Boron-dipyrromethene based reversible and reusable selective chemosensor for fluoride detection

Article abstract of DOI:10.1021/ic402767j

We synthesized benzimidazole substituted boron-dipyrromethene 1 (BODIPY 1) by treating 3,5-diformyl BODIPY 2 with o-phenylenediamine under mild acid catalyzed conditions and characterized by using various spectroscopic techniques. The X-ray structure analysis revealed that the benzimidazole NH group is involved in intramolecular hydrogen bonding with fluoride atoms which resulted in a coplanar geometry between BODIPY and benzimidazole moiety. The presence of benzimidazole moiety at 3-position of BODIPY siginificantly altered the electronic properties, which is clearly evident in bathochromic shifts of absorption and fluorescence bands, improved quantum yields, increased lifetimes compared to BODIPY 2. The anion binding studies indicated that BODIPY 1 showed remarkable selectivity and specificity toward F^- ion over other anions. Addition of F^- ion to BODIPY 1 resulted in quenching of fluorescence accompanied by a visual detectable color change from fluorescent pink to nonfluorescent blue. The recognition mechanism is attributed to a fluoride-triggered disruption of the hydrogen bonding between BODIPY and benzimidazole moieties leading to (i) noncoplanar geometry between BODIPY and benzimidazole units and (ii) operation of photoinduced electron transfer (PET) from benzimidazole moiety to BODIPY unit causing quenching of fluorescence. Interestingly, when we titrated the nonfluorescent blue 1-F^- solution with TFA resulted in a significant enhancement of fluorescence intensity (15-fold) because the PET quenching is prevented due to protonation of benzimidazole group. Furthermore, the reversibility and reusability of sensor 1 for the detection of F^- ion was tested for six cycles indicating the sensor 1 is stable and can be used in reversible manner.

Full text of DOI:10.1021/ic402767j

Article
pubs.acs.org/IC
Boron-Dipyrromethene Based Reversible and Reusable Selective
Chemosensor for Fluoride Detection
Sheri Madhu and Mangalampalli Ravikanth*
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
S
* Supporting Information
ABSTRACT: We synthesized benzimidazole substituted boron-dipyrromethene 1 (BODIPY 1) by treating 3,5-diformyl
BODIPY 2 with o-phenylenediamine under mild acid catalyzed conditions and characterized by using various spectroscopic
techniques. The X-ray structure analysis revealed that the benzimidazole NH group is involved in intramolecular hydrogen
bonding with fluoride atoms which resulted in a coplanar geometry between BODIPY and benzimidazole moiety. The presence
of benzimidazole moiety at 3-position of BODIPY siginificantly altered the electronic properties, which is clearly evident in
bathochromic shifts of absorption and fluorescence bands, improved quantum yields, increased lifetimes compared to BODIPY 2.
The anion binding studies indicated that BODIPY 1 showed remarkable selectivity and specificity toward F− ion over other
anions. Addition of F− ion to BODIPY 1 resulted in quenching of fluorescence accompanied by a visual detectable color change
from fluorescent pink to nonfluorescent blue. The recognition mechanism is attributed to a fluoride-triggered disruption of the
hydrogen bonding between BODIPY and benzimidazole moieties leading to (i) noncoplanar geometry between BODIPY and
benzimidazole units and (ii) operation of photoinduced electron transfer (PET) from benzimidazole moiety to BODIPY unit
causing quenching of fluorescence. Interestingly, when we titrated the nonfluorescent blue 1-F− solution with TFA resulted in a
significant enhancement of fluorescence intensity (15-fold) because the PET quenching is prevented due to protonation of
benzimidazole group. Furthermore, the reversibility and reusability of sensor 1 for the detection of F− ion was tested for six cycles
indicating the sensor 1 is stable and can be used in reversible manner.
sharp bands with large molar absorption coefficients in the
absorption spectra, decent fluorescence quantum yields,
reasonably long excited singlet state lifetimes, and good
chemical and photochemical stability in both solution and
solid states.14 The properties of the BODIPY dyes can be fine-
tuned by introducing suitable substituents at the right positions
on the BODIPY core.15 These dyes have numerous applications
such as acting as light-harvesting arrays, biological probes,
supramolecular fluorescent gels, fluorescent switches, and
sensors.16 In continuation of our work on BODIPYs, we have
developed a new BODIPY based chemosensor 1 for fluoride
ion which is found to be reversible and reusable as
demonstrated by various spectroscopic techniques. Further-
more, BODIPY 1 also can be used as colorimetric sensor for F−
ion, as it shows striking color change from fluorescent pink to
INTRODUCTION
■
Fluoride is a common ingradient in anesthetics, hypnotics,
psychiatric drugs, rat and cockroach poisons, and military nerve
gases and is a contaminant in drinking water.1 Excess fluoride
exposure may cause collagen break down, bone disorder,
thyroid activity, depression.2 Thus, detection of fluoride with
use of simple preparation and minimal instrumental assistance
is desirable toward practical applications.3 Various kinds of
fluoride chemosensors have been developed based on urea4 or
thiourea,5 amide,6 phenol,7 cationic borane,8 pyrrole based
macrocycles,9 boron-dipyrromethenes (BODIPYs),10 and so
forth. Most of these sensors are normally irreversible and one
time use for sensing fluoride ion.11 A perusal of the literature
reveals that there are very few scattered sensors available in the
literature which can be used as reversible and reusable systems
for sensing fluoride ion.12 Our group is involved in the design
and synthesis of new BODIPYs for sensing various anions and
cations.13 BODIPYs have very attractive features that include
Received: November 5, 2013
Published: January 22, 2014
© 2014 American Chemical Society
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nonfluorescent blue upon binding with F− ion. Although
several F− ion sensors are available in the literature, the sensor
reported here absorbs and emits in higher wavelength region
and it is reversible compared to the earlier reported probes.
RESULTS AND DISCUSSION
To synthesize the target BODIPY 1 (Scheme 1), we need
access to 3,5-diformyl BODIPY 2 which was prepared by
■
Scheme 1. Synthesis of BODIPY 1
following our previously reported method.13a The reaction of 1
equiv of BODIPY 2 with 1 equiv of o-phenylenediamine in
CH3OH in the presence of a catalytic amount of trifluoroacetic
acid at room temperature for 1 h. The progress of the reaction
was monitored by TLC analysis, which showed complete
disapperance of fluorescent yellow spot corresponding to
BODIPY 2 with an appearance of a less polar, new pink
fluorescent spot corresponding to the desired BODIPY 1 along
with a minor spot corresponding to 3,5-bis(acetal) BODIPY.
Column chromatography on basic alumina using petroleum
ether/ethyl acetate (90:10, v/v) afforded BODIPY 1 as a
golden yellow solid in 38% yield. The molecular ion peak at
473.1953 in high resolution mass spectra confirmed the identity
of BODIPY 1 (Figure S1 in Supporting Informaton). 1H NMR;
1H−1H COSY; and 13C, 19F, and 11B NMR spectroscopy were
used to characterize compound 1 in detail (Figure S2−S7 in
Supporting Information). Figure 1 shows the comparison of 1H
1
NMR spectra of BODIPY 1 and 2 and H−1H COSY NMR
1
spectrum of BODIPY 1. In H NMR, BODIPY 2 showed two
1
sets of signals for four pyrrole protons because of symmetric
substitution. However, BODIPY 1 is unsymmetric and showed
four sets of resonances for four pyrrole protons, which
experienced upfield/downfield shifts compared to BODIPY 2.
All protons in 1H NMR spectrum of BODIPY 1 were identified
based on the coupling constants, peak integration, and cross-
Figure 1. (a) Comparison of partial H NMR spectra of BODIPYs 1
and 2. (b) Partial 1H−1H COSY NMR spectrum of BODIPY 1
recorded in CDCl3.
1
peak correlations observed between the resonances in H−1H
6.76 ppm was assigned to b′-type pyrrole proton, as this
resonance showed cross-peak correlation with b-type resonance
at 6.96 ppm. The NH proton of benzimidazole moiety
appeared as broad unresolved triplet at 11.08 ppm. The four
aryl protons of benzimidazole moiety also identified similarly
based on cross-peak correlations in 2D NMR spectra. The two
doublets at 7.65 and 7.84 ppm corresponding to f-type and i-
type protons, respectively, showed cross-peak correlation with
multiplet resonance at 7.36 ppm, which corresponds to g-type
and h-type protons. The −OCH3 protons of the acetal group
appeared as a singlet at 3.51 ppm, whereas −CH proton
appeared as a singlet at 6.01 ppm. Thus, 1D and 2D NMR
spectroscopy were very helpful in deducing the molecular
structure of BODIPY 1. The BODIPY 1 showed a typical
quartet at −137.7 ppm in 19F NMR and a typical triplet at 0.31
ppm in 11B NMR spectra (Figure S4−S5 in SI).
COSY (Figure 1) and NOESY spectra (Figure S7 in SI). In 1H
NMR of BODIPY 1, the −CH3 resonance appeared as singlet
at 2.49 ppm showed cross-peak correlation with doublet
1
resonance at 7.33 ppm in H−1H COSY spectrum which we
identified as c-type meso-aryl protons. The c-type proton
resonance at 7.33 ppm showed cross-peak correlation with
multiplet resonance at 7.48 ppm in COSY spectrum which we
recognized as d-type meso-aryl protons. The d-type protons in
turn showed NOE correlation with doublet resonance at 7.59
ppm which corresponds to a-type pyrrole protons. The a-type
protons showed cross-peak correlation with doublet at 7.05
ppm which we identified as a′-type pyrrole proton. Similarly,
the d-type proton resonance at 7.48 ppm showed NOE
correlation with doublet resonance at 6.96 ppm which we
assigned to b-type pyrrole protons. The doublet resonace at
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We obtained single crystals for BODIPY 1 by slow
evaporation of n-hexane/CHCl3 (1:1 v/v) over a period of
one week and crystallized in a monoclinic P21/c unit cell. The
crystal structure of BODIPY 1 (CCDC 969275) is shown in
Figure 2. As shown in Figure 2, the BODIPY framework is
spectra of BODIPY 1 recorded in CHCl3 are shown in Figure
3a. The absorption spectra of BODIPY 1 showed a character-
istic strong band corresponding to a S0→S1 transition at 580
nm with one vibronic component on the higher energy side at
540 nm which was clearly separated from the main transition.
In addition, an ill-defined band at ∼420 nm corresponding to a
S0→S2 transition is also present. The absorption study of
BODIPY 1 in different solvents of varying polarity indicated
that the absorption band is broad and weak and shifted to blue
in polar solvents like CH3OH compared to nonpolar solvents
like hexane in which the absorption band is relatively sharp and
strong (Figure S8 in SI).
The BODIPY 1 showed strong emission band which is
slightly Stoke-shifted and experienced a bathochromic shift of
∼40 nm with decent quantum yield (Φ = 0.31) compared to
BODIPY 2 (Φ = 0.23), indicative of a rigid structure of
BODIPY 1 formed by a intramolecular hydrogen bonding
between benzimidazole NH and boron-bound fluoride atoms
(inset in Figure 3a). With the increase in the polarity of the
solvent, the fluorescence band of BODIPY 1 was hypsochromi-
cally shifted and quantum yields were decreased. However, in
the case of CH3CN and CH3OH, the BODIPY 1 showed
strong fluorescence with a quantum yield in the range of 0.47−
0.54. The singlet state lifetimes of BODIPY 1 in different
solvents were measured using time correlated single photon
counting (TCSPC) technique and the fluorescence decay of
BODIPY 1 in different solvents were fitted to single
exponential decay (Figure S9 in SI). The singlet state lifetime
in different solvents is in the range of 1.8−6.2 ns. Thus,
absorption and fluorescence studies indicated that the BODIPY
1 absorbs and emits in the visible region with very decent
quantum yield and singlet state lifetime. The electrochemical
properties of BODIPY 1 were followed by cyclic voltammetry
in CH2Cl2 using tetrabutylammonium perchlorate as the
supporting electrolyte (Figure 3b). The BODIPY 1 showed
one reversible and one irreversible reduction at −0.56 and
−1.52 V, respectively, but did not show any oxidation. As clear
from Figure 3b, the reduction waves of BODIPY 1 were shifted
toward more negative by ∼450 mV compared to BODIPY 2
indicating the BODIPY 1 is less electron deficient due to the
presence of benzimidazole moiety.
Figure 2. Molecular structure of BODIPY 1. Thermal ellipsoids are
drawn at the 50% probability level.
composed of two planar pyrrole rings and the central six-
membered boron ring. The dihedral angle between the meso-
aryl ring and the BODIPY core is 52° in BODIPY 1. The B−N
distances for BODIPY 1 are in the range of 1.54−1.55 Å, which
indicates the usual delocalization of the positive charge. The
benzimidazole moiety is also in the same plane as the BODIPY
core. The striking feature of BODIPY 1 is the presence of
intramolecular hydrogen bonding between the boron bound
fluoride groups and the NH protons of the benzimidazole
moiety. Each fluoride in BODIPY 1 is involved in hydrogen
bonding with NH of the benzimidazole moiety with N−H···F
distances are 2.45 Å (N−H3···F1) and 2.15 Å (N−H3···F2).
The intramolecular hydrogen bonding helps the benzimidazole
moiety and BODIPY core in one plane which led to effective
electronic communication between the BODIPY core and the
benzimidazole moiety.
Sensing of Fluoride Ion. It is well-known that the
imidazole and benzimidazole contains hydrogen donor moiety
like NH within the ring, which is useful for selective binding of
anions, and BODIPY moiety is a good fluorophore. Therefore,
the novel properties of benzimidazole and BODIPY in a single
molecular framework was very useful to create a sensor in
which the anion binding properties of benzimidazole can be
monitored by following the changes in the spectral properties
of BODIPY unit. We investigated the anion sensing ability of
BODIPY 1 with various anions such as F−, Cl−, Br−, I−, OH−,
Spectral and Electrochemical Properties of BODIPY 1.
The absorption and emission properties of BODIPY 1 were
studied in different solvents of varying polarity and relevant
data is presented in Table 1. The absorption and emission
Table 1. Photophysical Data of BODIPY 1 Recorded in Different Solvents
solvent
λabs (nm)
λem (nm)
Δvst (cm−1
)
log ε
ϕ
τ (ns)
kr (109 s−1
)
knr (109 s−1
)
χ2
Hexane
CHCl3
CH3CN
C6H6
581
579
568
583
583
566
575
592
596
591
599
599
587
601
320
493
685
458
458
632
752
3.32
3.93
3.59
3.47
3.33
3.66
3.56
0.38
0.31
0.47
0.22
0.20
0.54
0.18
5.2
4.7
5.8
4.1
4.3
6.2
1.8
0.073
0.087
0.081
0.054
0.047
0.087
0.100
0.119
0.147
0.091
0.190
0.186
0.074
0.456
0.98
1.00
1.04
1.04
0.99
1.02
1.08
Toluene
MeOH
DMSO
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Figure 3. (a) Comparison of normalized absorption and emission spectra (inset) of BODIPYs 1 and 2 (5 μM) recorded in CHCl3. (b) Comparison
of reduction waves of the cyclic voltammograms of BODIPYs 1 and 2 in dichloromethane containing 0.1 M tetrabutylammonium perchlorate as the
supporting electrolyte recorded at a 50 mV s−1 scan rate.
Figure 4. Color change induced upon addition of excess equivalents (7.5 × 10−5 M) of various anions (tetrabutylammonium salts) to BODIPY 1 (5
× 10−6 M) in CH3CN solution under daylight (left) and UV lamp (right).
Figure 5. (a) Absorption, (b) emission spectral changes of BODIPY 1 (5 μM) upon addition of increasing equivalents of F− ions (0−15 equiv) in
CH3CN solution. The spectra were recorded after the addition of F− ion to BODIPY 1 for 10 min. Excitation wavelength used was 530 nm. (c) Plots
of relative emission intensity changes (I0/I) of BODIPY 1 (5 μM) to F− ion and common anions that could interfere with the emission intensity at
deferent concentrations. (d) The histograms showing the competitive fluorescence titration response of BODIPY 1 in the presence of various other
anions. {[BODIPY 1-F‑] +An‑}; [BODIPY 1] = 5 μM, [F−] = 15 μM, [An−] = 30 μM.
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Scheme 2. Proposed Mechanism of Fluorescence On/Off for BODIPY 1 upon Addition of Fluoride Causes a Disruption of
Intramolecular H-Bond and Operation of Photoinduced Electron Transfer Process
HPO4 , HSO4 , SO42−, N3 , ClO4 , CH3COO−, and NO3− (as
tetrabutylammonium salts) in CH3CN solution. Our visual
inspection under both ambient and UV light showed that upon
the addition of various above-mentioned anions (Figure 4), a
discernible color change from fluorescent pink to non-
fluorescent blue color of BODIPY 1 solution occurs only in
the presence of F− ion, which indicates that BODIPY 1 has the
potential to act as specific sensor for F− ion. We carried out the
specific selectivity of BODIPY 1 for F− ion by using various
spectroscopic and electrochemical techniques.
sensitivity of BODIPY 1 for F− ion was further evaluated by
using the linear dynamic response and the detection limit
(LOD) was found to be 93 nM (Figure S11−S12 in SI).
To investigate the selective binding of F− ion to BODIPY 1,
fluorescence spectra of BODIPY 1 in the presence of various
above-mentioned tetrabutylammonium anions were recorded.
As shown in Figure 5c, only F− ion induced a prominent
intensity change, whereas addition of various other anions
under identical conditions led to almost no change in
fluorescence intensity, which clearly indicates that BODIPY 1
acts as a highly selective sensor for the detection of F− ion over
other anions. Moreover, competitive binding experiments
performed by addition of excess amounts of other anions did
not interfere with either the binding of F− to the BODIPY 1 or
the subsequent fluorescence quenching (Figure 5d), which
demonstrates that BODIPY 1 can specifically detect F− ions
even in the presence of other analyte ions. We also tested the
sensing behavior of BODIPY 1 with various other metal
fluoride salts such as KF, CsF, and HgF2 (Figure S13 in SI).
The studies showed that various metal fluorides showed similar
spectral changes like TBAF indicating that counter cations do
not play any role in the sensing of F− ion. To determine the
binding stoichiometry of BODIPY 1, fluorescence measure-
ments were carried out in the presence of varying mole-
fractions of F− ion in acetonitrile (Figure S14 in SI). Job’s plot
analyses reveal a maximum at ∼0.3, indicative of the formation
of a 1:2 complex between BODIPY 1 and F− ion.
−
−
−
−
Selective Detection of F− Ion by Absorption,
Fluorescence, NMR and Electrochemical Studies. The
absorption spectral titration of BODIPY 1 was carried out by
addition of increasing amounts of F− ions in CH3CN solution.
The systematic changes in the absorption spectra of BODIPY 1
on addition of TBAF in CH3CN are shown in Figure 5a.
Addition of increasing amounts of F− ion to acetonitrile
solution of BODIPY 1 resulted in the significant decrease in the
intensity of absorption bands at 568 and 420 nm with one clear
isosbestic point at 428 nm supporting the binding of F− ion to
BODIPY 1. The changes in the absorbance at different
equivalents of F− ion were plotted and shown as an inset in
Figure 5a. The sigmoidal nature of the plot serves as an
indicator of analyte binding. Under identical conditions, the
addition of various above-mentioned anions (excess equiv-
alents) did not produce any changes in the absorption spectra
of BODIPY 1 or color of the solution. Similarly, the selective
sensing of F− ion by BODIPY 1 was also followed
systematically by fluorescence titration experiments. The
BODIPY 1 is decently fluorescent with a quantum yield of
∼0.5. Upon addition of increasing amounts of F− ion, the
fluorescence intensity of BODIPY 1 at 591 nm was gradually
decreased and almost quenched in the presence of 15
equivalents of F− ion in CH3CN (Figure 5b). The significant
changes in the fluorescent intensity of BODIPY 1 with a clear
color change from fluorescent pink solution to nonfluorescent
blue solution indicates the following: (i) the binding of F− ion
with benzimidazole NH atom resulted in the increase of
electron density on imidazole moiety due to deprotonation,
which causes the fluorescence quenching via photoinduced
electron transfer (PET), and (ii) the binding of F− ion leads to
the disruption of hydrogen bond between fluoride atoms of
BODIPY and benzimidazole moiety, which may result in
noncoplanar geometry between BODIPY and benzimidazole
causing quenching of fluorescence (Scheme 2). The association
constant of BODIPY 1 by F− ion was estimated by Stern−
Volmer equation, and the corresponding association constant,
Ka, was found to be 3.1 × 106 M−1 (Figure S10 in SI). The
The fluoride anion binding to BODIPY 1 was also monitored
1
by H NMR titration studies in DMSO-d6. The systematic
1
changes in H NMR signals of BODIPY 1 upon addition of
increasing amounts of fluoride ion were shown in Figure 6. As
is clear from Figure 6 that upon addition of only 0.5 equiv of F−
ion, the signal corresponding to the benzimidazole NH proton
experienced downfield shift and broadened. However, upon
addition of 5 equiv of F− ion, we noticed one triplet at ∼16.7
ppm suggesting the formation of HF2− ion which confirmed the
deprotonation of the benzimidazole NH proton. The
deprotonation of benzimidazole NH proton was also evident
in the upfield shifts of various BODIPY core protons.
Furthermore, 19F NMR titration spectra also provide clear
evidence for interaction of F− ion with BODIPY 1 upon
titration with increasing equivalents of F− ion in DMSO-d6 as
shown in Figure 7. The sharp signal at −108 ppm is due to the
tetrabutylammonium fluoride titrant; i.e., the respective signal is
due to unbound fluoride anions, whereas the BODIPY 1
showed a typical quartet at −137 ppm. Upon addition of 1.5
equiv of F− ion to BODIPY 1, the quartet signal intensity at
−137 ppm decreased and at the same time a new quartet signal
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Figure 8. (a) Square wave voltammograms of BODIPY 1 (1.2 mM) in
the presence of increasing amounts of F− (TBAF) ions (0−1.2 equiv)
in CH3CN containing 0.1 M TBAP as supporting electrolyte recorded
at 50 mV s−1 scan rate.
Figure 6. Partial 1H NMR titration spectra of BODIPY 1 (1.6 × 10−2
M) upon addition of increasing amounts of F− (TBAF) ion (0−5
equiv) in DMSO-d6.
decrease of current intensity of reduction wave at −0.56 V and
at the same time appearance of new reduction with gradual
increase in the current intensity at −0.85 V. This significant
shift of potential toward more negative by ∼300 mV indicates
that F− ion interaction enhances electron density on
benzimidazole due to deprotonation. However, no change in
the reduction of BODIPY 1 was observed upon addition of
various other anions. These results supported the use of
BODIPY 1 as a specific electrochemical sensor for F− ion.
Reversibility and Reusability of the Sensor. It is very
beneficial if a sensor can be reversible in nature and be reusable
for sensing selective anion. A perusal of the literature revealed
that most of the F− ion sensors available in the literature are
irreversible and can be used only one time. To test the
reversibility and reusability of our BODIPY 1, we carried out
systematic titration studies of BODIPY 1-F− complex upon
addition of increasing amounts of TFA by absorption and
fluorescence techniques. The absorption spectral titration of 1-
F− complex with increasing addition of TFA (0−12 equiv) in
CH3CN solution is shown in Figure 9a. Addition of increasing
equivalents of TFA to a solution of 1-F− complex resulted in
the significant increase in the absorption bands at 566 and 428
nm. The sigmoidal nature of the plot of changes in the
absorbance at 566 nm versus different equivalents of TFA
shown as an inset in Figure 9a serves as an indicator of
protonation of the imidazole moiety. Similarly, we also
monitored the systematic titration studies of BODIPY 1-F−
complex upon addition of increasing equivalents of TFA by
fluorescence studies in CH3CN solution. Upon addition of
increasing amounts of TFA to [BODIPY 1-F−] complexes, the
fluorescence emission gradually increased (Figure 9b), and 15-
fold enhancement was observed on addition of 12 equiv of
TFA. Thus, upon F− ion addition, the benzimidazole moiety is
electron-rich via deprotonation and engaged in photoinduced
electron transfer (PET) quenching of the BODIPY excited
state, and upon titration with TFA, the PET quenching is
prevented because of protonation of benzimidazole group and
the absorption and fluorescence properties were reverted
(Scheme 2). We aslo studied the reversibility of the sensor
upon addition of F− ion and TFA in acetonitrile at different
temperatures by employing fluorescence techniques. Initially,
we have recorded the fluorescene spectra of BODIPY 1 upon
addition F− ion at various temperatures ranging from 25 to 60
°C (Figure S15a in SI) and our studies revealed that there is no
significant effect of temperature on quenching of fluorescence
of BODIPY 1. Similarly, we also monitored the revesible
Figure 7. 19F NMR titration spectra of BODIPY 1 (1.6 × 10−2 M)
upon addition of increasing amounts of F− (TBAF) ion (0−5 equiv) in
DMSO-d6.
at −145 ppm appeared with equal intensity. However, upon
addition of 5 equiv of F− ion to BODIPY 1, the signal at −137
ppm completely disappeared and signal at −145 ppm became
−
prominent along with the formation of HF2 ion, which
showed a clear signal at −148 ppm.17 This significant upfield
shift of ∼8 ppm in 19F NMR spectrum clearly supports the
disruption of the intramolecular hydrogen bond between
boron-bound fluoride atoms and benzimidazole NH moiety
upon interaction of BODIPY 1 with F− ion, which is in
1
agreement with the H NMR titration experiment.
We further tested BODIPY 1 for F− sensing by following
changes in the reduction potential of BODIPY 1 upon addition
of increasing amounts of F− by square wave voltammetry. The
systematic changes in the reduction wave of BODIPY 1 upon
increasing addition of F− ion is shown in Figure 8. Progressive
addition of F− ion to BODIPY 1 resulted in a significant
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Figure 9. (a) Absorption and (b) emission spectral changes of BODIPY 1-F− solution (5 μM) upon addition of increasing equivalents of TFA (0−
15 equiv) in CH3CN solution. The spectra were recorded after the addition of TFA to BODIPY 1 for 10 min. Excitation wavelength used was 530
nm.
Figure 10. Fluorescence experiment showing the reversibility and reusability of the BODIPY 1 for sensing F− sequentially: (a) relative fluorescence
intensity (λmax = 592 nm) obtained during the titration of BODIPY 1 with F− and H+ (TFA) in CH3CN solution; (b) visual color changes after each
sequential addition of F− and H+ ions.
phenomena by addition of TFA to a BODIPY 1-F− complex at
different temperatures and studies clearly showed the recovery
of fluorescene (Figure S15b in SI) occur indicating that
BODIPY 1 can efficiently acts as a reversible probe even at
variable temperatures.
and boron-bound fluoride atoms. 1H and 19F NMR also
supported the presence of intramolecular hydrogen bonding in
BODIPY 1. We explored the potential use of benzimidazole
substituted BODIPY for sensing anions by following various
spectroscopic and electrochemical techniques. Our studies
clearly showed that BODIPY 1 can act as a specific sensor for
the detection of F− ion over various other anions. Addition of
F− ions to a solution of BODIPY 1 led to a complete quenching
of fluorescence of the BODIPY moiety accompanied by a
visually detectable color change from fluorescent pink to
nonfluorescent blue with a detection limit of 93 nM.
Competitive binding experiments in the presence of various
anions demonstrate that BODIPY 1 can specifically detect F−
ions. Interestingly, systematic addition of TFA to non-
fluorescent blue solution resulted in a change to fluorescent
pink due to the significant enhancement of fluorescence
intensity supporting the reversible nature of sensor 1. The
reversibility and reusability of BODIPY 1 for sensing the F− ion
has been demonstrated by sequential addition of F− ions
followed by TFA over six cycles, which also indicates that
BODIPY 1 can be used as a reversible and reusable sensor for
F− ion.
The reversible and reusable response of BODIPY 1 was
demonstrated by carrying out four alternate cycles of titration
of BODIPY 1 with F− ion followed by addition of TFA (Figure
10). Titration of BODIPY 1 with F− ion resulted in quenching
of fluorescence due to the formation of [BODIPY 1-F−]
complex, thus acting as an OFF switch. However, titration of
the [BODIPY 1-F−] complex with TFA resulted in protonation
of the benzimidazole moiety and forms BODIPY 1, which is
accompanied by significant increase in the fluorescence
intensity, thus acting as an ON switch (Figure 10a). The
repeated demonstration of the OFF/ON behavior of
fluorescence as well as visual color changes from fluorescent
pink to nonfluorescent blue and then back to fluorescence pink
as shown in Figure 10b demonstrates the reversibility and
reusability of sensor 1.
CONCLUSIONS
■
In summary, we have synthesized benzimidazole substituted
BODIPY 1 from 3,5-diformyl BODIPY 2 under simple reaction
conditions. The presence of the benzimidazole moiety at the 3-
position of BODIPY alters the electronic properties of the dye
significantly. The absorption and emission bands were
bathochromically shifted with decent quantum yield and longer
singlet state lifetime compared to BODIPY 2. The crystal
structure revealed that the BODIPY and benzimidazole
moieties are in the same plane and showed the presence of
intramolecular hydrogen bonding between benzimidazole NH
EXPERIMENTAL SECTION
■
The information regarding “Materials and Methods” and “X-ray
Crystallography” was provided in the Supporting Information.
General Procedure for Acetal Substituted BODIPY Imidazole
(1). A mixture of 3,5-diformyl BODIPY 2 (0.150 g, 1.03 mmol) and
1,2-phenylenediamine (0.05 g, 1.03 mmol) in dry methanol (15 mL)
was warmed until some parts of the solids were dissolved, giving a
bright yellow suspension. Neat TFA (0.1 g, 2.06 mmol) was added
dropwise to yield a dark violet solution. The mixture was stirred for 1
h, and then a few drops of triethylamine were added to neutralize the
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Inorganic Chemistry
Article
excess acid. The aldehydes present at 3 and 5 positions of BODIPY are
highly reactive and electrophilic in nature. Since the reaction was
carried out in methanol as solvent, the methanol acted as a nucleophile
and reacted with one of the formyl groups and converted to acetal
moiety. The crude product was purified using flash basic alumina
column chromatography with petroleum ether/ethylacetate (90:10, v/
v) and afforded pure BODIPY 1 as a golden yellow solid. Yield 38%.
1H NMR (400 MHz, CDCl3, δ in ppm): 2.49 (s, 3H; −CH3), 3.51 (s,
6H; −OCH3), 6.01 (s, 1H; −CH), 6.76−6.78 (d, 3J (H, H) = 4.32 Hz,
1H; py), 6.95−6.96 (d, 3J (H, H) = 4.28 Hz, 1H; py), 7.04−7.05 (d, 3J
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3
(H, H) = 4.52 Hz, 1H; py), 7.34−7.36 (d, J (H, H) = 7.84 Hz, 4H;
Ar), 7.47−7.49 (d, 3J (H, H) = 8.04 Hz, 2H; Ar), 7.59−7.60 (d, 3J (H,
3
H) = 4.44 Hz, 1H; Py), 7.63−7.65 (d, J (H, H) = 7.76 Hz, 1H; Ar),
7.84−7.86 (d, 3J (H, H) = 7.92 Hz, 1H; Ar), 11.08 (br t, 1H; NH). 11
B
1
NMR (128.3 MHz, CDCl3, δ in ppm): 0.31 (t, J (B−F) = 33 MHz,
1B). 19F NMR (376.4 MHz, CDCl3, δ in ppm): −137.7 (q, 1J (F−B) =
33.4 MHz, 2F). 13C NMR (100 MHz, CDCl3, δ in ppm): 14.3, 21.7,
22.9, 29.5, 29.9, 32.1, 54.0, 97.9, 118.4, 122.8, 125.1, 129.4, 130.8,
131.1, 131.9, 135.4, 141.6, 143.5. HRMS. Calcd for C26H24BF2N4O2
[(M+1)+]: m/z 473.1960. Found: m/z 473.1953. Anal. Calcd for
C26H23BF2N4O2: C, 66.12; H, 4.91; N, 11.86. Found: C, 66.27; H,
5.11; N, 11.73.
ASSOCIATED CONTENT
* Supporting Information
■
S
“Materials and Methods” and “X-ray Crystallography” and
characterization data for the synthesized new compound, and
spectroscopic data related to F-ion sensing. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ravikanth@chem.iitb.ac.in.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.R. thanks the Department of Atomic Energy (India) for
financial support and S.M. acknowledge the IIT Bombay for
PhD fellowships.
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