Development of an indole-based boron-dipyrromethene fluorescent probe for benzenethiols

Article abstract of DOI:10.1021/jp109845g

Discrimination between chemically related benzenethiols and aliphatic thiols represents a big problem. In this paper, a fluorescent probe, Bodipy-1, containing an indole-based Bodipy as a fluorophore and a 2,4- dinitrobenzenesulfonyl group as a recognition unit was constructed to achieve the selectivity between them. The Bodipy group in the prepared probe was selectively released through aromatic nucleophilic substitution by thiolate anions from benzenethiols, resulting in blue-red switching in the emission spectra in buffer solutions; that is, two new peaks of the phenol/phenolate state of Bodipy-2 at 565 and 629 nm appeared in emission spectra. By varying the pH value from 6.6 to 8.8, the intensity ratio of I₅₆₅/I ₆₂₉ varies from 2.0 to 0.3 after complete conversion to Bodipy-2, a ca. 7-fold emission ratio change. This ratiometric emission property by varying the pH value makes Bodipy-1 a promising probe to discriminate benzenethiols from aliphatic thiols by careful selection of the reaction pH.

Full text of DOI:10.1021/jp109845g

642
J. Phys. Chem. B 2011, 115, 642–647
Development of an Indole-Based Boron-Dipyrromethene Fluorescent Probe for
Benzenethiols
Chunchang Zhao,* Yu Zhou, Qiuning Lin, Linyong Zhu,* Peng Feng, Yulin Zhang, and
Jian Cao
Key Laboratory for AdVanced Materials and Institute of Fine Chemicals, East China UniVersity of Science &
Technology, Shanghai 200237, People’s Republic of China
ReceiVed: October 14, 2010; ReVised Manuscript ReceiVed: NoVember 29, 2010
Discrimination between chemically related benzenethiols and aliphatic thiols represents a big problem. In
this paper, a fluorescent probe, Bodipy-1, containing an indole-based Bodipy as a fluorophore and a 2,4-
dinitrobenzenesulfonyl group as a recognition unit was constructed to achieve the selectivity between them.
The Bodipy group in the prepared probe was selectively released through aromatic nucleophilic substitution
by thiolate anions from benzenethiols, resulting in blue-red switching in the emission spectra in buffer
solutions; that is, two new peaks of the phenol/phenolate state of Bodipy-2 at 565 and 629 nm appeared in
emission spectra. By varying the pH value from 6.6 to 8.8, the intensity ratio of I565/I629 varies from 2.0 to
0.3 after complete conversion to Bodipy-2, a ca. 7-fold emission ratio change. This ratiometric emission
property by varying the pH value makes Bodipy-1 a promising probe to discriminate benzenethiols from
aliphatic thiols by careful selection of the reaction pH.
Introduction
fluorescent probe that can display a ratiometric response to
benzenethiols using an external stimulus.
The development of a highly sensitive and selective recogni-
tion of thiols has emerged as a significant interest in areas
ranging from the petrochemical industry to food quality control
and medicine. For instance, considerable efforts have been
devoted to the development of fluorescent sensors or probes
for aliphatic thiols in the past decades, mainly designed for
glutathione (GSH), cysteine (Cys), and homocysteine (HCys),1-6
which play a crucial role in maintaining redox homeostasis in
biological systems.7-10 Bezenethiols are useful chemicals in the
production of pesticides, polymers, and pharmaceuticals.11
However, exposure to benzenethiols can induce systemic
injuries, including shortness of breath, muscular weakness,
nausea, vomiting, coma, and even death, through targeting of
the central nervous system, kidney, and liver.12 Despite having
a high toxicity, only a few fluorescent sensors that selectively
respond to the widely existent benzenethiols have been reported.
Aliphatic thiols and benzenethiols are a class of molecules with
close physical and chemical properties. As a result, most
fluorescent probes exhibit poor selectivity toward them. Re-
cently, a pioneering work regarding the discrimination of
benzenethiols over aliphatic thiols was reported by Wang etc.
based on the analyst reactivity and reaction conditions.13 On
the basis of this concept, another two probes with a high
sensitivity and selectivity toward benzenethiols were construc-
ted,14,15 all of which respond to benzenethiols with a change
only in flouorescent intensity. Nevertheless, as if the change in
fluorescence intensity is the only detection signal, factors, such
as environmental conditions and the probe concentration, can
interfere with the signal output. In this case, the design of
molecules that can induce prominent spectroscopic changes,
such as ratiometric fluorescence changes, upon selective binding
with benzenethiols is highly desirable for ease of quantifications
and signal transductions. We, therefore, want to develop a
It is known that, for the design of selective and sensitive
sensors for discrimination between chemically closely related
analysts, conjugation of a well-established and efficient recogni-
tion site with a suitable signaling moiety is the most rewarding
approach.16 As a signaling unit, fluorophores generally have
many desirable characteristics in both sensitivity and ease of
signal transduction. Among many of the fluorophores, boron-
dipyrromethene (Bodipy) fluorescent dyes have been recognized
as the promising ones for the construction of molecular sensors
because of their excellent characteristics, such as intense
fluorescence quantum yields, sharp absorption and fluorescence
emission spectra, and high photo- and chemostability.17 How-
ever, no reports have been published concerning Bodipy as a
signaling unit for discrimination of thiols, so far. Therefore, in
this paper, we attempted to choose Bodipy as a fluorophore and
the strongly electron-withdrawing 2,4-dinitrobenzenesulfonyl
(DNBS) group as a recognition unit to achieve a sensitive and
selective fluorescent probe toward benzenethiols. The DNBS
group is now frequently employed as an efficient recognition
site for thiols due to its unique and high reactivity toward thiolate
anions.2f,8a,13-15,18 The fluorogenic reaction of these probes occurs
through deprotection of the DNBS through aromatic nucleophilic
substitution by thiolate anions (Scheme 1), so the degree of thiol
dissociation is important for a thiol probe to work.13-15,18 Thus,
careful selection of the reaction pH at 6-8 would allow a
judiciously designed probe to discriminate benzenethiols over
thiols. This selectivity is possible because the pKa value of
thiophenol is about 6.5, which is lower than that of aliphatic
thiols (pKa of ca. 8.5), and under pH 6-8, the corresponding
more nucleophilic thiolate for benzenethiols is the dominant
species; in comparison, the aliphatic thiols remain largely in
the less reactive neutral form.
Here, we report the synthesis of a novel benzenethiol probe,
Bodipy-1, having an indole-based Bodipy as a fluorophore and
a DNBS group as a recognition unit, whose function is based
* To whom correspondence should be addressed. Phone: 86-2164252388.
E-mail: zhaocchang@ecust.edu.cn (C.Z.), linyongzhu@ecust.edu.cn (L.Z.).
10.1021/jp109845g  2011 American Chemical Society
Published on Web 12/17/2010

Development of a Bodipy Probe for Benzenethiols
J. Phys. Chem. B, Vol. 115, No. 4, 2011 643
SCHEME 1: Probe Bodipy-1 and the Releasing Bodipy-2 by Benzenethiols at pH 7.4 in Sodium Phosphate Buffer
Solutions
SCHEME 2: Synthesis of Bodipy-2a
a Reagents and conditions: (a) BnCl, K2CO3, acetone, reflux 15 h; (b) N3CH2COOEt, NaOEt, EtOH, 0 °C to rt, 6 h; (c) toluene, reflux, 2 h; (d)
LiAlH4, THF, 1 h; (e) 2,4-dimethylpyrrole, CH3COOH, heat to 40 °C, 1 h; (f) 10%Pd(OH)2/C, MeOH/EtOAc, rt, 3d; (g) DDQ, CH2Cl2, rt, 1 h,
NEt3, BF3-OEt2, 30 min; (h) 10%Pd/C, MeOH/EtOAc, rt, 24 h.
upon the well-established effective thiolysis of DNBS. The
Bodipy group in the prepared probe can be selectively released
through aromatic nucleophilic substitution by thiolate anions
from benzenethiols (Scheme 1), resulting in a higher quantum
yield and phenol/phenolate-dependent blue-red switching in
the absorption and emission spectra in buffer solutions. Com-
pared to the reported examples of the fluorescent sensing of
thiols, which function by just the enhancement of fluorescence
signals, our probe with ratiometric emission properties by
varying the pH conditions can eliminate ambiguities by self-
calibration of the two emission bands, as if the change in
fluorescence intensity is the only detection signal; factors, such
as instrumental efficiency, environmental conditions, and the
probe concentration, can interfere with the signal output.
probe (4 µM) in 3 mL of neutral aqueous conditions (0.2 M
sodium phosphate buffer, pH 7.4) was prepared by mixing 20
µL of the probe stock solution and 10 mL of 0.2 M phosphate
buffer (pH ) 7.4). The solutions of various testing species were
prepared from cysteine, PhNH2, PhOH, glucine, PhCH2SH, or
CH3PhSH.
Materials and General Methods. All reagents and solvents
were commercially available and used without further purifica-
tion. Proton NMR and 13C NMR were measured on a Bruker
AV-400 spectrometer with chemical shifts reported in parts per
million (in (CD3)2CO or CDCl3; TMS as an internal standard).
Mass spectra were measured on an HP 1100 LC-MS spectrom-
eter. All pH measurements were made with a Sartorius basic
pH meter PB-10. Fluorescence spectra were determined on a
VARIAN CARY Eclipse Fluorescence spectrophotometer.
Absorption spectra were determined on a VARIAN CARY 100
Bio UV-visible spectrophotometer.
Experimental Methods
Fluorometric Measurements. A stock solution of the probe
(2 × 10-3 M) was prepared in THF. The test solution of the

644 J. Phys. Chem. B, Vol. 115, No. 4, 2011
Zhao et al.
Results and Discussion
Synthesis of a Novel Indole-Based Boron-Dipyrromethene
Fluorescent Dye, Bodipy-2. Although Bodipy derivatives are
among the most widely studied fluorescent dyes, very few
investigations have been carried out on their analogues based
on an indole building block. In our search for new fluorophores
with excellent photophysical properties, we first prepared
Bodipy-2 and examined its photophysical properties in buffer
solutions. Scheme 2 depicts the preparation of the dipyr-
romethane intermediate 5 and its subsequent production of
Bodipy-2 in dichloromethane. The intermediate 4 was prepared
in three steps, commencing from p-hydroxybenzenaldehyde
according to the literature procedure.19 Treatment of the ester 4
with an excess of sodium aluminum hydride in tetrahydrofuran
afforded the corresponding 2-methanol 5 in 92% isolated yield.
This alcohol was found to be sensitive to acidic environments
and liable to condense with excess 2,4-dimethylpyrrole in glacial
acetic acid to produce the dipyrromethane 6. Beginning with
dipyrromethane 6, two synthetic routes were adopted to get
Bodipy-2. In the first approach, the condensation product 6 was
first oxidized with DDQ, followed by neutralization with NEt3,
and subsequently treated with BF3-Et2O to afford the desired
Bodipy compound. However, deprotection of phenol carried out
using 10% Pd/C in methanol/EtOAc to give Bodipy-2 was
unsuccessful. The double bond in the meso position was indeed
reduced to a single bond. The second route was then investi-
gated. Prior to the oxidation with DDQ, compound 6 was first
deprotected to furnish the intermediate dipyrromethane 7. This
intermediate was then subject to oxidation with DDQ, followed
by neutralization with NEt3, and subsequently treated with BF3-
Et2O to afford the desired Bodipy-2.
Effect of pH on the Photophysical Properties of Bodipy-
2. The pH dependence of Bodipy-2 was next investigated in
sodium phosphate buffer solutions, as shown in Figure 1.
UV-vis of Bodipy-2 revealed the absorption with a maximum
at 520 nm, attributed to the neutral form of Bodipy-2. However,
as the pH values were increased from 5 to 9, the absorption
band at 520 nm was progressively decreased, which was
accompanied with a simultaneous increase of the absorption at
600 nm, coming from the phenolate form (Ph-OH f PhO-).
Varying the pH also gave rise to an isosbestic point at 546 nm,
indicating the existence of an equilibrium between phenol and
the phenolate form in buffer solutions within the physiological
pH value. In the same way as the absorption spectra, when
Bodipy-2 was excited at the wavelength of the absorption
isosbestic point (546 nm), the fluorescence spectra of Bodipy-2
exhibited a phenol/phenolate-dependent blue-red switching, as
shown in Figure 1b. When the pH value was increased from 5
to 9, the spectrum showed a decrease of the emission band at
565 nm and a simultaneous increase of the emission band at
629 nm. The likely characteristic isoemission point at 602 nm
makes Bodipy-2 an excellent ratiometric pH indicator. More
importantly, the characteristic two emission bands between pH
values 5 and 9 make Bodipy-2 a very interesting fluorophore.
Along with its releasing during the sensing process, the pH-
dependent blue-red switching emission appears, and the
emission intensity ratio of the two bands remain unchanged as
the pH value is constant.
Figure 1. (a) Absorption spectra of Bodipy-2 in buffer solution as a
function of pH. (b) Corresponding emission spectra (λex ) 546 nm).
The pH was adjusted by NaH2PO4 and Na2HPO4.
the target probe Bodipy-1 in 86% yield (Scheme 3). The 2,4-
dinitrobenzenesulfonyl (DNBS) group was chosen as protection
group due to its unique and high reactivity toward thiolate anions
to release the fluorophore.2f,8a,13-15,18
Sensing Properties of Bodipy-1 to Benzenethiols. The
fluorescent response of Bodipy-1 to benzenethiols was first
performed in a sodium phosphate buffer solution with pH 7.4,
containing Bodipy-1 at a concentration of 4 × 10-6 M. The
fluorescence spectra were recorded at several intervals during
the reaction of Bodipy-1 with benzenethiols. As expected,
Bodipy-1 displayed fluorescence (λem ∼ 530 nm) with a very
low quantum yield in the absence of thiols (Figure 2a). In sharp
contrast to Bodipy-2, almost no change in fluorescence intensity
was observed for Bodipy-1 over the pH range from 5 to 9.
Additionally, a negligible percentage of Bodipy-1 decomposed
to the corresponding Bodipy-2 after 1 h of incubation in buffer
solution at room temperature. However, upon addition of
benzenethiols to the buffer solution, immediately, a dramatic
change in the fluorescence spectra was observed, and two new
peaks of the phenol/phenolate state of Bodipy-2 at 565 and 629
nm appeared, resulting from the generation of Bodipy-2 by the
reaction of Bodipy-1 with benzenethiols. The intensity of the
peaks increased rapidly and reached a maximum after about 20
min. Therefore, in this work, an assay time of 20 min was
adapted as the optical measurement condition. Notably, the
intensity ratio of I565/I629 remained unchanged (Figure S1,
Supporting Information) after all Bodipy-1 was consumed,
which indicated that the ratio of phenol and phenolate is constant
Design and Synthesis of a Fluorescent Probe for Thiols.
One significant advantage of Bodipy-2 is the facile synthesis
of derivatives by modification of the phenol function. Using
this strategy, we designed and synthesized Bodipy-1, which was
achieved by reaction of Bodipy-2 with 2,4-dinitrobenzenesulfo-
nyl chloride in the presence of Et3N in dichloromethane to afford

Development of a Bodipy Probe for Benzenethiols
J. Phys. Chem. B, Vol. 115, No. 4, 2011 645
SCHEME 3: Synthesis of Bodipy-1
under the measurement condition. On the other hand, once
varying the pH to another measurement condition, the intensity
ratio of I565/I629 changed immediately and kept constant as the
measurement condition did not change anymore (Figure S1,
Supporting Information). When the pH value was increased from
an acidic to a basic condition, the ratio of I565/I629 decreased
accordingly, which indicated that, in a basic condition, the
phenolate form is the dominant species. On the basis of this
observation, we next tested the ratiometric detection of ben-
zenethiols by varying the pH value as shown in Figure 2b. The
ratio of two emission band intensities (I565/I629) upon excitation
at 520 nm varies from 2.0 in the presence of benezenethiols at
pH 6.6 to 0.3 at pH 8.8 after complete conversion to Bodipy-2,
a ca. 7-fold emission ratio change.
The reported examples of fluorescent sensing of benzenethiols
function by just the enhancement of fluorescence signals.
However, as the change in fluorescence intensity is the only
detection signal, factors, such as instrumental efficiency,
environmental conditions, and the probe concentration, can
interfere with the signal output. The probe reported here gave
ratiometric emission properties after the interaction with ben-
zenethiols by varying the pH value, so Bodipy-1 can eliminate
these ambiguities by self-calibration of the two emission bands.
We then study the fluorescence sensitivity of Bodipy-1 to
various concentrations of benzenethiols (Figure 3). When higher
concentrations of benzenethiols was used, a more dramatic
enhancement of the two peaks at 565 and 629 nm was observed,
and the enhancement of fluorescence intensity reached the
maximum as 4 equiv of benzenethiols was used. Further using
higher concentrations than 4 equiv of benzenethiols did not result
in additional enhancement of fluorescence intensity while a
negligible decrease was promoted (Figure S2a, Supporting
Information). The detection limit for benzenethiols was deter-
mined as 9.5 × 10-7 M under the experimental conditions
(Figure S2b, Supporting Information),4a,15,20 which is sufficiently
low to allow the fluorogenic detection of micromolar concentra-
tions of benzenethiols.
Although the above studies have indicated that the sensing
response of the probe to benzenethiols is due to the thiolysis of
ester Bodipy-1 to Bodipy-2, further evidence should be
provided. It is well known that the 2,4-dinitrobenzenesulfonyl
ester can be readily cleaved by a thiolate anion through
nucleophilic aromatic substitution, releasing the unprotected
dye.2f,8a,13-15,18 In our case, HPLC analysis was first performed
to confirm the production of Bodipy-2 in the reaction. As shown
in Figure 4, the retention times of standard Bodipy-2 and
p-thiocresol were at 5.35 and 11.61 min, separately. The process
of the thiolysis reaction was also monitored by HPLC at different
Figure 2. (a)Time course for the change in the fluorescence intensities
of Bodipy-1 (4 µM) in the presence of 2 equiv of benzenethiols, such
as CH3PhSH, in the neutral aqueous conditions (0.2 M phosphate buffer,
pH 7.4). (b) Response of Bodipy-1 (4 µM) to 2 equiv of benzenethiols,
such as CH3PhSH, at different pH values. The spectra were obtained
20 min later after addition of thiocresol. The pH value was adjusted
by NaH2PO4 and Na2HPO4. Inset: the ratiometric fluorescence change
as a function of pH.
Figure 3. Emission spectra of Bodipy-1 (4 µM) after addition of
different concentrations of benzenethiols, such as p-thiocresol (0-6
equiv), in a phosphate buffer (0.2 M, pH 7.4) at room temperature.

646 J. Phys. Chem. B, Vol. 115, No. 4, 2011
Zhao et al.
Figure 5. Fluorescence responses of Bodipy-1 (4 µM) in the presence
of 2 equiv of various analysts (phNH2, phOH, glucine, cysteine,
PhCH3SH, and p-thiocresol) in a phosphate buffer (0.2 M, pH 7.4) at
room temperature. The spectra were obtained 20 min later after addition
of analysts.
These results demonstrated that the DNBS groups serve as a
key part in the probe sensing process.
Selectivity Studies of Bodipy-1 toward Benzenethiols.
Fluorescence spectroscopy was employed to investigate the
selectivity of Bodipy-1 for benzenethiols over relevant aliphatic
thiols and other common nucleophiles. Shown in Figure 5 are
the fluorescence change of the probe upon the addition of
CH3PhSH, glucine, PhNH2, PhOH, cysteine, and PhCH2SH at
pH 7.4. The fluorescence spectra were obtained by excitation
at 520 nm. As expected, Bodipy-1 displays selective fluores-
cence enhancement in the presence of CH3PhSH, whereas a
negligible fluorescence enhancement is promoted by PhCH2SH.
On the other hand, no noticeable change was observed for the
addition of the representative nucleophiles, such as glucine,
PhNH2, PhOH, and cysteine. The reactivity in the order of
CH3PhSH > PhCH2SH > cysteine can be rationalized according
to the change of the fluorescence above. This order indicates
that the degree of thiol dissociation is important for Bodipy-1
to function as a thiol probe. The nucleophilic thiolate is an
essentially reactive form for the thiolysis of ester Bodipy-1 to
Bodipy-2. The pKa value of thiophenols is around 6.5, whereas
that of aliphatic thiols is about 8.5. In a neutral reaction medium
at pH 7.4, the high degree of dissociation of thiophenols results
in the predominant generation of the corresponding thiolate
anions, which can effectively react with 2,4-dinitrobenzene-
sulfonate. However, under the same reaction conditions, the
aliphatic thiols remain as a less reactive neutral form and thus
the cleavage of the sulfonate is very slow, which indicates that
careful selection of the reaction pH would allow Bodipy-1 to
discriminate benzenethiols from aliphatic thiols. Moreover,
Bodipy-1 can readily detect CH3PhSH in the presence of
glucine, PhNH2, PhOH, and cysteine (Figure 6), indicating that
these relevant components will not interfere with Bodipy-1
detection of benzenethiols at pH 7.4.
Figure 4. Chromatographic profile of (A) CH3PhSH, (B) Bodipy-2,
and (C-E) Bodipy-1 in the absence and presence of 2 equiv of
CH3PhSH as a function of incubation time on the Agilent Eclipse XDB-
C18: 5 µm, 4.6 × 150 mm column. Incubation time: (C) 0 min, (D) 5
min, (E) 10 min. Solvent: 60:40 (v/v) ) methanol/buffer solution (18
g of NaOAc in 9.8 mL of acetic acid and 1000 mL of H2O), pH 5.0.
time courses. In the absence of p-thiocresol, Bodipy-1 gave a
retention time at 3.89 min. However, upon addition of p-
thiocresol, the peak at 3.89 min decreased in intensity until it
disappeared after 10 min; meanwhile, a new peak with the
retention time at 5.35 min appeared, coming from the formation
of Bodipy-2.
Further support for the identity of the released Bodipy-2
comes from the comparison of the emission spectra of the
reaction mixture with the standard Bodipy-2. We found that
they have the identical spectra. Furthermore, the pH titration
displayed that they have the same behavior (Figure S3,
Supporting Information). All the results of these experiments
indicated that the reaction product was indeed the Bodipy-2.
Finally, mass spectrometry analysis of the mixture of Bodipy-1
with benzenethiols in buffer solutions was adopted. The mass
spectrum displayed two peaks at m/z 124.0350 and 290.0359,
consistent with that of p-thiocresol and 2,4-dinitrobenzene
p-methylbenzene sulfide, respectively. Although attempts to get
that of Bodipy-2 were unsuccessful, the formation of 2,4-
dinitrobenzene p-methylbenzene sulfide indeed indicated the
deprotection of DNBS as reported by the literature.2f,8a,13-15,18
To confirm the key role of the 2,4-dinitrobenzensulfonyl
(DNBS) protecting group in the sensing process, compound 11,
with a trifluorosulfonyl protecting group instead of DNBS, was
synthesized. Different from that for Bodipy-1, compound 11
showed almost no fluorescence and fluorescence change in the
absence and presence of benzenethiols under the same measur-
ing conditions (Figure S4, Supporting Information), which
means that compound 11 showed no response to benzenethiols.
Conclusions
A novel fluorescent probe, Bodipy-1, containing an indole-
based Bodipy as a fluorophore and the strongly electron-
withdrawing 2,4-dinitrobenzenesulfonyl group as a recognition
unit was synthesized. The probe displayed fluorescence (λem
∼
530 nm) with a very low quantum yield due to the quenching
effect by the DNBS. However, the Bodipy group in the prepared
probe was selectively released immediately upon addition of

Development of a Bodipy Probe for Benzenethiols
J. Phys. Chem. B, Vol. 115, No. 4, 2011 647
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Figure 6. Selectivity of Bodipy-1 toward p-thiocresol and other
nucleophiles: (1) Bodipy-1 only, (2) CH3PhSH, (3) glucine, (4) PhNH2,
(5) PhOH, (6) cysteine, and (7) PhCH2SH. Red bar: the furorescence
intensity of only a single analyte (2 equiv) with Bodipy-1 (4 µM).
Green bar: the fluorescence intensity of a mixture of a nucleophilic
reagent (2 equiv) and CH3PhSH (2 equiv) with Bodipy-1 (4 µM).
benzenethiols to the buffer solution, and a dramatic change with
a higher quantum yield and a phenol/phenolate-dependent
blue-red switching was observed. That is, two new peaks of
the phenol/phenolate state of Bodipy-2 at 565 and 629 nm
appeared in the emission spectrum and two at 520 and 600 nm
in the absorption spectrum, resulting from the generation of
Bodipy-2 by the reaction of Bodipy-1 with thiolate anions from
benzenethiols. By varying the pH value, the intensity ratio of
I565/I629 changed immediately and kept constant as the measure-
ment condition did not change anymore. The ratio of two
emission band intensities (I565/I629) upon excitation at 520 nm
varies from 2.0 in the presence of benezenethiols at pH 6.6 to
0.3 at pH 8.8 after complete conversion to Bodipy-2, a ca. 7-fold
emission ratio change. This ratiometric emission property makes
Bodipy-1 a promising probe to discriminate benzenethiols over
aliphatic thiols by careful selection of the reaction pH. The
reported examples of the fluorescent sensing of benzenethiols
function by just the enhancement of fluorescence signals.
However, as the change in fluorescence intensity is the only
detection signal, factors, such as instrumental efficiency,
environmental conditions, and the probe concentration, can
interfere with the signal output. The probe reported here gave
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Acknowledgment. We gratefully acknowledge the financial
support by the National Science Foundation of China (Grant
No. 20902021), the Scientific Research Foundation for the
Returned Overseas Chinese Scholars (State Education Ministry),
Key Laboratory of Photochemical Conversion and Optoelec-
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Supporting Information Available: A detailed description
of the synthesis and photophysical data of Bodipy-1 and
Bodipy-2. This material is available free of charge via the
Internet at http://pubs.acs.org.
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