Sensor for nitrophenol based on a fluorescent molecular clip

Article abstract of DOI:10.1021/ol900858d

We report the synthesis, X-ray crystal structure, and photophysics of a fluorescent molecular clip (1). Binding studies of 1 toward phenols 2a-j were carried out using fluorescence, ¹H NMR, and IR spectroscopy, which revealed a high affinity and selectivity for 4-nitrophenol (2a) due to formation of the 1-2a complex driven by H-bonding and π-π stacking interactions.

Full text of DOI:10.1021/ol900858d

ORGANIC
LETTERS
2009
Vol. 11, No. 12
2603-2606
Sensor for Nitrophenol Based on a
Fluorescent Molecular Clip
Nengfang She,† Meng Gao,† Liping Cao,† Anxin Wu,*,† and Lyle Isaacs*,‡
Key Laboratory of Pesticide & Chemical Biology, Ministry of Education,
Central China Normal UniVersity, 152 Luoyu Road, Wuhan 430079, P. R. China, and
Department of Chemistry and Biochemistry, UniVersity of Maryland,
College Park, Maryland 20742
chwuax@mail.ccnu.edu.cn; lisaacs@umd.edu
Received April 19, 2009
ABSTRACT
We report the synthesis, X-ray crystal structure, and photophysics of a fluorescent molecular clip (1). Binding studies of 1 toward phenols
1
2a-j were carried out using fluorescence, H NMR, and IR spectroscopy, which revealed a high affinity and selectivity for 4-nitrophenol (2a)
due to formation of the 1·2a complex driven by H-bonding and π-π stacking interactions.
Contemporary supramolecular systems aim to meld form
with function. Accordingly, supramolecular design principles
are being employed for the development of molecular devices
and machines,1 drug delivery systems,2 artificial ion chan-
nels,3 supramolecular catalysts,4 and optical sensing systems.5
A popular route to the development of optical (colorimetric
or fluorescence) sensors relies on indicator displacement
assays which have been used to sense a variety of molecular
and ionic guests.6 Alternatively, fluorophores may be incor-
portated into the covalent structure of the host and thereby
participate directly in the sensing process.7 In all of these
application areas, host systems that exhibit high affinity and
highly selective binding processes are essential.
Glycolurilsas the essential component of Nolte’s molec-
ular clips,8 Rebek’s capsules,9 and the cucurbit[n]uril family
of macrocycles10shas become a popular building block for
the preparation of new host systems.11 Of particular relevance
† Central China Normal University.
‡ University of Maryland.
(1) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. 2007,
46, 72–191. Saha, S.; Stoddart, J. F. Chem. Soc. ReV. 2007, 36, 77–92. Ko,
Y. H.; Kim, E.; Hwang, I.; Kim, K. Chem. Commun. 2007, 1305–1315.
(2) Haag, R.; Kratz, F. Angew. Chem., Int. Ed. 2006, 45, 1198–1215.
(3) Sakai, N.; Mareda, J.; Matile, S. Acc. Chem. Res. 2008, 41, 1354–
1365. Davis, A. P.; Sheppard, D. N.; Smith, B. D. Chem. Soc. ReV. 2007,
36, 348–357.
(4) Patureau, F. W.; Kuil, M.; Sandee, A. J.; Reek, J. N. H. Angew.
Chem., Int. Ed. 2008, 47, 3180–3183. Pluth, M. D.; Bergman, R. G.;
Raymond, K. N. Science 2007, 316, 85–88.
(6) Wright, A. T.; Anslyn, E. V. Chem. Soc. ReV. 2006, 35, 14–28.
(7) Swager, T. M. Acc. Chem. Res. 2008, 41, 1181–1189.
(8) Rowan, A. E.; Elemans, J. A. A. W.; Nolte, R. J. M. Acc. Chem.
Res. 1999, 32, 995–1006.
(5) de Silva, A. P.; Vance, T. P.; West, M. E. S.; Wright, G. D. Org.
Biomol. Chem. 2008, 6, 2468–2480.
(9) Rebek, J., Jr. Angew. Chem., Int. Ed. 2005, 44, 2068–2078.
10.1021/ol900858d CCC: $40.75
Published on Web 05/18/2009
 2009 American Chemical Society

to the work reported here are the reports by Nolte on the
use of U-shaped glycoluril-based molecular clips as receptors
for 1,3-dihydroxybenzene derivatives whose complexes
benefit from Ar-OH···OdC H-bonds, π-π stacking interac-
tions, and a cavity effect.12 In ongoing efforts, we aim to
combine the outstanding recognition properties of glycoluril
derived systems with the high sensitivity and low detection
limits of fluorescence spectroscopy.13 In this paper, we
describe the design, synthesis, and highly selective recogni-
tion properties of fluorescent molecular clip 1 toward
4-nitrophenol 2a. Compound 2aswhich is of interest because
of its mutagenic properties and its use in active ester coupling
reactionsshas previously been complexed by cyclodextrin,
calixarene, and cucurbit[n]uril molecular containers.14,15
Scheme 1 shows the synthesis of fluorescent molecular
clip 1. In brief, dihydroxyfumaric acid 3 was oxidized with
conditions17 gave molecular clip 1 in 72% yield. By virtue
of the extended π-system of the o-xylylene sidewalls and
the attached phenylethynyl arms, clip 1 is highly fluorescent.
We were fortunate to obtain X-ray quality crystals of
molecular clip 1 which allowed us to confirm the structural
assignment and elucidate the three-dimensional structure of
1 by single-crystal X-ray diffraction (Figure 1). The crystal
Scheme 1
.
(a) Synthesis of Molecular Clip 1 and (b) Guests 2
Used in This Study
Figure 1. Cross-eyed stereoview of the structure of 1 in the crystal.
Color code: C, gray; H, white; N, blue; O, red.
structure of molecular clip 1 clearly reveals the presence of
a deep cavity, which is defined by two extended aromatic
sidewalls. To achieve efficient packing in the crystal,
molecular clip 1 undergoes dimerization18 by the reciprocal
insertion of the aromatic sidewall of one clip into the cleft
of the opposing clip. The distance between the ureidyl CdO
(10) Lagona, J.; Mukhopadhyay, P.; Chakrabarti, S.; Isaacs, L. Angew.
Chem., Int. Ed. 2005, 44, 4844–4870. Lee, J. W.; Samal, S.; Selvapalam,
N.; Kim, H.-J.; Kim, K. Acc. Chem. Res. 2003, 36, 621–630.
(11) Chiang, P. T.; Cheng, P. N.; Lin, C. F.; Liu, Y. H.; Lai, C. C.;
Peng, S. M.; Chiu, S. H. Chem.sEur. J. 2006, 12, 865–876. Chiang, P. T.;
Chen, N. C.; Lai, C. C.; Chiu, S. H. Chem.sEur. J. 2008, 14, 6546–6552.
(12) Reek, J. N. H.; Priem, A. H.; Engelkamp, H.; Rowan, A. E.;
Elemans, J. A. A. W.; Nolte, R. J. M. J. Am. Chem. Soc. 1997, 119, 9956–
9964. Jansen, R. J.; Rowen, A. E.; de Gelder, R.; Scheeren, H. W.; Nolte,
R. J. M. Chem. Commun. 1998, 121–122. Sijbesma, R. P.; Kentgens,
A. P. M.; Nolte, R. J. M. J. Org. Chem. 1991, 56, 3199–3201. Sijbesma,
R. P.; Kentgens, A. P. M.; Lutz, E. T. G.; van der Mass, J. H.; Nolte, R. J. M.
J. Am. Chem. Soc. 1993, 115, 8999–9005. Jansen, R. J.; de Gelder, R.;
Rowan, A. E.; Scheeren, H. W.; Nolte, R. J.; M., J. Org. Chem. 2001, 66,
2643–2653.
(13) Hu, S.-L.; She, N.-F.; Yin, G.-D.; Guo, H.-Z.; Wu, A.-X.; Yang,
C.-L. Tetrahedron Lett. 2007, 48, 1591–1594. She, N.; Gao, M.; Cao, L.;
Yin, G.; Wu, A. Synlett 2007, 16, 2533–2536. Lagona, J.; Wagner, B. D.;
Isaacs, L. J. Org. Chem. 2006, 71, 1181–1190. Liu, S.; Shukla, A. D.; Gadde,
S.; Wagner, B. D.; Kaifer, A. E.; Isaacs, L. Angew. Chem., Int. Ed. 2008,
47, 2657–2660.
(14) Buva´ri-Barcza, A.; Ra´k, E.; Me´sza´ros, A.; Barcza, L. J. Inclusion
Phenom. Macrocycl. Chem. 1998, 32, 453–459. Saki, N.; Akkaya, E. U.
J. Inclusion Phenom. Macrocycl. Chem 2005, 53, 269–273. Kunsa´gi-Ma´te,
S.; Szabo´, K.; Lemli, B.; Bitter, I.; Nagy, G.; Kolla´r, L. Thermochim. Acta
2005, 425, 121–126. Zheng, L.-M.; Zhang, Y.-Q.; Zeng, J.-P.; Qiu, Y.;
Yu, D.-H.; Xue, S.-F.; Zhu, Q.-J.; Tao, Z. Molecules 2008, 13, 2814–2822.
(15) Molecular clips are also well known to bind electron-deficient
aromatic guests. See: Zimmerman, S. C. Top. Curr. Chem. 1993, 165, 71–
102. Klaerner, F.-G.; Kahlert, B. Acc. Chem. Res. 2003, 36, 919–932.
(16) Burnett, C. A.; Lagona, J.; Wu, A.; Shaw, J. A.; Coady, D.;
Fettinger, J. C.; Day, A. I.; Isaacs, L. Tetrahedron 2003, 59, 1961–1970.
(17) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N.
Synthesis 1980, 627–630.
bromine in acetic acid to the corresponding diketone which
was esterified to give 4. Transformation of 4 into diethoxy-
lcarbonyl glycoluril 5 proceeded smoothly according to the
literature procedure.16 Alkylation of 5 with 2,3-bis(bromom-
ethyl)-1,4-dibromobenzene under basic conditions (DMSO,
t-BuOK) gave 6 in 39% yield. Installation of the 4-meth-
oxyphenylethynyl arms under Sonogashira cross-coupling
2604
Org. Lett., Vol. 11, No. 12, 2009

oxygen atoms in 1 amounts to 5.6 Å; this spacing is
appropriate for H-bonding to 1,3-dihydroxybenzenes.10 The
dihedral angle between the two phenyl rings of the sidewalls
is 26° and the distance between the centroids of o-xylylene
is 6.1 Å. These geometrical features enable 1 to engage in
π-π interactions and H-bonds with suitable guests.12
Molecular clip 1 displayed strong fluorescence emission
in THF as shown in Figure 2. The excitation and emission
of the electron rich sidewalls of 1 relative to 2b-j, we
performed competition experiments. Figure 2b shows the
fluorescence intensity of solutions containing 1 (10 µM), 2a
(160 µM), and individually 2b-j (160 µM). Clearly, the
presence of 2b-j does not significantly effect the overall
fluorescence intensity which establishes that molecular clip
1 binds more tightly to 2a than to phenols 2b-j.20
Given the interesting ability of 2a to quench the fluores-
ence of 1, we decided to investigate this process in more
detail. We first performed a titration where the emission
spectrum of 1 (10 µM) was monitored as the concentration
of 2a was increased (Figure 3). The inset to Figure 3 shows
Figure 3. Fluorescence spectra (excitation at 339 nm) of 2 (10 µM)
Figure 2. (a) Fluorescence emission changes of 1 (10 µM) in
in THF-MeOH (9:1, v/v) in the presence of 0, 1.0, 2.0, 4.0, 6.0,
8.0, 10.0, 12.0, 14.0, 16.0, 18.0, 20.0, 22.0, 24.0, 26.0, 28.0, 30.0,
35.0, and 40.0 equiv of 4-nitrophenol predissolved in MeOH. Inset:
Stern-Volmer plot of the emission data.
THF-MeOH (9:1, v/v) in the presence of 2a-j (400 µM). (b)
Fluorescence intensity at 389 nm of a solution of 1 (10 µM), 2a
(160 µM), and 2b-j (160 µM). λex ) 339 nm.
wavelengths were 339 and 388 nm, respectively. The
quantum yield of 1 (Φ ) 0.65) was determined by comparing
the ratio of the fluorescence emission intensity at 389 nm to
UV-vis absorbance at the excitation wavelength used
relative to that of quinine (Φ ) 0.546) as reference.19
Once the synthesis, structural characterization, and pre-
liminary photophysical investigations of 1 were complete,
we decided to investigate the use of 1 as a fluorescence
sensor for phenols 2. Figure 2 shows the changes in
fluorescence intensity of molecular clip 1 upon addition of
40 equiv of 2a-j. Quite interestingly, significant fluorescence
quenching is observed for 4-nitrophenol 2a but not for any
of the other phenols 2b-j. To gauge whether the fluores-
cence quenching observed for 2a was due to a higher binding
constant for 1·2a or whether the electron-poor aromatic ring
of 2a was simply more efficient at quenching the fluorescence
the linearity of the Stern-Volmer plot which confirms the
formation of a single type of complex between 1 and 2a.
The stoichiometry of the complex between 1 and 2a is 1:1
based on the fluorescence Job plot shown in Figure 4.21 The
Figure 4. Job plot for mixtures of 1 and 2a ([1] + [2a] ) 20 µM).
(18) Reek, J. N. H.; Elemans, J. A. A. W.; de Gelder, R.; Beurskens,
P. T.; Rowan, A. E.; Nolte, R. J. M. Tetrahedron 2003, 59, 175–185. Wang,
Z.-G.; Zhou, B.-H.; Chen, Y.-F.; Yin, G.-D.; Li, Y.-T.; Wu, A.-X.; Isaacs,
L. J. Org. Chem. 2006, 71, 4502–4508. Chen, Y.; She, N.; Meng, X.; Yin,
G.; Wu, A.; Isaacs, L. Org. Lett. 2007, 9, 1899–1902. Wu, A.; Chakraborty,
A.; Fettinger, J. C.; Flowers, R. A.; Isaacs, L. Angew. Chem., Int. Ed. 2002,
41, 4028–4031.
association constant for the formation of 1·2a (Ka ) 1.09 ×
104 ( 1.82 × 102 M-1) in THF-MeOH (9:1) was deter-
mined from a Benesi-Hildebrand plot.22
(19) Ree, M.; Kim, J.-S.; Kim, J. J.; Kim, B. H.; Yoon, J.; Kim, H.
Tetrahedron Lett. 2003, 44, 8211–8215. Demas, J. N.; Crosby, G. A. J.
Phys. Chem. 1971, 75, 991–1024.
To further elucidate the geometry of the 1·2a complex we
1
1
performed H NMR and IR studies. Figure 5 shows the H
Org. Lett., Vol. 11, No. 12, 2009
2605

Figure 6. Cross-eyed stereoview of the MMFF-minimized model
of 1·2a. Color code: C, gray; H, white; N, blue; O, red.
As described above, molecular clip 1 binds strongly and
selectively to electron deficient guest 2a driven by a
combination of H-bonding and π-π stacking interactions.
In addition to the strong and selective binding observed
between 1 and 2a, it is critical that 1 undergo a significant
modulation in fluorescence upon binding. In this case, we
believe that the formation of the 1·2a complex results in
electron or energy transfer from the excited state of the
4-methoxyphenylethynyl-conjugated sidewalls of 1 to the
aromatic ring of electron-poor 2a. For the other phenols
examined, 2b-j, a combination of weaker binding and lower
fluorescence modulation upon binding result in the high
selectivity observed for 2a in this fluorescence assay.
In conclusion, we have synthesized a molecular clip 1 with
extended ethynylated o-xylylene sidewalls which exhibits
strong fluoresence emission. By virtue of the aromatic
sidewalls and the glycoluril ureidyl CdO groups, 1 exhibits
high binding affinity and selectivity toward electron-poor
aromatic phenol 2a driven by a combination of H-bonding
and π-π interactions as revealed by the detailed fluores-
Figure 5.
1H NMR spectra (400 MHz, CDCl3, 298 K) recorded
for (a) 2a, (b) 1·2a, and (c) 1.
NMR spectra recorded for 1, 2a, and the 1•2a complex. We
find that protons on the aromatic ring of 2a undergo upfield
shifting (Ha, -0.164 ppm; Hb, -0.154 ppm) whereas the OH
group (Hc, +0.553 ppm) undergoes significant downfield
shifting. The upfield shifts indicate that 2a binds in the cavity
of 1 by π-π interactions whereas the downfield shift
indicates that the OH group of 2a is involved in H-bonding
interactions with the ureidyl CdO group of 1. Infrared
spectroscopy was also used to investigate the H-bonding
between 1 and 2a (Supporting Information). Molecular clip
1 displays two νCdO stretching vibrations at 1750 and 1726
cm-1 due to the presence of ureidyl and CO2Et functional
groups.12 The carbonyl stretching vibration band of the host
is influenced by hydrogen bonding. IR spectra of molecular
clip 1 mixed with 4-nitrophenol shows that νCdO splits into
three bands (1755, 1723, and 1690 cm-1). This splitting
indicates that the carbonyl groups of host 1 in the complexes
are involved in hydrogen bonding. Overall, the binding of
2a in the cavity of molecular clip 1 described here is due to
three factors, namely H-bonding between the phenolic OH
and the urea carbonyl group, π-π stacking interactions
between the electron-poor aromatic guest and the electron-
rich sidewalls of the host, and a cavity effect.12,23 The
MMFF-minimized geometry of the 1·2a complex is shown
in Figure 6.
1
cence, H NMR, and IR studies. The high efficiency of the
sensing mechanism is due to the interplay of the H-bonds to
the ureidyl CdO groups of the glycoluril which are stronger
for more acidic electron-poor phenols and the fluorescence
quenching which is also enhanced by electron-poor aromatic
guests. Overall, the work highlights the advantages of a
sensor design wherein the determinants of binding affinity
and fluorescence modulation reinforce one another.
Acknowledgment. A.-X.W. thanks the National Labora-
tory of Applied Organic Chemistry, Lanzhou University, and
the National Natural Science Foundation of China (Grant
No. 20672042) for funding. L.I. thanks the National Science
Fountation (CHE-0615049) for financial support.
(20) The fact that the fluorescence intensity of a solution of 1·2a (10
mM, Ka ) 1.09 × 104 M-1) and 2a (150 µM) does not change significantly
(<10%) upon addition of 2b-j (160 µM) allows us to determine an upper
limit on the binding constants for the 1·2b-1·2j complexes (Ka e 1.09 ×
103 M-1).
Supporting Information Available: Synthesis of molec-
ular clip 1 and details of the fluorescence and IR experiments.
Crystallographic information file for 1 (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
(21) Job, P. Ann. Chim. 1928, 9, 113–204.
(22) Benesi, H.; Hildebrand, H. J. Am. Chem. Soc. 1949, 71, 2703–
2707.
(23) The larger π-surface area of 2a could also enhance its π-π
interactions with 1. See: Liu, T.; Schneider, H.-J. Angew. Chem., Int. Ed.
2002, 41, 1368–1370
.
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Org. Lett., Vol. 11, No. 12, 2009