Salt (LiF regulated fluorescence switching

Article abstract of DOI:10.1002/ejoc.201100359

A supramolecular host-guest complex stimulated by lithium cation and fluoride anion, functioning as reliable fluorescence "Off-On-Off" switching system has been developed. Nonfluorescent, receptor-bound fluorophore is exclusively displaced by second guest, fluoride anion to release thefluorophore that becomes highly fluorescent in guest concentration dependent way. Again, this process is completely reversed upon exposure to Li⁺. The entire process is an excellent model for fluorescence-based logic system. A reliable fluorescence "Off-On-Off" switching system regulated by lithium cation and fluoride anion has been developed.

Full text of DOI:10.1002/ejoc.201100359

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201100359
Salt (LiF) Regulated Fluorescence Switching
Punidha Sokkalingam,[a] Jaeduk Yoo,[a] Hyonseok Hwang,[a] Phil Ho Lee,[a]
Young Mee Jung,[a] and Chang-Hee Lee*[a]
Keywords: Luminescence / Fluorescence switching / Fluorescence indicator / Calix[4]pyrrole / Coumarin / Lithium fluoride
A supramolecular host-guest complex stimulated by lithium
cation and fluoride anion, functioning as reliable fluores-
cence “Off-On-Off” switching system has been developed.
Nonfluorescent, receptor-bound fluorophore is exclusively
displaced by second guest, fluoride anion to release the
fluorophore that becomes highly fluorescent in guest concen-
tration dependent way. Again, this process is completely re-
versed upon exposure to Li+. The entire process is an excel-
lent model for fluorescence-based logic system.
tive method for sensing anions even in biological systems.[9]
In fact, many fluorescent indicators combined with various
macrocyclic receptors have been successfully applied along
these lines.[10] However, a viable F– sensor based on the F-
IDA approach has yet to be described. In an early work by
Gale, Twyman, and Sessler, calix[4]pyrrole, a tetrapyrrolic
macrocycle with a well-recognized ability to bind certain
anions in organic media, was exploited for fluoride anion
detection using a simple colorimetric IDA.[11] However, this
system requires the use of excess indicator and thus dis-
played low sensitivity. This issue led us to target an F– sen-
sor that would display a sensitive “Turn-On” fluorescence
upon exposure to fluoride anion. Although there are a few
reports that describe a fluorescence-based anion sensors in
so-called “Turn-Off” fashion,[12] to the best of our knowl-
edge, no calix[4]pyrrole-based receptors that display a fluo-
rescence “Turn-On” response to F– have been reported.
Here, we report a simple-to-construct, calix[4]pyrrole-
based, “Turn-On” F-IDA sensor that facilitates detection
of F– with unprecedently high sensitivity and selectivity. At
the same time, this fluorescence “Turn-On” behavior can
be completely reversed (into “Turn-Off) upon exposure to
lithium cation (Li+). This fluorescence “Off-On” process
can be modulated repeatedly by Li+ and F– without di-
minished fluorescence intensity. The present system thus
functions as a novel molecular On/Off switch whose func-
tion is regulated via the choice of added anions and cation.
Such ion-switched systems are rare, but beginning to attract
on account of some very recent attention.[13]
Introduction
The development of stimuli-controlled supramolecular
logic devices is a challenging task since they require molecu-
lar systems that provide a differential recognition for vari-
ous substrates. Molecular logic, in which initiation of event
is followed by recognition, has been demonstrated with
various systems including rotaxanes,[1] molecular grid[2] and
basic logic devices.[3] However, it is seldom that the input-
output signal is completely controlled by inorganic salt. In
recent years, anions, particularly inorganic ones are gaining
increasing attention for their pivotal role in medicine, bio-
logy[4] and environmental science.[5] Consequently, enor-
mous efforts have been devoted to designing and synthesiz-
ing chemosensors so as to achieve selective recognition and
sensing of anions. Among the anions of biological interest,
fluoride anion (F–) has attracted special attention due to its
potential toxicity in various living systems, especially hu-
mans.[6] Although much progress has been made in the area
of F– recognition and sensing, there are only few easy-to-
use chemosensors that operate with high sensitivity. Indeed,
it remains a considerable challenge to produce receptors
that convert a given binding event into a readable signal,
particularly the one that translates the recognition event
into an increasing easy-to-monitor fluorescence intensity.[7]
One simple and seemingly attractive approach to producing
such a sensor for the fluoride anion involves the use of an
“Indicator Displacement Assay” (IDA). This approach, pi-
oneered by Anslyn and co-workers[8] can be modified to
allow “Turn-On” detection via the use of appropriately cho-
sen fluorophores or chromophores. The Fluorescence-IDA
(F-IDA) approach has been recognized lately as more sensi-
Results and Discussion
[a] Department of Chemistry and Institute of Molecular Science &
Fusion Technology, Kangwon National University,
Chun-Chon 200-701 Korea
Our system consists of a modified calix[4]pyrrole bearing
cis-4-fluorophenyl pickets (1) as the receptor and 2-oxo-4-
E-mail: chhlee@kangwon.ac.kr
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100359.
(trifluoromethyl)-2H-chromen-7-olate (2–) as
a highly
fluorescent indicator (Scheme 1). When the highly fluores-
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C.-H. Lee et al.
SHORT COMMUNICATION
cent chromenolate anion 2– was bound to the calix[4]pyr- was observed (Figure 1, a) due to the formation of a strong
role cavity through hydrogen donding, it became completely receptor-indicator complex [1·2–]. When, the complex [1·2–]
nonfluorescent presumably as the result of photo-induced was titrated with F–, complete recovery of fluorescence was
electron transfer (PET) from calix[4]pyrrole to bound 2–. observed (Figure 1, b) indicating that the binding affinity
Direct interaction between p-fluoro moieties of calix[4]pyr- of fluoride anion with receptor 1 is stronger than that of
role and chromenolate could be a contributing factor for with 2–. The complete recovery of the original spectral fea-
the quenching. If the binding affinity of an analyte anion is tures (1000-fold increase) is ascribed to the F–-driven dis-
comparable with 2–, then the bound 2– can be displaced by placement of the chromenolate 2– from the calix[4]pyrrole
the analyte anion that will eventually result in recovering binding site via the formation of a competitive receptor-
the original fluorescence of 2–. To exploit this supramolec- fluoride complex [1·F–]. The fluorescence enhancement was
ular displacement event for more useful analyte anion de- found to be linear at low F– concentrations (0–100 nm, Fig-
tection, the relative binding affinities of 2– and the anionic ure S14). The detection limit of 2.3 ppb was calculated;
analyte would have to be appropriately tuned; specifically, such a detection limit represents a dramatic improvement
the chromenolate anion would need to have sufficient affin- in sensitivity relative to the prior reported system.[8] Consis-
ity with 1, but still display an affinity that was slightly lower tent with fluorescence studies, the formation of receptor-
than that of analyte anion. Gratifyingly, these expectations indicator complex [1·2–] was initially verified by a blue shift
were realized as described further below. The identity of the of the λmax from 433 nm to 390 nm observed in UV/Vis
receptor 1 was verified by standard spectroscopic methods spectroscopic studies. Addition of F– into complex [1·2–]
as well as by single-crystal X-ray crystallography (SI). The produced a red-shifted absorption spectrum (390 nm Ǟ
electron withdrawing p-fluorophenyl groups present in 1 are 433 nm), which is consistent with that of free 2–. These
expected to create a suitable binding cavity and contribute events were further supported by structural analysis ob-
for enhanced binding affinity.[14] Proton NMR titration tained from single-crystal X-ray crystallography (SI) for
indicated that calix[4]pyrrole 1 forms stable complex with [1·F–] complex. Unfortunately, attempted growing crystal of
chromenolate 2– with a receptor/guest stoichiometry of 1:1 complex [1·2–] was not successful.
(SI). As shown in Figure 1, when highly fluorescent solu-
In order to verifiy the selectivity towards F–, we exposed
tion of chromenolate 2– was titrated with receptor 1 in ace- [1·2–] complex to the other anions including Cl–, Br–, I–,
tonitrile, receptor-dependent quenching of the fluorescence CN–, SCN–, AcO–, NO3 , PhCO2 , PF6 , HP2O73–, HSO4 ,
–
–
–
–
Scheme 1. Formation of strong supramolecular complex between calix[4]pyrrole (1) and chromenolate anion [2–, as tetrabutylammonium
(TBA) salt].
Figure 1. (a) Fluorescence quenching of fluorescent indicator 2– (2.1 μm) upon titration with host 1 in acetonitrile at λex = 410 nm; inset
shows Stern–Volmer plot of the fluorescence quenching (KSV = 4.78ϫ106). (b) Recovery of fluorescence upon titration with F– (as its
tetrabutyl ammonium salt) in acetonitrile at λex = 410 nm, [1] = 5.7 μm, [2–] = 2.1 μm; inset shows a plot of I/I0 vs. TBAF concentration.
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Salt (LiF) Regulated Fluorescence Switching
and H2PO4 . No detectable changes either in fluorescence [1·F–] and 2–. It is important to note here that during the
–
or absorbance were observed, suggesting that the displace- titration of 1 or [1·2–] with F–, the N-H signals were shifted
ment of receptor-bound indicator 2– is only possible by downfield with a doublet splitting but not broadening at
fluoride anion. Moreover, when complex [1·2–] was exposed all, thus, virtually ruling out the possibility of deproton-
to a mixture of anions containing Cl–, Br–, NO3– and AcO–, ation (SI). No significant spectral changes were observed in
no appreciable changes in fluorescence was observed, but a the 1H NMR spectrum upon addition of other anions other
strong fluorescence emission appeared when the mixture than fluoride to the complex [1·2–] (SI). Fortuitously, all
was loaded with F–, indicating that [1·2–] complex is highly these observations were in good agreement with the absorp-
specific towards F– even in the presence of mixture of tion and fluorescence results noted above.
anions.
To explore counter cation effects on this sensing process,
we tested the effect of metal ion to the fluorescence changes.
Surprisingly, addition of Li+ (as its perchlorate salt) to the
highly fluorescent mixture of [1·F–] and [2–] resulted in com-
plete quenching of the fluorescence which was analogous to
what was seen in the case of mixing of receptor 1 with 2–.
This result can be rationalized by the disappearance of the
host-bound fluoride anion complex (i.e., [1·F–]) and the ap-
pearance of host-bound chromenolate complex [1·2–] as the
result of re-displacement of bound F– by chromenolate 2–
(Figure 2). Decomplexation of F– from receptor 1 by Li+ is
ascribed to the stronger ion-pairing interaction between F–
and Li+ which suggests that the current host-indicator com-
plex can be considered as a molecular “On-Off” switching
system controlled by inorganic salts (F– and Li+)
Figure 2. Quenching of fluorescence emission of the solution con-
(Scheme 2). In agreement, the red-shifted absorption spec-
–
taining [1·F– + 2–] upon titration with Li+ (as its ClO4 salt) in
trum corresponding to mixture of [1·F–] and [2–] was again
acetonitrile. [1] = 5.7 μm, [2–] = 2.1 μm, [TBAF] = 6.9 μm; λex
=
moved back to original spectrum pertaining to [1·2–] upon
addition of Li+ (SI). It is important to note here that the
solubility of LiF in acetonitrile has been found to be
90 μm.[15] Since we have used considerably smaller amounts
410 nm.
In order to determine the energetics associated with the
of F– and Li+ [6.9 μm F–, 8.6 μm Li+ for fluorescence ti- formation of [1·2–] and [1·F–], Isothermal Titration Calo-
tration (Figure 2) and, 40 μm F–, 47.6 μm Li+ for UV/Vis rimetry (ITC) and UV/Vis spectroscopic measurements
titration (Figure S17)], we did not see LiF getting precipi- were performed. The obtained binding isotherm for the
tated out from the solution. 1H NMR titration studies con- binding of receptor 1 with 2– and 1 with F– displayed clean
ducted in CD3CN further corroborated this entire switching exothermic 1:1 stoichiometric binding for the formation of
process. For example, the pyrrole N-H signal of free host 1 either [1·2–] or [1·F–]. As anticipated, the affinity constant
was shifted from δ = 7.80 ppm to 11.43 ppm upon addition for the formation of [1·F–] (K = 5.96ϫ106 m–1) was slightly
of 1.04 equiv. of chromenolate anion 2– indicating the 1:1 larger than that of the formation of [1·2–] (K
=
stoichiometric association of receptor 1 with 2–. When F– 4.69ϫ106 m–1), providing a basis for the observed prefer-
(as its tetrabutyl ammonium salt) was added into the solu- ence for the formation of [1·F–]. The receptor 1 displayed a
tion of complex [1·2–], on the other hand, the N-H signal much larger binding affinity with 2– than does simple meso-
was shifted further downfield, i.e, from δ = 11.43 ppm to octamethyl calix[4]pyrrole (K = 6.66ϫ104 m–1). This is in
12.56 ppm, indicating displacement of 2– by F– to form the agreement with design expectations and serves to confirm
new complex [1·F–]. Then, the reformation of complex [1·2– that receptor 1 is superior receptor as far as the anionic
] was achieved upon addition of Li+ into the mixture of guests involved in the present study are concerened.
Scheme 2. Schematic representation of reversible fluorescence “Off-On-Off” switching of supramolecular complex [1·2–] by F– and Li+.
Eur. J. Org. Chem. 2011, 2911–2915
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C.-H. Lee et al.
SHORT COMMUNICATION
Since nonfluorescent solutions of complex [1·2–] become
highly fluorescent on addition of stoichiometric quantities
of the fluoride anion and complete quenching of the
fluorescence is possible upon addition of lithium ion, the
current system could be a good model for molecular logic
controlled by specific cation and anion. Indeed, the fluores-
cence can be completely switched “On” and “Off” via re-
versible displacement (Figure 3). The cycle can be repeated
over many times starting with chromenolate 2– followed by
Chemical shifts are reported in parts per million (ppm). When peak
multiplicities are given, the following abbreviations are used: s, sin-
glet; br. s, broad singlet; d, doublet; t, triplet; m, multiplet. 13C
NMR spectra were proton decoupled and recorded on a 100 MHz
Bruker spectrometer using TMS as the internal standard. Fluores-
cence spectra were recorded using LS-55B (Perkin–Elmer) model
spectrometer. Pyrrole was distilled at atmospheric pressure from
CaH2. All other chemicals and solvents were purchased from com-
mercial sources and were used as such, unless otherwise mentioned.
Column chromatography was performed over silica gel (Merck,
addition of receptor 1 to form nonfluorescent complex 230–400 mesh). All titrations (UV/Vis, fluorescence and ITC) were
[1·2–] without loss of fluorescence intensity. Addition of F–
performed using HPLC grade CH3CN purchased from Aldrich.
to the solution of the complex fully reinstates the original
fluorescence and subsequent addition of Li+ results in a
complete quenching of the fluorescence once again. The re-
sponse time of this On/Off operation is instantaneous with
the emission intensity of the “On” state remaining virtually
the same across repeated cycles.
Synthetic Procedures
Tetrabutylammonium 2-Oxo-4-(trifluoromethyl)-2H-chromen-7-ol-
ate (2–): Commercially available 7-hydroxy-4-(trifluoromethyl)cou-
marin 2 (0.1 g, 0.43 mmol) in MeOH (10 mL) was added into a
solution of tetrabutylammonium hydroxide (0.17 g, 0.65 mmol) in
MeOH (10 mL) at room temperature. After addition was complete,
stirring continued at room temperature for 30 min. The solvent was
then removed in vacuo and the residue triturated with Et2O af-
forded the tetrabutylammonium salt of coumarin 2– as a yellow
powder that was dried under high vacuum and gave satisfactory
spectroscopic data; yield 0.15 g (73%). 1H NMR (300 MHz,
CD3CN): δ = 7.25 (d, J = 2.0 Hz, 1 H, coumarin-H), 6.37 (d, J =
2.0 Hz, 1 H, coumarin-H), 6.12 (s, 1 H, coumarin-H), 5.94 (s, 1 H,
coumarin-H), 3.10–2.99 (m, 8 H, CH2), 1.64–1.54 (m, 8 H, CH2),
1.38–1.33 (m, 8 H, CH2), 0.96 (t, J = 7.18 Hz, 12 H, CH3) ppm.
1H NMR (400 MHz, CDCl3) δ = 7.32 (d, J = 2.0 Hz, 1 H, couma-
rin-H), 6.50 (d, J = 2.0 Hz, 1 H, coumarin-H), 6.35 (s, 1 H, couma-
rin-H), 5.98 (s, 1 H, coumarin-H), 3.15–3.11 (m, 8 H, CH2), 1.58–
1.54 (m, 8 H, CH2), 1.40–1.45 (m, 8 H, CH2), 0.97 (t, J = 7.32 Hz,
12 H, CH3) ppm. 13C NMR (100 MHz, CDCl3) δ = 162.4. 158.8,
142.4, 142.1, 125.6, 123.9, 121.2, 120.1, 104.8, 58.7, 23.9, 19.7, 13.6
ppm. C26H40F3NO3·0.6 H2O: calcd. C 64.73, H 8.61, N 2.90; found
C 64.90, H 8.48, N 2.73.
Figure 3. Repeated memory cycles starting with chromenolate 2–,
adding receptor 1 (to form the nonfluorescent complex [1·2–]), fol-
lowed by the addition of first F– and then Li+ in CH3CN. The
response time is instantaneous and the emission intensity in the
“On” state remains virtually the same with repeated cycles. Note
that the emission intensity change is about 1000 fold. (λex
=
410 nm). [2–] = 2.1 μm.
5-(4-Fluorophenyl)-5-methyldipyrromethane (3): To the mixture
of commercially available 4-fluoroacetophenone (2.0 mL,
16.48 mmol) and pyrrole (11.4 mL, 164.80 mmol) kept under argon
atmosphere at 0 °C, trifluoroacetic acid (1.2 mL, 16.51 mmol) was
added dropwise. After addition was complete, stirring continued at
0 °C to 25 °C for 12 h. The reaction mixture was quenched by add-
ing aqueous NaOH (0.1 n, 50 mL) and extracted with CH2Cl2
(50 mLϫ2). The organic layer was dried with anhydrous Na2SO4
Conclusions
In summary, we have developed a new calix[4]pyrrole
based fluorescent “Turn-On” chemosensor displaying re-
markable sensitivity and selectivity towards F–. The “Turn-
On” mechanism is based on the displacement of fluorescent
indicator from indicator–calix[4]pyrrole complex upon ad- and the solvent was removed under reduced pressure to afford the
dition of F– to form a strong [1·F–] complex that results in
a full reinstatement of the original indicator fluorescence.
crude reaction mixture, which was purified by silica gel column
chromatography (hexane/EtOAc = 9.5:0.5) to afford pure product
3 as a white solid; yield 2.76 g (66%). 1H NMR (400 MHz, CDCl3)
The receptor-bound fluoride anion can be easily displaced
δ = 7.71 (br. s, 4 H, pyrrole-NH), 7.07–7.02 (m, 2 H, Ar-H), 6.96–
6.90 (m, 2 H, Ar-H), 6.63–6.62 (m, 2 H, pyrrole-H), 6.17–6.14 (m,
2 H, pyrrole-H), 5.94–5.92 (m, 2 H, pyrrole-H), 2.00 (s, 3 H, CH3)
by treatment with Li+ resulting in reformation of the indi-
cator complex and a complete quenching of the fluores-
cence. This switchable “Off-On” operation was completely
reversible and could be carried out through many repeated
memory cycles. We expect that the current “Turn-On” and
“Turn-Off” ion-mediated events would provide useful in-
sights into the design and development of more selective
chemosensors and could lead to the development of molec-
ular scale information storage devices.
ppm. 13C NMR (100 MHz, CDCl3) δ = 163.2, 160.8, 143.6, 143.6,
137.7, 129.5, 129.5, 117.6, 115.3, 115.1, 108.7, 106.9, 44.7, 29.4
ppm. MALDI-MS Calcd. for C16H15FN2 254.12, found 255.03 (M
+ 1).
5,15-(4-Fluorophenyl)-5,10,10,15,20,20-hexamethylcalix[4]pyrrole
(1): To the solution of compound 3 (1.5 g, 5.90 mmol) in acetone
·
(300 mL) was added BF3 OEt2 (1.5 mL, 11.80 mmol) and the mix-
ture stirred for 12 h at room temperature. The reaction mixture was
quenched upon addition of triethylamine (3.5 mL, 25.36 mmol).
Excess acetone was removed under reduced pressure and the mix-
ture was combined with water and extracted with CH2Cl2
(100 mLϫ2). The organic layer was dried with anhydrous Na2SO4
Experimental Section
1
General: H NMR spectra were recorded on a 400 and 300 MHz
Bruker NMR spectrometer using TMS as the internal standard.
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Salt (LiF) Regulated Fluorescence Switching
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and the solvent was removed in vacuo. Column chromatography
on silica afforded a clear separation of two isomers. The first minor
fraction containing trans isomer (0.10 g, 3%) was collected in
9.7:0.3 hexane/EtOAc mixture. While the second major fraction
containing the required cis isomer 1 was collected as a white solid
in 9.4:0.6 hexane/EtOAc mixture; yield 0.73 g (21%). 1H NMR
(300 MHz, CD3CN) δ = 7.81 (br. s, 4 H, pyrrole-NH), 6.97–6.90
(m, 8 H, Ar-H), 5.86–5.84 (m, 4 H, pyrrole-H), 5.62–5.60 (m, 4 H,
pyrrole-H), 1.64 (s, 6 H, CH3), 1.56 (s, 6 H, CH3), 1.49 (s, 6 H,
1
CH3) ppm. H NMR (400 MHz, CDCl3) δ = 7.20 (br. s, 4 H, pyr-
role-NH), 6.95–6.87 (m, 8 H, Ar-H), 5.94–5.92 (m, 4 H, pyrrole-
H), 5.62–5.60 (m, 4 H, pyrrole-H), 1.87 (s, 6 H, CH3), 1.61 (s, 6 H,
CH3), 1.53 (s, 6 H, CH3) ppm. 13C NMR (100 MHz, CDCl3) δ =
162.6, 160.2, 143.7, 143.7, 138.5, 136.4, 129.0, 128.9, 114.4, 114.2,
106.0, 103.3, 44.2, 35.1, 30.1, 27.9, 27.9 ppm. MALDI-MS Calcd.
for C38H38F2N4 588.31, found 589.21 (M + 1).
Supporting Information (see footnote on the first page of this arti-
cle): Synthetic schemes, copies of NMR spectra, single-crystal X-
ray structures and spectroscopic data to support fluoride sensing
behaviour. The crystal data for the structural refinement of 1 and
[1·F–] are also provided.
CCDC-820611 (for 1) and -820612 (for [1·F–]) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Acknowledgments
This work was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF 2009-
008713). The Central Instrumentation Facility at KNU and BK21
program are acknowledged for support.
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Received: March 15, 2011
Published Online: April 26, 2011
Eur. J. Org. Chem. 2011, 2911–2915
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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