Colorimetric fluoride sensors based on deprotonation of pyrrole-hemiquinone compounds

Article abstract of DOI:10.1039/c001509a

The combination of pyrrole and hemiquinone units produced a novel prototype of efficient colorimetric fluoride sensors, and the presence of the tautomerism effect provides additional means of modulating the sensing behavior.

Full text of DOI:10.1039/c001509a

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Colorimetric fluoride sensors based on deprotonation
of pyrrole–hemiquinone compoundsw
Quanguo Wang, Yongshu Xie,* Yubin Ding, Xin Li and Weihong Zhu*
Received 25th January 2010, Accepted 17th March 2010
First published as an Advance Article on the web 16th April 2010
DOI: 10.1039/c001509a
The combination of pyrrole and hemiquinone units produced a
novel prototype of efficient colorimetric fluoride sensors, and the
presence of the tautomerism effect provides additional means of
modulating the sensing behavior.
Compounds 1 and 2 were readily synthesized and characterized
by 1H, 13C NMR and HRMS (Scheme S1, Fig. S1–S6w). Anion
sensing measurements have revealed that they can selectively
recognize fluoride through vivid color changes. In view of the
unique structures, the easy synthesis and the highly selective
recognition of Fꢀ, 1 and 2 may be developed as a novel prototype
of colorimetric fluoride sensors.
The design and synthesis of anion receptors have become of
increasing interest due to the important roles of anions in
many biological and chemical processes.1 Considerable atten-
tion has been focused on the design of colorimetric hosts that
can selectively recognize anion species through visible color
changes with the advantages that the responses can be con-
veniently detected by the naked eye.2–5
Changes in electronic spectra upon adding various anions
were measured in DMSO. The addition of fluoride induced a
color change from orange to bright blue (Fig. 1), and the peak
centered at 496 nm in the UV-vis spectra (Fig. 2) gradually
decreased, with a new band developed at 568 nm, which can be
ascribed to internal charge transfer (ICT) originated from the
deprotonation of the N–H moiety,15 showing the FHFꢀ signal
Hydrogen containing polar groups, such as ureas and thioureas,
have been widely employed as hydrogen-bond donors in the
design of anion receptors.6 Pyrrole, as a versatile building unit
for macrocyclic dyes, has an NH moiety, which may be utilized to
bind anions. In fact, pyrrole-based macrocyclic dyes are well
documented as cation and anion binding agents.7–9 However, only
a few examples have been reported utilizing easily accessible
acyclic oligopyrroles incorporating chromophores or fluorophores
as colorimetric or fluorometric anion receptors.2j,10,11
1
in the H NMR spectra (Fig.ꢀS7w). In contrast, Clꢀ, Brꢀ, Iꢀ,
CNꢀ, CH3COOꢀ and H2PO4 bind 1 only through hydrogen
bonds, and the addition of these anions did not induce obvious
changes in the color and the UV-vis spectra of 1. The selectivity
for Fꢀ over other anions, especially CNꢀ and CH3COOꢀ, is
important because many reported fluoride sensors suffer from
the deleterious interference of these anions. The excellent
selectivity of 1 for fluoride over other anions is evident from
the pronounced differences in absorbance and color changes.
Compared to 1, sensor 2 has a more complicated struc-
ture containing two kinds of NH protons with different
chemical environments and its sensing behavior showed marked
Among various anions, fluoride has caused intense interest
owing to its important clinical roles and theoretical interest.12
As the most electronegative atom, fluoride can form the
strongest hydrogen bond with protons and tend to deprotonate
polar NH groups. Thus, various colorimetric sensors containing
NH arrays have been developed to detect fluoride based on the
deprotonation mechanism.13
Recently, we have developed anion receptors with various
binding moieties.14 In this work, we designed acyclic oligopyrrole–
hemiquinone compounds 1 and 2 (Scheme 1) as fluoride
sensors utilizing the conjugated hemiquinone–pyrrole struc-
tures as novel chromophores and the oligopyrrole moieties as
the recognition site.
Fig. 1 A photograph showing the color change of 1 (25 mM) in
DMSO upon addition of 40 eq. of various anions.
Scheme 1 Structures of sensors 1 and 2.
Key Laboratory for Advanced Materials and Institute of Fine
Chemicals, East China University of Science and Technology,
Shanghai, P. R. China. E-mail: yshxie@ecust.edu.cn;
Fax: (+86) 21-6425-0772
w Electronic supplementary information (ESI) available: Full experi-
mental and crystallographic data and Fig. S1–S13. CCDC 740202. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c001509a
Fig. 2 UV-vis spectral changes of 1 (10 mM in DMSO) observed upon
the addition of 0–180 eq. Fꢀ (TBA salt) in DMSO.
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Scheme 2 Tautomerism of sensors 1 and 2.
When we further checked the solvent-dependence of the
sensing behavior, interesting solvent-induced tautomerism was
observed for both 1 and 2 (Scheme 2). In CDCl3, the molar
fractions of isomers 1 and 1a are 88.5 and 11.5%, respectively
(Fig. S16w). With the addition of DMSO-d6, the peaks for 1a
gradually disappeared to afford the spectra of pure 1, and the
NH peaks were shifted downfield, indicative of intermolecular
hydrogen-bonding interactions, which may be the driving
force for the tautomerism shift.17 The conversion of 1a into
1 with increasing solvent polarity is consistent with DFT
calculations, which have indicated that the energy difference
between 1 and 1a in polar solvents are much larger than that in
nonpolar solvents (Table S2w). Molar fractions of 2 and 2a in
CDCl3 are 68.2 and 31.8%, respectively (Fig. S17w). Similar
tautomerism behavior was also observed when DMSO-d6
was added.
Fig. 3 UV-vis spectral changes of 2 (10 mM in DMSO) observed upon
addition of Fꢀ (TBA salt) in DMSO.
Anion binding behavior of 1 and 2 in CHCl3 and CH2Cl2
shows a significant difference from that in DMSO due to the
coexistence of the two tautomers. For example, when chloride
was added to a DMSO solution of 2, the hydrogen bonding
interactions induced very small electronic spectra changes
(Fig. S18w), and the NH peaks in the 1H NMR spectra shifted
very little (from 11.45 to 11.54 ppm) when 1 equivalent of Clꢀ
was added to a DMSO-d6 solution of 2 (Fig. S19w). In
contrast, when chloride was added to a CH2Cl2 solution of
2, the binding was accompanied with a tautomerism shift,
showing much larger electronic spectra changes (Fig. S18w),
difference from that of 1. The spectral changes of a DMSO
solution of 2 upon addition of Fꢀ are shown in Fig. 3. The
initial addition (o 40 eq.) resulted in a decrease of the bands at
459 and 571 nm and an increase of a new band at 740 nm,
meanwhile the color changed from wine-red to gray. Upon
further addition (>40 eq.), the band at 740 nm decreased and
a new band developed at 624 nm. The solution color further
changed from gray to green (Fig. S8w), which can be ascribed
to the second deprotonation process. The blue shift from
740 to 624 nm can be rationalized by the different energy levels
for the mono- and di-deprotonated species, as indicated by the
DFT calculations (Fig. S9 and Table S1w). Clꢀ, Brꢀ, and Iꢀ can
be readily discriminated from Fꢀ based on the fact that no
obvious change in the UV-vis spectra could be observed even
when large equivalents of these anions were added.
1
and the signal of the peripheral NH protons of 2 in the H
NMR spectra shifted significantly downfield from 8.60 to
12.31 ppm (Fig. 4), indicative of the binding at these NH
moieties.
The proposed binding of chloride at the peripheral NH
moieties was further evidenced by X-ray diffraction analyses of
a single crystal of 2ꢃ2MeOHꢃH2O (Fig. 5).z In this crystal, the
O atom of water was hydrogen-bonded to two NH donors.
For the quinone oxygens O1 and O2, the C–O bond lengths
are 1.255(5) and 1.247(5), respectively, indicative of CQO
On the other hand, the addition of CNꢀ, CH3COOꢀ or
ꢀ
H2PO4 to a DMSO solution of 2 could induce only mono-
deprotonation, with a new band developed at 740 nm (Fig. S10w).
The difference between these anions and Fꢀ may be ascribed to
the unique stability of the hydrogen bonded complex
[HF2]ꢀ.13a,16 Thus, Fꢀ may be discriminated from these anions
based on the further color change from gray to green caused by
dideprotonation. These observations are in sharp contrast to the
fact that Fꢀ could induce only monodeprotonation of 1. This
may be ascribed to the difference in NH acidity between 1 and 2,
i.e., the molecule of 1 contains only one electron-withdrawing
hemiquinone group, whereas, 2 has two hemiquinone groups,
which induce stronger acidity. The detection limits for Fꢀ in
DMSO are 2 ꢂ 10ꢀ4and 2 ꢂ 10ꢀ5 M for 1 and 2, respectively.
When H2O-containing solvents such as DMSO–H2O, were
used, similar sensing behavior (Fig. S11–Fig. S15w) was ob-
served, but the deprotonation processes are associated with
larger equivalents of Fꢀ due to the competition effect of H2O,
as observed for most fluoride chemosensors based on the
H-bonding mechanism.
Fig. 4 1H NMR spectral changes of 2 (6.4 mM) upon addition of
chloride anion (TBA salt) in CDCl3.
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z Crystal data for 2ꢃ2CH3OHꢃH2O: C44H61N3O5, Mr = 711.96 g molꢀ1
,
0.45 ꢂ 0.43 ꢂ 0.40 mm3, monoclinic, P21/c, a = 9.9033(8), b =
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298(2) K, 18262 data measured on a Bruker SMART Apex diffracto-
meter, of which 7471 were unique (Rint = 0.0678); 484 para-
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2
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ꢁc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 3669–3671 | 3671