A new carbazole-based Schiff-base as fluorescent chemosensor for selective detection of Fe3+ and Cu2+

Article abstract of DOI:10.1016/j.saa.2013.02.043

A new carbazole-based Schiff-base (1) as a multi-functional fluorescent chemosensor was designed, synthesized and characterized, which can selectively recognized Fe³⁺ and Cu²⁺ ions over a number of other metal ions. Compound 1 could detect Fe³⁺ and Cu²⁺ by UV-Vis method and fluorescence method. The stoichiometry ratio of 1-Fe³⁺ and 1-Cu²⁺ are 2:1 and 1:1, respectively, by the method of Job's plot. Moreover, the detection limits were calculated to be 4.23 × 10 ^-6 mol/L for Fe³⁺ ion and 5.67 × 10^-6 mol/L for Cu²⁺ ion. In the presence of Fe³⁺/Cu ²⁺ ions, the fluorescence enhancement was attributed to the inhibited CN isomerization and the obstructed excited state intramolecular proton transfer (ESIPT) of compound 1. At the same time, the interactions of compounds 1 with other ions were also investigated and unobvious UV-Vis absorption and fluorescence spectral changes were observed. Thus a new kind of chemosensor for Fe³⁺/Cu²⁺with high sensitivity and selectivity was introduced.

Full text of DOI:10.1016/j.saa.2013.02.043

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 186–192
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A new carbazole-based Schiff-base as fluorescent chemosensor for selective
detection of Fe3+ and Cu2+
⇑
Lianlian Yang, Weiju Zhu, Min Fang, Qing Zhang, Cun Li
College of Chemistry and Chemical Engineering & AnHui Province Key Laboratory of Environment-friendly Polymer Materials, Anhui University, Hefei 230601, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
"
We designed and synthesized a new
carbazole-based Schiff base sensor.
Compound 1 could be used as a
multi-functional sensor for detecting
Fe3+/Cu2+ by both using UV–Vis and
fluorescence methods.
"
"
We found that the bonding mode
between compound 1 and Fe3+/Cu2+
is different.
a r t i c l e i n f o
a b s t r a c t
Article history:
A new carbazole-based Schiff-base (1) as a multi-functional fluorescent chemosensor was designed, syn-
thesized and characterized, which can selectively recognized Fe3+ and Cu2+ ions over a number of other
metal ions. Compound 1 could detect Fe3+ and Cu2+ by UV–Vis method and fluorescence method. The
stoichiometry ratio of 1-Fe3+ and 1-Cu2+ are 2:1 and 1:1, respectively, by the method of Job’s plot. More-
over, the detection limits were calculated to be 4.23 ꢀ 10ꢁ6 mol/L for Fe3+ ion and 5.67 ꢀ 10ꢁ6 mol/L for
Cu2+ ion. In the presence of Fe3+/Cu2+ ions, the fluorescence enhancement was attributed to the inhibited
C@N isomerization and the obstructed excited state intramolecular proton transfer (ESIPT) of compound
1. At the same time, the interactions of compounds 1 with other ions were also investigated and unob-
vious UV–Vis absorption and fluorescence spectral changes were observed. Thus a new kind of chemo-
sensor for Fe3+/Cu2+with high sensitivity and selectivity was introduced.
Received 16 November 2012
Received in revised form 30 January 2013
Accepted 20 February 2013
Available online 1 March 2013
Keywords:
Schiff-base
Fe3+, Cu2+ ion sensor
Fluorescence enhancement
Red shift
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
one of the most essential elements for normal physiological func-
tion in human body, because it plays an important role in many
The design and synthesis of fluorescent chemosensors are active
and fascinating research areas in supramolecular chemistry, organ-
ic chemistry, drug delivery, biological chemistry and environmen-
tal chemistry [1–3]. Fluorescent chemosensors have high
selectivity and sensitivity for detecting metal ions in aqueous
and nonaqueous media [4–7]. Iron and copper are the first and
the third abundant transition metal elements respectively. Iron is
cellular processes including DNA and RNA synthesis, energy gener-
ation and oxygen transport [8–10]. The deficiency of Fe3+ causes
intelligence decline, liver damage, hemochromatosis, Parkinson’s,
anemia, diabetes and cancer [11–13]. However, iron overload can
cause deleterious conditions such as b-thalassaemia and Friedr-
eich’s ataxia where the presence of excess iron leads to the gener-
ation of reactive oxygen species via the Fenton reaction [14]. On
the other side, copper is known as an essential element in both bio-
logical and industry systems [15]. It is one of the most essential
trace mental nutrients to sustain normal human health and sever
as an essential cofactor for a wide variety of enzymes in all living
⇑
Corresponding author. Tel.: +86 551 63861279; fax: +86 551 63861260.
E-mail address: cun_li@126.com (C. Li).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.02.043

L. Yang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 186–192
187
organisms [13,16]. However, high concentration of copper with re-
dox property could turn into a biologically hazard because of its
ability to generate reactive oxygen species, which causes irritation
of nose and throat, nausea, vomiting, and diarrhea [17]. Therefore,
how to effectively detect iron and copper is of great significance in
biology, chemistry, environmental science and medicine. There is a
pressing need for analytical methods with speed, accurateness and
low-cost. Fluorescent chemosensor could better meet the above
requirements. In recent years, it is gradually becoming a new hot-
spot that analysis and detection for metal ions in environmental
systems by taking advantage of fluorescent chemical sensor [18–
20].
land). Chemical shifts (d) were given in ppm relative to CDCl3
7.26 (1H). All spectral characterizations were carried out in
HPLC-grade solvents at 20 °C within a 10 mm quartz cell. UV–Vis-
ible absorption spectra were recorded on a UV-3600 spectropho-
tometer (Shimadzu, Japan). Fluorescence emission spectra were
recorded on a LS-55 FL spectrophotometer (Perkin Elmer, USA).
Stock solutions of metal salts were prepared in distilled water.
The stock solutions for compound 1 (1 mM) were prepared in HPLC
grade CH3CN and were used to prepare the CH3CN solution.
Synthesis
Different kinds of mechanisms such as photoinduced electron
transfer (PET), metal-to-ligand charge transfer (MLCT), internal
charge transfer (ICT), photoinduced proton transfer (PPT), ex-
cited-state intramolecular proton transfer (ESIPT), chelation-en-
hanced fluorescence (CHEF) effect and C@N isomerization have
been utilized for designing fluorescent chemosensors [21–26].
Schiff bases, condensation of amines and reactive aldehydes, are
important compound with various potential applications [27].
Schiff bases with a bridged C@N structure easily isomerize in the
excited state and tend to exhibit very weak fluorescence due to
the C@N isomerization [28,29]. But when they form complexes
with some special metal ions, the C@N isomerization is inhibited,
and then Schiff bases show strong fluorescence. At present, the
most carbazole derivatives are used for photoelectric functional
materials. There are few reports on fluorescent chemosensor of
the carbazole derivatives for selective detection of metal ions.
Moreover, Fe3+ and Cu2+ ions are well known fluorescent quench-
ers due to their paramagnetism, which could influence the infor-
mation of ‘turn-on’ fluorescent sensors for their detection [25,28].
Therefore, in this paper we designed and synthesized com-
pound 1 with a bridged C@N structure and carbazole ring (fluoro-
phores unit). 4-Nitrophenol linked alkylcarbazole through the C@N
bond, so the compound 1 formed complexes with some metal ions
through the oxygen atom of hydroxyl and the nitrogen atom of
C@N double bond. It can regulate coordination ability by using
the difference of modulation between ligand and the metal ions
to achieve the selective recognition for special metal ion. Com-
pound 1 is poorly fluorescent, may due to isomerization of the
C@N double bond in the excited state, and ESIPT involving the phe-
nolic protons [6]. When compound 1 forms complexes with some
special metal ions, the absorbance or the fluorescence intensity
maybe changed. In our work, a variety of complexes between com-
pound 1 and the special ions could be detected by UV–Visible
absorption method and fluorescence emission spectrum method,
so compound 1 could be used as a multi-functional sensor for
detecting the special ions by both using UV–Vis and fluorescence
methods.
Synthesis of 9H-hexylcarbazole (3)
One gram NaH was added in batches to the stirred solution of
carbazole (4.0 g, 24 mmol) in anhydrous DMF (20 mL) and the
solution was stirred for another 20 min. Then DMF (10 mL) solu-
tion containing 4 mL (28 mmol) 1-bromohexane was added drop-
wise. After complete addition, the reaction mixture was heated
to reflux for 3 h. When the mixture was cooled down to room tem-
perature, it was poured into water and carefully adjusted the pH
value to neutral with 1:1 (HCl/H2O, v/v) concentrated hydrochloric
acid. Then the product was filtered and recrystallized from ethanol.
The yield was 76.58% (4.62 g, 76.58%). FT-IR (Fig. S1, KBr, cmꢁ1):
3049 (@CAH), 2856-2953 (ACH2, ACH3), 1459 (ACH3), 1323
(ACH2A), 1621-1594 (structure of carbozle). 1H NMR (Fig. S2,
CDCl3, 400 Hz): dH 8.09 (d, J = 7.60 Hz, 2H), 7.38–7.48 (m, 4H),
7.21 (d, J = 8.00 Hz, 2H), 4.28 (t, 2H), 1.86 (m, 2H), 1.33 (m, 6H),
0.86 (t, 3H).
Synthesis of 3-formyl-9H-hexylcarbazole (2)
To a stirred solution of compound 3 (2.51 g, 10 mmol) in anhy-
drous DMF (30 mL), phosphorus oxychloride (POCl3, 0.2 mol) was
added dropwise at 0 °C, and the mixture was stirred for another
30 min at this temperature. Then the reaction mixture was stirred
at 70 °C for 2 h. When the mixture was cooled down to room tem-
perature and stirred for 16 h, it was poured into ice water by rap-
idly stirring and carefully neutralized with potassium bicarbonate.
The solution was extracted with ethyl acetate (3 ꢀ 50 mL) and
dried over anhydrous magnesium sulfate. The crude product was
purified by column chromatography (neutral alumina; ethyl ace-
tate/petroleum ether, 1/10, v/v) to give 2 (1.60 g, 57.35%). FT-IR
(Fig. S3, KBr, cmꢁ1): 1692 (AC@O), 1375 (ACH3), 1472 (ACH2),
1621-1594 (structure of carbozle). 1H NMR (Fig. S4, CDCl3,
400 Hz): dH 10.10 (s, 1H), 8.61 (s, 1H), 8.16 (d, J = 7.60 Hz, 1H),
8.01 (d, J = 8.40 Hz, 1H), 7.45–7.55 (m, 3H), 7.32 (t, 1H), 4.33 (t,
2H), 1.85–1.93 (m, 2H), 1.26–1.42 (m, 6H), 0.86 (t, 3H).
Experimental
Synthesis of the Schiff base (1)
2-Amino-4-nitro-phenol (0.15 g 1 mmol) was added to the
solution of 2 (0.34 g 1.2 mmol) in ethanol (2 mL). The resulting
reaction mixture was stirred at room temperature for 2 h during
which a pale yellow solid was obtained. The solid compound was
filtered, washed, and recrystallized from the mixed solvent of chlo-
roform and ethanol (9:1). The yield was 85.46%. Characterization of
compound 1: FT-IR (Fig. S5, KBr, cmꢁ1): 3302 (AOH), 3057 (@CAH),
2952-2856 (ACH2, ACH3), 1627 (ACH@N). 1H NMR (Fig. S6;
400 MHz; CDCl3; Me4Si): dH 10.10 (s, 1H), 8.93 (s, 1H), 8.71 (s,
1H), 8.29 (d, 1H), 8.12–8.21 (m, 3H), 7.46–7.57 (m, 3H), 7.35 (t,
1H), 7.11 (d, J = 8.80 Hz, 1H), 4.36 (t, 2H), 1.88–1.95 (m, 2H),
1.25–1.46 (m, 6H), 0.88 (t, 3H). Elemental analysis: Anal. Calcd.
for C25H25N3O3: C 72.27, H 6.06, N 10.11, found: C 72.28, H 6.08,
N 10.13.
Materials and instruments
Reagents were purchased as reagent-grade from Sinopharm
Chemical Reagent Co. Ltd., (Shanghai, China) and used without fur-
ther purification unless otherwise state. DMF was freshly distilled
from molecular sieve. Double distilled water was used for spectral
detection. CH3CN was HPLC grade without fluorescent impurities.
The solutions of metal ions were prepared from nitrate salts except
for FeCl3.
The FT-IR spectra were recorded with KBr pellets on a 170 sx
spectrometer (Nicolet, USA). Elemental analysis were determined
on a Vario EL III spectrometer (Elementar, Germany). 1H NMR spec-
tra were obtained on an AV-400 spectrometer (Bruker, Switzer-

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Spectrometric procedure
The stock solution of compound 1 (1 mM) was prepared in HPLC
grade CH3CN solvent, then 0.5 mL sample was diluted in a 50 mL
volumetric flask to be used for spectroscopic study. 3 mL diluted
solution was transferred to a 10 mm quartz cell, and then metal
ions were added for testing. UV–Visible and fluorescent titration
and competition experiments were carried out by adding
10.0 mM Fe3+/Cu2+ to solution of 1 (10.0
miscellaneous cations including Na+, K+, Ag+, Ca2+, Hg2+, Zn2+
Co2+, Mn2+, Mg2+ and Cr3+ (10.0
M, respectively).
lM) in the presence of
,
l
The association constant (Ka) of compound 1 with metal ions
was determined using the Benesi–Hildebrand equation [20] as
follows:
1
1
1
¼
þ
F ꢁ F0 KaðFmax ꢁ F0Þ½Mꢂ Fmax ꢁ F0
F and F0 represent the fluorescent intensity of compound 1 in the
presence and absence of metal ions, respectively. Fmax is the satu-
rated fluorescent intensity of compound 1 in the presence of excess
amount of metal ions. [M] is the concentration of metal ions added.
Job’s plot analyses were used to determine the stoichiometry
between compound 1 and Fe3+/Cu2+ [6].
Fig. 1. UV–Vis absorption spectrum of 1 (10.0
ions (10.0 M) in CH3CN solution.
lM) upon addition of various metal
l
such as Na+, K+, Ag+, Ca2+, Hg2+, Zn2+, Co2+, Mn2+, Mg2+, Cu2+, Fe3+
and Cr3+ showed selective response toward Fe3+ and Cu2+ ions. As
shown in Fig. 1, addition of 1 equiv. Fe3+ and Cu2+ resulted in an
obvious change indicating compound 1 had higher binding affinity
toward Fe3+ and Cu2+ than other surveyed metal ions. The absorp-
tion spectrum of compound 1 displayed well-defined bands at 286
and 370 nm (molar extinction coefficient: 2.61 ꢀ 104 Mꢁ1 cmꢁ1
and 2.98 ꢀ 104 Mꢁ1 cmꢁ1 respectively). The 286 nm band peak re-
Results and discussion
Synthesis and characterization
The compound 1 was synthesized with a good yield (85.46%) by
condensation of 2-amino-4-nitrophenol with 3-formyl-9H-hexylc-
arbazole (Scheme 1). Then compound 1 was characterized by IR, 1H
NMR and elemental analysis. The IR spectrum of the Schiff-base
exhibited two characteristic bands in the region 3302 and
1627 cmꢁ1 corresponding to AOH and AC@N groups respectively.
The carbon skeleton of carbazole showed a characteristic IR band
at 1588 and 1517 cmꢁ1 (Fig. S5, Supplementary data). The 1H
NMR spectrum of compound 1 showed one singlet for imino
(ACH@N) proton at 8.71 ppm (Fig. S6, Supplementary data). Com-
pound 1 is poorly fluorescent, may due to isomerization of the C@N
double bond in the excited state, and ESIPT involving the phenolic
protons [29].
flected a p–
pꢃ transition of the C@N group while the 370 nm band
reflected an intermolecular charge transfer (CT) band of the entire
conjugated molecule [2]. Addition of 1 equiv of Fe3+ to a solution of
compound 1, the observed absorption band of compound 1 at
286 nm (molar extinction coefficient: 3.25 ꢀ 104 Mꢁ1 cmꢁ1) in-
creased in absorbance and the other band at 370 nm (molar extinc-
tion coefficient: 1.93 ꢀ 104 Mꢁ1 cmꢁ1
) showed a decrease in
absorbance. Thus, there was a little blue shift of about 6 nm (from
370 nm to 364 nm). While addition of 1 equiv of Cu2+ to a solution
of compound 1, the observed absorption band of compound 1 at
286 nm (molar extinction coefficient: 2.83 ꢀ 104 Mꢁ1 cmꢁ1) in-
creased in absorbance and the other band at 370 nm (molar extinc-
tion coefficient: 2.01 ꢀ 104 Mꢁ1 cmꢁ1
) showed a decrease in
UV–Vis absorption spectral studies
absorbance. There was a blue shift of about 29 nm (from 370 nm
to 341 nm). The phenomenon of a blue shift maybe suggest that
the complex of 1-Fe3+ and the 1-Cu2+ were formed in CH3CN
solution.
The interaction between compound 1 and cations was investi-
gated by using UV–Vis and fluorescence spectrometries. The bind-
ing ability of compound 1 in CH3CN (1.0 ꢀ 10ꢁ5 M) with cations,
Scheme 1. The synthetic route of compound 1.

L. Yang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 186–192
189
Fig. 4. (a) Fluorescence titration of 1 (10.0
in CH3CN solution (kex = 290 nm); inset: changes of fluorescence upon addition of
l
M) with various concentration of Fe3+
Fig. 2. (a) UV–Vis absorption spectrum of 1 (10.0
in CH3CN solution; inset: changes of absorbance upon addition of Fe3+ at 370 nm;
(b) the UV–Vis absorption spectrum of 1 (10.0
M) with gradual addition of Cu2+ in
CH3CN solution; inset: changes of absorbance upon addition of Cu2+ at 370 nm.
l
M) with gradual addition of Fe3+
Fe3+ at 423 nm; (b) fluorescence titration of 1 (10.0
lM) with various concentration
of Cu2+ in CH3CN solution (kex = 290 nm); inset: changes of fluorescence upon
l
addition of Cu2+ at 423 nm.
To get more information on binding mechanism of compound 1
toward Fe3+/Cu2+, spectrometric titration experiments were per-
formed in the presence of Fe3+/Cu2+ (Fig. 2). Addition of Fe3+ to a
solution of compound 1 in CH3CN solution (1.0 ꢀ 10ꢁ5 M) showed
decrease close to 100% at 370 nm and about a 127% increase for the
286 nm band with a clear isosbestic point at 296 nm. The absorp-
tion peak at 370 nm had completely disappeared when 1.5 equiv
Fe3+ was added to a solution of compound 1. And a new absorption
peak appeared at 273 nm (Fig. 2a) and its intensity gradually in-
creased when the sequential addition of Fe3+ was reached to
1.0 equiv. On the other hand, successive addition of Cu2+ to a solu-
tion of compound 1 in CH3CN solution leaded to a stepwise de-
crease in absorbance at 370 nm and increase in absorbance at
286 nm with a clear isosbestic point at 296 nm. The absorbance
at 370 nm had a blue shift (from 370 nm to 328 nm). Moreover, a
new absorption peak appeared at 274 nm (Fig. 2b) and its intensity
gradually increased when the sequential addition of Cu2+ was
reached to 1 equiv. So it can be speculated that the stable complex
of 1-Fe3+ and the 1-Cu2+ are formed in CH3CN solution. The associ-
ation constant of Fe3+/Cu2+ ions with compound 1 which deter-
Fig. 3. Fluorescence responses of compound 1 (10.0
the addition of 5 equiv various metal ions (kex = 290 nm).
lM) in CH3CN solution upon

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L. Yang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 186–192
Fig. 6. (a) The Job’s plot for 1 and Fe3+ (kex = 290 nm); (b) the Job’s plot for 1 and
Cu2+ (kex = 290 nm).
Fig. 5. (a) Stern–Volmer plots for 1 for estimation of enhancement constants in
presence of Fe3+ (kex = 290 nm); (b) Stern–Volmer plots for 1 for estimation of
enhancement constants in presence of Cu2+ (kex = 290 nm).
time, there was a red shift from 423 nm to 452 nm. On the other
hand, the addition of 5 equiv of Fe3+ caused a 10.7-fold increase
of fluorescence intensity and a red shift from 423 nm to 441 nm.
The fluorescence intensity of adding Cr3+ was slightly enhanced
about 2.8-fold. This mean that the compound 1 exhibited high fluo-
rescent selectivity to Cu2+/Fe3+ over other metal ions. It indicated
that binding of Cu2+ or Fe3+ to compound 1 prevented C@N isom-
erization and excited-state intramolecular proton transfer (ESIPT)
between the enol-imine and the keto-imine forms and led to
enhancement.
mined by the Benesi–Hildebrand plot (Fig. S7, Supplementary data)
came as 8.33 ꢀ 104 Mꢁ1 and 2.57 ꢀ 104 Mꢁ1, the result was indi-
cated a strong and stable complexation of compound 1 and Fe3+
/
Cu2+ ions. Their detection limits were calculated to be 3.02 ꢀ 10ꢁ6
-
mol/L for Fe3+ ion and 3.62 ꢀ 10ꢁ6 mol/L for Cu2+ ion (Fig. S8, Sup-
plementary data) [30,31].
The fluorescence spectral studies
In addition, the effect of Fe3+/Cu2+ on the fluorescence proper-
ties of compound 1 in the CH3CN solution was investigated
In CH3CN solution, the fluorescence emission intensity of 1 was
recorded at 423 nm, but it was weak, which was probably due to
the fact that C@N isomerization was the predominant decay pro-
cess of the excited states of 1 with an unbridged C@N structure
and also due to excited-state intramolecular proton transfer
(ESIPT). Surprisedly a dramatic enhancement of the fluorescent
intensity was observed in the presence of Cu2+/Fe3+. While the
(Fig. 4). Fig. 4 showed that upon the continuous addition of Fe3+
/
Cu2+ ions, the intensity at 423 nm (kmax) was gradually enhanced
with a small red-shift. The results indicated that compound 1 can
serve as a good fluorescence probe for Fe3+/Cu2+ in CH3CN solution.
This enhancement of fluorescence intensity can be explained as
follows: The fluorescence intensity of compound 1 is weak in CH3-
CN solution, which may be due to ESIPT involving the phenolic pro-
tons and C@N isomerization. The C@N isomerization is the
predominant decay process of the excited state for compound 1.
Upon addition of Fe3+ or Cu2+, the coordination of compound 1
with the metal ion inhibits the C@N isomerization and prevents
addition of other metal ions, such as Na+, K+, Ag+, Ca2+, Hg2+
,
Zn2+, Co2+, Mn2+, Mg2+ and Cr3+, had no distinct influence on fluo-
rescent intensity (Fig. 3). As shown in Fig. 3, the addition of 5 equiv
of Cu2+ caused the great enhancement of fluorescence intensity
and the fluorescence intensity increased 20.5-fold. At the same

L. Yang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 186–192
191
Scheme 2. The binding mode of compound 1 and Fe3+/Cu2+
.
ESIPT. Additionally, the reaction of a chelating agent with a metal
ion induces rigidity of the resulting molecule and tends to produce
a large chelation-enhanced fluorescence (CHEF) effect. However,
other ions fail to form such framework, probably because of several
combined influences leading to selective recognition for Fe3+ and
Cu2+, such as the suitable coordination geometry conformation of
the Schiff base sensor, the appropriate ion radius and sufficient
binding energy of the Fe3+ and Cu2+
.
The sensitivity was assessed by a Stern–Volmer plot (Fig. 5). A
Stern–Volmer plot between fluorescence intensity vs. concentra-
tion of Fe3+ and Cu2+ exhibited linear relationship (R2 = 0.98 and
R2 = 0.99, respectively), suggesting limit of detection of
4.23 ꢀ 10ꢁ6 M and 5.67 ꢀ 10ꢁ6 M, and indicating that compound
1 is potentially useful for detection of Fe3+ and Cu2+. To further
determination of stoichiometry between compound 1 and Fe3+
/
Cu2+, Job’s plot (Fig. 6) analyses were used. A maximum fluores-
cence was observed when the molar fraction of Fe3+ reached
0.33, which is indicative of a 2:1 stoichiometry complexation be-
tween 1 and Fe3+. When molar fraction of Cu2+ was 0.5, the fluores-
cence at 423 nm got to maximum, indicating that forming a 1:1
complex between 1 and Cu2+ (Scheme 2). The association constant
(Ka) of 1-Fe3+ was determined by nonlinear curve fitting and 1-Cu2+
was determined using the Benesi–Hildebrand equation which
came as 9.27 ꢀ 104 Mꢁ1 and 9.15 ꢀ 104 Mꢁ1, respectively (Fig. S9,
Supplementary data). These further confirmed the extent of coor-
dination between 1 and Fe3+/Cu2+. And compound 1 could use as
a sensor for Fe3+/Cu2+ by both using UV–Vis and fluorescence
methods. In other words, compound 1 showed good selectivity to-
ward Fe3+/Cu2+ in ground state as well as in excited.
The competitive binding studies
The results of UV–Visible competitive binding studies are de-
picted in Fig. S10 (Supplementary data). These miscellaneous com-
petition cations did not induce significant absorption changes of 1
in the absence of Cu2+ and Fe3+, which indicate that the compound
1 can be used as the high selective chemosensor for Cu2+ and Fe3+
by using familiar UV–Vis method. With the same method, fluores-
cent competition experiment was shown and depicted in Fig. 7.
These miscellaneous competition cations did not induce significant
fluorescence intensity changes of compound 1 in the absence of
Fe3+/Cu2+. One could easily understand that the effects on emission
intensity of compound 1 upon the addition of higher concentra-
tions of various cations were almost negligible except for Fe3+
/
Cu2+. It was clear that the interference of other ions was negligible
small during the detection of Fe3+/Cu2+
.
Conclusion
Fig. 7. Metal ions selectivity of 1 (10.0 lM) in CH3CN solution. (a) The black bars
represent the fluorescent intensity of 1. The red bars represent the fluorescent
intensity of 1 and various ions. The blue bars represent the fluorescent intensity of 1
and Fe3+ (kex = 290 nm). (b) The black bars represent the fluorescent intensity of 1.
The red bars represent the fluorescent intensity of 1 and various ions. The blue bars
represent the fluorescent intensity of 1 and Cu2+ (kex = 290 nm). (For interpretation
of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
In conclusion, an imine-linked carbazole-based chemosensor
was synthesized and characterized for recognition of Fe3+/Cu2+
Studies showed that compound 1 showed good selectivity toward
Fe3+/Cu2+ over other competing ions in ground state as well as in
excited state. In addition, the binding stoichiometry was confirmed
.

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Acknowledgments
This research was financially supported by the Key University
Natural Science Research Project of the Natural Science Fund of An-
hui Province (No. 11040606M33), the Dr. Research Project Fund of
Anhui University (No. 02303319), Youth Scientific Research Fund
of Anhui University and the Excellent Youth Fund in University
of Anhui Province (No. 2012SQRL025), the Graduate Academic
Innovation Research Project Fund of Anhui University (No.
yph100080).
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