A sandwich anion receptor by a BODIPY dye bearing two calix[4]pyrrole units

Article abstract of DOI:10.2478/s11696-011-0033-2

A novel dicalix[4]pyrrolyl-substituted 4,4-difluoro-4-bora-3a,4a-diaza-s- indacene (BODIPY) dye I with an absorption peak at approximately 670 nm and an emission peak at about 690 nm was prepared. As an anion receptor, I displayed a red shift in absorption spectra and fluorescence quenching in varying degrees in the presence of F^-, AcO^-, H₂PO ^-₄ , or Cl^-. Compared with the parent calix[4]pyrrole, a representative anion receptor, I exhibited a stronger affinity to these anions due to the formation of a sandwich complex through multiple hydrogen-bonding interactions.

Full text of DOI:10.2478/s11696-011-0033-2

Chemical Papers 65 (4) 553–558 (2011)
DOI: 10.2478/s11696-011-0033-2
ORIGINAL PAPER
A sandwich anion receptor by a BODIPY dye bearing
two calix[4]pyrrole units
a
a
a,cYongjun Lv, Jian Xu, Yong Guo, a,bShijun Shao*
aKey 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, China
bState Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, China
cGraduate School of the Chinese Academy of Sciences, Chinese Academy of Science, Beijing 100039, China
Received 11 November 2010; Revised 17 February 2011; Accepted 22 February 2011
A novel dicalix[4]pyrrolyl-substituted 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dye
I with an absorption peak at approximately 670 nm and an emission peak at about 690 nm was
prepared. As an anion receptor, I displayed a red shift in absorption spectra and fluorescence
quenching in varying degrees in the presence of F−, AcO−, H2PO4−, or Cl−. Compared with the
parent calix[4]pyrrole, a representative anion receptor, I exhibited a stronger affinity to these anions
due to the formation of a sandwich complex through multiple hydrogen-bonding interactions.
c
ꢀ 2011 Institute of Chemistry, Slovak Academy of Sciences
Keywords: anion receptor, BODIPY, calix[4]pyrrole, fluorescence
Introduction
taining two calix[4]pyrrole units as an anion recep-
tor.
The construction of optical anion receptors which
convert the signal on molecular recognition into ab-
sorbance and fluorescence changes has been of impor-
tance (Martínez-Mán˜ez & Sancenón, 2003; Gunnlaug-
sson et al., 2006; Gale, 2008; Caltagirone & Gale,
2009; Mikláš et al., 2009), because anions play es-
sential roles in many biological and chemical pro-
cesses (Bianchi et al., 1997). The range between
650–900 nm, the so-called “optical window”, has
many advantages such as low light-scattering, sig-
nificant reduction of background interruption and
deep penetration of light (Ballou et al., 2005).
However, only a few long-wavelength chemosensors
have been reported for the detection of anions
(Zhang et al., 2008a; Qian et al., 2009). It is, there-
fore, worth constructing novel anion receptors in
the long-wavelength region. Hence, we report on
a novel dicalix[4]pyrrolyl-substituted 4,4-difluoro-4-
bora-3a,4a-diaza-s-indacene (BODIPY) dye I con-
The BODIPYs are very attractive fluorophores due
to their sharp absorption and emission bands, high ex-
tinction coefficients, high fluorescence quantum yields
and high stability in photo and chemical reactions.
In addition, the parent BODIPY can be readily mod-
ified to extend the degree of π-electron conjugation
which can introduce a pronounced red shift in ab-
sorbance and fluorescence (Loudet & Burgess, 2007).
Recently, many anion receptors based on BODIPY
have been reported (Coskun et al., 2003; Ekmekci et
al., 2008; Shiraishi et al., 2009). On the other hand,
calix[4]pyrrole derivatives have been widely used to
recognise and sense anions such as F−, AcO−, H2PO4−,
or Cl− via multiple hydrogen bonds. Nevertheless, this
anion recognition event only occurs in the ultraviolet
region, which cannot be performed sensitively through
the optical signal (Gale et al., 1996). Though several
calix[4]pyrrole-based fluorescent anion receptors have
been reported, these receptors only sense anions in
*Corresponding author, e-mail: sjshao@licp.cas.cn

554
Y. Lv et al./Chemical Papers 65 (4) 553–558 (2011)
Br
B
Br
N
CHO
H
NH
HN
Br
N
F
N
F
H
N
a
c
b
N
F
N
F
B
II
N
H
N
H
O
Cl
HN
NH
HN
NH
H
N
H
N
I
Fig. 1. Procedure for synthesis of receptor I: a) 2,4-dimethylpyrrole, r.t., 12 h; b) TEA, BF3 (OEt)2, r.t., 12h; c) AcOH, piperidine,
Mg(ClO4)2, toluene, r.t., 12 h.
the short-wavelength region (Miyaji et al., 1999, 2000;
Anzenbacher et al., 2000). There is a further challenge
to design and build improved receptors to allow the
anion recognition element in the near infrared region
(NIR).
With these considerations, we set out to con-
struct target receptor I incorporating BODIPY flu-
orophore and two calix[4]pyrrole units, according to
the fluorophore–spacer–receptor concept (Kollmanns-
berger et al., 1998). The procedure is depicted in
Fig. 1.
123.2, 129.8, 131.1, 132.4, 133.9, 140.0, 142.9, 155.9.
MS (ESI), m/z: 403.1 [M+H+].
Synthesis of 3,5-di[2-(β-octamethylcalix[4]
pyrrolyl)ethenyl]-8-(4-bromo)phenyl-4,4-
difluoro-1,7-dimethyl-3a,4a-diaza-4-bora-s-
indacene (I)
Compound II (1.00 g, 2.19 mmol) and formyl-
calix[4]pyrrole (Nishiyabu & Anzenbacher, 2006)
(1.76 g, 4.38 mmol) were refluxed in a mixture of dry
toluene (100 mL), piperidine (0.50 mL), acetic acid
(100 %) (0.40 mL), and Mg(ClO4)2 (cat.). Any wa-
ter formed during the reaction was removed azeotrop-
ically by heating overnight in a Dean–Stark apparatus.
Solvents were removed and the crude product was pu-
rified by column chromatography (CH2Cl2 : hexane,
ϕr = 2 : 1) to yield black solid 0.28 g I (10 %): M.p.
Experimental
All the tetrabutylammonium (Bu4N+) salts with
different anions were purchased from Alfa Aesar
Chemical (Beijing, China), stored in a desiccator un-
der vacuum and used without further purification.
Acetonitrile was HPLC grade, and other chemical ma-
terials were analytical pure and obtained from Al-
1
> 300◦C. H NMR (400 MHz, CDCl3), δ: 1.45 (s, 6H,
2CH3), 1.51 (s, 24H, 8CH3), 1.53 (s, 12H, 4CH3), 1.67
(s, 12H, 4CH3), 5.84 (d, 2H, J = 3.0 Hz), 5.89 (d, 2H,
J = 3.0 Hz), 5.90–5.94 (m, 6H), 5.96 (d, 2H, J = 3.2
Hz), 6.42 (d, 2H, J = 3.2 Hz), 6.50 (s, 2H, 2NH), 6.89–
6.91 (m, 4H, 4NH), 7.18 (s, 2H, 2NH), 7.21–7.23 (m,
4H, 2H for ArH, 2H for vinylic), 7.28 (s, 1H), 7.31 (s,
1H), 7.57–7.64 (m, 4H, 2H for ArH, 2H for vinylic).
13C NMR (100 MHz, CDCl3), δ: 14.9, 28.7, 28.7, 29.3,
29.7, 35.1, 35.2, 35.3, 37.4, 101.9, 102.7, 103.0, 103.3,
103.6, 103.8, 115.0, 117.3, 117.4, 117.5, 122.9, 130.8,
132.2, 132.5, 134.5, 134.7, 137.3, 137.4, 137.8, 137.9,
138.0, 138.1, 139.2, 139.3, 140.6, 153.5. MS (ESI) m/z:
1280.4 [M+, 86 %], 1313.3 [M+Na+, 100 %]. Anal.
Calc. for I (C77H86N10BBrF2) (%): C, 64.98; H, 6.32;
N, 9.75. Found: C, 65.07; H, 6.23; N, 9.83.
1
addin Reagent (Shanghai, China). H NMR and 13C
NMR spectra were determined in CDCl3 on a Varian
INOVA 400 MHz spectrometer from Varian (USA).
ESI-MS studies were carried out using a Waters Mi-
cromass ZQ-4000 spectrometer from Waters (USA).
The fluorescence spectra were recorded on a Perkin–
Elmer LS55 spectrometer from Perkin–Elmer (USA).
UV-VIS spectra were determined on a Perkin–Elmer
Lambda 35 spectrometer from Perkin–Elmer (USA).
Elemental (C, H, N) analyses were made on a Vario-
EL from Elementar (Germany).
Synthesis of 8-(4-bromo)phenyl-4,4-difluoro-
1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-
indacene (II)
Results and discussion
Compound II was prepared following the published
procedure (Zhang et al., 2008b). Compound II (30 %):
The spectroscopic characteristics of receptor I were
first investigated in different solvents as given in Ta-
ble 1. Receptor I shows an intense absorption band
at 673 nm (ε = 1.04 × 105 M−1 cm−1) and a maxi-
mum emission at 703 nm in CH3CN. Compared with
1
M.p. 176–178◦C. H NMR (400 MHz, CDCl3), δ: 1.39
(s, 6H, 2CH3), 2.53 (s, 6H, 2CH3), 5.97 (s, 2H, 2CH),
7.16 (d, 2H, J = 6.4 Hz, ArH), 7.62 (d, 2H, J = 6.4
Hz, ArH). 13C NMR (100 MHz, CDCl3), δ: 14.6, 121.4,

Y. Lv et al./Chemical Papers 65 (4) 553–558 (2011)
555
Table 1. Spectroscopic characteristics of the two BODIPY dyes in different solvents
λabs(max)
λem(max)
∆
maxν
ε · 105
Dye
Solvent
nm
cm−1
M−1 cm−1
CH3CN
EtOH
THF
673
672
674
670
703
694
691
691
654
472
365
454
1.04
0.96
0.76
0.77
I
CH2Cl2
CH3CN
EtOH
THF
499
501
502
503
512
513
515
514
509
467
503
425
1.03
0.56
0.90
1.17
II
CH2Cl2
A
A
0.4
0.2
0.0
0.4
0.2
5, 6, 7, 8, 9
4
1
2, 3
0.0
400
600
800
400
600
Wavelength/nm
800
Wavelength/nm
B
0.00
Fig. 2. Changes in the UV-VIS absorption spectra of receptor
I in CH3CN (5 × 10−6 M) in the presence of various
B
anions (4.8 × 10−5 M): F− (1), AcO− (2), H2PO−
4
(3), Cl− (4), none (5), Br− (6), I− (7), ClO−4 (8), and
HSO−4 (9).
-0.03
-0.06
the parent BODIPY II, I displays a red shift of about
170 nm in absorption and emission spectra, which can
be assigned to the increased degree of π-electron con-
jugation of the skeleton after the attachment of two
calix[4]pyrrole units. Moreover, the small Stokes shift
(∆maxv range from 365 cm−1 to 654 cm−1) indicates
that there is no difference in the structure of receptor
I in the ground state and in the first excited state.
As shown in Fig. 2, UV-VIS spectra of I were mea-
sured with the respective anions as (Bu4N+) salts in
CH3CN. Without anions, receptor I exhibits a typi-
cal absorption band at 673 nm as well as a weaker
absorption band at 353 nm. In the presence of 4.8 ×
10−5 M of F−, AcO−, H2PO4−, Cl−, and Br−, the
absorption band at 673 nm of receptor I decreased, a
new red-shifted absorption peak developed at 688 nm,
681 nm, 680 nm, and 676 nm, respectively. By co−n-
trast, the addition of other anions, such as I−, ClO4 ,
and, HSO−4 showed almost no change in absorption
spectra. The appearance of the new red-shifted ab-
sorption bands indicated the formation of hydrogen-
bonded complexes between receptor I and anions.
0
2
4
c
105/(mol L−1)
•
Fig. 3. (A) UV-VIS titration of receptor
I in CH3CN
(5 × 10−6 M) upon the addition of F− (0–4.8 × 10−5
M); (B) Non-linear fitting curve as a function of [F−
]
monitored by the decrease in absorption with increase
in [F−] at 673 nm.
Fig. 3 shows the UV-VIS titration profiles of re-
ceptor I with F− addition. Upon the dropwise addi-
tion of F−, the intensities of the bands at 353 nm
and 673 nm reduced gradually along with two new
increased absorption bands at 370 nm and 693 nm,
respectively, and reached its maximum value after the
addition of 4.8 × 10−5 M of F−. Three clear isosbestic
points at 359 nm, 549 nm, and 683 nm, indicative of
the presence of only two absorbing species in the so-
lution, were observed. Similar absorption titrations of

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Y. Lv et al./Chemical Papers 65 (4) 553–558 (2011)
Table 2. Association constants and detection limits for recep-
tor I with respective anions (as Bu4N+ salts) in
CH3CN (5 × 10−6 M)
Charge transfer
Ka
Kb
Detection limit
M
N
N
N
N
Anion
H
H H
H
N
M−1
F
F
B
Br
F−
3.19 × 105
6.68 × 104
4.40 × 104
4.81 × 104
8.57 × 102
4.45 × 105
8.73 × 104
5.29 × 104
3.54 × 104
7.18 × 102
3.48 × 10−5
5.04 × 10−4
1.01 × 10−4
1.21 × 10−4
2.20 × 10−3
N
H
H
H H
AcO−
N N N
N
H2PO−
4
Cl−
Br−
Charge transfer
a) The K values were determined by absorption changes with
increase in concentration of anions at 673 nm; b) The K val-
ues were determined by fluorescence quenching with increase in
concentration of anions at 703 nm.
Fig. 4. The interaction model of receptor I and anion.
450
300
150
0
6, 7, 8, 9
5
receptor I with AcO−, H2PO4−, Cl−, and Br− were
also conducted (see supplementary Figs. S1–S4). The
stoichiometry of the complexation of I with each an-
ion was determined to be 1 : 1 by Job’s analysis (see
supplementary Fig. S5), and the association constants
were evaluated through non-linear least square curve-
fitting of the titration data in Table 2 (Connors, 1987).
These results suggest that the selectivity and sensi-
tivity of receptor I towards anions are similar to those
of the parent calix[4]pyrrole, a representative anion re-
ceptor. However, compared with calix[4]pyrrole, I dis-
played stronger affinity to F−, AcO−, H2PO4−, and
Cl−, which may be ascribed to the formation of sand-
wich complexes between I and anions. As depicted in
Fig. 4, receptor I with two calix[4]pyrrole motifs pro-
vides a more open and less rigid cavity to sandwich
an anion via multiple hydrogen bonding interactions.
For further investigation, the emission spectra
of receptor I with various anions in CH3CN were
recorded (Fig. 5). Receptor I itself exhibits an
emission at 703 nm in the NIR. The remarkable
fluorescence-quenching was observed in the presence
4
2, 3
1
650
700
Wavelength/nm
750
Fig. 5. Fluorescence spectra of receptor I in CH3CN (5 × 10−6
M, excited at 672 nm) in the presence of various anions
(1.6 × 10−3 M): F− (1), AcO− (2), H2PO− (3), Cl−
4
(4), Br− (5), none (6), I− (7), ClO4− (8), and HSO−4
(9).
of F−, AcO−, H2PO−4 , Cl−, and Br−, which can be
attributed to the occurrence of intramolecular charge
transfer (ICT) from calix[4]pyrrole to BODIPY in-
duced by the coordination of special anions with re-
ceptor I. The quenching occurred to a varying degree
and the quenching degree, on a per molar basis, fol-
A
500
400
300
200
100
0
B
0.8
0.4
0.0
0
6
12
640 660 680 700 720 740 760 780
Wavelength/nm
c
105/(mol L−1)
•
Fig. 6. (A) Fluorescence titration of receptor I in CH3CN (5 × 10−6 M, excited at 673 nm) upon the addition of F−
(0–1.6 × 10−3 M); (B) Non-linear fitting curve as a function of [F−] monitored by the decrease in fluorescence with
increase in [F−] at 703 nm.

Y. Lv et al./Chemical Papers 65 (4) 553–558 (2011)
557
0
-2
-4
-6
for NH protons at δ: 6.50, 6.90, and 7.18, while with
the addition of 5 eq. or excess of F−, a large downfield
shift was observed and new NH signals appeared at
δ: 13.02, 12.94, 12.88, and 12.45, respectively. These
results indicated that hydrogen-bonding interactions
occurred between receptor I and F−. Moreover, it was
found that the signal of vinyl CH protons showed
about δ 0.6 downfield from δ 7.20 to δ 7.80, which
demonstrated that vinylic CH protons might be in-
volved in the hydrogen bonds.
0.0
0.3
0.6
0.9
[I]/([I] + [A−])
Conclusions
Fig. 7. Job’s plots of receptor I with F− ( ), AcO−
(
),
•
In summary, we here report on a novel BODIPY
dye I incorporating two calix[4]pyrrole units, which
can serve as an anion rece−ptor for selective binding
and sensing of F−, H2PO4 , AcO−, and Cl− with
the effects of a red shift in absorption spectra and
fluorescence-quenching in varying degrees in the near
infrared region. The anion selectivity of I is similar
to the parent calix[4]pyrrole, but it displays enhanced
affinity towards F−, H2PO4−, AcO−, and Cl− due to
the formation of a sandwich complex through multiple
hydrogen-bonding interactions.
H2PO− (ꢀ), and Cl− (ꢁ) measured by fluorescence
4
spectroscopy at 703 nm (λex = 673 nm) in CH3CN. [I]
+ [A−] = 5 × 10−6 M.
Acknowledgements. This work was supported by the Na-
tional Natural Science Foundation of China (Grant No.
20972170 to S.-J. Shao), the open fund of State Key Lab-
oratory of Oxo Synthesis & Selective Oxidation (Grant No.
OSSO2008kjk6 to S.-J. Shao) and by the Natural Science Foun-
dation of Gansu province (No. 096RJ2A033 to Y. Guo).
Fig. 8. Partial 1H NMR spectra of receptor I in the presence
of 0 eq., 0.5 eq., 5eq., 10 eq., and 15 eq. of F−
.
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