A deep cavitand with a fluorescent wall functions as an ion sensor

Article abstract of DOI:10.1021/ol2021127

The synthesis and characterization of a deep cavitand bearing a fluorescent benzoquinoxaline wall is reported. Noncovalent host-guest recognition events are exploited to sense small charged molecules including acetylcholine. The cavitand also exhibits an

Full text of DOI:10.1021/ol2021127

ORGANIC
LETTERS
2011
Vol. 13, No. 19
5232–5235
A Deep Cavitand with a Fluorescent Wall
Functions as an Ion Sensor
Orion B. Berryman, Aaron C. Sather, and Julius Rebek Jr.*
The Skaggs Institute for Chemical Biology and Department of Chemistry,
The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla,
California 92037, United States
jrebek@scripps.edu
Received August 3, 2011
ABSTRACT
The synthesis and characterization of a deep cavitand bearing a fluorescent benzoquinoxaline wall is reported. Noncovalent hostÀguest
recognition events are exploited to sense small charged molecules including acetylcholine. The cavitand also exhibits an anion dependent
change in fluorescence that is used to differentiate halide ions in solution.
Developing sensors for biologically relevant ions and
small molecules is a timely topic of research.1 The versa-
tility and sensitivity of photoluminescence have made it a
favorite readout of sensor systems. Photophysical proper-
ties of luminescent molecules can be fine-tuned through a
number of different interactions such as heavy atom
effects; electron-, proton-, and energy-transfer processes;
destabilization of excited states; and changes in electron
density.2 These interactions can quench (“turn-off”) or
enhance (“turn-on”) the intensity of the fluorescent emis-
sion; these processes have been studied in many cavitands
and capsules.3 Numerous fluorescent sensors have been
reported, and different analytes are targeted through
covalent bonding or noncovalent interactions.4 The con-
trolled microenvironments of host molecules makes them
appealing choices to sense analytes through noncovalent
interactions. In particular, fluorescence spectroscopy and
cavitand molecules offer unique opportunities to study
noncovalent recognition processes. One approach has
been to use the displacement indicator method5 with
nonfluorescent cavitands in combination with fluorescent
guests.6 Another method showed that calix[4]arenes with
pyrene substituents appended to the lower rim exhibited
changes in excimer emission upon guest binding.7 Other
calixarenes and resorcinarenes with fluorophores ap-
pended to the periphery have been shown to function as
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r
10.1021/ol2021127
Published on Web 09/13/2011
2011 American Chemical Society

sensors.8 Recently, a resorcin[4]arene with four organo-
boron“walls”was reportedtoexhibita hypsochromic shift
upon guest binding.9 Here we detail an approach where the
fluorescent functionality is one of the walls of a deep
cavitand host, in which the guest analyte is placed in direct
contact with the fluorophore. Upon guest binding the
cavitand displays a bathochromic shift and an anion
dependent change in fluorescence.
different from that of the parent octa-amide system.11
The fluxional behavior of cavitand 1 is a result of the
extended benzoquinoxaline wall and the absence of two
amide substituents. Interestingly, the addition of a guest
molecule such as acetylcholine chloride (4) results in
sharper signals characteristic of a kinetically stable com-
plex (Figure 2, top).
Cavitand 1 was synthesized in one step from known
hexa-amide cavitand 3.10 The fourth wall is attached to the
cavitand by two nucleophilic aromatic substitution reac-
tions between 2,3-dichlorobenzoquinoxaline and the free
phenols on 3 (Figure 1). Cavitand 1 features six amides
positioned on the upper rim of the cavitand which form
intramolecular hydrogen bonds, stabilizing the vase con-
formation. The fluorescent benzoquinoxaline completes
the cavitand’s concave structure and functions as the
spectroscopic signal for hostÀguest studies. A control
molecule (2), featuring the benzoquinoxaline functionality
without the well-defined molecular space of cavitand 1,
was also synthesized (Figure 1).
Figure 2. Select portions of the 1H NMR spectrum of cavitand 1
in CDCl3 at 300 K (bottom) and in the presence of acetylcholine
chloride (4) (top). A CAChe MM3 minimized model of 1 and
acetylcholine is provided in the upper left inset.
Cavitand 1 and control 2 display similar spectro-
scopic properties dictated by the benzoquinoxaline
fluorophore. Both molecules are characterized by a
broad absorbance centered at 365 nm that tails into
the visible region resulting in yellow compounds. The
green fluorescence of the 2,3-dichlorobenzoquinoxa-
line starting material is shifted to a blue fluorescence
once integrated into compounds 1 and 2. When excited
at 365 nm these molecules emit a broad emission
centered at 460 nm with quantum yields of ΦF = 0.24
for cavitand 1 and ΦF = 0.19 for control 2. The
structured microenvironment of the cavitand improves
the quantum yield of this fluorophore. The character-
istic blue fluorescence of these molecules is used to
probe hostÀguest interactions in this system.
Figure 1. Synthesis of cavitand 1 (top). Control and guest
molecules (bottom).
1H NMR spectra in CDCl3 reveal that cavitand 1 adopts
a folded vase conformation but with broadened signals
(Figure 2, bottom). This suggests a dynamic behavior
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O.; Verboom, W.; Reinhoudt, D. N. J. Org. Chem. 2001, 66, 3998–4005.
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T. Chem. Commun. 2010, 46, 3604–3606.
Cavitand 1 functions as a sensor for alkyl ammonium
salts that are small enough to complement the receptor
cavity. The addition of 5 equiv of acetylcholine chloride
(4) to a CHCl3 solution of cavitand 1 produces a bath-
ochromic shift and an increase in fluorescence. The
change in emission is attributed to the proximity of the
cation and anion to the fluorophore (see below). A
titration with this analyte results in isosbestic conver-
sion (Supporting Information (SI)). The fluorescence
of control molecule 2 is not altered by the same addition of
guest, highlighting the necessity of the hostÀguest interac-
tion (Figure 3). Tetra-N-butylammonium bromide;which
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J. Am. Chem. Soc. 2000, 122, 4573–4582.
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G.; Rebek, J., Jr. J. Am. Chem. Soc. 1998, 120, 12216–12225.
Org. Lett., Vol. 13, No. 19, 2011
5233

Figure 3. Acetylcholine chloride (4) induced fluorescence en-
hancement of cavitand 1 (left) and static fluorescene of control 2
(right, red and blue lines overlap completely) in CHCl3 at 298 K.
Figure 4. Select regions of the 1H NMR spectra of cavitand 1 in
CDCl3 with 2 equiv each of N-propylquinuclidinium iodide
(top), bromide (middle), and chloride (bottom). The shifted
hydrogen bonding amide protons are noted by blue circles.
does not fit into 1;had minimal effect on the fluorescence
spectrum (see SI).
Fluorescent sensors for anions are increasingly studied.12
The guest binding and emission of 1 are dependent on
the guest counteranion. To study this dependence we
turned to N-propylquinuclidinium halide guests. 1H
NMR and NOESY spectra reveal that N-propylquinu-
clidinium binds to 1 with the propyl group directed
toward the open end of the cavitand. The associa-
tion constants of N-propylquinuclidinium guests 5À7
are 8.0 Â 103 MÀ1 for 5, 4.9 Â 103 MÀ1 for 6, and 2.3 Â
103 MÀ1 for 7. This trend is in accord with the most
basic chloride forming the strongest hydrogen bonds
cationic charge polarizes the environment around the
fluorophore resulting in altered emission. The anions
collisionally quench fluorescence to varying degrees de-
pending on their location and association with the upper
rim of the cavitand. This interplay results in a balance
between emission enhancement and quenching ultimately
dictated by the anion.
1
with the amide protons on the upper rim of 1. The H
NMR of these complexes corroborate this assertion
with the greatest downfield shifts of the amide protons
observed for the complex with 5 (Figure 4, bottom).
The fluorescence spectrum of 1 also showed a strong
dependence on the counteranion of the guest.13 Emission
spectra were obtained for cavitand 1 with guests 5À7
(Figure 5). The chloride guest (5) produced an increase in
fluorescence, while the bromideguest (6) led only toa small
increase. Iodide is well-known for its fluorescence quench-
ing capability,14 and the iodide guest (7) resulted in a
significant decrease in fluorescence which is likely due to
a collisional quenching process of the excited state. The
addition of other iodide salts to the cavitand (N-methyl-
quinuclidinium iodide or N-propyl-N-methylpiperidinium
iodide) also resulted in a decrease in fluorescence. The
changes in emission are likely due to subtle effects from
both the cation and anion. The cation binds to the host
which rigidifies the structure and reduces the nonradiative
energy transfer. Additionally, the close proximity of the
Figure 5. Anion dependent fluorescence of cavitand 1 in CHCl3.
N-Propylquinuclidinium chloride (5, green line) and bromide
(6, red line) both produce fluorescence enhancement while
N-propylquinuclidinium iodide (7, purple line) quenches
fluorescence.
In conclusion, we report a deep cavitand containing a
fluorescent benzoquinoxaline as one of the walls. The
cavitand displays fluxional behavior in chloroform, but
complexes kinetically stable on the NMR time scale are
formed when good guests are introduced. The end result is
ꢀ~
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(12) Martinez-Manez, R.; Sancenon, F. Chem. Rev. 2003, 103, 4419–
4476.
(13) The UVÀvis spectra on the other hand showed negligible
dependence on the counteranion (see SI).
(14) Ide, G.; Engelborghs, Y. J. Biol. Chem. 1981, 256, 11684–11687.
5234
Org. Lett., Vol. 13, No. 19, 2011

a deep cavitand that functions as a fluorescent sensor for
ions. Future studies will explore other guest molecules for
selective recognition.
by the NIH (F32GM087068). A.C.S. is a Skaggs Pre-
doctoral Fellow.
Supporting Information Available. Experimental pro-
cedures, UVÀvis, fluorescence studies, and NMR data.
This material is available free of charge via the Internet at
http://pubs.acs.org
Acknowledgment. This work was financially sup-
ported by the NIGMS (GM27953). A Ruth L. Kirsch-
stein Postdoctoral fellowship for O.B.B. was provided
Org. Lett., Vol. 13, No. 19, 2011
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