Design and synthesis of a neutral fluorescent macrocyclic receptor for the recognition of urea in chloroform

Article abstract of DOI:10.1021/ol050034h

(Chemical Equation Presented) An artificial macrocyclic fluorescent receptor 1 has been designed and synthesized for the recognition of urea. 1 shows significant fluorescence quenching on complexation with urea and thiourea in chloroform and thus may be used as a synthetic fluorescent molecular sensor for their determination in a nondegradative way.

Full text of DOI:10.1021/ol050034h

ORGANIC
LETTERS
2005
Vol. 7, No. 7
1283-1285
Design and Synthesis of a Neutral
Fluorescent Macrocyclic Receptor for
the Recognition of Urea in Chloroform
Shyamaprosad Goswami,* Reshmi Mukherjee,† and Jayanta Ray
Department of Chemistry, Bengal Engineering and Science UniVersity,
Shibpur, Howrah-711 103, West Bengal, India
spgoswamical@yahoo.com
Received January 7, 2005
ABSTRACT
An artificial macrocyclic fluorescent receptor 1 has been designed and synthesized for the recognition of urea. 1 shows significant fluorescence
quenching on complexation with urea and thiourea in chloroform and thus may be used as a synthetic fluorescent molecular sensor for their
determination in a nondegradative way.
Urea is the chief nitrogenous end product of the metabolic
breakdown of proteins in all mammals and some fishes and
not only occurs in the urine of all mammals but also in their
blood, bile, milk, and perspiration. The design and synthesis
of an artificial receptor for urea for possible use as a
fluorescent sensor1 is important in the clinical identification
of diabetes and kidney and thyroid diseases or to extract
excess blood urea into a nonpolar solvent. Potentiometric
sensors, based on gel-immobilized enzymes, and calorimetric
nonenzymatic sensor would be preferred to determine urea
in a nondegrative way. This communication presents such a
neutral receptor that shows appreciable fluorescence quench-
ing upon binding of urea.
We have designed3 a novel neutral fluorescent macrocyclic
receptor 1 having two pyridylamide moieties and a 1,10-
phenanthroline moiety as hydrogen bonding (HB) ligands
to encircle urea by combined hydrogen bonding to urea
carbonyl (HB acceptor) as well as its four NH atoms (HB
donor) and also possible sideways hydrogen bonds by ether
oxygens (and for possible secondary electrostatic interac-
tions).4 The incorporation of the fluorescent chromophore
phenanthrolene in receptor 1 is aimed to allow possible
hydrogen bonding (as a hydrogen bond acceptor) with the
two anti hydrogens of urea carbonyl as well as the other
two separate pyridine amides with isophthaloyl spacer, where
pyridine nitrogens act as hydrogen bond acceptors for the
remaining two hydrogens of urea and the two amide NH
bonds of 1 act as two hydrogen bond donors to carbonyl
2-
+
sensors decompose urea into CO3 and NH4 ions in the
former case and CO2 and NH3 in the later.2 A neutral
* Fax: + 91-33-2668-4564/2916.
† Presently at University of Zurich, Switzerland.
(1) (a) De Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley,
A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997,
97, 1515. (b) Fabbrizzi, L. Acc. Chem. Res. 1999, 32, 846. For recent works,
see: (c) Mello, J. V.; Finney, N. Angew. Chem., Int. Ed. 2001, 40, 1536.
(d) Miyaji, H.; Sato, W.; Sessler, J. L. Angew. Chem., Int. Ed. 2000, 39,
1777. (e) Pugh, V. J.; Hu, Q.-S.; Pu, L. Angew. Chem., Int. Ed. 2000, 39,
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York, 1997; pp 23, 50-53.
10.1021/ol050034h CCC: $30.25
© 2005 American Chemical Society
Published on Web 03/08/2005

oxygen of urea. Thus, it can bind urea strongly to form a
stable and less polar complex compared to the individual
components5 engaging all the possible polar hydrogen
bonding groups3f to solubilize urea in chloroform.
The suggested mode of complexation of urea with receptor
1 and the CPK model of the energy-minimized structure of
the urea complex are shown in Figure 1, which shows the
Figure 1. Structure of receptor 1 and the proposed urea complex
(left) and a CPK model of the energy-minimized structure of the
complex (right).
Figure 2. (A) 1H NMR of receptor 1. (B) 1H NMR of urea complex
with 1.
complementarity of urea with receptor 1 to form a stable
complex. The synthesis of receptor 1 is delineated in Scheme
1. 1,10-Phenanthroline-dimethanol 36 is coupled with 2-piv-
occurs in the 1:1 complex from δ 9.81 to 11.15 ppm (∆δ )
1.34 ppm), and also the peri proton of isophthaloyl (C2H)
moiety shifts downfield from δ 8.80 to 9.09 ppm (∆δ )
0.29 ppm) possibly due to the close approach of the urea
carbonyl oxygen for strong complexation as suggested in
Figure 1. The strong complexation is also evident from the
better resolution of the peaks in the NMR spectrum. Two
overlapping doublets at δ 8.4 appear as two separate doublets
with the middle peaks merging on complexation, which
reduces free rotation compared to the uncomplexed receptor.
During the study of binding constant measurement by the
dilution method, it was observed that on dilution of the 1:1
complex, there is no shift of the amide and urea protons in
Scheme 1. Synthesis of Receptor 1
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Madhukar, P.; Madura, J. D.; Thummel, R. P. J. Am. Chem. Soc. 1990,
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Bell, T. W.; Beckles, D. L.; CraggLiu, P. J.; Maioriello, J.; Papoulis, A.
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aloylamino-6-bromomethylpyridine7 (obtained by NBS bro-
mination of 2-pivaloylamino-6-methylpyridine) by sodium
hydride to give 4, which on hydrolysis forms the diamine 5.
High-dilution coupling of 5 with isophthaloyl chloride affords
the desired macrocyclic receptor 1 in 8-10% yield.
(4) Jorgensen, W. L.; Pranata, J. J. Am. Chem. Soc. 1990, 112, 2008.
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Abstr. 1991, 115, 136387g.
The 1H NMR experiment in CDCl3 reveals that the
receptor 1 forms a strong 1:1 complex to solubilize urea,
which is evident from the appearance of four urea protons
at δ 6.36 (Figure 2). The downfield shift of amide protons
1284
Org. Lett., Vol. 7, No. 7, 2005

the NMR spectra, which implies the formation of a strong
urea complex.
The fluorescence spectra of receptor 1 and its urea and
thiourea complexes in chloroform at 298 K are shown in
Figure 3, using 10-6 M concentration. Receptor 1, when
i.e., the phenanthroline9 part of 1, may be greater due to the
fact that thiourea NH is more acidic8 than urea. It may also
be noted that S1 f T1 intersystem crossing in the phenan-
throline moiety will be more efficient in the presence of the
S atom of thiourea due to large spin-orbit coupling.
The absorbance effects at different dilutions of the urea
complex with receptor 1 at λmax 272 and 239 nm, respec-
tively, were shown to give straight lines. To determine the
approximate binding constant (Ka) of 1 with urea by the
fluorescence technique,10 at first a 10-6 M 1:1 urea complex
solution was prepared by dilution of the solution, used in
NMR showing a 1:1 complex. Different compositions (by
volume) of the urea complex of 1 in chloroform were then
prepared by dilution of the above solution and also by
dilution of the residue obtained by evaporating the NMR
solution of the 1:1 complex. The solubility of urea in
chloroform is so limited that an accurate value of Ka cannot
be determined. Here, the binding of receptor 1 is so tight
(as also suggested from the fact that there is no change in
the NMR spectrum of the 1:1 complex on dilution) that
significant dissociation does not occur as a result of the minor
change in concentration, and thus the emission intensity
should decrease linearly as a function of concentration, which
is actually the case here. However, the fluorescence experi-
ments suggest that Ka for urea cannot be less than the order
of 105. For thiourea, Ka (≈103) is much less than that for
urea. This suggests that receptor 1 has a stronger affinity
for urea than for thiourea.
Figure 3. Fluorescence spectra in CHCl3 (6.64 × 10-6 M/L in
each case) at 298 K for (a) receptor 1, (b) the urea complex of 1,
and (c) the thiourea complex of 1. The emission spectrum for c is
depicted in the inset.
excited at 272 nm (λmax of 1 in chloroform at 298 K), gives
emission maxima around 365 and 384 nm. On complexation
with urea and thiourea, significant fluorescence quenching
takes place in both cases as shown in Figure 3, where the
fluorescence maximum (λF) of the urea complex is slightly
blue shifted and the thiourea complex is slightly red shifted
with respect to 1. To study the effect of free urea concentra-
tion in solution, we recorded different fluorescence spectra
of urea complex solutions having increasing urea concentra-
tions, and we observed about 50% quenching in all cases
with respect to 1 [Figure 3]. There was no change in the
quenching and shift of λF when the fluorescence spectra of
1 in chloroform at 298 K were recorded in the presence of
excess urea or in ethyl alcohol, dilute HCl, or water. This
implies that quenching was due to the urea complex of 1.
To validate this implication, excitation and absorption spectra
of the complex were also compared. So by quenching, shift
of λF, and also by comparison of absorption and excitation
spectra, fluorescence response by urea binding was detected.
Though the overall binding of urea with 1 is greater than
that of thiourea, the quenching by thiourea complex of 1 is
greater than that of the urea complex, which may be due to
the greater tendency of thiourea for ground-state complex
formation through hydrogen bond interaction with the
fluorophore, i.e., the phenanthroline9 part of 1. Thus, though
thiourea makes a weaker hydrogen bond with sulfur com-
pared to oxygen, interaction of thiourea with the fluorophore,
We have thus developed a nonenzymatic neutral fluores-
cent synthetic probe for notoriously insoluble urea in
chloroform. The 1,10-phenanthroline moiety of 1 possibly
lies in the binding zone to influence the fluorescence property
during complexation, showing the potential applicability of
1 as a fluorescent sensor for urea and thiourea in a
nondegradative way.
Acknowledgment. We acknowledge DST, Government
of India, for financial assistance. We appreciate the help of
Dr. Avijit K. Adak and Swapan Dey of our laboratory. We
are grateful to the reviewers for their valuable suggestions.
Supporting Information Available: Experimental details
and spectral data (1H and 13C NMR and mass spectra) for 1
and its urea complex (1H NMR), 4, figure for fluorescence
spectra of receptor 1 in chloroform with varying amounts
of urea and their one representative calculation for ap-
proximate Ka determination for the urea complex of 1, the
linear regression analysis, and also the corresponding change
of fluorescence spectra on dilution. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL050034H
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