Chromofluorescent probes for selective detection of fluoride and acetate ions

Article abstract of DOI:10.1021/ol802352j

(Graph Presented) Chromofluorescent probe 2 has been developed for selective and ratiometric estimation of F^- (CH₃CN) ions. The presence of a lipophilic dodecyl appendage at imidazolium nitrogen in 3 successfully allows the selective

Full text of DOI:10.1021/ol802352j

ORGANIC
LETTERS
2008
Vol. 10, No. 24
5549-5552
Chromofluorescent Probes for Selective
Detection of Fluoride and Acetate Ions
Subodh Kumar,* Vijay Luxami, and Ashwani Kumar
Department of Chemistry, Guru Nank DeV UniVersity, Amritsar 143 005, India
subodh_gndu@yahoo.co.in
Received October 11, 2008
ABSTRACT
Chromofluorescent probe 2 has been developed for selective and ratiometric estimation of F- (CH3CN) ions. The presence of a lipophilic dodecyl appendage
at imidazolium nitrogen in 3 successfully allows the selective determination of AcO- ions in protic (CHCl3-MeOH, 1:1) solvent.
The development of artificial optical receptors for selective anion
recognition has attained significant interest, as anions play a
fundamental role in a wide range of chemical and biological
processes.1 Recently, considerable efforts have been made to
develop colorimetric2 and fluorescent3 molecular probes for the
recognition of anions. The colorimetric probes have been attracted
for their “naked eye” detection of target species and offer qualitative
and quantitative information using inexpensive equipment.4 The
emission-based chemosensors are particularly attractive as fluori-
metric techniques, in general exhibit a high degree of specificity
(via choice of excitation and emission wavelengths), and enable
access to a wealth of information (e.g., spectral and intensity
changes, fluorescence lifetime, etc.) in a rather straightforward
manner and fast measurement protocols. Especially, the probes with
visible region excitation and emission have attained significance
for their reduced scattering and low background emissions.
However, only scarce examples of metal ion probes with red
emission have been reported,5 and to the best of our knowledge,
anion probes with red emission are not known.
(1) (a) de Silva, A. P.; Gunaratne, H. Q.; Gunnlaugsson, T.; Huxley,
A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997,
97, 1515–1566. (b) Fabrizzi, L.; Poggi, A. Chem. Soc. ReV. 1995, 197–
202. (c) Martinez-Manez, R.; Sancenon, F. Chem. ReV. 2003, 103, 4419–
4476. (d) Callan, J. F.; de Silva, A. P.; Magro, D. C. Tetrahedron 2005,
61, 8551–8588. (e) Martinez-Manez, R.; Sancenon, F. Coord. Chem. ReV.
2006, 250, 3081–3093. (f) Gunnlaugsson, T.; Glynn, M.; Tocci, G. M.;
Kruger, P. E.; Pfeffer, F. M. Coord. Chem. ReV. 2006, 250, 3094–3117.
(g) Gale, P. A.; Garcia-Garrido, S. E.; Garric, J. Chem. Soc. ReV. 2008, 37,
151–190.
(2) (a) Yun, S.; Ihm, H.; Kim, H. G.; Lee, C. W.; Indrajit, B.; Oh, K. S.;
Gong, Y. J.; Lee, J. W.; Yoon, J.; Lee, H. C.; Kim, K. S. J. Org. Chem.
2003, 68, 2467–2470. (b) Boiocchi, M.; Boca, L. D.; Gomez, D. E.;
Fabbrizzi, L.; Licchelli, M.; Monzani, E. J. Am. Chem. Soc. 2004, 126,
16507–16514. (c) Jose, D. A.; Kumar, D. K.; Ganguly, B.; Das, A. Org.
Lett. 2004, 6, 3445–3448. (d) Cho, E. J.; Ryu, B. J.; Lee, Y. J.; Nam, K. C.
Org. Lett. 2005, 7, 2607–2609. (e) Esteban-Gomez, D.; Fabbrizzi, L.;
Licchelli, M. J. Org. Chem. 2005, 70, 5717–5720. (f) Evans, L. S.; Gale,
P.; Light, M. E.; Quesada, R. New J. Chem. 2006, 30, 1019–1025. (g)
Thangadurai, T. D.; Singh, N. J.; Hwang, I.; Lee, J. W.; Chandaran, R. P.;
Kim, K. S. J. Org. Chem. 2007, 72, 5461–5464. (h) Yeo, H. M.; Ryu, B. J.;
Nam, K. C. Org. Lett. 2008, 10, 2931–2934. (i) Wang, T.; Bai, Y.; Yan, X.
Org. Biomol. Chem. 2008, 6, 1751–1755. (j) Ali, H. D. P.; Kruger, P. E.;
Gunnlaugsson, T. New J. Chem. 2008, 32, 1153–1161. (k) Day, J. K.;
Bresner, C.; Coombs, N. D.; Fallis, I. A.; Ooi, L.; Aldridge, S. Inorg. Chem.
2008, 47, 793–804.
(3) (a) Gunnlaugsson, T.; Davis, A. P.; O’Brien, J. E.; Glynn, M. Org.
Lett. 2002, 4, 2449–2452. (b) Bruseghini, I.; Fabbrizzi, L.; Licchelli, M.;
Taglietti, A. Chem. Commun. 2002, 1348–1349. (c) Gunnlaugsson, T.;
Davis, A. P.; Hussey, G. M.; Tierney, J. Org. Biomol. Chem. 2004, 2, 1856–
1863. (d) Liu, S.-Y.; Fang, L.; He, Y.-B.; Chan, W.-H.; Yeung, K.-T.;
Cheng, Y.-K.; Yang, R.-H. Org. Lett. 2005, 7, 5825–5828. (e) Gunnlaugsson,
T.; Davis, A. P.; O’Brien, J. E.; Glynn, M. Org. Biomol. Chem. 2005, 3,
48–56. (f) Zhao, Y.; Zhang, B.; Duan, C,; Lin, Z.; Meng, Q. New J. Chem.
2006, 30, 1207–1213. (g) Sun, X. H.; Li, W.; Xia, P. F.; Luo, H.-B.; Wei,
Y.; Wong, M. S.; Cheng, Y.-K.; Shuang, S. J. Org. Chem. 2007, 72, 2419–
2426. (h) Garcia-Garrido, S. E.; Caltagirone, C.; Light, M. E.; Gale, P. A.
Chem. Commun. 2007, 1450–1452. (i) Kim, D.-S.; Ahn, K. H. J. Org. Chem.
2008, 73, 6831–6834.
10.1021/ol802352j CCC: $40.75
Published on Web 11/25/2008
2008 American Chemical Society

In molecular probes, the binding affinity and thus the
selectivity between an anion and the host is attributed to
hydrogen bonding and/or electrostatic interactions and is also
significantly affected by the topology of the ligating sites,6 viz.,
probes 2 and 3, which enable naked eye and dual channel
(absorption and fluorescence) detection of F- and AcO- ions,
respectively. Significantly, in these probes, the appearance of
absorption and emission maxima due to free probe (λabs 405 nm,
ε ) 6600 M-1 cm-1; λem 475 nm; ꢀ ) 0.11) and their complexes
with the anion (λabs ) 480 nm, ε ) 6000 M-1 cm-1; λem ) 580
nm, ꢀ ) 0.38) at different wavelengths has enabled elaboration
of ratiometric approach (Figure 1). The probes 2 and 3 constitute
the first examples where N-arylimidazoilium9 salts have found
application as anion sensors.
1-(Chloroacetylamido)-anthracene-9,10-dione (1)13 on heat-
ing with 1-benzylimidazole and 1-dodecylimidazole in DMSO
for 4-5 h gave respective chemosensors 2 (80%, mp > 300
°C, M+ m/z 404) and 3 (78%, mp > 300 °C, M+ m/z 482 for
cation); for spectral data see Figures S1-S8 in Supporting
Information. Evidently, the initially formed imidazolium salts
4a and 4b undergo intramolecular condensation at the an-
thraquinone carbonyl group to form respective compounds 2
and 3. The appearance of upfield 1H doublet at δ 6.68 in 2 and
at δ 6.58 in 3 demonstrates that the Hx proton (Scheme 1) faces
Figure 1. (A) UV-vis spectra of 2 (50 µM) in CH3CN-DMSO (20:
1) and 2 + F- (100 µM). (B) Fluorescence spectra of 2 (10 µM) and
2 + F- (20 µM). The inset shows the color changes on addition of
fluoride ions under (A) visible light and (B) irradiation of UV light.
See Figure S10 in Supporting Information for 3.
that of the anion (size and spherical, trigonal, or tetrahedral
geometries). In contrast to ammonium and guanidinium7 based
anion probes, quaternary ammonium,8,12c imidazolium,9 ben-
zimidazolium,10 pyridinium,11 etc. salts based anion probes
remain unaffected by the pH of the medium and so can find
applicability under physiological conditions.
Scheme 1. Synthesis of Chemosensor 2 and 3
Our work aims12 at the design and synthesis of molecular probes
with colorimetric and fluorimetric assays to selectively detect the
presence of a target anion over the wide range of other interfering
anions. Here, we have developed new N-aryl imidazolium based
(4) (a) Yu, X.; Lin, H.; Cai, Z.; Lin, H. Tetrahedron Lett. 2007, 48,
8615–8618. (b) Lin, Z.; Ou, S.; Duan, C.; Zhang, B.; Bai, Z. Chem.
Commun. 2006, 624–626.
the ring currents of imidazolium ring. The proximity of this
CH with imidazolium ring has been ascertained by X-ray crystal
structure of PF6 salt of 2 (Figure S9, Supporting Information).
(5) (a) Xu, Z.; Qian, X.; Cui, J. Org. Lett. 2005, 7, 3029–3032. (b) Xu,
K.; Tang, B.; Huang, H.; Yang, G.; Chen, Z.; Li, P.; An, L. Chem. Commun
2005, 5974–5976.
(6) (a) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 486–
516. (b) Bowman-James, K. Acc. Chem. Res. 2005, 38, 671–678, and
references therein.
(7) (a) Schmuck, C. Coord. Chem. ReV. 2006, 250, 3053–3067. (b)
Llinares, J.; Powell, D.; Bowman-James, K. Coord. Chem. ReV. 2003, 240,
57–75. (c) Fitzmaurice, R. J.; Kyne, G. M.; Douheret, D.; Kilburn, J. D.
J. Chem. Soc., Perkin Trans. 1 2002, 841–864. (d) Schmuck, C.; Bickert,
V. J. Org. Chem. 2007, 72, 6832–6839. (e) Raker, J.; Glass, T. E. J. Org.
Chem. 2002, 67, 6113–6116.
(8) (a) Worm, K.; Schmidtchen, F. P. Angew. Chem., Int. Ed. 1995, 34,
65–66. (b) Hossain, M.; Kang, S. K.; Powell, D.; Bowman-James, K. Inorg.
Chem. 2003, 42, 1397–1399
.
(9) (a) Yoon, J.; Kim, S. K.; Singh, N. J.; Kim, K. S. Chem. Soc. ReV.
2006, 35, 355–360, and references therein. (b) Khatri, V. K.; Upreti, S.;
Pandey, P. S. Org. Lett. 2006, 8, 1755–1758. (c) Xu, Z.; Kim, S.; Lee,
K. H.; Yoon, J. Tetrahedron Lett. 2007, 48, 3797–3800. (d) Yoon, J.; Kim,
S. K.; Singh, N. J.; Lee, J. W.; Yang, Y. J.; Chellappan, K.; Kim, K. S. J.
Org. Chem. 2004, 69, 581–583. (e) Lee, H. N.; Singh, N. J.; Kim, S. K.;
Kwon, J. Y.; Kim, Y. Y.; Kim, K. S.; Yoon, J. Tetrahedron Lett. 2007, 48,
169–172. (f) Singh, N. J.; Jun, E. J.; Chellappan, K.; Thangadurai, D.;
Chandran, R. P.; Hwang, I.; Yoon, J.; Kim, K. S. Org. Lett. 2007, 9, 485–
488.
Figure 2. (a) Effect of incremental addition of fluoride ions on the
UV-vis spectrum of 2 (50 µM) in CH3CN-DMSO (20:1). (b)
Effect of incremental addition of acetate ions on the UV-vis
spectrum of 3 (50 µM) in CHCl3-MeOH(1:1).
(10) (a) Wong, W. H.; Vicker, M.; Cowley, A. R.; Paul, R. L.; Beer,
P. D. Org. Biomol. Chem. 2005, 3, 4201–4208. (b) Joo, T. Y.; Singh, N.;
Lee, G. W.; Jang, D. O. Tetrahedron Lett. 2007, 48, 8846–8850.
(11) (a) Kumaresh, G.; Masanta, G.; Chattopadhyay, A. P. Tetrahedron
Lett. 2007, 48, 6129–6132. (b) Dickson, S. J.; Wallace, E. V. B.; Swinburne,
A. N.; Paterson, M. J.; Lloyd, G. O.; Beeby, A.; Belcher, W. J.; Steed,
J. W. New J. Chem 2008, 32, 786–789. (c) Filby, M. H.; Dickson, S. J.;
Zaccheroni, N.; Prodi, L.; Bonacchi, S.; Montalti, M.; Paterson, M. J.;
Humphries, T. D.; Chiorboli, C.; Steed, J. W. J. Am. Chem. Soc. 2008,
130, 4105–4113, and references therein.
The chemosensor 2 (50 µM, CH3CN-DMSO (20:1)) ex-
hibited absorption bands at λmax 308 nm (ε 8200) and 405 nm
(12) (a) Kaur, N.; Kumar, S. Chem. Commun. 2007, 3069–3070. (b)
Kaur, S.; Kumar, S. Chem. Commun. 2002, 2840–2841. (c) Luxami, V.;
Sharma, N.; Kumar, S. Tetrahedron Lett. 2008, 49, 4265–4268. (d) Luxami,
V.; Kumar, S. Tetrahedron Lett. 2007, 48, 3083–3087. (e) Kaur, N.; Kumar,
S. Dalton Trans. 2006, 3766–3771.
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Org. Lett., Vol. 10, No. 24, 2008

(ε 6600). On addition of tetrabutylammonium fluoride
(TBA F) (100 µM) to a solution of 2, the color of the solution
changed from light yellow to orange. The addition of acetate
(100 equiv) and dihydrogen phosphate (500 equiv) to the
chemosensor 2 showed significantly smaller changes than that
observed for 2 equiv of F- anion (Figure S11, Supporting
Information). The other anions, e.g., Cl-, Br-, I-, NO3-, HSO4-,
CN-, and ClO4-, caused no significant change in the absorption
spectrum of 2.
On gradual addition of TBAF to a solution of 2 (50 µM,
CH3CN-DMSO (20:1), the absorbance at 405 nm underwent
gradual decrease in its intensity with concomitant increase at
480 nm with isobestic point at 440 nm (Figure 2a). This red
shift by 75 nm points to the significant stabilization of the
intramolecular charge transfer the excited state achieved through
interaction of 2 with fluoride anions. On addition of water, the
reversal in spectral changes indicated both the reversibility of
the process and poor complexation of 2 with F- ion. Evidently,
the solvation of fluoride ions in protic solvent decreased its
binding with chemosensor 2.
The addition of tetrabutylammonium hydroxide (TBA OH)
to the solution of chemosensor 2 gave the similar change in
its UV-vis spectrum (Figure S12, Supporting Information)
as that observed with the TBAF and points to the fluoride
ion induced deprotonation14 of amide NH responsible for
the spectral and visible color changes.
The plot of absorbance of 2 at 308 and 480 nm against
concentration of fluoride ions shows a straight line that attains
a plateau after addition of 100 µM (2 equiv) of fluoride ions.
stants15 log ꢁLF ) 6.2 ( 0.3, log ꢁLF2 ) 12.0 ( 0.4; log ꢁL2AcO
) 11.4 ( 0.2, log ꢁLAcO ) 5.7 ( 0.1; log ꢁLAcO2 ) 10.7 ( 0.2.
This shows that among 1:1 complexes of 3 with F- and AcO-
ions, the F- ions are associated only five times better than AcO-
ions. Also, probe 3 exhibits nearly 10 times stronger association
with F- ions than that observed for probe 2. Therefore, the
presence of the lipophilic dodecyl appendage in 3 increases its
binding capacity toward F- and AcO- anions.
To evaluate the role of the dodecyl chain in 3 on its association
with anions in the presence of protic solvents,16 we studied the
effect of different amounts of MeOH on spectra of 3-F- and
3-AcO- complexes in chloroform. We have found that a
CHCl3-MeOH (1:1) solution of 3 does not show any spectral or
color change on addition of F- ions, but on addition of AcO- ions,
a new absorption band at 475 nm is observed that is hypsochro-
mically shifted by only 10 nm in comparison to that observed in
pure CHCl3. On gradual addition of TBA OAc to a solution of 3
(50 µM, CHCl3-MeOH; 1:1), the absorbance at 405 nm under-
went gradual decrease in its intensity with concomitant increase at
475 nm, and the spectral fitting of these absorbance data (Figure
2b, Figure S18, Supporting Information) revealed the formation
of 1:1, 1:2, and 1:3 (3:AcO-) stoichiometric complexes with
stability constants log ꢁLAcO ) 5.15 ( 0.2, log ꢁLAcO2 ) 10.6 (
0.2, and log ꢁLAcO3 ) 14.4 ( 0.2. Therefore, 3 can be used as a
probe for selective estimation of AcO- ions. Probably, due to
greater solvation of fluoride ions in protic solvent, the association
of F- ions is completely inhibited, but AcO- ion shows reasonable
complexation with 3.
The “OFF-ON” switching behavior on addition of F- ions
to a solution of 2 at 405 and 480 nm and on addition of acetate
ions to 3 provides dual absorption channel for elaborating a
ratiometric approach17 that permits signal rationing and allows
the estimation of analyte independent of the concentration of
the receptor (Figure 3a). Chemosensor 2 (CH3CN) can be used
for estimation of fluoride ions between 5 and 100 µM, and
chemosensor 3 (CH3CN-MeOH, 1:1) for the estimation of
acetate ions between 2 and 200 µM (Figure 3b).
Chemosensor 2 (10 µM, CH3CN-DMSO (20:1)) on excitation
at 410 nm exhibited an emission band at λem 480 nm. The addition
of Cl-, Br-, I-, NO3-, CH3COO-, HSO4-, H2PO4-, and ClO4-
anions (0.01 M) to a solution of 2 caused insignificant changes in
its fluorescence spectrum. Interestingly, on addition of fluoride ions
(20 µM), the fluorescence intensity at 480 nm (green emission)
was turned off and simultaneously a new red-shifted fluorescence
emission band at 580 nm appeared (∆λmax 100 nm) (Figure 4a).
The color of this solution under UV-vis radiation appears red due
to the 580 nm emission band. The nonlinear regression analysis
of the spectral data obtained on titration of solution of 2 with
fluoride ions shows the formation of 2:1, 1:1, and 1:2 (2:F)
Figure 3. (a) Absorbance ratiometric (50 µM) responses (A480/A405)
of chemosensor 2 and (b) 3 (A475/A405) upon incremental addition
of fluoride and acetate ions respectively.
The spectral fitting (Figure S13, Supporting Information) of
these absorbance data shows the formation of 1:1 and 1:2
(2:F) stoichiometric complexes [log ꢁLF2 ) 8.45 ( 0.34 and
log ꢁLF ) 5.19 ( 0.12].
The chemosensor 3 (50 µM, CH3CN) on addition of fluoride
and acetate ions gave similar UV-vis changes as observed in
the case of 2 with only fluoride ions. The spectral information
obtained from titration of 3 with fluoride and acetate ions
(Figures S14 -S17, Supporting Information) on nonlinear
regression analysis using specfit 32 gave complexation con-
(15) As receptors 2 and 3 on addition of anions undergo deprotonation,
it is not a strict binding phenomenon and so the log ꢁ values need to be
taken with caution.
(16) (a) Hu, A.; Guo, Y.; Xu, J.; Shao, S. Org. Biomol. Chem. 2008, 6,
2071–2075. (b) Gunnlaugsson, T.; Kruger, P. E.; Jensen, P.; Tierney, J.;
Ali, H. D. P.; Hussey, G. M. J. Org. Chem. 2005, 70, 10875–10878.
(17) (a) Jang, Y. J.; Jun, E. J.; Lee, Y. J.; Kim, Y. S.; Kim, J. S.; Yoon,
J. J. Org. Chem. 2005, 70, 9603–9606. (b) Ojida, A.; Nonaka, H.; Miyahara,
Y.; Tamaru, S.; Sada, K.; Hamachi, I. Angew. Chem., Int. Ed. 2006, 45,
5518–5521.
(13) Ma, M.; Sun, Y.; Sun, G. Dyes Pigments 2003, 58, 27–35.
(14) (a) Gunnlaugsson, T.; Kruger, P. E.; Jensen, P.; Pfeffer, F. M.;
Hussey, G. M. Tetrahedron Lett. 2003, 44, 8909–8913. (b) Gunnlaugsson,
T.; Kruger, P. E.; Lee, T. C.; Parkesh, R.; Pfeffer, F. M.; Hussey, G. M.
Tetrahedron Lett. 2003, 44, 6575–6578.
Org. Lett., Vol. 10, No. 24, 2008
5551

stoichiometric complexes [log ꢁL2F ) 11.5 ( 0.4, log ꢁLF ) 6.22
( 0.24, and log ꢁLF2 ) 11.37 ( 0.32].
The chemosensor 3 (10 µM, CH3CN) upon excitation at 410
nm gave similar emission changes on addition of fluoride and
acetate ions (40 µM) (Figures S19 and S20, Supporting Informa-
tion) as observed in the case of 2 with only fluoride ions. On
Figure 5. (a) Fluorescence ratiometric (10 µM) responses (I580/I480)
of 2 and (b) 3 upon incremental addition of fluoride ions and acetate
ions, respectively.
1H NMR titration experiments were performed to under-
stand the character of the receptor-anion interactions. The
1H NMR (DMSO-d6) spectrum of 2 on addition of fluoride
ions showed upfield shift of protons of nitrogen bearing ring
of anthrone moiety. This upfield shift clearly points to the
generation of negative charge on nitrogen on addition of
Figure 4. (a) Effect of incremental addition of fluoride ions on the
1
fluoride ions (Figure 6). Similar H NMR spectral changes
fluorescence spectrum of 2 (10 µM) in CH3CN. Inset shows plot of
fluorescence intensity versus [F-] (points show the experimental results
and line is curve fit). (b) Effect of incremental addition of acetate ions on
fluorescence spectrum of 3 (10 µM) in CHCl3-MeOH (1:1).
shifting from aprotic (CH3CN) to protic solvent CHCl3-MeOH
(1:1), the chemosensor 3 selectively showed red emission with
AcO- ions only. On addition of acetate ions the fluorescence
intensity at 480 nm (green emission) was turned off and simulta-
neously a new red-shifted fluorescence emission band at 580 nm
appeared (∆λmax 100 nm) (Figure 4b). The additon of F- and other
anions did not cause any significant change in the fluorescence
spectrum of 3. The spectral fitting of the fluorescence data obtained
by titration (Figure 4b) of solution of 3 with acetate ions shows
the formation of 2:1, 1:1, and 1:2 (3:AcO-) stoichiometric
complexes [log ꢁL2AcO ) 11.5 ( 0.4, log ꢁLAcO ) 6.22 ( 0.24,
and log ꢁLAcO2 ) 11.37 ( 0.32].
Figure 6.
1H NMR spectra of chemosensor 2 on addition of TBA
F in DMSO-d6.
The “OFF-ON” switching behavior at 480 and 580 nm on
addition of F- to a solution of 2 and acetate ions to a solution of
3 provides a dual emission channel for elaborating a ratiometric
approach (Figure 5a). The ratio of emission intensities (I580/I480)
of 2 (CH3CN) varies from 0.09 to 5.3 on gradual addition of
fluoride ions, showing 60-fold emission ratio change and can be
used to estimate F- ions between 2 and 40 µM. In the case of
chemosensor 3 (CHCl3-MeOH, 1:1), the ratio of emission
intensities varies from 0.1 to 2.3 on gradual addition of AcO- ions,
indicating a 23-fold emission ratio change, and can be used to
estimate acetate ions between 2 and 25 µM (Figure 5b).
We have further evaluated the selectivity of probes 2 and 3 for
estimation of the fluoride and acetate ions in the competitive media
of other anions. For these studies the number of solutions of
chemosensor 2 (CH3CN) having a different concentration of
fluoride ions and 500 µM other anions (Cl-, Br-, AcO-, NO3-,
H2PO4-, HSO4-, CN-, and ClO4-) were prepared. The fluores-
cence intensity of each solution was measured at 580 nm, and the
result showed that the intensity of the fluorescence was almost
identical to that obtained in the absence of anions (Figure S21,
Supporting Information). Similarly, probe 3 could be used to
estimate acetate ions in CHCl3-MeOH (1:1) without any interfer-
ence by other anions (Figure S22, Supporting Information).
have been observed on addition of F- or AcO- anions to 3
(Figures S23-S24, Supporting Information), pointing to
similar anion-receptor interactions in both 2 and 3. These
interactions are in consonance with stabilization of the
intramolecular charge transfer excited state responsible for
red-shifted absorption bands.
Thus, we have synthesized N-arylimidazolium based
chromofluorescent probes where the presence of lipophilic
dodecyl appendage on imidazolium nitrogen tuned the
selectivity for AcO- ions in protic solvents. Further effect
of substituents on imidazolium ring on the anion sensing
behavior is under investigation.
Acknowledgment. We thank DST and UGC for financial
assistance.
Supporting Information Available: Synthesis and spec-
tral data of chemosensors 2 and 3, X-ray structure of 2. PF6
1
and selected UV-vis, fluorescence, and H NMR titrations
data.This material is available free of charge via the Internet
at http://pubs.acs.org.
OL802352J
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