8168
K. Ghosh, S. Adhikari / Tetrahedron Letters 47 (2006) 8165–8169
O2N
S
a
N
H
N
H
O
O
O
O
O2N
S
N
H
N
H
b
c
F
O2N
S
N
N
O
O
Figure 8. Partial 1H NMR (400 MHz) of 1 (6 mM) in CDCl3 containing 0.03% DMSO-d6, (b) 1:1 complex with Fꢀ, (c) in the presence of excess Fꢀ.
field chemical shift of the thiourea protons (Dd Ha 3.21,
Hb 3.37 ppm) of 1 in the 1:1 complex with benzoate indi-
cated a strong hydrogen bonding interaction. In the case
of the 1:1 complex of fluoride with 1, both the thiourea
protons (Ha and Hb) underwent downfield shifts (Dd Ha
1.98, Hb 1.44 ppm) and became broad and then dis-
appeared in the presence of excess fluoride, suggesting
either deprotonation or strong hydrogen bonding. The
relatively smaller downfield shifting of the thiourea
protons in 2 upon addition of benzoate and fluoride
indicated a weaker binding. The electron donating reso-
nance effect of the phenyl group increases the charge
density on the carboxylate group allowing it to form
strong hydrogen bonds with the acidic thiourea protons
of 1. Fluoride, on the other hand, due to its small size
and high charge density, initially forms a hydrogen-
bonded complex (see Fig. 8b) and then, in the presence
of excess Fꢀ, causes deprotonation (shown in Fig. 8c) to
form the L2ꢀ species (L = coumarin receptor).
Acknowledgments
We thank the CSIR [01(1922)/04/EMR-II], New Delhi,
India, for financial support and DST-FIST for provid-
ing the facilities in the Department.
References and notes
1. (a) Supramolecular Chemistry of Anions; Bianchi, E.,
Bowman, J. K., Garcia-Espana, E., Eds.; Wiley-VCH:
New York, 1997; (b) Martinez-Manez, R.; Sancenon, F.
Chem. Rev. 2003, 103, 4419; (c) Evans, L. S.; Gale, P. A.;
Light, M. E.; Quesada, R. Chem. Commun. 2006, 965; (d)
Kirk, K. L. Biochemistry of Halogens and Inorganic
Halides; Plenum Press: New York, 1991, p 58; (e)
Chemosensors for Ion and Molecular Recognition; Desver-
gne, J.-P., Czarnik, A. W., Eds.; Kluwer: Dordrecht, 1997;
Vol. 492, (f) Sohn, H.; Letant, S.; Sailor, M. J.; Trogler,
W. C. J. Am. Chem. Soc. 2000, 122, 5399.
2. (a) Lee, S. H.; Kim, H. J.; Lee, Y. O.; Vicens, J.; Kim, J. S.
Tetrahedron Lett. 2006, 47, 4373; (b) Causey, C. P.; Allen,
W. E. J. Org. Chem. 2002, 67, 5963, and references cited
therein.
3. (a) Gomez, D.; Fabbrizzi, L.; Licchelli, M. J. Org. Chem.
2005, 70, 5717; (b) Fan, E.; van Arman, S. A.; Kincaid, S.;
Hamilton, A. D. J. Am. Chem. Soc. 1993, 115, 369; (c) Cho,
E. J.; Ryu, B. J.; Lee, Y. J.; Nam, K. C. Org. Lett. 2005, 7,
2607, and references cited therein; (d) Kondo, S.-I.;
Hiroka, Y.; Kurumatani, N.; Yano, Y. Chem. Commun.
2005, 1720; (e) Lee, D. H.; Lee, H. Y.; Hong, J.-I.
Tetrahedron Lett. 2002, 43, 7273.
4. Best, M. D.; Tobey, S. L.; Anslyn, E. V. Coord. Chem.
Rev. 2003, 240, 3.
5. Linares, J. M.; Powell, D.; Bowman-James, K. Coord.
Chem. Rev. 2003, 240, 57.
6. (a) Gale, P. A. Coord. Chem. Rev. 2003, 240, 1; (b) Sessler,
J. L.; Davis, J. M. Acc. Chem. Res. 2001, 34, 989; (c)
Suksai, C.; Tuntulani, T. Chem. Soc. Rev. 2003, 32, 192;
(d) Yoon, J.; Kim, S. K.; Singh, J.; Kim, K. S. Chem. Soc.
Rev. 2006, 35, 355.
7. (a) Ghosh, K.; Masanta, G. Tetrahedron Lett. 2006, 47,
2365; (b) Ghosh, K.; Adhikari, S. Tetrahedron Lett. 2006,
47, 3577; (c) Ghosh, K.; Masanta, G. Chem. Lett. 2006,
34, 414.
However, these non-covalent interactions altogether
enhance the efficiency of the PET process. The high de-
gree of fluorescence quenching is believed to result from
the increase in the reduction potential of the thiourea
receptor moieties after anion recognition. This increases
the rate of electron transfer from the HOMO of the thio-
urea-anion complex to the coumarin-excited state and
encourages the PET process. It appears that the depro-
tonated species LHꢀ/L2ꢀ, being more electron rich com-
pared to the hydrogen-bonded complex with benzoate,
activates the PET process more efficiently and shows a
greater quenching. The origin of the color change in
the host solution of 1 is ascribed to the charge-transfer
interactions between the electron-rich thiourea–anion
complex donor unit and the electron-deficient p-nitro-
phenyl moiety.
In conclusion, ‘fluorophore-spacer-receptor’ model based
thiourea linked coumarin receptors 1 and 2 have been
presented. The receptor 1 shows a strong binding to
benzoate over fluoride ions and can report the molecular
recognition events both by changes in fluorescence and
color. Further research in this direction is underway in
our laboratory.
8. de Silva, A. P.; McCoy, C. P.; Rademacher, J. T.; Rice, T.
E. Chem. Rev. 1997, 97, 1515.