Colorimetric and fluorescence turn-on sensor for biologically important anions based on carbazole derivative

Article abstract of DOI:10.1016/j.inoche.2011.11.025

A colorimetric and fluorescent anion chemosensor containing carbazole group has been designed and synthesized based on photoinduced electron transfer (PET). The strong basic anions such as F^-, AcO^- and H ₂PO₄^- resulted in significant red-shift in absorption band and enhancement in fluorescent emission intensity of the compound 1, synchronously accompanied by a "naked eye" color change from light yellow to orange-yellow in organic medium. The determination limit of sensor 1 toward H₂PO₄^- is 1.0 × 10 ^{- 6} mol?L^{- 1}. ¹H NMR titration experiments shed light on the nature of the interaction between 1 and the anions. Theoretical investigation further illustrated the possible binding mode of host-guest and the roles of molecular frontier orbitals in anion binding.

Full text of DOI:10.1016/j.inoche.2011.11.025

Inorganic Chemistry Communications 16 (2012) 37–42
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Colorimetric and fluorescence turn-on sensor for biologically important anions based
on carbazole derivative
b
a
a
c
d
Xuefang Shang a, , Xinjuan Li , Jie Han , Shenyu Jia , Jinlian Zhang , Xiufang Xu
⁎
a
Department of Chemistry, Xinxiang Medical University, East of Jinsui Road, Xinxiang, Henan, 453003, China
Department of Chemistry, Henan Normal University, East of Construction Road, Xinxiang, Henan, 453007, China
School of Pharmacy, Xinxiang Medical University, East of Jinsui Road, Xinxiang, Henan, 453003, China
Department of Chemistry, Nankai University, Tianjin, 300071, China
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
A colorimetric and fluorescent anion chemosensor containing carbazole group has been designed and synthe-
sized based on photoinduced electron transfer (PET). The strong basic anions such as F−, AcO− and H2PO4−
resulted in significant red-shift in absorption band and enhancement in fluorescent emission intensity of
the compound 1, synchronously accompanied by a “naked eye” color change from light yellow to orange-
Received 13 September 2011
Accepted 22 November 2011
Available online 3 December 2011
yellow in organic medium. The determination limit of sensor 1 toward H2PO4− is 1.0×10−6 mol·L−1 1H
.
Keywords:
NMR titration experiments shed light on the nature of the interaction between 1 and the anions. Theoretical
investigation further illustrated the possible binding mode of host–guest and the roles of molecular frontier
orbitals in anion binding.
Colorimetric sensor
Fluorescence
PET
© 2011 Elsevier B.V. All rights reserved.
Naked-eye
Carbazole
Determination limit
1. Introduction
[20,21]. The mechanisms, which control the response of the chromo-
phore or fluorophore to substrate binding, include photoinduced
The research field of chemosensors continues to expand with new
synthetic molecules capable of recognizing anions which play an im-
portant role in a wide range of chemical and biological processes
[1–5]. Among various biologically important anions, phosphates
anion has received growing attention for its established role in a vari-
ety of fundamental processes such as energy transduction, signal pro-
cessing, genetic information storage, and membrane transport [6].
Phosphates can also be found in many chemotherapeutic and anti-
viral drugs [7–9]. It is for sure that phosphate originating from the
over use of agricultural fertilizers can also lead to eutrophication in
inland waterways [10]. Hence, to recognize and sense phosphate
ions and phosphorylated biomolecules is to be more and more impor-
tant than other biologically functional anions [11–13].
Nowadays, a colorimetric receptor for special anionic species is in-
creasing interest due to its simplicity and high sensitivity [14–17].
Colorimetric sensing is especially attractive, as it may allow “naked-
eye” detection of the analyte without resorting to any expensive
equipment [18,19]. In general, these chemosensors are constructed
according to the receptor–chromophore general binomial, which in-
volves the binding site and a chromophore or fluorophore which
can translate the receptor–anion association into an optical signal
electron transfer (PET) [22], fluorescence (Förster) resonance energy
transfer (FRET) [23], excimer/exciplex formation [24] or extinction,
and photoinduced charge transfer (PCT) [25]. Among these signaling
mechanisms, the PET mechanism is often exploited for the sensors
showing the fluorescence response to the guest binding owing to
that such fluorescent host molecules based on PET is mostly structur-
ally simple [26].
Various types of artificial receptors have been developed that em-
ploy hydrogen bonds offered by specific binding sites as in amides
[27], thioamide [28], azamacrocycles [29], urea or thiourea [30], and
pyrroles [31,32] to bind anions with size and shape selectivity in var-
ious media. Phenol group is among the most frequently used frag-
ments to design neutral receptors for the selective recognition of
anions as it is able to strongly bind anions using directional hydrogen
bonding interactions even in the aqueous solution [33,34]. Although
anion receptors containing phenol group as a binding site have been
researched, recognition mechanism based on the synergism of phenol
and carbazole-NH is rarely reported in “naked-eye” detection of an-
ions [35,36].
In this paper, we reported a colorimetric and fluorescent anion
sensor 1 (N, N′-di(methylimino-1′-2′-hydroxyl-3′,5′-dibromidoben-
zene)-3, 6-dichloro-1,8-diami-nocarbazole) providing phenol group
and carbazole-NH as binding sites and conducting the PET signaling
mechanism. We reasoned that the OH group and NH group were in-
corporated to the conjugated system with the carbazole moiety
⁎
Corresponding author: Tel.: +86 373 3029128; fax: +86 373 3029959.
E-mail address: xuefangshang@126.com (X. Shang).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.11.025

38
X. Shang et al. / Inorganic Chemistry Communications 16 (2012) 37–42
(fluorophore) which made the sensor 1 capable of sensing anions op-
tically in organic medium possible. Just as expected, the sensor 1
showed “naked-eye” color changes from light yellow to orange and
the fluorescence turn-on changes upon the addition of anions in or-
ganic solvent. Although several PET sensors for anions are known,
such carbazole-NH and phenol group conjugated systems, employing
neutral anion receptors and exhibiting an ideal PET behavior and
color changes as signaling events, detectable by the “naked eye”
upon anion binding, have been rarely reported.
2. Experimental
Most of the starting materials were obtained commercially and all
reagents and solvents were of analytical grade. All anions, in the form
of tetrabutylammonium salts [such as (n-C4H9)4NF, (n-C4H9)4NCl, (n-
C4H9)4NBr, (n-C4H9)4NI, (n-C4H9)4NAcO, (n-C4H9)4NH2PO4], were
purchased from Sigma-Aldrich Chemical Co., and stored in a desicca-
tor under vacuum containing self-indicating silica, and used without
any further purification. Tetra-n-butylammonium salts were dried
for 24 h in vacuum with P2O5 at 333 K. Dimethyl sulfoxide (DMSO)
was distilled in vacuum after dried with CaH2. C, H, N elemental ana-
lyses were made on Vanio-EL. 1H NMR spectra were recorded on a
Varian UNITY Plus-400 MHz Spectrometer. ESI-MS was performed
with a MARINER apparatus. UV–vis titration experiments were
made on a Shimadzu UV2550 Spectrophotometer at 298 K. The affin-
ity constant, Ks, was obtained by non-linear least square calculation
method for data fitting.
Fig. 1. Changes in the absorption spectra of compound 1 (2.6×10−5 mol·L−1) in ab-
sence and presence of H2PO4- in DMSO.
for 0.5 h. The suspension was filtered, giving pure product. Yield: 72%;
mp: 202–204 °C (lit. 205 °C).
2.2. 3, 6-Dichloro-1, 8-dinitrocarbazole (3) [37]
To
a
three-neck, 250 ml round-bottomed flask, 3, 6-
dichlorocarbazole (8.35 g, 35 mmol), acetic acid (28 ml) and acetic
anhydride (21 ml) were added and then, the flask was equipped
with a magnetic stirrer, a dropping funnel and a thermometer. The
solution was cooled to 1 °C and 100% HNO3 (4.5 ml, 0.11 mol) began
to be added dropwise while stirring and the temperature did not ex-
ceed 1 °C. Following the addition of 1/8 of the whole volume of HNO3,
the cooling bath was replaced with an oil bath. The mixture was heat-
ed to 60 °C and half of HNO3 was added at this temperature. During
the addition of the remaining HNO3, the reaction mixture was further
heated, until it reached to 75 °C. After the addition was completed,
the temperature of the solution was elevated to 110 °C for next
10 min. Afterward, the suspension was filtered while hot and the
product was washed with boiling acetic acid (10 ml) and diethyl
ether. Yield: 86%; mp: 282–283 °C (lit. 284 °C); 1H NMR (DMSO-d6):
δ 11.30 (s, 1H, NH), 8.98 (s, 2H), 8.49 (d, 2H); elemental analysis
calcd. for C12H5Cl2N3O4: C, 44.20; H, 1.55; N, 12.89; found: C, 44.36;
Compound 1 was synthesized according to the route shown in
Scheme 1.
2.1. 3,6-Dichlorocarbazole (2) [37]
To a three-neck, 250 ml round-bottomed flask, carbazole (10 g,
0.06 mol) and CH2Cl2 (100 ml) were added and the flask was
equipped with a mechanical stirrer and a thermometer. The resulting
mixture was cooled to 0 °C and then SO2Cl2 (9.6 ml, 0.12 mol) was
added dropwise while the solution was being vigorously stirred
(note: the temperature did not exceed 2 °C). After the addition, the
cooling bath was removed and the reaction mixture was stirred for
another 4 h at room temperature. Then, the solid precipitate was fil-
tered, washed with CH2Cl2 and dried to give raw product. The raw
3,6-dichlorocarbazole was suspended in 250 ml of hexane and boiled
H, 1.61; N, 12.85. ESI-MS (m/z): 324.30 (M-H)−
.
Cl
Cl
Cl
Cl
CH Cl
AcOH, (Ac) O
2
2
2
SO Cl
2
HNO
3
2
N
H
N
H
N
O N
2
NO
H
2
3
2
Br
NH NH H O
Pd/C
Hc
2
2 2
Br
CHO
OH
Ha
Hd
Cl
Hb
N
Br
Cl
Cl
OH
OH
Br
NH
Br
Br
C H OH
2 5
N
N
H N
2
H
NH
2
Cl
4
1
Scheme 1. General synthetic route to the target compound 1.

X. Shang et al. / Inorganic Chemistry Communications 16 (2012) 37–42
39
Fig. 2. Color changes of 1 (2.6×10−5 mol·L−1) in DMSO in absence and presence of different anions (A: 1 only; B: 1+10 equiv. F−; C: 1+ 10 equiv. Cl−; D: 1+ 10 equiv. Br−; E: 1+
10 equiv. I−; F: 1+10 equiv. AcO−; G: 1+10 equiv. H2PO4−).
2.3. 3, 6-Dichloro-1, 8-diaminocarbazole (4)
detail through UV–vis spectroscopy and fluorescence emission. Fig. 1
shows the changes in UV–vis spectra of 1 (2.6×10−5 mol·L−1) upon ti-
tration with the H2PO4- solution. Obviously seen from Fig. 1, the UV–vis
spectrum of free 1 exhibited two broad absorption bands centered at
340 nm and 410 nm. The first band (340 nm) could be assigned to the
excitation of the π electrons in the carbazole system and the latter
band was due to intramolecular charge-transfer (ICT) transitions within
the whole structure of compound 1 [25]. With the stepwise addition of
H2PO4− ions to the DMSO solution of 1 (2.6×10−5 mol·L−1), the absor-
bance values at 340 nm and 410 nm were reduced in intensity and si-
multaneously a new absorbance band at 470 nm was developed
gradually. The red-shift phenomenon that occurred was induced by
H2PO4− ion in the absorption spectrum of compound 1. Noticeably, the
color of the solution changed from light-yellow to orange-yellow
which was attributed to the significant development of absorption
peak at 470 nm (Fig. 2). Interestingly, the addition of protic solvent
(such as H2O or MeOH) to the solution of compound 1 bound to
H2PO4− made orange yellow solution change back to light yellow,
which suggested hydrogen bonding donor solvent destroyed the bind-
ing between compound 1 and H2PO4−. The above observations also
demonstrated that the interactions between compound 1 and H2PO4−
were hydrogen bonding interaction in essence [38,39]. In addition,
there were two well-defined isosbestic points at 320 and 423 nm, re-
spectively, indicating the stable complex having a certain stoichiometric
ratio between compound 1 and H2PO4− anion formed [40]. Particularly,
the presence of AcO− and F− induced similar changes in UV–vis spec-
trum of compound 1 with those originating from the addition of
H2PO4−, but addition of excess equiv of Cl−, Br− and I− ions resulted
in slight changes in the UV–vis spectrum of compound 1 (Fig. 3).
To a three-neck, 250 ml round-bottomed flask, 3, 6-Dichloro-1,8-
dinitrocarbazole (1.62 g, 5 mmol) and Pd/C were suspended in
C2H5OH (100 ml). The solution was refluxing and NH2NH2·H2O
(10 ml) resolved in C2H5OH (30 ml) was added dropwise. After the
addition was completed, the mixture was refluxing for 12 h until
the solution was changed to colorless. Then, the hot suspension was
filtered and washed with ethanol. The solvent in filtrate was removed
by rotary evaporator and the gray-white compound was obtained and
recrystallized from ethanol. Yield: 59%; 1H NMR (DMSO-d6): δ 10.67
(s, 1H, NH), 8.91 (s, 2H), 8.36 (s, 2H); 5.42 (s, 4H, NH2) elemental
analysis calcd. for C12H9Cl2N3: C, 54.16; H, 3.41; N, 15.79; found: C,
54.28; H, 3.66; N, 15.65.
2.4. N,N′-Di(methylimino-1′-2′-hydroxyl-3′,5′-dibromidobenzene)-3,
6-dichloro-1,8-diaminocarbazole (1)
3, 6-Dichloro-1,8-diaminocarbazole (265 mg, 1 mmol) and 3,5-
dibromo-salicylaldehyde (556 mg, 2 mmol ) were suspended in
35 ml ethanol. The mixture was heated under refluxing for 4 h. The
yellow precipitate was separated by filtration. The solid was washed
with diethyl ether and dried under vacuum. 1H NMR (DMSO-d6): δ
13.24 (s, 2H, OH), 12.04 (s, 1H, NH), 9.06 (s, 2H), 8.29 (s, 2H), 7.98
(s, 4H), 7.53 (s, 2H). Elemental analysis: Calc. for C26H13Br4Cl2N3O2:
C, 39.53; H, 1.66; N, 5.32; Found: C, 39.31; H, 1.74; N, 5.69. ESI-MS
(m/z): 783.56 (M-H)−
.
3. Results and discussion
3.1. UV–vis titration and “naked-eye” observation
3.2. Fluorescence titration
The anion sensing ability of compound 1 with halide anions (F−
,
The photophysical response of compound 1 toward the addition of
anions tested was also investigated in DMSO. Just as Fig. 4 shows, the
Cl−, Br− and I−), AcO− and H2PO4− in dry DMSO was investigated in
Fig. 3. The spectral changes of compound 1 (2.6×10−5 mol·L−1) induced by the addi-
tion of 10 equiv of anions tested, respectively.
Fig. 4. Changes in the emission spectra of compound 1 (2.6×10−5 mol·L−1) in ab-
sence and presence of F− in DMSO.

40
X. Shang et al. / Inorganic Chemistry Communications 16 (2012) 37–42
Br
500 nm
444 nm
Br
Br
Br
Cl
Cl
N
Br
O
Cl
-
Br
Br
N
N
O
H
O
O
H
H
anion
anion
H
N
N
H
N
H
H
O
Br
Br
N
-
O
Br
Br
N
Cl
Cl
N
Cl
Br
Scheme 2. Proposed host–guest binding mode.
fluorescence spectrum of compound 1 exhibited emission band cen-
tered at 500 nm when excitated at 444 nm. Upon the addition of F−
ions (0–10 equiv) to the solution of compound 1, the fluorescence
got enhanced. There were similar changes in the fluorescence emis-
sion of 1 induced by the addition of AcO− and H2PO4− ions with
those of F−. To account for such fluorescence enhancement, the PET
mechanism was exploited. As we all knew, the fluorescence of the
anion PET chemosensor generally was ‘turn off’ rather than ‘turn on’
upon ion sensing unlike most PET sensors for cations. In the case of
compound 1, the excited state of the fluorophore was not, or only to
a minor extent, enhanced by electron transfer (ET) from the receptor
to the fluorophore and the receptor–anion interaction. Upon the
further addition of F−, it appeared that the deprotonated species
LH−/L2−, being more electron rich compared to the hydrogen-
bonded complex with fluoride, activated the PET process more effi-
ciently and showed up a greater enhancing (Scheme 2). Nevertheless,
fluorescence emission of sensor 1 was insensitive to addition of ex-
cess equiv of Cl−, Br− and I− ions (Fig. 5).
expected from their basicity, H2PO4−, AcO− and F− will bind more
strongly than the other anions studied; in addition, the tetrahedral
configuration of H2PO4− ion may well match 1 in terms of shape and
could form multiple hydrogen bonding interaction with compound
1. Consequently, H2PO4− ion can be selectively recognized from
other anions based on its affinity constant. In addition, the affinity
constants obtained by UV–vis data are in the same order of magni-
tude with it obtained by fluorescence data.
In cation recognition, determination limit was reported by many
literatures. While, determination limit of anion is almost not reported.
Thus, the determination limit of sensor 1 to H2PO4− was studied
(Fig. 6). The interaction of sensor 1 with H2PO4− could be detected
down to at least a concentration of 1.0×10−6 mol·L−1, at this time
the concentration of H2PO4− ion is 2.0×10−6 mol·L−1. And the corre-
sponding absorption intensity increase to 1.3 times, which showed
that sensor 1 could potentially be used as a sensor for monitoring
H2PO4− levels in physiological and environmental systems.
3.4. 1H NMR titration
3.3. Affinity constant
To further shed light on the nature of the interaction between
compound 1 and the anions, as an example, 1H NMR spectral changes
upon the addition of F− as their tetrabutylammonium salts to the
DMSO-d6 solution of 1 (1×10−2 mol·L−1) were investigated. Obvi-
ously observed from Fig. 7, the peak at 12.04 ppm, which was
assigned to ―NH, broadened and exhibited a downfield shift to
12.28 ppm upon the addition of different equiv of F− indicating the
strong hydrogen-bonding interactions occurred between compound
1 and F− ion [45]. The hydroxyl proton peak at 13.24 ppm, exhibited
an upfield shift to 12.75 ppm and then thoroughly disappeared with
the increase of F− ion. Such a significant upfield was rarely seen in
the ever-reported PET anion sensors. The reason may be as following:
In compound 1, hydroxyl group formed intramolecular hydrogen
bond with the near Br atom, which can be proved by theoretical in-
vestigation. With the addition of 0.5 equiv of F- ion, intramolecular
hydrogen bond was broken which induced deshielding effect and
the proton peak of hydroxyl group shifted to the upfield direction.
The job-plot analysis indicated that the spectral change could be
ascribed to the formation of 1:1 host–guest complexation. The
obtained affinity constants were listed in Table 1 using the method
of non-linear least squares calculation according to the UV–vis data
[41–43]. The selectivity trend of binding ability of compound 1 to an-
ions followed the order of: H2PO4− >AcO− >F− >>Cl− ~Br− ~I−. It is
apparent that the selectivity for specific anions can be rationalized on
the basis of the anion's basicity and the interactions between the host
and the anionic guests. However, multiple hydrogen-bond interac-
tions are also necessary in high-affinity anion binding sites [44]. As
Table 1
Affinity constants of compound 1 with various anions.
Anionsa
Ks (UV)
Ks (fluorescence)
H2PO4
AcO
F
Cl
Br
I
(2.13 0.19)×104
(2.02 0.11)×104
(1.79 0.26)×104
(1.98 0.24)×104
(1.08 0.18)×104
(0.98 0.17)×104
ND
ND
ND
NDb
ND
ND
a
Fig. 5. Changes in intensity of the emission band at 480 nm in presence of 10 equiv of
anions tested.
All anions were added in the form of tetra-n-butylammonium (TBA) salts.
The affinity constant could not be determined.
b

X. Shang et al. / Inorganic Chemistry Communications 16 (2012) 37–42
41
2.133
2.522
2.491
Fig. 6. The change of absorption intensity of sensor 1 in 475 nm with the increase of
Fig. 8. Optimized geometry of compound 1.
H2PO4− concentration.
induced electron transition of frontier orbital. The highest HOMO
density in compound 1 is mainly localized on the carbazole moiety.
While the highest LUMO density is mainly localized on one phenyl
ring moiety and carbazole moiety, this demonstrates that it is the
electron transition of the highest HOMO that arouse the red-shift
phenomenon in UV–vis spectra of 1-anion.
With the further addition of F- ion, one hydroxyl moiety was deproto-
nated due to the formation of FHF-. The proton peak of FHF- did not
appear which may be related with the small quantity water in
DMSO-d6 [46]. The carbazole protons (Ha: 8.19 ppm; Hb: 7.53 ppm)
exhibited a slight upfield shift which indicated the increase of the
electron density on them owing to the through-bond effects. The
proton peaks of phenyl (Hc and Hd, 7.98 ppm) were overlapped and
then splitted when the anion was absence and presence, respectively.
With the increase of F− ion, the proton peaks of phenyl splitted to
two peaks which shifted to the downfield and upfield direction,
respectively.
According to the results from UV–vis spectral, fluorescent and 1H
NMR titrations, the proposed host–guest binding mode in the solu-
tion was depicted in Scheme 2. In the complex structure, an anion
interacted with compound 1 via N―H and O―H hydrogen bonds.
With the further addition of anion, hydroxyl group deprotonated
and the hydrogen bond still existed in carbozale–NH and anion.
3.5. Theoretical investigation
The geometry of compound 1 was optimized (Fig. 8) using density
functional theory at B3LYP/3-21G level with Gaussian03 program
[47]. From Fig. 8, the intramolecular hydrogen bonds indeed exist in
compound 1 between carbozale-NH and hydroxyl group in benzene
ring or hydroxyl group and the bromide atom. This result can explain
the 1HNMR titration phenomena in the interacted process of host–
guest.
In addition, selected frontier orbitals for compound 1 are shown in
Fig. 9. We introduced molecular frontier orbital in order to explain
UV–vis absorption spectra in the interacted process of host–guest
Fig. 7. 1H NMR spectra of compound 1 in DMSO-d6 (1×10−2 mol·L−1) upon addition
of molar equiv of F−
.
Fig. 9. Molecular orbital level of compound 1, a) HOMO; b) LUMO.

42
X. Shang et al. / Inorganic Chemistry Communications 16 (2012) 37–42
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In conclusion, a colorimetric and fluorescent anion sensor, which
allows so-called naked-eye detection in a straightforward and inex-
pensive manner, offering qualitative and quantitative information
without using expensive equipment, has been developed. And what
is more attractive is that the sensor is based on photoinduced electron
transfer (PET) fluorescence enhancement (turn-on), which offers the
potential for high sensitivity. In addition, the determination limit of
sensor 1 toward H2PO4− is 1.0×10−6 mol·L−1 which indicates that
this sensor could potentially be useful as a probe for monitoring
H2PO4− levels in physiological and environmental systems. What is
more, the sensor is expected to have many opportunities in detection
of H2PO4− ion in real life owing to the simplicity and sensitivity of the
analysis.
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Acknowledgments
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fumi Unno, Yumihiko Yano, Anion recognition by 2,2′-binaphthalene derivatives
bearing urea and thiourea groups at 8- and 8′-positions by UV–vis and fluores-
cence spectroscopies, Tetrahedron 67 (2011) 943–950.
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This work was supported by the Nature Science Fund of Henan
Province (2010B150024), Young Teacher Fund of Henan Province
and Doctorial Starting Fund of Xinxiang Medical University.
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