Highly selective, colorimetric probes for cyanide ion based on β-formylBODIPY dyes by an unprecedented nucleophilic addition reaction

Article abstract of DOI:10.1016/j.saa.2020.119102

Two β-formylBODIPYs with strong daylight excitable fluorescence and highly selective visual and colorimetric responses to cyanide anion (CN^–) have been prepared. NMR titration experiments have been performed to study the sensing mechanism for t

Full text of DOI:10.1016/j.saa.2020.119102

Journal Pre-proofs
Highly Selective, Colorimetric Probes for Cyanide Ion Based on β –Formyl-
BODIPY Dyes by an Unprecedented Nucleophilic Addition Reaction
Qinghua Wu, Shengyuan Wang, Erhong Hao, Lijuan Jiao
PII:
S1386-1425(20)31081-7
https://doi.org/10.1016/j.saa.2020.119102
SAA 119102
DOI:
Reference:
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Spectrochimica Acta Part A: Molecular and Bio-
molecular Spectroscopy
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Accepted Date:
31 July 2020
15 October 2020
15 October 2020
Please cite this article as: Q. Wu, S. Wang, E. Hao, L. Jiao, Highly Selective, Colorimetric Probes for Cyanide
Ion Based on β –FormylBODIPY Dyes by an Unprecedented Nucleophilic Addition Reaction, Spectrochimica
Acta Part A: Molecular and Biomolecular Spectroscopy (2020), doi: https://doi.org/10.1016/j.saa.2020.119102
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Highly Selective, Colorimetric Probes for Cyanide Ion Based
on FormylBODIPY Dyes by an Unprecedented Nucleophilic
Addition Reaction
Qinghua Wu Methodology Investigation Data Curation Writing - Original draft, Shengyuan Wang
Investigation Data Curation, Erhong Hao Writing - Conceptualization Funding acquisition Writing -
Review & Editing, Lijuan Jiao Supervision Funding acquisition Project administration Writing -
Review & Editing*,jiao421@ahnu.edu.cn
Laboratory of Functionalized Molecular Solids, Ministry of Education, Anhui Key Laboratory of Molecular Based
Materials, College of Chemistry and Material Science, Anhui Normal University, Wuhu, 241002, China.
*Corresponding author.
Highlights
An unprecedented nucleophilic addition between cyanide anion and -position of BODIPY core was
found.
Formyl groups on the BODIPY core enhanced the reactivity toward nucleophilic addition by cyanide
anion
The probes showed visual, colorimetric and fluorescent sensing of cyanide anion.
Abstract
Two -formylBODIPYs with strong daylight excitable fluorescence and highly selective visual and
colorimetric response to cyanide anion (CN-) have been prepared. NMR titration experiments have
been performed to study the sensing mechanism for these two dyes. Surprisingly, cyanide anion is
nucleophilic addition to the -position of BODIPY core (the azafulvene framework) in aqueous system
instead of the expected classical nucleophilic addition to the formyl moiety of the probes. This
1

nucleophilic addition of cyanide anion to the azafulvene framework causes the interruption of the -
conjugation of the BODIPY system, which brings a significant blue-shift (up to 104 nm) of the
absorption maxima. A broadened and decreased absorption as well as ratiometrical fluorescence
response (with maxima shifts from 523 to 670 nm) were observed with the titration of cyanide anion
to probe 1b.
Introduction
Cyanide anion has been widely used in industry, including the extracting gold from its ores, the
electroplating and the manufacture of synthetic rubber and fibers [1-2]. In addition, the nitrogen
metabolic pathways of many organisms, such as bacteria, fungi and algae also leads to the formation
of cyanide anion [3]. On the other hand, cyanide anion itself is highly toxic. This may be attributed to
the fact that it can rapidly interact with a heme unit in the active site of cytochrome a3, which inhibits
the cellular respiration in mammals and leads to symptoms including vomiting, convulsion, loss of
consciousness, and even death [4-7]. The World Health Organization (WHO) has regulated the
allowable concentration of cyanide anion in drinking water to be less than 1.9 M [8]. Therefore, the
facile and sensitive detection of cyanide has attracted wide research attentions, especially the facile
quantitative and qualitative detection of cyanide anion in the environmental and biological samples.
Although a set of sophisticated methods, including the mass spectrometry, electrochemistry as well
as chromatography have been successfully used to detect cyanide [3, 9-15], however, the major
limitations of these methods were that they need expensive instrumentation or tedious sample
2

pretreament. Considering the easy accessibility and operational simplicity of colorimetric and
fluorescent detection of cyanide anion, which allows naked-eye detection of cyanide anion without
resorting to some expensive spectroscopic instrumentation [16-19], extensive research efforts have
been devoted to the development of reaction-based colorimetric and fluorescence probes by taking
advantage of the nucleophilic reactivity of cyanide anion [20-21]. However, the low sensitivity issue
remains a problem, which limits their applications in the real sample analysis.
BODIPY (boron dipyrromethene) dyes, have received increasing research interests in a wide
research area, including sensors, imaging, labeling, photosensitizers due to their facile synthesis, easy
functionalization, good stability, and tuneable photophysical properties, including large molar
extinction coefficients [22-35]. Herein, we report the synthesis of two formylBODIPYs 1a and 1b
(Scheme 1) and their highly selective visual and colorimetric response to cyanide anion over a series
of common competition ions. NMR titration experiments indicate that this selective sensing of cyanide
anion is based on an unprecedented nucleophilic addition of cyanide anion to the position of
BODIPY core (the azafulvene framework, path b in Scheme 1a), which is in great contrast to those
classical additions of cyanide anion to the formyl groups of the BODIPY core (path a in Scheme
1a).
Experimental Section
General materials, instruments, and methods were similar to our previous reports [26-28]. Detail
information and UV–vis and fluorescence titration methods are available in the Supporting
Information.
Synthesis and characterization: meso-MesitylBODIPY and BODIPYs 1a and 1b were synthesized
using a slightly modified literature procedure [36-37].
3

BODIPY 1a was prepared by reacting meso-mesitylBODIPY (50 mg, 0.16 mmol) in 10 mL 1,2-
dichloroethane with the reaction mixture of DMF (3.0 mL) and POCl3 (3.0 mL) for 10 h at 80 °C.
After workup, the crude product was collected and purified using column chromatography (silica gel,
hexane/ CH2Cl2 = 3/1, v/v) to give 1a in 81% yield (44 mg). 1H NMR (300 MHz, CDCl3) δ 9.81 (s,
1H), 8.25 (s, 1H), 8.16 (s, 1H), 7.03- 6.91 (m, 4H), 6.65 (s, 1H), 2.37 (s, 3H), 2.09 (s, 6H); 13C NMR
(75 MHz, CDCl3) δ 184.9, 150.4, 150.0, 143.0, 139.7, 137.9, 136.2, 135.1, 133.7, 131.9, 128.8, 128.6,
127.4, 121.8, 21.3, 20.1, HRMS (EI): calcd. for C19H17BF2N2O, [M]+ 338.1402; found 338.1408.
BODIPY 1b was prepared in 75% isolated yield (29 mg) using a similar procedure described above
for 1a, from the reaction of 1a (85 mg, 0.25 mmol) with the mixture of DMF (6.0 mL) and POCl3 (6.0
mL). 1H NMR (300 MHz, CDCl3) δ 9.87 (s, 2H), 8.46 (s, 2H), 7.28 (s, 2H), 7.02 (s, 2H), 2.39 (s, 3H),
2.10 (s, 6H); 13C NMR (75 MHz, CD3COCD3) δ 185.3, 154.3, 147.0, 136.4, 134.5, 132.5, 128.9, 128.8,
20.7, 19.4. HRMS (EI): calcd. for C20H17BF2N2O2, [M]+ 366.1351; found 366.1356.
Results and Discussion
First, formylBODIPY 1a was synthesized by Vilsmeier-Haack reaction of meso-mesitylBODIPY
in 81% yield following our previously reported procedure. Further Vilsmeier-Haack reaction of 1a
gave ’diformylBODIPY 1b in 75% yield. The symmetrical substituted pattern of 1b was
confirmed by NMR spectroscopy, in which a singlet signal at 9.87 ppm for the two protons of aldehyde
group, and two sets of singlet signals at 8.46 and 7.02 ppm for the  and  pyrrolic protons on the
BODIPY core were observed. The solvent-dependent photophysical properties of 1a and 1b at various
common organic solvents with different polarities have been studied as summarized in Figures S1-2
and Table S1 in Supporting Information. The formylBODIPYs 1a and 1b showed absorption
maxima centered at 501 and 509 nm in toluene, respectively. There is a small red-shift of the absorption
bands with respect to that of the starting meso-mesitylBODIPY. The increase of the polarity of solvents
(from toluene to methanol) leads to a slightly blue-shift in absorption bands for both 1a and 1b (Table
4

S1). No obvious solvent-dependent fluorescence emission was observed for BODIPYs 1a and 1b,
except a slightly increase of the fluorescence quantum yields for 1b with increase of the polarity of
solvents (0.66 in toluene, 0.72 in chloroform, 0.63 in THF and 0.12 in acetonitrile, Table S1).
Next, the sensing behavior of BODIPYs 1a and 1b to cyanide anion was studied in a CH3CN-
HEPES buffer solution (v/v = 9:1, pH = 7.4). In this buffer solution, BODIPY 1a (1 × 10-5 M) showed
an absorption peak at 492 nm (Figure 1a), which gradually disappeared with the increase of the
concentration of cyanide anion. Meanwhile a new absorption band centered at 388 nm gradually
increases with an isosbestic point at 425 nm. This blue-shift of the absorption spectra is in good
consistence with an apparent visible color change from light green to yellow under ambient light
irradiation condition that can be visualized by naked eyes (inserted picture in Figure 1a). As shown in
Figure 1b, the increase of the concentration of cyanide anion leads to the gradual decrease of the
fluorescence intensity of 1a at 517 nm. There is a good linear relationship between the fluorescence
intensity at 517 nm and the CN- concentration in the range of 0-450 M (R2 = 0.992, Figure S3) with
a detection limit at 2.5 × 10-5 M (S/N = 3).
Furthermore, the spectrum changes of BODIPY 1b after reaction with CN- were also investigated
by absorption and fluorescence emission spectroscopy. Surprisingly, the increasing addition of CN-
leads to the gradual decrease of the absorption band of 1b centred at 498 nm and an appearance of a
broader new band centred at 513 nm with a distinct isosbestic point at 455 nm (Figure 2a). This clearly
indicates a chemical reaction between probe 1b and CN−. In addition, there is a gradual decrease of the
fluorescence emission of 1b centered at 523 nm with an appearance of a gradual appearance of a new
band centered at 670 nm with a distinct isosbestic point at 615 nm with the increase of the concentration
of CN- (Figure 2b). This can be visualized by naked eyes since there is a significant visible color
5

change from light orange to pink was observed under day light irradiation condition. This indicates
that probe 1b could be used as a ratiometrical fluorescence probe for cyanine ion (Figure S4).
To demonstrate the high specificity of these two reaction-based probes toward CN−, some
common competing anions were comparatively studied under the same conditions. As shown in
Figures 3 and 4, probes 1a and 1b are highly selective toward CN- over common competing anions.
-
2-
-
The addition of other common competing anions including AcO-, Br-, Cl-, ClO4 , F-, H2PO4 , HSO3 ,
-
-
HSO4 , NO3 , SCN-, SO42- leads to negligible variations in both the absorption and fluorescence spectra
for both probes 1a and 1b. It is worth noticed that although the addition of F- leads to the decrease of
fluorescence emission for 1b, no spectra shift was observed. This is in great contrast to CN- which
brings not only the decrease of the fluorescence emission but also a red-shift of the spectra..
To demonstrate the high selectivity of probe 1a and 1b toward to CN−, the competition
experiments were performed (Figures S5 and S6). The fluorescence competition experiments
indicated that the presence of other competing anions only leads to a negligible to slight variation of
the fluorescence response induced by CN-. Therefore, probes 1a and 1b may be used for the selective
detection of cyanine anion in the presence of other common competing species.
In addition, the visible light (daylight) excitation and emission (green light) properties of probes
1a and 1b renders the naked-eye detection of CN- (Figure 5). Under daylight irradiation condition, the
addition of CN- to probes 1a and 1b leads to a dramatic colour change from light green (visible
emission by excitation of daylight) to yellow (for 1a), and from yellow to pink (for 1b) respectively.
By contrast, the addition of other competition anions causes negligible variations of the colour.
6

The significantly blue shifts of absorption maxima indicated that the BODIPY core conjugation was
broken, especially in the reaction of BODIPY 1a and cyanine ion. Since the direct addition of formyl
group on BODIPY 1a with cyanine ion (Scheme 1a, path a) can not affect the conjugation of the
BODIPY core, thus we speculate that the cyanide ion may attack azafulvene ring of BODIPY core as
shown in Figure 6. In fact, our group have recently disclosed that BODIPY derivative with electron
withdrawn group at position shows enhanced reactivity toward the direct oxidative nucleophilic
substitution and can efficiently react with various amines regioselectively at the -position of BODIPY
core at room temperature [38].
To further probe the proposed reaction mechanism, 1H NMR titration experiments were carried out.
The variation of the 1H NMR spectra for probes 1a and 1b with the addition of CN- were recorded in
CDCl3 at room temperature (Figures 6 and S7), respectively. The addition of CN-, brings a significant
up-field shift of the proton signals corresponding to the aldehyde proton (Ha) and aromatic protons
(Hb-Hf) in comparison with those in probes 1a and 1b, respectively. As shown in Figure 6, the addition
of 1 equiv CN- leads to the disappearance of the chemical shift of the aldehyde proton (Ha) at 9.82 ppm
and the appearance of a new signal at 9.62 ppm (Ha’). This confirms that the formyl group in 1a remains
after the reaction CN-. Significantly, a upfield shift of the azafulvene proton (Hc) from 8.16 ppm to
5.51 ppm (Hc’) was observed with the addition of CN-. This may be attributed to the formation of sp3
hybridized carbon anion due to the nucleophilic addition to the C=N double bond of the azafulvene
framework. This result is in good agreement with the blue shifted absorption bands observed with the
1
addition of CN-. Probe 1b shows a similar H NMR titration behaviour to that of probe 1a (Figure
S5), in which the aldehyde groups remain untouched with the reaction with CN-. Our initial attempts
to prepare and isolate the proposed possible -CN substituted BODIPYs via the atmospheric oxidation
of 1a-CN and 1b-CN (Scheme 1) by exposure the reaction mixture to the air were failed. Fortunately,
7

the addition of a strong oxidant Pb(OAc)4 to the reaction mixture of 1b and CN- smoothly promotes
the oxidation reaction to afford the corresponding -CN substituted BODIPY as stable product in 75%
yield (Scheme S1). The formation of this oxidation product as was confirmed by NMR spectra and
HRMS.
Conclusion
In summary, we have prepared two new colorimetric and ratiometric probes 1a and 1b for the
selective “naked-eye” detection of cyanide anion over other common competition anions based on an
unprecedented nucleophilic addition of cyanide ion to the azafulvene framework of the BODIPY
chromophore instead of the expected formyl moieties. The addition of cyanide ion brings a remarkably
dual change: a blue-shift in absorption (from 492 to 388 nm) and an efficient quenching in the
fluorescence for probe 1a, and a ratiometrical red-shift of the fluorescence emission band for probe 1b
(from 523 to 670 nm). Possible mechanism of these reaction based cyanide probes were studied. Both
the 1H NMR titration and the oxidative nucleophilic addition products indicates that probes 1a and 1b
underwent an unprecedented nucleophilic addition of cyanide anion to the -position of BODIPY core
(the azafulvene framework) instead of classical -formyl addition products.
Acknowledgements
This work is supported by the National Nature Science Foundation of China (Grants 21871006,
21672006 and 21672007).
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Figure 1. (a) Absorption titration and (b) fluorescence titration spectra of 1a (10 μM) with the addition
of CN- from 0 to 500 M in CH3CN-HEPES buffer (v/v = 9:1, pH = 7.4). Data was recorded 10 min
after the addition of CN-. Inserted pictures: color changes of probe 1a (10 M) in the presence of 50
equiv of cyanide anion under daylight (left) and UV irradiation (right) conditions.
Figure 2. (a) Absorption titration and (b) fluorescence titration spectra of 1b (10 μM) with the addition
of CN- from 0 to 300 μM in CH3CN-HEPES buffer (v/v = 9:1 , pH = 7.4). Each spectrum was collected
11

10 min after addition of CN-. Inserted pictures: color changes of probe 1b (10 M) in the presence of
30 equiv of cyanide anion under daylight (left) or UV irradiation (right) conditions.
Figure 3. (a) Overlaid absorption and (b) fluorescence emission spectra (right, ex = 460 nm) of 1a
(10 μM) in the presence of 50 equiv CN- and some common competing anions (100 equiv) in CH3CN-
-
-
HEPES buffer (v/v = 9:1, pH = 7.4). [0: Blank, 1: AcO-, 2: Br-, 3: Cl-, 4: ClO4 , 5: F-, 6: H2PO4 , 7:
-
-
-
2-
HSO3 , 8: HSO4 , 9: NO3 , 10: SCN-, 11: SO4 , 12:CN-].
Figure 4. (a) Overlaid absorption and (b) fluorescence emission spectra (right, ex = 470 nm) of 1b
(10 μM) in the presence of 30 equiv CN- and some common competing anions (100 equiv) in CH3CN-
-
-
HEPES buffer (v/v = 9:1, pH = 7.4). [0: Blank, 1: AcO-, 2: Br-, 3: Cl-, 4: ClO4 , 5: F-, 6: H2PO4 , 7:
-
-
-
2-
HSO3 , 8: HSO4 , 9: NO3 , 10: SCN-, 11: SO4 , 12:CN-].
Figure 5. Images of 1a (a) and 1b (b) (10 μM) in the presence of CN- (50 equiv for 1a and 30 equiv
for 1b) and 100 equiv of various competing anions in CH3CN-HEPES buffer (v/v = 9:1, pH = 7.4)
under daylight (top) and handhold UV-irradiation (bottom) conditions. [0: Blank, 1: AcO-, 2: Br-, 3:
-
-
-
-
-
2-
Cl-, 4: ClO4 , 5: F-, 6: H2PO4 , 7: HSO3 , 8: HSO4 , 9: NO3 , 10: SCN-, 11: SO4 , 12: CN-]
Figure 6. Proposed mechanism and 1H NMR titration spectra of BODIPY 1a (8 mM) with and without
1
the addition of cyanide anion (0.5 and 1.0 equiv, respectively) in CDCl3 solution at 25°C. H NMR
spectra was immediately recorded upon addition of TBACN.
Scheme 1. (a) Chemical structures of probes 1a and 1b and the proposed nucleophilic addition sensing
mechanisms. (b, c) Overlaid absorption (black line) and emission (red line) spectra of probes 1a-CN
12

(b, 10 M 1a in the presence of 50 equiv CN-) and 1b-CN (c, 10 M 1b in the presence of 30 equiv
CN-) in CH3CN-HEPES solution (v/v = 9:1, pH = 7.4), respectively. Inserted pictures: the
corresponding photos of probes 1a-CN (b) and 1b-CN (c) under daylight (left) or 360 nm light
irradiation (right) conditions.
13