A new fluorene-based Schiff-base as fluorescent chemosensor for selective detection of Cr3?+ and Al3?+

Article abstract of DOI:10.1016/j.saa.2017.08.007

2-((9H-fluoren-2-ylimino) methyl)phenol (F3) was synthesized by condensation reaction of 9H-fluoren-2-amine and 2-hydroxybenzaldehyde in EtOH and characterized by its melting point, ¹H-,¹³C NMR and molecular mass. F3 exhibits a high selectivity for detection of Cr^3?+ and Al^3?+ ions as a fluorescent chemosensor and showed a single emission band at 536?nm upon excitation at 333?nm according to fluorescence emission studies. The addition of Cr^3?+ and Al^3?+ make a significant increase in fluorescent intensity at 536?nm in CH₃CN, while other metal ions have almost no influence on the fluorescence. The fluorescence enhancement was attributed to the inhibited C[dbnd]N isomerization and the obstructed excited state intra-molecular proton transfer (ESIPT) of compound F3. Job's plot and DFT calculations data showed that the binding stoichiometries of F3 with Cr^3?+ and Al^3?+ are 2:1. The association constants (K_a) for Cr^3?+ and Al^3?+ were calculated and found to be 8.33?×?10⁴?M^??1 and 5.44?×?10⁴?M^??1, respectively. The detection limits were also calculated for Cr^3?+ and Al^3?+ and found to be 2.5?×?10^??7?mol/L and 3.1?×?10^??7?mol/L, respectively.

Full text of DOI:10.1016/j.saa.2017.08.007

Accepted Manuscript
A new fluorene-based Schiff-base as fluorescent chemosensor for
selective detection of Cr3+ and Al3+
Mahmood Tajbakhsh, Gholam Babaei Chalmardi, Ahmadreza
Bekhradnia,
Rahman
Hosseinzadeh,
Nahid
Hasani,
Mohammadreza Azizi Amiri
PII:
DOI:
Reference:
S1386-1425(17)30636-4
doi: 10.1016/j.saa.2017.08.007
SAA 15364
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date:
Revised date:
Accepted date:
4 May 2017
6 July 2017
1 August 2017
Please cite this article as: Mahmood Tajbakhsh, Gholam Babaei Chalmardi, Ahmadreza
Bekhradnia, Rahman Hosseinzadeh, Nahid Hasani, Mohammadreza Azizi Amiri , A new
fluorene-based Schiff-base as fluorescent chemosensor for selective detection of Cr3+ and
Al3+, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2017),
doi: 10.1016/j.saa.2017.08.007
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ACCEPTED MANUSCRIPT
A new fluorene-based Schiff-base as fluorescent chemosensor for selective detection of Cr3+
and Al3+
Mahmood Tajbakhsha*, Gholam Babaei Chalmardia, Ahmadreza Bekhradniab, Rahman
Hosseinzadeha, Nahid Hasania, Mohammadreza Azizi Amiria
a
Department of Organic Chemistry, Faculty of Chemistry, University of Mazandaran, Babolsar 47416-95447, Iran
b
Pharmaceutical Sciences Research Center, Department of Medicinal Chemistry, Mazandaran University of
Medical Sciences, Sari, Iran
E-mail: Mahmood.Tajbakhsh32@gmail.com
Fax: +98 112 5242002
Tel: +98 112 5342301
Graphical Abstract
N
C
H
O
Cr3+
O
Cr3+
N C
H
isomerization
inhibited
O
H
ESIPT OFF
Fluorescence
ON
N
CH
Al3+
isomerization
allowed
N
C
H
ESIPT ON
O
Al3+
Fluorescence
OFF
O
N C
H
isomerization
inhibited
ESIPT OFF
Fluorescence
ON
Highlights
A novel fluorene-based Schiff-base sensor for Cr3+ and Al3+ ions has been designed,
synthesized, and characterized.
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DFT calculations together with the experimental results obtained from Job’s plot method
showed a 2:1 binding ratio for both Cr3+ and Al3+ with Schiff base F3.
The sensing mechanism is explained by excited state intra-molecular proton transfer (ESIPT)
and C=N isomerization.
The association constants (Ka) for Cr3+ and Al3+ were calculated and found to be 8.33 × 104 M-
1 and 5.44 × 104 M-1, respectively.
The detection limit of sensor towards Cr3+ and Al3+ were 2.5 × 10-7 mol/L and 3.1 × 10-7
mol/L, respectively.
Abstract
2-((9H-fluoren-2-ylimino) methyl)phenol (F3) was synthesized by condensation reaction of 9H-
fluoren-2-amine and 2-hydroxybenzaldehyde in EtOH and characterized by its melting point, 1H-
,13C-NMR and molecular mass. F3 exhibits a high selectivity for detection of Cr3+ and Al3+ ions
as a fluorescent chemosensor and showed a single emission band at 536 nm upon excitation at
333 nm according to fluorescence emission studies. The addition of Cr3+ and Al3+ make a
significant increase in fluorescent intensity at 536 nm in CH3CN, while other metal ions have
almost no influence on the fluorescence. The fluorescence enhancement was attributed to the
inhibited C=N isomerization and the obstructed excited state intra-molecular proton transfer
(ESIPT) of compound F3. Job’s plot and DFT calculations data showed that the binding
stoichiometries of F3 with Cr3+ and Al3+ are 2:1. The association constants (Ka) for Cr3+ and Al3+
were calculated and found to be 8.33 × 104 M-1 and 5.44 × 104 M-1, respectively. The detection
2

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limits were also calculated for Cr3+ and Al3+ and found to be 2.5 × 10-7 mol/L and 3.1 × 10-
7mol/L, respectively.
Keywords: Fluorescence, Chemosensor, Chromium and Aluminum ions, ESIPT, Schiff-base, DFT
1. Introduction
Fluorescent chemosensors have received considerable attention, as they provide a
convenient way for detection of metal ions [1–4]. Aluminum is one of the most abundant metals
in the earth and is widely dispersed and used in the environment [5]. Nevertheless, as a non-
essential and toxic element for living systems, the incremental increase of Al3+ concentrations is
harmful to plant growth. Furthermore, exposure of the human body to high concentration of Al3+
is hazardous to health and can lead to a wide variety of diseases, such as Alzheimer, osteoporosis
and etc [6–8]. The average daily intake of aluminum in humans is around 3–10 mg as reported
by World Health Organization (WHO) [9]. Therefore, development of convenient and selective
methods for determination of Al3+ is highly desirable [10]. As an essential heavy metal,
chromium plays a vital role in the metabolism of carbohydrates, proteins, lipids and nucleic acids
in biological systems [11]. The deficiency of chromium in the diet will increase risk factors
associated with diabetes and cardiovascular diseases [12]. Chromium has also realized as a wide
application in various industries such as pigment manufacturing, leather tanning, wood treatment
and dyeing [13]. So, its importance toxic effects of Cr3+ and Al3+ in living systems leads to a
growing interest in the development of appropriate analytical methods for their qualitative and
quantitative detection in different biological and environmental samples. Although, the
conventional techniques such as flame atomic absorption spectroscopy (FAAS) [14], inductively
coupled plasma atomic emission spectroscopy (ICP-AES) [15], etc. Have been extensively
employed for determination of Cr3+ and Al3+, they are costly, complicated and not convenient for
routine monitoring of Cr3+ and Al3+.Thus, significant efforts have been considered for using of
the fluorescent chemosensors to provide an easy and quick analysis of these elements [16-17].
Up to now, fluorene derivatives are mostly used for photoelectric functional materials and there
are a few reports on fluorescent chemosensor of the fluorene derivatives for selective recognition
of the metal ions [18-20]. Moreover, Cr3+ and Al3+ ions are well-known fluorescence quenchers
due to their paramagnetism, which could affect the information of ‘turn-on’ fluorescent sensors
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for their detection [21-23]. Many studies have been reported on designing turn on sensors
because turn-off sensors usually produce a low signal output upon binding making them liable to
interfere with the temporal separation of similar complexes with time-resolved fluorometry [24-
27]. Different kinds of mechanisms such as photoinduced electron transfer (PET), metal to
ligand charge transfer (MLCT), internal charge transfer (ICT), photoinduced proton transfer
(PPT), excited state intra-molecular proton transfer (ESIPT), chelation-enhanced fluorescence
(CHEF) effect and imine bond isomerization have been employed for designing fluorescent
chemosensors [28–33]. Schiff bases that can be easily prepared by condensation of different
types of amines and aldehydes have found widespread applications in coordination chemistry
[34]. Schiff bases bearing a bridged C=N structure could easily isomerize in the excited state and
tend to exhibit very weak fluorescence due to the C=N isomerization [35, 36]. However, the
C=N isomerization is inhibited when this functional group can coordinate to metal ions to form
the complexes that results in much stronger fluorescence signals. The ESIPT process usually
involves the transfer of a proton donor (–OH, –NH2) to an adjacent proton acceptor (–C=O, –N=)
through a six-membered ring of hydrogen-bonding configuration. In the presence of Cr3+ and
Al3+, the proton of the donor will be removed due to strong resonance-assisted hydrogen bonding
(RAHB) and, as a result, the excited-state intra-molecular proton transfer process is inhibited and
a significant emission increment or decrement can be observed [37-41]. In continuation of our
studies on developing novel chemosensors [42-46], we report here the design and synthesis of a
new fluorene-based Schiff-base (F3) which was synthesized by condensation of 9H-fluoren-2-
amine and 2-hydroxybenzaldehyde (Scheme 1). Chelation of the resulting Schiff-base, 2-((9H-
fluoren-2-ylimino)methyl)phenol (F3), with Cr3+ and Al3+, inhibited the ESIPT process and C=N
isomerization and showed a great fluorescence enhancement with turn-on at 536 nm, which is
attributed to the formation of a 2:1 stoichiometric complex in acetonitrile.
2. Experimental
2.1. General and reagents
1
Melting points were measured on an Electrothermal 9100 apparatus. H and 13C spectra were
measured with Bruker DRX-400 AVANCE spectrometer at 400.1 and 100.6 MHz, respectively.
Mass spectra were obtained on an Agilent 5975c spectrometer operating at 70 eV. The UV-Vis
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spectra were obtained using a Perkin-Elmer lambda-EZ 201 and a Jasco FP-200 spectra
fluorometer was used to record fluorescence emission spectra. Data were recorded on-line and
analyzed by Excel software on a PC computer. Fluorescence intensity measurements were
performed at room temperature in CH3CN.
2.2. Synthesis
2.2.1. Synthesis of 2-nitro-9H-fluorene (F1)
Fluorene (60 g, 361 mmol) and 500 mL AcOH were added to a 1 L three-necked flask equipped
with a thermometer and addition funnel. The reaction mixture was heated to 50 °C under
nitrogen atmosphere. Then, nitric acid (80 mL) was added over 20 min via the addition funnel.
The temperature was subsequently raised to 85 °C and maintained for 5 min. The resulting
mixture was removed from the heater and then permitted to cool to room temperature over 2 h.
Then, the resulting yellow suspension was filtered, washed with 50 mL AcOH containing 1.3 g
KOAc, slurried in water and filtered again. The yellow product was dried in a vacuum oven,
affording 58 g (77% yields) of desired product. The melting point was 152 158 °C. (Lit 155–
156°C) [47]. 1H NMR (400 MHz, CDCl3), δ, ppm: 8.42 (1H, d, J=1.2 Hz), 8.31 (1H, dd, J=2Hz,
13
J=8Hz), 7.90-7.87 (2H, m), 7.65-7.62 (1H, m), 7.49-7.43 (2H, m), 4.01 (2H, S). C NMR (100
MHz, CDCl3), δ, ppm: 148.08, 146.74, 144.82, 143.92, 139.46, 128.87, 127.43, 125.43, 123.13,
121.34, 120.48, and 119.88. The mass spectrum show peak at m/z = 211.1 corresponding to
compound F1 (Fig. S1-S2).
2.2.2. Synthesis of 9H-fluoren-2-amine (F2)
In a two-necked round-bottomed flask (500 ml) equipped with a reflux condenser and a dropping
funnel, a suspension of 2-nitro-9H-fluorene (10.12 g, 48 mmol), palladium on carbon 5% (5 g),
and ethanol (250 mL) was prepared. The mixture was heated, and while being stirred
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magnetically, hydrazine hydrate 85% (35 ml) in ethanol (50 ml) was added dropwise over a 1.5 h
period through the dropping funnel while maintaining the temperature at about 50 ºC. The
reaction mixture was then refluxed for 2 h and filtered while hot. On cooling, the filtrate gave
light yellow colored crystals of the title diamine compound, which was recrystallized from
ethanol and dried under vacuum to give 5.4 g (62% yield) (Scheme1). mp 129-134 °C. 1H NMR
(400 MHz, CDCl3), δ, ppm: 7.66 (1H, d, J=7.6 Hz), 7.59 (1H, d, J=8 Hz), 7.49 (1H, d, J=7.6 Hz),
7.36-7.32 (1H, m), 7.22 (1H, ddd, J=1.2 Hz, J=7.6 Hz), 6.91 (1H, t, J=0.4 Hz), 6.74 (1H, dd,
J=2Hz, J=8 Hz), 3.83 (2H, S), 3.76 (2H, S, NH2). 13C NMR (100 MHz, CDCl3), δ, ppm: 145.74,
145.16, 142.26, 142.14, 133, 126.63, 125.08, 124.75, 120.66, 118.58, 113.97, and 111.81. The
mass spectrum show peak at m/z = 181.1 corresponding to compound F2 (Fig. S3-S4).
2.2.3. Synthesis of 2-((9H-fluoren-2-ylimino)methyl)phenol (F3)
A solution of 9H-fluoren-2-amine (0.181 g,1 mmol) in absolute ethanol was added to an ethanol
solution of 2-hydroxybenzaldehyde (0.122 g, 1 mmol). The mixture was refluxed for 2 h and
then cooled to room temperature. The solvent was evaporated and the yellow product was
1
recrystallized from ethanol to yield 0.23 g (81%) of F3 was (Scheme1). mp 142-147 °C. H
NMR (400 MHz, CDCl3), δ, ppm:13.43 (1H, S), 8.74 (1H, S), 7.84 (2H, t, J=8.4 Hz), 7.76 (1H,
d, J=7.2 Hz), 7.52 (1H, d, J=1.2 Hz), 7.45-7.38 (3H, m), 7.37-7.32 (2H, m), 7.06 (1H, d, J=8.4
Hz), 6.99 (1H, ddd, J=1.2 Hz, J=7.2 Hz), 3.94 (2H, S).13C NMR (100 MHz, CDCl3), δ,
ppm:161.70, 161.16, 147.19, 144.63, 143.43, 141.08, 140.81, 133.01, 132.19, 126.95, 126.85,
125.09, 120.56, 120.36, 119.94, 119.37, 119.08, 117.84 and 117.25. The mass spectrum show
peak at m/z = 285.1 corresponding to compound F3 (Fig. S5-S6).
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HNO3
NO2
acetic acid
F1
Pd/C, NH2NH2
NO2
NH2
ETOH
F2
CHO
HO
OH
ETOH
NH2
N
C
H
F3
Scheme 1. The synthetic route of compound F3.
2.3. Spectrometric procedure
Stock solutions (10 µM) of nitrate salts of Na+, K+, Bi2+, Hg2+, Co2+, Cu2+, Zn2+, Cr3+, Cd2+, Fe3+,
Al3+ and Ag+ in CH3CN were prepared. A solution of Schiff-base (F3) (10 µM) in acetonitrile
was also prepared. Test solutions were prepared by placing 2 mL of the probe stock solution into
cuvettes, adding 2 mL of each metal ion stock. For all measurements, excitation wavelength was
at 333 nm.
3. Results and discussion
3.1. Spectral studies
Fig. S7 and Fig.1 show the changes in the absorption and fluorescence spectra of F3 in
CH3CN solution, upon the addition of various cation salts. The absorption spectrum of F3 (10
µM) exhibits three bands at 228 nm, 330 nm and 363 nm. The absorption intensity of F3
decreased with the addition of various heavy metal ions and Cr3+ and Al3+. In all the cases, a
hypochromic effect in the UV-Vis absorption spectra of F3 was observed (Fig. S7). Because of
the ambiguous absorption spectra, it was difficult to identify specific cations over other metal
ions by UV-Vis analysis [48]. Therefore, to investigate the selectivity of F3 for Cr3+ and Al3+ in
the presence of other metal ions, the fluorescence response of F3 toward various metal ion
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solutions (10 µM) was studied in CH3CN (Fig. 1). The fluorescence emission intensity of F3 was
recorded at 536 nm when it was excited at 333 nm. Upon addition of Cr3+ and Al3+, F3 shows a
large fluorescence enhancement at 536 nm. The reason for this phenomenon is attributed to the
formation of a 2:1 complex, resulted from the cation-induced inhibition of the excited state intra-
molecular proton transfer (ESIPT) process and C=N isomerization [35-41]. Other metal ions
including Na+, K+, Bi2+, Hg2+, Co2+, Cu2+, Zn2+, Cd2+, Fe3+ and Ag+ had almost no influence on
the fluorescence.
a)
b)
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Fig. 1. (a) Fluorescence responses of compound F3 (10µM) in CH 3CN solution upon the addition of various metal
ions (excitation = 333 nm, 2 equiv). (b) Bar graph representing the change of the relative emission intensity of F3 at
536 nm upon treatment with various metal ions (λex = 333 nm).
To explore the selectivity of F3 for Cr3+ and Al3+, the absorption in a mixture of other
metal ions was measured in solution of CH3CN, resulted in a significant fluorescence intensity
enhancement (Fig. 2). These observations demonstrated that compound F3 could be used as an
efficient and selective fluorescence chemosensor for Cr3+ and Al3+ ions.
Fig. 2. Fluorescence intensity of F3 (10 µM) in the absence of metal ions (red curve), and presence of 2 equiv. of all
kinds of competitive metal ions (blue curve) in CH3CN (λex = 333 nm). The green curve represents the addition of
Al3+ to the above mixture. The black curve represents the addition of Cr3+ to the above mixture. Each spectrum was
acquired 1 minute after cations addition at room temperature.
To further investigate, the chemosensing properties of receptor F3, the fluorescence
titration of F3 with Cr3+ and Al3+ were performed. By increasing the concentrations of Cr3+ and
Al3+, the sensitivity of F3 (10 µM) in CH3CN towards Cr3+ and Al3+ ions were clearly observed
in Fig. 3. The fluorescence spectrum of F3 (λem=536 nm) showed rapid turn-on responses and the
inset illustrated relative fluorescence intensity changes as a function of Cr3+ and Al3+
concentration.
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a)
Al3+
b)
Cr3+
Fig. 3. (a) Fluorescence titration of F3 (10 µM) with various concentration of Al3+ (0-2 equiv) in CH3CN solution
(excitation = 333 nm); inset: changes of fluorescence upon addition of Al3+ at emission=536 nm. (b) Fluorescence
titration of F3 (10 µM) with various concentration of Cr3+ (0-2 equiv) in CH3CN solution (excitation = 333 nm);
inset: changes of fluorescence upon addition of Cr3+ at emission=536 nm.
The association constant (Ka) of compound F3 with metal ions was determined using the
Benesi–Hildebrand equation [49] as follows:
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F and F0 represent the fluorescence intensity of compound F3 in the presence and absence of
metal ions, respectively. Fmax indicates the saturated fluorescent intensity of F3 in the presence of
excess amounts of metal ions. [M]. Using the Benesi–Hildebrand equation, the association
constants for F3-Cr3+ and F3-Al3+ were calculated from the fluorescence titration results and
obtained 8.33 ×104 M-1 and 5.44 × 104 M-1 respectively. Furthermore, the limitation of detection
was calculated according to fluorescence titration. To determine the S/N ratio, the emission
intensity of the complexes (F3-Cr3+ and F3-Al3+) were measured and the standard deviation of
blank measurements were calculated. The limitation of detection was then calculated according
to 3 × δb/m, where δb is the standard deviation of blank solutions, and "m" is the slope between
intensity versus sample concentration [50]. The detection limits for Cr3+ and Al3+ were calculated
to be 2.5× 10-7 mol/L and 3.1× 10-7mol/L, respectively. Fluorescence quantum yield was
measured by assessment the emission and absorption intensities of the probe in respect to those
of a fluorescence standard, fluorescein in 0.1 N NaOH. For all fluorescence measurements, the
excitation wavelength was 333 nm with excitation and emission slit widths of 3.0 nm.
Accordingly, the quantum yields of 14.8% and 12.9% were determined for the F3-Cr3+ and the
F3-Al3+complexes. However, low quantum yields were obtained for the combination of the
receptor with other metal cations as well as for the free receptor and no worth to report. Ligand
F3 as a fluorescence probe can be widely used to measure the low concentrations (µM) of Cr+3
and Al+3 cations in biochemical chemical environmental and forensic analysis. The sensitivity of
fluorescence detection is approximately one thousand times greater than absorption
spectrophotometric methods. This means, the fluorimetey may even be a more powerful method
for Cr+3 and Al+3 cations characterization than UV/Vis spectrophotometery. Leading greater
limits of detection, while potentially using less Cr+3 and Al+3 cations. It might be important
especially when working with valuable or limited-quantity of the cations.
3.2. Binding mode studies
The stoichiometric relationship between the ligand F3 with Cr3+ and Al3+ were confirmed
by the Job’s plot. The intensity of fluorescence emission was measured by varying the mole
fraction of F3. As shown in Fig. 4, the fluorescence emission intensity showed a maximum when
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the molar fraction of F3 were 0.67, demonstrates the formation of 2:1 ratio of F3 with Cr3+and
Al3+.
a)
b)
Fig. 4. Job‘s plot titration of F3 with Al3+, the total concentration of F3 and Al3+ is 10 µM (a) and Cr3+, the total
concentration of F3 and Cr3+ is 10 µM (b) at room temperature.
1
In order to get further insight into the binding event of F3 with Cr3+ and Al3+, H NMR
spectra of F3 in the absence and presence of Cr3+ and Al3+ were measured in CD3CN (Fig. 5).
The Proton shift of F3 at 13.43 ppm can be assigned to the hydroxyl proton, which is involved in
the intra-molecular hydrogen bond (OH…N) [51]. The phenolic hydroxyl proton at 13.43 ppm
disappeared when 2 equiv of Cr3+ and Al3+ were added to the F3 solution. This observation
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implies that the phenolic OH groups are involved in the formation of complex between the F3
with Cr3+ and Al3+ [52]. The signals of other protons in F3 and complexes did not show any
significant change. The binding mode of F3 with Cr3+and Al3+ is meaningful for the design of
new Cr3+ and Al3+ fluorescent chemosensors.
a)
F3+2eq Al3+
F3 +1.5eq Al3+
F3+1eq Al3+
F3 +0.5eq Al3+
OH
b)
F3+2eq Cr3+
F3+1.5eq
F3+0.5eq Cr3+
OH
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Fig. 5. 1H NMR spectra of F3 (10 µM) with Al3+ and Cr3+ in CD3CN: (a) F3 andAl3+ (0-2 equiv) (b) F3 and Cr3+ (0-2
equiv). Each spectrum was acquired 1 minute after cations addition at room temperature.
3.3. Theoretical study
3.3.1. The optimization studies
The given complexes (F3–Al3+ and F3-Cr3+) have produced as fine powders, so far we
were not able to obtain the single crystals of the compounds to determine the electronic
structures by X-ray crystallography [10]. To get insight into the proposed binding modes, the full
geometry optimizations were performed without any symmetry constraints by density functional
calculations (DFT) [53] in the framework of the Becke three-parameter hybrid exchange and
Lee–Yang–Parr correlation functional (B3LYP), employing Gaussian 09 package [54]. Whole
the atoms were characterized by a split-valence Pople basis set plus polarization and diffuse
functions, 6-31+G (d, p), but a double-ζ quality LANL2DZ basis set was employed for both Cr
(III) and Al (III) atoms. The optimized structure of the F3 Schiff base and its complexes are
shown in Fig 6, indicating 2:1 stoichiometry between the sensor F3 and Cr3+/Al3+. Also, the
selected calculated bond distances and the bond angles are shown in Table 1. As shown in Fig. 6,
the coordination environments around both Cr (III) and Al (III) are tetrahedral with some distortion,
by nitrogen atoms of imine groups (N1 and N2) and two deprotonated phenolic oxygens (O1 and
O2). The calculated bond angles show appreciable deviation from ideal tetrahedral value of 109.5º,
suggesting that tetrahedral geometry is slightly distorted due to chelation effects.
To find the mole percent of the sensor to moles of the metal, the fluorescence titration
experiments were performed using 10 µM of F3 with varying concentrations of Cr3+ and Al3+ in
solution CH3CN which are shown a relationship between absorbance and mole ratio of 2:1,
consistent with the theoretical results.
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Fig. 6. The optimized structures of the F3 (a), F3–Al3+ (b) and F3-Cr3+ (c).
Table 1: Selected bond lengths (Å) and bond angles () for the structures of F3
(ligand), F3–Al3+ and F3-Cr3+
F3–Al
F3-Cr
Cr-O1
F3
Al-O1
Al-O2
Al-N1
Al-N2
N1-C1
O2-C2
N1-C3
O1-C4
N2-C5
N2AlO1
N1AlO2
N1AlO1
N2AlO2
1.757
1.775
1.941
1.933
1.328
1.351
1.447
1.360
1.424
99.90
95.94
109.71
113.35
1.833
1.841
1.999
2.004
1.321
1.351
1.446
1.350
1.324
88.84
88.60
118.66
118.41
N1-C1
N1-C2
O1-C4
C2-C3
C3-C4
C1-C5
C5-C6
C6-C7
C1NC2
C3C4O
NC1C5
1.407
1.294
1.344
1.450
1.423
1.409
1.387
1.515
121.60
121.70
117.46
177.12
179.13
Cr-O2
Cr-N1
Cr -N2
N1-C1
O2-C2
N1-C3
O1-C4
N2-C5
N2 Cr O1
N1 CrO2
N1 CrO1
N2CrO2
C1NC2C3
C1C5C6C7
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3.3.2. The TD-DFT calculations
The excited-state electronic spectra of the F3 (Schiff base) and the F3–Al3+ and F3-Cr3+
complexes, optimized with inclusion of acetonitrile solvent, were calculated by
TDDFT/IEFPCM method to determine the energies and compositions of the molecular orbitals
(MOs) [55]. The assignment of the most important occupied (HOMOs) and unoccupied
(LUMOs) orbitals involved in electronic transitions in α and β spin states in acetonitrile solvent,
with the fragment Mulliken contributions expressed in terms of the compositions from the metal
central atoms (Cr and Al), Fluorene-part (Flou), CN, Phenyl-part (Ph) and OH were presented in
Fig 7. The HOMO orbital in Schiff base is mostly characterized with π-orbitals of Flou fragment
but the LUMO is exhibited by a mixture of π*-orbitals of CN (39%), Ph (32%) and Flou (27%)
characters. In both α- and β-spin states of the F3-Cr3+, the HOMOs are significantly composed
with the π orbitals of Flou character, while the LUMOs, in α-spin are importantly localized on Cr
(64%) character and also in β-spin, are indicated of CN (39%), Cr (29%) and Ph (27%)
characters. On the other hand, the HOMOs and LUMOs orbitals of the F3-Al3+ in α-spin state,
are predominately composed by an admixture of π-character of Ph (34%) and CN (44%)
contributions but in β-spin state, both HOMO and LUMO orbitals are composed with π- and π*-
characters of Flou (89% and 73%) fragments, respectively. As seen in Fig 7, both HOMO’s and
LUMO’s energies in the F3-Al3+ and F3-Cr3+ compounds are more stabilized than the F3-Schiff
base, so the F3 can convenience convert to F3–Al3+ and F3-Cr3+ compounds by narrowing the
energy gap between the HOMO and LUMO.
The theoretical UV–Vis spectra of the titled compounds in the ranges of 180-700 nm are
shown in Figure 8. The d–d band pattern suggests that Cr (III) and Al (III) centersare distorted as
a tetrahedral geometry. Generally, three possible d–d band expected for d3 configuration in a
16

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4
4
4
tetrahedral geometry which are T2 (F)→4T1(F), T2(F)→4T1(P) and T2(F)→4A2(F). Thus, the
theoretical absorption spectrum at 333.6 (shoulder), 387.9 and 515.4 nm for F3-Cr3+ complex
and the bands at 335.8, 422.9 and 569.5 nm for F3-Al3+ complex are attributed to first, second
and third ligand-field transitions, respectively. The exhibited electronic excited state at 326.0 nm
corresponding to F3 ligand are in agreement with the experimental spectrum at 333 nm together
with its fluorescence emission recorded at 536 nm. By complexation of the F3 ligand to Cr3+ and
Al3+, this excited spectrum is shifted to 333.6 nm (F3-Cr3+) and 335.8 nm (F3-Al3+), respectively
(as shown in Figure 6). The major transitions at 333.6 nm (ligand) is due the electron transition
from (HOMO-1) to (LUMO) orbitals which occur from dominant excitation characters of π-
Phenyl (67%), π-OH (16%) and π-Flourene (16%) to π*-Phenyl (32%), π*-CN (39%) and π*-
Flourene (27%).
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Fig. 7. The energy (eV), character and some contours of the HOMO and LUMO molecular orbitals of the
compounds. Positive values of the HOMOs and LUMOs contour are represented in pink and yellow and the
negative values in blue and green, respectively.
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Fig. 8. The calculated UV-Vis spectra of the F3 (red line), F3–Al3+ (blue line) and F3-Cr3+ (green line) compounds.
3.4. The proposed mechanism
According to the above experiments, it could be suggested that the F3 chelates with Cr3+
and Al3+ through interactions with imine nitrogen and the oxygen of the phenolic hydroxyl
groups [56–59]. The sensing mechanism was estimated using the degree of chelation. In solution,
the free F3 exhibited weak fluorescence emission because of ESIPT and C=N isomerization. The
isomerization of C=N bond is the predominant decay process of the excited state for the
compound F3. Upon addition of Cr3+ and Al3+ the compound F3 coordinates with the metal ions
and inhibits the C=N bond isomerization. The C=N isomerization mechanism for the Schiff-base
receptors have been reported for detection of metal ions various types [60]. The coordination
between Cr3+ and Al3+ with F3 a deprotonates the phenolic hydroxyl groups preventing the intra
hydrogen bonding with imine nitrogen (OH...N) and also inhibits the intra-molecular proton
1
transfer in the excited state (Scheme 2) [61, 62]. This phenomenon has been confirmed by H
NMR analysis as described in section (3. 2).
N
C
H
O
M3+
H
O
M3+
M= Cr or Al
O
N
CH
N C
H
ESIPT
isomerization
allowed
NO ESIPT
isomerization
inhibited
Fluorescence
OFF
Fluorescence
ON
Scheme. 2. Proposed mechanism for detection of Cr3+ and Al3+ by F3.
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3.5. Solvent effect
Optical sensing nature of chemosensors was found to be depended on the solvent nature,
so the response of ligand F3 toward Cr3+ ion was also examined in different solvents such as
DMSO, DMF, methanol, acetonitrile, acetonitrile/H2O at maxima of emission intensity. As
showed in fig. 9, the optimum fluorescence enhancement occurred only in case of acetonitrile.
The aprotic solvents (DMSO, DMF) and protic solvents (methanol, H2O) decrease significant the
intensity of fluorescent, which might be due to hydrogen bonding of this solvents with hydroxyl
groups, rather than general solvent effects [63]. However, the highest fluorescent intensity was
observed in acetonitrile. So acetonitrile was chosen as solvent [64, 65].
Fig. 9. Solvent effect on fluorescent intensity of F3 (10 µM) with Al3+ (2 equiv) and Cr3+ (2 equiv) in different
solvent methanol, DMSO, DMF, acetonitrile/H2O and acetonitrile.
3.6. Effect of pH on the binding affinity of Cr3+ to F3
Since pH of the medium plays a crucial role in fluorescence detection, the effect of pH in
the range of (2–13) on the fluorescence response of F3 was investigated in CH3CN. The changes
in the fluorescence intensity at 536 nm of F3 in the absence and presence of Cr3+ and Al3+ ions
were plotted as a function of pH (Fig. 10). The overall fluorescence intensity of F3–Cr3+ and F3-
20

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Al3+ complexes remained higher than the free receptor F3. The highest fluorescent intensity was
obtained at pH 8.5. The emission intensity was decreased over the pH 8.5, possibly due to the
formation of Cr(OH)3 and Al(OH)3 that precipitate from the solution and will become
unavailable to the chelating agent. Similarly, under the pH 8.5, F3 might react with H+ and show
slow affinity for binding to Cr3+ or Al3+, resulting a decrease in fluorescence intensity. Therefore,
F3 can tolerate the detection of Cr3+ and Al3+ in the pH 8.5 [66].
a)
b)
Fig. 10. Fluorescence response for F3 (10 µM) in the absence and presence of Al3+ (2 equiv) (a) and Cr3+ (2 equiv)
(b) as a function of pH in CH3CN at 536 nm.
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3.7. Comparison
The 2-((9H-fluoren-2-ylimino) methyl) phenol (F3) for fluorescence sensing of Cr3+ and
Al3+ are very interesting. In addition, we compared the present sensor with those previously
reported chemosensors for Cr3+ and Al3+ (Table 2). Chemosensor F3 exhibited better response
relatively.
Table 2: Comparative analysis of chemosensor F3 with previously reported sensors.
Ionophores
mechanism
Methods of
detections
Medium of
detection
Detection
limit
Binding
Constant
M-1
Sensing
response
pH
range
(M)
Ref. 67
Ref. 68
Ref. 69
-
fluorescent
fluorescent
CH3CN/H2O
(8:2)
EtOH/H2O
(3:7)
7.94 × 10-5
1.6 × 104
Cr3+ (Turn on)
Al3+ (Turn on)
Cr3+ (Turn on)
7
3.20 × 10-6
1.21 × 10-6
-
7.4
2-7
ICT and
CHEF
PET
colorimetric
and
MeOH
1.01 × 104
fluorescent
colorimetric
and
fluorescent
colorimetric
and
7.3 × 10-7
1.1 × 10-5
2.63 × 104
1.4 × 104
Al3+ (Turn on)
Cr3+ (Turn on)
2-7
-
Ref. 69
Ref. 70
PET
-
MeOH
MeOH
fluorescent
fluorescent
Ref. 71
Ref. 72
CHEF
EtOH-H2O
(95:5)
CH3CN
3.6 × 10-6
2 × 104
Al3+ (Turn on)
Cr3+ (Turn on)
2.75 × 10-6
5.97 × 104
-
ESIPT and
C=N
colorimetric
and
isomerization
MLCT
fluorescent
fluorescent
fluorescent
fluorescent
fluorescent
10-6
9.86 × 10-6
10-6
1.14 ×104 Al3+ (Turn on)
-
Ref. 73
Ref. 74
Ref. 75
F3
CH3CN
CH3CN
H2O /CH3CN
CH3CN
-
2.1 × 103
13.98
Cr3+ (Turn on)
Al3+ (Turn on)
Cr3+ (Turn on)
6-8
-
8.5
ICT
ESIPT and
C=N
2.5 × 10-7
8.33 ×104
isomerization
ESIPT and
C=N
3.1 × 10-7
5.44 × 104
Al3+ (Turn on)
8.5
fluorescent
CH3CN
F3
isomerization
4. Conclusion
In summary, a highly selective and sensitive fluorescent sensor based on fluorene derivative for
detection of Cr3+ and Al3+ has been developed. The sensor F3 shows a great fluorescence turn-on
upon binding to Cr3+ or Al3+ that inhibits ESIPT and C=N isomerization. In addition, the binding
22

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stoichiometry was confirmed as 2:1 (F3/Cr3+) and 2:1 (F3/Al3+) by Job’s plot method together
with theoretical study. The association constants (Ka) for Cr3+ and Al3+ were calculated and found
to be 8.33 × 104 M-1 and 5.44 × 104 M-1, respectively. The detection limit of sensor towards Cr3+
and Al3+ were 2.5 × 10-7 mol/L and 3.1 × 10-7 mol/L, respectively exhibiting more efficient than
other reported Cr3+ and Al3+ sensors [67–75]. However the present chemosensor system with the
remarkable photophysical properties could extend applications of fluorescent sensor for Cr3+ and
Al3+.
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