6'
light
light
S
1
H2N
N
N
2
S
H2N
N
1'
N
N
H
Mn+
N
3
Mn+
1 (nonfluorescent)
Scheme 2 Sensing mechanism of 1 towards metal ions
1 (fluorescent)
123

M. Gao et al.
Fig. 9 Distributions of HOMO and LUMO of 1 and 1 ? Al3? determined by DFT calculations
Pb2?, Zn2?, and Ni2?. The sensor could be applied for monitoring Al3? or Fe3?
within a wide pH range. The coordination between 1 and Al3? or Fe3? increased the
rigidity of the molecular skeleton and suppressed the PET process, resulting in
fluorescence enhancement. The results indicated 1 to be a promising ‘‘turn-on’’
optical sensor for Al3? or Fe3?
.
Acknowledgments This project was supported by the National Natural Science Foundation of China
(NSFC) (21102037).
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