A novel colorimetric and fluorometric anion sensor based on BODIPY-calix[4]pyrrole conjugate

Article abstract of DOI:10.1007/s10847-011-9946-1

A novel fluorescent anion sensor 1 based on boradiazaindacenes (BODIPY) derivative was synthesized and its absorption and fluorescence properties were investigated in various solvents. 1 exhibited a red shift of absorption spectrum and fluorescence quenching in varying degree in the presence of F^-, AcO^-, H₂PO₄^- and Cl^- due to multiple hydrogen bonding interactions between these anions and calix[4]pyrrole receptor. As an anion sensor in the visible region, 1 displayed the similar selectivity and sensitivity toward anions compared to the parent calix[4]pyrrole. However, 1 can be used as an effective dual responsive optical sensor for F^- via chromogenical and fluorogenical signals.

Full text of DOI:10.1007/s10847-011-9946-1

J Incl Phenom Macrocycl Chem (2012) 72:95–101
DOI 10.1007/s10847-011-9946-1
ORIGINAL ARTICLE
A novel colorimetric and fluorometric anion sensor based
on BODIPY-calix[4]pyrrole conjugate
•
Yongjun lv Jian Xu Yong Guo Shijun Shao
•
•
Received: 18 January 2011 / Accepted: 1 March 2011 / Published online: 20 April 2011
Ó Springer Science+Business Media B.V. 2011
Abstract A novel fluorescent anion sensor 1 based on
boradiazaindacenes (BODIPY) derivative was synthesized
and its absorption and fluorescence properties were inves-
tigated in various solvents. 1 exhibited a red shift of
absorption spectrum and fluorescence quenching in varying
degree in the presence of F-, AcO-, H2PO4- and Cl- due
to multiple hydrogen bonding interactions between these
anions and calix[4]pyrrole receptor. As an anion sensor in
the visible region, 1 displayed the similar selectivity and
sensitivity toward anions compared to the parent
calix[4]pyrrole. However, 1 can be used as an effective
dual responsive optical sensor for F- via chromogenical
and fluorogenical signals.
Keywords Calix[4]pyrrole Á BODIPY Á Fluorescence Á
Colorimetric Á Anion sensor
Introduction
Design and synthesis of optical chemosensors capable of
selectively recognizing and sensing anions have attracted
increasing interest in the last few years, as anions play an
important role in medical, biological, and environmental
researches [1–5]. Many reported anion receptors generally
follow the approach of the covalent attachment of a sig-
naling unit to an anion receptor [6]. Based on this strategy,
some chromophores and fluorophores capable of absorbing
and emitting in the visible region have been employed to
response anion binding by giving rise to colorimetic
receptors and fluorometric receptors. Among many sig-
naling fluorophores, BODIPY dyes are very attractive
functional groups due to their high extinction coefficients,
fluorescence quantum yields, stability against light and
chemical reactions, and excitation at approximately
500 nm. Moreover, the parent BODIPY dye can be easily
modified to afford a variety of chemosensors, light-har-
vesting systems, energy transfer cassettes, fluorescent bi-
olabeling reagents, and photodynamic therapy [7, 8]. In
spite of this, fluorescent anion sensors based on BODIPY
moiety are still relatively rare [9–12]. Therefore, we set out
to construct a novel colorimetric and fluorometric anion
sensor that contains a fluorescent BODIPY moiety and a
calix[4]pyrrole anion binding site. As is well known,
calix[4]pyrrole, a class of molecule known for over a
century, are effective anion binding agents for halides,
carboxylates and phosphates via multiple hydrogen bond-
ing interactions in solution [13–15]. However, the parent
calix[4]pyrrole only response anion recognition events in
Electronic supplementary material The online version of this
article (doi:10.1007/s10847-011-9946-1) contains supplementary
material, which is available to authorized users.
Y. lv Á J. Xu Á Y. Guo Á S. Shao (&)
Key Laboratory of Chemistry of Northwestern
Plant Resources and Key Laboratory for Natural Medicine
of Gansu Province, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000,
People’s Republic of China
e-mail: sjshao@licp.cas.cn
S. Shao
State Key Laboratory for Oxo Synthesis and Selective
Oxidation, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000,
People’s Republic of China
Y. lv
Graduate School of the Chinese
Academy of Sciences, Beijing 100039,
People’s Republic of China
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J Incl Phenom Macrocycl Chem (2012) 72:95–101
Synthesis of 3-[2-(b-octamethylcalix[4]pyrrolyl)ethenyl]-
4,4-difluoro-8-(4-bromo)phenyl-1,5,7-trimethyl-3a,4a-
diaza-4-bora-s-indacene (1)
the ultraviolet region, and these changes are unpredictable
and difficult to rationalize. As a result, it is desirable to
design calix[4]pyrrole-based sensors by attaching various
fluorescent reporter groups such as anthracene, dansyl,
rhodamine B, fluorescein, and anthraquinone moieties [16–
18], and these systems show relatively short-wavelength
absorption and emission spectra changes. To obtain an
optical anion sensor in the longer wavelength region, we
herein report a novel BODIPY-calix[4]pyrrole conjugate 1
and its spectroscopic properties and particular behaviors of
anion recognition and sensing. The target compound 1 and
a reference 2 were prepared according to the route shown
in Scheme 1.
Compound 3 (1.00 g, 2.19 mmol) [19] and formylca-
lix[4]pyrrole (0.88 g, 2.19 mmol) [20] were refluxed in a
mixture of toluene (80 mL), piperidine (0.5 mL), glacetic
acid (0.5 mL), and Mg(ClO4)2 (cat.). Any water formed
during the reaction was removed azeotropically by heat-
ing in a Dean-Stark apparatus overnight. After completion
of the reaction monitored by TLC, solvents were evapo-
rated under reduced pressure, and the crude product was
purified by column chromatography on silica gel using
eluent (CH2Cl2/Hexane, 1:1). The red colored fraction
was collected to afford purple solid 1. Yield 40%. M.p.
1
[300 °C. H NMR (400 MHz, CDCl3): dH 1.40 (s, 3H,
Experimental section
CH3), 1.44 (s, 3H, CH3), 1.50 (s, 6H, CH3), 1.51 (s, 6H,
CH3), 1.53 (s, 6H, CH3), 1.66 (s, 6H, CH3), 2.57 (s, 3H,
CH3), 5.84 (d, J = 3 Hz, 1H, pyrrolic CH), 5.88 (d,
J = 3 Hz, 1H, pyrrolic CH), 5.91–5.94 (m, 3H, pyrrolic
CH), 5.96 (d, 3.2 Hz, 2H, pyrrolic CH), 6.38 (s, 1H, NH),
6.50 (s, 1H, CH), 6.90 (s, 2H, 2NH), 7.16 (s, 1H, NH),
7.18–7.24 (m, 4H, 2H for ArH, 1H for vinylic, 1H, CH),
7.60–7.63 (m, 3H, 2H for ArH, 1H for vinylic). 13C NMR
(100 MHz, CDCl3): dC 14.5, 14.6, 15.0, 28.6, 28.7, 29.3,
29.8, 35.1, 35.2, 35.3, 37.4, 101.8, 102.8, 103.0, 103.1,
103.6, 103.7, 103.8, 114.5, 117.3, 117.8, 120.5, 123.0,
130.3, 130.8, 132.3, 132.8, 134.4, 136.9, 137.2, 137.3,
137.6, 138.1, 138.3, 138.7, 139.2, 139.4, 140.2, 142.7,
152.9, 155.8. FT-IR (KBr): 3406.05, 3104.88, 2966.64,
2924.01, 2865.72, 1602.60, 1544.04, 1508.36, 1488.84
cm-1. HRMS (ESI, m/z) calcd for C48H52BBrF2N6 840.
3608 (M ? H?), Found 840.3600. Anal. Calcd. for 1
(C48H52N6BBrF2): C, 67.99; H, 7.04; N, 9.37. Found: C,
68.06; H, 6.93; N, 9.37.
Materials and methods
All chemicals were obtained from commercial suppliers
and used without further purification. CH2Cl2 was distilled
from NaH, THF was distilled from LiAlH4, and CH3CN
was distilled from P2O5 prior to use. Silica gel (mesh
200–300) was used for TLC analysis and flash column
chromatography. Proton and carbon NMR spectra were
performed on a Varian INOVA 400 MHz spectrometer in
CDCl3 using TMS as the internal standard. HRMS (ESI)
studies were carried out using a Bruker Daltonics APEX
spectrometer. The emission spectra were recorded on a
Perkin Elmer LS55 spectrometer. UV–vis spectra were
determined on a Perkin Elmer Lambda 35 spectrometer.
FT-IR spectra were performed on a Nexus 870 spectrom-
eter. C, H, N elemental analyses were made on a Vario-EL.
Synthesis of 3-[2-(2-pyrrolyl)ethenyl]-4,4-difluoro-8-(4-
bromo)phenyl-1,5,7-trimethyl-3a,4a-diaza-4-bora-s-
indacene (2)
The procedure is similar to the synthesis of 1. Compound 3
(1.00 g, 2.19 mmol) reacted with 2-formylpyrrole (0.21 g,
2.19 mmol) to afford black solid 2. Yield 42%. M.p.
1
[300 °C. H NMR (400 MHz, CDCl3): dH 1.42 (s, 3H,
CH3), 1.43 (s, 3H, CH3), 2.55 (s, 3H, CH3), 5.97 (s, 1H,
CH), 6.26 (m, J = 4.4 Hz, 1H, pyrrolic CH), 6.45 (m,
J = 4.4 Hz, 1H, pyrrolic CH), 6.53 (s, 1H, CH), 6.90 (s,
J = 4.0 Hz, 1H, pyrrolic CH), 7.11 (s, 2H, vinylic CH)
7.18 (d, J = 8.4 Hz, 2H, ArH), 7.62 (d, J = 8.4 Hz, 2H,
ArH), 8.76 (s, 1H, NH). 13C NMR (100 MHz, CDCl3): dC
14.6, 15.0, 110.9, 112.6, 114.1, 117.6, 120.9, 122.2, 123.2,
126.7, 130.2, 130.5, 132.3, 134.2, 137.4, 141.1, 142.6,
153.7, 154.1. FT-IR (KBr): 3362.70, 2918.33, 2851.39,
Scheme 1 The synthetic route of compounds 1 and 2
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J Incl Phenom Macrocycl Chem (2012) 72:95–101
97
1603.48, 1541.15, 1496.11 cm-1. HRMS (ESI, m/z) calcd
for C24H21BBrF2N3 479.1089 (M ? H?), Found 479.1092.
Anal. Calcd. for 2 (C24H21N3BBrF2): C, 58.91; H, 4.60; N,
8.15. Found: C, 58.85; H, 4.65; N, 8.12.
(kAbs = 499 nm). This red shift can be attributed to the
increased degree of p-electron conjugation of the two
compounds due to the attachment of the conjugated pyrrole
unit to BODIPY core. The high molar absorption coeffi-
cients of 1 and 2 suggested that the long-wavelength of the
absorption spectra were ICT bands during S0 ? S1
transition.
Results and discussion
Fluorescence quantum yields of 1 and 2 were estimated
in different solvents using rhodamine B (Af = 0.49 in
EtOH) as a standard [28]. As the solvent polarity increased,
the Af value became small, which can be assigned to the
solvent relaxation of the fluorescent state in the polar sol-
vent. Both 1 and 2 showed identical emission maxima at
600–620 nm, which revealed their similar ICT nature of
excited states. The Stokes shift for sensor 1 or 2 was small
(Dmaxv = 338–1061 cm-1) indicating that there was no
obvious difference in the structure of sensor in the ground
state S0 and in the first excited state S1 [29]. However, the
fluorescence quantum yield of 1 was smaller than that of 2,
especially in CH3CN, which may be ascribed to a more
efficient ICT process for the system of 1 in the polar
solvent.
Design and synthesis of sensors
The ‘‘fluorophore-spacer-receptor’’ format, one of the most
popular models to build fluorescent sensors and switchers
[21, 22], was configured for the design of sensor 1. In this
case, BODIPY unit was a fluorophore, calix[4]pyrrole unit
served as an anion binding site, and vinyl fragment acted as
a spacer. It was predicted that intramolecular charge
transfer (ICT) process would occur from the electron-rich
calix[4]pyrrole unit to the electron-withdrawing BODIPY
unit [23, 24]. Moreover, this ICT process can be enhanced
by the anion coordination of calix[4]pyrrole unit, which
would result in changes in absorption and fluorescence
spectra [25, 26]. For a control test, compound 2 was syn-
thesized as a reference to study the anion recognition
mechanism.
In a word, both 1 and 2 exhibited almost similar
absorption and fluorescence characteristics. These results
indicated that, for sensor 1, the conjugated pyrrole group of
calix[4]pyrrole subunit made substantial contributions to
its improved spectroscopic properties.
The target compound 1 was prepared by a little modified
Knoevenagel reaction in satisfied yields. Intermediate 3
reacted with formyl-calix[4]pyrrole and 2-formylpyrrole to
obtain compounds 1 and 2, respectively.
Spectroscopic properties of sensors
Anion sensing studies of senors
As shown in Table 1, spectroscopic properties of 1 and 2
were investigated in various solvents. Their absorption
maxima showed minor solvent-dependent shifts, indicating
a weak ICT character for the lowest electronic excitation in
the ground state [27]. Both 1 and 2 displayed an absorption
band centered at 586–595 nm having about a 90 nm red
shift in comparison to that of the parent BODIPY dye 3
The anion binding and sensing properties of sensor 1 were
firstly studied by UV–vis spectroscopic techniques. Sensor
1 (1 9 10-5 M in CH3CN) itself displayed an intense
absorption peak at 587 nm which can be assigned to the
ICT absorption band. Upon addition of anions such as F-,
AcO-, H2PO4-, Cl-, Br-, I-, ClO4 and HSO4 (as tet-
-
-
rabutylammonium salts) to the solution of sensor 1, only
Table 1 Spectroscopic characteristics of two BODIPY dyes (1 and 2) in different solvents
Dyes
Solvent
kAbs (max/nm)
kem (max/nm)
D
maxv (cm-1
)
Af
e (M-1cm-1
)
1
CH3CN
EtOH
587
591
592
589
589
586
595
595
590
590
626
622
617
617
604
619
623
620
613
602
1061
843
684
770
422
910
755
678
636
338
0.036
0.100
0.187
0.200
0.219
0.176
0.207
0.210
0.249
0.299
0.97 9 105
0.86 9 105
0.79 9 105
1.02 9 105
1.84 9 105
1.34 9 105
1.12 9 105
0.88 9 105
1.52 9 105
1.78 9 105
THF
CH2Cl2
Cyclohexane
CH3CN
EtOH
2
THF
CH2Cl2
Cyclohexane
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J Incl Phenom Macrocycl Chem (2012) 72:95–101
F-, AcO-, H2PO4 and Cl- induced obvious absorption
spectral changes over other anions tested (Fig. 1).
-
During the titration of F- with a solution of sensor 1 in
Fig. 2, the absorption maximum at 587 nm decreased,
along with the formation of a new red-shift band centered
at 602 nm, which was responsible for simultaneously
changing the solution of 1 from fuchsia to blue (Fig. 3).
Likewise, a new red-shift absorption band appeared at
597 nm for AcO-, at 596 nm for H2PO4 or 592 nm for
-
Cl-, respectively (Fig. S1–S3). Naked eye-detectable color
change from fuchsia to purple was observed for AcO- and
-
H2PO4
.
The 1:1 binding stoichiometry of sensor 1 with F-,
AcO-, H2PO4-, and Cl- (Fig. 4) was established by Job
plot method, respectively. According to 1:1 model, the
Fig. 3 The color changes and the fluorescence emission responses of
1 (1 9 10-5 M in CH3CN) upon addition of various anions (10 equiv.
excess)
association constants of sensor 1 interacting with these
anions were determined (Figs. 2 and S1–S3 inset) and the
values were given in Table 2 [30]. The affinity of sensor 1
toward
various
anions
was:
F- [ AcO- [ H2-
PO4- [ Cl- [[ Br-, I-, ClO4- and HSO4-, in consistent
with that of the parent calix[4]pyrrole [31]. These results
suggested that the anion-sensor complexations formed
between sensor 1 and putative anions, which might be
based on hydrogen bonds.
For a further investigation, fluorescence titration exper-
iment of sensor 1 (5 9 10-6 M) with F- was conducted in
CH3CN (Fig. 5). With gradual addition of F-, the emission
intensity at 626 nm decreased without any significant
changes in the emission wavelength, which reflected that
there was no obvious structure difference in sensor 1 before
and after binding F- in the first excited state. Fluorescence
quenching can be ascribed to the occurrence of ICT process
from calix[4]pyrrole unit upon binding F- to BODIPY unit.
Quenching was also observed in varying degree upon
addition of AcO-, H2PO4-, and Cl- to sensor 1 (Fig. S4–
S6). In this stance it can be seen that the addition of F-
caused the greatest fluorescence quenching, followed by the
Fig. 1 Changes in the UV–vis absorption spectrum of 1 (1 9 10-5
in CH3CN) upon addition of various anions (10 equiv. excess)
M
addition of AcO-, H2PO4 and Cl- (Fig. 3). Using these
-
quenching data and Nonlinear curve fitting, association
constants were calculated for sensor 1 interacting with F-
AcO-, H2PO4 and Cl-, as illustrated in Table 2 (Figs. 5
-
and S4–S6 inset). In general, good agreement was observed
between these K values and those obtained using UV–vis
changes. In contrast, addition of Br-, I-, ClO4-, and
-
HSO4 showed no significant fluorescence response.
The reversibility of the anion binding process was
examined by adding water to the system of sensor 1, in
result, the absorption and fluorescence spectra can revert to
the original spectra. This is presumably because water
Fig. 2 Changes in the UV–vis absorption spectrum of 1 (1 9 10-5
M
in CH3CN) upon addition of F- from 0 to 1.1 9 10-5 mol L-1. Inset:
Nonlinear curve fitting as a function of [F-] monitored at 587 nm
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J Incl Phenom Macrocycl Chem (2012) 72:95–101
99
Fig. 4 Job plots of sensor 1
with various anions measured
by UV–vis spectroscopy at
587 nm in CH3CN.
[1] ? [Anion] = 1 9 10-5
M
could compete for anion with NH moieties through
hydrogen bonding interactions.
Table 2 Association constants for sensor 1 with putative anions in
CH3CN as determined from UV–vis at 587 nm and fluorescence
titrations at 626 nm
In addition, the nature of interactions between sensor 1
and F- was evident by means of 1H NMR titration in
CDCl3, as shown in Fig. 6. With gradual addition of tet-
rabutylammonium fluoride to sensor 1, pyrrolic NH pro-
tons resonance broadened and downfield shifted
significantly from 6.377, 6.897 and 7.162 to 12.52 and
13.05 ppm, consistent with the formation of anion-sensor
hydrogen bonds. Moreover, it was also found that pyrrolic
CH protons upfield shifted and split by interaction with
fluoride. These results indicated that hydrogen bonding
interactions may occur between sensor 1 and F- involving
calix[4]pyrrole-anion complex.
Anion
K (M-1) determined by
absorption changes
K (M-1) determined by
fluorescence quenching
F-
6.78 9 104
2.27 9 104
4.16 9 103
1.88 9 103
7.34 9 104
3.42 9 104
9.11 9 103
2.59 9 103
AcO-
H2PO4
Cl-
-
For a comparison, a control experiment using reference
2 was carried out to investigate whether the interaction of
sensor 1 with anions underwent deprotonation under sim-
ilar experimental conditions. There were no significant
absorption and fluorescence changes observed upon addi-
tion of various anions tested (Fig. 7). These results indi-
cated that the only one pyrrole unit, as an anion binding
site, can not effectively bind anions through deprotonation
or hydrogen bonds. In the case of calix[4]pyrrole-based
sensor 1, the pyrrole unit that conjugated to BODIPY
fluorophore should cooperate with other three pyrrole units
to effectively bind anions through multiple hydrogen
bonds. In other words, bearing calix[4]pyrrole unit as an
effective and sensitive anion binding unit, 1 can be used as
a colorimetric and fluorometric anion sensor.
Fig. 5 Changes in the emission spectrum of 1 (5 9 10-6 M in
CH3CN) upon addition of F- from 0 to 6.4 9 10-6 M. Inset:
Nonlinear curve fitting as a function of [F-] monitored at 626 nm.
Excitation was at 587 nm
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J Incl Phenom Macrocycl Chem (2012) 72:95–101
a new fluorescent anion sensor 1 based on BODIPY-
calix[4]pyrrole conjugate was reported, and it can be used
as a colorimetric and fluorometric anion sensor especially
for F- based on hydrogen bonding interactions. Though the
anion binding behavior of sensor 1 toward specific anions
was the same as that of the parent calix[4]pyrrole, the
introduction of the BODIPY group had a positive effect on
the anion sensing properties of calix[4]pyrrole receptor,
which allowed the detection of anions via dual optical
methods in the visible region. As a result, the recognition
of F- gave rise to the fabrication of both color change from
fuchsia to blue and fluorescence quenching, which would
have a potential application to develop a naked-eye
detection material. Further work to that end is in progress.
Fig. 6 1H NMR spectra of sensor 1 in the presence of 0.2, 0.5, 1.0
and 1.5 equiv. of F-. The NH and b-pyrrolic CH proton resonances in
free and complexed sensor 1 are labeled as follows: NH free, CH free,
NH complex, and CH complex
Acknowledgments This work was supported by the National Nat-
ural Science Foundation of China (Grant No. 20972170 to S-J. Shao),
the open fund of State Key Laboratory of Oxo Synthesis & Selective
Oxidation (Grant No. OSSO2008kjk6 to S-J. Shao) and by the Natural
Science Foundation of Gansu province (No. 096RJ2A033 to Y. Guo).
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