A new colorimetric and fluorescent chemosensor based on Schiff base-phenyl-crown ether for selective detection of Al3+ and Fe3+

Article abstract of DOI:10.1016/j.ica.2018.12.036

A novel chemosensor (L) based on Schiff base-phenyl-crown ether was synthesized and evaluated as a chemoselective Al³⁺ and Fe³⁺ sensor in ethanol through “turn-on” fluorescence enhancement and sharp colorimetric responses. Adding 10 equiv. Al³⁺ or Fe³⁺ could change the color of the sensor L from colorless to orange yellow, and increase the fluorescence intensity of L at 596 nm by 160-fold or 130-fold, respectively. The selective response of L to Al³⁺ and Fe³⁺ was not interfered by other ions, and their bonding was reversible. The results of fluorescence spectral titration data showed that the association constants (lgK_s) of L for Al³⁺ and Fe³⁺ were 8.95 and 8.67 in ethanol, and the corresponding detection limits were 0.33 and 0.36 μM, respectively. The Job's plot and mass spectrometry revealed that the sensor L formed a 2:1 complex with Al³⁺ or Fe³⁺. The proposed binding modes of L with Al³⁺ and Fe³⁺ were demonstrated by means of the infrared spectroscopy and ¹H NMR titration. The results of DFT/B3LYP calculations showed that the binding with Al³⁺ and Fe³⁺ makes the sensor L accompanied a mechanism with the inhibition of C[dbnd]N isomerization (cis-trans) and the activation of chelation enhanced fluorescence (CHEF).

Full text of DOI:10.1016/j.ica.2018.12.036

InorganicaChimicaActa487(2019)322–330
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
A new colorimetric and fluorescent chemosensor based on Schiff base-
phenyl-crown ether for selective detection of Al3+ and Fe3+
Guangcui Dong, Kaifei Duan, Qiang Zhang⁎, Zizhong Liu
College of Chemical and Environmental Science, Inner Mongolia Normal University, Hohhot 010022, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A novel chemosensor (L) based on Schiff base-phenyl-crown ether was synthesized and evaluated as a chemo-
selective Al3+ and Fe3+ sensor in ethanol through “turn-on” fluorescence enhancement and sharp colorimetric
responses. Adding 10 equiv. Al3+ or Fe3+ could change the color of the sensor L from colorless to orange yellow,
and increase the fluorescence intensity of L at 596 nm by 160-fold or 130-fold, respectively. The selective re-
sponse of L to Al3+ and Fe3+ was not interfered by other ions, and their bonding was reversible. The results of
fluorescence spectral titration data showed that the association constants (lgKs) of L for Al3+ and Fe3+ were 8.95
and 8.67 in ethanol, and the corresponding detection limits were 0.33 and 0.36 μM, respectively. The Job’s plot
and mass spectrometry revealed that the sensor L formed a 2:1 complex with Al3+ or Fe3+. The proposed
binding modes of L with Al3+ and Fe3+ were demonstrated by means of the infrared spectroscopy and 1H NMR
titration. The results of DFT/B3LYP calculations showed that the binding with Al3+ and Fe3+ makes the sensor L
accompanied a mechanism with the inhibition of C]N isomerization (cis-trans) and the activation of chelation
enhanced fluorescence (CHEF).
Schiff base-phenyl-crown ether
Chemosensor
Colorimetric/fluorescent
Al3+/Fe3+
1. Introduction
environment, the development of sensitive colorimetric and fluorescent
chemosensors which can detect aluminium ions with high selectivity
The design and construction of chemosensors with high selectivity
and sensitivity for various metal ions have become the focus of nu-
merous studies because of their application value in cell biology,
medicine, analytical chemistry and environmental sciences [1–8]. Of
various chemosensors known to date, the optical chemosensors based
on fluorometric and chromogenic analysis have attracted more and
more attention due to their high sensitivity, ultra-fast response, real-
time measurements, convenient operation as well as low costs [9–12].
Therefore, it is very meaningful that great efforts have been devoted to
the study and develop colorimetric and fluorescent chemosensors.
Among metals, aluminum is the most prevalent metallic element
[13]. It is widely used in many fields, from industrial fields such as
water treatment, food additives and pharmaceuticals to our daily life
such as various packaging items, aluminum alloy products and so on
[14,15]. However, there is growing evidence that excessive absorption
of aluminum is harmful to human health. It can lead to diseases such as
Parkinson's disease, Alzheimer’s disease, muscular dystrophy, osteo-
malacia, and dialysis encephalopathy [16–19]. In addition, high con-
centrations of aluminium in the environment can inhibit the growth of
plant roots in soil [20]. Therefore, it is of great importance to develop
the efficient methods for the detection of aluminum in organism and
has important application value. Iron has become an indispensable
element in all living systems due to its vital role in many biological
processes [21,22], such as many enzymes use iron as a catalyst for
oxygen metabolism, electron transfer and the synthesis of DNA and
RNA. However, its lack and excess of normal permissible limits can lead
to physical disorders and health problems, such as anemia and hemo-
chromatosis [23], breast cancer [24], liver damage, and diabetes [25].
Thus, iron plays an important role in biological and environmental
systems. It is becoming more and more important to carry out high
selectivity and high sensitivity detection of iron [26–29].
Schiff base compounds can form stable complexes with some metal
ions, which is due to the fact that nitrogen atoms in their molecules can
be used as good bonding sites for metal ions, which has been widely
used in ion recognition [30–33]. Although many Schiff base derivatives
have been used to detect various metal ions [34–37], the Schiff base
type colorimetric and fluorescent chemosensors for the detection of
Al3+ and Fe3+ ions are rare [38–42]. Furthermore, compared with
other metal cations, the detecting of Al3+ has always been problematic
due to the lack of spectroscopic characteristics, poor coordination
ability comparing to transition metals, strong hydration ability, and
interferences by Fe3+ and Cu2+ [43–45]. Herein, a novel colorimetric
⁎ Corresponding author.
E-mail address: zhangq@imnu.edu.cn (Q. Zhang).
https://doi.org/10.1016/j.ica.2018.12.036
Received 24 July 2018; Received in revised form 17 December 2018; Accepted 21 December 2018
Availableonline23December2018
0020-1693/©2018ElsevierB.V.Allrightsreserved.

G. Dong et al.
InorganicaChimicaActa487(2019)322–330
Density functional theory (DFT) structural optimizations were per-
formed by B3LYP method using Gaussian 09 program [51].
2.3. Synthesis and characterization
The sensor L was synthesized according to the route shown in
Scheme 1 and characterized by 1H NMR, MALDI-TOF-MS and FT-IR.
2.3.1. Synthesis of intermediates
Compounds triethylene glycol di(p-toluenesulfonate), N-phenylaza-
15-crown-5 (1) and 4-formylphenylaza-15-crown-5 (2) were synthe-
sized according to modified methods reported by literature [52,53].
2.3.2. Synthesis of bis-[4- (N-aza-15-crown-5) benzaldehyde]-azine (L)
20 drops of triethylamine was added to ethanol solution (60 mL)
containing 4-formylphenyl-aza-15-crown-5 (1.29 g, 4 mmol), then
stirred for 20 min at 50 °C. Then, the solution of ethanol (30 mL) con-
taining hydrazine hydrate (80%, 0.1 mL, 2 mmol) was added to the
above mixed solution drop by drop during 1 h, then the reaction mix-
ture was refluxed for an additional 5 h. After completion, the mixture
was evaporated under reduced pressure to remove the solvent, and the
residue was recrystallized from acetonitrile and the desired product was
gained as a yellow flake crystal. Yield: 52.5%, mp: 169–170 °C. 1H NMR
(500 MHz, DMSO-d6) (Fig. S3): δ (ppm), 3.51(s, 8H), 3.56–3.59 (m,
24H), 3.66 (t, J = 6.0 Hz, 8H), 6.72 (d, J = 9.0 Hz, 4H), 7.62 (d,
J = 8.5 Hz, 4H), 8.48 (s, 2H). FT-IR (KBr, cm−1) (Fig. S21): 1605.22
(C]N). TOF-MS m/z (Fig. S4): Calcd. for C34H50N4O8: [M+H]+
643.7988; [M+Na]+ 665.7812. Found: [M+H]+ 643.3716; [M
+Na]+ 665.3544.
Scheme 1. The synthesis route of bis-[4-(N-aza-15-crown-5) benzaldehyde]-
azine (L).
and fluorescent chemosensor (L) based on Schiff base phenyl crown
ether was designed and synthesized. The synthesis route is shown in
Scheme 1. This novel colorimetric and fluorescent chemosensor L has
high selectivity to Al3+ and Fe3+, compared with other metal ions
detected.
2. Experimental
2.1. Materials and instrumentation
All organic solvents and chemicals that were of dry and analytical
reagent grade were obtained from commercial suppliers and used
without further purification. All of the solutions of metal ions (Li+, K+
Na+, Co2+, Ba2+, Cd2+, Hg2+, Ca2+, Cu2+, Mg2+, Mn2+, Ni2+, Sr2+
,
,
3. Results and discussions
Pb2+, Zn2+, Al3+, and Fe3+) were prepared from the corresponding
metal chloride salt. Elemental analyses were done on Vario instrument
Micro Elemental analyzer. Melting points were determined using a
Beijing XT5 microscopic melting point apparatus. The UV-vis spectra
and fluorescence spectra were determined on a Hitachi U-2900 UV-vis
3.1. The selective sensing
The selective sensing ability of sensor L to different metal ions (Li+
,
,
K+, Na+, Co2+, Ba2+, Cd2+, Hg2+, Ca2+, Cu2+, Mg2+, Mn2+, Ni2+
Sr2+, Pb2+, Zn2+, Al3+, and Fe3+) was studied by UV-vis spectroscopy
in ethanol solvent. As shown in Fig. 1, the free sensor L showed a strong
absorption peak at about 387 nm, when 10 equiv. of Al3+ or Fe3+ ions
were added to the L solution, the absorption peak of L at 387 nm was
decreased significantly, while a new absorption peak occurred at
491 nm, and this absorption peak of L enhanced by Al3+ was twice as
much as that by Fe3+. This interaction of Al3+ or Fe3+ with sensor L
resulted in the color of L solution to change from colorless to light
spectrophotometer and
a Hitachi F-4600 Fluorescence spectro-
photometer, respectively. 1H NMR spectra were measured on a Bruker
Advance III 500 MHz instrument spectrometer using TMS as the in-
ternal standard. The mass spectra were recorded on a Waters Xevo G2-
XS QTOF MS spectrometer. The Fourier transform infrared (FT-IR)
spectra were carried out on a Nicolet FT-IR 6700 spectrometer (Nicolet
Co., USA).
2.2. Calculating methods
The limit of detection (LOD) was calculated by using the equation,
(3δblank/K), where δblank was the standard deviation of the blank
measurements (by ten times), K was the slope of the fit lines in titration
experiment [46–48].
According to the linear fitting of the spectral titration data, the as-
sociation constant Ks was calculated based on a Li’s Eq. (1) [49,50]
shown below:
n
1
Imax
Imax Imin
I
= nKs·
;
=
1
[M]m·[L]nT
1
(1)
Herein, Imin, I and Imax were the fluorescence (or absorption) in-
tensities of L in the absence of M, at an intermediate concentration of
the M, and at a concentration of complete interaction, respectively. [M]
was the concentration of free metal ions added. Where (L) and (M) are
assumed to form a complex (MmLn) with a complexation ratio of m: n. α
was defined as the ratio between the free ligand concentration [L] and
the initial concentration of ligand [L]T, which can be determined by
changes in the fluorescence (or absorption) intensity in the presence of
different concentrations of M. Ks was the association constant.
Fig. 1. UV-vis absorption spectra of L (8.2 μM) in the presence of 10.0 equiv. of
various metal ions in ethanol.
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Fig. 2. Color changes of L (8.2 μM) in the presence of 10 equiv. of various metal ions in ethanol. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
Fig. 3. Fluorescence emission spectra (λex. = 500 nm) of sensor L (8.2 μM) in
Fig. 4. The fluorescence intensity histogram of
L
(8.0 μM) and
L
the presence of 10.0 equiv. of various metal ions in ethanol. Inset: pictures of
(8.0 μM) + Al3+ (10 equiv.) at 596 nm in the presence and absence of other
matal ions (10 equiv.) under the identical experiment conditions in ethanol
(λex = 500 nm).
the fluorescence corresponding to L, L + Fe3+ and L + Al3+ (λex = 500 nm).
orange for Fe3+ ions and from colorless to orange yellow for Al3+ ions,
which could be easily observed by the naked eye (Fig. 2). Other metal
ions did not cause significant changes in the absorption spectra of the
sensor L, which indicated that L can be used as a colorimetric selective
sensor for Al3+ and Fe3+ and for detect their presence.
In ethanol solvent, the selective fluorescence spectral behaviour of
sensor L to various metal ions has been investigated at room tem-
perature, as shown in Fig. 3. The emission spectra of free sensor L
showed only a very weak fluorescence on excitation at 500 nm. When
10 equiv. of various metal ions were added to L solution, only Al3+ and
Fe3+ could induce L to produce 160-fold and 130-fold fluorescence
enhancement at 596 nm. The light green fluorescence of sensor L, the
light orange fluorescence induced by Fe3+ and the deep orange fluor-
escence induced by Al3+ were easily distinguished by the naked eye
(Inset of Fig. 3). Importantly, other metal ions could not activate the
fluorescence of L except for Al3+ and Fe3+. Obviously, both Al3+ and
Fe3+ exhibited efficient “turn-on” fluorescence behavior during the
course of fluorescence response test. The observed fluorescence en-
hancement may be attributed to the inhibition of C]N isomerization
(cis-trans) and the formation of a rigid structure by binding L with one
of Al3+ and Fe3+ causing a highly efficient chelation enhanced fluor-
escence effect (CHEF) [54,55].
Fig. 5. The fluorescence intensity histogram of
L
(8.0 μM) and
L
(8.0 μM) + Fe3+ (10 equiv.) at 596 nm in the presence and absence of other
matal ions (10 equiv.) under the identical experiment conditions in ethanol
(λex = 500 nm).
3.2. Competitive selectivity experiments
In order to investigate whether the sensor L can selectively detect
Al3+ and Fe3+ under the condition of the coexistence of other metal ions,
competitive selectivity experiments were carried out in ethanol solvent.
Upon the addition of 10 equivalent of Al3+ ions to L solution in the
presence of equivalent amount of other metal ions, the fluorescence
enhancement at 596 nm induced by Al3+ was retained and not interfered
by other added competitive metal ions (Fig. 4). The same results were
obtained from the competition experiments measured by UV-vis spectra
(Fig. S5). Similarly, the presence of other competing ions showed neg-
ligible disturbance to the detection of Fe3+ except for Al3+. Only when
Al3+ were added to the solution of sensor L and Fe3+, the fluorescence
intensity and UV-vis absorption of the corresponding system were ob-
viously increased (Figs. 5 and S6). It showed Al3+ could displace Fe3+ in
its metal-ligand complex, L-Fe3+, indicating the order of stability of
corresponding metal-ligand complexes (L-Al3+ > L-Fe3+).
3.3. Reversibility study
As an excellent colorimetric and fluorescent chemical sensor, it was
the prerequisite of its practical application that it can reversibly bind
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Fig. 7. UV-vis absorption changes of L (8.0 μM) in ethanol upon gradual ad-
dition of Al3+ (0–20 equiv.).
1347.6586 and 1373.9893 in the MALDI-TOF-MS (Figs. S13 and S14),
+
these could be assigned to [2L + Al3++Cl−-H+
]
(calcd. 1347.08)
and [2L + Fe3+ + CH3OH]+ (calcd. 1373.47). Therefore, the results of
mass spectrometry further confirmed the 2:1 binding stoichiometry
between L and Al3+ or Fe3+
.
3.5. The spectroscopic titration experiments
The UV-vis spectral titration experiments were performed to in-
vestigate the binding properties of L with Al3+ and Fe3+ in ethanol
(Figs. 7 and 8). With the concentration of Al3+ or Fe3+ was increased in
L solution, the absorption band of L at 387 nm was gradually decreased
in intensity, whereas the absorption band at 491 nm was gradually in-
creased, and a roughly isosbestic point could be clearly observed at
428 nm. When Al3+ and Fe3+ were increased to 20 and 25 times of the
concentration of sensor L, the absorption band of L at 491 nm reached
the maximum absorption, respectively. These observations clearly in-
dicate that the free sensor L in the equilibrium between them un-
doubtedly formed the corresponding aluminum complex or iron com-
plex.
Fig. 6. (a) Job’s plot for determining the stoichiometry of sensor L and Al3+ in
ethanol (the total concentration of L and Al3+ was 29.0 μM, λex = 500 nm). (b)
Job’s plot for determining the stoichiometry of sensor L and Fe3+ in ethanol
(the total concentration of L and Fe3+ was 27.0 μM, λex = 500 nm).
with the analyte. Thus, the reversibility of the binding process of the
sensor L toward Al3+/Fe3+ was confirmed by adding an Al3+/Fe3+
bonding agent, Na2EDTA. After addition of 10 equiv. Na2EDTA to the
ethanol solution of L with Al3+ (10 equiv.) and L with Fe3+ (10 equiv.),
respectively, the absorption band at 491 nm of L-Al3+ complex and L-
Fe3+ complex were disappeared and recovered to 387 nm (Figs. S7 and
S8). Similarly, when the same amount of Na2EDTA as Al3+ and Fe3+
was added to the system of L-Al3+ and L-Fe3+, the fluorescence emis-
sion intensity of the sensor L induced by Al3+ and Fe3+ at 596 nm was
reduced and recovered, all this implied that the free L was regenerated
(Figs. S9 and S10). These results showed that the changes of the ab-
sorption and fluorescence spectra of L induced by Al3+ and Fe3+ ions
were reversible. Therefore, the sensor L could be used as a reversible
colorimetric and fluorescent chemosensor for Al3+ and Fe3+ ions in
ethanol.
To get a further insight into the fluorescence sensing properties of
sensor L, a quantitative investigation of the binding of sensor L with
Al3+ and Fe3+ was studied by fluorescence titration. Figs. 9 and 10
3.4. Stoichiometry measurements
As shown in Fig. 6a and b, the Job's plot completed by fluorescence
spectra measurement showed that the binding stoichiometry for the
sensor L to Al3+ or Fe3+ was both 2:1. The Job's plot measured by UV-
vis spectrometry showed the same stoichiometric result (Figs. S11 and
S12). In addition, the binding stoichiometry for the sensor L to Al3+
and Fe3+ was further confirmed by the appearance of a peak at m/z
Fig. 8. UV-vis absorption changes of L (8.0 μM) in ethanol upon gradual ad-
dition of Fe3+ (0–25 equiv.).
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Fig. 9. Fluorescence spectra changes of L (8.0 μM) in ethanol upon addition of
increasing concentration of Al3+ (0–20 equiv.), λex = 500 nm.
Fig. 11. Association constant (Ks) of L (8.0 μM) for Al3+(a) and Fe3+(b)
through absorption intensity, assuming 2:1 stoichiometry for association be-
tween L and Al3+ (or Fe3+) in ethanol.
3.6. 1H NMR titration, IR analysis of the binding modes of sensor L with
Fig. 10. Fluorescence spectra changes of L (8.0 μM) in ethanol upon addition of
Al3+ and Fe3+
increasing concentration of Fe3+ (0–25 equiv.), λex = 500 nm.
In order to confirm the binding modes between the sensor L and
Al3+ / Fe3+, the 1H NMR spectrometric titration experiments were
carried out in the mixed solvent of DMSO-d6/CDCl3 (9:1 v / v). During
the titration, the concentration of L was kept constant, and the amounts
of Al3+ or Fe3+ increased gradually. As shown in Fig. 12a and b, when
increasing the amounts of Al3+ or Fe3+ to the solution of sensor L, it
was observed that the-HC = NeN = CHe group proton signal of sensor
L at 8.478 ppm was shifted to the high field to 8.454 ppm and
8.430 ppm, respectively. With the increasing of the equivalent number
of Al3+ or Fe3+ ions, a new peak appeared at the low field of about
9.6 ppm, which may be attributed to the interaction of azino-group with
Al3+ or Fe3+ ions [37]. In addition, the proton signal at about 8.32 ppm
can be attributed to the chloroform solvent peak. These results showed
that the two nitrogen atoms on the azino-group in the sensor L were
involved in the bonding with Al3+ and Fe3+, respectively.
showed that with the addition of increasing concentration of Al3+ or
Fe3+, the fluorescence intensity of L at about 596 nm increased gra-
dually. When the concentration of Al3+ or Fe3+ increased to 20 or 25
times of the concentration of the sensor L, the fluorescence intensity
reached the maximum, respectively.
The association constants (lgKs) of L-Al3+ and L-Fe3+ in the ethanol
solution were obtained by the linear fitting method according to the
UV-vis and the fluorescence spectrum titration data (Figs. 7–10), re-
spectively. The lgKs values of L-Al3+ and L-Fe3+ measured by UV–vis
spectroscopy were 8.97 and 8.82, respectively (Fig. 11), and the lgKs
values measured by fluorescence spectrometry were 8.95 and 8.67,
respectively (Figs. S15 and S16). In addition, based on the UV-vis and
fluorescence titration measurements, the detection limits of L for Al3+
and Fe3+ were calculated to be 3.5 μM and 4.4 μM respectively by
absorption spectrometry, and 0.33 μM and 0.36 μM by fluorescence
spectrometry, respectively (Figs. S17, S18, S19 and S20). Therefore, the
detection limit was low enough for the sensor L to detect the con-
centration of Al3+ or Fe3+ in environment and biological system by
using UV-vis spectroscopy and fluorescence spectroscopy. And when
fluorescence spectrometry was used, it was far below than the max-
imum allowable Al3+ concentration (7.41 μM) in drinking water lim-
ited by the World Health Organization (WHO) [56].
In order to provide further evidence to confirm the binding modes of
L to Al3+ and Fe3+, the IR spectra of L-Al3+ and L-Fe3+ were collected
and compared with those of L. In IR spectra, the C]N vibrational sharp
peak of the sensor L at 1605.22 cm−1 (Fig. S21), which was weakened
and shifted to 1607.29 cm−1 (Fig. S22) and 1597.59 cm−1 (Fig. S23)
respectively in its corresponding L-Al3+ and L-Fe3+ complexes. The
changes further indicated that the nitrogen atom in the C]N group
were involved in the bonding with Al3+ and Fe3+
.
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Fig. 12. (a) 1H NMR titrations of L with the addition of Al3+ (0, 0.25, 0.53 and 1.0 equiv.) in DMSO‑d6/CDCl3(9:1, v/v) mixed solvents; (b) 1H NMR titrations of L
with the addition of Fe3+ (0, 0.51, 1.03 and 1.73 equiv.) in DMSO-d6/CDCl3(9:1, v/v) mixed solvents.
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Scheme 2. Proposed binding mode of L for Al3+ and Fe3+
.
Fig. 13. Optimized geometry and molecular orbitals for L and L-Al3+
.
Based on the experimental results of Job's plots, mass spectrometry,
6–311 g as basis set and in ethanol using conductor-like polarizable
continuum model (CPCM) [57–59]. The energy-minimized structures of
L and its complexes with Al3+ and Fe3+ were optimized, respectively.
The spatial distributions and orbital energies of the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) of L and its corresponding metal complexes were also calcu-
lated and shown in Figs. 13 and S24. Obviously, the strong absorption
band of chemical sensor L at 387 nm should correspond to the transition
of HOMO → LUMO (ΔE = 3.21 eV), while the HOMO-LUMO energy
1H NMR titration and IR analysis, the possible binding modes of sensor
L with Al3+ and Fe3+ were given in Scheme 2.
3.7. DFT calculations
In order to further confirm the proposed mechanism of the chemical
sensor L and its corresponding metal-ligand complexes, we performed
quantum chemical computational studies by DFT/B3LYP method using
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band gap of L-Al3+ complex (ΔE = 2.47 eV) was significantly lower
than that of sensor L (Fig. 13) which supports the movement of ab-
sorption band to longer wavelength on coordination to Al3+ (387 nm in
free L to 491 nm in L-Al3+). As shown in Fig. 13, the HOMO and LUMO
of sensor L in the ground state show obvious π and π* features, re-
spectively. Since the isomerization process of cis structure and trans
structure of L only need to overcome the low energy barrier, the C]N
isomerization of L was very easy, which leads to the fluorescence
quenching of sensor L, and L was mainly in the form of a more stable
trans structure in equilibrium. The sensing mechanism of L towards
Al3+ and Fe3+ resulted from the formation of metal-ligand complexes,
which restricted the free rotation of C]N relative to benzene ring,
increased the rigidity of molecular assembly and inhibited the non-ra-
diative processes such as C]N isomerization. Chelation enhanced
fluorescence (CHEF) effect and the inhibition of C]N isomerization
played a very important role in activation of fluorescence.
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4. Conclusion
A new crown ether benzaldehyde based Schiff base L was synthe-
sized and can be used as a colorimetric and fluorescent chemosensor for
Al3+ and Fe3+ detection in ethanol. The binding force of the sensor L to
Al3+ ion was greater than that of Fe3+ ion, and the detection limit of
the two ions reached the level of micro-molar concentration. The sensor
L used diazo atoms in its molecule to bind with Al3+ and Fe3+, and
their stoichiometric ratios were both 2: 1. The free L and its metal ion
complexes have the characteristic of absorption spectra by ππ* transi-
tion. The very weak fluorescence of sensor L was due to the easily oc-
curring isomerization of C]N. The DFT calculation supported the
sensing mechanism of the chelation enhancement fluorescence (CHEF)
effect of L on Al3+ and Fe3+, and the inhibiting the isomerization of
C]N by chelation.
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