Fluorescence Sensing of Anions
metal-to-ligand charge transfer (MLCT), excimer/exciplex
1
0
SCHEME 1. Structure and Synthesis of Sensors TABO and
PUBO
4
a,11
12
formation,
and tuning proton transfer for fluorescence
chemosensors. But the differentiation is always poor due to the
5c
same spectral outputs for the anion-receptor interactions. Here
we report two new anion fluorescence chemosensors based on
inhibition of the excited-state intramolecular proton transfer
(ESIPT). By careful selection of the acidity and the hydrogen-
bonding donor ability of the protons that are crucial to the ESIPT
process, the above biologically interesting anions can be
distinguished from the different spectral outputs for anion-
sensor interactions.
Although inhibition of the ESIPT has been well exploited
1
3
14
for cation sensing, it was poorly applied for anion sensing.
In view of the attracting nature of the ESIPT and the know-
ledge of the hydrogen bond formation/neat proton transfer
existing in the anion recognition processes gained by Fabbrizzi
and co-workers15 and more recently by us, it is possible to
develop new anion chemosensors based on the influence of
anion binding on the ESIPT process. In the ESIPT mole-
cules, the intramolecular hydrogen bonds (IHB) formed in the
ground state between the hydrogen-bonding donors (e.g., -OH,
12
-NH2, etc.) and acceptors (dN-, >CdO, etc.) in close
(3) (a) Gunnlaugsson, T.; Kruger, P. E.; Jensen, P.; Tierney, J.; Ali, H.
D. P.; Hussey, G. M. J. Org. Chem. 2005, 70, 10875-10878. (b) Gomez,
D. E.; Fabbrizzi, L.; Licchelli, M.; Monzani, E. Org. Biomol. Chem. 2005,
3
, 1495-1500. (c) Jose, D. A.; Kumar, D. K.; Ganguly, B.; Das, A. Org.
proximity are vital to the ESIPT process.16 For example, in
Lett. 2004, 6, 3445-3448. (d) Lee, D. H.; Im, J. H.; Lee, J. H.; Hong, J. I.
Tetrahedron Lett. 2002, 43, 9637-9640.
recent years the general family of 2-(2′-hydroxyphenyl)benz-
(4) (a) Kim, S. K.; Bok, J. H.; Bartsch, R. A.; Lee, J. Y.; Kim, J. S.
17
oxazole (HBO) has been studied extensively to elucidate the
Org. Lett. 2005, 7, 4839-4842. (b) Gale, P. A. Chem. Commun. 2005,
761-3772. (c) Camiolo, S.; Gale, P. A.; Hursthouse, M. B.; Light, M. E.;
Warriner, C. N. Tetrahedron Lett. 2003, 44, 1367-1369.
5) (a) Lee, D. H.; Lee, K. H.; Hong, J. I. Org. Lett. 2001, 3, 5-8. (b)
Lee, D. H.; Lee, H. Y.; Lee, K. H.; Hong, J. I. Chem. Commun. 2001,
mechanism of the ESIPT process, and the disturbances of the
ESIPT in them have been ingeniously elaborated for sensing
purposes. Deprotonation of the hydroxyl proton is the result of
3
(
13a
13b
the coordination with lithium and zinc
ions, or of the
1
2
188-1189. (c) Zhang, X.; Guo, L.; Wu, F. Y.; Jiang, Y. B. Org. Lett.
1
4c
reaction with basic fluoride ions, so the ESIPT process is no
longer possible, and a substantial blue shift in the peak emission
would be expected. A wealth of information available about
the photophysics of benzoxazole derivatives has prompted us
to choose this class of molecules as an attractive starting point
to explore the influence of anion binding on the ESIPT process.
But for an anion sensor, if the sensing process is only based
on proton transfer, the anion differentiation will definitely be
poor, especially for the same spectral outputs with the addition
of excess anions, due to the similar basicity and surface charge
003, 5, 2667-2670.
(6) Chellappan, K.; Singh, N. J.; Hwang, I. C.; Lee, J. W.; Kim, K. S.
Angew. Chem., Int. Ed. 2005, 44, 2899-2903.
7) Sessler, J. L.; Camiolo, S.; Gale, P. A. Coord. Chem. ReV. 2003,
40 (1-2), 17-55.
8) (a) Wen, Z. C.; Jiang, Y. B. Tetrahedron 2004, 60, 11109-11115.
b) Wu, F. Y.; Li, Z.; Guo, L.; Wang, X.; Lin, M. H.; Zhao, Y. F.; Jiang,
Y. B. Org. Biomol. Chem. 2006, 4, 624-630.
9) (a) Thiagarajan, V.; Ramamurthy, P.; Thirumalai, D.; Ramakrishnan,
(
2
(
(
(
V. T. Org. Lett. 2005, 7, 657-660. (b) Gunnlaugsson, T.; Ali, H. D. P.;
Glynn, M.; Kruger, P. E.; Hussey, G. M.; Pfeffer, F. M.; dos Santos, C. M.
G.; Tierney, J. J. Fluoresc. 2005, 15, 287-299.
(10) Sun, S. S.; Anspach, J. A.; Lees, A. J.; Zavalij, P. Y. Organometallics
5c,12
density of the anions.
So in order to selectively differentiate
2
002, 21, 685-693.
11) Wu, J. S.; Zhou, J. H.; Wang, P. F.; Zhang, X. H.; Wu, S. K. Org.
Lett. 2005, 7, 2133-2136.
12) Peng, X.; Wu, Y.; Fan, J.; Tian, M.; Han, K. J. Org. Chem. 2005,
0, 10524-10531.
(
the anions by utilizing the ESIPT mechanism, the anion receptor
moieties of the newly designed sensors should have the
following characteristics: receptor moieties should form IHB
in the ground state with the adjacent hydrogen-bonding accep-
tors; the acidity of the receptors should be weak enough to avoid
the deprotonation of sensors upon interaction with the basic
(
7
(13) (a) Obare, S. O.; Murphy, C. J. New J. Chem. 2001, 25, 1600-
1
5
604. (b) Henary, M. M.; Fahrni, C. J. J. Phys. Chem. A 2002, 106, 5210-
220. (c) Fahrni, C. J.; Henary, M. M.; VanDerveer, D. G. J. Phys. Chem.
A 2002, 106, 7655-7663. (d) Henary, M. M.; Wu, Y. G.; Fahrni, C. J.
Chem. Eur. J. 2004, 10, 3015-3025. (e) Wu, K. C.; Lin, Y. S.; Yeh, Y. S.;
Chen, C. Y.; Ahmed, M. O.; Chou, P. T.; Hon, Y. S. Tetrahedron 2004,
-
-
anions such as F or CH3COO but should be strong enough
to induce a fast rate of ESIPT and increase the quantum yields
6
0, 11861-11868. (f) Taki, M.; Wolford, J. L.; O’Halloran, T. V. J. Am.
T 18
of the tautomer emission Φfl . In our new sensor 2-(2′-
Chem. Soc. 2004, 126, 712-713. (g) Zhang, X. B.; Peng, J.; He, C. L.;
Shen, G. L.; Yu, R. Q. Anal. Chim. Acta 2006, 567, 189-195.
phenylureaphenyl)benzoxazole (PUBO, 2, Scheme 1), the pKa
of the NHR fragment in it satisfies the above requirements, which
(14) (a) Tong, H.; Zhou, G.; Wang, L. X.; Jing, X. B.; Wang, F. S.;
Zhang, J. P. Tetrahedron Lett. 2003, 44, 131-134. (b) Zhou, G.; Cheng,
Y. X.; Wang, L. X.; Jing, X. B.; Wang, F. S. Macromolecules 2005, 38,
14
is higher than an -OH group and lower than an -NH2
2
148-2153. (c) Lee, J. K.; Na, J.; Kim, T. H.; Kim, Y. S.; Park, W. H.;
Kim, J.; Lee, T. S. Mater. Sci. Eng., C 2004, 24, 261-264.
15) (a) Boiocchi, M.; Del Boca, L.; Gomez, D. E.; Fabbrizzi, L.;
Licchelli, M.; Monzani, E. J. Am. Chem. Soc. 2004, 126, 16507-16514.
b) Boiocchi, M.; Del Boca, L.; Esteban-Gomez, D.; Fabbrizzi, L.; Licchelli,
(16) Schulman, S. G. Acid-Base Chemistry of Excited Singlet States.
In Modern Fluorescence Spectroscopy; Wehry, E. L., Ed.; Plenum Press:
New York, 1976; pp 239-275.
(17) (a) Das, K.; Sarkar, N.; Ghosh, A. K.; Majumdar, D.; Nath, D. N.;
Bhattacharyya, K. J. Phys. Chem. 1994, 98, 9126-9132. (b) Roberts, E.
L.; Dey, J.; Warner, I. M. J. Phys. Chem. 1996, 100, 19681-19686. (c)
Rios, M. A.; Rios, M. C. J. Phys. Chem. A 1998, 102, 1560-1567.
(
(
M.; Monzani, E. Chem. Eur. J. 2005, 11, 3097-3104. (c) Amendola, V.;
Esteban-Gomez, D.; Fabbrizzi, L.; Licchelli, M. Acc. Chem. Res. 2006,
3
9, 343-353.
J. Org. Chem, Vol. 72, No. 1, 2007 63