Quaternary ammonium salt-based chromogenic and fluorescent chemosensors for fluoride ions

Article abstract of DOI:10.1016/j.tetlet.2008.04.147

Chemosensors 5-7 possessing a quaternary ammonium cation (for electrostatic interactions) and an N-H group(s) (for H-bonding) as recognition sites and an anthracene-9,10-dione as both a chromogenic and fluorescent moiety exhibit absorption and emission changes with fluoride ions only. No significant response to other anions such as Cl^-, Br^-, I^-, NO₃^-, CH₃COO^-, HSO₄^-, H₂ PO₄^- and ClO₄^- is observed. The dual emission at λ_max 580 nm (free 5/6) and λ_max 510 and 540 nm (5/6 + F^-) in chemosensors 5 and 6 enables ratiometric analysis of fluoride ions.

Full text of DOI:10.1016/j.tetlet.2008.04.147

Tetrahedron Letters 49 (2008) 4265–4268
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Quaternary ammonium salt-based chromogenic and fluorescent
chemosensors for fluoride ions
*
Vijay Luxami, Nidhi Sharma, Subodh Kumar
Department of Chemistry, Guru Nanak Dev University, Amritsar 143 005, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Chemosensors 5–7 possessing a quaternary ammonium cation (for electrostatic interactions) and an N–H
group(s) (for H-bonding) as recognition sites and an anthracene-9,10-dione as both a chromogenic and
fluorescent moiety exhibit absorption and emission changes with fluoride ions only. No significant
Received 23 February 2008
Revised 13 April 2008
Accepted 26 April 2008
Available online 1 May 2008
ꢀ
response to other anions such as Clꢀ, Brꢀ, Iꢀ, NO3ꢀ, CH3COO , HSO4ꢀ, H2PO4 and ClO4 is observed.
The dual emission at kmax 580 nm (free 5/6) and kmax 510 and 540 nm (5/6 + Fꢀ) in chemosensors 5
and 6 enables ratiometric analysis of fluoride ions.
ꢀ
ꢀ
Ó 2008 Elsevier Ltd. All rights reserved.
The development of artificial optical receptors for selective an-
ion recognition1 has attained significant interest, as anions play a
fundamental role in a wide range of chemical and biological pro-
cesses.2 Binding affinities and thus selectivities between anions
and their hosts are generally attributed to hydrogen bonding
and/or electrostatic interactions in addition to selectivities arising
due to the topologies of the ligating sites,1e,3 viz. a viz. that of the
anion (size and spherical, trigonal or tetrahedral structures). The
H-bonding is more influential in providing selective binding
through complimentarity and consequently a number of neutral
H-bond donor moieties, for example, amide,4 urea,1c,5 thiourea5a,6
and pyrrole7 or their combinations have been incorporated in the
design of anion receptors.
we have designed quaternary ammonium cation (for electrostatic
interactions) and N–H (for H-bonding) based chromogenic and
fluorescent chemosensors 5–7 which are selective for fluoride ions.
A solution of amine 115b and benzyl chloride 3 in ethanol on
stirring at 80 °C for 24 h followed by filtration of separated solid
gave the chemosensor 5,17 as a red solid in 80% yield (Scheme 1).
The downfield shift of NCH3 and both NCH2 signals to d 3.11, d
3.64 and d 4.03 in comparison to amine 1 (d 2.35, d 2.68, d
3.43),15b and the appearance of a new NCH2 singlet at d 4.69 and
aromatic protons confirmed the formation of quaternary ammo-
nium salt 5. Chemosensors 6 (92%) and 7 (85%) were synthesized17
X-
The presence of positively charged ammonium or guanidinium8
groups or metal ions9 complements the electrostatic interactions.
In such receptors, where the presence of both electrostatic and
H-bonding interactions provides a favourable environment, the
existence of multiple charged species at neutral pH and the possi-
bility of anion promoted deprotonation complicates the situation.
As a result, quaternary ammonium,10,11 imidazolium,12 benzimi-
dazolium,13 pyridinium14 salts, etc. which are not affected by the
pH of the medium have received significant attention. However,
a few mixed amine/amide-quaternary amine systems have been
explored as the anion receptors. Bowman-James has shown that
quaternary ammonium–amide receptors11 exhibit higher affinities
for almost all anions compared to their neutral counterparts.
Recently, we15 and others5,16 have shown that the modulation
of electron-deficient (amide/urea) or electron-rich (amine/pyr-
idine) functionalities in a host based on anthracene-9,10-dione
and 1,8-naphthalimide derivatives led to the development of
respective anion and cation chemosensors. In the present study,
Me
Me
Me
N
N
R
Me
O HN
O HN
EtOH
80 oC
+ R
X
O
O
1
3 R = H, X = Cl
4 R = NO2 X = Br
5 R = H, X = Cl (80%)
6 R = NO2 X = Br (92%)
Me
Me
N
N
Me
Me
O HN
O HN
EtOH
+
2Cl-
Cl reflux
NH O
NH O
Me
3
Me
Me
N
N
Me
2
7 (85%)
* Corresponding author. Tel.: +91 0183 2258802; fax: +91 0183 2258820.
E-mail address: subodh_gndu@yahoo.co.in (S. Kumar).
Scheme 1.
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.04.147

4266
V. Luxami et al. / Tetrahedron Letters 49 (2008) 4265–4268
by similar reactions of amine 1 with halide 4 and amine 218 with
halide 3. In chemosensor 7,17 the appearance of only four signals
in the aliphatic region confirmed that a bis-quaternary salt has
been formed.
Cl-/ Br-
N
Cl-/ Br-
N
R
R
_
O
H
N
OH N
The effect of a proximal positive charge on the spectroscopic
behaviour of 5–7 was studied using absorption and steady-state
fluorescence spectroscopy. In CH3CN–DMSO (9:1), chemosensors
5–7 showed absorption spectra shifted to shorter wavelengths
(Dkab 10–20 nm) with respect to their parent amines 1 and 2 (Table
1). In fluorescence spectroscopy, the amine 1 shows a Stokes shift
of 30 nm but the diamine 2 and quaternary ammonium salts 5–7
exhibit larger (80–105 nm) Stokes shifts. The emission maxima in
the case of 7 (585 nm) appeared at a shorter wavelength (Dkem
10 nm) with respect to that of amine 2 (595 nm). However, in
the case of amine 1, kem was observed at 525 nm which after con-
version to ammonium salts 5 and 6 appeared at 580–585 nm. The
quantum yields (U) of the quaternary ammonium salts 5–7 were
between 0.019 and 0.024 which are nearly 4–5 times larger for 5
and 6, and 19 times larger in the case of 7 than those observed
in their parent amines 1 and 2 (Table 1).
+ hν
ESIPT
+
O
O
Excited state
hν
Cl-/ Br-
N
Cl-/ Br-
N
R
R
H
O
O
N
O HN
O
Ground State
Scheme 2. Probable mechanism for the enhanced emission in 5.
This increased U in chemosensors 5–7 is consistent with the
earlier reported19 interaction of the anthracene-9,10-dione chro-
mophore with a positively charged species. In anthracene-9,10-
*
dione the low-lying n–p transition is generally non-fluorescent
which on co-ordination of the chromophore associated non-
bonded electrons (C@O of anthracene-9,10-dione) with any of
*
the electropositive centres is destabilized. As a result the p–p state
becomes the lowest energy transition and emission is switched
‘ON’. Chemosensors 5–7 constitute the first examples, where intra-
molecular interactions of an anthracene-9,10-dione derivative
with a quaternary ammonium cation switches ‘ON’ the emission.20
Such inversion of electronic transition states by intermolecular
interactions has been used earlier for cations sensing only19
(Scheme 2).
The chemosensor 5 (50 lM, CH3CN–DMSO (9:1)) exhibits two
absorption bands at kmax 310 nm (e 6000) and 480 (e 6600) nm.
Addition of tetrabutylammonium fluoride (TBAF) (1000 lM) to a
solution of 5 resulted in a change in colour of the solution from yel-
low to pink. The absorption bands of 5 centred at 310 nm and
480 nm underwent red shifts to 340 nm (Dkmax = 30 nm) and
515 nm (Dkmax = 35 nm) which reflects the stabilization of the
charge transfer excited state following the interaction of 5 with
fluoride ions. On keeping the solution for 24 h, the colour of the
solution turned fluorescent green (Fig. 1) and the UV–vis spectrum
of this solution attained a structured absorption band with maxima
at 364, 473, 505, 534, 573 and 646 nm. Addition of the other
Figure 1. UV–vis spectra of 5 (50 lM) in CH3CN–DMSO (9:1) (a) 5 only, (b) 5 + Fꢀ
(500 lM) (immediately), (c) 5 + Fꢀ (500 lM) (after 24 h) (inset shows the respective
colour changes).
increased gradually. A plot of the absorbance at 340 nm and 480
nm verses concentration of fluoride ions shows a straight line
which attains a plateau after addition of 1000 lM fluoride ions
(Figs. S3 and S4). The spectral fitting of the absorbance data shows
the formation of higher stoichiometric complexes [LF4
(logbLF4 = 12.11 0.27) and LF6 (logbLF6 = 17.76 0.5)]. Chemosen-
sor 6 possessing a p-nitro group showed similar changes on addi-
tion of fluoride anions to those observed with 5.
anions, for example, Clꢀ, Brꢀ, Iꢀ, NO3ꢀ, CH3COOꢀ, HSO4ꢀ, H2PO4
ꢀ
ꢀ
and ClO4 caused no significant change in the absorption spectrum
of 5. Significantly, amine 1 did not show any change in its absorp-
tion spectrum even on addition of 0.01 M fluoride and other anions
(Figs. S1 and S2). Therefore, the presence of a quaternary ammo-
nium salt as an appendage in chemosensor 5 is conspicuously
responsible for its selective interaction towards fluoride anion.
On gradual addition of fluoride anions to a solution of 5 (50 lM,
CH3CN–DMSO (9:1)), the absorbance at 340 nm and 515 nm
Chemosensors 5 and 6 (50 lM, CH3CN–DMSO (9:1)) on excita-
tion at 450 nm exhibited emission band at kem 580 nm. Addition
ꢀ
of Clꢀ, Brꢀ, Iꢀ, NO3ꢀ, CH3COO , HSO4ꢀ, H2PO4 and ClO4 anions
(0.01 M) to solutions of 5 and 6 led to insignificant changes in their
fluorescence spectra. However, on addition of fluoride (1000 lM),
the fluorescence intensity at 580 nm was completely turned off
and a new blue-shifted fluorescence spectrum with two new emis-
sion bands at 505 and 540 nm appeared (Fig. 2).
ꢀ
ꢀ
Table 1
UV–vis and fluorescence data for 1, 2 and 5–7
UV–vis (CH3CN–DMSO; 9:1)
Fluorescence (CH3CN–DMSO; 9:1)
kmax (nm)
e
kmax (nm)
U
On gradual addition of fluoride to the solution of 5, the emission
intensity at 580 nm decreased slowly up to 600 lM of fluoride ions.
On further addition of fluoride, a new structured emission spec-
trum with two emission maxima at 510 and 540 nm appeared,
which went off scale at greater than 1000 lM of fluoride ions.
1
5
6
2
7
495
480
485
515
495
8600
6600
5400
14,100
10,200
525
585
580
595
585
0.005
0.020
0.024
0.001
0.019

V. Luxami et al. / Tetrahedron Letters 49 (2008) 4265–4268
4267
Figure 2. Fluorescence emission spectra of 5 (50 lM) in CH3CN–DMSO (9:1) upon
addition of fluoride.
Figure 4. Fluorescence emission spectra of 7 (50 lM) in CH3CN–DMSO (9:1) upon
addition of fluoride. Inset shows a plot of fluorescence vs [Fꢀ] (points show the
experimental results and the line is curve fit).
Spectral fitting of these data (Fig. S5) showed the formation of
higher stoichiometric complexes LF3 and LF6 (logbLF3 = 9.43 0.07
and logbLF6 = 22.5 0.3) and is in agreement with UV studies.
Therefore, in the case of chemosensor 5, upon incremental addi-
tion of Fꢀ ions, the emission intensity at 580 nm is gradually
‘switched off’ and at 505 nm is ‘switched on’. This situation of dual
emission provides the opportunity for elaborating 5 as a ratiomet-
ric chemosensor and allows the estimation of analyte independent
of the concentration of the receptor. Therefore, chemosensor 5 can
be used for estimation of 500–1200 lM fluoride ions and other an-
ions do not interfere in its estimation (Fig. 3).
The appearance of structured band in UV–vis spectrum and en-
hanced emission at 510 nm, on addition of CsF (solid) to solution of
5 in DMSO and enhanced emission of solution of 5 in pure CH3CN
on addition of TBAF, ruled out participation of tetrabutylammo-
nium cation and DMSO in achieving such photophysical changes.
Klymchenko et al.20 have suggested that in a flavone-based sensor,
the participation of long range coulomb interactions between qua-
ternary ammonium moiety and added anions is responsible for
such behaviour and has proposed its application in developing
new chemosensors.
The UV–vis spectrum of 7 on addition of fluoride and other an-
ions showed insignificant changes in its absorbance and kmax. The
solution of chemosensor 7 (50 lM, CH3CN–DMSO (9:1)) on excita-
tion at kmax 450 nm exhibited emission at kmax 585 nm. Addition of
Clꢀ, Brꢀ, Iꢀ, NO3ꢀ, CH3COOꢀ, HSO4ꢀ, H2PO4 and ClO4 anions
(1000 equiv) resulted in insignificant changes in the emission of
7 but on addition of fluoride ions, the emission was quenched
(Fig. 4). Spectral fitting of these data showed the formation of LF2
only (logbLF2 = 6.23 0.07).
ꢀ
ꢀ
Thus, chemosensors 5–7 bearing quaternary ammonium and
amine NH moieties exhibit unique changes in their absorption
and fluorescence properties, on addition of fluoride ions and emu-
late new dual channel chemosensors for Fꢀ ions. The effect of addi-
tional functionalities on sensing behaviour is under investigation.
Acknowledgements
We thank DST, New Delhi for the Ramanna fellowship, FIST pro-
gramme and CDRI Lucknow for FAB mass spectra.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2008.04.147.
Figure 3. (a) Fluorescence ratiometric response (I510/I585) of 5 (50 lM) upon addi-
References and notes
ꢀ
tion of tetrabutylammonium salts of anions in CH3CN–DMSO (9:1) [A ¼ NO3
;
B = Iꢀ; C ¼ HSO4ꢀ; D ¼ H2PO4ꢀ; E ¼ ClO4ꢀ; F = Clꢀ; F = Brꢀ; G = AcOꢀ; I = Fꢀ]. (b)
Ratiometric response of 5 towards Fꢀ anions; inset shows the effect of Fꢀ between
500 and 1000 lM.
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17. Compound 5: red solid; Yield 80%, mp 50 °C; FAB Mass 385 (M+); 1H NMR
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CH2), 4.69 (s, 2H, CH2), 7.43 (d, J = 8.4 Hz, 1H, ArH), 7.44–7.59 (m, 6H, ArH),
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8.21 (d, J = 7.5 Hz, 1H, ArH), 9.75 (br s, 1H, NH). 13C NMR (DMSO-d6, 75 MHz): d
36.0, 49.3, 61.7, 68.2, 112.9, 115.7, 118.5, 136.3, 127.8, 128.9, 130.3, 132.3,
133.0, 133.6, 134.1, 134.5, 135.8, 150.4, 182.7, 184.3. Elemental analysis for
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C25H25ClN2O2 requires: C, 71.33; H, 5.99; N, 6.66. Found: C, 71.45; H, 5.85; N,
6.50.Compound 6: red solid; Yield 92%, mp 230 °C; FAB Mass 430 (M+); 1H
NMR (DMSO-d6, 300 MHz): d 3.19 (s, 6H, CH3), 3.76 (t, J = 6.3 Hz, 2H, CH2), 4.06
(q, J = 6.3 Hz, 2H, CH2), 4.89 (s, 2H, CH2), 7.39 (d, J = 8.4 Hz, 1H, ArH), 7.58 (d,
J = 7.5 Hz, 1H, ArH), 7.69 (t, J = 8.4 Hz, 1H, ArH), 7.80–7.87 (m, 2H, ArH), 7.93 (d,
J = 8.4, 2H, ArH), 8.16 (d, J = 7.5 Hz, 1H, ArH), 8.22 (d, J = 7.5 Hz, 1H, ArH), 8.34
(d, J = 8.4 Hz, 2H, ArH), 9.80 (br s, 1H, NH). 13C NMR (DMSO-d6, 75 MHz): d 50.1,
56.8, 63.2, 66.6, 113.9, 116.4, 118.7, 124.2, 124.3, 126.9, 133.1, 134.1, 134.8,
134.9, 135.0, 135.1, 136.3, 149.5, 150.9, 183.4, 185.2. Elemental analysis for
C
25H24BrN3O4 requires: C, 58.83; H, 4.74; N, 8.23. Found: C, 58.80; H, 4.78; N,
8.18.Compound 7: red solid; Yield 85%, mp 280 °C; FAB Mass 563 (M++1); 1H
NMR (DMSO-d6, 300 MHz): d 3.11 (s, 12H, 4 ꢁ CH3), 3.64 (t, J = 6.3 Hz, 4H,
2 ꢁ CH2), 4.03 (q, J = 6.3 Hz, 4H, 2 ꢁ CH2), 4.71 (s, 4H, 2 ꢁ CH2), 7.35 (d,
J = 8.4 Hz, 2H, ArH), 7.51–7.63 (m, 12H, ArH), 7.72 (t, J = 8.4 Hz, 2H, ArH), 9.71
(br s, 2H, 2 ꢁ NH); 13C NMR (DMSO-d6, 75 MHz): 36.4, 50.0, 62.2, 68.1, 113.2,
116.0, 117.9, 118.1, 129.7, 131.2, 133.6, 136.0, 136.7, 150.6, 185.0. Elemental
analysis for C36H42Cl2N4O2 requires: C, 68.24; H, 6.68; N, 8.84. Found C, 68.25;
H, 6.60; N, 8.90.
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20.
A similar effect of quaternary ammonium cations on the fluorescence of
flavone derivatives has been reported earlier. Klymchenko, A. S.; Demchenko,
A. P. J. Am. Chem. Soc. 2002, 124, 12372–12379.