Triphenylamine-based simple chemosensor for selective fluorometric detection of fluoride, acetate and dihydrogenphosphate ions in different solvents

Article abstract of DOI:10.1007/s10847-010-9866-5

Triphenylamine-based new chemosensors 1 and 2 have been designed and synthesized for fluorometric detection of anions. The urea-amide conjugates in 1 and 2 are involved in binding of anions via hydrogen bonding. UV-vis and fluorescence titration experimen

Full text of DOI:10.1007/s10847-010-9866-5

J Incl Phenom Macrocycl Chem (2011) 70:97–107
DOI 10.1007/s10847-010-9866-5
ORIGINAL ARTICLE
Triphenylamine-based simple chemosensor for selective
fluorometric detection of fluoride, acetate
and dihydrogenphosphate ions in different solvents
•
Kumaresh Ghosh Indrajit Saha
Received: 21 April 2010 / Accepted: 9 September 2010 / Published online: 25 September 2010
Ó Springer Science+Business Media B.V. 2010
Abstract Triphenylamine-based new chemosensors 1 and
2 have been designed and synthesized for fluorometric
detection of anions. The urea-amide conjugates in 1 and 2 are
involved in binding of anions via hydrogen bonding. UV–vis
and fluorescence titration experiments revealed that the sen-
sor 1 has the selectivity for acetate (AcO-), dihydrogen-
phosphate (H2PO4-) and fluoride (F-) over the other anions
examined in the present study, in CHCl3. In comparison,
receptor 2 is non responsive for the same anions under similar
conditions. In more polar solvent CH3CN containing 0.1%
DMSO, the receptor 1 shows a greater selectivity towards
fluoride. The color of the solution of 1 is changed from col-
orless to light yellow and finally to yellowish brown only in
thepresence of fluoride in CH3CN containing10%DMSO. In
pure DMSO and CH3CN solvents, almost colorless solution
of 1 is transformed into blood red and reddish brown in the
presence of 30 equivalent amounts of F-, respectively.
chemically important anions is emerging as a research
area of great importance [1–5]. Recently, considerable
attention has been focused on this aspect in supramolec-
ular chemistry. The chemosensors of abiotic origin that
signal the presence of analytes in solution through the
change in luminescence properties as well as color of the
solution are demanding in recent days [6, 7]. In particular,
the sensing of fluoride ions has attracted growing atten-
tion, because of its great potential in biological and
environmental applications [8, 9]. Fluoride, the smallest
anion, has unique chemical properties, and its recognition
and detection is critically important because it is associ-
ated with nerve gases, the analysis of drinking water, the
refinement of uranium used in nuclear weapons manu-
facture, dental care and treatment of osteoporosis. For the
recognition of this particular anion, a number of receptors
that differentiate fluoride from other anions, such as
chloride, phosphate or nitrate etc. through the change in
electrochemical or optical responses are known [10 and
references cited therein]. Similarly, the sensors that bind
carboxylates with significant affinities are also known [3].
In spite of considerable progress in this horizon, there is a
demand of new abiotic receptors of different architectures
that can detect fluoride as well as carboxylates in solution.
During the course of our work on anion recognition [11–
14], we report here simple and easy-to-make receptors 1
and 2 that bind fluoride and acetate with moderate asso-
ciation constants in CHCl3. In more polar CH3CN con-
taining 0.1% DMSO, the receptor 1 is also selective
towards F-. In the present study, differential quenching of
emission of triphenylamine motif in 1 upon complexation
of anions is the basic feature for distinction of the anions.
Receptor 2, in comparison, did not show any marked
change with F- and AcO- ions like 1 under similar
condition.
Keywords Triphenylamine-based receptors Á Anion
recognition Á Fluoride recognition Á Fluorescence
quenching Á Colorimetric detection
Introduction
The design and synthesis of simple molecular receptor
that is capable of sensing various biologically and
Electronic supplementary material The online version of this
article (doi:10.1007/s10847-010-9866-5) contains supplementary
material, which is available to authorized users.
K. Ghosh (&) Á I. Saha
Department of Chemistry, University of Kalyani, Kalyani,
Nadia 741235, India
e-mail: ghosh_k2003@yahoo.co.in
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J Incl Phenom Macrocycl Chem (2011) 70:97–107
N
N
Binding site
Binding site
O
NHa
O
NHa
N
Hb
N
Hc
N
Hb Hc
N
N
O
O
NO2
1
2
Experimental
All the compounds were characterized using 1H NMR, 13
NMR, FTIR, MS, and UV–vis spectroscopic tools.
C
The receptors 1 and 2 were accomplished according to the
Scheme 1. Reaction of the amines as obtained according to
the Scheme 2, with the acid chloride 7 yielded the desired
compounds 1 and 2. Coupling of o-phenylenediamine with
p-nitrophenyl isocyanate (obtained from p-nitro aniline by
reaction with triphosgene in the presence of Et3N in dry
CH2Cl2) gave the amine 8 (Scheme 2). Subsequent reac-
tion of the amine 8 with triphenylamine carbonyl chloride 7
(obtained from the triphenylamine after performing a series
of reactions such as formylation of triphenylamine, oxi-
dation of the aldehydic group followed by reaction with
oxalyl chloride in dry CH2Cl2) yielded the desired com-
pound 1 in 59% yield (Scheme 1).
Materials and methods
All the starting materials were used directly from com-
mercial sources. Solvents were distilled prior to use, and
dried according to the literature procedure when required.
Silica gel GF 254 was used for TLC. Melting points are
uncorrected. Spectroscopic grade CHCl3, CH3CN and
DMSO solvents were used to record the fluorescence and
UV–vis spectra.
Synthesis of 4-(diphenylamine)benzaldehyde 3 [15]
On the other hand, reaction of 8-aminoquinoline and o-
nitrophenyl isocyanate followed by reduction of the nitro
group furnished the amine 10 (Scheme 2). Subsequent
reaction of the amine 10 with the triphenylamine carbonyl
chloride 7 gave the compound 2 in 60% yield (Scheme 1).
To a solution of triphenylamine (8 g, 32.65 mmol) in dry
DMF (65 mL), POCl3 (31 mL, 332.65 mmol) was added
dropwise from a dropping funnel at 0 °C over a period of
30 min. The reaction mixture was stirred at room
Scheme 1 Reagents and
conditions: (i) POCl3, DMF,
reflux, 8 h; (ii) a. KMnO4,
acetone–water, 4 h, b. dil. HCl;
(iii) Oxalyl chloride, 1 drop
DMF, dry CH2Cl2, 15 h; (iv) 8,
dry THF–DMF, Et3N, 4 h; (v)
10, dry THF, Et3N, 4 h
OHC
OHC
CHO
N
N
N
N
(i)
CHO
CHO
CHO
4
3
5
(iv)
1
N
N
(iii)
(ii)
3
(v)
COCl
2
COOH
7
6
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J Incl Phenom Macrocycl Chem (2011) 70:97–107
99
Scheme 2 Reagents and
conditions: (i) a. Triphosgene,
NH2
NH
NH2
Et3N/dry CH2Cl2, 30 min. b.
excess o-phenylenediamine, dry
CH2Cl2, 3 h; (ii) dry CH2Cl2,
stirring, 1 h; (iii) SnCl2Á2H2O,
EtOAc–EtOH (1:1) stirring, 4 h
(i)
O
(a)
NH
NO2
NO2
8
NH2
NH
NO2
NH
NH2
(iii)
N
NO2
NCO
(ii)
O
NH
O
NH
(b)
N
N
10
9
1-(2-Aminophenyl)-3-(4-nitrophenyl) urea 8
temperature for 1 h. After additional stirring for 8 h at
100 °C, the reaction mixture was poured into ice and was
neutralized with 30% aqueous NaOH solution. The aque-
ous part was extracted with CH2Cl2 (4 9 200 mL). The
combined organic layer was dried over anhydrous Na2SO4
and concentrated on a rotary evaporator. The crude product
was purified by silica gel column chromatography using
15% ethyl acetate in petroleum ether as eluent to give
monoaldehyde 3 as a pale yellow solid (5.6 g, yield: 33%),
mp 140 °C.
To a stirred solution of triphosgene (2.15 g, 7.25 mmol) in
dry CH2Cl2 (20 mL), p-nitroaniline (1 g, 7.25 mmol) in
dry CH2Cl2 (15 mL) was added dropwise for 10 min fol-
lowed by dropwise addition of triethylamine (2.17 mL,
15.6 mmol) in dry CH2Cl2 (10 mL). After additional stir-
ring for 25 min at room temperature, the reaction mixture
was added dropwise to a refluxing solution of o-phenyl-
enediamine (6.26 g, 58 mmol) in dry CH2Cl2 (40 mL).
After heating the mixture for 3 h, the solvent was removed
by evaporation. The reaction mixture was poured into
water, extracted with CHCl3 (3 9 30 mL). Organic layer
was dried over anhydrous Na2SO4 and was removed under
reduced pressure. Purification of the crude product by silica
gel column chromatography using 3% CH3OH in CHCl3 as
eluent afforded the compound 8 (1.04 g, yield: 53%), mp
190–192 °C.
Synthesis of 4-(diphenylamine)benzoic acid 6
To a stirred solution of monoaldehyde 3 (1.5 g, 5.49 mmol)
in 50 mL acetone–water (4:1 v/v), KMnO4 (4 g,
25.16 mmol) was added portion wise at 60 °C. The reaction
mixture was stirred under refluxing condition for 4 h. The
solvent was removed under vacuum and water (25 mL) was
added and filtered. The filtrate was acidified with dilute HCl
to give white precipitate. The precipitate was filtered and
washed with water for several times and dried under vacuum
to give the monoacid 6 (1.2 g, yield: 75%), mp: 256 °C.
1H NMR (400 MHz, DMSO-d6) d 9.52 (s, 1H, urea NH),
8.17 (d, 2H, J = 8 Hz), 7.91 (s, 1H, urea NH), 7.66 (d, 2H,
J = 8 Hz), 7.28 (d, 1H, J = 8 Hz), 6.86 (t, 1H, J = 8 Hz),
6.73 (d, 1H, J = 8 Hz), 6.55 (t, 1H, J = 8 Hz); FTIR
(KBr, cm-1) 3412, 3320, 1668, 1613, 1488.
4-(Diphenylamino)benzoyl chloride 7
Receptor 1
To a stirred solution of compound 6 (0.500 g, 1.73 mmol)
in dry CH2Cl2 (15 mL), oxalyl chloride (0.18 mL,
2.1 mmol) was added followed by addition of one drop of
dry DMF. After stirring the reaction mixture at room
temperature for 15 h under nitrogen atmosphere, excess
oxalyl chloride was removed completely under vacuum to
give 4-(diphenylamino)benzoyl chloride 7 (0.520 g) in
98% yield. It was directly used in next step without any
characterization.
To the solution of acid chloride 7 (0.250 g, 0.81 mmol) in
dry THF (10 mL), 1-(2-aminophenyl)-3-(4-nitrophenyl)
urea 8 (0.220 g, 0.81 mmol), dissolved in dry DMF–THF
mixture solvent (1:1 v/v), was added dropwise followed by
addition of Et3N (0.113 mL, 0.81 mmol). After stirring the
reaction mixture for 4 h at room temperature, solvent was
evaporated on a rotary evaporator. Aqueous NaHCO3
solution (20 mL) was then added and the aqueous layer
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J Incl Phenom Macrocycl Chem (2011) 70:97–107
was extracted with CH2Cl2 (3 9 30 mL). Organic layer was
dried over anhydrous Na2SO4 and evaporated under reduced
pressure. Purification of the crude mass was done by column
chromatographic using 2% MeOH in CHCl3 as eluent to
afford the receptor 1 in 59% yield (0.260 g), mp 228 °C.
1H NMR (400 MHz, DMSO-d6) d 9.98 (s, 1H), 9.92(s,
1H), 8.16 (d, 2H, J = 8 Hz), 8.13 (s, 1H, NH), 7.93 (d, 2H,
J = 8 Hz), 7.87 (d, 1H, J = 8 Hz), 7.66 (d, 2H,
J = 8 Hz), 7.36(t, 4H, J = 8 Hz), 7.30 (d, 1H, J = 8 Hz),
7.24 (t, 1H, J = 8 Hz), 7.14 (t, 3H, J = 8 Hz), 7.09 (d, 4H,
J = 8 Hz), 6.96 (d, 2H, J = 8 Hz); 13C NMR (100 MHz,
DMSO-d6) d 165.2, 152.1, 150.4, 146.4, 146.3, 140.9,
133.7, 129.8, 129.4, 128.8, 127.1, 126.2, 125.2, 125.1,
124.3, 123.4, 122.6, 120.0, 117.3 (one carbon in the aro-
matic region is unresolved.); FTIR (KBr, cm-1): 3299,
1702, 1625, 1591, 1564, 1529. MS(ESI) C32H25N5O4
requires 543.2, found 566.3 (M ? Na?), 544.3 (M ? H)?.
1HNMR(400 MHz,CDCl3)d9.39(s, NH, 1H), 8.60–8.63
(m, 2H), 8.10 (dd, 1H, J1 = 8 Hz, J2 = 2 Hz), 7.49 (t, 1H,
J = 8 Hz), 7.41–7.29 (m, 3H), 7.15 (t, 1H, J = 8 Hz),
6.82–6.87 (m, 2H), 6.76 (br s, NH, 1H), 4.11 (br s, NH, 2H);
FTIR (KBr, cm-1) 3434, 3327, 1653, 1622, 1594, 1543.
Receptor 2
To the solution of acid chloride 7 (0.250 g, 0.81 mmol) in
dry THF (10 mL), the 1-(2-aminophenyl)-3-(quinolin-8-yl)
urea 10 (0.225 g, 0.81 mmol), dissolved in dry THF, was
added dropwise followed by addition of Et3N (0.113 mL,
0.81 mmol) and the reaction mixture was stirred for 4 h at
room temperature. After completion of reaction, as moni-
tored by TLC, solvent was evaporated on rotary evapora-
tor. Aqueous NaHCO3 solution (20 mL) was added to the
residue and the aqueous layer was extracted with CH2Cl2
(3 9 30 mL). Organic layer was dried over anhydrous
Na2SO4 and evaporated on a rotary evaporator. The crude
product was purified by column chromatography using
0.5% CH3OH in CHCl3 as eluent to afford the receptor 2 in
60% yield (0.270 g). mp 186 °C.
1-(2-Nitrophenyl)-3-(quinolin-8-yl) urea 9
To
a
stirred solution 8-aminoquinoline (0.500 g,
3.05 mmol) in dry CH2Cl2 (25 mL), ortho-nitrophenyl
isocyanate (0.439 g, 3.05 mmol) was added. After stirring
the reaction mixture for 1 h at room temperature, it was
concentrated on rotary evaporator. Pure 1-(2-nitrophenyl)-
3-(quinolin-8-yl) urea 9 (0.890 g, yield: 95%) was isolated
as yellow solid after recrystallization from ethyl acetate–
petroleum ether mixture solvent (1:3 v/v), mp 166 °C.
1H NMR (400 MHz, CDCl3) d 10.24 (s, NH, 1H), 9.50
(s, NH, 1H), 8.87–8.85 (dd, 1H, J1 = 8 Hz, J2 = 4 Hz),
8.76 (d, 1H, J = 8 Hz), 8.59 (d, 1H, J = 8 Hz), 8.26 (d,
1H, J = 8 Hz), 8.21 (d, 1H, J = 8 Hz), 7.68 (t, 1H,
J = 8 Hz), 7.58 (t, 1H, J = 8 Hz), 7.54–7.49 (m, 2H), 7.13
(t, 1H, J = 4 Hz); 13C NMR (100 MHz, CDCl3); d 151.2,
148.1, 138.2, 136.5, 136.4, 135.9, 135.8, 134.6, 128.0,
127.3, 125.8, 121.8, 121.7, 121.6, 120.9, 115.5; FTIR
(KBr, cm-1): 3338, 3303, 3272, 1658, 1597, 1583; MS
(ESI) C16H12N4O3 requires 308.1 found 309.2 (M ? H)?.
1H NMR (00 MHz, DMSO-d6) d 9.97 (s, NH, 1H), 9.70
(s, NH, 1H), 9.25 (s, NH, 1H), 8.84 (dd, 1H, J1 = 6 Hz,
J2 = 3 Hz), 8.54 (dd, 1H, J1 = 12 Hz, J2 = 6 Hz), 8.34
(dd, 1H, J1 = 9 Hz, J2 = 3 Hz), 7.89 (d, 2H, J = 9 Hz),
7.80 (d, 1H, J = 9 Hz), 7.59–7.55 (m, 1H), 7.50–7.49 (m,
2H), 7.42 (d, 1H, J = 9 Hz), 7.33 (t, 4H, J = 9 Hz),
7.18–7.04 (m, 8H), 6.92 (d, 2H, J = 9 Hz); 13C NMR
(100 MHz, DMSO-d6) d 169.8, 159.3, 155.6, 152.5, 151.5,
143.2, 141.2, 140.7, 136.2, 135.7, 134.2, 133.5, 132.9,
132.1, 131.8, 130.3, 130.1, 129.9, 129.6, 129.1, 128.8,
126.3, 125.4, 124.8, 120.0; FTIR (KBr, cm-1) 3311, 3059,
1676, 1629, 1591,1526, 1488, 1454; MS (ESI) C35H27N5O2
requires 549.2 found 550.3 (M ? H)?.
Results and discussion
1-(2-Aminophenyl)-3-(quinolin-8-yl) urea 10
Both the receptors 1 and 2 contain hydrogen-bonding sites
for complexation of anions and triphenylamine as a sig-
naling unit (see the different components as indicated in
structures 1 and 2) to transduce the recognition events. In
an effort to understand the anion binding behaviors of 1
1-(2-Nitrophenyl)-3-(quinolin-8-yl) urea
9
(0.700 g,
2.27 mmol) was taken in ethyl acetate–ethanol (1:1, v/v)
and SnCl2 (2.56 g, 11.35 mmol) was added to it. The
reaction mixture was stirred for 4 h at room temperature.
After completion of the reaction, solvent was evaporated
under reduced pressure. Then NaHCO3 solution was added
to the reaction mixture to make the solution alkaline. The
aqueous layer was extracted with ethyl acetate
(3 9 30 mL) and concentrated on a rotary evaporator. The
crude mass was purified by silica gel column chromatog-
raphy using 2% MeOH in CHCl3 to give the pure com-
pound 10 (0.510 g, 80%), mp 172 °C.
1
and 2, UV–vis, fluorescence and H NMR studies were
performed in less polar CHCl3 and more polar CH3CN
solvents. Figure 1 represents the change in fluorescence
ratio of 1 and 2 in the presence of four equivalent amounts
of a particular anion in CHCl3. As evident from Fig. 1,
while the receptor 1 has greater affinity for F-, AcO-, and
-
H2PO4 ions, the receptor 2 exhibits poor interaction with
F- and H2PO4-. In receptor 2, the quinoline ring nitrogen
has a possibility of forming an intramolecular hydrogen
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J Incl Phenom Macrocycl Chem (2011) 70:97–107
101
500
400
300
200
100
0
400
450
500
550
600
650
Wavelength (nm)
Fig. 2 Change in emission spectra of 1 (c = 3.55 9 10-5 M) upon
addition of F- (as tetrabutylammonium salt) in CHCl3
Fig. 1 Fluorescence ratio (I0 - I/I0) of receptor 1 (c = 3.55 9
10-5 M) and 2 (c = 4.05 9 10-5 M) at 462 nm upon addition of
4.0 equiv. of a particular anion in CHCl3
500
400
300
200
100
0
bond with the adjacent urea proton1 and thereby has a least
tendency in forming strong and directed hydrogen bond
with the incoming anions along with the urea protons
(Supporting information). In 1, the electron withdrawing p-
nitrophenyl group increases the acidity of urea proton (Hc),
for which the affinity of the receptor 1 toward anionic
guests increases. Molecular modeling study,2 in this regard,
shows that all the hydrogen bond donors are deeply
involved in complexation (see Supporting information).
Upon gradual addition of anions to the solution of 1 in
CHCl3, the emission of triphenylamine moiety gradually
decreased and it was found to be pronounced with tetra-
butylammonium fluoride, acetate and dihydrogenphosphate
salts. Figures 2, 3 and 4 represent the change in emission of
400
450
500
550
600
650
Wavelength (nm)
Fig. 3 Change in emission spectra of 1 (c = 3.55 9 10-5 M) upon
1 during titrations with F-, AcO- and H2PO4 in CHCl3,
-
addition of CH3COO- (as tetrabutylammonium salt) in CHCl3
respectively. Other anions such as Cl-, Br-, I- and HSO4
-
interacted weakly (see Supporting information).
600
500
400
300
200
100
0
In case of 2, the quenching of emission of triphenyl-
amine unit was small when titrated with F-, H2PO4
-
.
Even, emission of 2 was hardly perturbed in the presence of
other anions studied (see Supporting information). Fig-
ures 5 and 6 show the change in emission of 2 upon
titration with F-, H2PO4-, respectively.
However, during interaction no other spectral change in
emission was noticed for both the receptors. The quenching
of emission upon complexation, in both cases, is due to the
activation of photo-induced electronic transfer process
(PET), occurring in between the binding site and triphen-
ylamine motif (fluorophore). To realize this, we optimized
the structures by AM1 method and identified the position
400
450
500
550
600
650
Wavelength (nm)
1
Fig. 4 Change in emission spectra of 1 (c = 3.55 9 10-5 M) upon
Energy minimization was performed by Chem3D Ultra 10.0.
2
-
addition of H2PO4 (as tetrabutylammonium salt) in CHCl3
See footnote 1.
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J Incl Phenom Macrocycl Chem (2011) 70:97–107
acetate
800
700
600
500
400
300
200
100
0
350
300
250
200
150
100
50
dihydrogenphosphate
fluoride
hydrogensulfate
chloride
bromide
iodide
0
400
450
500
550
600
0
2
4
6
8
10
12
14
16
Wavelength (nm)
[G]/[H]
Fig. 5 Change in emission spectra of 2 (c = 4.05 9 10-5 M) upon
addition of F- (as tetrabutylammonium salt) in CHCl3
Fig. 7 Fluorescence titration curves ([Guest]/[Host] versus change in
emission) of 1 (c = 3.55 9 10-5 M) measured at 462 nm in CHCl3
1.5
1.0
0.5
0.0
800
600
400
200
0
400
450
500
550
600
250
300
350
400
450
Wavelength (nm)
Wavelength (nm)
Fig. 6 Change in emission spectra of 2 (c = 4.05 9 10-5 M) upon
Fig. 8 Change in absorbance spectra of 1 (c = 1.14 9 10-5 M)
-
addition of H2PO4 (as tetrabutylammonium salt) in CHCl3
upon addition of I- (as tetrabutylammonium salt) in CHCl3
of HOMO and LUMO. For example, in 1 the HOMO and
LUMOs are on the triphenylamine and 4-nitrophenylurea
motifs, respectively (supporting information). The
LUMO ? 1 resides on the triphenylamine unit. Thus the
LUMO of the 4-nitrophenylurea part falls in between the
HOMO and LUMO ? 1 of the fluorophore triphenylamine
unit and participates in the PET process in the usual
manner [3] by accepting electrons from the excited state of
triphenylamine. We presume that such electron transfer
becomes activated upon complexation. The similar situa-
tion was noticed with receptor 2 (Supporting information).
The significant quenching of emission in 1 upon com-
plexation is clearly reflected in the Stern-Volmer plots
(Supporting information).
sharp break of the curves at [G]/[H] = 1 corroborated 1:1
stoichiometry of the complexes. Job plots [16] further
confirmed this (see Supporting information). In Job plots,
the broadening nature of the maxima of the curves at 0.55
instead of 0.5, suggested 1:1 binding stoichiometry.
Concurrent UV–vis titrations of 1 and 2 with the same
anions in CHCl3 showed gradual decrease in absorbance.
During titrations of 1 and 2 with I- sharp isosbestic points
were observed (Fig. 8 for 1 and Fig. 9 for 2). The
appearance of such isosbestic points signifies the formation
of new species in solution, which may remain in equilib-
rium with the free receptors. Figure 10, collectively, shows
the change in absorbance of 1 at 320 nm upon titration with
all the anions examined. The almost linear nature of the
titration curves indicates the weak interaction between the
For stoichiometry of the complexes, Fig. 7 represents
the titration curves for 1 with the anions in CHCl3. The
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J Incl Phenom Macrocycl Chem (2011) 70:97–107
103
2.0
1.2
0.8
0.4
0.0
1.5
1.0
0.5
0.0
300
400
280
320
360
400
Wavelength (nm)
Wavelength (nm)
Fig. 11 Change in absorbance spectra of 1 (c = 1.14 9 10-5 M)
Fig. 9 Change in absorbance spectra of 2 (c = 4.05 9 10-5 M)
upon addition of F- (as tetrabutylammonium salt) in CHCl3
upon addition of I- (as tetrabutylammonium salt) in CHCl3
Table 1 Association constant (Ka in M-1) values for receptors 1 and
2 in CHCl3 by fluorescence method
0.5
(a)
(b)
(c)
Guests
Ka for 1a
Ka for 2b
(d)
(e)
(f)
0.4
0.3
0.2
0.1
0.0
Acetate
4.26 1.04 9 105
R2 = 0.99263
nd
(g)
Dihydrogen phosphate 7.94 0.66 9 104
R2 = 0.9968
1.11 0.106 9 104
R2 = 0.9934
acetate
(a)
(b)
1.14 0.06 9 104
R2 = 0.9980
3.71 0.599 9 103
R2 = 0.9872
dihydrogenphosphate
(c) hydrogensulfate
Hydrogen sulfate
(d)
(e)
fluoride
Fluoride
6.63 0.28 9 104
R2 = 0.9991
2.16 0.48 9 104
R2 = 0.9609
chloride
(f) bromide
(g)
iodide
Chloride
1.24 0.026 9 104 nd
R2 = 0.9997
0
5
10
15
20
25
Bromide
5.31 0.59 9 103
R2 = 0.9932
nd
[G]/[H]
Iodide
2.14 0.57 9 103
R2 = 0.9722
nd
Fig. 10 UV–vis titration curves [Guest]/[Host] versus change in
absorbance) of 1 (c = 1.14 9 10-5 M) measured at 320 nm
nd not determined due to minor and irregular change
a
Determined at 462 nm
receptor and anions in ground state. Similar situation was
found with the receptor 2 (see Supporting information).
Figure 11 demonstrates the spectral change in absorbance
for 1 upon increasing amounts of F-. A transient isosbestic
point at 340 nm, which on progression of titration is
destroyed, was observed.
b
Determined at 445 nm
Br- [ I-. We believe that this order is due to the combined
effect of both basicity and size or shape of the anions. Fig-
ure 12, for example, represents the binding constant curve
for the receptor 1 with acetate ion. On the other hand,
receptor 2 shows moderate binding constant values for
H2PO4-, F-, HSO4- but the values are less compared to the
case of 1.
However, in order to determine the binding potencies of
the receptors 1 and 2, we used the fluorescence titration
results. The binding constant values were determined using
non-linear curve-fitting methods [17] and the results are
accumulated in Table 1. As can be seen from Table 1,
receptor 1 exhibits higher binding constant value for AcO-.
Other anions such as H2PO4-, F-, HSO4- also show mea-
surable affinity towards 1. The association constant values
for 1 follow the order AcO- [ H2PO4- [ F- [ Cl- [
It is mentionable that change in polarity of the solvent
may affect both the binding and the signal intensity of the
chromophoric groups present in the receptor. To realize
this, we performed the fluorescence and UV–vis studies of
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J Incl Phenom Macrocycl Chem (2011) 70:97–107
500
450
400
350
300
250
200
150
100
600
500
400
300
200
100
0
400
450
500
550
600
650
1.0x10-4 2.0x10-4 3.0x10-4 4.0x10-4 5.0x10-4
0.0
Wavelength (nm)
[G] in M
Fig. 14 Change in emission spectra of 1 (c = 4.05 9 10-5 M) upon
addition of F- (as tetrabutylammonium salt) in CH3CN containing
0.1% DMSO
Fig. 12 Binding constant curve of receptor 1 with acetate anion in
CHCl3
Fig. 15 Change in color in CH3CN containing 10% DMSO of (a) 1
(c = 4.60 9 10-3 M), (b) 1 with 1 equiv. F-, (c) 1 with 10 equiv. F-,
(d) on addition of MeOH to the complex
Fig. 13 Fluorescence ratio (I0 -I/I0) of receptor 1 (c = 4.05 9
10-5 M) at 462 nm upon addition of 10 equiv. of a particular anion in
CH3CN containing 0.1% DMSO
complexation with the more basic F- ion increases the
electron density around the amide-urea binding site and this
facilitates the intramolecular charge transfer. Particularly, in
the presence of excess F- ion, deprotonation of the urea
proton Hc in 1 likely occurs enhancing such charge transfer
and resulting in the sharpest, most intense color change.
Upon addition of methanol to the colored solution of 1
containing F- in CH3CN containing 10% DMSO, the color
of the solution discharged immediately and returned to the
initial color of the receptor solution (Fig. 15d). This indi-
cated the reversible nature of the complexation. It is
important to mention that the change in color of the receptor
solution of 1 in the presence of F- is dependent on the
polarity of the solvent used. While almost colorless solution
of 1 (c = 4.10 9 10-3 M) in DMSO is changed to blood red
coloration in the presence of 30 equiv. amounts of F-, the
receptor 1 in CH3CN containing 0.1% DMSO. The emis-
sion of 1 at 462 nm (kex = 330 nm) was hardly perturbed
upon titration with the anions except F- (Fig. 13). The
spectral change in emission of 1 in the presence of F- is
represented in Fig. 14. Upon addition of F-, the emission
of 1 was quenched significantly (*59%) along with a
change in color of the solution from colorless to light
yellow. When concentrated solutions of 1 and F- in
CH3CN containing 10% DMSO were mixed together the
color changed from colorless to yellow and finally to yel-
lowish brown (Fig. 15).
The origin of the color change of 1 upon complexation
could be ascribed to a charge-transfer interaction between
the electron rich amide-urea (donor unit) and the electron
deficient p-nitrophenyl moiety [18, 19]. We hypothesize that
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J Incl Phenom Macrocycl Chem (2011) 70:97–107
105
colorless solution of 1 (c = 4.10 9 10-3 M) in CH3CN is
transformed into deep reddish brown (Fig. 16) in the pres-
ence of 30 equiv. amounts of the same anion. In comparison,
in the presence of AcO-, the almost colorless solutions of 1
in both DMSO and CH3CN were transformed into faint
yellow colored solutions.
cases the downward running of the titration curves
(Fig. 17) is attributed to the several reasons which are (i)
weak interaction, (ii) disruption of the complexes in the
presence of high concentration of anions that leads to more
than one equilibrium in solution and also (iii) dilution
effect.
However, the association constant of 1 with F- was
found to be 1.52 0.13 9 104 M-1 in CH3CN containing
0.1% DMSO, which is slightly less in comparison to the
value found in CHCl3. This is attributed to the fact that
more polar solvent CH3CN containing 0.1% DMSO redu-
ces the strength of interaction between host and guest and
thereby decreases the magnitude of association constant.
Moreover, the change in fluorescence intensity of 1 upon
titration with all anions except F- (Fig. 17) was too small
to determine the association constant accurately. In major
Simultaneous UV–vis experiments were performed in
CH3CN containing 0.1% DMSO to understand the binding
interaction of 1 in the ground state. The spectral change in
absorbance for 1 upon increasing amounts of F- is
0.6
0.5
0.4
0.3
0.2
0.1
0.0
250
300
350
400
450
500
Wavelength (nm)
Fig. 18 Change in absorption spectra of 1 (c = 4.05 9 10-5 M)
upon addition of F- (as tetrabutylammonium salt) in CH3CN
containing 0.1% DMSO
Fig. 16 Change in color of (a) 1 (c = 4.10 9 10-3 M) in DMSO,
(b) 1 with 30 equiv. F- in DMSO, (c) 1 with 30 equiv. F- in CH3CN
400
300
200
100
0
fluoride
acetate
dihydrogenphosphate
bromide
chloride
-100
-200
hydrogensulfate
iodide
0
2
4
6
8
10 12 14 16 18 20 22
[G]/[H]
Fig. 17 Fluorescence titration curves ([Guest]/[Host] versus change
in emission) of 1 (c = 4.05 9 10-5 M, measured at 472 nm) in
CH3CN containing 0.1% DMSO
Fig. 19 Partial 1H NMR (400 MHz, CDCl3 containing 1.6% DMSO-
d6) of (a) 1 (c = 4.80 9 10-3 M) and in presence of equivalent
amount of (b) F-, (c) AcO-, (d) H2PO4
-
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J Incl Phenom Macrocycl Chem (2011) 70:97–107
Fig. 20 Proposed binding
modes for 1 with a AcO-, b F-
-
and c H2PO4
N
N
N
HO
OH
P
O-
O
O-
O
O
NH
O
NH
O
NH
H
H
H
H
H
N
H
N
N
N
N
N
O
O
O
NO2
NO2
NO2
= F-
(a)
(b)
(c)
displayed in Fig. 18. As can be seen from Fig. 18, two
isosbestic points at 353 and 268 nm are observed. During
the titration of 1 with AcO- and I- in CH3CN containing
0.1% DMSO, similar isosbestic points were noticed.
Due to weak binding and poor change in emission of 2
when interplayed in CHCl3 in the excited state with the
anions examined in the present study, we did not proceed
further with the receptor 2 by considering the change in
polarity of the solvent.
and also investigated their binding properties toward a
series of anions. Between 1 and 2, chemosensor 1 is found
to be effective for sensing AcO-, F- and H2PO4 in
-
CHCl3 solvent. However, the chemosensor 1 exhibits a
greater selectivity in the sensing process when CHCl3 is
replaced by more polar solvent CH3CN containing 0.1%
DMSO. Moreover, the chemosensor 1 in the different
solvents such as DMSO, CH3CN, CH3CN containing 10%
DMSO effectively detected F- by showing different color
changes.
The binding of anions was also realized from 1H NMR of 1
in the presence and absence of anions in CDCl3 containing
1.6% DMSO-d6 (due to solubility problem of 1 in the NMR
concentration range). To the individual receptor solution of 1
in CDCl3 containing 1.6% DMSO-d6, various anions were
added as their tetrabutylammonium salts in equivalent
amount. In the presence of the anionic guests, the amide
proton Ha, urea protons Hb and Hc underwent downfield
chemical shift (DdNHa = 0.2–1.72, DdNHb = 0.34–1.67,
DdNHc = 0.48–1.66). Figure 19, for instance, demonstrates
Acknowledgments KG acknowledges financial support from CSIR,
New Delhi, India. IS thanks CSIR for providing a fellowship. We also
acknowledge infrastructure support from DST under DST-FIST
program.
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