A Schiff-base receptor based chromone derivate: Highly selective fluorescent and colorimetric probe for Al(III)

Article abstract of DOI:10.1016/j.saa.2019.03.101

Upon excitation of the visible light, probes show colorimetric and fluorescent responses to the specific metal ion, which can be easily detected by the naked eye. Owing to the excitation of the visible light at 423 nm, a novel and simple Schiff-base receptor based chromone derivative called 7-methoxychromone-3-carbaldehyde-(indole-3-formyl) hydrazone (MCIH2) had been investigated as a selective and sensitive probe for Al³⁺ with colorimetric and fluorescent responses. Upon addition of Al³⁺ to compound MCIH2 solution, compound MCIH2 could respond to Al³⁺ with a good selective colorimetric signal, which was easily observed from colorless to yellow-green by the naked eye. Furthermore, a remarkable fluorescence emission enhancement with an “OFF-ON” signal by over 700-fold was triggered, but other various metal ions had no such significant effects on the fluorescence emission. In addition, the detection limit of compound MCIH2 for recognizing Al³⁺ was evaluated to be as low as 1 × 10^?7 M level, which was sufficiently low for sensing Al³⁺ widely distributed in various environmental and biological systems.

Full text of DOI:10.1016/j.saa.2019.03.101

Accepted Manuscript
A Schiff-base receptor based chromone derivate: Highly selective
fluorescent and colorimetric probe for Al(III)
Long Fan, Jing-can Qin, Chao-rui Li, Zheng-yin Yang
PII:
DOI:
Reference:
S1386-1425(19)30350-6
https://doi.org/10.1016/j.saa.2019.03.101
SAA 16991
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 2 January 2019
Revised date:
Accepted
date:
23 March 2019
28 March 2019
Please cite this article as: L. Fan, J.-c. Qin, C.-r. Li, et al., A Schiff-base receptor based
chromone derivate: Highly selective fluorescent and colorimetric probe for Al(III),
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/
10.1016/j.saa.2019.03.101
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ACCEPTED MANUSCRIPT
A Schiff-base receptor based chromone derivate: Highly selective
fluorescent and colorimetric probe for Al(III)
Long Fana,b, Jing-can Qina, Chao-rui Lia , Zheng-yin Yanga,*
a
College of Chemistry and Chemical Engineering, State Key Laboratory of Applied Organic
Chemistry, Lanzhou University, Lanzhou 730000, P.R. China
b Quality and Technical Supervision and Inspection of Jin Chang, Jin Chang 737100, People’s
Republic of China
**Corresponding author. Tel.: +86 931 8913515; Fax: +86 931 8912582; e-mail:
yangzy@lzu.edu.cn (Z.Y. Yang)
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Abstract:
Upon excitation of the visible light, probes show colorimetric and fluorescent responses to
the specific metal ion, which can be easily detected by the naked eye. Owing to the excitation of
the visible light at 423 nm, a novel and simple Schiff-base receptor based chromone derivative
called 7-methoxychromone-3-carbaldehyde-(indole-3-formyl) hydrazone (MCIH2) had been
investigated as a selective and sensitive probe for Al3+ with colorimetric and fluorescent
responses. Upon addition of Al3+ to compound MCIH2 solution, compound MCIH2 could
respond to Al3+ with a good selective colorimetric signal, which was easily observed from
colorless to yellow-green by the naked eye. Furthermore, a remarkable fluorescence emission
enhancement with an “OFF-ON” signal by over 700-fold was triggered, but other various metal
ions had no such significant effects on the fluorescence emission. In addition, the detection limit
of compound MCIH2 for recognizing Al3+ was evaluated to be as low as 1 × 10-7 M level, which
was sufficiently low for sensing Al3+ widely distributed in various environmental and biological
systems.
Keywords: Chromone; Colorimetric and fluorescent probe; Aluminum ion; Selectivity;
Sensitivity
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Graphical Abstract
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1. Introduction
Up to date, a series of analytical methods, including atomic absorption spectrometry (AAS)
[1], inductively coupled plasma atomic emission spectrometry (ICP-AES) [2], inductively
coupled plasma mass spectroscopy (ICP-MS) [3], technology of electrochemistry [4], volumetric
precipitation method [5] and colorimetric technique [6], have been applied for sensing various
envionmentally and biologically relevant metal ions, but the shortcomings of expensive
apparatus and equipment, high instrumental maintenance cost and complicated sample
preparations in these methods restrict their applications for environmental monitoring and
biological assays [7-9]. In comparison, owing to the advantages of simplicity, rapid response,
good selectivity, high sensitivity and non-destructive property, fluorescence sensing method has
drawn appealing attention among researchers in the broad fields of biochemistry [10], analytical
chemistry [11], medical science [12] and environtology [13], and a large number of fluorescent
probes for sensing a variety of metal ions have been developed so far [14-18].
Among various main group and transition metal in the periodic table, aluminum is
extensively distributed in the environment around us, which is always caused by the effulent
from industrial, agricultual and household waste water [19-21]. As the third abundant element in
the earth’s crust [22], the use of aluminum is found in many aspects, such as foodstuff packaging
bags [23], food ingredients [24], healing drugs [25] and aluminum alloys [26]. Therefore, the
most common state of aluminum (Al3+) can accumulate in the human bodies through the food
chain. As Al3+ is useless in various biochemical processes, the superabundant Al3+ is harmful to
kidney [27], liver [28], central nervous system [29] and immunity system [30], inducing a series
of neurodegenerative diseases like cerebropathy [31], Alzheimer’s disease [32] and Parkinson’s
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disease [33], and histological diseases like brittle-bone disease [34] and myonosus [35].
Additionally, the risk of breast cancer even increases, which is deadly to human beings [36].
Thus, the development of Al3+ fluorescent probes with simplicity, good selectivity and high
sensitivity is highly desirable for the purpose of environment protection and disease prevention
[37-38].
A large deal number of fluorescent probes for recognizing and sensing Al3+ have been
reported, but most of them suffer from the excitation of light at the wavelength in the ultraviolet
range, which hardly display UV-vis absorption spectra and color changes with addition of Al3+.
Thus, these probes respond to Al3+ with single fluorescence signals [39-42]. Nevertheless, upon
excitation of the visible light, the probes show colorimetric and fluorescent responses to Al3+,
which can be easily detected by the naked eye [43]. Bearing these considerations in mind, we
designed and synthesized a novel and simple Schiff-base receptor based chromone derivative
called 7-methoxychromone-3-carbaldehyde-(indole-3-formyl) hydrazone (MCIH2), which could
be identified as a colorimetric and fluorescent probe towards Al3+ with good selectivity and high
sensitivity due to the excitation of the visible light at 423 nm. Additionally, this compound
MCIH2 exhibited perfect reversibility and regeneration for sensing Al3+, which was a
prerequisite in developing compound MCIH2 for practical applications.
2. Experimental
2.1. Materials and instrumentation
All chemicals were obtained from commercial suppliers and used with no further
1
purification needed. A Bruker 400 MHz instrument was used to measure H NMR spectra by
utilizing TMS as an internal standard and DMSO-d6 as a solvent. A Bruker esquire 6000
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spectrometer was utilized for determining ESI-MS spectra in ethanol. A VERTEX-70
spectrometer with KBr disks was used for acquiring FT-IR spectra in the range of 4000-400 cm-1.
A Shimadzu UV-240 spectrophotometer was used to conduct UV-vis absorption spectra
recorded at 298 K. A Hitachi RF-4500 spectrophotometer equipped with quartz cuvettes of 1 cm
path length was utilized for recording fluorescence spectra. A Beijing XT4-100x microscopic
melting point apparatus was used to measure the values of melting points with no rectification.
2.2. Synthesis
7-methoxychromone-3-carbaldehyde was obtained according to the procedure of the
previously reported literature [44]. Synthesis of the final product MCIH2 was based on the
following method (Scheme 1): An ethanol solution (12 mL) of 1H-indole-3-carboxylic acid
hydrazide (0.175 g, 1 mmol) was added dropwise to another ethanolic solution (20 mL) of
7-methoxychromone-3-carbaldehyde (0.204 g, 1 mmol). After refluxing the reaction mixture
under agitating on an electric mixer for 4 h, the precipitated pale yellow solid was filtered,
washing with ethanol for five times. Subsequently, the recrystallization of the crude product
from ethanol furnished 7-methoxychromone-3-carbaldehyde-(indole-3-formyl) hydrazone
(MCIH2) as a pale yellow powder, which was dried under vacuum. Yield: 73.6%. m.p: 224-226
oC. 1H NMR (400 MHz, DMSO-d6) (Fig. S1): δ (ppm): 11.71 (s, 1H, H7), 11.45 (s, 1H, H9), 8.71
(s, 1H, H5), 8.41 (s, 1H, H6), 8.17 (d, 2H, H8, 10), 7.98 (d, 1H, H4), 7.44 (d, 1H, H11), 7.17 (d, 1H,
H12), 7.15 (d, 1H, H13), 7.13 (s, 1H, H3), 7.08 (d, 1H, H2), 3.95 (s, 3H, H1). MS (ESI): m/z 362.1
[M + H+]+. FT-IR (KBr pellet, cm-1) (Fig. S2): 3254, 1621, 1601, 1561.
2.3. Analysis
Ethanol was utilized for preparing stock solutions (5 mM) of the nitrate salts of Al3+, Li+,
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Na+, K+, Ag+, Ba2+, Ca2+, Co2+, Cd2+, Mn2+, Cr3+, Fe3+, Cu2+, Hg2+, Pb2+, Ni2+, Mg2+, Zn2+ and
Fe2+. Stock solution of compound MCIH2 (5 mM) was also prepared in ethanol. In the processes
of all the UV-vis absorption and fluorescence measurements, to prepare test solutions, we placed
20 μL of the stock solution of compound MCIH2 into cuvettes and added an appropriate aliquot
of each ions stock, which was followed by the dilution with ethanol to 2 mL. For all fluorescence
measurements, the excitation wavelength was set at 423 nm and the fluorescence emission
spectra were recorded over the range of 430-650 nm. Both the excitation and emission slit widths
were set at 3.0 nm.
3. Results and discussion
3.1. Electronic absorption spectra
The UV-vis absorption responses of compound MCIH2 to various metal ions including
Al3+, Li+, Na+, K+, Ag+, Ba2+, Ca2+, Co2+, Cd2+, Mn2+, Cr3+, Fe3+, Cu2+, Hg2+, Pb2+, Ni2+, Mg2+,
Zn2+ and Fe2+ were conducted to explore the colorimetric properties of MCIH2 towards these
metal ions. From Fig. 1 showing the UV-vis absorption spectra of compound MCIH2 with
addition of these different metal ions, we could clearly observe that only the introduction of Al3+
induced a significant enhancement in the absorption bands at 403 nm, 424 nm and 448 nm with a
decrease in another absorption band centered at 314 nm, resulting in a color change from
colorless to yellow-green, which could be easily detected by the naked eye. However, the
addition of 5.0 equiv. of other representative metal ions investigated did not give rise to obvious
UV-vis absorption spectra and color changes (Fig. 1). In addition, with incremental addition of
Al3+ to the solution of compound MCIH2, the absorbance at 403 nm, 424 nm and 448 nm
gradually increased with a continuous decrease in the absorbance at 314 nm, causing an
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appearance of a clearly isosbestic point at 351 nm, which indicated the formation of a stable
MCIH2-Al3+ complex (Fig. 2) [45-47]. These results suggested that compound MCIH2 could
be served as a selective potential colorimetric probe for “naked-eye” detection of Al3+.
3.2. Emission studies
Before investigating the fluorescence properties of compound MCIH2 towards various
metal ions, the effect of solvent on the fluorescence emission spectra of compound MCIH2 was
carried out. As depicted in Fig. S3, colorless and nonfluorescent solutions were obtained by
dissolving compound MCIH2 in a series of organic solvents including ethanol, methanol,
acetonitrile, N, N-dimethyl formamide (DMF), tetrahydrofuran (THF) and dimethylsulfoxide
(DMSO). Nevertheless, upon excitation at the visible light of 423 nm, the separate addition of
Al3+ to the ethanolic and methanolic solution of compound MCIH2 both led to a certain
enhancement in the fluorescence emission band centered at 489 nm, but the emission intensity in
ethanol was higher than that in the case of methanol (Fig. S3). Therefore, ethanol was chosed as
the superior solvent for the following fluorescence investigations.
In fluorescence emission study, the selectivity of compound MCIH2 towards various metal
ions, such as Al3+, Li+, Na+, K+, Ag+, Ba2+, Ca2+, Co2+, Cd2+, Mn2+, Cr3+, Fe3+, Cu2+, Hg2+, Pb2+,
Ni2+, Mg2+, Zn2+ and Fe2+ was firstly determined. As can be seen from Fig. 3, the free compound
MCIH2 hardly exhibited fluorescence emission bands in the range of 430-650 nm and was
almost non-fluorescent in ethanol. Fascinatingly, the treatment of Al3+ (1.0 equiv.) into
compound MCIH2 solution induced a remarkable enhancement in the fluorescence emission
band centered at 489 nm and an apparent fluorescence color change to yellow-green with excited
at the visible light of 423 nm, but negligible changes in the fluorescence emission spectra and
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fluorescence color changes were observed when compound MCIH2 was treated with an
excessive amount of other cations (5.0 equiv.) (Fig. 3). These results proved good selectivity of
compound MCIH2 towards Al3+ over other various metal ions.
Subsequently, the fluorescence emission titration spectra of compound MCIH2 in the
existence of increasing amounts of Al3+ were conducted to explore the mechanism for the
detection of Al3+ by MCIH2. As illustrated in Fig. 4, the fluorescence emission of compound
MCIH2 alone was extremely weak with a low quantum yield value of 0.060 in ethanol, which
was due to the fluorescence quenching by the photoinduced electron-transfer (PET) phenomenon
from lone pair electrons of the Schiff-base nitrogen atom to the chromone moiety. Nevertheless,
when increasing amounts of Al3+ (0-1.0 equiv.) were injected to the solution of compound
MCIH2, gradual enhancement in the fluorescence emission band centered at 489 nm was
observed, and its intensity increased by more than 700-fold with the value of fluorescence
quantum yield changing to 0.367 (Fig. 4), which was attributed to the chelation of compound
MCIH2 with Al3+, leading to the chelation-enhanced fluorescence (CHEF) process caused by
the interruption of the PET phenomenon, thereby restoring the fluorescence emission of the
chromone moiety in compound MCIH2 [48-49]. According to the fluorescence spectra of
compound MCIH2 with addition of increasing concentrations of Al3+ (0.10-0.30 μM) showing a
certain fluorescence emission enhancement in ethanol, the detection limit of compound MCIH2
for recognizing Al3+ was evaluated to be as low as 1 × 10-7 M level (Fig. S4), which was
sufficiently low for sensing Al3+ widely distributed in various environmental and biological
systems [50-51].
So as to test the possibility of utilizing compound MCIH2 as a practical selective
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fluorescent probe for Al3+, competition experiments were carried out by treating compound
MCIH2 with 1.0 equiv. of Al3+ in the existence of 5.0 equiv. of other metal ions in ethanol, and
the results were illustrated in Fig. 5. Compound MCIH2 responding for Al3+ in the existence of
Fe2+ was relatively low but clearly detectable, but the fluorescence emission intensity at 489 nm
of compound MCIH2 in the existence of Al3+ with other various metal ions was nearly identical
to that in the case of Al3+ alone, indicating relatively low interferences from the existence of
other metal ions for sensing Al3+ (Fig. 5). Thus, this compound MCIH2 was shown to be a
promising selective fluorescent probe for Al3+ in the presence of most competing metal ions.
3.3. Binding studies
In order to determine the chemical reaction stoichiometry between compound MCIH2 and
Al3+, the method of continuous variation (Job's plot) was used. During this experimental process,
the total concentration of compound MCIH2 and Al3+ was kept constant at 20.0 μM with a
continuous variable molar fraction of the host ([MCIH2]/[MCIH2] + [Al3+]). From Fig. S5
showing the relationship of fluorescence emission intensity at 489 nm as a function of the molar
fraction of the host, we could clearly see that the value approached a maximum when the molar
fraction of compound MCIH2 was 0.67, which meant that compound MCIH2 formed a 2 : 1
complex with Al3+. Additionally, the formation of this 2 : 1 complex was further confirmed by
the appearance of a characteristic peak at m/z 747.1 assignable to [2MCIH2 + Al3+ – 2H+]+ in
the ESI-MS spectrum of MCIH2-Al3+ complex (Fig. S6). On a basis of 2:1 ratio stoichiometry
between compound MCIH2 and Al3+, the association constant K of compound MCIH2 with
Al3+ was then calculated to be 2.5 × 103 M-1 from fluorescence titration experiments with a good
linear relationship by the Benesi-Hildebrand equation (Eq. (1)) (Fig. S7) [52-53], where Fmax, F,
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and Fmin are fluorescence emission intensities of compound MCIH2 in the existence of Al3+ at
saturation, any intermediate Al3+ concentration, and in the absence of Al3+.
(1)
Moreover, FT-IR spectrum of MCIH2-Al3+ complex was then conducted to investigate the
binding mode of compound MCIH2 with Al3+. Compared with that of compound MCIH2
shown in Fig. S3, FT-IR spectrum of MCIH2-Al3+ complex exhibited a blue-shift of the
stretching vibration band of the carbonyl group (C=O) from the chromone moiety, the carbonyl
group (C=O) from amide and the -C=N- group by 26 cm-1, 40 cm-1 and 18 cm-1 respectively.
Furthermore, the absorption band at 696 cm-1 and 502 cm-1 or so emerged, which was assigned
to ν(Al-O) and ν(Al-N) respectively, demonstrating the formation of coordination bonds between
Al3+ and oxygen or nitrogen atom (Fig. S2) [54]. From the results above, it was concluded that
two oxygen atoms of the carbonyl groups (C=O) and the Schiff-base nitrogen atom in compound
MCIH2 participated in the coordination with Al3+ and formed a stable MCIH2-Al3+ complex.
1
In addition, H NMR titration experiments provided additional evidence for the interaction
between compound MCIH2 and Al3+, which were carried out by measuring 1H NMR spectra of
compound MCIH2 with and without addition of Al3+ in DMSO-d6 solution and recording the
corresponding spectral changes. We found that the lone pair electron from Schiff-base nitrogen
atom caused the photo-induced electron transfer (PET) phenomenon. However, upon
complexation of compound MCIH2 with Al3+, the electron cloud density of the Schiff-base
nitrogen atom was decreased, inducing the inhibition of the PET phenomenon, which resulted in
the up-field shift of the proton signals of H4, H6, H7 and H9 in compound MCIH2. As shown in
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Fig. 6, upon addition of 1.0 equiv. of Al3+ to compound MCIH2 solution, the proton signals of
H4, H6, H7 and H9 were shifted up-field by 0.05, 0.08, 0.02 and 0.07 respectively, while the
signals of other protons almost remained unchanged (Table 1), demonstrating the direct
involvement of the oxygen atom of carbonyl group (C=O) from the chromone moiety, the
oxygen atom of carbonyl group (C=O) from amide and the Schiff-base nitrogen atom in the
coordination with Al3+.
Finally, reversibility is a prerequisite in developing novel probes for practical applications,
which was discussed by introducing a strong Al3+ chelation agent called Na2EDTA to a mixture
of compound MCIH2 and Al3+ in ethanol. As can be seen from Fig. S10, the addition of
Na2EDTA to the mixture of MCIH2 and Al3+ resulted in nearly complete diminution of
fluorescence emission intensity at 489 nm, and this value was recovered by the sequential
addition of Al3+ again (Fig. S8), suggesting the perfect reversibility and regeneration of this
compound MCIH2 towards Al3+, which were important for the fabrication of devices for sensing
Al3+.
4. Conclusion
In conclusion, A novel and simple Schiff-base receptor based chromone derivative called
7-methoxychromone-3-carbaldehyde-(indole-3-formyl) hydrazone (MCIH2) was designed,
synthesized and recognized as a colorimetric and fluorescent probe for sensing Al3+ in ethanol.
Owing to the possibility of exciting the chromone moiety in compound MCIH2 at the visible
light of 423 nm, this compound MCIH2 could be served as a selective potential colorimetric
probe for “naked-eye” detection of Al3+. Moreover, this compound MCIH2 was shown to be a
promising selective and sensitive fluorescent probe for Al3+ in the presence of most competing
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metal ions, because most of metal ions did not interfere with the detection of Al3+ by compound
MCIH2, and the detection limit of compound MCIH2 for recognizing Al3+ was as low as 1 ×
10-7 M level. Additionally, the 2 : 1 ratio stoichiometry between compound MCIH2 and Al3+
was determined by the methods of continuous variation (Job's plot) and ESI-MS spectrum of
MCIH2-Al3+ complex, and the binding mode of compound MCIH2 with Al3+ was explored in
detail by FT-IR spectra and 1H NMR titration experiments. Furthermore, the perfect reversibility
and regeneration of this compound MCIH2 towards Al3+ were important for the fabrication of
devices for sensing Al3+. The colorimetric and fluorescent properties of this compound MCIH2
towards Al3+ might provide new ideas for the development of novel probes for sensing various
metal ions, anions and biomolecules with colorimetric and fluorescent responses.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (21761019), Young
Foundation of Jinchang (jrc201811).
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Figure captions
Fig. 1. Change in UV-vis absorption spectra of compound MCIH2 in ethanol (50.0 μM) after
addition of increasing amounts of Al3+ (0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0 equiv.).
Fig. 2. (top) Changes in UV-vis absorption spectra of compound MCIH2 (50.0 μM) in ethanol
upon addition of various metal ions (5.0 equiv.); (bottom) Corresponding photographs of
compound MCIH2 (50.0 μM) in ethanol after addition of various metal ions (5.0 equiv.).
Fig. 3. (top) Changes in fluorescence emission spectra of compound MCIH2 (50.0 μM) in
ethanol upon addition of Al3+ (1.0 equiv.) and other various metal ions (5.0 equiv.) with an
excitation wavelength of 423 nm; (bottom) Corresponding photographs of compound MCIH2
(50.0 μM) in ethanol after addition of Al3+ (1.0 equiv.) and other various metal ions (5.0 equiv.)
under a UV lamp at 423 nm.
Fig. 4. Change in fluorescence emission spectra of compound MCIH2 (50.0 μM) in ethanol
upon addition of increasing amounts of Al3+ (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 equiv.)
with an excitation wavelength of 423 nm. Inset: Corresponding photographs of compound
MCIH2 in the absence and existence of Al3+ (1.0 equiv.) under a UV lamp at 423 nm.
Fig. 5. Relative fluorescence emission intensity at 489 nm of compound MCIH2 (50.0 μM) and
its complexation with Al3+ (1.0 equiv.) in the existence of various metal ions (5.0 equiv.). Blue
bar: MCIH2 and MCIH2 with other various metal ions stated; Red bar: MCIH2 with Al3+ in the
existence of other various metal ions stated.
Fig. 6. (top) Proposed binding mode for interaction of Al3+ with compound MCIH2; (bottom) 1H
NMR spectra of compound MCIH2 in the absence and existence of Al3+ (1.0 equiv.) in
21

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DMSO-d6.
Scheme 1. Synthetic route of 7-methoxychromone-3-carbaldehyde-(indole-3-formyl) hydrazone
(MCIH2).
22

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Fig. 1. Change in UV-vis absorption spectra of compound MCIH2 in ethanol (50.0 μM) after
addition of increasing amounts of Al3+ (0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0 equiv.).
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Fig. 2. (top) Changes in UV-vis absorption spectra of compound MCIH2 (50.0 μM) in ethanol
upon addition of various metal ions (5.0 equiv.); (bottom) Corresponding photographs of
compound MCIH2 (50.0 μM) in ethanol after addition of various metal ions (5.0 equiv.).
24

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Fig. 3. (top) Changes in fluorescence emission spectra of compound MCIH2 (50.0 μM) in
ethanol upon addition of Al3+ (1.0 equiv.) and other various metal ions (5.0 equiv.) with an
excitation wavelength of 423 nm; (bottom) Corresponding photographs of compound MCIH2
(50.0 μM) in ethanol after addition of Al3+ (1.0 equiv.) and other various metal ions (5.0 equiv.)
under a UV lamp at 423 nm.
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Fig. 4. Change in fluorescence emission spectra of compound MCIH2 (50.0 μM) in ethanol
upon addition of increasing amounts of Al3+ (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 equiv.)
with an excitation wavelength of 423 nm. Inset: Corresponding photographs of compound
MCIH2 in the absence and existence of Al3+ (1.0 equiv.) under a UV lamp at 423 nm.
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Fig. 5. Relative fluorescence emission intensity at 489 nm of compound MCIH2 (50.0 μM) and
its complexation with Al3+ (1.0 equiv.) in the existence of various metal ions (5.0 equiv.). Blue
bar: MCIH2 and MCIH2 with other various metal ions stated; Red bar: MCIH2 with Al3+ in the
existence of other various metal ions stated.
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Fig. 6. (top) Proposed binding mode for interaction of Al3+ with compound MCIH2; (bottom) 1H
NMR spectra of compound MCIH2 in the absence and existence of Al3+ (1.0 equiv.) in
DMSO-d6.
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Scheme 1. Synthetic route of 7-methoxychromone-3-carbaldehyde-(indole-3-formyl) hydrazone
(MCIH2).
29