Benzoselenadiazole fluorescent probes-near-ir optical and ratiometric fluorescence sensor for fluoride ion

Article abstract of DOI:10.1021/ol403082p

A highly selective and sensitive near-IR optical sensor, benzoselenadiazole based diarylamine (TBS-HN), for fluoride (F-) has been designed and synthesized. TBS-HN also shows turn-on ratiometric fluorescence signaling in the presence of F-by inhibiting th

Full text of DOI:10.1021/ol403082p

Letter
pubs.acs.org/OrgLett
Benzoselenadiazole Fluorescent Probes − Near-IR Optical and
Ratiometric Fluorescence Sensor for Fluoride Ion
Chinnusamy Saravanan,† Shanmugam Easwaramoorthi,‡ Chuen-Yo Hsiow,† Karen Wang,†
Michitoshi Hayashi,† and Leeyih Wang*,†
†Centre for Condensed Matter Sciences, National Taiwan University, Taipei, 10617 Taiwan (R.O.C.)
‡Chemical Laboratory, CSIR- Central Leather Research Institute (CLRI), Adyar, Chennai 600 020, India
S
* Supporting Information
ABSTRACT: A highly selective and sensitive near-IR optical
sensor, benzoselenadiazole based diarylamine (TBS-HN), for
fluoride (F−) has been designed and synthesized. TBS-HN
also shows turn-on ratiometric fluorescence signaling in the
presence of F− by inhibiting the excited state intramolecular
proton transfer (ESIPT) processes.
he design and synthesis of artificial receptors that are able
signaling pathways, the ESIPT based sensors have a significant
Tto sense anions by the naked eye has currently been advantage due to the larger Stokes shift, ratiometric
fluorescence between the ESIPT and S0←S1 emission. Many
F− sensors have been reported using the inhibition of the
ESIPT process previously.13
evolving as a forefront research topic because of the significant
role of anions in the broad range of chemical and biological
processes.1 Among the anions, fluoride (F−) is an attractive
target for sensor design owing to its role in dental care,
osteoporosis, skeletal fluorosis, etc.2 So far the reported
molecules that sense F− can broadly be classified into four
types based on the nature of molecular level interaction
between the receptor and F−: (1) suitably preorganized N−H,
C−H, and O−H groups which are capable of sensing anions
through hydrogen bonding interactions,3 (2) recognition
through anion−π interactions,4 (3) Lewis acid−base inter-
actions,5 and (4) anion induced chemical reactions.6 Notably,
the H-bonding interactions between the N−H/O−H fragment
of the receptor and the F− ion are observed only at lower F−
concentrations; however an excess of F− ion leads to
deprotonation by means of Brønsted acid−base interactions,
which is also an efficient and sensitive method.7 In this case, the
receptors generally are comprised of a highly acidic N−H
proton, which is expected to facilitate spontaneous F− induced
deprotonation. On the other hand, diarylamine based receptors,
to the best of our knowledge, have not been reported so far for
this purpose. The N−H proton becomes weakly acidic due to
the strongly electron-donating N-aryl groups, which eventually
prevents F− sensing through deprotonation.
Although anion sensing by means of deprotonation of the
N−H proton of diarylamine is rather difficult, appropriate
modification of the aryl group with judicious choice of an
electron-withdrawing substituent would probably increase the
acidity of N−H proton, which in turn can be used for sensing
applications. Hence, we have synthesized a diarylamine
containing a strongly electron-withdrawing 2,1,3-benzoselena-
diazole moiety, 7-(4-hexylthiophen-2-yl)-N-(4-nitrophenyl)-
benzo[c][1,2,5]selenadiazol-4-amine (TBS-HN), as a receptor
for F−. Molecules with 2,1,3-benzoselena- and benzothia-
diazoles are known to show intense fluorescence, large molar
extinction coefficients, and good photostability.14 Particularly,
upon introduction of these electron-withdrawing moieties into
the low band gap polymers, a clear ICT absorption band was
observed in the visible region.15 Hirao et al. demonstrated that
benzothiadiazole-aniline oligomers have a hydrogen bonding
interaction between the NH proton and nitrogen in the
benzothiadiazole moiety.16 Notably, in the presence of F−, the
oligomers undergo reduction followed by substitution and form
the fluoride substituted polyaniline rather than the deprotona-
tion of NH protons.17 In the present investigation, we have
shown that TBS-HN senses the F− ion with high sensitivity and
selectivity through the inhibition of the ESIPT process.
Interestingly, it also shows ratiometric signaling of fluorescence
for the fluoride ion.
The signaling mechanism for sensing could be the UV−
visible absorption and fluorescence spectral changes upon
modulations in the photoinduced electron transfer (PET),8
intramolecular charge transfer (ICT),9 excimer/exciplex for-
mation,10 metal-to-ligand charge transfer (MLCT),11 and
excited-state intramolecular proton transfer (ESIPT)12 pro-
cesses before and after F− binding. Among the reported
Received: October 29, 2013
© XXXX American Chemical Society
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The TBS-HN was synthesized as shown in Scheme 1 by
following the method previously reported for the preparation of
emission peak at 709 nm remains quite insensitive to the
excitation energy except the relative intensity. This feature
suggests the presence of at least two species, viz. neutral and
zwitter ionic tautomers that are responsible for the observed
excitation energy dependent fluorescence in polar solvents. In
contrast, in nonpolar solvent toluene, fluorescence from TBS-
HN is independent of the excitation energy (SI, Figure S3).
Further, replacing the NH proton by a methyl group (TBS-
MN) in the diarylamino group resulted in the excitation energy
independent fluorescence, both in polar, acetonitrile, and
nonpolar, toluene, solvents (SI, Figure S4). A similar
phenomenon has already been reported for N-(2-(benzo[d]-
oxazol-2-yl)phenyl)-4-methyl benzene sulfonamide, which also
shows the ESIPT process.13a Thus, based on the above facts we
can conclude that the TBS-HN exhibits the ESIPT process and
also possesses significant intramolecular charge transfer
interactions.
Scheme 1. Synthesis of TBS-HN
phenylenediamines.18 Briefly, refluxing TBS with 4-nitroaniline
in toluene in the presence of Pd2(dba)3, dppf, and t-BuONa
afforded TBS-HN in quantitative yield. The TBS-MN, an N-
methyl analogue of TBS-HN, was obtained in 96% yield
through the reaction between TBS-HN and 7 equiv of tert-
butylammonium fluoride in DMSO followed by the addition of
2 equiv of CH3I. The structure of the TBS-HN and TBS-MN
1
was confirmed by the H and 13C NMR and HRMS spectral
The sensing ability of the TBS-HN was tested in DMSO
solvent using various anions as their tetrabutylammonium salt.
Upon addition of about 100 equiv of F− to the TBS-HN
solution, the intense red color turned into dark blue as shown
in Figure 1a, which in fact can be observed by the naked eye.
techniques (Supporting Information (SI), Figures S12−S19).
The UV−visible absorption spectra of TBS-HN dissolved in
toluene show intense peaks at 344, 383, and 520 nm (SI, Figure
S1). The time-dependent density functional theory calculations
using the Gaussian 09 program19 at the B3LYP level with the
PCM model suggest that the lowest electronic transition
originated mostly from the highest occupied molecular orbital
(HOMO) to the lowest unoccupied molecular orbital (LUMO)
(Table S1). The major contribution to the absorptions at 383
and 520 nm originated through the charge transfer transition
from the thiophene→nitro and thiophene→benzoselenadiazole
groups, respectively. This feature suggests that TBS-HN
possesses a substantial charge transfer character, which is a
desired factor for anion sensing through alternations in PET
processes. Further, the fluorescence spectra of TBS-HN
measured in toluene solvent show two emission peaks at 662
and 705 nm (shoulder), which in fact do not have a mirror
image relationship with the absorption spectrum (SI, Figure
S1). The larger Stokes shift value ca. 4125 cm−1 (142 nm)
suggests significant structural changes between the ground and
excited states upon photoexcitation. Indeed, both of the above
parameters suggest the possibility of intramolecular proton
transfer processes from the Franck−Condon state. This
Figure 1. Visible color changes (a) and the corresponding absorption
spectra (b) of TBS-HN (3 × 10−5 M) in the presence of 100 equiv of
various anions (as their TBA salts) in DMSO.
1
−
−
−
interpretation has been further supported by the H NMR
Importantly, other anions Cl−, Br−, I−, HSO4 , H2PO4 , NO2 ,
CH3COO− have no impact on the color of TBS-HN. We have
further evaluated the sensing behavior using UV−visible
absorption and fluorescence spectroscopy. As can be seen in
Figure 1b, the addition of F− to TBS-HN resulted in the
appearance of intense new bands at 498 nm and at 691 nm at
the expense of a peak at 402 nm. Addition of F− to TBS-HN
shifts the low-energy absorption band bathochromically by 181
nm. The observed excellent sensitive and selective detection of
F− over the other anions with a larger spectral shift and the
appearance of a new peak close to the near-IR region are
certainly advantageous for their applications in biological
systems. To understand the nature of anion−receptor
interaction, UV−visible titration experiments were carried out
with the incremental addition of F− to the DMSO solution of
TBS-HN. Upon progressive addition of F−, a new band at 691
nm appeared with clear isosbestic points at 369, 439, and 535
nm (SI, Figure S5). The appearance of an isosbestic point
clearly defines the existence of only two species, i.e., a free and
F− bound receptor. The observed spectral changes are
attributed to the interaction between the F− ion with the
amino proton of TBS-HN through hydrogen bonding
data, which indicate the intramolecular hydrogen bonding
between the amino-hydrogen with the imine nitrogen on the
benzoselenadiazole moiety. This forms a strain-free, five-
membered ring including the hydrogen bond, an ideal factor
that facilitates the proton transfer processes.20 In addition, the
reduced electron density at the amino nitrogen in the LUMO
weakens the NH bond, making the proton dissociation more
facile, while the relatively higher electron density at the
benzoselenadiazole nitrogen attracts the proton.
Since the ESIPT occurs in the femto- to picosecond time
scale, the observed fluorescence from the ESIPT chromophore
is expected to be from the zwitter ionic tautomer (SI, Scheme
S2). Nonetheless, ESIPT strongly depends on conformations,
the nature of the solvent medium, and its ability to form
intermolecular hydrogen bonds between the chromophore.21
To our surprise, the fluorescence spectrum changes with the
excitation wavelength, particularly in the polar solvent
acetonitrile: The emission band shifted from 667 to 617 nm,
when the excitation wavelength changes from 380 to 440 nm.
However further red-shifted excitation energy shifts the
emission from 617 to 667 nm (SI, Figure S2). Notably, the
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Letter
interactions. Since the N−H proton becomes strongly acidic as
the amino nitrogen is bonded to the strongly electron-
withdrawing 4-nitrophenyl and benzoselenadiazole moiety,
the addition of excess F− would lead to the deprotonation of
the N−H proton (Scheme 2).
TBS-HN can also act as a fluorimetric sensor as
demonstrated by the fluorescence titration experiments in
DMSO. In the absence of an anion, TBS-HN exhibited strong
emission at 671 nm by exciting the solution at 440 nm. The
gradual addition of F− quenches the fluorescence at 671 nm
with a concomitant, progressive, enhancement of a new band at
478 nm. The fluoride induced deprotonation induces the
disappearance of normal and tautomer fluorescence and favors
the new emission from the deprotonated state. Thus, the
sensing mechanism in this case would be the inhibition of
ESIPT processes. A ratiometric fluorescence response can be
derived by plotting the intensities at two different wavelengths
Scheme 2. Schematic Representation of Interaction of TBS-
HN with F− in DMSO
I478/I671 which is calculated to be 25
1 (Figure 3). The
The charge transfer interactions such as thiophene→nitro
and thiophene→benzoselenadiazole groups can be nullified
after the deprotonation of the N−H proton due to the
delocalization of the negative charge toward the acceptor end,
but the absorption is tailing over the near-IR region. This can
be explained on the basis of theoretical predictions that the shift
in absorption maxima occurs only because of alternations in the
electronic energy levels after deprotonation (SI, Table S2).
To understand the interaction between the F− and receptor
1
further, H NMR titration experiments were carried out in
DMSO-d6. Figure 2 shows the partial 1H NMR spectra of TBS-
Figure 3. Fluorescence spectra of TBS-HN (3 × 10−5 M) in the
presence of TBAF (0.06 M) in DMSO. Insets: ratios of fluorescent
intensities at 478 and 671 nm as a function of F− concentration.
fluoride ion detection limit of TBS-HN was found to be in the
range of the submillimolar level. Importantly, similarly to
absorption spectral changes, the fluorescence spectra of TBS-
HN remain unaltered by the other anions such as Cl−, Br−, I−,
HSO4 , H2PO4 , NO3 , and CH3COO−. From these
observations it can be concluded that TBS-HN could sense
F− selectively by colorimetric and ratiometric fluorescence
methods (SI, Figure S6).
−
−
−
1
Figure 2. Partial H NMR titration spectra of TBS-HN in DMSO-d6
in the presence of various equivalents of TBAF.
We have synthesized TBS-HN, which exhibits ESIPT and
CT processes. It senses the F− with high sensitivity and
selectivity by means of optical, and off-on, ratiometric,
fluorescence methods. In the presence of F−, TBS-HN
develops a new absorption band in the near-IR region, which
is beneficial in terms of using this sensor in a biological
medium. We believe that the combination of near-IR optical,
turn on, ratiometric fluorescence and the involvement of the
ESIPT process in a single system is certainly a promising
feature in the sensing of anions.
HN upon addition of F− as tetrabutylammonium fluoride
(TBAF) in DMSO-d6. The N−H signal completely disappears
when the concentration of F− is 0.4 equiv; however, the
chemical shift values of aromatic protons remain unaltered (SI,
Figure S9). Further gradual addition of F− steadily shifts the
aromatic protons to the upfield region until 6 equiv (Figure 2).
Meanwhile, a new peak, characteristic of [HF2]−, started to
appear at 16.5 ppm (triplet) in the presence of 2 equiv of F−
(SI, Figure S11). This indicates that F− interacts with TBS-HN
through hydrogen bonding at lower concentrations, but at
higher concentrations it induces the deprotonation of the N−H
proton. After deprotonation, the lone pair electron on the
nitrogen has two possible ways to migrate as shown in Scheme
S3 (SI). As shown in Figure 2, the magnitude of upfield shifting
of protons in nitrobenzene is higher than that of the
benzoselenadiazole ring, which suggests that the negative
charge is favorably localized on the nitrobenzene rather than
the benzoselenadiazole (SI, path B, Scheme S3). According to
the NMR spectra of TBS-HN before and after the addition of
F−, the positions of the resonance signals of the protons were
assigned (SI, Figures 20−25).
ASSOCIATED CONTENT
* Supporting Information
■
S
The experimental details, synthetic procedures, characterization
data, computational data, UV−vis and fluorescence data. This
materials is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: leewang@ntu.edu.tw.
Notes
The authors declare no competing financial interest.
C
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ACKNOWLEDGMENTS
■
The authors thank the National Science Council of Taiwan,
Republic of China (NSC 102-2113-M-002-003-MY3) and
suprainstitutional project “STRAIT” of CSIR-CLRI for financial
support. This manuscript is CSIR-CLRI Contribution No.
1020.
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