A distyryl BODIPY derivative as a fluorescent probe for selective detection of chromium(III)

Article abstract of DOI:10.1016/j.tetlet.2010.03.013

A new boradiazaindacene (BODIPY) derivative (1a) bearing simple NO bidentate ligands has been synthesized. The 1a molecule behaves as a fluorescent probe for Cr³⁺ and shows strong red fluorescence upon coordination with Cr³⁺, while showing almost no fluorescence for other metal cations.

Full text of DOI:10.1016/j.tetlet.2010.03.013

Tetrahedron Letters 51 (2010) 2545–2549
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A distyryl BODIPY derivative as a fluorescent probe for selective detection
of chromium(III)
*
Dongping Wang, Yasuhiro Shiraishi , Takayuki Hirai
Research Center for Solar Energy Chemistry and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A new boradiazaindacene (BODIPY) derivative (1a) bearing simple NO bidentate ligands has been synthe-
sized. The 1a molecule behaves as a fluorescent probe for Cr3+ and shows shows a strong red fluorescence
upon coordination with Cr3+, while showing almost no fluorescence for other metal cations.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 21 January 2010
Revised 25 February 2010
Accepted 4 March 2010
Available online 7 March 2010
Keywords:
BODIPY
Fluorescence
Probe
Trivalent chromium
Trivalent chromium (Cr3+) is an essential nutrient for humans
and plays an important role in the metabolism of carbohydrates,
lipids, proteins, and nucleic acids.1 Insufficient intake of Cr3+ in-
creases the risk for diabetes and cardiovascular diseases, whereas
excessive intake causes genotoxic effects.2 Accurate and rapid
determination of Cr3+ amount in the environment is therefore nec-
essary. Traditional analytical methods for Cr3+ employ expensive
instruments such as atomic absorption spectrometry3 and induc-
tively-coupled plasma atomic emission spectrometry.4 Although
length light is beneficial for intracellular fluorescence imaging
due to low light scattering and low background signal.9
In the present work, we synthesized a BODIPY-based fluores-
cent probe containing simple NO bidentate ligands, 1a (Scheme
1). Upon addition of Cr3+, the probe shows a strong fluorescence
in the red visible region (610–720 nm). The Cr3+ selectivity of the
probe is much higher than the early reported probes and thus al-
lows selective Cr3+ detection.
The distyryl probe, 1a, was synthesized via three steps accord-
ing to the procedure summarized in Scheme 1 with 66% overall
yield (see Synthesis, Supplementary data). A Knoevenagel-type
condensation of a BODIPY derivative, 2,10 with 4-acetamidobenzal-
dehyde affords a distyryl BODIPY compound, 3a, in 91% yield.
Deprotection of the acetyl groups of 3a affords 4a in 97% yield.
Condensation of 4a with salicylaldehyde followed by reduction
with NaBH4 gives rise to the probe, 1a, in 75% yield. To clarify
the coordination and sensing properties of 1a, a monostyryl probe,
1b, was also synthesized in a similar manner to 1a with 15%
overall yield. The structures of these probes were fully character-
ized by 1H NMR, 13C NMR, MS, and HRMS analysis (Figs. S1–S12,
Supplementary data).
these methods enable accurate and selective detection of Cr3+
,
these are usually time-consuming and require complicated and te-
dious sample preparation.
Fluorescence analysis with probe molecules is one of the alter-
native methods because it enables easy, simple, and rapid analysis
of metal cations with inexpensive instruments.5 Several kinds of
fluorescent probes have been proposed so far, but there are only
five reports of Cr3+ probe.6 In addition, these probes show poor
selectivity for Cr3+; the fluorescence intensity obtained with Cr3+
is four times less than that obtained with other metal cations.
BODIPY is a dye which has been studied extensively for applica-
tion to various kinds of functional materials because of its high
fluorescence quantum yields and large extinction coefficients.7
Several BODIPY-based fluorescent probes have been proposed so
far for the detection of various analytes such as cations, anions,
and molecules.7 Of recent particular interest is the synthesis of
BODIPY-based probes that show absorption and emission in red
visible or near infrared (NIR) region.8 This is because long wave-
Figure 1a shows the fluorescence spectra (kex = 560 nm) of 1a
(5 lM) measured in CH3CN with 40 equiv of respective metal cat-
ions. Without cations, 1a shows almost no fluorescence, where the
fluorescence quantum yield (UF) is determined to be 0.003.11 This
is because, as usually observed,5c electron transfer from the amine
nitrogens to the photoexcited BODIPY moiety quenches the fluo-
rescence. Addition of Cr3+, however, creates a strong fluorescence
(
UF = 0.69) in the red visible region at 610–720 nm with an emis-
* Corresponding author. Tel.: +81 6 6850 6271; fax: +81 6 6850 6273.
E-mail address: shiraish@cheng.es.osaka-u.ac.jp (Y. Shiraishi).
sion maximum at 643 nm. As shown in the inset picture, bright
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.03.013

2546
D. Wang et al. / Tetrahedron Letters 51 (2010) 2545–2549
ii
iii
i
N
F
N
F
N
F
N
F
N
F
N
F
B
B
B
3a
4a
1a
O
O
NH
H2N
NH
OH
HN
NH2
HN
HO
N
F
N
F
B
2
N
F
N
i
ii
N
N
F
iii
N
F
N
F
B
B
B
F
F
3b
4b
1b
O
HN
NH2
HN
HO
Scheme 1. Reagents and conditions: (i) 4-acetamidobenzaldehyde, piperidine, acetic acid, reflux, overnight; (ii) (1) 1 M HCl/MeOH, 100 °C, 6 h; (2) Et3N, BF3ÁOEt2, CH2Cl2, rt,
1 h; (iii) (1) salicylaldehyde, CH2Cl2/MeOH, rt; (2) NaBH4, rt, 5 min.
red fluorescence is observed upon Cr3+ addition. In contrast, addi-
tion of other metal cations scarcely shows fluorescence (UF <0.01).
Figure 1b shows the fluorescence enhancement factor (F/F0) de-
fined as the ratio of the fluorescence intensity measured with
and without metal cations. With Cr3+, the enhancement factor is
determined to be 2870. In contrast, the factors for other cations
are less than 70. This suggests that the factor for Cr3+ is more than
40 times larger than that for other metal cations, which is much
larger than the value obtained with early reported Cr3+ probes
(<4 times).6 The above-mentioned findings clearly indicate that
1a behaves as a highly selective fluorescent Cr3+ probe.
ligand usually coordinates with Cr3+ in a 1:1 stoichiometry. This
implies that two 1b molecules are involved in the coordination
with Cr3+ and produce a 2:1 complex.
In contrast, coordination of 1a with Cr3+ produces two kinds of
complexes; a 2:1 complex is produced at low Cr3+ amount, but fur-
ther Cr3+ addition leads to a transformation into a 2:2 complex via
sequential coordination with another Cr3+. As shown in Figure 3a,
there is no isosbestic point observed throughout the titration with
0–40 equiv of Cr3+. Detailed titration results (Fig. S16, Supplemen-
tary data), however, reveal that two isosbestic points exist at
674 nm in the range of 0–2 equiv of Cr3+ and at 643 nm in the
range of 7–40 equiv of Cr3+, respectively, whereas no isosbestic
point is observed at 2–7 equiv of Cr3+. The coordination sequence
of 1a with Cr3+ can be explained to be analogous fashion to that of
1b, as shown in Scheme 2. The 674 nm isosbestic point is due to
the formation of a 2:1 complex. In contrast, the 643 nm isosbestic
point is due to the transformation of the complex to a 2:2 complex
via a coordination with another Cr3+. The absence of isosbestic
point at 2–7 equiv of Cr3+ is because all three species (free 1a,
2:1 complex, and 2:2 complex) exist in solution at once. At the
present stage, we have not obtained direct evidence of the forma-
tion of 2:1 and 2:2 complexes; ESI-MS analysis of a CH3CN solution
containing 1a and Cr3+ does not show clear mass assigned to the
complexes, and Job’s plot analysis (Fig. S17, Supplementary data)
does not provide clear stoichiometry. These are probably because
of a weak binding affinity between 1a and Cr3+. However, the isos-
bestic points observed during the titration of 1a (Fig. 3a) support
the coordination sequence of 1a shown in Scheme 2.
As shown in Figure S13 (Supplementary data), the monostyryl
BODIPY derivative, 1b, also shows almost no fluorescence without
cations, but addition of Cr3+ creates
a strong fluorescence
(enhancement factor: 1380), as does 1a. However, 1b shows a fluo-
rescence enhancement upon the addition of Hg2+ (enhancement
factor: 844) and Fe2+ (410). This indicates that Cr3+ selectivity of
1b is poor and two NO bidentate ligands for 1a are necessary for
selective Cr3+ detection.
Figure 2 shows the result of fluorescence titration of 1a with
Cr3+. The stepwise Cr3+ addition leads to an increase in the
643 nm fluorescence, and the increase is saturated upon the addi-
tion of 40 equiv of Cr3+. The fluorescence increase takes place
immediately after Cr3+ addition (within 10 s), indicating that 1a
enables rapid detection of Cr3+
.
Figure 3a shows the result of absorption titration of 1a with
Cr3+. Without cations, 1a shows an intense absorption band cen-
tered at 692 nm (
the 692 nm band decreases and a blue-shifted band appears at
e ,
57,100 MÀ1 cmÀ1). Upon the addition of Cr3+
As shown in Figure 2b, fluorescence intensity enhancement is
very weak at 0–2 equiv of Cr3+ where the 2:1 complex exists
mainly. In contrast, strong fluorescence enhancement is observed
at higher Cr3+ concentration where the 2:2 complex exists mainly.
The weak fluorescence of the 2:1 complex is probably because the
uncoordinated amine nitrogens of 1a quench the fluorescence via
an electron transfer to the photoexcited BODIPY moieties.5c This
leaves that the 2:2 complex is the major emitting species in the
present system.
628 nm (e
64,300 MÀ1 cmÀ1). As shown in the inset picture, the
solution color changes from green to blue upon Cr3+ addition. As
shown in Figure 3b, the spectral change almost stops upon the
addition of 40 equiv of Cr3+, which is similar to the fluorescence
titration result (Fig. 2b).
As shown in Figure S15 (Supplementary data), absorption titra-
tion of the monostyryl BODIPY derivative, 1b, with Cr3+ also shows
a blue shift of the absorption band (603–573 nm), as is the case for
1a. During the spectral change of 1b, a clear isosbestic point is ob-
served at 581 nm, indicating that coordination of 1b with Cr3+ pro-
duces a single component. As reported,6b,c,12 a N2O2 tetradentate
Figure 4 shows the effect of water addition on the Cr3+-induced
fluorescence enhancement of 1a. The fluorescence intensity mea-
sured with 40 equiv of Cr3+ decreases with an increase in the water

D. Wang et al. / Tetrahedron Letters 51 (2010) 2545–2549
2547
b3000
a
400
300
200
100
0
Cr3+
2500
2000
1500
1a only, Li+, Na+, Mg2+, Mn2+
,
Fe3+, Co2+, Ni2+, Cu2+, Zn2+
Ag+, Cd2+, Hg2+, Pb2+
,
1a
1a + Cr3+
1000
500
0
Fe2+
1a
Na+ Mn2+ Fe3+ Ni2+ Zn2+ Cd2+ Pb2+
650
700
750
Li+ Mg2+ Fe2+ Co2+ Cu2+ Ag+ Hg2+ Cr3+
Wavelength / nm
Figure 1. (a) Fluorescence spectra (kex = 560 nm) of 1a (5 lM) measured in CH3CN with 40 equiv of respective metal cations. (b) Fluorescence enhancement factor (F/F0),
where F and F0 are the fluorescence intensity measured at 643 nm with and without metal cations, respectively. (Inset) Change in fluorescence color of the solution upon Cr3+
addition.
b
a
400
300
200
100
0
400
300
200
100
0
0
10
20
[Cr3+]/[1a]
30
40
650
700
750
Wavelength / nm
Figure 2. (a) Change in fluorescence spectra (kex = 560 nm) of 1a (5
22, 24, 26, 28, 30, 32, 34, 36, 38 and 40 equiv. (b) Change in fluorescence intensity monitored at 643 nm.
l
M) in CH3CN upon addition of Cr3+, where the Cr3+ amount is 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20,
a
b0.35
0.4
0.3
0.2
0.1
0.0
628 nm
643 nm
674 nm
0.30
0.25
0.20
0.15
0.10
0.05
0.00
1a
1a + Cr3+
692 nm
300
400
500
600
700
800
0
10
20
[Cr3+]/[1a]
30
40
Wavelength / nm
Figure 3. (a) Change in absorption spectra of 1a (5 l
M) in CH3CN upon addition of Cr3+, where the Cr3+ amount is 0, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, 4,
5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38 and 40 equiv. (b) Change in absorbance at 692 nm and 628 nm. (Insert) Color change of the solution upon Cr3+
addition. The change in 3D absorption spectra upon Cr3+ addition is shown in Figure S14 (Supplementary data).

2548
D. Wang et al. / Tetrahedron Letters 51 (2010) 2545–2549
N
F
N
F
N
F
N
F
B
B
Cr3+
Cr3+
N
F
N
F
B
H
H
H
H
H
H
O
O
H
H
O
O
N
N
N
N
N
N
N
N
0-7 equiv
> 2 equiv
O
O
H
H
O
O
H
H
H
H
H
H
NH
OH
HN
HO
F
F
F
F
B
B
N
N
N
N
1a
2:1 complex
Scheme 2. A possible sequence for coordination between 1a and Cr3+
2:2 complex
.
gands with Cr3+. However, with 1% water, strong fluorescence still
remains(enhancementfactor:1110), indicatingthattheprobe 1a al-
lows the fluorometric detection of Cr3+ for samples containing less
than 1% water.
400
The probe 1a enables selective detection of Cr3+ even in the
presence of many other metal cations. Figure 5 shows the fluores-
cence enhancement factor (F/F0) measured with Cr3+ (40 equiv)
and other metal cation (40 equiv) in CH3CN containing 1% water.
The Cr3+-induced fluorescence enhancement is unaffected in the
vol% water
0
300
200
100
0
5
presence of many other cations (Li+, Na+, Mg2+, Mn2+, Fe2+, Co2+
,
Ni2+, Zn2+, Ag+, Cd2+, and Pb2+). In contrast, some cations such as
Fe3+, Cu2+, and Hg2+ strongly quench the fluorescence. As shown
in Figure S18 (Supplementary data), addition of these three cations
leads to a significant blue shift of the absorption spectra of 1a to
450–580 nm even in the presence of Cr3+, which is much shorter
than that obtained only with Cr3+ (550–720 nm). This clearly indi-
cates that 1a coordinates with these cations (Fe3+, Cu2+, and Hg2+
)
650
700
750
Wavelength / nm
more strongly than Cr3+. This suppresses the coordination with Cr3+
and, hence, results in almost no fluorescence enhancement.
In summary, we found that a new distyryl BODIPY derivative, 1a,
behaves as a selective and red-emitting Cr3+ probe. The probe allows
selective Cr3+ detection in the presence of contaminating metal cat-
ions other than Fe3+, Cu2+, and Hg2+. Detailed coordination structure
between 1a and Cr3+ still remains to be clarified. In addition, there
are several problems for practical application of the probe.13 How-
ever, the simple molecular design presented here using a NO biden-
tate ligand may contribute to the development of more efficient and
more useful fluorescent probe for Cr3+ detection.
Figure 4. Fluorescence spectra (kex = 560 nm) of 1a (5
lM) measured with 40 equiv
of Cr3+ in CH3CN in the presence of 0, 0.5%, 1%, 2% and 5% water.
1200
1000
800
Acknowledgments
600
This work was supported by the Grant-in-Aid for Scientific Re-
search (No. 21760619) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan (MEXT).
400
200
Supplementary data
0
Li+ Mg2+ Fe2+ Co2+ Cu2+ Ag+ Hg2+ Cr3+
Na+ Mn2+ Fe3+ Ni2+ Zn2+ Cd2+ Pb2+
Supplementary data (general methods, synthesis, Figure S1–
S18) associated with this article can be found, in the online version,
at doi:10.1016/j.tetlet.2010.03.013.
Figure 5. Fluorescence enhancement factor (F/F0) of 1a (5 lM) measured in CH3CN
containing 1% water with 40 equiv of Cr3+ and 40 equiv of other respective metal
cations. F and F0 are the fluorescence intensity monitored at 643 nm with and
without cations, respectively.
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13. A referee pointed out that the probe works in solution with <1% water. In
addition, the Cr3+-induced fluorescence is quenched by some metal cations
(Cu2+, Hg2+, and Fe3+). These are the critical problems for practical application.
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addition, the emission enhancement factor of the probe for Cr3+ as compared to
other cations is much higher than that of early reported probes. We therefore
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