A colorimetric and fluorometric fluoride sensor based on a BODIPY-phenol conjugate

Article abstract of DOI:10.1007/s11426-011-4237-7

A boradiazaindacenes (BODIPY)-phenol conjugate, 1, can act as a colorimetric and fluorometric sensor for sensitive and selective measurement of F^- over AcO^- and H2PO ₄ ^- in CH ₃CN. Sensor 1 gives resp

Full text of DOI:10.1007/s11426-011-4237-7

SCIENCE CHINA
Chemistry
May 2011 Vol.54 No.5: 797–801
doi: 10.1007/s11426-011-4237-7
• ARTICLES •
A colorimetric and fluorometric fluoride sensor based
on a BODIPY-phenol conjugate
LV YongJun1,3, XU Jian1, GUO Yong1* & SHAO ShiJun1,2*
1Key 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
2State Key Laboratory for Oxo Synthesis and Selective Oxidation; Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, China
3Graduate University of the Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100039, China
Received November 11, 2010; accepted December 25, 2010
A boradiazaindacenes (BODIPY)-phenol conjugate, 1, can act as a colorimetric and fluorometric sensor for sensitive and se-
lective measurement of F over AcO and H2PO in CH3CN. Sensor 1 gives response to F in a 1:1 ratio via the deprotonation
4
of the phenolic OH proton, which results in color change from pale yellow to light green and quenching of bright green fluo-
rescence.
colorimetric, fluorescence, fluoride, sensor, BODIPY
the limit is lack of reversibility [6–9]. Alternatively, few
1 Introduction
sensors can selectively sense F over AcO and H2PO4, due
to their similar basicity, based on hydrogen-bond or depro-
tonation [1, 10–14]. Thus, we expect to develop a selective
anion sensor for fluoride through both the acid-base interac-
tion and hindrance effect between anion and the binding site
modified by the introduction of a larger bulky group.
As known, phenolic unit has been investigated widely as
a good anion receptor via hydrogen bonding interactions or
salt complex [15, 16]. Several phenol-based boradiazainda-
cenes (BODIPY) conjugates were reported as pH indicators
[17, 18], but their potential capabilities as sensing anions
are not extensively explored. On the other hand, BODIPY is
an advantageous fluorophore with many excellent photo-
physical properties, and numerous BODIPY-based deriva-
tives have been synthesized as anion, mental and pH probes
[19].
In recent years, the design and synthesis of sensors capable
of binding and sensing fluoride selectively have drawn con-
siderable attention in supramolecular chemistry [1], because
fluoride anion plays very important roles in clinical treat-
ments for osteoporosis and fluoride toxicity [2]. To date,
considerable effort has been devoted to the development of
artificial fluoride sensors through visible, optical and elec-
trochemical responses [3–5]. Most of synthetic sensors gen-
erally involve the covalent linking of a signaling fragment
to a neutral anion receptor containing urea, thiourea, amide,
indole, or phenolic hydroxyl subunits, which can provide
hydrogen-bond donor sites for selective binding some ani-
ons, especially these anions F, AcO and H2PO4. The se-
lectivity can be related to the structure of the hydrogen bond
complex and the basicity of the anions. Although some re-
ported reaction-based sensors selectively recognize fluoride,
Hence, we synthesized a BODIPY-phenol conjugate 1
and investigated its anion reorganization properties by
UV-vis and fluorescence spectroscopy. With the larger
bulky tert-butyl groups ortho to the phenolic OH, com-
*Corresponding author (email: guoyong@licp.cas.cn; sjshao@licp.cas.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2011
chem.scichina.com
www.springerlink.com

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Lv YJ, et al. Sci China Chem May (2011) Vol.54 No.5
pound 1 was expected to trigger color and fluorescence
changes upon complexation with fluoride over other large
anions such as AcO and H2PO4. Moreover, a reference 2
was also prepared according to published procedures (Scheme
1) [20].
68.93; H, 7.37; N, 5.79.
Compound 2 yield 34 %. 1H NMR (400 MHz, CDCl3) :
1.441 (s, 6H, 2CH3), 2.550 (s, 6H, 2CH3), 5.301 (s, 1H, OH),
5.979 (s, 2H, 2CH), 6.943 (d, J = 7.6 Hz, 2H, 2ArH), 7.114
(d, J = 7.6 Hz, 2H, 2ArH). 13C NMR (100 MHz, CDCl3) :
14.56, 116.10, 121.13, 127.11, 129.35, 131.79, 141.73,
143.19, 155.28, 156.29. FT-IR (KBr): 1408.33, 1439.30,
1468.34, 1511.70, 1542.91, 1608.07, 2874.06, 2922.57,
2963.32, 3418.38 cm1. ESI-MS: m/z (%) 339.3 (M  H+,
100). Anal. calcd for 2 (C19H19BF2N2O): C, 66.20; H, 5.38;
N, 7.67. Found: C, 66.26; H, 6.32; N, 7.64.
2 Experimental
2.1 Materials and methods
All the tetrabutylammonium salts with different anions were
purchased from Alfa Aesar Chemical Co., stored in a des-
iccator under vacuum, and used without further purification.
3 Results and discussion
1
CH3CN used in the context was chromatographic pure. H
NMR and 13C NMR spectra were performed on a Varian
INOVA 400 MHz spectrometer in CDCl3. ESI-MS studies
were carried out using a Waters Micromass ZQ-4000 spec-
trometer. The fluorescence spectra were recorded on a
Perkin Elmer LS55 spectrometer. UV-vis spectra were de-
termined on a Perkin Elmer Lambda 35 spectrometer. C, H,
N elemental analyses were made on a Vario-EL.
As shown in Table 1, both 1 and 2 show a typical narrow
absorption band at approximately 500 nm, which corre-
sponds to the S0S1 transition of BODIPY subunits, and an
emission band at about 510 nm in tested solvents, which is
in keeping with classical BODIPY derivatives [21]. Fluo-
rescein (f = 0.925) in 1 mol L1 NaOH was used as a stan-
dard to estimate the fluorescence quantum yields f [22].
Compound 1 shows a relatively high fluorescence quantum
yield compared to 2, which can be ascribed to the introduc-
tion of bulky tert-butyl groups hindering - stacking of the
fluorophore [23].
2.2 Synthesis
A solution of corresponding benzaldehyde (0.61 g, 2.62
mmol), 2,4-dimethyl pyrrole (0.52 g, 5.24 mmol) and one
drop of trifluoroacetic acid (TFA) in dry CH2Cl2 (50 mL)
was stirred overnight under N2 atmosphere. After oxidation
by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (0.59 g,
2.62 mmol), Et3N (2 mL) and BF3-OEt2 (2 mL) were added
in ice-water bath. After being stirred for 2 h, the mixture
was washed with water several times, dried over MgSO4,
and concentrated at reduced pressure. The residue was puri-
fied by column chromatography (silica gel, CH2Cl2) to give
the target compound.
The interaction of sensor 1 with different anions was in-
vestigated in CH3CN by observing the changes in the ab-
sorption spectra. Upon addition of anions such as F, H2PO4,
Cl, Br, I, AcO, HSO4 and ClO4 (as their tetrabutylam-
monium salts) to sensor 1, only F remarkably changed the
absorption spectrum, whereas the other anions tested only
induced negligible responses (Figure 1). During the titration
of sensor 1 with F as shown in Figure 2, the absorption
band at 496 nm decreased and two new bands emerged at
470 and 670 nm, which was responsible for simultaneously
changing the solution of sensor 1 from pale yellow to light
green (Figure 3). Two isosbestic points at 481 and 519 nm
were observed, indicating the formation of F-sensor com-
plex. Two novel bands can be assigned to the deprotonated
form of BODIP-phenolate as was confirmed by the Brøn-
sted acid-base reaction of sensor 1 with a strong base
[Me4N]OH. According to the Job plot, sensor 1 interacted
with F in a 1:1 ratio (Figure 2 inset). The equilibrium con-
stant (or proton-dissociation constant) K for F was deter-
mined to be 1.65 × 104 at 496 nm and 1.66 × 10 4 at 670 nm
[24]. In brief, sensor 1 can selectively bind F over other
basic anions such as AcO and H2PO4, owing to the basicity
1
Compound 1 yield 40%. H NMR (400 MHz, CDCl3)
:1.408 (s, 6H, 2CH3), 1.433 (s, 18H, 6CH3), 2.557 (s, 6H,
2CH3), 5.368 (s, 1H, OH), 5.981 (s, 2H, 2CH), 7.032 (s, 2H,
2ArH). 13C NMR (100 MHz, CDCl3) : 14.34, 14.54, 30.41,
34.52, 120.92, 124.38, 125.52, 131.89, 137.08, 143.05,
154.29, 154.88. FT-IR (KBr): 1413.73, 1440.79, 1464.76,
1509.80, 1542.84, 2868.15, 2916.46, 2951.53, 3421.05 cm1
ESI-MS: m/z (%) 453.4 (M + H+, 100). Anal. calcd for 1
(C26H31BF2N2O): C, 69.11; H, 7.34; N, 5.83. Found: C,
and smaller radius of F and the larger steric hindrance of
two tert-butyl groups.
For further investigation, the recognition properties of
sensor 1 with various anions were conducted by fluorescence
technology (Figure 4). In the presence of F, the fluorescence
Scheme 1 The synthetic route of two compounds 1 and 2.

Lv YJ, et al. Sci China Chem May (2011) Vol.54 No.5
799
Table 1 Spectroscopic characteristics of the two compounds 1 and 2 in different solvents
Compound
Solvent
CH3CN

Abs (max/nm)
496

em (max/nm)
509
f
 × 104 (L mol1 cm1)
1
0.525
0.574
0.873
0.870
0.715
8.84
9.48
8.67
9.02
8.06
EtOH
497
512
THF
499
511
CH2Cl2
cyclohexane
499
511
501
512
2
CH3CN
EtOH
497
498
500
501
502
511
511
513
514
514
0.420
0.458
0.429
0.704
0.651
3.66
2.80
7.22
7.35
7.55
THF
CH2Cl2
cyclohexane
Figure 3 Color changes and fluorescence responses of sensor 1 (3.1 ×
105 mol L1) with various anions (4 × 104 mol L1) in CH3CN.
Figure 1 Absorption spectra of sensor 1 (3.1 × 105 mol L1) upon the
addition of respective anions (4 × 104 mol L1).
Figure 4 Fluorescence spectra of sensor 1 (3.1 × 106 mol L1) upon the
addition of respective anions (6.9 ×105 mol L1). Excitation was at 496 nm.
Figure 2 UV-vis titration of sensor 1 (3.1 × 105 mol L1) upon the addi-
tion of F (0–4 × 104 mol L1). Inset: A job plot of sensor 1 with F in
CH3CN.
to the BODIPY core. No fluorescence change was observed
for other tested anions. As shown in Figure 5, the intensity
of the peak at 509 nm decreased upon gradual addition of F
and complete quenching (f = 0.018) took place in the
presence of 20 equiv of F. Based on the change in fluores-
cence intensity, the detection limit for F was determined to
be 2.0 × 106 mol L1 (based on S/N = 3).
intensity of 1 significantly decreased along with the bright
green fluorescence quenching (Figure 3). Likewise, similar
quenching was observed by addition of [Me4N]OH. Quench-
ing can be attributable to the occurrence of photoinduced
electron transfer (PET) mechanism from the phenolate salt

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Lv YJ, et al. Sci China Chem May (2011) Vol.54 No.5
Figure 5 Fluorescence spectra of sensor 1 (3.1 × 106 mol L1) upon the
addition of F (0–6.9 × 105 mol L1) in CH3CN.
1
To firmly prove the interaction of sensor 1 with F, H
NMR experiments were conducted in acetonitrile-d3 and
CDCl3 (5:1) solution, and addition of CDCl3 was to improve
1
the solubility of 1 in acetonitrile-d3. A partial H NMR
spectrum of sensor 1 is shown in Figure 6. It was found that,
upon gradual addition of F, the signal of the phenolic OH
proton underwent downfield shift (= 0.34) and disap-
peared with the addition of 5.0 equiv of F. The above-
mentioned results indicate that sensor 1 can interact with F
through deprotonation.
In Figure 7, control experiments were performed with
compound 2. In the presence of F or [Me4N]OH, a slight
response was developed in the absorption spectrum. Inter-
estingly, compared to 2, sensor 1 exhibited two novel ab-
sorption bands at 470 and 670 nm, respectively, which may
be attributed to the two allotropic forms (1a and 1b) of sen-
sor 1 after deprotonation (Figure 8) [25, 26]. The donor
ability of two tert-butyl groups can facilitate the formation
of 1b, which causes the increased degree of -electron con-
jugation of the sensor. Besides, addition of AcO or H2PO4
to the solution of 2 also resulted in the decreased fluores-
cence intensity at 511 nm to some extent (Figure 7(b)).
Figure 7 (a) Absorption spectra of 2 (3.1 × 105 mol L1) upon the addi-
tion of respective anions (4 × 104 mol L1); (b) fluorescence spectra of 2
(3.1 × 106 mol L1) upon the addition of respective anions (6.9 × 105 mol
L1) in CH3CN.
Figure 8 Proposed binding mechanism of sensor 1 with F.
Namely, reference 2 not only gave the fluorescence re-
sponse to F, but also AcO and H2PO4 under the similar
experimental conditions. During the titration of 2 with F in
Figure 9, fluorescence quenching was also observed upon
gradual addition of F due to the occurrence of PET process
after the deprotonation of 2. These findings support that the
existence of tert-butyl hindrance in molecule 1 can prevent
the affinity of large basic anions. As a result, sensor 1 can
allow sensitive and selective detection of F through color
and fluorescence changes.
Figure 6 1H NMR spectra of sensor 1 in the presence of 0, 1.0, 2.0, and
5.0 equiv of F.

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