Fluorometric recognition of both dihydrogen phosphate and iodide by a new flexible anthracene linked benzimidazolium-based receptor

Article abstract of DOI:10.3762/bjoc.7.34

A new flexible anthracene linked benzimidazolium-based receptor 1 has been designed, synthesized and its binding properties have been studied by NMR, UV-vis and fluorescence spectroscopic techniques. While the receptor 1 exhibits a greater change in emission in the presence of tetrabutylammonium dihydrogenphosphate in CH₃CN over the other anions studied, iodide is selectively preferred in CHCl₃ containing 0.1% CH₃CN. Upon complexation of dihydrogen phosphate and iodide, the emission of 1 gradually decreased without showing any other characteristic change in the spectra. Hydrogen bonding and charge-charge interactions interplay simultaneously in a cooperative manner for selectivity in the binding process.

Full text of DOI:10.3762/bjoc.7.34

Fluorometric recognition of both dihydrogen phos-
phate and iodide by a new flexible anthracene linked
benzimidazolium-based receptor
Kumaresh Ghosh*§ and Debasis Kar
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 254–264.
Department of Chemistry, University of Kalyani, Kalyani-741235, India
doi:10.3762/bjoc.7.34
Email:
Received: 26 August 2010
Accepted: 26 January 2011
Published: 25 February 2011
Kumaresh Ghosh* - ghosh_k2003@yahoo.co.in
* Corresponding author
§ Fax: +913325828282; Tel: +913325828750
Associate Editor: P. J. Skabara
Keywords:
© 2011 Ghosh and Kar; licensee Beilstein-Institut.
License and terms: see end of document.
anthracene coupled receptor; benzimidazolium-based receptor;
dihydrogenphosphate recognition; iodide recognition
Abstract
A new flexible anthracene linked benzimidazolium-based receptor 1 has been designed, synthesized and its binding properties have
been studied by NMR, UV–vis and fluorescence spectroscopic techniques. While the receptor 1 exhibits a greater change in emis-
sion in the presence of tetrabutylammonium dihydrogenphosphate in CH3CN over the other anions studied, iodide is selectively
preferred in CHCl3 containing 0.1% CH3CN. Upon complexation of dihydrogen phosphate and iodide, the emission of 1 gradually
decreased without showing any other characteristic change in the spectra. Hydrogen bonding and charge–charge interactions inter-
play simultaneously in a cooperative manner for selectivity in the binding process.
Introduction
The selective recognition of anionic species by artificial abiotic NH groups in neutral amines [13], amides [14], ureas/thioureas
receptors is a rapidly growing area in supramolecular chemistry [15,16], indoles [17-19] and pyrroles [20] as well as in guani-
[1-6]. Anion recognition has gained significant importance as it dinium [21] and imidazolium [22] groups are well established.
plays an important role in a wide range of biological, environ- In addition, the use of the benzimidazolium motif [23,24] in
mental and chemical processes [7-10]. Sensors based on anion- anion recognition is also known. During the course of our work
induced changes in fluorescence appear to be particularly attrac- on anion recognition, we used this motif along with the other
tive in anion recognition due to the simplicity and high detec- functionalities for selective recognition of carboxylates, and
tion limit of fluorescence [11,12]. In devising such sensors, dihydrogen phosphate [25]. Selective recognition of dihy-
various functional sites with hydrogen bond donors and accep- drogen phosphate is an important aspect of supramolecular
tors are of considerable importance. Among the different chemistry and various receptors of different designs for this
binding motifs for anions, the hydrogen bonding properties of anion have been reported in the literature [26-33].
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Figure 1: Synthesized compounds 1 and 2.
In an effort to investigate the anion recognition behavior of the Results and Discussion
benzimidazolium group in the presence of a flexible spacer, we The receptor 1 was obtained according to Scheme 1. Initially,
report herein the design and synthesis of a new chemosensor 1 2-chloroethylamine was reacted with isophthaloyl dichloride to
(Figure 1), which shows selective recognition of dihydrogen afford the diamide 4 in 80% yield. Subsequent reaction of 4
phosphate in CH3CN among the other anions studied. The with anthracene coupled benzimidazole 3 (prepared in 64%
selectivity is changed on changing the polarity of the solvent, yield from benzimidazole and 9-chloromethylanthracene in the
and the receptor 1 exhibits a preference for iodide when CH3CN presence of NaH in dry THF) gave the dichloride salt 5 in 55%
is replaced by CHCl3 containing 0.1% CH3CN. The results yield. Anion exchange using NH4PF6 in warm aqueous CH3OH
were compared with the monomeric unit 2 (Figure 1).
gave the desired compound 1 in 90% yield. Similarly, model
Scheme 1: Synthesis of 1 and 2.
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Beilstein J. Org. Chem. 2011, 7, 254–264.
compound 2 was obtained in 64% yield from the reaction 30%, respectively. During titration experiments, no other
between 6 and 3 in dry CH3CN followed by anion exchange changes such as excimer or exciplex formation were observed.
using NH4PF6 in aqueous CH3OH. All the compounds were The large quenching in emission of 1 upon increasing H2PO4−
characterized unequivocally by 1H NMR, 13C, mass spectrom- concentration is illustrated in Figure 4.
etry and FTIR.
In receptor 1, two amide NHs and two benzimidazolium protons
(C+–H) are accessible for the complexation of anions. The
residual charge on each benzimidazolium motif, in principle,
will stabilize the complex by charge-charge interaction. It is
quite rational that the binding group in each arm of flexible
receptor 1 could either be involved cooperatively or functions
individually for complexation of anions. Thus the selectivity in
the binding process is related with the disposition of the binding
groups around the isophthaloyl spacer. Figure 2, for example,
shows the energy-optimized geometry of the complex of 1 with
H2PO4− in the gas phase [34]. In the complex, benzimida-
zolium protons (C+–H) and amide protons are cooperatively
involved in hydrogen bonding with H2PO4−. Anthracene, being
a fluorophore in 1, has the advantage of being considered as a
flat hydrophobic fluorophore probe for sensing of anions by the
change in intensity due to the photo-induced electron transfer
(PET) mechanism.
Figure 3: Change in fluorescence emission of 1 (c = 5.78 × 10−5 M) in
the presence of 2 equiv of tetrabutylammonium salts of different guests
in CH3CN.
Figure 2: Energy optimized geometry of 1 with H2PO4− (E = 92.14
kcal/mol, a = 2.45 Å, b = 1.94 Å, c = 2.34 Å, d = 2.82 Å).
Figure 4: Change in emission spectra of 1 (c = 5.78 × 10−5 M) in pres-
ence of increasing amounts of H2PO4− in CH3CN; Inset: Plot of fluo-
rescence intensity vs concentration of H2PO4−.
As expected, we observed a change in emission of 1 (c = 5.78 ×
10−5 M) in CH3CN upon the addition of anions as their tetra-
butylammonium salts. Receptor 1 (c = 5.78 × 10−5 M) in
CH3CN gave a structured emission band when excited at
369 nm. Upon the addition of 2 equiv of each particular anion
to the receptor solution of 1, a large change in emission of the
anthracene group was observed for H2PO4− (Figure 3): Other
anions perturbed the emission of 1 only weakly. Upon the add-
ition of 2 equiv of tetrabutylammonium salts of H2PO4−, F−,
Br− and I−, the emission of 1 was quenched by 72, 18, 14 and
Interestingly, the emission of 1 is quenched up to the addition of
an equivalent amount of H2PO4− ions. Further addition caused
an increase in emission. This is presumably due to a change in
conformation upon complexation or decomplexation of anion,
although deprotonation of the bound H2PO4− cannot be ignored
[35]. We believe that initially, the binding sites of 1 interact
cooperatively to make a 1:1 complex according to the suggested
mode A, which in turn, changes to mode B with 2:1 (guest:host)
stoichiometry in the presence of excess H2PO4− (Figure 5).
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Beilstein J. Org. Chem. 2011, 7, 254–264.
Figure 5: Suggested modes of binding of the H2PO4− ion into the cleft of 1.
The flexible nature of the binding site contributes to this aspect.
For an example of a related system, see [36]. The stoichiometry
of the complexes was confirmed by Job plots (see Supporting
Information File 1) as well as from the break of the titration
curves at [G]/[H] = 2 (Figure 6).
Figure 7: Fluorescence Job plot of 1 with H2PO4−.
Figure 8 displays the change in emission of 2 upon addition of 1
equiv of the same anions in CH3CN. In the presence of excess
H2PO4− ions, the change in emission of 2 was less compared to
the case of 1 (see Supporting Information File 1).
Figure 6: Plot of change in emission of 1 at 420 nm vs the ratio of
guest to host concentration in CH3CN.
Figure 7, for example, represents the Job plot [37] for H2PO4−, The quenching of emission of 1 upon complexation is attrib-
which corresponds to a clear-cut case for 2:1 stoichiometry of uted to the activation of a PET process occurring between the
the complex. In the binding process, cooperative interaction of binding site and the excited state of anthracene. The
the two benzimidazolium motifs in 1 are primarily necessary for Stern–Volmer plot in Figure 9 illustrates the quenching
a large change in emission. This was proved by considering the phenomena with anions such as H2PO4−, F−-, Br− and I−. The
model compound 2, where only one binding site is present for non-linear nature of the curves in Figure 9 indicates that both
interaction. Under similar experimental conditions, the emis- static (hydrogen bonding effects) and dynamic quenching
sion of 2 was only slightly changed.
(bimolecular collision) take place during binding.
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Beilstein J. Org. Chem. 2011, 7, 254–264.
Concurrent UV–vis studies of 1 with the anions exhibited only a
small change in absorbance of anthracene and predicted 1 as a
PET system [38]. In many cases, the change of absorbance
during the titration with the anions was irregular. Figure 10
corroborates the irregular change in absorbance of 1 on titration
with H2PO4− and importantly, in the ground state, all the anions
showed 1:1 binding (Supporting Information File 1). However,
the selectivity of 1 towards the anions studied was established
by determining the binding constant values from fluorescence
titration data (Table 1) [39]. As can be seen from Table 1, the
receptor 1 shows a marginal selectivity for H2PO4−. The
receptor 1 also binds the larger sized iodide ion with 2:1
(guest:host) stoichiometry. By comparison, the change in emis-
sion of 1 in the presence of the smaller sized F− is attributed to
its greater charge density which causes strong hydrogen
bonding followed by deprotonation. Although receptor 1
demonstrates a similar order of binding with F−, I− and H2PO4−
Figure 8: Change in fluorescence emission of 2 (c = 3.93 × 10−5 M) in
the presence of 1 equiv of tetrabutylammonium salts of different guests
in CH3CN.
Figure 9: Stern–Volmer plots for 1 (c = 5.78 × 10−5 M) with H2PO4−,
F−, Br− and I− at 420 nm (up to the addition of 2 equiv of guest) in
CH3CN.
Figure 10: Change in UV–vis spectra of 1 (c = 5.78 × 10−5 M) in pres-
ence of increasing amounts of H2PO4− in CH3CN.
Table 1: Binding constant values of 1 with the guests by fluorescence method.
Guestsa
Ka (M−1)d in CH3CN
Ka (M−1)d in CHCl3 containing 0.1% CH3CN
Acetate
—b
—b
—b
—b
Benzoate
Dihydrogen phosphate
Hydrogen sulfate
Perchlorate
Fluoride
5.56 × 103c
—b
1.29 × 104
—b
—b
—b
4.06 × 103c
—b
—b
Chloride
—b
Bromide
—b
5.05 × 103
6.90 × 104
Iodide
2.41 × 103c
aTetrabutylammonium salts were used; bNot determined due to minor change; cConsidering K11; dError: ≤ ±10%.
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Beilstein J. Org. Chem. 2011, 7, 254–264.
in CH3CN, the greater fluorometric change of 1 in the presence quenching is not only due to the heavy atom effect but also
of H2PO4− is quite worth mentioning for its fluorometric involves some hydrogen bonding effects. Figure 12 is the
distinction from other anions in the present study.
Stern–Volmer plot of the quenching process. It was also noted
that while the monomeric benzimidazolium unit 2 was ineffec-
For the application of this simple receptor in aqueous system, tive in CH3CN, it showed measurable changes in emission in
we carried out the complexation study of 1 in aq CH3OH CHCl3 containing 0.1% CH3CN (Supporting Information
(CH3OH:H2O = 4:1 v/v) with different phosphate salts. Surpris- File 1). These observations thus intimate that solvent polarity is
ingly, the change in emission of 1 was found to be negligible an important aspect for monitoring the sensing behavior of
(see Supporting Information File 1). This suggested weak or no benzimidazolium-based receptors. In our opinion, CH3CN in
interactions of 1 in aq CH3OH. However, on changing the the present case participates in H-bonding with the polar C+–H
polarity of the medium both 1 and 2 responded more efficiently bond of benzimidazolium motif and reduces the possibility of
and behaved differently. When CH3CN was replaced by CHCl3 host–guest interactions [46]. This is clearly reflected in the
containing 0.1% CH3CN, the change in emission of 1 upon binding constant values in Table 1. Due to the presence of a
complexation of the same anions was very sharp and found to minimum amount of CH3CN in CHCl3 the binding constant
be sharpest for the iodide ion. Figure 11 shows the change in values for the selected anions are greater in magnitude. In the
emission of 1 in presence of particular anions. The large series I− shows a higher value of Ka. We presume that it is due
quenching of emission in the presence of I− is a characteristic to the dimension of the open cavity of 1 in CHCl3 containing
feature of 1 for the fluorometric identification of I− among the 0.1% CH3CN for which I− anion fits sterically with 1:1 stoi-
other anions in CHCl3 containing 0.1% CH3CN. Iodide is an chiometry. Other complexes in CHCl3 containing 0.1% CH3CN
important halide that plays an important role in several bio- had also 1:1 stoichiometry.
logical processes such as neurological activity and thyroid func-
tion. The iodide content of urine and milk is often required to
provide information for nutritional, metabolic, and epidemio-
logical studies of thyroid disorder [40]. In relation to this, very
few reports on iodide recognition are known in the literature
[41-45].
Figure 12: Stern–Volmer plots for 1 (c = 5.78 × 10−5 M) with H2PO4−,
F−, Br− and I− at 420 nm (up to the addition of 2 equiv of guest) in
CHCl3 containing 0.1% CH3CN.
Time resolved fluorescence measurements were additionally
carried out to study the dynamics of the flexible receptor 1
(λexc = 369 nm) in the presence and absence of the anions such
as H2PO4− in CH3CN. The emission decay profile of 1 moni-
tored at 420 nm could be fitted bi-exponentially with two
constants τ1 = 3.42 ns (12.22%), τ2 = 7.76 ns (87.78%). The
Figure 11: Change in fluorescence emission of 1 (c = 5.78 × 10−5 M)
in the presence of 2 equiv of tetrabutylammonium salts of different
guests in CHCl3 containing 0.1% CH3CN.
Iodide binding induced quenching of emission is attributed to faster decay component (3.42 ns) is due to the anthracene
the i) complementarity in size of iodide with the pseudocavity moiety [47] and the relatively stable component with longer
formed by the receptor-binding site and ii) heavy atom effect of lifetime (7.76 ns) is attributed to the benzimidazolium motif of
iodide, which is also true for Br−. But the small quenching of 1. However, in the presence of 1 equiv of H2PO4−, the lifetime
emission in the presence of Br− suggests that I− induced of both the components marginally decreases (Table 2). In the
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Beilstein J. Org. Chem. 2011, 7, 254–264.
Table 2: Fluorescence decay times (τ) and pre-exponential factors (c) for 1 in the presence and absence of anions.
τ1 (c)
τ2 (c)
τ3 (c)
χ2
Receptor 1a
3.42 ns (12.22%)
3.36 ns (11.40%)
3.49 ns (11.74%)
1.45 ns (2%)
7.76 ns (87.78%)
7.72 ns (88.60%)
7.75 ns (88.26%)
8.90 ns (98%)
—
1.13
1.18
1.13
1.12
1.10
1 + Bu4N+H2PO4− (1:1)a
1 + Bu4N+H2PO4− (1:2)a
Receptor 1b
—
—
—
1 + Bu4N+I− (1:1)b
2.15 ns (5.82%)
8.95 ns (92.86%)
0.28 ns (1.32%)
aIn CH3CN; bIn CHCl3 containing 0.1% CH3CN.
presence of 2 equiv of H2PO4−, the lifetime of the anthracene hydrogen bonds with I− in the open cavity of 1. The faster
component increases without showing any marked change in decay component (0.28 ns) is assumed to be either due to a very
the pre-exponential factor. This was also true for the benzimida- short-lived species or an artifact or for tunneling of extra energy
zolium component. Figure 13 shows the decay profiles. The to the bulk by a non-radiative pathway [47,48].
change in the lifetimes of the components in 1 in the presence
of H2PO4− ions can be correlated with the change in emission
intensity in Figure 4. The fluorescence dynamics of 1 was also
carried out for 1 in the presence and absence of I− in CHCl3
containing 0.1% CH3CN. On changing the solvent combination,
the lifetimes of the anthracene and benzimidazolium compo-
nents of 1 changed (Table 2). Interestingly, while the faster
decay component (1.45 ns) due to anthracene moiety
contributed a small pre-exponential factor (2%), the relatively
stable benzimidazolium component (8.90 ns) had a larger
contribution (98%) to the total emission of the molecule.
However, upon addition of equivalent amounts of I− the decay
profile followed a tri-exponential fitting that indicated three
emitting species with lifetimes τ1 = 2.15 ns (5.82%),
τ2 = 8.95 ns (92.86%) and τ3 = 0.28 ns (1.32%) (Figure 14). A
small increase in the lifetimes of both the anthracene and benz-
Figure 14: Fluorescence decays (at λmax = 420 nm) of receptor 1 upon
the addition of I− ion ([H] = 5.78 × 10−5 M, [G] = 4.55 × 10−3 M) in
CHCl3 containing 0.1% CH3CN.
imidazolium moieties in 1 is attributed to the formation of
The expected strong interaction of 1 with H2PO4− was estab-
lished additionally by 1H NMR. Although the guest is primarily
bound by the benzimidazolium motif on account of a
charge–charge interaction, there is undoubtedly a degree of
cooperation from the amide protons as evidenced by the down-
field change in their chemical shift positions in 1H NMR.
Figure 15 indicates the change in chemical shift of the inter-
acting amides and benzimidazolium protons upon complexa-
tion of H2PO4− ions and shows the broadening of the signals of
1 in the 1H NMR spectrum. A precipitate appeared during the
course of the study due to insolubility in the NMR concentra-
tion range and this was one reason for not determining the
binding constant values by the NMR method. However, both
the amide and benzimidazolium protons (0.51 ppm) were found
to move downfield and thereby supported our binding proposi-
tion as indicated in Figure 5. The exact position of amide
protons Ha was difficult to determine upon complexation.
Figure 13: Fluorescence decays (at λmax = 420 nm) of receptor 1 upon
the addition of H2PO4− ion ([H] = 5.78 × 10−5 M, [G] = 4.33 × 10−3 M)
in CH3CN solvent.
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Beilstein J. Org. Chem. 2011, 7, 254–264.
Experimental
Hydrogen bonding and deprotonation of 1 in the presence of F−
was also evidenced from 1H NMR (Supporting Information General methods
File 1, Figure 11S). In the presence of 1 equiv of F− the benzim- Solvents were distilled prior to use, and dried according to the
idazolium protons in 1 moved downfield by 0.30 ppm which literature procedure when required. Chromatographic separa-
became 0.59 ppm when 2 equiv of F− were added to the tions were performed on silica gel (60–120 mesh). All melting
receptor solution.
points were determined in open capillaries and are uncorrected.
1H NMR spectra were recorded on Bruker 400 and 300 MHz
spectrometers. 13C NMR spectra were recorded on a Bruker
400 MHz spectrometer. FT IR spectra were recorded on a
Perkin-Elmer L120-00A spectrometer as KBr discs. UV–vis
spectra were recorded on Perkin-Elmer Lambda-25 and fluores-
cence spectra on a Perkin-Elmer LS 55 spectrofluorometer, res-
pectively.
Synthesis of 3 [49]
To a solution of benzimidazole (0.300 g, 2.54 mmol) in dry
THF (15 mL), NaH (0.14 g) was added and the mixture re-
fluxed for 1 h under a nitrogen atmosphere. The reaction mix-
ture was then cooled to room temperature, 9-chloromethylan-
thracene (0.700 g, 3.09 mmol) in THF (15 mL) added and then
heated under reflux for 10 h. The THF was removed, water
added and the mixture extracted with CHCl3 (3 × 30 mL). The
organic layer was dried over anhydrous Na2SO4 and concen-
trated on a rotary evaporator. Purification of the crude product
by silica gel column chromatography with 20% ethyl acetate in
petroleum ether as eluent gave compound 3 (0.500 g, yield
64%). 1H NMR in CDCl3 (400 MHz) δ 8.61 (s, 2H), 8.10 (d,
4H, J = 8 Hz), 7.82 (d, 1H, J = 8 Hz), 7.71 (d, 1H, J = 8 Hz),
Figure 15: Partial 1H NMR (300 MHz, CDCl3 containing 4% CD3CN)
spectra of (a) 1 (c = 1.38 × 10−3 M), (b) 1:1 and (c) 2:1 (guest:host)
complexes with H2PO4− [see labeled structure 1].
Conclusion
In conclusion, we have designed a new type of flexible receptor 7.51 (m, 4H), 7.42 (t, 1H, J = 8 Hz), 7.35(t, 1H, J = 8 Hz), 6.19
1, which uniquely responses the recognition of H2PO4− and I− (s, 2H); 13C NMR (CDCl3, 100 MHz) δ 144.0, 142.2, 134.2,
by exhibiting large quenching of emission of anthracene group 131.4, 131.0, 129.7, 129.5, 127.5, 125.4, 123.6, 123.1, 123.0,
in solvents of different polarities. Two benzimidazolium motifs 122.5, 120.6, 109.5, 41.3; m/z (ES+): 308.9 [M]+.
with an isophthaloyl spacer group in 1, act cooperatively for
successful detection of H2PO4− in CH3CN. In comparison, Synthesis of 4
under the same experimental condition, a single benzimida- A mixture of 2-chloroethylamine hydrochloride (704 mg,
zolium unit in 2 is inefficient in sensing H2PO4− ion. By 6 mmol) in dry CH2Cl2 (30 mL) containing triethylamine
contrast, in the less polar solvent CHCl3 containing 0.1% (1.35 mL) was stirred for 1 h to give the free amine. To this
CH3CN, the receptor shows a greater fluorescence quenching solution, isophthaloyl dichloride (600 mg, 2.9 mmol) was added
for I−. Such differential quenching of 1 in the presence of and the reaction mixture stirred at room temperature for 8 h.
H2PO4− and I− in different solvent combinations is presumably The solvent was evaporated and water added. The aqueous layer
due to the activation of the PET process occurring between the was extracted with CHCl3 (3 × 50 mL), the combined organic
binding site and the excited state of anthracene function at the phases was washed with water, dried over anhydrous Na2SO4
different rates. We believe at present that the different degrees and concentrated in vacuo. The resulting crude solid was puri-
of solvation of the anions as well as the effective dimension of fied by column chromatography with 50% ethyl acetate in
the binding site which in turn, controls the selectivity in binding petroleum ether as eluent to give pure 4 (0.67 g, yield 78%), mp
process in the excited state are the key factors that regulate the 145 °C, 1H NMR (CDCl3, 400 MHz): δ 8.25 (s, 1H), 7.94 (dd,
PET to different extents. The binding affinity and selectivity of 2H, J1 = 8 Hz, J2 = 4 Hz), 7.53 (t, 1H, J = 8 Hz), 6.84 (br t, 2H,
this simple fluororeceptor are associated with the combined -NH-), 3.84–3.79 (m, 4H), 3.75–3.71 (m, 4H); 13C NMR
effects of semi-rigid structures of receptor, charge-charge inter- (CDCl3 containing few drops DMSO-d6, 100 MHz) δ 166.0,
actions, and the involvement of both N–H---O and C–H---O 134.21, 129.8, 128.3, 126.2, 42.8, 41.3; FTIR: ν cm−1 (KBr):
hydrogen bonds. Further work is underway in our laboratory. 3288, 3066, 2915, 2863, 1639, 1539; m/z (LCMS): 289 (M)+.
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Beilstein J. Org. Chem. 2011, 7, 254–264.
Synthesis of 1
46.4, 43.2, 37.2; FTIR: ν cm−1 (KBr): 3436, 1718, 1654, 1563,
To a stirred solution of 4 (0.1 g, 3.8 mmol) in dry CH3CN (20 1453. m/z (LCMS): 394.2 [(M − PF6−) ]+.
mL), compound 3 (0.236 g, 7.6 mmol) was added and the reac-
tion mixture heated under reflux for 3 days. During the heating General procedure of fluorescence and UV–vis titrations:
the dichloride salt 5 precipitated and was removed by filtration. Stock solutions of the receptors were prepared in different
Repeated recrystallization of the salt from CH3CN gave almost solvents such as CH3CN and CHCl3 containing 0.1% CH3CN
pure 5 (0.184 g, 59.7% yield). Compound 5 (0.184 g) was then and 2.5 ml of the individual receptor solution was placed in the
dissolved in hot CH3OH (20 mL) followed by addition of cuvette. Stock solutions of anions were prepared in the same
NH4PF6. After stirring the reaction mixture for 30 min, the solvents, and were added individually in different amounts to
CH3OH was evaporated to reduce the volume. The precipitated the receptor solution. For fluorescence, the receptor solutions
salt was filtered, washed with cold water and dried. Repeated prepared in dry CH3CN were irradiated at the excitation wave-
recrystallization of 1 from CH3CN afforded the pure 1 (0.21 g, length 369 nm maintaining the excitation and emission slits 8
90% yield), mp 210 °C, 1H NMR (DMSO-d6, 400 MHz): δ 8.94 and 8, respectively and the filter was 1% attenuated. Upon the
(s, 2H), 8.83 (s, 2H), 8.50–8.10 (m, 14H), 7.78–7.72 (m, 4H), addition of anions, the change in emission and absorbance of
7.59–7.49 (m, 12H), 6.63 (s, 4H), 4.46 (br s, 4H), 3.59 (br s, the receptors were noted. Similarly, for emission, the receptors
4H) [due to solubility problem signals were broad in nature]; dissolved in CHCl3 containing 0.1% CH3CN were irradiated at
13C NMR (DMSO-d6, 100 MHz): 141.8, 134.3, 132.2, 132.0, the excitation wavelength 369 nm maintaining the excitation
131.5, 131.47, 131.43, 131.1, 130.8, 129.7, 128.2, 128.1, 127.3, and emission slits 10 and 5, respectively.
127.1, 126.0, 125.9, 123.7, 122.0, 114.6, 114.1, 47.0, 43.7,
38.7; FTIR: ν cm−1 (KBr): 3433, 3146, 2954, 1721, 1652, 1565, Method for Job plots: The stoichiometry was determined by
1531; m/z (ES+): 1123.7 (M − 1)+, 979.5 [(M − PF6−) ]+.
the continuous variation method (Job Plot) [35]. In this method,
solutions of host and guests of equal concentrations were
prepared in dry CH3CN and CHCl3 containing 0.1% CH3CN.
Synthesis of 2
A mixture of 2-chloroethylamine hydrochloride (460 mg, 3.9 Then the host and guest solutions were mixed in different
mmol) in dry CH2Cl2 (20 mL) containing triethylamine (1 mL) proportions maintaining a total volume of 3 mL of the mixture.
was stirred at room temperature for 1 h to give the free amine. The related compositions for host:guest (v/v) were 3:0, 2.8:0.2,
Acetyl chloride (0.582 mL, 5.9 mmol) was added to the reac- 2.5:0.5, 2.2:0.8, 2:1, 1.8:1.2, 1.5:1.5, 1:2, 0.8:2.2, 0.5:2.5,
tion mixture which was then stirred at room temperature for 8 h. 0.2:2.8. All the prepared solutions were kept for 1 h at room
After completion of the reaction, the solvent was evaporated temperature with occasional shaking. Then the emission and
and the residue extracted with CHCl3 (3 × 20 mL). The organic absorbance of the solutions of different compositions was
extracts were washed with NaHCO3 solution (3 × 15 mL) and recorded. The concentration of the complex, i.e., [HG] was
dried over anhydrous Na2SO4. The solvent was removed under calculated using the equation [HG] = ΔI/I0 × [H] or [HG] = ΔA/
vacuum and the residue purified by silica gel column chroma- A0 × [H] where ΔI/I0 and ΔA/A0 indicate the relative emission
tography with 80% ethyl acetate in petroleum ether as eluent to and absorbance intensities. [H] corresponds to the concentra-
afford the compound 6 (0.309 g, 64% yield). The chloro-amide tion of pure host. The mole fraction of the host (XH) was plotted
6 (150 mg, 1.2 mmol) was heated under reflux in dry CH3CN against the concentration of the complex [HG]. In the plot, the
(20 mL) with the anthracene-coupled benzimidazole 3 (570 mg, mole fraction of the host at which the concentration of the
1.85 mmol) for 4 days. The precipitated chloride salt was host–guest complex concentration [HG] is maximum, gives the
filtered, washed with water and dried (0.51 g, 64.4%). The com- stoichiometry of the complex.
pound 6 in MeOH was subsequently treated with aqueous
NH4PF6 solution to carry out the anion exchange reaction. The
Supporting Information
solution was heated with stirring for 20 min until a precipitate
appeared. Filtration of the precipitate followed by thorough
Supporting Information File 1
washing with ether afforded compound 2 in 95% yield (0.61 g),
mp 203 °C (decomposition); 1H NMR (DMSO-d6, 400 MHz): δ
8.93 (s, 1H), 8.82 (s, 1H), 8.41 (d, 1H, J = 8 Hz), 8.33 (d, 2H, J
= 8 Hz), 8.27 (d, 2H, J = 8 Hz), 8.08 (d, 1H, J = 8 Hz)
7.84–7.78 (m, 2H), 7.63 (br m, 5H), 6.67 (s, 2H), 4.32 (br s,
2H), 3.50 (br s, 2H), 2.32 (s, 3H); 13C NMR (DMSO-d6, 100
MHz): 169.4, 141.2, 131.8, 131.4, 131.1, 131.0, 130.4, 129.3,
127.8, 126.8, 126.6, 125.6, 123.3, 121.5, 114.2, 113.5, 57.4,
Supplementary Data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-7-34-S1.pdf]
Acknowledgements
We thank CSIR, Government of India for financial support.
K.G. thanks DST, Government of India for providing facilities
262

Beilstein J. Org. Chem. 2011, 7, 254–264.
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in the department under FIST program and also to UGC for the
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